Neuronal pericellular baskets: neurotransmitter convergence and regulation of network excitability
Authors: Rebecca A. Senft, Susan M. Dymecki
Journal: Trends in Neurosciences (2021)

A pericellular basket is a presynaptic array of boutons from single or multiple axons that encase the target, postsynaptic cell body and proximal dendrites.Pericellular baskets ensheathing cerebellar and cortical neurons were described by Ramón y Cajal in the late 1800s and early 1900s.Pericellular baskets typically form postnatally after initial innervation of a brain region, suggesting possible induction by maturing target neurons.Pericellular baskets often deploy multiple neurotransmitters either coexpressed in individual axons or separately deployed by different axons, suggesting complex target cell regulation.The proximity to the postsynaptic soma of boutons configuring a basket suggests temporally precise inhibition or excitation of the encased cell.Pet1 neurons of the raphe nuclei offer an example of pericellular basket projections. Pet1 neuron pericellular baskets frequently target distant inhibitory GABAergic interneurons and thus may exert privileged influence on target region networks and their excitability.Cell types targeted by Pet1 neuronal pericellular baskets suggest possible roles in regulating memory durability. A pericellular basket is a presynaptic configuration of numerous axonal boutons outlining a target neuron soma and its proximal dendrites. Recent studies show neurochemical diversity of pericellular baskets and suggest that neurotransmitter usage together with the dense, soma-proximal boutons may permit strong input effects on different timescales. Here we review the development, distribution, neurochemical phenotypes, and possible functions of pericellular baskets. As an example, we highlight pericellular baskets formed by projections of certain Pet1/Fev neurons of the serotonergic raphe nuclei. We propose that pericellular baskets represent convergence sites of competition or facilitation between neurotransmitter systems on downstream circuitry, especially in limbic brain regions, where pericellular baskets are widespread. Study of these baskets may enhance our understanding of monoamine regulation of memory, social behavior, and brain oscillations. A pericellular basket is a presynaptic configuration of numerous axonal boutons outlining a target neuron soma and its proximal dendrites. Recent studies show neurochemical diversity of pericellular baskets and suggest that neurotransmitter usage together with the dense, soma-proximal boutons may permit strong input effects on different timescales. Here we review the development, distribution, neurochemical phenotypes, and possible functions of pericellular baskets. As an example, we highlight pericellular baskets formed by projections of certain Pet1/Fev neurons of the serotonergic raphe nuclei. We propose that pericellular baskets represent convergence sites of competition or facilitation between neurotransmitter systems on downstream circuitry, especially in limbic brain regions, where pericellular baskets are widespread. Study of these baskets may enhance our understanding of monoamine regulation of memory, social behavior, and brain oscillations. The strength and temporal dynamics of neuronal input are modulated by multiple factors. Among them is the spatial arrangement of presynaptic axonal boutons on the postsynaptic cell, that is, the locations and densities of the boutons. One particularly striking bouton arrangement comprises what is called a pericellular basket, a presynaptic organization of boutons from one or multiple axons that surround the postsynaptic cell body and proximal dendrites. This innervation is typically dense, such that the shape of the postsynaptic soma can be discerned from the basket itself, like climbing vines around the trunk and limbs of a tree [1.Köhler C. et al.The distribution and origin of serotonin-containing fibers in the septal area: a combined immunohistochemical and fluorescent retrograde tracing study in the rat.J. Comp. Neurol. 1982; 209: 91-111Crossref PubMed Google Scholar,2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar]. Historically, this configuration has also been referred to as a pericellular nest or pericellular array [3.Marin-Padilla M. Three-dimensional reconstruction of the pericellular nests (baskets) of the motor (area 4) and visual (area 17) areas of the human cerebral cortex: a Golgi study.Z. Für Anat. Entwicklungsgeschichte. 1974; 144: 123-135Crossref PubMed Google Scholar, 4.Gall C. Moore R.Y. Distribution of enkephalin, substance P, tyrosine hydroxylase, and 5-hydroxytryptamine immunoreactivity in the septal region of the rat.J. Comp. Neurol. 1984; 225: 212-227Crossref PubMed Scopus (146) Google Scholar, 5.Hornung J.P. et al.Distribution of two morphologically distinct subsets of serotoninergic axons in the cerebral cortex of the marmoset.J. Comp. Neurol. 1990; 297: 165-181Crossref PubMed Scopus (93) Google Scholar]. Pericellular baskets are thought to confer privileged control over the targeted cell via the high number of boutons and their location proximal to the soma and axon hillock, potentially overriding effects of more distal inputs [6.Strüber M. et al.Strength and duration of perisomatic GABAergic inhibition depend on distance between synaptically connected cells.Proc. Natl. Acad. Sci. U. S. A. 2015; 112: 1220-1225Crossref PubMed Scopus (25) Google Scholar]. Pericellular baskets in the cerebellum, hippocampus, and cortex were originally described by Ramón y Cajal and later determined to be GABAergic [7.Curtis D.R. et al.GABA and hippocampal inhibition.Br. J. Pharmacol. 1970; 40: 881-883Crossref PubMed Scopus (138) Google Scholar]. Since this foundational work, diverse neuron types have been reported to configure pericellular baskets at their axon termini. These include certain monoaminergic neuron subtypes [2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar,8.Aznar S. et al.Non-serotonergic dorsal and median raphe projection onto parvalbumin- and calbindin-containing neurons in hippocampus and septum.Neuroscience. 2004; 124: 573-581Crossref PubMed Scopus (33) Google Scholar], some neuropeptide-releasing neurons [4.Gall C. Moore R.Y. 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These basket-extending neurons have cell soma residing in regions such as the median raphe (MR) nucleus [1.Köhler C. et al.The distribution and origin of serotonin-containing fibers in the septal area: a combined immunohistochemical and fluorescent retrograde tracing study in the rat.J. Comp. Neurol. 1982; 209: 91-111Crossref PubMed Google Scholar,8.Aznar S. et al.Non-serotonergic dorsal and median raphe projection onto parvalbumin- and calbindin-containing neurons in hippocampus and septum.Neuroscience. 2004; 124: 573-581Crossref PubMed Scopus (33) Google Scholar,10.Szőnyi A. et al.Median raphe controls acquisition of negative experience in the mouse.Science. 2019; 366eaay8746Crossref PubMed Scopus (10) Google Scholar,14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar], cerebral cortex [15.Armstrong C. Soltesz I. 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The presence of pericellular baskets is phylogenetically widespread, being found in reptiles, songbirds, rodents, non-human primates, and humans [2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar,3.Marin-Padilla M. Three-dimensional reconstruction of the pericellular nests (baskets) of the motor (area 4) and visual (area 17) areas of the human cerebral cortex: a Golgi study.Z. Für Anat. Entwicklungsgeschichte. 1974; 144: 123-135Crossref PubMed Google Scholar,18.Font C. et al.Septal complex of the telencephalon of the lizard Podarcis hispanica. II. Afferent connections.J. Comp. Neurol. 1997; 383: 489-511Crossref PubMed Scopus (31) Google Scholar, 19.Goodson J.L. et al.Chemoarchitectonic subdivisions of the songbird septum and a comparative overview of septum chemical anatomy in jawed vertebrates.J. Comp. 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We discuss pericellular baskets as sites of convergence of neurotransmitter systems, suggesting that their privileged control over postsynaptic neuron excitability is complex and may span different timescales if the different neurotransmitters signal ionotropically (e.g., 'fast' glutamatergic signaling) versus metabotropically (e.g., 'slow' serotonergic signaling). We consider functional roles for pericellular baskets, for example, in the regulation of target neuron activity in the hippocampus and septum, possibly shaping brain theta rhythm and memory formation. We close with a set of questions, intending to stimulate future advances in this exciting area. A diverse set of neurotransmitters have been detected singly or coexpressed in boutons comprising pericellular baskets. These include serotonin (5-hydroxytryptamine [5-HT]), dopamine, noradrenaline, acetylcholine, glutamate, GABA, enkephalin (Met- and Leu-), substance P, somatostatin, neuropeptide Y (NPY), and cocaine- and amphetamine-regulated transcript (CART) peptide [2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar,4.Gall C. Moore R.Y. Distribution of enkephalin, substance P, tyrosine hydroxylase, and 5-hydroxytryptamine immunoreactivity in the septal region of the rat.J. Comp. Neurol. 1984; 225: 212-227Crossref PubMed Scopus (146) Google Scholar,14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. 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Precedent for cotransmission deploying glutamate has been reported in monoaminergic, cholinergic, and GABAergic neurons [26.Trudeau L.-E. El Mestikawy S. Glutamate cotransmission in cholinergic, GABAergic and monoamine systems: contrasts and commonalities.Front. Neural Circuits. 2018; 12: 113Crossref PubMed Scopus (27) Google Scholar], perhaps applying to pericellular basket terminals as well. Pericellular baskets, even of different neurotransmitter phenotypes, share certain cytoarchitectural features. Boutons are characteristically large, and typically, 20–30 of them decorate the target soma [12.Acsády L. et al.Unusual target selectivity of perisomatic inhibitory cells in the hilar region of the rat hippocampus.J. Neurosci. 2000; 20: 6907-6919Crossref PubMed Google Scholar,14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar]. The contained synapses are typically symmetric, as revealed in electron micrographs of septal and hippocampal pericellular baskets [27.Beauvillain J.C. et al.GABA and enkephalin in the lateral septum of the guinea pig: light and electron microscopic evidence for interrelations.J. Comp. Neurol. 1991; 308: 103-114Crossref PubMed Scopus (16) Google Scholar, 28.Dinopoulos A. et al.Serotonergic innervation of the mature and developing lateral septum of the rat: a light and electron microscopic immunocytochemical analysis.Neuroscience. 1993; 55: 209-222Crossref PubMed Scopus (33) Google Scholar, 29.Fasano C. et al.Regulation of the hippocampal network by VGLUT3-positive CCK- GABAergic basket cells.Front. Cell. 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Excitatory control also seems possible by some pericellular baskets using glutamate and/or 5-HT. These neurotransmitters may trigger excitatory postsynaptic receptors such as ionotropic and metabotropic glutamate receptors [26.Trudeau L.-E. El Mestikawy S. Glutamate cotransmission in cholinergic, GABAergic and monoamine systems: contrasts and commonalities.Front. Neural Circuits. 2018; 12: 113Crossref PubMed Scopus (27) Google Scholar,31.Crupi R. et al.Role of metabotropic glutamate receptors in neurological disorders.Front. Mol. Neurosci. 2019; 12: 20Crossref PubMed Scopus (53) Google Scholar] or the excitatory ionotropic 5-HT receptor 3A (5-HT3aR) [32.Koyama Y. et al.Building a 5-HT3A receptor expression map in the mouse brain.Sci. 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Additionally, cultured 5-HT neurons have been found to release glutamate at asymmetric synapses [36.Johnson M.D. Synaptic glutamate release by postnatal rat serotonergic neurons in microculture.Neuron. 1994; 12: 433-442Abstract Full Text PDF PubMed Scopus (109) Google Scholar], suggesting asymmetric synapses may be more common in cases of cotransmission of glutamate and serotonin. The degree to which neurochemically distinct pericellular baskets target the same soma is largely unknown. In the septum, different neurotransmitter systems form pericellular baskets in broadly similar distributions [1.Köhler C. et al.The distribution and origin of serotonin-containing fibers in the septal area: a combined immunohistochemical and fluorescent retrograde tracing study in the rat.J. Comp. Neurol. 1982; 209: 91-111Crossref PubMed Google Scholar,2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar,4.Gall C. Moore R.Y. Distribution of enkephalin, substance P, tyrosine hydroxylase, and 5-hydroxytryptamine immunoreactivity in the septal region of the rat.J. Comp. Neurol. 1984; 225: 212-227Crossref PubMed Scopus (146) Google Scholar], raising the possibility that different basket systems interact by projecting to the same downstream target neurons or by axo-axonic synapses onto other baskets. A convergence-organization model suggests that different neurotransmitter systems may compete with each other or facilitate modulation of the targeted cell, either by affecting postsynaptic cellular processes or by inhibiting or exciting other basket terminals. Indeed, multiple neurochemically distinct fibers (serotonergic versus nonserotonergic) have been observed as making baskets on the same septal cells [8.Aznar S. et al.Non-serotonergic dorsal and median raphe projection onto parvalbumin- and calbindin-containing neurons in hippocampus and septum.Neuroscience. 2004; 124: 573-581Crossref PubMed Scopus (33) Google Scholar,14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar], supporting the idea of basket convergence (Figure 1). An alternative possibility is that separate basket systems 'tile' innervated regions, targeting largely distinct postsynaptic cells. A distributed, nonoverlapping pattern of basket systems would suggest high target specificity for postsynaptic cell types, and possibly even repulsive or nonpermissive environments underlying the development of basket stratification. In the septum, glutamatergic pericellular baskets (expressing vesicular glutamate transporter 3 [VGLUT3]) rarely overlap topographically with baskets immunopositive for parvalbumin (PV), tryptophan hydroxylase 2 (TPH2; the rate-limiting enzyme for 5-HT synthesis), calretinin, or choline acetyltransferase [2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar]. However, they do occasionally target the same somata as do separate, tyrosine hydroxylase-positive (TH+) (presumably dopaminergic) baskets [2.Riedel A. et al.Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum.J. Chem. Neuroanat. 2008; 36: 177-190Crossref PubMed Scopus (8) Google Scholar] (Figure 1A). The extent of pericellular basket convergence may vary between different basket systems or as a function of region. Brain stem raphe neurons defined molecularly by expression of Pet1 (also known as Fev) are referred to as Pet1 neurons [37.Jensen P. et al.Redefining the serotonergic system by genetic lineage.Nat. Neurosci. 2008; 11: 417-419Crossref PubMed Scopus (182) Google Scholar, 38.Okaty B.W. et al.Multi-scale molecular deconstruction of the serotonin neuron system.Neuron. 2015; 88: 774-791Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar, 39.Okaty B.W. et al.A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons.eLife. 2020; 9e55523Crossref PubMed Scopus (19) Google Scholar, 40.Okaty B.W. et al.Embracing diversity in the 5-HT neuronal system.Nat. Rev. Neurosci. 2019; 20: 397-424Crossref PubMed Scopus (46) Google Scholar] and include a neuron subset that collectively forms pericellular baskets in the septum, hippocampus, and cerebral cortex [14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar]. Pet1 encodes for a transcription factor master regulator of differentiation of the serotonergic fate [41.Deneris E. Gaspar P. Serotonin neuron development: shaping molecular and structural identities.Wiley Interdiscip. Rev. Dev. Biol. 2018; 7e301Crossref Scopus (40) Google Scholar, 42.Liu C. et al.Pet-1 is required across different stages of life to regulate serotonergic function.Nat. Neurosci. 2010; 13: 1190-1198Crossref PubMed Scopus (123) Google Scholar, 43.Wyler S.C. et al.Pet-1 switches transcriptional targets postnatally to regulate maturation of serotonin neuron excitability.J. Neurosci. 2016; 36: 1758-1774Crossref PubMed Google Scholar]. Recent findings, however, show that a substantial subset of basket-forming Pet1 neurons express low or undetectable levels of serotonergic pathway genes such as Tph2 and Slc6a4, the latter encoding the serotonin reuptake transporter. Rather, these Pet1 neurons express high levels of VGLUT3, enabling glutamate packaging into synaptic vesicles [26.Trudeau L.-E. El Mestikawy S. Glutamate cotransmission in cholinergic, GABAergic and monoamine systems: contrasts and commonalities.Front. Neural Circuits. 2018; 12: 113Crossref PubMed Scopus (27) Google Scholar,44.Amilhon B. et al.VGLUT3 (vesicular glutamate transporter type 3) contribution to the regulation of serotonergic transmission and anxiety.J. Neurosci. 2010; 30: 2198-2210Crossref PubMed Scopus (129) Google Scholar]. This pattern contrasts some other Pet1 neurons, which coexpress serotonergic and glutamatergic identity genes, permitting glutamate and serotonin cotransmission [38.Okaty B.W. et al.Multi-scale molecular deconstruction of the serotonin neuron system.Neuron. 2015; 88: 774-791Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar]. These Pet1-neuron somas that are high in Vglut3 but low in Tph2 transcripts reside in the MR and comprise part of the Pet1 neuronal population that derives developmentally from the hindbrain compartment referred to as rhombomere 2 (r2) [14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar,37.Jensen P. et al.Redefining the serotonergic system by genetic lineage.Nat. Neurosci. 2008; 11: 417-419Crossref PubMed Scopus (182) Google Scholar,38.Okaty B.W. et al.Multi-scale molecular deconstruction of the serotonin neuron system.Neuron. 2015; 88: 774-791Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar]. Pet1 neurons arising from r2 are referred to as 'r2-Pet1' neurons [37.Jensen P. et al.Redefining the serotonergic system by genetic lineage.Nat. Neurosci. 2008; 11: 417-419Crossref PubMed Scopus (182) Google Scholar] and can be accessed genetically by exploiting the overlap (intersection) of expression of two driver transgenics, the r2-specific r2Hoxa2-cre [45.Awatramani R. et al.Cryptic boundaries in roof plate and choroid plexus identified by intersectional gene activation.Nat. Genet. 2003; 35: 70-75Crossref PubMed Google Scholar] and the Pet1-specific Pet1-Flpe [37.Jensen P. et al.Redefining the serotonergic system by genetic lineage.Nat. Neurosci. 2008; 11: 417-419Crossref PubMed Scopus (182) Google Scholar]. Pericellular baskets are characteristic of a subgroup within the r2-Pet1 neuron population, the VGLUT3-positive, TPH2-low or -negative subset of r2-Pet1 neurons referred to as r2-Pet1Vglut3-high [14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar]. The more classical, serotonergic subgroup of r2-Pet1 neurons, referred to as r2-Pet1Tph2-high, expresses high levels of TPH2, 5-HT, and SLC6A4 and are low or negative for VGLUT3. This group does not form pericellular baskets and projects to brain regions different from the basket-forming r2-Pet1Vglut3-high cells [14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar]. The majority of baskets from r2-Pet1Vglut3-high cells are immunoreactive for VGLUT3 but not 5-HT, perhaps unsurprisingly given their soma transcriptome just mentioned [14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar,38.Okaty B.W. et al.Multi-scale molecular deconstruction of the serotonin neuron system.Neuron. 2015; 88: 774-791Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar]. Interestingly, some of the r2-Pet1Vglut3-high-targeted postsynaptic cells are ensheathed by additional baskets that are serotonergic and derive from non-r2 Pet1 neurons (Figure 1B). These baskets are likely formed by other MR serotonergic neuron subgroups referred to as r1En1-Pet1 or r3Egr2-Pet1 neurons [37.Jensen P. et al.Redefining the serotonergic system by genetic lineage.Nat. Neurosci. 2008; 11: 417-419Crossref PubMed Scopus (182) Google Scholar,46.Bang S.J. et al.Projections and interconnections of genetically defined serotonin neurons in mice.Eur. J. Neurosci. 2012; 35: 85-96Crossref PubMed Scopus (113) Google Scholar]. Thus, in some cases, axons from developmentally distinct subsets of Pet1 neurons (derived from different hindbrain rhombomeres) converge and ensheathe the same target cell, with one deploying glutamate and the other deploying 5-HT. Such 'composite' pericellular baskets are prevalent in the septum [14.Senft R.A. et al.Neurochemically and hodologically distinct ascending VGLUT3 versus serotonin subsystems comprise the r2-Pet1 median raphe.J. Neurosci. 2021; 41: 2581-2600Crossref PubMed Scopus (0) Google Scholar] (Figure 1C) and likely explain at least a portion of the baskets formed by serotonergic and nonserotonergic fibers reported decades ago [8.Aznar S. et al.Non-serotonergic dorsal and median raphe projection onto parvalbumin- and calbindin-containing neurons in hippocampus and sept