Influence of the paraventricular and suprachiasmatic nuclei and olfactory bulbs on mela .tonin responses in the golden hamster

Removal of the pineal, or denervation of this gland by superior cervical ganglionectomy, blocks testicular regression in golden hamsters exposed to short photoperiods. Aspiration of the olfactory bulbs or lesions of the suprachiasmatic or paraventricular nuclei of the hypothalamus (SCNx or PVNx) have similar effects. We have examined the effects of these operations on pineal melatonin content and gonadal responses to var-ious patterns of exogenous melatonin in order to examine the roles played by the olfactory bulbs, the SCN, and the PVN in hamster photoperiodism. SCNx and PVNx significantly reduced pineal melatonin content throughout the dark phase, while bulbectomy did not significantly affect melatonin concentrations at the time of the nocturnal peak. Bulbectomy significantly delayed the nightly onset of locomotor activity in hamsters exposed to 14L: 1OD, but not that of animals housed in 1OL:14D. Although bulbectomy reduced the gonadal response to one or three daily injections of melatonin, these individuals exhibited significant testicular re-gression in response to melatonin as long as injections fell in the evening. In contrast, destruction of the PVN rendered hamsters unresponsive to one daily melatonin injection, but equally responsive to three injections, regardless of the time of day at which these injevtions were given. Whereas exposure of bulbectomized ham-sters to 30 weeks of short days made them refractory to subsequent melatonin challenge, PVNx hamsters remained sensitive to appropriately timed melatonin treatments regardless of their photoperiodic history. Many, but not all hamsters that experienced complete SCN lesions remained sensitive to three daily mela-tonin injections. The results indicate that these operations differ fundamentally in their influence on hamster photoperiodism: destruction of the SCN or PVN intereferes with melatonin synthesis, and removal of the olfac-tory bulbs overrides reproductive responses to a normal melatonin signal