Variation in tolerance to the antinociceptive, hormonal and thermal effects of morphine after a 5-day pre-treatment of male rats with increasing doses of morphine

Intimate associations between the endogenous opiate systems and the growth hormone-releasing hormone system in the human hypothalamus

Although it is a general consensus that opioids modulate growth, the mechanism of this phenomenon is largely unknown. Since endogenous opiates use the same receptor family as morphine, these peptides may be one of the key regulators of growth in humans by impacting growth hormone (GH) secretion, either directly, or indirectly, via growth hormone-releasing hormone (GHRH) release. However, the exact mechanism of this regulation has not been elucidated yet. In the present study we identified close juxtapositions between the enkephalinergic/endorphinergic/dynorphinergic axonal varicosities and GHRH-immunoreactive (IR) perikarya in the human hypothalamus. Due to the long post mortem period electron microscopy could not be utilized to detect the presence of synapses between the enkephalinergic/endorphinergic/dynorphinergic and GHRH neurons. Therefore, we used light microscopic double-label immunocytochemistry to identify putative juxtapositions between these systems. Our findings revealed that the majority of the GHRH-IR perikarya formed intimate associations with enkephalinergic axonal varicosities in the infundibular nucleus/median eminence, while endorphinergic-GHRH juxtapositions were much less frequent. In contrast, no significant dynorphinergic-GHRH associations were detected. The density of the abutting enkephalinergic fibers on the surface of the GHRH perikarya suggests that these juxtapositions may be functional synapses and may represent the morphological substrate of the impact of enkephalin on growth. The small number of GHRH neurons innervated by the endorphin and dynorphin systems indicates significant differences between the regulatory roles of endogenous opiates on growth in humans.

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.

Morphine withdrawal alters anterior pituitary hormone secretion, brain endopeptidase activity and brain monoamine metabolism in the rat

Rats were made tolerant to morphine by a 5-day regimen with increasing doses. The time course of changes in serum anterior pituitary hormone levels, brain endo- and exopeptidase activity, levels of brain biogenic amines and body weight were studied during abrupt morphine withdrawal. Cold stimulated secretion of thyrotropin and the secretion of growth hormone were both decreased whereas that of prolactin was increased. In the hypothalamus both prolyl endopeptidase and dipeptidyl peptidase IV activities were concomitantly increased. The hypothalamic 5 hydroxyindole acetic acid levels were also increased. Changes in hormone secretion, peptidase activity and monoamine turnover had returned to baseline levels by 92 hr. Our results indicate that morphine withdrawal and the associated stress produce alterations in anterior pituitary thyrotropin and growth hormone secretion. Concomitant increases in hypothalamic prolyl endopeptidase and dipeptidyl peptidase activities may contribute to these changes.

Differential disappearance of tolerance to thermal, hormonal and locomotor effects of morphine in the male rat

Development and disappearance of tolerance to various effects of morphine was studied by comparing the effect of acute morphine at 6 h and at 92 h after cessation of a 5-day regimen with increasing doses of morphine. After the 6-h lag time, tolerance manifested to the thermal, locomotor depressant and hormonal (stimulation of growth hormone and prolactin secretion) effects of morphine. The hypokinetic effect of morphine was replaced by a hyperkinetic effect and increased locomotor activity was evident following the challenge dose of morphine. Tolerance disappeared in different ways during the 92-h lag time. Tolerance persisted (hypothermic and hypokinetic effect) or disappeared considerably (prolactin secretion) during the 92-h withdrawal period. Tolerance to some effects also faded completely, and in contrast, even sensitization to various effects of morphine (growth hormone secretion, hyperthermic effect) could be seen after the 92-h withdrawal period. In addition, the original hypokinetic effect of morphine was replaced by a hyperkinetic effect (i.e., enhanced locomotor activity), which was even stronger after the 92-h lag time. The observed dissociation, which has not been seen to such an extent before, may be due to the differential modulation of the subtypes of mu-opioid receptors or differences in the adaptive mechanisms, e.g. conditioning, in various brain areas. Faster recovery of tolerance to an inhibitory than to a stimulatory effect of morphine during the withdrawal period may partially explain the sensitization to some effects of morphine.

N-nitro-L-arginine attenuates development of tolerance to antinociceptive but not to hormonal effects of morphine

The effect of N-nitro-L-arginine methyl ester on thyrotropin (TSH), growth hormone (GH) and prolactin levels in serum was studied after a single dose in male rats. The effect of repeated N-nitro-L-arginine methyl ester on the development of tolerance to the multiple effects of morphine was also examined. N-Nitro-L-arginine methyl ester (25 mg/kg i.p.) decreased cold-stimulated (rats were kept 30 min at +4 degrees C) TSH levels but did not alter basal TSH, GH or prolactin levels. Acute or repeated N-nitro-L-arginine methyl ester treatments were not antinociceptive alone, neither did they modify the effect of acute morphine. A 5-day treatment with increasing doses of morphine induced antinociceptive tolerance. This was attenuated by N-nitro-L-arginine methyl ester co-administration on the 3rd day, less so on the 5th day. Repeated morphine produced tolerance to the effect on TSH (decrease) and prolactin (increase) levels, but sensitization to the GH-elevating effect, measured after a 4-day delay. A 5-day treatment with N-nitro-L-arginine methyl ester did not modify the development of tolerance to the hormonal effects of morphine. In the case of TSH, it is difficult to draw a conclusion because the 5-day N-nitro-L-arginine methyl ester and morphine treatment generally decreased basal TSH levels. Sensitization to the hyperthermic effect of morphine occurred after a 4-day delay and this was not altered by N-nitro-L-arginine methyl ester. These results suggest that both nitric oxide-dependent and independent mechanisms are involved in the development of tolerance to the various effects of morphine.