SPLANCHNIC AND VAGAL DENERVATION ATTENUATE CENTRAL FOS BUT NOT AVP RESPONSES TO INTRAGASTRIC SALT IN RATS

We have recently reported that an acute intragastric hypertonic saline load increases plasma arginine vasopressin (P-AVP) and Fos immunoreac tivity in several central nuclei, including the supraoptic nucleus (SO N), paraventricular nucleus (PVN), nucleus of the solitary tract (NTS) , area postrema (AP), and lateral parabrachial nucleus (LPBN). We hypo thesized that these responses are mediated by stimulation of periphera l osmoreceptors with splanchnic and/or vagal afferent projections. To test this hypothesis, we examined the effect of bilateral subdiaphragm atic vagotomy and bilateral splanchnic denervation on the P-AVP and Fo s immunoreactivity responses to intragastric hypertonic saline infusio n in awake rats. Compared with responses in sham rats, Fos immunoreact ivity responses were significantly reduced in vagotomized rats in the AP, SON, and PVN, whereas normal Fos levels were observed in the LPBN. However, vagotomized rats exhibited a normal increase in P-AVP Splanc hnic-denervated rats also exhibited similar changes in P-AVP in respon se to intragastric hypertonic saline compared with sham-denervated rat s, and no differences were observed in Fos immunoreactivity in the LPB N, SON, and PVN compared with sham rats. However, splanchnic-denervate d rats were observed to have significantly lower Fos staining in the N TS and AP compared with sham rats. The inability of splanchnic or vaga l denervation alone to block the P-AVP response to intragastric hypert onic saline suggests that either peripheral osmoreceptors project via both splanchnic and vagal afferents to mediate AVP release or that the observed response of P-AVP is due to the activation of central osmore ceptors in the absence of measurable changes in plasma osmolality.