An electrophysiological study of amygdalohypothalamic projections to the ventromedial nucleus of the rat.

The influence of the amygdala on the activity of single neurons within the hypothalamic ventromedial nucleus (HVM) was studied in pentobarbital or urethane anesthetized rats. The results are summarized as follows: (1) Stimulation of different amygdaloid nuclei or of the stria terminalis (ST) evoked a prominent field potential within HVM and altered the spike discharge patterns of the majority of HVM neurons. (2) More than 80% of 428 HVM neurons tested with single amygdala shocks exhibited excitation or excitation-inhibition sequences; the remainder displayed inhibitory responses of 100-150 msec duration at latencies slightly longer than for most of the observed excitatory responses. ST stimulation also evoked excitation or excitation-inhibition sequences from 85% of 240 HVM neurons tested; of the remainder, those with spontaneous activity displayed inhibitory responses with durations of 100-150 msec at latencies slightly longer than for most observed excitatory responses. (3) Evoked potential interaction studies suggested that stimulation of either ST or the amygdala activated the same population of HVM neurons. Single cells tested with both amygdala and ST stimulation displayed similar patterns of response. HVM field potentials and single unit responses to amygdala stimulation were markedly diminished by lesions of ST. Thus, in the rat, only one pathway, i.e., the stria terminalis, contains amygdalofugal fibres to the ventromedial hypothalamic nucleus. (4) The orthodromic responses of HVM neurons were dependent on the frequency of amygdala stimulation. Less than 50% of HVM neurons responded to amygdala stimuli at frequencies greater than 33Hz. Many cells could not be activated at stimulation frequencies greater than 10 Hz, and the spontaneous discharges from certain HVM neurons were effectively abolished at this stimulation frequency. (5) Evidence of prominent postsynaptic inhibition was present throughout HVM. Seventeen HVM neurons displayed amygdala evoked unitary activity different from that of the majority of HVM neurons, and these cells were considered to represent possible inhibitory neurons. In contrast to most HVM neurons activated via probable monosynaptic amygdalohypothalamic pathways, these putative inhibitory neurons were apparently activated via polysynaptic pathways. (6) In summary, these results suggest that the amygdala exerts a prominent monosynaptic influence on the activity of many HVM neurons, coupled with polysynaptic activation of powerful local postsynaptic inhibitory mechanism. In the rat, these amygdala evoked events depend on the integrity of the stria terminalis.