Effects of pinealectomy on neurohypophysial hormones in the SFO and plasma of dehydrated rats exposed to 12 hours of light

Abstract

Magnocellular neurons in the supraoptic and paraventricular nuclei synthesize and release vasopressin and oxytocin in response to dehydration. Pinealectomy has been observed to decrease the distribution in the supraoptic nuclei of thiamine diphosphate-phosphohydrolase, an enzyme specific for the Golgi apparatus that correlates positively with neurosecretory activity. Based upon these studies we postulated that pinealectomy would alter the concentration of neurohypohysial hormones in plasma elevated by 48 hr of water deprivation. In addition, we investigated the possibility that pinealectomy would affect vasopressin concentration in another circumventricular organ, the subfornical organ (SFO) and in an adjacent fiber tract of the limbic system, the hippocampal commissure-fornix (HC-F). Adult, male, Sprague-Dawley rats exposed to a 12 hr light/dark cycle were either unoperated (controls; C), sham-operated (Sham; S) or pinealectomized (PX) three weeks prior to testing. Food and water consumption and urinary excretion of Na and K were measured for 7 days. On the fifth day, half of the animals in each treatment group (C, S, PX) were deprived of water for 48 hr. Animals were decapitated on day 8. Vasopressin and oxytocin in plasma were extracted using bentonite and acetone-ether, respectively, then quantified by radioimmunoassay. The SFO and HC-F were microdissected from each brain. Like tissues from 4 rats were pooled, homogenized in 0.1 N HCl, and centrifuged. The supernatant was neutralized and vasopressin was quantified by radioimmunoassay. Dehydration resulted in antidiuresis, increased urine concentrations of Na and K, a decreased ratio of Na:K in urine, and reduced food consumption of similar magnitudes in all groups (C, S, PX; p > 0.05). The plasma concentration of both vasopressin and oxytocin was increased ( p < 0.05) after 48 hr of water deprivation in C, S and PX rats. PX altered only one measured parameter. Oxytocin concentration in plasma was increased ( p < 0.05) in pinealectomized rats deprived of water for 48 hr. No difference ( p > 0.05) in vasopressin concentration (pg/mg protein, pg/mg wet wt) in the SFO area was measured in pinealectomized or dehydrated rats. Pinealectomy influences release of oxytocin but not vasopressin from the hypothalamo-neurohypophysial system of the dehydrated rat exposed to a 12 hr light photoperiod.