Monoamines and the control of sexual behaviour.

1 The availability of large numbers of drugs in the 1960s and 1970s resulted in an explosion of psychopharmacological studies of motivated behaviour, including sexual behaviour. The effects of some drugs, particularly those affecting serotonin and dopamine, on patterns of sexual activity in males and females of a number of non-human species were impressive and encouraged the belief, held by many, that these cerebral monoamines were of special importance in regulating the expression of sexual motivation. Before summarizing the major consequences of manipulating mono-aminergic activity in the brain, it is important to point out that such experimental investigations were undertaken within a conceptual framework which acknowledged that sexual behaviour is hormone-dependent. Thus, it was well known that gonadal steroids critically determine whether or not a male or female displays sexual activity in the presence of an appropriate stimulus. That the hypothalamus is a major target for these behavioural effects of sex steroids was similarly widely accepted, but the precise mechanism of action remained largely a mystery. It was the latter problem which the consequences of aminergic manipulations appeared to address although, as we shall see, the narrow interpretation of the results of many of these psychopharmacological experiments was, in retrospect, misleading. It was Meyerson, in a series of innovative experiments, who first showed that inhibiting serotonin (5-HT) synthesis enhanced the display of'heat' in female rats (Meyerson, 1964). Indeed, the effect was so great that progesterone, the hormone which together with oestradiol controls oestrous behaviour in this species, could be replaced by drugs causing 5-HT depletion. Conversely, elevating 5-HT levels prevented the display of oestrus normally induced by the ovarian hormones. These results have been confirmed subsequently in a number of species and have been extended to include the male-for example, castrated male rats treated with doses of testosterone too low to restore their sexual behaviour can be rendered sexually active by injection of 5-HT-depleting drugs or receptor antagonists (Malmnas, 1973). The interpretation of these data, still held by many, was that hormones induce changes in sexual behaviour by depressing 5-HT transmission-indeed, Meyerson spoke of 5-HT neurones as a 'heat inhibition system' and continues to present experimental evidence directly linking progesterone and 5-HT in female rats. There is no doubt that decreasing 5-HT concentrations in the brain can have profound effects on sexual behaviour and not only in rodents. Thus, treatment of female rhesus monkeys made unreceptive by hormone withdrawal with …