Epileptogenesis Reduces the Sensitivity of Presynaptic " ' 1-Aminobutyric Acids Receptors on Glutamatergic Afferents in the Amygdala 1

Intracellular recordings were obtained from the basolateral amygdala in in vitro rat brain slice preparations to examine whether ,-aminobutyric acid (GABA)s receptors are altered after in vivo kindling-induced epileptogenesis. Stimulating the stria terminalis evoked excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials in control neurons, and epileptiform bursting or enhanced EPSPs, but no IPSPs, in neurons from animals 4 to 8 weeks after the last kindled seizure. Baclofen (0.1 nM-100 ,uM) depressed EPSPs in control and kindled basolateral amygdala neurons, but the ECso appeared to be shifted 100-fold from 5 nM in control to 500 nM in kindled neurons. Further analysis suggested a high-affinity component may be affected in kind led neurons . The absence of IPSPs in kindled neurons could not account for this shift, because effects of baclofen on EPSP amplitude were reduced in kindled animals even when GABAA receptors were blocked with bicuculline methiodide (30 ,uM) and postsynaptic GABAs receptors with intracellular guanosine 5'In the central nervous system, GABA is an inhibitory neurotransmitter. Two classes of GABA receptors, GABAA and GABAB, exist in the mammalian brain. GABAA receptors are important in regulating neuronal activity, because blockade of these receptors induces epileptiform activity (Traub et al., 1987). However, the specific roles for GABAH receptors in brain functioning are still being defined. GABAB receptors are thought to mediate slow IPSPs (Dutar and Nicoll, 1988a; Stevens et ai., 1987), and to negatively regulate excitatory (Collins et ai., 1982; Olpe et ai., 1982) and inhibitory transmission (Davies et al., 1990; Thompson and Gahwiler, 1989). Recent evidence suggests that the presynaptic GABAB receptors Received for publication February 24, 1992. 1 This research was supported by National Institutes of Health grant NS2464:3 to P. S. G. 0-3-thiotriphosphate (10 mM); 6-cyano-2,3-dihydroxy-7 -nitroquinoxaline (10 ,uM) was also present to block bicuculline methiodide-induced bursting. Membrane responses to exogenously applied N-methyl-o-aspartate and C1'-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid were not affected by baclofen. Baclofen also hyperpolarized basolateral amygdala neurons and reduced membrane input resistance with an ECso of 1 ,uM in control and kindled neurons. Postbut not presynaptic effects of baclofen were blocked by 2-hydroxy-saclofen (100 ,uM) and pertussis toxin pretreatment. In conclusion, kindling-induced epileptogenesis reduces the sensitivity of presynaptic GABAs receptors, an effect which may contribute to the enhancement of excitatory transmission in kindled animals. Furthermore, different pharmacological properties of preand postsynaptic receptors in the amygdala suggest two distinct populations of GABAs receptors whose long-lasting responses to kindling-induced seizures are different. located on glutamatergic afferents (Calabresi et al., 1990) or GABAergic neurons (Deisz and Prince, 1989; Davies et ai., 1990) can be activated by endogenous GABA. Kindling is a model of limbic epilepsy and results from repetitive electrical stimulation of certain brain areas which eventually leads to epileptiform responses (Goddard et ai., 1969). Seizure activity induced by kindling enhances NMDA receptor-mediated synaptic potentials and glutamatergic excitatory transmission, in general (Gean et ai., 1989; Rainnie et ai., 1992; Mody and Heinemann, 1987; Mody et al., 1988). However, although enhanced glutamatergic transmission in kindling has been shown, clear evidence for long-lasting biochemical changes in NMDAand non-NMDA-sensitive glutamate binding in kindling is lacking. Neurochemical studies have shown inconsistent results. NMDA agonist binding and NMDA-stimulated norepinephrine release are unchanged in the amygdala and the ABBREVIATIONS: GABA, y-aminobutyric acid; IPSP, inhibitory postsynaptic potential; NMDA, N-methyl-D-aspartate; BLA, basolateral amygdala; ACSF. artificial cerebrospinal fluid; ST, stria terminalis; KAc, potassium acetate; GTP-y-S, guanosine 5' -O-3-thiotriphosphate; AMPA, a-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid; BMI, bicuculline methiodide; CNQX, 6-cyano-2,3-dihydroxy-7-nitroquinoxaline; TTX, tetrodotoxin; PTX, pertussis toxin; 2-0H-saclofen, 3-amino-2-(4-chlorophenyl)-2-hydroxypropane sulfonic acid; RMP, resting membrane potential; Ri, input resistance; Tm, membrane time constant; EPSP, excitatory postsynaptic potential; f-IPSP, fast inhibitory postsynaptic potential; s-IPSP, slow inhibitory postsynaptic potential.