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Three different calcitonins: salmon calcitonin, eel calcitonin and the semi-synthetic analog [Asu1,7]eel calcitonin have been evaluated for their ability to affect phosphoinositide hydrolysis in primary cultures of anterior pituitary cells and in the osteoblast-like UMR-106 cells. In both cellular systems a repeated treatment with any form of calcitonin induced an inhibition of inositol phospholipid turnover. Eel calcitonin and its analog were always more potent than salmon calcitonin, but the efficacy of the three polypeptides was comparable. In cultured anterior pituitary cells, the inhibitory effect on phosphoinositide hydrolysis observed after chronic treatment with calcitonin was accompanied by a reduction of prolactin release. In contrast, a single treatment of cultured anterior pituitary cells with eel calcitonin or its analog [Asu1,7]eel calcitonin induced an increase of inositol phosphate accumulation, while salmon calcitonin was inactive. Accordingly, eel and [Asu1,7]eel calcitonin, but not salmon calcitonin, induced a slight but significant stimulation of prolactin secretion. In UMR-106 cells, the three calcitonins exhibited similar potency and efficacy in reducing parathyroid hormone-stimulated 4 beta[3H]-phorbol-12,13-dibutyrate ([3H]PdBu) binding, an indirect index of protein kinase C activation. Taken together, these results suggest that, either at the pituitary or in osteoblast-like cells, some of the effects exerted by calcitonin may be ascribed to an interference with the intracellular events initiated by modulation of phosphoinositide turnover.

We have recently demonstrated that calcitonin, a putative neuromodulator, may influence extrapyramidal motor system by decreasing nigro-striatal dopaminergic function. Since calcitonin is extensively used in aged patients, we have investigated whether calcitonin might influence extrapyramidal motor behavior (haloperidol-induced catalepsy and apomorphine-induced hyperactivity) in rats of different ages. Intracerebroventricular injection of salmon calcitonin (1 micrograms/kg) prevented apomorphine-induced hyperactivity in 2, 7, 18 or 21 month old rats, but potentiated haloperidol-induced catalepsy only in 2 or 7 month old rats. In addition, in all the animals salmon calcitonin significantly decreased the secretion of prolactin, an anterior pituitary hormone that may act at central level enhancing nigro-striatal dopaminergic activity.

The effects of different doses of (Asu1,7) eel-calcitonin, iv injected, on gastric secretion were studied in conscious rats with pyloric occlusion. Moreover, we evaluated the activity of this analogue on gastric ulcer formation by restraint stress. It was found that 2.5 or 5 Ul/kg (Asu1,7) eel-calcitonin decreased gastric acid secretion and inhibited the development of stress-induced ulcers in rats. In the isolated rat stomach the peptide at the concentrations of 1 nM to 1 microM did not modify acetylcholine, histamine or 5-hydroxytriptamine-induced contractions. These results suggest that this peripheral activity of (Asu1,7) eel-calcitonin does not involve a direct interference with cholinergic, histaminergic or serotonergic pathways at gastric level.

Salmon calcitonin, centrally injected, increased plasma renin activity and blood pressure in rats. It is possible that the peptide through an action on the central serotonergic tone stimulated plasma renin activity with subsequent enhancement of blood pressure. Conversely, peripheral administration of the peptide increased plasma renin activity but did not cause changes in blood pressure. Probably, the enhancement of renin may be a physiological response to the renal activity of calcitonin; however, this activity does not seem to involve urinary prostaglandins.

The effects of different doses of Asu(1,7) eel-calcitonin, peripherally injected, on gastric secretion were studied in conscious Brattleboro rats, which are genetically deficient in vasopressin. Moreover, we evaluated the activity of this analogue on gastric ulcer formation by restraint stress. We found that 5 IU/kg Asu(1,7) eel-calcitonin decreased gastric secretion and inhibited the development of stress-induced ulcers in Brattleboro rats. These data suggest that vasopressin does not play a role in the gastrointestinal activity of Asu(1,7) eel-calcitonin.

The effects of calcitonin on neurochemical parameters related to the tuberoinfundibular dopaminergic system have been investigated in an attempt to elucidate how calcitonin decreases serum prolactin levels. Intracerebroventricular human or salmon calcitonin injection decreases serum prolactin, medial basal hypothalamic dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) and hypophysial DA and increases hypophysial DOPAC. Results suggest that calcitonin may decrease prolactin secretion via the tuberoinfundibular dopaminergic system.

The effects of i.v.c. injection of human and salmon calcitonin on biochemical and behavioral parameters related to the extrapyramidal motor system, were investigated in male rats. Calcitonin injection resulted in a potentiation of haloperidol-induced catalepsy and a partial prevention of apomorphine-induced hyperactivity. Moreover calcitonin induced a significant decrease in nigral GAD activity but no change in striatal DA and DOPAC concentration or GAD activity. The results are discussed in view of a primary action of calcitonin on the striatonigral GABAergic pathway mediating the DA-related behavioral messages of striatal origin.

The acute effects of peripherally administered methysergide or phentolamine on the analgesic activity of salmon calcitonin (sCT) injected into the lateral ventricle were investigated in male rats. Methysergide, but not phentolamine, significantly (P less than 0.05) antagonized the analgesic activity of sCT at 60 and 120 min after the administration of the peptide. The results obtained suggest that the analgesic activity of sCT may involve central serotonergic system(s), while the central noradrenergic system does not seem to be needed for this activity.

Stimulation of inositol phospholipid hydrolysis by norepinephrine or 5-hydroxytryptamine was reduced in hippocampal or cortical slices from rats repeatedly injected with (Asu1.7)eel-calcitonin (2.5 IU/kg i.p.). This effect was specific, as the basal or carbamylcholine-stimulated inositol phospholipid hydrolysis was unchanged in slices from calcitonin-injected animals. The reduced responsiveness to norepinephrine did not reflect a decreased number or affinity of alpha 1-adrenergic recognition sites, suggesting that calcitonin treatment leads to a reduced coupling between alpha 1-adrenoceptors and phospholipase C.

The effect of peripherally administered cyproheptadine or reserpine and the administration of 5,7-dihydroxytryptamine (5,7-DHT) in the nucleus raphe dorsalis on the analgesic activity of salmon calcitonin (sCT) injected into the lateral ventricle were investigated in male rats. Cyproheptadine or reserpine, given respectively 30 min or 24 h before the peptide, completely abolished the analgesic activity at all the times studied. However, when reserpine was given before the peptide it increased the effect of sCT at 30 (P less than 0.01), 60 (P less than 0.001), 120 (P less than 0.01) and 180 (P less than 0.01) min. 5,7-DHT injected in the nucleus raphe dorsalis 15 days before the peptide led to complete abolition of the analgesic activity. If neurotoxin was injected 4 days before sCT, the effect of the peptide was significant (P less than 0.05) only at 60 min. The results obtained confirm that the analgesic activity of sCT may involve central serotonergic pathway(s), and that the midbrain raphe nuclei 5-HT content is an important focus for this activity.

We have investigated the effects of salmon calcitonin injected intracerebroventricularly on adrenocorticotropic (ACTH) secretion in rats. Salmon calcitonin was able to increase significantly (p less than 0.05) ACTH secretion at the dose of 5 ng/rat and (p less than 0.01) at the doses of 12.5, 25 and 50 ng/rat; the effect (50 ng/rat) was maximal at 30 min and still present after 60 and 120 min. The maximal dose of calcitonin produced a significant (p less than 0.05) increase in serum ACTH concentration also in stressed animals. It is suggested that calcitonin may increase serum ACTH secretion by central norepinephrine and/or 5-hydroxytryptamine pathways that regulate corticotropic releasing factor activity.

Intracerebroventricular (i.c.v.) injection of (Asul,7)eel-calcitonin, a synthetic analogue of eel-calcitonin, at doses of 3 or 6 IU/rat, (but not at 1.5 IU/rat) determined analgesic activity, in male rats, as evaluated by the 'hot-plate' test. Also electrophysiological studies carried out on 13 neurons belonging to the nucleus raphe magnus (NRM) showed that i.c.v. administration of the same doses of (Asul,7)eel-calcitonin increased the spontaneous cellular firing rate. No modification of the discharge frequency was observed when 1.5 IU/rat (Asul,7)eel-calcitonin was used. Authors suggest that the analgesic activity induced by i.c.v. injection of (Asul,7)eel-calcitonin is due, at least in part, to mechanisms affecting NRM neurons.

The acquisition of active avoidance response was studied in rats injected intracerebroventricularly with calcitonin (CT), parathyroid hormone (PTH) or PTH-related fragments. PTH and PTH65-84 facilitated the acquisition of shuttle-box active avoidance behavior as indicated by the number of conditioned avoidance responses and the percentage of animals reaching the learning criterion. PTH1-34 and PTH44-68, on the contrary, inhibited the acquisition of active avoidance behavior, while CT totally suppressed this behavior in the rat. It is possible that the opposite effects of PTH and CT on learning ability of the rat depend on a different action on calcium metabolism.

The effect of (Asu1,7)E-CT a deaminodicarba-analogue of the synthetic eel-calcitonin on the nociceptive transmission has been studied in mice. The analogue was intracerebroventricularly (0.0.5-0.01-0.02 U.I./kg) or intravenously (0.02-0.04-0.05 -0.1 U.I./kg) injected. This synthetic derivative of eel-calcitonin increased the antinociceptive effect also after peripheral administration. Moreover, preliminary studies on the time-course of this analogue showed that the dose of 0.1 U.I./kg i.v. injected, was able to elicit antinociceptive effect, already 5 min after the administration.