Attempts to antagonize the effect of histamine on the cold-stimulated thyrotropin secretion in male rats

Histamine given directly into the 3rd ventricle inhibits the enhancement of thyrotropin secretion induced by cold-exposure (4 degrees, 30 min.) in male rats. This effect was antagonized neither by mepyramine, a H1-receptor antagonist, nor by cimetidine, a H2-receptor antagonist. Histamine would not have an indirect mechanism of action through adrenergic alpha 1- or alpha 2-receptors in rat hypothalamus, since pretreatment with neither phenoxybenzamine nor yohimbine had any effect on histamine suppressed TSH cold-response. Further, it was also tested if histamine could decrease the TSH secretion through cholinergic-, GABA-, serotonergic- or dopaminergic receptors but no results supporting such a mechanism of action were obtained. The effect of histamine was not modified by pretreatment with naloxone or desipramine either. Imidazole acetic acid, IAA, a metabolite of histamine, had no effect on cold-induced TSH secretion. It is concluded that the effect of exogenous histamine on cold-stimulated TSH secretion is not mediated through H1- or H2-receptors. Histamine may decrease brain noradrenergic activity which is important in the generation of TSH cold-response. In addition, the effect of exogenous histamine might be due to decreased endogenous histaminergic activity in rat brain.

Studies on the site and mechanism of the inhibitory action of intracerebral histamine on the cold-stimulated thyrotropin secretion in male rats

The locus of the inhibitory action of histamine on cold-stimulated TSH secretion was studied in male rats. Various histaminergic drugs were given either into the median eminence (ME), the third ventricle, the rostral and caudal part of anterior hypothalamus (rAH and cAH) or the posterior hypothalamus (PH) of conscious rats. When infused into ME, histamine (5 and 10 micrograms/rat) decreased and mepyramine (2.5 micrograms) increased the cold-stimulated TSH secretion while a larger dose of mepyramine (10 micrograms/rat), impromidine (0.05 and 0.1 microgram/rat), 2-pyridylethylamine (2-PEA; 5 and 10 micrograms/rat) and cimetidine (2.5 and 7.5 micrograms/rat) had no effect. The inhibitory action of histamine (5 micrograms/rat) was most marked 30 min after the infusion into ME and it remained significant for at least 50 min. Neither cimetidine (50 and 100 mg/kg i.p.) nor mepyramine (10 and 20 mg/kg i.p.) antagonized the inhibitory action of histamine (5 micrograms/rat) in ME. Histamine (10 micrograms/side) inhibited the cold-stimulated TSH secretion likewise when infused bilaterally either into rAH, cAH or PH. TSH secretion induced by exogenous TRH (100 ng i.p.) was also inhibited by histamine (1 microgram/rat) given into the third ventricle. In conclusion, histamine seems to have an inhibitory action on the cold-stimulated TSH secretion. This action is apparently mediated through areas close to the third ventricle. The mechanism of this action seems to be fairly non-specific, i.e. it is mediated through neither H1- nor H2-receptors.

Inhibition of the TSH secretion by histamine in male rats

The effect of histamine (HA) on the cold-stimulated and the TRH-induced TSH secretion was studied in male rats utilizing HA and specific HA antagonists and histaminergic drugs as tools. L-histidine (1000 and 1500 mg/kg ip) inhibited the cold-stimulated but not the TRH-induced TSH secretion. Mepyramine (5-50 mg/kg ip) and cimetidine (50-250 mg/kg ip) had no effect on the TSH-cold-response, but the TRH-induced TSH secretion was enhanced by them. HA (1 mg/kg sc), 2-pyridylethylamine (2PEA; 10 mg/kg ip) and impromidine (1 mg/kg sc) decreased the TRH-induced TSH secretion but did not affect the TSH cold-response. When infused into the third ventricle HA (1-50 micrograms/rat), impromidine (0.1 and 1.0 microgram/rat) and 2PEA (50 micrograms/rat) decreased the cold-stimulated TSH-secretion. The locus of the inhibitory action of HA seems to be both at the anterior pituitary, where it is mediated through H1- and H2-receptors and in the hypothalamus, where the effect appears to be mediated through H2-receptors. At least a part of this particular action might be explained by other mechanisms since neither H1- nor H2-receptor antagonists were able to counteract the effect of HA.

Studies on the mechanism of the enhanced cold-induced TSH secretion in spontaneously hypertensive rats

Various noradrenergic and tryptaminergic antagonists as well as pinealectomy significantly inhibited cold-induced TSH secretion in SHR as in control rats.

Effects of some putative amino acid neurotransmitters on the stimulated TSH secretion in male rats

The importance of several amino acids (glycine, L-glutamic acid, L-serine, taurine and beta-alanine) in the regulation of the stimulated secretion of TSH was studied in male rats using both peripheral and central administration of the amino acids. Glycine (10-200 mg/kg i.p.), L-glutamic acid (10-500 mg/kg i.p.) and L-serine (500 mg/kg i.p.) decreased significantly the cold-induced TSH secretion whereas beta-alanine (1-500 mg/kg i.p.) and taurine (10-100 mg/kg i.p.) were not effective. The effect of L-glutamic acid (100 mg i.p.) was partially antagonized by bicuculline (1 mg/kg i.p.) but not by picrotoxin (1 or 2 mg/kg i.p.). Only glycine (50 and 100 mg/kg i.p.) inhibited the TRH-stimulated TSH secretion. When the intracerebroventricular route was used, L-serine (50 micrograms/rat) decreased the TSH could response whereas glycine and L-glutamic acid (1-50 micrograms/rat) had no clear effect. We conclude that glycine, glutamate and serine inhibit the cold-induced TSH secretion in the male rat. The action of serine and glycine is possibly mediated through the periventricular hypothalamus and the anterior pituitary, respectively. The inhibition caused by glutamate seems to be partially mediated through the bicuculline-sensitive GABA receptors in the hypothalamus. Taurine and beta-alanine play no role in the control of rat TSH secretion.