bluuebunny/crossref_metadata_2025 huggingface dataset:
Endogenous GABA attenuates CNS white matter dysfunction following anoxia
Authors: Fern R , Waxman SG , Ransom BR
Published: April 2018

We studied the effects of GABA on anoxia-induced injury in CNS white matter using optic nerves exposed to 60 min of anoxia. Injury was assessed by recording pre- and postanoxic compound action potentials (CAPs). GABA (1 microM) significantly increased postanoxic CAP recovery when applied 60 min prior to anoxia. This effect was bicuculline (100 microM) insensitive, mimicked by baclofen (1 microM), blocked by GABA-B antagonists, and not mimicked by selective GABA-A agonists. GABA therefore acted at GABA-B receptors. High concentrations of GABA and baclofen did not influence recovery, possibly indicating GABA-B receptor desensitization at high agonist concentrations. Pertussis toxin (PTX) treatment reduced postanoxic CAP recovery in the presence of 1 microM GABA to control levels, indicating the recruitment of a G- protein-linked intracellular pathway. Protein kinase C (PKC) activation with 12-myristate 13-acetate (PMA) mimicked the effects of GABA. Inhibition of PKC with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7) or staurosporine reduced postanoxic recovery in the presence of GABA to lower levels than under control conditions, confirming the involvement of PKC in the protective effect of GABA and indicating that this GABA-B receptor/G-protein/PKC protective pathway might be active under control conditions. This was confirmed by the observation that GABA-B receptor blockade, in the absence of exogenous GABA, significantly reduced postanoxia recovery. Thus, activation of the protective mechanism under control conditions is due to endogenous GABA release. Increasing the level of endogenous extracellular GABA by blocking GABA uptake with 1 mM nipecotic acid also protected against anoxia. We propose a model where release of GABA in white matter helps to limit nerve fiber injury during anoxia via recruitment of a G- protein/PKC pathway with subsequent phosphorylation of an unknown target protein.
Regulation of protease nexin-1 expression in cultured Schwann cells is mediated by angiotensin II receptors
Authors: Bleuel A , de Gasparo M , Whitebread S , Puttner I , Monard D
Published: April 2018

Protease nexin-1 (PN-1) is a potent inhibitor of serine proteases, such as thrombin and plasminogen activators, which is secreted into the extracellular space. Since PN-1 is induced following lesion of the sciatic nerve, the effect of substances known to accumulate at the site of injury was examined in primary cultures of Schwann cells. Among the cytokines, growth factors, mitogens, neurotrophins, and neuroactive peptides analyzed, only angiotensin II (Ang II), calcitonin gene- related peptide (CGRP), and vasoactive intestinal peptide (VIP) were found to regulate the expression of PN-1 on Schwann cells. While Ang II and CGRP caused downregulation, VIP acted as a positive modulator of PN- 1. Displacement of Ang II binding using the selective ligands losartan and CGP 42112 led to a severalfold increase of PN-1 protein and mRNA over basal levels, indicating that the observed effect was mediated by specific binding sites. Indeed, the presence of AT1 and AT2 angiotensin receptor subtypes was demonstrated in cultured Schwann cells as well as in the rat sciatic nerve. Moreover, the detection of angiotensinogen- and renin-mRNA in these cultures suggested an endogenous production of Ang II. This data identified one of the mechanisms regulating PN-1 synthesis. Altogether our results indicate that neuropeptides can differentially control the proteolytic activity of the microenvironment, providing new aspects of neuron-glia interactions in the intact tissue and following nerve injury.
Cutaneous activation of the inhibitory L30 interneurons provides a mechanism for regulating adaptive gain control in the siphon withdrawal reflex of Aplysia
Authors: Fischer TM , Carew TJ
Published: April 2018

The functional role of inhibition in the neural network underlying the siphon withdrawal response (SWR) of Aplysia was assessed by examining a recurrent circuit comprised of identified inhibitory interneurons (L30s), and excitatory interneurons (L29s). We previously showed that activity-dependent potentiation of the L30 inhibitory synapse onto L29 can regulate the net excitatory input elicited by tactile siphon stimulation onto siphon motor neurons (LFS cells) (Fischer and Carew, 1993a). To explore the functional significance of L30 potentiated inhibition, we have examined how a behaviorally relevant stimulus that activates the L30 interneurons modulates the SWR circuit. Utilizing a reduced preparation, we show that weak tactile stimulation of the tail strongly activates the L30s, and leads to significant potentiation of the L30 synapse. Next, we demonstrate that similar weak tail stimulation produces significant inhibition of siphon tap-evoked responses in both L29 interneurons and LFS motor neurons. We further show that this form of inhibition is transient, having a time course of approximately 60 sec. Finally, we directly tested the role of the L30s in mediating this form of inhibition by hyperpolarizing two (of three) L30 interneurons during tail stimulation. L30 inactivation significantly attenuated tail stimulation-induced inhibition of siphon- evoked input to both L29 interneurons and LFS motor neurons. Based on these results, we suggest that L30-potentiated inhibition may have an important adaptive role in optimizing the signal-to-noise ratio for activation of the SWR circuit by providing stabilization of SWR responsiveness under a wide range of environmental conditions.
A single identified interneuron gates tail-shock induced inhibition in the siphon withdrawal reflex of Aplysia
Authors: Wright WG , Carew TJ
Published: April 2018

The marine mollusc Aplysia has proven very useful for a mechanistic analysis of behavioral modification. Among the stimuli used to modify the behavior of Aplysia, a noxious stimulus, tail shock, is one of the most effective. In addition to the extensively analyzed facilitatory effects of tail shock, recent work has demonstrated that it also produces marked transient inhibition in reflex responses. Here we report that functional removal (by hyperpolarization or voltage clamp) of a single inhibitory interneuron, L16, can eliminate most, if not all, of the inhibition in the siphon withdrawal reflex circuit produced by tail shock. In addition, this interneuron is strongly activated by tail shock. Finally, direct intracellular activation of L16 does not, in itself, reliably produce inhibition, suggesting that L16 plays a gating role which is necessary for the expression of inhibition in the siphon withdrawal circuit. These results support the idea that behaviorally relevant neural modulation can be gated by a small number of neurons, in this case, by a single identified cell. Moreover, they indicate that in Aplysia, as in many other systems, the modulatory effects of a noxious stimulus are often funneled through a restricted neural locus before being distributed to the circuits actually responsible for generating the behavioral output.
Representation of curved surfaces in responses of mechanoreceptive afferent fibers innervating the monkey's fingerpad
Authors: Goodwin AW , Browning AS , Wheat HE
Published: April 2018

The aim was to elucidate how the population of digital nerve afferents signals information about the shape of objects in contact with the fingerpads during fine manipulations. Responses were recorded from single mechanoreceptive afferent fibers in median nerves of anesthetized monkeys. Seven spherical surfaces were used, varying from a highly curved surface (radius, 1.44 mm; curvature, 694 m-1) to a flat surface (radius, infinity; curvature, 0 m-1). These were applied to the fibers' receptive fields, which were located on the central portion of a fingerpad. When the objects were located at the centers of the receptive fields, the responses of the slowly adapting fibers (SAIs) increased as the curvature of the surface increased and as the contact force increased. All SAIs behaved in the same way, differing only by a scaling factor (the sensitivity of the individual afferent). Responses of the rapidly adapting afferents were small and did not vary systematically with the stimulus parameters, and most Pacinians did not respond at all. Stimuli were applied at different positions in the receptive fields of SAIs to define the response profiles of the afferents (response as a function of position on the fingerpad). All SAIs had similarly shaped profiles for the same surface curvature and the shape differed for different curvatures. These profiles reflected the shape of the stimulus. An increase in contact force scaled these profiles upward. Thus, the population of digital nerve fibers signals unambiguous information about the shape and contact force of curved surfaces contacting the fingerpad.
Receptor encoding of moving tactile stimuli in humans. II. The mean response of individual low-threshold mechanoreceptors to motion across the receptive field
Authors: Essick GK , Edin BB
Published: April 2018

The mean firing rate evoked in 70 cutaneous, low-threshold mechanoreceptors in the human median, radial, and inferior alveolar nerves by stimulus motion across the skin was quantitatively studied. Moving stimuli, controlled for velocity, direction, and length of skin traversed, were provided by a servo-controlled motor that carried a brush across the receptive field. Each unit was studied with stimuli delivered at multiple velocities from 0.5 to 32 cm/sec in at least two opposing directions. A power function provided an excellent description of the MFR-versus-velocity relationship. The exponent n was interpreted to reflect the receptor's sensitivity to changes in stimulus velocity, and the multiplicative constant c, the predicted response to stimuli moving at 1.0 cm/sec. The fast adapting mechanoreceptors exhibited higher sensitivity to stimulus velocity than the slowly adapting mechanoreceptors. The mean velocity at which the fast adapting units were predicted to first respond to movement was also higher. Estimates of n, c, or both differed significantly for stimuli delivered in opposing directions for more than 70% of the mechanoreceptors. No direction of motion consistently led to power function parameters with higher values so as to suggest a “preferred” regional direction of motion for the entire population. Neither the directional difference in n nor c could be attributed to directional differences in the forces applied across the receptive fields. These findings suggest that information about velocity and direction is represented in the mean firing rate responses evoked in the population of mechanoreceptors activated by a moving tactile stimulus.
Soluble factors from the olfactory bulb attract olfactory Schwann cells
Authors: Liu KL , Chuah MI , Lee KK
Published: April 2018

Olfactory Schwann cells (OSCs) extend processes that ensheathe bundles of olfactory axons as they course from the olfactory epithelium to the olfactory bulb (OB). Results of morphological and immunohistochemical studies have led to speculation that OSCs may be involved in guiding the olfactory axons to their target tissue. In this study we have explored this possibility by investigating the relationship between OSCs and the OB. Olfactory Schwann cells labelled with 1,1′-dioctadecyl 3,3,3′,3′-tetramethylindocarbocyanine perchlorate (Dil) were injected into the nasal region of E14 rat embryos and entire embryos were cultured for 24 hr. It was found in some embryos, that the OSCs had migrated toward the presumptive OB. Cocultures of neonatal OB explants on OSC monolayers showed that the OSCs were attracted to the OB and formed a ring-like aggregate around the explant after 48 hr culture. This attraction was absent when a piece of cerebrum was used in place of the OB. When medium conditioned by neonatal OBs was placed in the lower compartment of the chemotaxis chamber, OSCs seeded in the upper compartment migrated through the pores of the nucleopore filter to reach the underside which was in contact with the conditioned medium. After 6 hr of incubation, scanning electron microscopy was performed on the underside of the nucleopore filters. Cell counts of OSCs showed that the cell density was significantly higher when medium conditioned by OBs was used instead of unconditioned medium or medium conditioned by cerebrum. The results of these experiments show that the OSCs migrate toward the OB under the influence of soluble factor(s) secreted by the target tissue.
Delayed neuronal death in the CA1 pyramidal cell layer of the gerbil hippocampus following transient ischemia is apoptosis
Authors: Nitatori T , Sato N , Waguri S , Karasawa Y , Araki H , Shibanai K , Kominami E , Uchiyama Y
Published: April 2018

The CA1 pyramidal neurons in the hippocampus are selectively vulnerable to transient ischemic damage. In experimental animals, the CA1 pyramidal neurons undergo cell death several days after brief forebrain ischemia. It remains, however, unknown whether this delayed neuronal death is necrosis or apoptosis. To investigate the degenerating processes of the CA1 pyramidal neurons in gerbil hippocampus after brief ischemia, lysosomal and nuclear alterations in the cells were examined using immunocytochemistry, in situ nick-end labeling, and Southern blotting. By light and electron microscopy, immunoreactivity for cathepsins B, H, and L, representative lysosomal cysteine proteinases, increased in the CA1 pyramidal neurons 3 d after ischemic insult, which showed cell shrinkage. By morphometric analysis, the volume density of cathepsin B-positive lysosomes markedly increased 3 d after ischemic insult, while that of autophagic vacuole-like structures also increased at this stage, suggesting that cathepsin B- immunopositive lysosomes increasing in the neurons after ischemic insult are mostly autolysosomes. Nuclei of the CA1 neurons were nick- end labeled by biotinylated dUTP mediated by terminal deoxytransferase 3 and 4 d after ischemic insult, but not in the prior stages. Simultaneously, dense chromatin masses appeared in nuclei of the neurons. By Southern blotting, laddering of DNA occurred only in CA1 hippocampal tissues obtained 4 d after ischemic insult. Confocal laser scanning microscopy demonstrated that the fragmented DNA in the CA1 pyramidal layer was phagocytosed by microglial cells. The results suggest that delayed death of the CA1 pyramidal neurons after brief ischemia is not necrotic but apoptotic.
Characterization of K+ currents and the cAMP-dependent modulation in cultured Drosophila mushroom body neurons identified by lacZ expression
Authors: Wright NJ , Zhong Y
Published: April 2018

Electrophysiological analysis of cultured neurons provides a potential approach toward understanding the physiological defects that may contribute to abnormal behavior exhibited by mutants of the fruit fly Drosophila. However, its application has been restricted by an inability to identify a particular functional or anatomical subpopulation of neurons from the CNS. To study neurons composing the CNS mushroom body proposed as a center for insect olfactory learning, we utilized a Drosophila enhancer detector line that expresses a lacZ reporter gene in these neurons and identified them in acutely dissociated larval CNS cultures by vital fluorescent staining. The patch-clamp analysis suggests that whole-cell voltage-activated K+ currents can be classified into two types in identified mushroom body neurons. Type 1 current comprises a TEA-sensitive slowly inactivating current and noninactivating component while type 2 current contains a 4- AP-sensitive transient A-current and a noninactivating component. Application of cAMP analogs induced distinct modulation of type 1 and type 2 currents. Our results demonstrate that the expression of the lacZ gene and the subsequent staining do not significantly alter the different types of K+ currents. This initial characterization provides a basis for further analysis of mutations that impair learning and memory resulting from an abnormal cAMP cascade preferentially expressed in the mushroom body.
Cloning and characterization of a calcium channel alpha 1 subunit from Drosophila melanogaster with similarity to the rat brain type D isoform
Authors: Zheng W , Feng G , Ren D , Eberl DF , Hannan F , Dubald M , Hall LM
Published: April 2018

We report the complete sequence of a calcium channel alpha 1 subunit cDNA cloned from a Drosophila head cDNA library. This cDNA encodes a deduced protein containing 2516 amino acids with a predicted molecular weight of 276,493. The deduced protein shares many features with vertebrate homologs, including four repeat structures, each containing six transmembrane domains, a conserved ion selectivity filter region between transmembrane domains 5 and 6, and an EF hand in the carboxy tail. The Drosophila subunit has unusually long initial amino and terminal carboxy tails. The region corresponding to the last transmembrane domain (IVS6) and the adjacent cytoplasmic domain has been postulated to form a phenylalkylamine-binding site in vertebrate calcium channels. This region is conserved in the Drosophila sequence, while domains thought to be involved in dihydropyridine binding show numerous changes. The Drosophila subunit exhibits 78.3% sequence similarity to the rat brain type D calcium channel alpha 1 subunit, and so has been designated as a Drosophila melanogaster calcium channel alpha 1 type D subunit (Dmca1D). In situ hybridization shows that Dmca1D is highly expressed in the embryonic nervous system. Northern analysis shows that Dmca1D cDNA hybridizes to three size classes of mRNA (9.5, 10.2, and 12.5 kb) in heads, but only two classes (9.5 and 12.5 kb) in bodies and legs. PCR analysis suggests that the Dmca1D message undergoes alternative splicing with more heterogeneity appearing in head and embryonic extracts than in bodies and legs.
Migratory paths of neurons and glia in the embryonic chick spinal cord
Authors: Leber SM , Sanes JR
Published: April 2018

To study the migration of chick spinal cord neurons, we labeled individual cells in the ventricular zone with recombinant retroviruses, then identified their progeny histochemically. First, we analyzed cell mixing in the ventricular zone. Some clones labeled at early neural tube stages spread widely along both the dorsoventral and rostrocaudal axes. However, clones labeled later were confined to narrow domains along both axes. These results imply that displacement of cells within the ventricular zone becomes progressively restricted. Second, we studied the migration of cells out of the ventricular zone by infecting embryos at a fixed stage and varying the time of analysis. At first, most clones consisted of radial arrays of cells, suggesting that the initial migration is predominantly radial. In many clones, however, neurons turned orthogonally from parental radial arrays and migrated along the path of circumferentially oriented axons. By hatching, clonally related cells in the gray matter were usually distributed in narrow transverse slabs, but some white matter glial cells had migrated longitudinally for up to several segments. We conclude that the dispersal of clonally related cells results from (1) early mixing of progenitors within the neural tube; (2) radial stacking of progeny in the ventricular zone; (3) migration of progeny from the ventricular zone in spoke-like routes; (4) circumferential migration of some neurons along axons; (5) short-distance dispersal of differentiating neurons; and (6) a late, longitudinal migration of glia through white matter tracts. Finally, we show that floor plate cells differ from other spinal cord cells in both their
Presynaptic enhancement of inhibitory synaptic transmission by protein kinases A and C in the rat hippocampus in vitro
Authors: Capogna M , Gahwiler BH , Thompson SM
Published: April 2018

The protein kinase C activator phorbol 12,13-dibutyrate (0.5 microM, PDBu) and the protein kinase A activator forskolin (20 microM) each increased evoked monosynaptic inhibitory postsynaptic current (IPSC) amplitude, without affecting its reversal potential, and increased the frequency of miniature IPSCs (mIPSCs), without affecting their amplitude or kinetics, as assessed with whole-cell recording form CA3 pyramidal cells in hippocampal slice cultures. The effects of forskolin and PDBu on both evoked IPSC amplitude and mIPSC frequency were additive and were antagonized by inhibitors of protein kinases A and C, respectively. The kinase activator-induced increases in mIPSC frequency were quantitatively comparable to the increases in evoked IPSC amplitude. The increases in mIPSC frequency were not attenuated by the voltage-dependent calcium channel blocker Cd2+ (100 microM). We conclude that stimulation of protein kinases A and C potentiates hippocampal inhibitory synaptic transmission through independent presynaptic mechanisms of action. Kinase-induced potentiation of spontaneous release does not require modulation of axon terminal Ca2+ channels. This mechanism may also contribute substantially to the potentiation of evoked release.
Regeneration of the auditory midbrain intercommissural projection in organotypic culture
Authors: Hafidi A , Sanes DH , Hillman DE
Published: April 2018

The aim of this study was to determine whether postnatal mammalian central neurons retain the capacity for axonal regeneration across a lesion site in organotypic cultures of the auditory midbrain. Brain slices from the gerbil inferior colliculus (IC) were obtained from postnatal 6–8 d old animal and cultured for 6–15 d in vitro. IC explants containing an intact commissural projection exhibited robust axonal and dendritic morphologies as assessed with biocytin labelling. In transected explants, the two lobes of the inferior colliculus were cut at the midline and then reapposed to one another in vitro. There was a robust regeneration of commissural fibers across the lesion site in 78% of the biocytin-labeled explants. Massive axonal regeneration was also revealed by immunostaining explants for Tau (100% of sections), an axon-specific microtubule-associated protein. Ultrastructural analyses demonstrated that biocytin-labeled regenerating fibers established de novo synaptic profiles in the contralateral lobe of the inferior colliculus. Finally, the distribution of astrocytes and oligodendrocytes were assessed by staining for glial fibrillary acidic protein (GFAP) and myelin- associated glycoprotein (MAG), respectively. GFAP-positive astrocytes were more widely distributed than in vivo, and oligodendrocytes remained immature, and evenly distributed in all explants. Taken together, these data demonstrate that the postnatal mammalian auditory midbrain can be maintained in vitro, and that central axons are capable of regenerating across the site of injury without the aid of an artificial substrate.
Hippocampus norepinephrine, caudate dopamine and serotonin, and behavioral responses to the stereoisomers of amphetamine and methamphetamine
Authors: Kuczenski R , Segal DS , Cho AK , Melega W
Published: April 2018

Microdialysis in behaving animals was used to concomitantly characterize the dopamine and 5-HT responses in the caudate and the norepinephrine response in the hippocampus to the D- and L-isomers of amphetamine and methamphetamine. Doses of all four drugs which promoted similar stereotypy responses produced a D-amphetamine-like response profile of dopamine and dopamine metabolites, suggesting that all these drugs interact with dopamine systems to facilitate the release of transmitter. However, in contrast to the similar behavioral profiles, the magnitude of the dopamine responses diverged significantly. In addition, all four drugs increased extracellular norepinephrine and 5- HT, but the relative responses differed markedly from dopamine and from each other. The contrasting structure-activity relationships for these drugs likely reflect their differential potency at the various neuronal uptake transporters in promoting either transmitter release, and/or uptake blockade. In addition, the interaction of each drug at the vesicular transporters, as well as the availability of a cytoplasmic pool of transmitter likely also contribute to the neurotransmitter response. Because of the particularly divergent transmitter response profiles exhibited by L-methamphetamine, its behavioral and neurotransmitter effects were characterized over a more extended range of doses. Although the duration of the increase in extracellular dopamine was clearly proportional to dose, the dose-dependent increases in the magnitude of the dopamine response did not parallel the behavioral profiles. The results of these studies indicate that, while the dopamine, norepinephrine and 5-HT responses to these drugs probably contribute to the expression of stimulant-induced behaviors, simple relationships between the neurotransmitter responses and the behavioral profiles were not evident.
Functional expression of Shaker K+ channels in cultured Drosophila "giant" neurons derived from Sh cDNA transformants: distinct properties, distribution, and turnover
Authors: Zhao ML , Sable EO , Iverson LE , Wu CF
Published: April 2018

Expression of transgenic Shaker (Sh) channels has not previously been examined in Drosophila neurons. We studied K+ current by whole-cell recording in cultured “giant” neurons derived from germline transformants. Independent lines were generated by using a P-element vector, in which transcription of the 29–4 cDNA, one of the Sh splicing variants (Iverson and Rudy, 1990), was under the control of a heat shock (HS)-inducible promoter. Transformants in wild-type and two different Sh mutant backgrounds all exhibited an HS-inducible, A-type K+ current that was characterized by a much slower recovery from inactivation and a higher sensitivity to 4-aminopyridine than native K+ currents of Sh 29–4 currents expressed in Xenopus oocytes. Despite similarities in the kinetic and pharmacological properties of the HS- induced current in all backgrounds examined, host-dependent differences in the peak current amplitude have been consistently observed between multiple lines of 29–4 ShM and 29–4 Sh120 that might reflect differential channel subunit assembly in different hosts. Isolation of the novel 29–4 currents allowed determination of the channel turnover rate in cultured neurons. These currents persisted for up to 3 d or more, comparable with the durations previously reported for Na+ and Ca2+ channels. Surprisingly, the percentage of cells expressing inactivating K+ currents remained approximately the same with or without HS induction, suggesting that some mechanisms exist to restrict functional expression of inactivating K+ channels, including transgenic Sh channels and those not encoded by the Sh locus, to certain types of neurons.
Mamillothalamic tract transection blocks anterior thalamic training- induced neuronal plasticity and impairs discriminative offidance behavior in rabbits
Authors: Gabriel M , Cuppernell C , Shenker JI , Kubota Y , Henzi V , Swanson D
Published: April 2018

Rabbits with bilateral transecting lesions of the mamillothalamic tract, control (tract-sparing and sham) lesions, or no lesions, and chronic, fixed-position anterior ventral (AV) and medial dorsal (MD) thalamic and posterodorsal subicular complex unit recording electrodes were trained to step in an activity wheel in response to a 0.5 sec tone (CS+) in order to avoid a brief foot shock. The rabbits also learned to ignore a different tone (CS-) not predictive of shock. Behavioral acquisition was significantly retarded in rabbits with mamillothalamic tract transection compared to controls. When trained, transected rabbits failed to avoid the shock more often than controls. Mamillothalamic tract transection abolished and control lesions attenuated AV thalamic discriminative training-induced activity (i.e., development with training of greater discharges in response to the CS+ than to the CS-). Transection and control lesions attenuated AV thalamic excitatory training-induced activity (greater elicited activity during training than during unpaired tone-shock presentations before training) as well as AV thalamic “spontaneous” baseline unit activity. CS-elicited discharge magnitude was reduced by control lesions and it was further reduced by tract transecting lesions. Significant lesion-related changes were not found in the subicular or MD thalamic neuronal receptor. Mamillothalamic tract afferent information flow is thus essential for AV thalamic discriminative training-induced activity, excitatory training-induced activity, tone- elicited discharges and maintenance of conditioned avoidance responses. The effects of the control lesions suggested that afferents which course in parallel with and near the mamillothalamic tract may contribute to AV thalamic spontaneous activity and excitatory training- induced activity.
Structural basis for explaining open-channel blockade of the NMDA receptor
Authors: Zarei MM , Dani JA
Published: April 2018

The open-channel structure of the N-methyl-D-aspartate (NMDA) receptor was investigated to explain apparently conflicting interpretations about ionic interactions within the pore. Patch-clamp techniques were applied to tissue-cultured rat hippocampal neurons from the CA1 region. A wide range of ammonium derivatives was studied to learn about the structure of the pore from permeability and open-channel blocking characteristics. We conclude that the pore is asymmetric, having high- affinity binding for organic cations from the outside and having a larger external entrance. The minimum cross-sectional area of the pore is rectangular (approximately 0.45 x 0.57 nm) and is the single- occupancy binding site(s) for small permeant cations. The narrow region extending from this minimum cross section is short, and its shape underlies the voltage dependencies of blocking cations. While occupying the blocking site, some open-channel blockers can interact with permeant cations at their binding site in the minimum cross section. A structurally based hypothesis is presented, explaining that the electrostatic interactions between the blocking site and permeant-ion site produce a high voltage dependence for blockade by some cations.
Target regulation of a motor neuron-specific epitope
Authors: Chen EW , Loera S , Chiu AY
Published: April 2018

In the adult rat nervous system, motor neurons are recognized specifically by a monoclonal antibody, MO-1. Because binding by MO-1 is lost following axotomy, contact with the target may regulate this motor neuron-specific epitope. To test this hypothesis, we examined the recovery of MO-1 immunoreactivity in hypoglossal neurons following unilateral damage to the hypoglossal nerve. During the first week following nerve crush, neurons in the ipsilateral hypoglossal nucleus lost all immunoreactivity for MO-1. Antibody binding returned with time, and by 4 weeks, 80% of the injured neurons had recovered the MO-1 epitope. Since motor neurons reinnervate their original targets readily following nerve crush, it appears that MO-1 binding is recovered when motor neurons return to their original target muscles in the tongue. When the hypoglossal nerve was cut and inserted into a foreign muscle nearby (the sternomastoid muscle), the MO-1 epitope was not detected in the injured neurons, even when examined 6 weeks after surgery. However, if the sternomastoid muscle was denervated prior to insertion of the hypoglossal nerve, thus allowing the hypoglossal nerve to synapse with this foreign target, increasing numbers of hypoglossal neurons reacquired MO-1 immunoreactivity with time. Our results suggest that the MO-1 epitope is only expressed in motor neurons that are in synaptic contact with skeletal muscle. Thus, a property that distinguishes mature motor neurons from other neuronal phenotypes appears to be regulated by direct synaptic interaction with the postsynaptic target.
TrkA expression in the CNS: evidence for the existence of several novel NGF-responsive CNS neurons
Authors: Holtzman DM , Kilbridge J , Li Y , Cunningham ET , Lenn NJ , Clary DO , Reichardt LF , Mobley WC
Published: April 2018

NGF acts as a neurotrophic factor by binding and activating its receptor on certain neuronal populations in the CNS and PNS. TrkA is a receptor for NGF. Recent findings in vitro indicate that this NGF- activated receptor tyrosine kinase transduces the NGF signal. To further define NGF actions in the CNS, we examined trkA expression in the adult rat brain. We found that trkA mRNA and immunoreactivity (IR) coincided in specific, defined neuronal populations in the forebrain and brainstem. In addition to cholinergic neurons in the basal forebrain and neostriatum, trkA expression was found in noncholinergic neurons in (1) the paraventricular anterior and reuniens thalamic nuclei, (2) the rostral and intermediate subnuclei of the interpeduncular nucleus (IPN), (3) scattered neurons in the ventrolateral and paramedian medulla, (4) the prepositus hypoglossal nucleus, and (5) the area postrema. NGF responsiveness was demonstrated for each of these populations. In contrast to trkA, p75NGFR was found only in a minority of NGF-responsive populations. Our data provide further evidence that expression of trkA marks NGF-responsive CNS neurons and suggests novel roles for NGF in the brain.
GABAergic and developmental influences on homosynaptic LTD and depotentiation in rat hippocampus
Authors: Wagner JJ , Alger BE
Published: April 2018

Low-frequency (1 Hz) stimulation (LFS) was used to elicit long-term depression (LTD) or depotentiation of excitatory transmission of the Schaffer collateral pathway in the CA1 region of the rat hippocampus. Both LTD and depotentiation were found to be homosynaptic and NMDA receptor (NMDAR) dependent. As NMDAR activation can be modulated by the inhibitory GABAergic system, we tested the hypothesis that GABA plays a role in regulating these phenomena. The GABAB antagonist CGP 35348 significantly inhibited LTD, but not depotentiation, in slices from young animals (indicating that the GABAB-mediated contribution was altered following HFS). The ability to express LTD was found to be developmentally dependent, as young animals (16–22 d) consistently expressed LTD, whereas LTD was not expressed in naive slices taken from mature (5–10 weeks) animals. The GABAA antagonist bicuculline did not affect LTD in the young animals, but did enhance LTD expression in slices from mature animals. LFS was also effective in decreasing, or depotentiating, responses that had undergone long-term potentiation (LTP) by high-frequency stimulation (HFS). In contrast to LTD, depotentiation was consistently expressed in slices from both the young and mature groups. Moreover, following an HFS train, LTD (compared to initial baseline response) could be induced in mature slices previously unable to express LTD in the naive state. Thus, the role of GABA in modulating the effects of LFS varied with the prior synaptic activity in the slice as well as with the maturity of the animal. Our results suggest that the influence of both age and prior synaptic activity (i.e., HFS) on LTD induction can be explained by changes in GABAergic systems in young versus mature, and naive versus tetanized slices.
Inositol 1,4,5-trisphosphate and ryanodine receptor distributions and patterns of acetylcholine- and caffeine-induced calcium release in cultured mouse hippocampal neurons
Authors: Seymour-Laurent KJ , Barish ME
Published: April 2018

The distributions of inositol 1,4,5-trisphosphate and ryanodine receptors (InsP3R and RyR) and the patterns of increase in intracellular calcium ion concentration ([Ca2+]i) elicited by their activation were compared in cultured hippocampal neurons. InsP3R and RyR were labeled using specific antibodies and formed small aggregations in the somata and dendrites of pyramidally shaped neurons. Both receptors were densest in somata. In dendrites the InsP3R and RyR were not distributed homogeneously; InsP3R was found in all regions, while RyR was least dense in fine processes. Increases in [Ca2+]i elicited by acetylcholine (to activate InsP3 receptors via muscarinic receptors) and caffeine (to stimulate ryanodine receptors) were measured in dendrites using Ca(2+)-sensitive fluorescent dyes and confocal microscopy. Ca2+ responses to acetylcholine were transient and observed in proximal and distal dendritic regions. In contrast, caffeine-induced responses were sustained and restricted to proximal dendrites. Thus the patterns of calcium release in fine dendrites mirrored the distributions of InsP3R and RyR. Calcium responses to both acetylcholine and caffeine were observed in the absence of external calcium and thus were dependent on Ca2+ release. Ca2+ responses showed localized fluctuations and variations in response delay times. Sequential activation of InsP3R and RyR in somata resulted in mutual occlusion of Ca2+ release. The existence of InsP3-gated and Ca(2+)- induced Ca2+ release as spatially distinct, but mutually interacting, mechanisms may be important in the generation of oscillations and propagating Ca2+ waves in somata and dendrites of hippocampal neurons.
Axonal activation-induced calcium transients in myelinating Schwann cells, sources, and mechanisms
Authors: Lev-Ram V , Ellisman MH
Published: April 2018

We have investigated the role of myelinating glia in events associated with propagation of the action potential at nodes of Ranvier using combinations of optical and electrophysiological recording methods. Calcium transients were observed in Schwann cells by fluorescent imaging of the nodal complex of fibers loaded with the calcium- sensitive dye fluo3-AM. To follow [Ca2+]i changes associated with neuronal activity at the node of Ranvier, nerves loaded with fluo3 were imaged during axonal activation using laser-scanning confocal microscopy. To elucidate sources of [Ca2+]i transients, we tested the effects of drugs known to alter [Ca2+]i. [Ca2+]i transients in Schwann cells were observed in response to axonal activation and these were subsequently blocked by ryanodine if ryanodine was present during a previous [Ca2+]i transient. Bath applications of caffeine induced [Ca2+]i transients which could be blocked by ryanodine. These findings indicate that calcium-activated calcium release occurs in Schwann cells in response to impulse activity.
IPSPs strongly inhibit climbing fiber-activated [Ca2+]i increases in the dendrites of cerebellar Purkinje neurons
Authors: Callaway JC , Lasser-Ross N , Ross WN
Published: April 2018

The interaction between the excitatory climbing fiber (CF) response and stellate cell inhibition was studied in guinea pig Purkinje cells in sagittal slices from the cerebellar vermis. Sharp microelectrode recordings from the soma or dendrites were combined with high-speed fluorescence imaging of intracellularly injected fura-2. In this way both the electrical responses and the associated [Ca2+]i changes could be monitored at the same time. Usually simultaneously activated inhibition caused almost no change to the somatically recorded CF response. However, the inhibition caused a strong reduction in the CF- associated [Ca2+]i increase which normally was widespread in the dendrites. This effect was graded; stronger inhibition caused a larger and more widespread reduction in the [Ca2+]i change that was greatest in the more distal dendrites. Sometimes the reduction was over 90% in the distal dendrites and occasionally it was localized to only a single dendritic branch. Both the inhibitory postsynaptic potential (IPSP) and the associated reduction in the CF-induced [Ca2+]i change were blocked by bicuculline, a GABAA receptor antagonist. Dendritic recordings showed that each CF response evoked a 2–3 msec wide action potential. The amplitude of this action potential was reduced in a graded manner by the IPSP in parallel with the reduction in the [Ca2+]i change. Varying the time between the activation of the IPSP and the CF response showed that both the reduction in the [Ca2+]i change and the action potential amplitude occurred in a narrow time window of about 8–10 msec, about the rise time of the IPSP. Together these results indicate that the CF response activates a fast dendritic Ca2+ spike that causes most of the [Ca2+]i increase, both of which can be blocked by an inhibitory shunting conductance. This interaction provides a means whereby Ca(2+)-dependent dendritic mechanisms can be modulated without affecting the immediate output of the Purkinje cell.
Development of memory and the hippocampus: comparison of food-storing and nonstoring birds on a one-trial associative memory task
Authors: Clayton NS
Published: April 2018

Food-storing birds, for example, marsh tits, Parus palustris, use memory to retrieve stored food and have a larger hippocampus relative to the rest of the telencephalon than do species that store little or no food such as the blue tit, P. caeruleus. The difference between food storers and nonstorers in relative hippocampal volume occurs after the young birds have fledged from the nest and is dependent upon some aspect of memory for retrieving caches of stored food. To test whether or not species differences in memory and volumetric changes in the hippocampus could be triggered by experience of memory tasks other than retrieval of stored food, groups of hand-raised marsh tits and blue tits were tested between days 35 and 192 posthatch on a one-trial associative memory task in which they were rewarded in phase II for returning to the feeder where they had eaten part of a peanut 20 min earlier. No species differences were found when the peanut was visible in phase I, but when the peanut was hidden in phase I, marsh tits performed better than blue tits, irrespective of whether or not they had had previous experience of storing and retrieving food. In dissociation trials (transformed array of feeders), marsh tits with food-storing experience responded preferentially to spatial cues, whereas blue tits responded equally to both spatial position and object- specific cues. These species differences are also found in wild-caught adults. However, marsh tits without food-storing experience responded equally to both spatial position and object-specific cues, which suggests that experience of storing and/or retrieving caches is required in order for marsh tits to develop the spatial preference seen in adult food storers. Both marsh tits with experience of the one-trial associative memory task and those that had also had food-storing experience had larger relative hippocampal volumes than did controls, independent of age. Of the marsh tits trained on the one-trial associative memory task, there was no difference between those that had had food-storing experience and those that had not. However, in blue tits, there was no effect of experience on relative hippocampal volume. No volumetric differences were observed in ectostriatum, which served as a control brain region. The results suggest that some aspect of memory for retrieving food (whether or not stored by the bird) directly influences growth of the hippocampal region in marsh tits, the food- storing species, but not in blue tits, the nonstoring species.
Neurexins are differentially expressed in the embryonic nervous system of mice
Authors: Puschel AW , Betz H
Published: April 2018

Expression of the major isoforms of three neurexin genes was analyzed in the developing embryonic nervous system of mice by Northern blot and in situ hybridization. Transcripts of all three genes were detected as early as embryonic day 10 (E10) and increased with maturation of the nervous system. RNAs of the major neurexin isoforms (alpha and beta) were found throughout the central nervous system exclusively in postmitotic neurons and at least 1 d before synapses are formed. In contrast, in the PNS the alpha- and beta-isoforms displayed differential expression patterns. Neurexin III mRNA showed a more restricted regional expression than neurexin I and II transcripts. These expression profiles are consistent with the hypothesis that the neurexins have a function in early neuronal differentiation and axogenesis.
Temporal relations of the complex spike activity of Purkinje cell pairs in the vestibulocerebellum of rabbits
Authors: Wylie DR , De Zeeuw CI , Simpson JI
Published: April 2018

Parasagittal zones in the vestibulocerebellum contain Purkinje cells whose complex spike (CS) activity is modulated in response to rotational optokinetic stimulation (OKS) about either the vertical axis (VA) or a horizontal axis (HA) that is approximately perpendicular to the ipsilateral anterior canal. In rabbits, there are two VA zones in both the ventral nodulus and flocculus, two HA zones in the flocculus, and one HA zone in the ventral nodulus. We investigated the temporal relationship of the CS activity of Purkinje cell pairs in the same or different zones of the vestibulocerebellum in ketamine-anesthetized pigmented rabbits. A synchronous temporal relationship was defined as the tendency of the CS of each Purkinje cell to fire within, at most, 2 msec of one another. Generally, neurons in the same zone showed a tendency to exhibit CS synchrony. Of 82 pairs consisting of two Purkinje cells in the same zone (e.g., two nodulus HA cells), 33 were synchronous. In contrast, none of 26 pairs consisting of two neurons in functionally different zones (e.g., a VA cell paired with an HA cell), showed CS synchrony. Pairs consisting of neurons in spatially separated VA zones in the ventral nodulus also showed a tendency to be synchronously related (6/16), as did pairs consisting of a nodulus VA cell and a flocculus VA cell (3/14). The CS synchrony was higher during OKS in the preferred direction than during spontaneous activity. This is the first demonstration that CS synchrony in the vestibulocerebellum can be manipulated with a natural sensory stimulus.
Regulation of TrkA and ChAT expression in developing rat basal forebrain: evidence that both exogenous and endogenous NGF regulate differentiation of cholinergic neurons
Authors: Li Y , Holtzman DM , Kromer LF , Kaplan DR , Chua-Couzens J , Clary DO , Knusel B , Mobley WC
Published: April 2018

TrkA is a receptor tyrosine kinase whose activation transduces NGF signaling. TrkA expression has been demonstrated in NGF-responsive adult basal forebrain cholinergic neurons (BFCNs). Several lines of evidence have suggested that endogenous NGF plays a role in the development and differentiation of these neurons. We examined TrkA expression during development. TrkA mRNA and protein were present in basal forebrain neurons during the entire postnatal period; the distribution of neurons bearing these markers was identical to that for those containing choline acetyltransferase (ChAT) mRNA, suggesting that, as in the adult, TrkA gene expression is localized to BFCNs. The expression of TrkA and ChAT followed a very similar temporal pattern, suggesting regulation by the same factor(s). We discovered that NGF administration in vivo activated TrkA receptors, and increased both TrkA and ChAT mRNA; conversely, anti-NGF infusions suppressed expression of both genes. These results suggest that endogenous NGF regulates expression of TrkA and ChAT. Finally, while NGF infusion increased the size of developing BFCNs, NGF antibodies inhibited the normal developmental increase. The results are evidence that endogenous NGF acts on developing BFCNs to enhance gene expression and cellular differentiation.