Bistable behavior of inhibitory neurons controlling impulse traffic through the amygdala: role of a slowly deinactivating K+ current

The intercalated cell masses of the amygdala are clusters of GABAergic neurons located strategically to influence behavioral responsiveness. Indeed, they receive glutamatergic sensory inputs from the basolateral amygdaloid complex and generate feedforward inhibition in neurons of the central amygdala that mediate important components of fear responses. In the present study, using whole-cell recording methods in coronal slices of the guinea pig amygdala, we show that the activity of intercalated neurons is a function of their recent firing history because they express an unusual voltage-dependent K(+) conductance (termed I(SD) for slowly deinactivating). This conductance activates in the subthreshold regime, inactivates in response to suprathreshold depolarizations, and deinactivates very slowly upon return to rest. As a result, after bouts of suprathreshold activity, these cells enter a self-sustaining state of heightened excitability associated with an increased input resistance and a membrane depolarization. In turn, these changes increase the likelihood that ongoing synaptic activity will trigger orthodromic action potentials. However, because each orthodromic spike "renews" the inactivation of I(SD), intercalated cells can remain hyperexcitable for a long time and, via the central amygdaloid nucleus, exert a lasting influence on behavior.

An inhibitory interface gates impulse traffic between the input and output stations of the amygdala

The central amygdaloid nucleus projects to brainstem and hypothalamic nuclei mediating fear responses and receives convergent sensory inputs from the basolateral amygdaloid complex. However, interposed between the basolateral complex and central nucleus is a string of interconnected GABAergic cell clusters, the intercalated cell masses. Here, we analyzed how intercalated neurons influence impulse traffic between the basolateral complex and central nucleus using whole-cell recordings, microstimulation, and local application of glutamate receptor antagonists in brain slices. Our results suggest that intercalated neurons receive glutamatergic inputs from the basolateral complex and generate feedforward inhibition in neurons of the central nucleus. As the position of the recording site was shifted medially, intercalated cells projected to gradually more medial sectors of the central nucleus and were maximally responsive to progressively more medial stimulation sites in the basolateral complex. Thus, there is a lateromedial correspondence between the position of intercalated cells, their projection site in the central nucleus, and the source of their excitatory afferents in the basolateral complex. In addition, basolateral stimulation sites eliciting maximal excitatory responses in intercalated neurons were flanked laterally by sites eliciting prevalently inhibitory responses via the activation of intercalated cells located more laterally. As a result, the feedforward inhibition generated by intercalated neurons and, indirectly, the amplitude of the responses of central neurons could be increased or decreased depending on which combination of amygdala nuclei are activated and in what sequence. Thus, the output of the central nucleus depends not only on the nature and intensity of sensory inputs but also on their timing and origin.

Reciprocal changes in the firing probability of lateral and central medial amygdala neurons

The amygdala is essential for classical fear conditioning. According to the current model of auditory fear conditioning, the lateral nucleus is the input station of the amygdala for conditioned auditory stimuli, whereas the central nucleus is the output station for conditioned fear responses. Yet, the lateral nucleus does not project to the central medial nucleus, where most brainstem projections of the amygdala originate. The available evidence suggests that the basal nuclei could transmit information from the lateral to the central medial nucleus. However, interposed between the basolateral complex and the central nucleus are clusters of GABAergic cells, the intercalated neurons, which receive inputs from the lateral and basal nuclei and contribute a massive projection to the central medial nucleus. Because it is impossible to predict the consequences of these connections, we correlated the spontaneous and auditory-evoked activity of multiple simultaneously recorded neurons of the lateral, basal, and central nuclei. The spontaneous activity of lateral and basolateral neurons was positively correlated to that of central lateral cells but negatively correlated to that of central medial neurons. In response to auditory stimuli, the firing probability of lateral and central medial neurons oscillated in phase opposition, initially being excited and inhibited, respectively. In light of previous anatomical findings, we propose that the lateral nucleus exerts two indirect actions on central medial neurons: an excitation via the basal nuclei and an inhibition via intercalated neurons.