Influence of melatonin and serotonin on some parameters of respiratory activity in rabbits

This study showed the effect of intravenous [iv] injection of melatonin in doses of 0.1, 0.5 and 1.0 mg/kg and serotonin in doses of 0.1, 0.3 and 0.5 mg/kg on the rate and depth of respiration in adult male rabbits. Intravenous injection of melatonin [1 mg/kg] caused a decrease in the frequency and an increase in the depth of respiration which resulted in a fall in PaCO2 and SaO2. Hypoventilation led also to an increase in PaCO2. The effect of melatonin on the rate and depth of respiration started immediately post-injection of melatonin and the maximum effect was recorded after 5 minutes and returned to the normal levels within 10 minutes of injection. Based on the results it can be concluded that melatonin as well as serotonin may have an important effect on the regulation of ventilation. Melatonin may affect respiration by acting directly on the muscle of respiration, while serotonin effect may be due to its direct action on chemoreceptors

Effect of melatonin on baroreflex control of heart rate in rabbits

The effect of intravenous injection of melatonin [0.1 mg/kg, 0.5 mg/Kg and 1 mg/Kg] on arterial blood pressure and heart rate in adult male rabbits was investigated in this study. Baroreflex function was assessed using phenylephrine [the pressor test] to induce moderate changes in arterial blood pressure and to alter the stimulation of baroreceptor sites. In addition, the effect of melatonin on sympathetic and parasympathetic efferent nerve activity was also investigated. Melatonin resulted in a lowering effect on arterial blood -pressure with a decrease in heart rate. The significant decrease of arterial blood pressure was observed at a dose as low as 0.5 mg/Kg. The lowering effect of melatonin on heart rate was detected at 1 mg/Kg. Baroreflex slope was not depressed significantly with a significant increase in pulse interval, until I mg/Kg melatonin. Sympathetic and vagal nerve activities were attenuated significantly by melatonin. This study indicated that baroreflex control of heart rate was depressed by intravenous administration of melatonin. There was a dissociation between the effect of melatonin on baroreflex sensitivity versus the effect of low blood pressure. Melatonin was found to alter the baroreflex heart rate response through its direct depressant effect on baroreceptors and on both sympathetic and parasympathetic efferent nerve pathways

Effect of caffeine on movements of the small intestine isolated from male rabbits

Caffeine [Caffeine-Sodium benzoate in doses of 0.6, 0.9 and 1.2 mg in 0.1 ml distilled water stimulated the amplitude of intestinal movements isolated from mature male rabbits. His stimulation increased with gradual increase in caffeine dosage. On the other hand, a small dose of caffeine [0.3 mg in 0.1 ml distilled water] was without effect on intestinal - movements while larger doses of caffeine [1.8 and 2.4 mg in 0.1 ml distilled water] decreased these movements. Avil [Pheniramine hydrogen maleate] which is a histaminic antagonist in a dose of 4.55 mg in.1 ml distilled water antagonized the effect of caffeine on the amplitude of intestinal movements, while other antagonists [atropine as anticholinergic drug and indomethacin as a prostaglandin inhibitor] failed to antagonize the effect of caffeine on isolated intestinal movements. These results led to the conclusion that caffeine acts through the release and potentiation of action of histamine on intestinal movements

Mechanism of decreased pressor responsiveness of estradiol in pregnant rabbits

The mechanism of decreased pressor responsiveness to oestradiol during pregnancy was examined in rabbits. Oesdtradiol injection resulted in a rise in mean arterial pressure [MAP] greater in nonpregnant than pregnant animals. Rabbits 25 days pregnant, showed marked blunting of the pressor response to oestradiol, whereas after 5 days of pregnancy there was a normal response and at 15 days an intermediate pressor response. Following treatment with indomethacin, sensitivity to the hypertensive action of oestradiol was perceptibly increased only in pregnant rabbits. Prostaglandin F2alpha [PGF2alpha] injection casued a significant decrease in MAP and vascular sensitivity [mean diastolic pressure] greater in nonpregnant than in pregnant rabbits. Plasma level of sodium was significantly increased after oestradiol and showed a nonsignificant change after PGF2alpha. Plasma level of potassium showed a nonsignificant change after the injection of oestradiol and PGF2alpha. Histological examination of renal or adrenal tissues of rabbits injected with oestradiol showed a picture similar to those of pregnant animals. Juxtaglomerular apparatus and zona glomerulosa showed signs of increased activity. There were no significant histologidal changes in the renal or adrenal tissues after PGF2alpha injection in oestradiol-treated rabbits when compared with those treated with oestradiol only. The only detect able change was the greatest increase in vascular sensitivity. These observations showt hat the altered responsiveness to oestradiol in pregnant rabbits appears to be - related to hyporesponsiveness angiotensin II and to an increased synthesis of vasodilator prostaglandins. There may be two of the mechanisms by which the maternal circulation adapts to pregnancy

effect of melatonin on capillary permeability in the rat skin

The effect of different doses of melatonin [0.1, 1, 10 and 20 ug] on the capillary permeability in rat skin was examined in this study. Intraperitoneal injection of melatonin resulted in a dose-dependent decrease in vascular permeability. 20 ug melatonin caused 100% reduction in the mean diameter of the blue reaction sites caused by intradermal injection of prostaglandin E2 [PGE2] and a significant decrease in the mean number of blood capillaries and their transverse diameter. The blueing discoloration of the response site was pale. Histological examination of these skin revealed a thin capillary wall, no endothelial bud formation and little cellular infiltration [neutrophils, plasma cells and lymphocytes]. Injection of the other three low doses [0.1, 1 and 10 ug] led to a weaker effect than the higher dose. Also, melatonin [20 ug] caused insignificant change in the increased vascular permeability due to local injection of histamine and serotonin. In addition, melatonin [20 ug] caused insignificant change in the mean systolic blood pressure and a significant decrease in the threshold of pain provoked by sub-planter injection of PGE2 into the sole of the rat paw. It was concluded that melatonin suppressed the local vascular permeability in part skin. Melatonin may exert its effect through the inhibition of PG synthesis