GABAergic synaptic transmission in projections from the basal forebrain and hippocampal formation to the amygdala: an in vivo iontophoretic study

We recorded extracellular responses from rat amygdaloid neurons in vivo after electrical stimulation of the basal forebrain and hippocampal formation. Iontophoretic application of the GABA_A receptor antagonist, bicuculline, lead to the appearance of short latency evoked bursts after stimulation of either region. This occurred whether the baseline response was inhibitory or excitatory. Bicuculline only affected an early phase of inhibition, leaving a longer latency, longer duration phase unchanged or even increased. By contrast, the GABA_B receptor antagonist, phaclofen, never produced such short latency evoked bursts. Both bicuculline and phaclofen increased the spontaneous rate of firing of amygdaloid neurons. The excitatory burst response to hippocampal formation stimulation of an amygdaloid candidate inhibitory neuron was blocked by CNQX (an antagonist of the AMPA subtype of glutamate receptor). Based on these and prior studies, it seems likely that the effects of hippocampal formation stimulation are mediated by feed-forward inhibition, in which GABAergic amygdaloid inhibitory neurons are excited by glutamatergic projections from the hippocampal formation. The effects of basal forebrain stimulation may be mediated by both feed-forward inhibition and direct, GABAergic inhibition.