Sexually differentiated actions of 3-PPP enantiomers on prolactin secretion

Adaptive properties and heterogeneity of dopamine D(2) receptors - pharmacological implications

In this review, we focus on the marked adaptability of dopamine D(2) receptors to varying agonist levels and we discuss the extent to which this phenomenon can account for the heterogeneity of these receptors in regard to function and pharmacological responsiveness. We emphasize the significance of a distinction between synaptic and extrasynaptic receptors in this context. For example, the application of this dichotomy appears to shed new light on the various subgroups of antipsychotic drugs and the mechanisms underlying their different profiles.

The enhanced responsiveness of hypophyseal DA receptors in female rats induced by repeated reserpine treatment is not due to decreased oestrogen secretion

In a previous study we have demonstrated that repeated reserpine treatment markedly enhances the intrinsic activity of the partial dopamine (DA) agonist (-)-3-PPP (preclamol) on pituitary DA receptors in female rats. This effect was attributed to the DA-depleting action of reserpine. However, since reserpine may also decrease oestrogen secretion, and since this hormone is known to affect dopaminergic transmission, experiments with ovariectomized, oestrogen-replaced female rats were undertaken. Ovariectomized rats were administered a depot preparation of oestradiol valerate in a dose that, according to literature data, yields physiological or slightly supraphysiological plasma concentrations of oestrogen. In spite of this treatment, repeated reserpine administration was found to substantially increase the intrinsic activity of (-)-3-PPP, as well as that of the partial DA agonist TDHL (terguride), on female pituitary DA receptors. It was concluded that repeated reserpine treatment increases pituitary DA receptor responsiveness in female rats by depleting DA, rather than by decreasing oestrogen secretion.

The partial dopamine receptor agonists (--)-3-PPP and transdihydrolisuride enhance prolactin secretion in female but not in male rats

The effects exerted by the partial dopamine (DA) receptor agonists (--)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(--)-3-PPP] and transdihydrolisuride (TDHL) on prolactin secretion were compared in male and female rats. Both agents were found to increase prolactin release more than tenfold in female rats. In male rats, in contrast, (--)-3-PPP and TDHL were either without effect or decreased prolactin secretion. It is suggested that this sexually differentiated prolactin response to partial DA agonists reflects a lower responsiveness of lactotroph DA receptors in female than in male rats.

The intrinsic activity of (-)-3-PPP (preclamol) on pituitary DA receptors in female rats is enhanced following chronic DA depletion

The present investigation was aimed at comparing the influence of the partial dopamine (DA) receptor agonist (-)-3-PPP (preclamol) on prolactin release in acutely hyperprolactinemic but otherwise intact female rats and female rats subjected to chronic DA depletion. One group of animals received daily vehicle injections (controls) and another group daily reserpine (1 mg/kg) injections for a period of seven days. On the eighth day all animals were administered 10 mg/kg of reserpine in order to eliminate endogenous DA and elevate serum prolactin levels. In the control group (-)-3-PPP lowered serum prolactin levels only moderately. In contrast, in the chronically reserpinized female rats, a pronounced reduction of prolactin secretion was observed. It is suggested that this increase in intrinsic activity of (-)-3-PPP following chronic DA depletion reflects an enhanced responsiveness of hypophyseal DA receptors, possibly due to conformational changes in the receptor molecules. Our observations lend further support to the hypothesis that an inverse relationship exists between the intrinsic activity of a mixed agonist/antagonist and the degree of previous receptor occupancy.

The intrinsic activities of the partial dopamine receptor agonists (-)-3-PPP and TDHL on pituitary dopamine receptors are lower in female than in male rats

The abilities of the mixed agonists/antagonists on dopamine (DA) receptors, (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [-)-3-PPP) and transdihydrolisuride (TDHL), to suppress serum prolactin levels in acutely hyperprolactinemic male and female rats were investigated. gamma-Butyrolactone was used to deplete endogenous DA and raise serum prolactin concentrations. Both (-)-3-PPP and TDHL were found to cause sexually differentiated responses: (-)-3-PPP reduced serum prolactin levels dose dependently and effectively in males but caused only a modest decrease of prolactin release in females. Moreover, (-)-3-PPP antagonized the prolactin-suppressing effects induced by the DA receptor agonist (+)-3-PPP in females. Likewise TDHL decreased prolactin secretion markedly in males while it had only slight effects in females. It can be concluded from these results that the intrinsic activities of the partial DA agonists (-)-3-PPP and TDHL are lower in female than in male rats, suggesting a reduced responsiveness of hypophyseal DA receptors in females. Since DA levels in the pituitary portal circulation are higher in female than in male rats, this study gives further support to the hypothesis claiming an inverse relationship between the intrinsic activity of a mixed agonist/antagonist and the degree of previous stimulation of its receptor.