BASAL PROLACTIN CONCENTRATIONS AND TUBEROINFUNDIBULAR DOPAMINE METABOLISM IN THE SIBERIAN HAMSTER HYPOTHALAMUS - EFFECTS OF PHOTOPERIOD ANDHYPOTHALAMIC KNIFE CUTS

Horizontal knife cuts located between the suprachiasmatic and paravent ricular nuclei of the hypothalamus induce a photoperiod-independent de cline in circulating prolactin (PRL) concentrations in Siberian hamste rs. The possible contribution of alterations in the inhibitory activit y of the tuberoinfundibular dopaminergic (TIDA) neurons was investigat ed in animals that received knife cuts or sham surgery. Animals also r eceived systemic infusions of saline or prolactin for 2 days before sa crifice to evaluate the effects of the lesions on the 'positive feedba ck' of prolactin on dopamine (DA) activity. Some sham animals were tra nsferred to a short photoperiod to evaluate the effects of photoperiod alone. Tissue harvested from the caudate, arcuate and median eminence was assayed for DA and dihydroxyphenylacetic acid (DOPAC), the major metabolite of DA, with high pressure liquid chromotography (HPLC). No differences were seen for any measure in the caudate. For the arcuate, knife cuts had no effect on DA and DOPAC, but induced lower DOPAC/DA ratios. in the median eminence, PRL infusion increased the DOPAC/DA ra tio. Saline-infused animals did not differ, and showed lower DOPAC/DA ratios than short-day animals. These results indicate that these knife cut-induced declines in basal PRL are not related to increased DA act ivity, and suggest that photoperiod effects upon TIDA cells may differ for the soma and terminal regions.