FOS-CONTAINING NEURONS IN MEDULLA AND PONS AFTER UNILATERAL STIMULATION OF THE AFFERENT ABDOMINAL VAGUS IN CONSCIOUS RABBITS

Vagal afferents originating in abdominal viscera initiate numerous centrally-mediated responses, including behavioural, cardiovascular and hormonal changes associated with satiety, and nausea and vomiting. The present work was undertaken to map the pontomedullary distribution of neurons expressing Fos immunoreactivity following unilateral electrical stimulation of abdominal vagal afferents in conscious unanaesthetized rabbits. After 2 h of stimulation of the anterior trunk of the abdominal vagus nerve (20 Hz, 0.5 mA, 0.5 ms duration, 4.5 min on, 0.5 min off), Fos-positive neurons were found in the area postrema, the nucleus tractus solitarius, the spinal nucleus of the trigeminal nerve, the caudal and the rostral ventrolateral medulla, the locus coeruleus, the subcoeruleus and the lateral parabrachial nucleus. In all these regions, more than 70% of Fos-containing neurons occurred on the ipsilateral side. In control animals only occasional Fos-immunoreactive neurons were observed, usually very faintly labelled. Simultaneous staining for both Fos and tyrosine hydroxylase revealed Fos immunoreactivity in catecholamine neurons, including A1, A2, C1, A5, subcoeruleus and locus coeruleus (A6) groups. Our findings complement functional studies in the rabbit, identifying Al neurons as part of the central pathway by which afferent abdominal vagal stimulation increases plasma vasopressin, and C1 neurons as part of the central pathway, whereby afferent abdominal vagal stimulation increases arterial pressure.