The effect of medial forebrain bundle lesion on thyrotropin secretion in the rat

L-type Cav1.2 calcium channel is involved in 6-hydroxydopamine-induced neurotoxicity in rats

Evidence suggested that L-type calcium channels may play a key role in the pathogenesis of dopaminergic neuron degeneration. In the present study, effects of L-type Cav1.2 calcium channel on 6-hydroxydopamine (6-OHDA)-induced neurotoxicity were investigated. By the semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) studies, we showed that the expression of L-type Cav1.2 calcium channel α1 subunit mRNA increased in the substantia nigra (SN) of 6-OHDA-lesioned rats. Treatment with nifedipine could improve the apomorphine-induced rotation behavior in 6-OHDA-lesioned rats. Using high-performance liquid chromatography electrochemical detection, we also observed that nifedipine partly restored 6-OHDA-induced dopamine depletion in the striatum of rats. These results suggest that the L-type Cav1.2 calcium channel is associated with the development and progression of dopaminergic neuron degeneration.

Neuroendocrine effects of dexmedetomidine: evidence of cross-tolerance between a mu-opioid agonist and an alpha 2-adrenoceptor agonist in growth hormone secretion of the male rat

The role of alpha 2-adrenergic receptors (adrenoceptors) in the secretion of growth hormone, prolactin and thyrotropin was studied using highly selective agonists and antagonists of the alpha 2-adrenoceptor. The interplay between opiates and alpha 2-adrenergic drugs in the acute secretion of growth hormone and prolactin, as well as the possible cross-tolerance between morphine (mu-opioid receptor agonist) and dexmedetomidine (alpha 2-adrenoceptor agonist) in growth hormone secretion were also evaluated. Dexmedetomidine dose-dependently increased plasma growth hormone and prolactin levels and decreased thyrotropin levels. The enhanced secretion of both growth hormone and prolactin was antagonized by atipamezole (an alpha 2-adrenoceptor antagonist) but not by prazosin (an alpha 1-adrenoceptor antagonist). Morphine (5 mg/kg)-induced stimulation of growth hormone secretion was antagonized by both naloxone (mu-opioid antagonist) and atipamezole. Naloxone, but not atipamezole, antagonized the morphine-induced increase in prolactin secretion. Dexmedetomidine increased growth hormone secretion in the saline pretreated rats, but did not do so in the morphine-tolerant rats. The stimulation of alpha 2-adrenoceptor enhances secretion of both growth hormone and prolactin. The adrenergic regulation of thyrotropin secretion still remains unclear. Evidently, adrenergic mechanisms are involved in the morphine-induced stimulation of growth hormone secretion, but not in the morphine-induced stimulation of prolactin secretion. In addition, there is a clear cross-tolerance between dexmedetomidine and morphine in growth hormone secretion of the rat.

The stimulatory effect of chronic lithium treatment on basal thyrotropin secretion in rats: in vivo antagonism by methylparaben

Chronic treatment of rats with lithium chloride was examined in order to determine its effects on hypothalamic monoamine and metabolite content, basal thyrotropin (TSH) secretion and thyroid function. The hypothalamic concentrations of noradrenaline (NA), dopamine (DA) and its metabolites, dihydroxyphenylacetic acid. (DOPAC) and homovanillic acid (HVA) in the lithium treated rats remained unaltered when compared to control levels. NA turnover and the NA metabolite, 3-methoxy-4-hydroxyphenylglycol (total MHPG), were significantly lower (p < 0.01), whereas both serotonin (5-HT) and its metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), were significantly higher (p < 0.01 and p < 0.02, respectively) in the lithium treated rat hypothalami than in controls. Chronic lithium treatment significantly elevated basal TSH levels (p < 0.05). This effect was antagonized by methyl p-hydroxybenzoate (methylparaben, p < 0.01), which did not itself affect basal TSH levels. Free serum T3 and T4 levels were not significantly affected by chronic lithium treatment, although T4 tended to be slightly lower than control levels. The monoamine changes observed in the hypothalamus of lithium treated rats did not appear to account for the elevated TSH levels observed in these rats since NA activity which is generally regarded as stimulatory was decreased and 5-HT which has an inhibitory effect on TSH secretion, was increased. The elevated TSH levels may have been due to a reduced negative feedback inhibition of TSH release by the mildly reduced circulating T4 levels caused by chronic lithium treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic control of the secretion of anterior pituitary hormones

The hypothalamic hypophysiotrophic neurones are densely innervated by adrenergic and noradrenergic nerve terminals. Activation of alpha 1-adrenoceptors located in the brain stimulates the secretion of ACTH, prolactin and TSH. The effects of the alpha 1-adrenoceptors seem to be exerted on hypothalamic neurones that secrete vasopressin, CRH-41 and TRH. These mechanisms are important in the physiological control of the secretion of ACTH and TSH in humans. alpha 2-Adrenoceptors are not involved in the control of secretion of these hormones under basal conditions in humans. However, alpha 2-adrenoceptors exert an inhibitory effect that acts as a negative feedback mechanism, limiting excessive secretion of these hormones. There is no convincing evidence for the involvement of beta-adrenoceptors in the control of the secretion of these three hormones in humans. Studies on cultured anterior pituitary cells suggested that adrenaline and noradrenaline may influence the secretion of ACTH, prolactin and TSH directly at the level of the pituitary. However, these effects are not demonstrable in humans, and are likely to be due to alterations in the pituitary adrenoceptors during culture. In the case of growth hormone, activation of alpha 2-adrenoceptors located in the brain stimulates secretion of this hormone both by increasing the secretion of GHRH and by inhibiting the secretion of somatostatin. Activation of beta-adrenoceptors inhibits the secretion of growth hormone via an increase in the secretion of somatostatin. The effects of the central alpha 2- and beta-adrenoceptors are important in the physiological control of growth hormone secretion in humans. A considerable amount of evidence implicates brain alpha 1-adrenoceptors in the control of secretion of the gonadotrophins in experimental animals, but, despite intensive study, no convincing evidence has been found in humans of reproductive age.

Alpha 2- and beta-adrenergic stimulation of corticosterone secretion in rats

Bilateral injection of 6-hydroxydopamine into the medial forebrain bundle (MFB) significantly decreased monoamine concentrations in the hypothalamus. The noradrenaline and serotonin content of the paraventricular nucleus (PVN) was also significantly reduced. These drastic decreases in neurotransmitter concentration did not alter basal secretion of corticosterone. Isoproterenol, a beta-adrenoceptor agonist (1 mg/kg, i.p.), significantly stimulated corticosterone release in saline and MFB lesioned rats. This stimulation did not differ significantly between the two groups. Clonidine, an alpha 2-adrenoceptor agonist, injected either intraperitoneally or intracerebrally just dorsal to the PVN, caused a dose-dependent increase in corticosterone secretion. The stimulation of corticosterone release by clonidine (250 micrograms/kg, i.p.) was antagonised by the selective alpha 2-adrenoceptor antagonist, yohimbine (1 mg/kg, i.p.) and significantly reduced by the MFB lesion. These results suggest that corticosterone secretion is stimulated by activation of alpha 2-adrenoceptors which occur on noradrenergic nerve terminals in the PVN.

Effects of ?2- and ?-adrenoceptor agonists on growth hormone secretion following lesion of the noradrenergic system of the rat

The aim of the present investigation was to lesion the noradrenergic system and to measure the effect on growth hormone (GH) secretion following peripheral administration of alpha 2- and beta-adrenoceptor agonists. Direct injection of these agonists into the paraventricular nucleus of the hypothalamus (PVN) and its effect on GH secretion were also investigated. Systemic administration of N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4, 60 mg/kg, injected i.p. 10 days prior to experimentation) significantly decreased the noradrenaline (NA) content of the hippocampus, frontal cortex and hypothalamus but had no effect on the dopamine (DA) or serotonin (5-HT) content of these areas. Bilateral injection of 6-hydroxydopamine (6-OHDA, 10 micrograms/microliters, 14 days prior to experimentation) into the medial forebrain bundle (MFB) caused a greater reduction of NA and also decreased the DA and 5-HT content of the hypothalamus. Analysis of the PVN of the hypothalami of rats following 6-OHDA lesion of the MFB showed significantly decreased NA and 5-HT content. Neither DSP4 treatment nor 6-OHDA lesion of the MFB affected the clonidine (250 micrograms/kg, i.p.) induced stimulation of GH secretion. Injection of isoproterenol (1 mg/kg, i.p.) had varying effects on GH secretion. It stimulated GH release in control rats but not in DSP4 or MFB lesioned rats. Direct injection of clonidine (0.1 microgram/microliters) into the PVN significantly stimulated GH secretion, whereas injection of isoproterenol (2.5 micrograms/microliters) into the PVN did not affect GH levels when compared to controls.(ABSTRACT TRUNCATED AT 250 WORDS)