The androgen testosterone (T) and its metabolite, dihydrotestosterone (DHT), are released from the gonads and act in the brain and periphery to control many male-typical traits, including male sexual behavior (MSB). The classical model of androgenic action asserts that T, or DHT binds to a single species of intracellular androgen receptor (AR), that acts as a transcription factor to induce transactivation of androgen-regulated genes. Accordingly, androgens may take hours or days to assert their full influence and transcriptional activity of the AR may persist long after circulating androgens decline. Despite decades of research on testicular factors that control MSB there are still major gaps in our knowledge of how androgens exert their effects on MSB via the AR; most related cellular and molecular data are derived from studies of prostate cancer cell lines grown on cell culture plates--the applicability of these data to other AR positive cell-types and to regulation of behavior is unknown. Consequently the pattern of endogenous gonadal release and its actions on the genome to control MSB requires more research.
In the present dissertation I tested the hypothesis that discontinuous hormone replacement paradigms that maintain endogenous-like hormone release profiles are more effective than constant release regimens at maintaining MSB in castrated male hamsters. Further, by employing a discontinuous T replacement regimen I developed a paradigm by which to investigate many general aspects of T action, including its interaction with the AR, and its efficacy in maintaining and restoring MSB.
Experiments 1 and 2 establish that endogenous-like T replacement regimens require a lower total concentration and duration of circulating androgen availability to effectively maintain and restore MSB than T replacement via other commonly employed regimens. I injected hamsters daily with T dissolved in an aqueous vehicle that raises circulating concentrations for only 7h per day; hamsters that received only 15 μg T in this manner maintained pre-operative- like MSB even though much higher doses are insufficient when administered in a long-lasting oil vehicle (Experiments 1 and 2). Additionally, a majority of hamsters injected with the low-dose of 50 μg T as infrequently as once per 7 days maintained the ejaculatory reflex despite having supraphysiological concentrations of circulating T for only 7 hours per week.
A follow-up study (experiment 3) suggests a more complex and potentially interesting regulation of the AR by its ligands than is typically appreciated. DHT does not maintain MSB in Syrian hamsters. I implanted 30 mm of DHT in Silastic tubing subcutaneously into hamsters, to generate circulating DHT concentrations an order of magnitude greater than those present endogenously, concurrent with daily T injections of 25 μg in aqueous solution. The high doses of constantly circulating DHT failed to inhibit the restoration of MSB by low-dose T treatment, instead surprisingly enhancing T's restoration of the behavior, suggesting that DHT and T do not compete for the same binding sites and the possibility of multiple AR species with different affinities for T and DHT or differential uptake of these hormones in the MSB neural circuit.
Discontinuous, low dose androgen treatment to castrated Syrian hamsters was then used to probe the timing of protein synthesis required for the expression of MSB (experiment 4). Hamsters were injected with 100 μg T once per week, while also being injected with the protein synthesis inhibitor anisomycin at varying times relative to the T injection. Anisomycin treatment from 6 to 12 hours after T injection blocked the maintenance of MSB, whereas injections before and up to 3 hours after T treatment were without significant effect. By taking into account the time course of anisomycin inhibition of protein synthesis, a time period of T-induced protein synthesis necessary for the maintenance of MSB was identified--12-15 h post-injection.
Finally, photoperiod markedly affects T and MSB in Syrian hamsters. Dopamine and norepinephrine are two critical neurotransmitter systems necessary for the expression of MSB: norepinephrine induces generalized arousal of the CNS necessary for motivated behavior, whereas dopamine must be released into the medial preoptic area (mPOA) to induce MSB. Experiment 5 tested whether upregulation of dopamine or norepinephrine signaling could bypass short-photoperiod induced sexual quiescence. Apomorphine and yohimbine were administered just prior to MSB tests to photoregressed hamsters, but failed to increase sexual behavior, suggesting that upregulation of these two systems is not sufficient to restore MSB during the season of reproductive quiescence.
Overall, the research herein demonstrates that T replacement therapy that more closely mimics endogenous T profiles is more effective at maintaining MSB and possibly other traits controlled by androgens. Exploitation of this paradigm may enhance understanding of hormone signaling of motivated behavior. Additionally, the present paradigm will facilitate specification of the genomic and proteomic underpinnings of MSB as well as molecular and cellular research into this complex social behavior.
