Differential Involvement of 5-Hydroxytryptamine (5HT) in Specific Hypothalamic Areas in the Mediation of Steroid-Induced Changes in Gonadotrophin Release and Sexual Behaviour in Female Rats

Abstract

Ovariectomised rats were treated with either oestradiol benzoate (OB) alone or OB followed by progesterone (P). The effects of the steroid treatments were noted on plasma LH concentration, lordosis quotient and concentration of 5-hydroxytryptamine (5HT) and 5-hydroxyindole acetic acid (5HIAA) in microdissected hypothalamic areas, i.e. the preoptic area (POA), arcuate nucleus (ARC), median eminence (ME), ventromedial nucleus (VMN), suprachiasmatic nucleus (SCN) and zona incerta (ZI). The ratio of 5HIAA:5HT or parallel changes in the concentrations of 5HT and 5HIAA were taken as indices of 5HT neuronal activity. 5 µg OB alone reduced LH release and had no effect on receptivity. This treatment reduced 5HT activity in the SCN and raised it in the ZI. After 5 µg OB plus 0.5 mg P, all the rats exhibited an LH surge and full sexual receptivity. 5HT activity was significantly raised in the ME and reduced in the ARC, VMN and ZI. By using submaximal steroid regimes (2 µg OB plus 0.05 mg P, or 50 µg OB alone) correlations could be made between sexual behaviour, LH release and 5HT activity. There was no correlation between LH release and receptivity indicating that the two functions are controlled by different systems. Correlating 5HT activity in the various sites with the two physiological parameters indicated whether the changes seen after the maximal steroid treatment were concerned with behaviour, LH release or both. The results indicate that the changes seen in the VMN, ARC and ME are correlated with sex behaviour and those in the VMN and ZI with increased LH release. Oestrogen appears to stimulate behaviour by reducing 5HT activity in the ARC and P by reducing 5HT activity in the VMN and increasing it in the ME. Oestrogen seems to exert its feedback effects on LH by altering a stimulatory 5HT system in the SCN and an inhibitory one in the ZI. P stimulates LH release by reducing 5HT in the VMN and the ZI.