Intrinsic and extrinsic connections of the rat central extended amygdala: an in vivo electrophysiological study of the central amygdaloid nucleus

Anatomical studies have shown that the central amygdaloid nucleus (CeA) is reciprocally connected with the lateral bed nucleus of the stria terminalis (BSTL), both structures being major components of the central extended amygdala. The CeA also receives projections from the insular cortex (InsCx) and the paraventricular thalamic nucleus (PVT). Extracellular unit activity was recorded from neurons in the lateral CeA (CeL) in urethane anaesthetized rats and their responses were studied after electrical stimulation of the BSTL, InsCx and PVT. The spontaneous activity of CeL neurons was low (1.69 spikes/s) and 40% of recorded cells were silent. The iontophoretic application of the GABAA antagonist, bicuculline, increased the firing rate of 20% of neurons. The BSTL stimulation induced an antidromic response in 33% of the tested cells. Orthodromic responses were obtained from 83% (BSTL stimulation), 70% (InsCx stimulation) and 85% (PVT stimulation) of tested cells, some of which responded to both BSTL and InsCx or PVT stimulations. Orthodromic responses mostly consisted in 1-3 orthodromic spikes followed by an inhibition. During iontophoretic application of bicuculline, stimulation induced additional short latency orthodromic spikes, even in cells that were previously unresponsive. However, the duration of the inhibition was never reduced. These results indicate that GABAergic neurotransmission may play a dominant role in both spontaneous and evoked electrical activities in the CeL, probably mediated by local circuit cells involved in a feed-forward inhibition. This organization, along with the reciprocal connections between the CeL and the BSTL, is considered in the context of the extended amygdala.