The Effect of Pinealectomy on Osmotically Stimulated Vasopressin and Oxytocin Release and Fos Protein Production within the Hypothalamus of the Rat

Abstract

Osmotically stimulated vasopressin and oxytocin release were measured in pinealectomized and sham operated male rats infused with hypertonic sodium chloride. Neuronal activation in the hypothalamic regions associated with oxytocin and vasopressin release was investigated by quantitative assessment of Fos protein production. The osmotically stimulated release of both vasopressin and oxytocin was significantly lower in pinealectomized animals as compared to sham operated controls. The slope of regression lines between plasma osmolality and hormone concentrations in the sham animals showed a 1.0 +/- 0.1 pmol per mosm/kg rise in vasopressin and 2.0 +/- 0.4 pmol per mosm/kg rise in oxytocin whilst in the pinealectomized animals these values were significantly lower at 0.4 +/- 0.1 pmol vasopressin per mosm/kg and 0.8 +/- 0.2pmol oxytocin per mosm/kg. The osmotic thresholds for hormone release were unaffected by pinealectomy. Fos production was also significantly lower in the supraoptic nucleus and organ vasculosum of the lamina terminalis in the pinealectomized rat at 62 +/- 20 and 59 +/- 9 Fos immunoreactive cells/section as compared to corresponding values of 202 +/- 31 and 123 +/- 20 Fos immunoreactive cells/section in the shams. These observations suggest that reduced hormone release in the pinealectomized animal is due to lowered responsiveness of central osmoregulatory mechanisms and that melatonin may therefore influence the activation of the magnocellular system.

Daily Rhythms of Spike Coding in the Rat Supraoptic Nucleus

“…An increased MSF of phasic cells is consistent with the reduction in neurohypophysial vasopressin content after pinealectomy, especially in the morning (37) and in dehydrated animals (43). Reduced neurohypophysial hormone reserves may account for an attenuated response to osmotic stimulation after the pinealectomy (9).…”

“…The normal daily pattern of vasopressin and oxytocin concentration is profoundly affected by constant light (6) and pinealectomy (7). Pinealectomy also attenuates neurohyphophyseal responses to osmotic stimulation, as measured by the release of vasopressin (8) and oxytocin (9). It is thus likely that the firing of magnocellular neurosecretory cells is modulated by circadian pacemakers as well as osmosensitive projections (10).…”

Pinealectomy Reduces Optic Nerve But Not Intergeniculate Leaflet Input to the Suprachiasmatic Nucleus at Night

“…Pinealectomy was performed as described by Windle et al . (29), with the difference that the animals were under halothane anaesthesia. At least 21 days were allowed between Px and the recordings described below.…”

The effect of exogenous melatonin on stimulated neurohypophysial hormone release in man

“…To date, no melatonin receptors have been demonstrated in the supraoptic and paraventricular nuclei, so that melatonin could be acting on pathways to the neurones producing oxytocin and vasopressin. The osmoregulatory complex could be involved as pinealectomy has been shown to attenuate the increase in neuronal activity in the organosum vasculosum of the lamina terminalis, as judged by Fos protein production (Windle et al ., 1996).…”

“…This again could be related to melatonin release as melatonin appears to affect stimulated release of the hormone. Thus the oxytocin and vasopressin response to infusion of hypertonic saline infusion of the rat is reduced following pinealectomy (Windle et al ., 1996), as is the response to hypovolaemia (Forsling et al ., 1996). Similarly, in man, stimulation of oxytocin and vasopressin release on hypoglycaemia is reduced on melatonin administration (Coiro et al ., 1997; Chiodera et al ., 1998a).…”