The effect of the acute and chronic administration of the opiate receptor antagonist naloxone on the development of fatigue during physical loading in rats]

In chronic experiments with rats during the exercises on a treadmill the ECG was registered and duration of the development of exhaustion was determined under control and under acute (1 mg/kg intraperitoneally 1 hour before the experiment) or chronic (1 mg/kg intraperitoneally, twice a day during 5 days) treatment with the opioid receptor antagonist naloxone. Chronic but not acute naloxone action resulted in increase of the fatiguability: the time of achievement of exhaustion decreased by 55.3% (P < 0.05). In this case the exhaustion developed at lower degree of heart rate than in control. Comparison of data obtained with the results of chronic treatment with the opioid antagonist permits to conclude that the chronic blockade increases the fatiguability to a great extent than chronic activation of opioid system. Possible mechanisms causing this difference are under discussion.     

Aspects of cardiovascular reflexes in pathologic states

Cardiovascular reflexes that are mediated by receptors in the heart and blood vessels control a variety of important hemodynamic and humoral functions. The action of these receptors can be shown to be abnormal in several pathologic states. Left atrial receptors exhibit a depressed discharge sensitivity in dogs with chronic congestive heart failure caused by an aortocaval fistula. The reflex effects of atrial receptor stimulation are also depressed in heart failure. Left ventricular receptor stimulation has been implicated in the abnormal vascular responses to exercise in patients with aortic stenosis. The arterial baroreflex control of heart rate is abnormal in animals and humans with various forms of hypertension. Arterial baroreceptors from hypertensive animals show a resetting of their pressure-discharge curve to higher pressures. The arterial baroreflex is also depressed in chronic heart failure. This effect may result from an abnormality of the efferent limb of the reflex arc or from changes in the interaction between baroreceptors and cardiac receptors centrally. A final possibility may be abnormal arterial baroreceptor discharge characteristics in heart failure.     

Changes in the autonomic balance of the regulation of the rate of heart contractions during physical loading in chronic suppression or potentiation of the endogenous opioidergic system in rats

The influence of endogenous opioid system on the rat's mean blood pressure (BP) and heart rate (HS) has been studied under the chronic infusion of the opioid receptor antagonist naloxone (1 mg/kg intraperitoneally, twice a day, during 6 days) or an inhibitor of captorile enkephalinases (20 mg/kg subcutaneously). Naloxone caused a significant decrease and captorile--increase of maximum meanings of HR during exercises (the running on the treadmill during 3.5 min by the velocity of 30 m/min), both compounds didn't exert a considerable effect on BP at rest and during exercises. It has been concluded that the endogenous opioid system plays an important role in the autonomic HR regulation during exercise.     

Effects of cocaine alone and in combination with haloperidol and SCH 23390 on cardiovascular function in squirrel monkeys

The potential involvement of D1 and D2 dopamine receptors in the effects of cocaine on cardiovascular function in squirrel monkeys was evaluated. A low dose of cocaine (0.1 mg/kg i.v.) produced increases in both blood pressure and heart rate. At the higher doses of cocaine (1.0-3.0 mg/kg) the heart rate response was biphasic, consisting of an early decrease followed by an increase in heart rate 10-20 min following injection. The dopamine D2 antagonist haloperidol (0.1 mg/kg i.m.) attenuated the heart rate increasing effect of cocaine, but doses as high as 0.03 mg/kg did not alter the blood pressure increase. The D1 antagonist SCH 23390 (0.01-0.03 mg/kg i.m.) did not attenuate either the blood pressure or heart rate increasing effects of cocaine. The D2 agonist quinpirole (1.0 mg/kg i.v.) produced increases in heart rate similar to cocaine, with little effect on blood pressure. Although effective against the heart rate increasing effect of cocaine, haloperidol (0.01 mg/kg) did not antagonize the heart rate increasing effects of quinpirole. The D1 agonist SKF 38393 (3.0 mg/kg i.v.) decreased heart rate and increased blood pressure. The blood pressure increasing effect of SKF 38393 was antagonized by 0.01 mg/kg SCH 23390. Haloperidol's ability to partially antagonize the tachycardiac response to cocaine suggests the involvement of D2 receptors in that response. However, the failure of haloperidol to antagonize quinpirole's tachycardiac effect suggests that non-dopaminergic mechanisms may also be involved in haloperidol's antagonism of cocaine's tachycardiac effect. The pressor effects of cocaine do not appear to be controlled by selective dopamine receptors.     

Paticipation of cannabinoid receptors in the regulation of cardiac rhythm and cardiac contractility

It has been found that i. v. administration of cannabinoid receptor (CB) agonists (HU-210, ACPA, anandamide, methanandamide) induced a decrease in the heart rate (HR) in anesthetized rats. Pretreatment with CB1 receptor antagonist SR141716A completely abolished a negative chronotropic effect of CB receptor agonist HU-210. The CB2 receptor antagonist SRI 44528 did not prevent a HU-210-induced decrease in the HR. Pretreatment with the ganglion blocker hexamethonium had no effect on the negative chronotropic action of HU-210. Addition of HU-210 (100 nM) to perfusion solution induced a decrease in the HR, left ventricular development pressure, rate of contractility and relaxation of isolated perfused rate heart without change in end diastolic pressure. These data suggest that cardiac CBI receptor activation induces a decrease in the HR both in vivo and in vitro. An occupancy of the same receptors mediates a negative inotropic effects of cannabinoids.     

Action of somatostatin (SRIF) on circulation and its interaction with the adrenergic system in rats]

An effect of somatostatin on arterial blood pressure, isolated heart, and its interaction with adrenergic system have been studied in rats. It was found, that somatostatin does not exert any significant effect on blood circulation. It modifies however catecholamines action. Somatostatin decreases the activity of alpha-adrenergic receptors agonist (norepinephrine), and potentiates the activity of beta-adrenergic receptors agonist (isoprenaline).     

Central muscarinic modulation of fetal blood pressure and heart rate

It has previously been demonstrated that the fetal lamb cardiovascular system can respond to peripheral muscarinic stimulation. However the role of central muscarinic mechanisms in modulating fetal cardiovascular function has not been described. Pilocarpine is a muscarinic agonist that readily crosses the blood-brain barrier and was therefore employed to examine both central and peripheral muscarinic mechanisms in modulating fetal cardiovascular function. Fetal lambs were prepared for chronic intrauterine recording of fetal blood pressure (FBP) and heart rate (FHR). Direct administration of pilocarpine to the fetus resulted in an immediate dose-dependent decrease in both systolic and diastolic blood pressure and a rapid fall in FHR. The initial phase of hypotension was very short-lived (1-2 min) and was subsequently followed by a significant increase in systolic, diastolic and pulse pressures (30-60 min). Fetal heart rate gradually returned to control levels by 30 min after pilocarpine administration. Atropine pretreatment was effective in completely blocking the cardiovascular actions of pilocarpine, while methylatropine was only able to block the initial hypotensive and bradycardiac response. A prolonged tachycardia was also unmasked by methylatropine pretreatment. These data suggest that the initial hypotension and bradycardia in response to pilocarpine administration are mediated via peripheral muscarinic receptors, while stimulation of central muscarinic receptors result in hypertension and tachycardia. These data confirm that, as in the adult, central cholinergic mechanisms are involved in the modulation of cardiovascular function in the developing fetus.     

Decrease of cyclic GMP in cerebellum by intrastriatal D-Ala2-met-enkephalinamide.

The bilateral intrastriatal injection of D-Ala²-Met-Enkephalinamide (DALA) at doses of ranging from 12 to 50 μg decreased cyclic-GMP content in the cerebellum and produced catalepsy. These effects were prevented by naltrexone, an opiate receptor antagonist but not by apomorphine, a dopamine agonist. The bilateral injection of DALA in the cerebellum, and substantia nigra neither decreased cerebellar cyclic GMP content nor produced catalepsy. The bilateral injection of DALA (20 ug) into the ventromedial thalamic nuclei caused marked catalepsy but failed to decrease cerebellar cyclic GMP. The results suggest that the effect of DALA on cGMP can be differentiated from the cataleptic response and that it is mediated by an action on opioid receptors located in the striatum, beyond DA receptors.     

The species characteristics of the beta-adrenergic regulation of gastric secretion in man and dog

In chronic experiment on dogs it has been established that the subcutaneous injection of equimolar doses of izadrine (nonselective beta-adrenergic agonist), alupent (moderately selective beta 2-adrenergic agonist) and salbutamol (predominantly beta 2-adrenergic agonist) suppresses the pentagastric secretion approximately in the same degree. The blockade of beta-adrenoreceptors by the anapriline intensifies the gastric secretion stimulated by pentagastrin. All investigated adrenoactive agents didn't effect the dogs' histamine gastric secretion. In healthy men the activation of beta 2-adrenergic receptors by alupent accompanied by the expressed intensification of basal, pentagastrin and submaximal histamine gastric secretion. The blockade of these receptors by anaprilin decreases the gastric secretion. It has been concluded that only beta 2-adrenoceptors take part in the gastric secretion regulation. Considerable specific differences in the reaction of gastric glands on the activation of beta-adrenoreceptors are revealed: in human beings it leads to the excitation, in dogs--to the suppression of secretory cells.     

Modulation of cardiac A1-adenosine receptors in rats following treatment with agents affecting heart rate

Effects of chronic treatment affecting heart rate on A₁ adenosine receptor levels and their functions were studied. Treatment of rats with isoproterenol for 10 days accelerated heart rate and increased the level of adenosine receptors, in both the atria and ventricles. Negative dromotropic response of isolated heart to adenosine was enhanced in isoproterenol-treated rats. Similar results were obtained following treatment with atropine sulfate, or swimming training but not after treatment with thyroxine. On the other hand, treatment with amiodarone, which normally causes a decrease in heart rate, also increased the level of adenosine receptors in both atria and ventricles. The sensitivity of the isolated heart to the negative dromotropic and chronotropic effects of adenosine was not enhanced in the amiodarone treated rats. Similar results were obtained following treatment with propranolol, while treatment with PTU (6-n-propyl-2-thiouracil) increased adenosine sensitivity in the isolated heart. It was concluded that the levels of A₁ adenosine receptors in the heart correspond to heart rate, and to cardiac efficiency. While an increase in heart rate was followed by up-regulation of A₁ adenosine receptors, a decrease in heart rate caused a moderate elevation of these receptors.     

A model for the interaction of muscarinic receptors, agonists, and two distinct effector substances

The binding of the agonist carbamylcholine to muscarinic receptors in rat heart myocytes from young and aged cultures and in rat atrial membranes has been measured in the absence and presence of GppNHp, pertussis toxin, and/or batrachotoxin. The effect of each of the added substances upon agonist binding was accounted for by a model according to which the receptor may form an equilibrium complex with agonist and either of two distinct effector substances, one of which is postulated to increase the affinity of receptor for agonist and the other of which is postulated to decrease the affinity of receptor for agonist.     

Effects of oral pergolide mesylate on nociceptive spinal cord reflexes in rats

The effects of orally administered pergolide mesylate on the flexor reflex were evaluated in chronic spinal rats. The mixed D1/D2 agonist pergolide (0.1 to 3.0 mg/kg) produced a dose-related decrease in the magnitude of the flexor reflex elicited by a tetanic stimulus. The effects of pergolide were blocked by haloperidol, demonstrating that the effects of pergolide were mediated through dopamine receptors. In contrast, the selective D2 agonist bromocriptine (3.0 to 30 mg/kg) had no effect on the flexor reflex. The present results are consistent with the interpretation that pergolide produces an antinociceptive action at the spinal cord level by stimulating both D1 and D2 dopamine receptors.     

Expression of orphan receptors GPR22 and GPR162 in streptozotocin-induced diabetic rats

Abstract

Aims/Introduction: Diabetes mellitus is a chronic degenerative disease characterized by high blood glucose levels as a result of problems in the action or insulin secretion. Although there are many treatments for this pathology, it has been associated with a high mortality rate. For this reason, it is important to try to identify new pathways that could be involved in diabetic complications. Recently, a new class of receptors has been reported, called orphan receptors because the associated ligand and signaling pathways are unknown, these receptors have been associated with certain pathologies. Therefore, the aim of this work was to study the expression of the orphan receptors GPR22 and GPR162 in heart, aorta, brain and kidney of diabetic rats. Materials and methods: We used Wistar male rats with 10–12 weeks of age. Diabetes was induced by a single dose of streptozotocin (60?mg/kg i.p.). After four weeks, the tissue was obtained and the expression of the mRNA was measured by RT-PCR. Results: Our results showed that the orphan receptors are expressed in a different way in heart, kidney, brain and aorta of diabetic and non-diabetic rats. Conclusions: We conclude that orphan receptors could be involved in the development of diabetes complications.     

Up-regulation of neuropeptide Y levels and modulation of glutamate release through neuropeptide Y receptors in the hippocampus of kainate-induced epileptic rats

Kainate-induced epilepsy has been shown to be associated with increased levels of neuropeptide Y (NPY) in the rat hippocampus. However, there is no information on how increased levels of this peptide might modulate excitation in kainate-induced epilepsy. In this work, we investigated the modulation of glutamate release by NPY receptors in hippocampal synaptosomes isolated from epileptic rats. In the acute phase of epilepsy, a transient decrease in the efficiency of NPY and selective NPY receptor agonists in inhibiting glutamate release was observed. Moreover, in the chronic epileptic hippocampus, a decrease in the efficiency of NPY and the Y(2) receptor agonist, NPY13-36, was also found. Simultaneously, we observed that the epileptic hippocampus expresses higher levels of NPY, which may account for an increased basal inhibition of glutamate release. Consistently, the blockade of Y(2) receptors increased KCl-evoked glutamate release, and there was an increase in Y(2) receptor mRNA levels 30 days after kainic acid injection, suggesting a basal effect of NPY through Y(2) receptors. Taken together, these results indicate that an increased function of the NPY modulatory system in the epileptic hippocampus may contribute to basal inhibition of glutamate release and control hyperexcitability.     

Adrenergic receptors and adenylate cyclase activity in hepatocytes of the streptozotocin-diabetic rat.

Effects of short and long exposure to the diabetic state induced by an injection of streptozotocin to young female rats on glucagon- and catecholamine-sensitive adenylate cyclase activity and adrenergic receptors of hepatic membranes have been studied. The short period of exposure to the diabetic state exhibited an increase in the sensitivity of the enzyme to isoproterenol without changes in the affinity and the number of beta-adrenergic receptors. The increased response of adenylate cyclase activity to isoproterenol was accompanied with a greater GTP-induced lowering of the affinity to the beta-adrenergic agonist in diabetic membranes than in the controls. The chronic diabetic state produced a decrease in the adenylate cyclase activity to hormonal or non-hormonal stimuli with a fall in the number of alpha- and beta-adrenergic receptors. These results suggest that the observed effects of the diabetic state on hormonally sensitive adenylate cyclase activities and their receptor binding sites of the hepatic membranes would vary depending on the duration and/or severity of the diabetic state experimentally induced.     

Effect of adaptation to physical loading on the adrenoreactivity of the isolated rat atrium

The experiments on the isolated atria of rats, previously adapted to swimming exercises, have established a significant decrease in the heart rate and an increase in positive inotropic effect of noradrenaline (4 X 10(-7) and 6 X 10(-7) M). After beta-adrenoceptor blockade with propranolol (2 X 10(-6) M) the inotropic effect of an alpha-adrenoceptor agonist mezaton on the isolated atrium of the adapted rats was greater than in nonadapted controls.     

Cysteine substitution mutagenesis and the effects of methanethiosulfonate reagents at P2X2 and P2X4 receptors support a core common mode of ATP action at P2X receptors

The agonist binding site of ATP-gated P2X receptors is distinct from other ATP-binding proteins. Mutagenesis on P2X(1) receptors of conserved residues in mammalian P2X receptors has established the paradigm that three lysine residues, as well as FT and NFR motifs, play an important role in mediating ATP action. In this study we have determined whether cysteine substitution mutations of equivalent residues in P2X(2) and P2X(4) receptors have similar effects and if these mutant receptors can be regulated by charged methanethiosulfonate (MTS) compounds. All the mutants (except the P2X(2) K69C and K71C that were expressed, but non-functional) showed a significant decrease in ATP potency, with >300-fold decreases for mutants of the conserved asparagine, arginine, and lysine residues close to the end of the extracellular loop. MTS reagents had no effect at the phenylalanine of the FT motif, in contrast, cysteine mutation of the threonine was sensitive to MTS reagents and suggested a role of this residue in ATP action. The lysine-substituted receptors were sensitive to the charge of the MTS reagent consistent with the importance of positive charge at this position for coordination of the negatively charged phosphate of ATP. At the NFR motif the asparagine and arginine residues were sensitive to MTS reagents, whereas the phenylalanine was either unaffected or showed only a small decrease. These results support a common site of ATP action at P2X receptors and suggest that non-conserved residues also play a regulatory role in agonist action.     

Agonist Interactions with Beta-Adrenergic Receptors Following Chronic Administration of Desipramine or the Atypical Antidepressants,Iprindole and Mianserin

Abstract

Desipramine (DMI), decreased the maximum number of beta-adrenergic receptors by approximately 10, 20, 30, and 20% in groups of rats treated i.p. with 5 mg/kg for 14 days or 10 mg/kg for 7, 14, or 21 days, respectively. In studies of agonist competition for beta-adrenergic receptors labelled with [¹²⁵I]-CYP, chronic DMI administration caused a selective decrease in those receptors normally found in the high affinity conformation in proportion to the dose of DMI administered. No change was observed in either the number of receptors in the agonist low affinity conformation or in the affinity of any drug for the high or low affinity conformations of the receptors. Therefore, chronic DMI caused a selective decrease in the beta-adrenergic receptors linked to adenylate cyclase but did not appear to change other properties of the receptors that would be manifested as a change in their ability to interact with adrenergic agonists. Neither iprindole (15 mg/kg i.p., 14 days) nor mianserin (10 mg/kg i.p., 14 days) decreased the number of receptors, the proportions of agonist high or low affinity receptors, or the affinity of competitor drugs for these receptors, suggesting a different mechanism for the reported loss of adenylate cyclase activity following these drugs than the down-regulation of receptors observed with chronic DMI treatment.     

Role of kappa-opioid receptors in the regulation of cardiac resistance to arrhythmogenic effects of ischemia and reperfusion

It was found that pretreatment of rats with selective agonist of kappa1-opioid receptors (OR) (-)--U--50.488 decreased the incidence of ischemic (10 min) and reperfusion (10 min) ventricular arrhythmias. The selective kappa2-OR agonist GR-89696 had no effect on the incidence of ventricular arrhythmias during a 10-min coronary artery occlusion and following reperfusion in anesthetized rats. The effect of (-)--U-50.488 was abolished by the selective kappa1-OR antagonist of non-binaltorphimine and the non-selective peripheral OR antagonist naloxone methiodide. Perfusion of isolated rat heart with (-)--U-50.488 did not affect arrhythmias during ischemia and reperfusion. The authors suggest that stimulation of kappa1-opioid receptors located outside the central nervous system increases heart resistance against arrhythmogenic action of ischemia/reperfusion, antiarrhythmic action of (-)--U-50.488 being mediated through extracardiac opioid receptors.     

Further characterization of the direct inhibitory effect of LHRH agonists at the testicular level in the rat

To further clarify the relative importance of the pituitary and gonadal sites of LHRH action, intact and hypophysectomized adult male rats were treated with hCG for 7 days, in the presence or absence of simultaneous treatment with increasing doses of the LHRH agonist [D-Ser(TBU)6des-Gly-NH2(10)]LHRH ethylamide, Buserelin (0.025, 0.25, 2.5 or 25 micrograms/rat, twice daily). Daily treatment of intact adult rats with hCG (25 IU) markedly increased ventral prostate and seminal vesicle weight, while a dose-dependent inhibition of the effect was observed following combined administration of Buserelin. In hypophysectomized rats, treatment with hCG resulted in a partial restoration of ventral prostate and seminal vesicle weight, while combined treatment with a high dose of the LHRH agonist (25 micrograms, twice daily) partially (P less than 0.05) inhibited the stimulatory effect of hCG. LH/hCG receptors were almost completely inhibited after hCG injection alone and a further decrease was observed in the presence of simultaneous LHRH agonist treatment. The hCG-induced stimulation of GH/PRL receptors was counteracted by Buserelin treatment in hypophysectomized animals. The present data demonstrate that although LHRH-induced LH release has been shown to play a major role in the loss of testicular functions induced by low doses of LHRH agonists in the rat, a direct inhibitory action of LHRH agonists can be exerted at the testicular level at high doses of the peptide.