Comparison of the effects of intraventricular taurine, GABA and homotaurine on serum prolactin levels in male rats

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Changes in mediobasal hypothalamic dopamine and GABA release: a possible mechanism underlying taurine-induced prolactin secretion

Taurine (Tau), a putative inhibitory amino acid neurotransmitter, has been shown to stimulate prolactin (PRL) release. Using ovariectomized, estrogen-replaced adult rats we investigated initially the effect of this amino acid, injected by different routes, on PRL secretion in vivo. Tau (100-500 mg/kg) had no effect on PRL release when given i.p.; 15 min after i.c.v. injection of Tau (3 mumoles), a significant increase in serum PRL levels was observed (78 +/- 9 ng/ml over basal levels, p < 0.01 vs. controls). In vitro (cultured anterior pituitary cells) PRL release was not affected by a 5 h incubation with Tau (10(-3)-10(-8) M). Basal dopamine (DA) or gamma-aminobutyric acid (GABA) output from superfused mediobasal hypothalamic fragments (MBH) was not affected by Tau (10(-3) M or 10(-5) M). However, during stimulation with KCl (50 mM), Tau (10(-3) M) significantly lowered DA release, and increased GABA output. It is concluded that Tau acts at a central level to increase PRL secretion, most probably by modulating the hypothalamic release of neurotransmitters controlling lactotroph function.

Effects of selective opioid receptor antagonists on morphine-induced changes in striatal and limbic dopamine metabolism

The effects of selective opioid receptor antagonists, beta-funaltrexamine (selective for mu receptor), naloxonazine (microliter) and naltrindole (delta) on morphine-induced changes in striatal and limbic dopamine (DA) metabolism were studied in rats. beta-Funaltrexamine (20 micrograms intracerebroventricularly) and naloxonazine (15 mg/kg intraperitoneally) were given 24 hr before morphine (15 mg/kg subcutaneously), and the rats were decapitated 60 min. after morphine. Naltrindole (1 mg/kg intraperitoneally) was given twice, 15 min. before and after morphine. Morphine significantly increased the concentrations of DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). This effect was significantly antagonized by pretreatment with beta-funaltrexamine but not by naloxonazine or naltrindole. However, naloxonazine attenuated the antinociceptive effect of morphine in the hot-plate test. The concentration of DA was not significantly altered by any of the drugs studied. These results show that selective blockade of mu-opioid receptors totally blocks the increase of striatal and limbic DA metabolism induced by morphine. It seems that mu 2-subtype of mu-opioid receptor predominantly mediates this effect. Blockade of delta-opioid receptor did not alter these effects of morphine.

Effects of intraventricular taurine, homotaurine and GABA on serum prolactin and thyrotropin levels in female and in male rats

Serum prolactin and thyrotropin levels of conscious, unrestrained male and female rats were compared after intracerebroventricular (i.c.v.) administration of taurine, gamma-aminobutyric acid (GABA) and homotaurine. The amino acids studied had no clear effect on serum basal thyrotropin levels in male or female rats. All amino acids elevated serum prolactin levels in female rats at the dose of 5 mumol/rat; homotaurine by about 18-fold, taurine and GABA by 3-fold. Only homotaurine elevated serum prolactin of male rats at this dose, but its effect was less pronounced (p < 0.01) in male than in female rats. Although homotaurine was clearly more potent than the two other amino acids, at the dose of 10 mumol/rat taurine and GABA also elevated serum prolactin in male rats. These findings show that there are gender-related differences in the responses of serum prolactin levels to homotaurine, taurine and GABA in rats. The tuberoinfundibular dopaminergic pathway, which exerts tonic inhibitory influence on prolactin secretion, is sexually differentiated. Hence the gender-related differences in the effects of the amino acids on prolactin secretion suggest that they might inhibit dopamine release from the median eminence. In case of homotaurine, the gender effect was most pronounced. The less clear dependence of GABA's effect on the gender is in accordance with the suggestions that GABA influences the secretion of serum prolactin by more than one mechanism.

Effects of taurine, homotaurine and GABA on hypothalamic and striatal dopamine metabolism

To elucidate the effects of taurine on hypothalamic and striatal dopaminergic neurotransmission we compared its effects to those of gamma-aminobutyric acid (GABA) and homotaurine (a GABAA-receptor agonist) on hypothalamic and striatal concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and, in the case of striatum, 3-methoxytyramine (3-MT) in rats. In addition, hypothalamic and striatal 5-hydroxytryptamine (5-HT) und 5-hydroxyindoleacetic acid, hypothalamic noradrenaline (NA) and 3-methoxy-4-hydroxyphenylglycol sulfate, and pituitary DA concentrations were also measured. The amino acids were injected into the lateral brain ventricles of conscious male rats in doses of 10 and 36 mumol/rat, and rat were sacrificed 15 and 60 min later, respectively. Homotaurine (by 11%) but not the other two amino acids elevated striatal DA, whereas hypothalamic DA was increased by both taurine (36%) and homotaurine (31%). All three amino acids at 36 mumol elevated striatal DOPAC, homotaurine (51%) more than taurine (31%) or GABA (30%), and hypothalamic DOPAC, both taurine (102%) and homotaurine (82%) clearly more than GABA (34%). Neither striatal nor hypothalamic HVA was altered by any of the amino acids. At 10 mumol the amino acids decreased striatal 3-MT by about 40%. At 36 mumol taurine and homotaurine reduced 3-MT by about 70%, whereas increasing the dose of GABA did not further reduce 3-MT. Both taurine and homotaurine at 36 mumol decreased hypothalamic NA content. Neither hypothalamic nor striatal 5-HT metabolism was altered. In the neurointermediate lobe of the pituitary gland taurine at 10 mumol but not at 36 mumol slightly (20%) increased DA.(ABSTRACT TRUNCATED AT 250 WORDS)