A re-evaluation of the effects of gonadal steroids on neuronal activity in the male rat

Single unit activity (SUA) was recorded from 77 cells located in the arcuate nucleus (ARC) and medial preoptic area (MPA) of anesthetized, intact male rats. Animals were administered vehicle, testosterone (T; 5 or 50 μg) or 17β-estradiol (E; 0.5 μg) intravenously and SUA was monitored for 8-12 min. T (50 μg) reduced SUA in 50% of ARC units and 44% of MPA units within 2.1 ± 0.46 and 3.3 ± 0.92 min, respectively. Inhibition of ARC SUA was more pronounced than MPA SUA. A small percentage (9%) of ARC units were excited by T. E reduced SUA in 29% of ARC units and 27% of MPA units. Single doses of 5 Mg T did not affect ARC activity. However, when followed within 10 min by an additional dose of 5 or 50 μg T, 30% and 43% of ARC units were inhibited, respectively. Doses (10 μg) of T produced plasma T concentrations within physiological limits, although 50 μg doses produced supraphysiological T levels. Neither dose affected circulating LH concentrations. We conclude that physiological and supraphysiological concentrations of T can rapidly affect SUA within the ARC.