Effects of testosterone on neuronal nitric oxide synthase and tyrosine hydroxylase

Male rat copulatory ability decreases dramatically following castration. Th is may be due in part to the impairment of medial preoptic area (MPOA) dopa mine (DA) release. Previous studies showed that extracellular DA levels in the MPOA of castrates were lower than in intact males, both during basal co nditions and in the presence of a receptive female. However, tissue levels of DA in the MPOA were higher in castrates than in intact males, suggesting that DA synthesis may be normal or increased in castrates, but that releas e may be compromised. The current study found that neither long term (2 mon ths) nor short term (2 weeks) castration had any effect on the number of ne urons in the DA A(14) area that were immunoreactive (ir) for tyrosine hydro xylase (TH), the rate limiting enzyme for DA synthesis. Therefore, castrati on may not affect DA synthesis in the MPOA. Tissue levels of neurotransmitt er reflect release, as well as synthesis. We previously reported that nitri c oxide (NO) may increase DA release in the MPOA. The present study tested whether castration affected the number of NO producing cells in the MPOA. L ong term, but not short term, castration significantly decreased the number of NADPH-d (nicotinamide adenine dinucleotide phosphate diaphorase) positi ve neurons and brain nitric oxide synthase immunoactive (bNOS-ir) neurons i n the medial preoptic nucleus (MPN). This suggests that in gonadally intact animals testosterone may activate NOS, which increases the production of N O. Long or short term castration had no effect on the numbers of bNOS-ir ne urons in the paraventricular nucleus (PVN) or medial amygdala. However, sho rt term castration decreased bNOS-ir neurons in the bed nucleus of stria te rminalis (BNST). Thus, one means by which testosterone promotes male sexual behavior may be by increasing production of NO in the MPOA, which increase s local DA release. (C) 1999 Elsevier Science B.V. All rights reserved.