Acute changes in norepinephrine content in the median eminence induced by orchidectomy or testosterone replacement

Median eminence catecholamines were measured in individual samples from intact, castrated, or sham-operated male rats, in order to correlate changes in catecholamine content with the early post-castration rise in serum LH levels. Both norepinephrine (NE) and dopamine (DA) were measured by an enzymatic-isotopic assay and advantage was also taken of a method recently developed that makes it possible to obtain the median eminence free of the surrounding hypothalamus. Determinations were made at 4, 8, or 24 h after surgery. All animals were killed between 1400 and 1600 h. It was observed that castration increases the ME content of NE significantly at all the times studied, with peak values 8 h after orchidectomy. Sham operation induced a significant decrease in NE content in the ME 4 h after surgery. By 8 h the NE values had returned to intact control levels. Both castration and sham operation induced a significant decrease in DA levels 4 h after surgery. By 8 h, DA values had returned to intact control levels in both groups. When the NE levels in castrated rats had reached the highest value, serum LH values had not departed from control levels, indicating that the change in NE content in the ME takes place prior to the expected rise in LH. Testosterone replacement therapy (40 mug/100 gBW) of castrated rats blocked the increase in NE content observed 8 h after castration. Dopamine levels were not affected by this treatment. These results support the concept that the NE contained in median eminence catecholaminergic terminals participates in the feed-back regulation of LH secretion in the male rat, while DA is apparently not involved under the present conditions.