Electrophysiology of limbic forebrain and paraventricular nucleus connections

The connections of forebrain structures with the hypothalamic paraventricular nucleus (PVN) were examined electrophysiologically in anaesthetized male rats. Single unit recordings from 336 neurons were made within the cingulate cortex (CC, n = 78), lateral septum (LS, 114), bed nucleus of the stria terminalis (BST, 27). bed nucleus proprioris commissurae (BCA, 27) and preoptic area (POA, 90). Following PVN stimulation, some cells in all regions were identified as projecting to the PVN. Antidromic (24%), orthodromic excitatory (44%) and inhibitory (22%) responses recorded from CC units demonstrated the presence of reciprocal pathways between PVN and CC. Thirty-eight percent of LS units were antidromically identified as projecting to the PVN and these appeared to show some discrete topographic organization. Fifty-seven percent of LS units responded orthodromically to stimulation of the PVN, the majority of such responses being excitatory. Within the bed nuclei, 24% of units were antidromically identified as projecting to PVN and 70% of cells in these regions responded orthodromically following PVN stimulation, excitatory responses predominating within the BST and inhibitory responses within the BCA. Within the POA, 38% of units were identified as projecting to the PVN and the remaining units were approximately divided between orthodromic excitatory and inhibitory responses. A small proportion of antidromically identified units (less than 20%) in all regions also exhibited responses suggesting the presence of reciprocal connections with the PVN.