Naoto Minamitani

Stimulatory effect of peptide histidine isoleucine amide 1–27 on proclactin release in the rat

Intravenous injection of pure peptide histidine isoleucine amide 1-27 (PHI) resulted in a prompt and significant increase of plasma prolactin (PRL) in conscious freely-moving male rats. Using a perifusion system of rat anterior pituitary tissues in vitro, effluent PRL levels were also increased by 10(-8)-10(-7) M PHI. A PRL releasing potency of PHI was almost similar with that of vasoactive intestinal polypeptide (VIP) or TRH both in vivo and in vitro. Coupled with the recent immunocytochemical studies showing the dense network of PHI immunoreactive fibers around the hypophysial portal vessels, PHI might be another candidate for PRL releasing factor.

Release of vasoactive intestinal polypeptide into the cerebrospinal fluid of the fourth ventricle of the rat: Involvement of cholinergic mechanism

Vasoactive intestinal polypeptide (VIP) is found abundantly in the cerebrospinal fluid (CSF). In order to clarify its source in the brain and the control mechanism of its release, cerebral ventricles of urethane-anesthetized male rats were locally perfused with the artificial CSF at a constant rate of 120 microliters/min by means of a push-pull cannula and immunoreactive VIP was continuously measured in the effluents obtained at 10 min intervals. The perfusion with veratridine, a depolarizing agent, at a rate of 5 x 10(-4) M/min, caused a significant (P less than 0.01) increase in the effluent VIP levels when the tip of a cannula was placed in the fourth ventricle but not when it was in the third or the lateral ventricle. The release of VIP into the fourth ventricle was also significantly (P less than 0.05) enhanced by the perfusion with acetylcholine (ACh) at a dose of 3.66 x 10(-5) to 1.83 x 10(-4) M/min. A simultaneous perfusion of hexamethonium (1.9 x 10(-4) M/min) blocked ACh-induced VIP release. These results indicate that VIP is released into the CSF from the wall of the fourth ventricle by a mechanism possibly involving nicotine-sensitive cholinergic pathways.

Central nervous system effect of calcitonin: Stimulation of prolactin release in rats

Effect of [Asu 1,7]eel calcitonin (CT) on prolactin (PRL) release was examined in male rats under urethane anesthesia. Intravenous injection of 4-20 micrograms [Asu1,7]eel CT did not modify plasma PRL levels. Injections of 0.5-2.5 micrograms [Asu1,7]eel CT into the lateral ventricle produce a significant and dose-related increase of plasma PRL within 10 min of injection. When intraventricularly injected in an equimolar dose (0.74 nmol/10 microliters), eel CT11-32, eel CT15-32, [Asu1,7]eel CT1-16 and [Asu1,7]eel CT1-9 showed 44.8, 25.7, 19.9 and 10.1% the potencies of [Asu1,7]eel CT, respectively, in stimulating activity of PRL release. The rise of plasma PRL after [Asu1,7]eel CT injection were significantly less or abolished not only in hypothalamic-lesioned rats but also in rats with complete deafferentation. Pretreatment with alpha-methyl-p-tyrosine (250 mg/kg, 12 h before) but not with p-chlorophenylalanine (300 mg/kg, 72 and 24 h before) resulted in a suppression of [Asu1,7]eel CT-induced PRL release. These results suggest the following: first, PRL release is stimulated by centrally injected [Asu1,7]eel CT, the action site of which may exist in the extrahypothalamic area; second, brain catecholamines may be involved in the mechanism of [Asu1,7]eel CT-evoked PRL release; third, the C-terminal portion of the peptide may play an important role in stimulating PRL release.

Attenuation by Hypocalcemia of Pulsatile Growth Hormone Secretion in Conscious Male Rats

The effect of hypocalcemia following parathyroidectomy (PTX) on growth hormone (GH) secretion was investigated in unrestrained, unanesthetized male rats bearing chronically implanted indwelling cannulae. During a 6-hour period, starting at about 10 a.m., control rats with a serum calcium (Ca) value of 8.11 +/- 0.38 mg/dl (mean +/- SEM) 2 weeks after sham-operation showed secretory bursts of GH similar to those observed in conscious intact rats. Under hypocalcemia of 4.88 +/- 0.32 mg/dl 2 weeks after PTX, GH secretory episodes were completely suppressed throughout the study. Plasma prolactin (PRL) levels were also decreased in PTX rats as compared with those of sham-operated rats. Daily food intake and body weight gain as well as serum T4 levels in PTX rats were not different from those of sham-operated and intact rats. Pituitary GH content of PTX rats was significantly lower than that of sham-operated and control rats. Pulsatile GH secretion was partially restored in PTX rats by raising serum Ca to 8.43 +/- 0.27 mg/dl through feeding with high Ca diet containing 7% Ca. Immediately after intravenous injection of antisomatostatin sheep serum, pulsatile GH surges recovered in PTX rats despite hypocalcemia of 4.48 +/- 0.74 mg/dl. The mean plasma 6-hour GH levels were significantly higher than those of normal sheep-serum-treated PTX rats (p less than 0.001). These findings suggest that the episodic release of GH is suppressed in hypocalcemic rats after PTX, at least partially via circulating endogenous somatostatin.

Effect of Various Catecholamine Antagonists on Prolactin Secretion in Conscious Male Rabbits

Abstract To clarify physiological roles of catecholaminergic systems in the control of rabbit prolactin (PRL) release, the effect of various catecholamine receptor antagonists on plasma PRL levels was examined in conscious, freely moving male rabbits. An intravenous (iv) injection of yohimbin (2.5 mg/kg body wt), an α2-adrenoreceptor antagonist, but not prazosin (2 mg/kg body wt), an α-adrenergic receptor antagonist, resulted in a significant elevation of plasma PRL. Conversely, propranolol (2.5 mg/kg body wt, iv), a nonselective β-adrenoreceptor antagonist, and metoprolol (2.6 mg/kg body wt, iv), a β1-adrenergic antagonist, slightly but significantly suppressed basal levels of plasma PRL. On the other hand, haloperidol (0.5 mg/ kg body wt, iv), pimozide (0.3 mg/kg body wt, iv), sulpiride (5 mg/kg body wt, iv), chlorpromazine (3 mg/kg body wt, iv), and YM-09151-2 (0.2 mg/kg body wt, iv), all dopamine receptor antagonists caused a significant increase in plasma PRL. These results suggest that dopaminergic and α2-adrenergic mechanisms exert a tonic inhibitory role and β-adrenergic mechanisms, probably β1 a tonic stimulatory role in the regulation of PRL release in the rabbit.

Ultradian Rhythm of Growth Hormone Secretion in Unrestrained, Conscious Male Rabbits: Role of Endogenous Somatostatint†

The profiles of growth hormone (GH) secretion were examined by obtaining serial blood samples every 15 min for a 5 to 24 h observation period from freely‐moving, conscious male rabbits chronically implanted with a right atrial cannula. The effects of restraint or surgical stress on GH secretion were also investigated in these animals. Four days after cannulation of the right atrium, plasma GH levels remained low without oscillation, during a 5 h observation period (1100 to 1600 h) with the mean (± SEM) value of 1.6±0.2 ng/ml. Individual rabbits exhibited a spontaneous, pulsatile GH secretion 7 days after the surgery. Mean 6 h GH levels were 5.6 ± 0.8 ng/ml at 7 days after the surgery, 6.3 ± 0.6 ng/ml at 14 days and 7.0 ± 1.2 ng/ml at 28 days. Therefore, the animals, 7 to 14 days after cannulation, were used to analyse the pulsatile pattern of GH secretion throughout 6 to 24 h. Two episodes of 45 min immobilization stress, separated by 75 min, caused a complete suppression of the spontaneous GH secretion (mean 6 h GH levels, 2.2 ± 0.1 ng/ml vs control, 5.0 ± 0.5 ng/ml, P<0.01). No surges appeared after the first restraint stress. In 14 non‐treated rabbits, plasma GH levels fluctuated in an episodic manner throughout the study with the peaks of 14.2 + 0.7 ng/ml, the nadirs of 2.6 ± 0.2 ng/ml and the peak to peak intervals of 2.20 ± 0.17 h. The iv administration of normal goat λ‐globulin (NGG) affected neither GH secretory patterns nor baseline levels of plasma GH. In contrast, the iv administration of anti‐sornatostatin goat λ‐globulin (ASG) caused a significant increase in the amplitude of plasma GH peaks (38.8±1.9 vs NGG‐treated, 13.7 ± 0.8 ng/ml, P<0.001) as well as the trough level (13.5 ± 0.6 vs NGG, 2.9 ± 0.1 ng/ml, P<0.001) during a 24 h observation period. Also, ASG treatment increased numbers of plasma GH peaks per day (18.8±2.7 vs NGG, 12.2 ± 0.8, P < 0.05) with concomitant shortening of the peak to peak interval (1.25 ± 0.10 vs NGG, 2.03±0.12h, P<0.01).

Peptide histidine methionine and vasoactive intestinal peptide levels in human cerebrospinal fluid : age-related changes and absence of a correlation with serum prolactin

Serum levels of prolactin (PRL), peptide histidine methionine (PHM) and vasoactive intestinal peptide (VIP) were measured in 97 subjects and cerebrospinal fluid (CSF) levels of PHM and VIP were measured in 50 subjects by specific radioimmunoassays to investigate correlations between them. The chromatographic studies revealed that PHM and C-terminal extended form of PHM, peptide histidine valine occurred in human serum and CSF. Significant age-related increases of CSF PHM (P < 0.02) were observed in both males and females, whereas an age-related decrease of serum PRL level was found in females (P < 0.01). In contrast, neither serum VIP, serum PHM nor CSF VIP changed significantly with age. There was a significant positive correlation (P < 0.002) between VIP and PHM in serum, but not in CSF. The close relation between VIP and PHM in serum meets ones expectation because of their derivation from a common precursor. In CSF, these two peptides did not change in parallel with each other. These results may reflect the alteration in metabolism of PHM in CSF. Finally, there was no correlation between serum PRL concentrations and either serum or CSF levels of the peptides, suggesting that neither VIP nor PHM levels in CSF as well as in serum can influence the basal PRL secretion in subjects without endocrine disorders.