Hippocampal input to a "visceral motor" corticobulbar pathway: an anatomical and electrophysiological study in the rat.

The hippocampus has previously been shown to influence cardiovascular function, and this effect appears to be mediated by the connection the hippocampus has with the infralimbic area of the medial frontal cortex (MFC), a region which projects directly to the nucleus of the solitary tract (NTS) in the dorsal medulla. In the present study, anatomical and electrophysiological techniques were utilized to determine the degree of convergence of hippocampal input to the MFC on neurons in the MFC which project to the NTS. Injections of the anterograde and retrograde neuroanatomical tracer wheat-germ agglutinin-horseradish peroxidase (WGA-HRP) into the NTS retrogradely labelled cells in the infralimbic and prelimbic regions of the MFC. Injections of WGA-HRP into the ventral hippocampus anterogradely labelled terminals in the MFC which, at the light microscopic level, closely overlapped the origin of the descending projection from the MFC to the brainstem. Electron microscopic analysis revealed that anterogradely labelled terminals make synaptic contact primarily on dendritic processes in the neuropil adjacent to retrogradely labelled cells. In addition, anterogradely labelled terminals did, in some cases, make synaptic contact on the somas of retrogradely labelled cells. Electrical stimulation of the NTS antidromically activated cells in the infralimbic and prelimbic areas of the MFC. The average latency of antidromic activation was 30 msec, corresponding to a conduction velocity of approximately 0.7 m/s. Electrical stimulation of the ventral hippocampus orthodromically activated cells in the MFC. With an appropriate delay between the hippocampal and NTS stimuli, the orthodromic and antidromic potentials could be made to collide. The results of this study establish a structural as well as functional link between the hippocampus and NTS-projection neurons in the MFC.