The Role of Brainstem and Limbic Structures in Regulation of Sexual Behavioural Patterns

Summary

There is a number of observations which clearly indicate that the influence of hormonal factors in sexual activity is more of a conditioning than a direct action. During the course of evolution the sexual drive seems to become less dependent on the actual level of gonadal hormones in the body, and thus is more marked in animals high in the phylogenic scale than in lower animals (Beach, 1951). Moreover, it has often been noted that the sexual drive and organised patterns of sexual behaviour persist for a period after gonadectomy in both sexes (Beach, 1951; Goldstein, 1957; Young, 1957). The sex steroids exert their influence upon neuro-anatomical structures which are fundamentally involved in the integration of elementary learning processes, e.g. motivation and the formation of tempory linkages; we may infer therefore that the gonadal hormones have a conditioning effect on these processes. The patterns of sexual behaviour are innate, and, if there is an adequate hormonal background, they become established at puberty in response to specific stimuli of environment. After a number of such experiences, sexual arousal continues to occur as the result of more or less specific environmental stimuli, even if there is a relative lack or a total absence of hormonal support. Nevertheless, there is no doubt that previous conditioning by the gonadal hormones plays a basic role in these events.

The sex steroids have various effects on the central nervous system. Oestrogens exert a facilitatory influence on the brainstem and forebrain connections, but have only little influence on the sensory input to the brainstem and the diencephalon. In contrast, progesterone by its biphasic effect appears to control the sensory input in two opposite ways; by facilitation and inhibition. Both oestrogen and testosterone administration have a facilitatory influence on the amygdalo-ventromedial hypothalamic connections.

Our previous studies have shown that the organization of motivated behavioural responses take place at the brainstem and diencephalic level, and that this neuro-anatomical mechanism consists of two systems which are connected to each other in a motivation-specific way. Under physiological conditions the activation of the ascending facilitatory system is accompanied by an integration of the patterns of motivated behaviour. This is followed by reinforcement, mediated by rebound activation of the descending inhibitory system of basal forebrain. Sex steroids affect both ascending and descending parts of this antagonistic neuro-anatomical mechanism in different ways: oestrogens facilitate the ascending activatory system but do not have any direct effect on the descending inhibitory influence of basal forebrain. Implantation studies revealed that the site of action of oestrogens is in the preoptic area, which has been postulated in earlier studies (Flerkó, 1967; Szentágothai et al., 1962). On the other hand, the administration of progesterone causes an initial facilitation of the sensory input from vaginal sources, followed by a marked raising of the threshold of the ascending activatory system; this effect appears to be mediated through the descending inhibitory influence of basal forebrain.

There is much evidence to show that the administration of oestrogen and progesterone alone may elicit sexual arousal after gonadectomy in different species. Taking into account that oestrogens facilitate “nonspecific connections of the brainstem and forebrain structures”, and this nonspecific activation means an increased sensitivity to environmental stimuli. The sexual arousal induced by oestrogen can be interpreted as a central or nonspecific arousal. In contrast to this, progesterone facilitates the sensory input for a while, and during this period sexual arousal may occur in some species. Androgens seem to play an important role in sexual arousal in both sexes (Whalen, 1966). Testosterone treatment in female rodents enhances the facilitatory influence of the amygdala upon brainstem and hypothalamic connections; this is in accord with earlier behavioural findings.

Our neurophysiological observations show that an integration of motivated behavioural responses takes place at the level of the brainstem and diencephalon, and that this neuro-anatomical mechanism consists of two antagonistically connected systems. Sex steroids can alter thresholds of these antagonistic connexions and may facilitate or inhibit the excitatory state of this neuro-anatomical substrate. Taking into account that the organization of other behavioural responses is also carried out within this neuro-anatomical substrate, the so-called nonspecific influence of sex steroids may be explained in the light of a common neurophysiological basis of motivated behavioural processes.