Descending projections of infralimbic cortex that mediate stimulation-evoked changes in arterial pressure.

The infralimbic cortex (IL) of the rat can modify autonomic nervous system activity, but the critical pathway(s) that mediate this influence are unclear. To define the potential pathways, the first series of experiments characterizes the descending projections of IL and the neighboring cortical areas using Phaseolus vulgaris leucoagglutinin (PHA-L). IL has prominent projections to the central nucleus of the amygdala (Ce), the mediodorsal nucleus of the thalamus (MD), the lateral hypothalamic area (LHA), the periaqueductal gray (PAG), the parabrachial nucleus (Pb), and the nucleus of the solitary tract (NTS). The density and selectivity of these projections suggest that the LHA and the PAG mediate the ability of the IL to regulate cardiovascular function. The second series of experiments demonstrates that locally anesthetizing neurons in either the LHA or PAG with lidocaine attenuates the hypotensive effects produced by electrical stimulation of the IL. Similarly, microinjections of cobalt chloride (a neurotransmission blocker) into the anterior portion of the LHA also decrease the arterial pressure responses to IL stimulation, suggesting that the ability of lidocaine to reversibly block the evoked response is due to inactivation of neurons in the LHA. These data indicate that hypotension evoked by stimulation of IL is mediated, at least in part, by direct or indirect projections to the LHA and through the PAG.