Methylatropine Pretreatment Research Articles

Stimulation of pulmonary vagal C-fibers by trans-4-methyl-β-nitrostyrene induces bradycardiac and depressor reflex in rats: Role of vanilloid TRPV1 receptors

Previously, we showed that the synthetic nitroderivative trans-4-methyl-β-nitrostyrene (T4MeN) induced vasorelaxant effects in rat isolated aortic rings. Here, we investigated the mechanisms underlying the cardiovascular effects of T4MeN in normotensive rats. In pentobarbital-anesthetized rats, intravenous (i.v.) injection of T4MeN (0.03–0.5 mg/kg) induced a rapid (onset time of 1–2 s) and dose-dependent bradycardia and hypotension. These cardiovascular responses to T4MeN were abolished by bilateral cervical vagotomy or selective blockade of neural conduction of vagal C-fiber afferents by perineural treatment of both cervical vagus nerves with capsaicin. Hypotension and bradycardia were also recorded when T4MeN was directly injected in the right, but not into the left ventricle. Furthermore, they were significantly reduced by i.v. pretreatment with capsazepine but remained unaltered by ondansetron or suramin. In conscious rats, the dose-dependent hypotension and bradycardia evoked by T4MeN were abolished by i.v. methylatropine pretreatment. In conclusion, bradycardiac and depressor responses induced by T4MeN has a vago-vagal reflex origin resulting from the vagal pulmonary afferents stimulation. The transduction mechanism seems to involve the activation of vanilloid TRPV1, but not purinergic (P2X) or 5-HT3 receptors located on vagal pulmonary sensory nerves.

Cardiovascular effects of the essential oil of Croton zehntneri leaves and its main constituents, anethole and estragole, in normotensive conscious rats

Cardiovascular effects of the essential oil of Croton zehntneri (EOCZ) were investigated in conscious rats. In these preparations, intravenous (i.v.) injections of EOCZ (1–20 mg kg − 1 ) and its main constituents anethole and estragole (both at 1–10 mg kg − 1 ) elicited brief and dose-dependent hypotension and bradycardia (phase I) that were followed by a significant pressor effect associated with a delayed bradycardia (phase II). The initial hypotension and bradycardia (phase I) of EOCZ were unchanged by atenolol (1.5 mg kg − 1 , i.v.) or l-NAME (20 mg kg − 1 , i.v.) pretreatment, but were respectively reversed into pressor and tachycardic effects by methylatropine (1 mg kg − 1 , i.v.) pretreatment. The subsequent pressor effect and the delayed bradycardia (phase II) remained unaffected by atenolol, but were abolished by l-NAME and methylatropine pretreatment, respectively. In rat endothelium-containing aorta preparations, the vasoconstrictor responses to phenylephrine were enhanced and reduced, respectively, by the lower (1–30 μg mL − 1 ) and higher (300–1000 μg mL − 1 ) concentrations of EOCZ. Only the enhancement of phenylephrine-induced contraction was abolished by either the incubation with l-NAME (50 μM) or in the absence of the endothelium. These data show, for the first time, that i.v. administration EOCZ induces an initial hypotension followed by a pressor response, two effects that appear mainly attributed to the actions of anethole and estragole. The EOCZ-induced hypotension (phase I) is mediated by a cholinergic mechanism and seems to result mainly from the concomitant bradycardia. The pressor response of EOCZ (phase II) seems to be caused by an indirect vasoconstrictive action of EOCZ most likely through inhibition of endothelial nitric oxide production.

Pancreatic islet blood flow in conscious rats during hyperglycemia and hypoglycemia

Anesthesia affects general hemodynamics and regulation of organ perfusion. We used colored microspheres to measure pancreatic islet blood flow in conscious rats at two time points, during either hyperglycemia or hypoglycemia. This method, using black and green microspheres, was validated by comparison with previous microsphere experiments and by lack of effect of a nonmetabolizable glucose analog, 3-O-methylglucose, on islet perfusion. Basal and glucose-stimulated islet blood flow levels were similar in pentobarbital sodium-anesthetized and conscious rats. However, the basal distribution of pancreatic blood flow was altered by anesthesia (fractional islet blood flow 5.8 +/- 0.4% in conscious rats, 7.9 +/- 0.8% in pentobarbital-anesthetized rats, P < 0.05). Insulin-induced hypoglycemia significantly increased whole pancreatic blood flow in conscious rats, whereas islet blood flow remained unchanged and fractional islet blood flow was decreased (5.8 +/- 0.5% in the basal state, 4.2 +/- 0.4% during hypoglycemia, P < 0.001). Methylatropine pretreatment significantly increased islet blood flow during hypoglycemia by 181%. This result suggests that prevention of hypoglycemia-induced increase in islet perfusion may be mediated, at least in part, by a cholinergic, vagal muscarinic mechanism.

Atropine sulfate increases pituitary responses to stress

The effects of atropine sulfate pretreatment on pituitary indices of stress response were examined. Pituitary cyclic AMP and plasma prolactin increases following 15 min of acute stress were used as measures of stress response. Over a range of doses (0, 5, 10, 30 and 60 mg/kg), pretreatment with atropine sulfate increased the measured stress responses to footshock but had little or no effect on resting or non-stressed levels of the substances measured. The effects of atropine on response to immobilization were tested only at 5 mg/kg. At this dose, atropine sulfate, but not methylatropine nitrate, increased pituitary cyclic AMP response to immobilization stress demonstrating that the potentiation of the pituitary cyclic AMP stress response was not limited to footshock stress and suggesting that this effect of atropine was central rather than peripheral. Neither atropine nor methylatropine pretreatment at this dose potentiated prolactic response to immobilization stress.

Microwave Facilitation of Methylatropine Antagonism of Central Cholinomimetic Drug Effects

Pilocarpine-induced hypothermia and oxotremorine-induced tremors in mice are central cholinomimetic drug effects that are readily blocked by the muscarinic antagonist atropine. However, the quaternary ammonium derivative of atropine, methylatropine, is unable to block these cholinomimetic drug effects by virture of its inability to penetrate the blood-brain barrier (BBB) and blood-cerebral spinal fluid barrier (B-CSFB). Dose-response curves for pilocarpine and oxotremorine effects are not appreciably affected either by pretreatment with methylatropine (1.0 mg/kg) or by exposure to moderate-level microwave irradiation (2.45 GHz, 23.7 W/kg, CW, 10-min exposure). However, in mice receiving both the methylatropine pretreatment and microwave irradiation, the dose-response curves for both pilocarpine and oxotremorine effects were significantly shifted to the right, signifying a central anticholinergic action by methylatropine. These data indicate that a single acute exposure to a thermogenic level of microwave irradiation facilitates methylatropine antagonism of centrally mediated cholinomimetic drug effects. One possible explanation for this observation is that microwave radiation may enhance passage of quaternary ammonium compounds like methylatropine across the BBB and B-CSFB.

Cardiovascular effects produced by injections of thyrotropin-releasing hormone in specific preoptic and hypothalamic nuclei in the rat

Microinjection of 1.4 pmol TRH (0.5 ng; 50–150 nl) into both the preoptic suprachiasmatic nucleus (pos) and the A6800–7000 region of the medial preoptic nucleus (pom) produced increases in blood pressure and heart rate of 7% and 19%, respectively; heart rate responses in these two areas were higher than those occurring in other areas tested. TRH induced a significant increase in blood pressure and heart rate in the posterior hypothalamic nucleus (nhp) and increased heart rate only in the anterior (nha) and dorsomedial (ndm) hypothalamic nuclei. A small decrease in both blood pressure and heart rate resulted with TRH injections in the A7050–7400 region of the pom. No changes in respiratory rate or rectal temperature were observed at any site with this dose of TRH. Preliminary studies into the mechanism of the cardiovascular actions of TRH suggested that inhibition of the parasympathetic nerves to the heart make a partial contribution to the TRH-induced heart rate increase in the pos and that adrenal catecholamine release mediates the TRH response in the nhp. Neither methylatropine pretreatment nor adrenalectomy prevented the response to TRH injected into the nha, suggesting that activation of the cardiac sympathetic nerves may mediate TRH actions in this region. In the ndm, neither methylatropine nor adrenalectomy prevented the response to TRH; however, there was a tendency for the response to be less after methylatropine. Therefore, both inhibition of the parasympathetic and activation of the sympathetic nervous systems may contribute to the response observed, but no adrenal involvement could be demonstrated. Discrete injections of 0.8 nmol TRH produced increases in heart rate and blood pressure in all preoptic and hypothalamic nuclei tested with accompanying changes in respiratory rate and rectal temperature in some areas. Lateral cerebral ventricle injections of as little as 2.8 pmol TRH produced increases in blood pressure and heart rate; cardiovascular responses to higher doses (0.8–22 nmol) in the ventricle were often accompanied by arousal, piloerection, “wet dog” shakes and changes in respiratory rate and rectal temperature. Previous immunohistochemical demonstration of nerve cells and fibers in the preoptic-hypothalamic area and the present finding of specific sites responsive to low dose TRH injections (1.4 pmol) both support a physiological role for this peptide in central control of the cardiovascular system.

Cardiovascular effects of discrete intrahypothalamic and preoptic injections of Bradykinin

Blood pressure and heart rate were monitored during discrete injections of bradykinin (5 nmol; 100–300 nl) in the hypothalamus and preoptic area of halothane anesthetized rats. In the paraventricular nucleus, bradykinin produced bradycardia without effecting blood pressure. The decrease in heart rate was abolished by pretreatment with methylatropine (IP), suggesting that the parasympathetic nervous system mediates this response. In contrast, in the dorsomedial and posterior hypothalamic nuclei, bradykinin increased both heart rate and blood pressure; methylatropine pretreatment (but not adrenalectomy) blocked these responses, suggesting that inhibition of the parasympathetic nervous system is responsible for the actions of bradykinin in these nuclei. In the preoptic suprachiasmatic nucleus, bradykinin produced an increase in heart rate only, which was attenuated by either methylatropine or adrenalectomy, indicating that both inhibition of the parasympathetic nervous system and adrenal catecholamine release contribute to the actions of bradykinin at this site. The increase in heart rate observed with bradykinin in the medial preoptic and anterior hypothalamic (A6400-6001 region) nuclei was not effected by either methylatropine or adrenalectomy, therefore activation of the sympathetic nervous system may be involved in responses in these regions. Finally, a 5 nmol dose of bradykinin potentiating factor (converting enzyme inhibititor; CEI) had effects similar to bradykinin when injected into the posterior hypothalamus, but no effect at any other brain site. CEI administration into brain sites 15 min prior to bradykinin injections failed to alter the bradykinin response. In summary, the central cardiovascular responses to bradykinin depend upon the specific site of injection and these sites correspond with the localization of bradykinin-like immunoreactivity previously reported by others.

In vivo characterization of the mechanisms that secrete enkephalin-like peptides stored in dog adrenal medulla

The stimulation of splanchnic nerve (SNS) induced a voltage-dependent increase of met 5-enkephalin-like peptides (met 5-enkephalin-IR) in the plasma from the blood withdrawn through an indwelling catheter in the lumbar adrenal vein. The characterization of this material by gel filtration followed by high pressure liquid chromatography indicated the presence of authentic met 5-enkephalin in the blood plasma extract. The content of this peptide was increased after SNS. The effect of SNS on plasma met 5-enkephalin-IR was antagonized by hexamethonium suggesting that the in vivo release of these peptides is mediated through a cholinergic mechanism impinging upon a nicotinic receptor. To investigate further the mechanism(s) regulating the in vivo release of met'-enkephalin-IR from the adrenal glands, treatment with acetylcholine receptor agonists and antagonists were made in intact or splanchnic nerve-transected dogs. Among the drugs studied, only the specific nicotinic agonists dymethylphenylpiperazinium (DMPP) induced, both in the intact and in the splanchnicectomized dogs, an increase of circulating met 5-enkephalin-IR. The involvement of opiate receptors in the release of met 5-enkephalin-IR from the adrenal gland was also studied. Morphine injection induced a very pronounced increase o circulating met 5-enkephalin-IR. This effect was blocked by naloxone or hexamethonium but not by methylatropine pretreatment and required an intact splanchnic nerve. These data suggest that the action of morphine might involve a centrally mediated reflex eliciting an increase of splanchnic nerve activity. This study shows that peptides having met 5-enkephalin-IR can be released into the circulation from the adrenal gland following splanchnic nerve stimulation probably through the activation of a nicotinic receptor located in the membrane of chromafin cells. The physiological role of these peptides presumably released from the adrenal medulla, together with catecholamines, remains to be investigated.

Pilocarpine, food deprivation, and induction of mouse killing by cats

Effects of three treatments that induce mouse killing by rats were examined with cats. Food deprivation induced about 50% killing after 24 hr and almost 100% killing by 72 hr. Pilocarpine (at doses of 1.0 mg/kg that produced marked side-effects, and whether or not methyl atropine pretreatment blocked those side-effects) and chlordiazepoxide (at doses of 1.0–2.0 mg/kg) did not induce any killing. Pilocarpine produced a dose-related inhibition of spontaneous mouse killing (as it does in rats), but this was antagonised by methyl atropine. The side-effects of pilocarpine and chlordiazepoxide did not seem to account for their failure to induce killing. In contrast with food deprivation, the mechanisms by which pilocarpine and chlordiazepoxide induce killing in rats may not have homologs in cats.

FURTHER STUDIES ON CHOLINERGIC MECHANISMS AND HORMONE-ACTIVATED COPULATORY BEHAVIOUR IN THE FEMALE RAT

SUMMARY Previous experiments have demonstrated that the cholinergic compounds pilocarpine and oxotremorine decrease the oestrogen plus progesterone-activated lordosis response in the female rat. The present study showed that pilocarpine and oxotremorine, in doses which did not interfere with overt behaviour, also facilitated the lordosis response when given 48 h after oestrogen and thus substituted for the progesterone treatment. This effect could be counteracted by atropine but not by methylatropine pretreatment. The facilitating effect was not obtained after adrenalectomy or hypophysectomy. It is therefore suggested that the facilitating effects of the cholinergic compounds on copulatory behaviour in the oestrogen-treated female rat is mediated by pituitary—adrenal mechanisms.

The effect of pilocarpine and oxotremorine on hormone-activated copulatory behavior in the ovariectomized hamster.

The effect of cholinergic compounds on hormone activated copulatory behavior in the ovariectomized hamster was studied. Pilocarpine and oxotremorine decreased the lordosis response elicited by estrogen plus progesterone at a time which coincided with the time for maximal muscarinic symptoms. This effect was prevented by atropine but not by methyl-atropine pretreatment. Pilocarpine could also activate the copulatory behavior after estrogen treatment only. This effect was obtained after the muscarinic symptoms had disappeared. These data are in general agreement with those previously found in the female rat. However, in contrast to the rat, the inhibitory effect of pilocarpine and oxotremorine on lordosis response in the hamster was not prevented by pretreatment with reserpine or p-chlorophenylalanine and the lordosis activating effect of pilocarpine was counteracted by methyl-atropine pretreatment. In both species, reserpine followed by pilocarpine treatment induced a clearcut tremor, which was not seen after either drug alone. It is suggested that the mechanism of action by which cholinergic drugs exert an inhibitory effect on hormone activated copulatory behavior is partly different in the rat and the hamster.