EXPRESSION OF C-FOS PROTEIN IN RAT-BRAIN ELICITED BY ELECTRICAL AND CHEMICAL-STIMULATION OF THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

The functional connectivity of the paraventricular nucleus of the hypo thalamus (PVN) was studied by assessing the expression of the immediat e early gene, c-fos, after unilateral stimulation of this structure in urethane-anesthetized rats. Electrical stimulation for 1 h (10 s on, 10 s off, 15-40 mu A at 20 Hz) was accompanied by increases in mean ar terial pressure (13-29 mm Hg). In these animals, ipsilateral increases in numbers of neurons with Fos-like immunoreactivity (FLI) were immun ohistochemically demonstrated in the insular cortex, lateral septum, m edial amygdala, hypothalamus, lateral division of the parabrachial nuc leus (PBN) of the pens and the nucleus of the tractus solitarius (NTS) and ventrolateral medulla (VLM). Numbers of cells with FLI were quant itated in five areas known for their roles in autonomic function: arcu ate nucleus, ventromedial hypothalamus, lateral PBN, NTS (at three lev els) and VLM (caudal and rostral). In each case, stimulation of the PV N led to significant differences in number of neurons with FLI on the side ipsilateral to the stimulation compared to the contralateral side . To eliminate effects associated with stimulation of fibers of passag e in the vicinity of the PVN, the results after electrical stimulation were compared to those obtained in animals in which the PVN was chemi cally stimulated unilaterally with the excitatory amino acid L-glutama te (5 one-minute infusions of 50 nl, 0.5 M glutamate over 1 h). Mean a rterial pressure was increased after each injection (7-13 mm Hg), and significant differences in numbers of neurons with FLI between sides w ere maintained in all five areas except the NTS caudal to, and at, the level of the area postrema. An increase in neurons with FLI in the pi riform cortex of all animals including controls may be due to injury-i nduced activation of target neurons from the PVN. These data illustrat e that electrical and chemical stimulation of the PVN leads to simulta neous activation of neurons in many targets. All of the target areas s tudied receive direct projections from the PVN, although multisynaptic projections may also contribute to activation of target neurons.