Ejaculation results in changes in prolactin (increase) and dopamine (temporary decrease), but does not result in changes in Testosterone. Although prolactin and dopamine are both involved with testosterone, they do not appear to influence testosterone levels acutely.
Dopamine is reduced temporarily as prolactin rises, as the two can be seen as antagonistic of each other. Levels shortly normalize. The post-orgasm 'orgasmic state' of euphoria is mediated by prolactin and mimicked by ectasy.
What does male orgasm result in, biochemically?
Higher serum Testosterone levels are seen during abstinence (3 weeks in cited study). Non-significant Luteinizing Hormone increases have also been noted. The difference between abstinence and non-abstinence appears to be about 0.5ng/ml when averaged out.
Although higher testosterone levels are seen with abstinence, orgasm does not acutely affect testosterone levels in the blood. Although there is not much evidence for the spike in testosterone during abstinence, there does not appear to be counter evidence at the moment; it is an understudied topic.
Orgasm can cause a significant spike in prolactin levels (approximately 10 to 15ng/ml) immediately after and upwards to 10-20 minutes later, at which it starts to decline. This spike is dependent on ejaculation, and does not occur under non-orgasmic arousal. This spike may serve to suppress further sexual desires.
Various cardiovascular parameters, such as heart rate and catecholamine (adrenaline, noradrenaline) levels are increased during sex/masturbation and orgasm. Some measure of increase is seen during arousal.
Other various markers, such as Vasopressin and Follicle-Stimulating Hormone (FSH) remain unchanged.
Biochemical markers do not differ significantly when comparing orgasm after abstinence and orgasm without abstinence. Slight increases were seen in heart rate and catecholamine (adrenaline) levels, but may be due to self-reported higher arousal on average.
How is testosterone related to orgasm?
Testosterone has minimal interactions with orgasm, but is seen as a positive regulator of sexual desire or libido alongside dopamine. Agents that increase dopamine levels or act like dopamine can increase frequency of erections and subjective sexual arousal. Prolactin is the opposite here, and is a negative regulator of sexual appetite.
The actual ejaculatory process and erection process is mediated by serotonin and Nitric Oxide, as well as various mechanical contractions in the pelvic and penile region. Drugs or supplements that interfere with serotonin reuptake (such as SSRIs) can reduce the orgasm response, and may be useful in treating premature ejaculation.