Epileptiform activity in the nucleus accumbens induced by GABA_A receptor antagonists in rat forebrain slices is of cortical origin

Abstract.

Extracellularly recorded field potentials, evoked by stimulation of cortico-nucleus accumbens border, were recorded in the nucleus accumbens (NAcc) in horizontal slices of rat ventral forebrain. The field excitatory postsynaptic potential (EPSP) event (N2) was calcium dependent, blocked by tetrodotoxin (1 µM), and reduced by over 70% by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 µM), the antagonist of AMPA-type glutamate receptors. The EPSP amplitude was enhanced by either of the GABA_A receptor antagonists, picrotoxin (10 µM; by 252±33%, n=18) and bicuculline methiodide (20 µM; by 216±34%, n=4). Additionally, picrotoxin (3–50 µM) and bicuculline methiodide (20 µM) promoted epileptiform activity within the NAcc, manifest as the emergence of additional late components, N3, N4 and N5, in the evoked synaptic waveform. In slices with the frontal cortex removed, picrotoxin (10–50 µM) and bicuculline methiodide (20 µM) were unable to promote epileptiform activity within the NAcc, although a smaller increase in the peak amplitude of the field EPSP (163±18%, n=6) was observed at the highest concentrations of picrotoxin (50 µM). Histological examination of the slice demonstrated that in such decorticated slices, the piriform cortex (PC) had been removed. We propose that stimulation of the cortico-NAcc border not only evokes an orthodromic EPSP in the NAcc, but also causes antidromic activation of cortical tissue. Disinhibition by GABA_A antagonists of circuits intrinsic to the cortex, possibly the piriform cortex, is the principal cause of the facilitation of the EPSP and of regenerative epileptiform activity in NAcc evoked by stimulation of cortical input.