Responses of amygdaloid central nucleus neurons to stimulation of the insular cortex in awake rabbits

Recent evidence suggests that the amygdaloid central nucleus may contribute importantly to autonomic regulation during emotional states, possibly via both direct and indirect central nucleus efferent projections to autonomic regulatory nuclei in the lower brainstem. Additional findings suggest that the insular cortex may participate in autonomic regulatory processes, possibly by exerting an influence upon central nucleus neuronal activity via its direct projections to the central nucleus. The present experiment was conducted to determine the effects of insular cortex stimulation upon extracellularly recorded central nucleus neuronal activity in conscious, drug-free, rabbits. Satisfactory recordings were obtained from 146 central nucleus neurons. These were classified as belonging to one of six general categories on the basis of rates and patterns of ongoing discharge, responses to an auditory stimulus, and location within the nucleus. Determinations were then made as to whether each neuron could be activated antidromically from a ventrolateral mesencephalic region through which descending central nucleus projections to the lower brainstem course, and hence, whether the neuron might contribute to these projections. The activity of each neuron was then assessed during single-pulse stimulation of the region of the insular cortex demonstrated previously to project to the central nucleus. Such stimulation produced no response, an initial increase, or an initial decrease in the activity of 33%, 53% and 14% of the 146 neurons, respectively. The predominant response consisted of a single spike with a short but variable onset latency, suggesting orthodromic activation via one or a few synapses. Included among the neurons that responded to stimulation of the insular cortex in this manner were seven of 22 central nucleus neurons identified as projecting to the lower brainstem. Responses consisting of initially decreased activity most often occurred in neurons that discharged infrequently and were sensory-responsive. The results of this experiment thus provide more detailed information than was available previously regarding the pervasive influence of the insular cortex upon central nucleus neuronal activity, and provide further support for the notion that the insular cortex may participate in autonomic regulatory processes by way of its direct projections to the central nucleus.