The effect of opiates and naloxone on food intake in virgin and lactating rats

Lactation provides an excellent model of non-obese hyperphagia. There is accumulating evidence that endogenous opioids play a role in the modulation of the hormonal changes that occur during lactation. Because endogenous opioids appear also to play a role in the regulation of feeding, we studied the effects of the opiate agonist, butorphanol tartrate, and an opiate antagonist, naloxone, on food intake in virgin female rats and in rats during early, mid and late lactation and during post-weaning. It has been reported that female rats are less sensitive to the suppressant effects of nalmefene, an opioid antagonist, than male rats. Therefore, we also examined the effect of naloxone, an opioid antagonist, on spontaneous nocturnal feeding and 24 hour food deprivation-induced food intake in virgin female rats. We found that female rats were relatively insensitive to the food suppressant effects of naloxone following 24 hour food deprivation, while male rats tested under similar conditions had a decreased intake in response to naloxone. Despite the marked hyperphagia that occurred during lactation, there were minimal alterations in the response to opiate agonists and antagonists during this time period. Our data suggest that endogenous opioids may not play a pivotal role in the hyperphagia of lactation.     

Effects of intraventricularly injected 6-hydroxydopamine on carotid sinus baroreceptor reflex in rabbits

Summary Effects of 6-hydroxydopamine (6-OH-DA) injected into the lateral brain ventricle on the carotid sinus baroreceptor reflex were studied in rabbits anesthetized with -chloralose and urethane. As short term effects, injection of 500 g/kg of 6-OH-DA caused a fall in blood pressure and heart rate, enhanced the depressor and bradycardia responses to electrical stimulation of the carotid sinus nerve (CSN), and inhibited the pressor response to carotid occlusion. These effects reached the maximum within 2 hrs and disappeared by the 4th hr. Intraventricular injection of noradrenaline (NA) could mimic most of these effects. At 4.5 hrs after injection of 6-OH-DA, NA content of the brain was definitely reduced: 21% of control in the hypothalamus and 14% in the pons-medulla. Rabbits treated with 6-OH-DA under pentobarbital anesthesia 24 hrs before showed a slight fall in resting blood pressure and almost normal baroreceptor function. Intraventricular application of phentolamine abolished the responses to CSN stimulation in 6-OH-DA pretreated as well as normal animals.These results suggest that the acute effects of 6-OH-DA are based on the increased release of NA from the affected nerve terminals and that noradrenergic neurons are involved in the central pathway of baroreceptor reflex. Moreover, the reflex may be functionally maintained by a small portion of brain NA content, even when noradrenergic neurons are greatly affected by 6-OH-DA.     

Ketanserin and the arterial baroreceptor reflex in normotensive subjects

Summary The effects of oral ketanserin 40 and 120 mg on the responses to baroreflex activation and deactivation by phenylephrine and nitroglycerin, respectively, were investigated in normotensive subjects. Plasma catecholamine levels were measured at the same times. Two hours after the administration of ketanserin, and regardless of its effect on arterial pressure (no change after 40 mg, decrease after 120 mg), there was no alteration either in resting heart rate or baroreflex sensitivity during baroreceptor activation or deactivation. The lack of reflex tachycardia in response to the drug-induced hypotension may be related to the ₁-adrenoceptor blockade-mediated sympathoinhibitory effect of ketanserin, which leaves unaffected both plasma catecholamines and the normal reactivity of the sympathetic system.     

Effects of α-adrenoceptor-stimulating drugs on baroreceptor reflexes in conscious cats

The action of some α-adrenoceptor stimulating drugs with a central effect (clonidine, α-methyldopa, reserpine) on baroreceptor reflexes was studied in conscious cats (both in the resting condition and when influenced by emotional tension or electrical stimulation of the hypothalamus). The sedative effect of these drugs was observed simultaneously with bradycardia and the increase of baroreceptor reflexes. Clonidine and reserpine (in 6–24 h after injection) lowered blood pressure while α-methyldopa (40 mg/kg) increased it. Confrontation with a dog or electrical hypothalamic stimulation produced hypertensive reactions and diminished the baroreflexes. All drugs reduced the emotional and hypertensive reactions caused by natural stress situations and restored baroreceptor reflexes. On the other hand, neither clonidine nor α-methyldopa changed the decrease of baroreceptor reflexes caused by electrical hypothalamic stimulation. It is supposed that central α-adrenoceptor stimulating drugs do not influence processes of hypothalamic modulation of baroreceptor reflexes. The increase in baroreflex activity after clonidine, α-methyldopa and reserpine appears to be due to a direct effect of the drugs on the central neurones mediating baroreceptor reflexes and to the tranquillizing action of these drugs.     

Further similarities between the action of clonidine and a central activation of the depressor baroreceptor reflex

Summary The relationship between the central action of the hypotensive drug clonidine and the depressor baroreceptor reflex was studied in cats anaesthetized with urethane. Activation of the depressor baroreceptor reflex was achieved by bilateral electrical stimulation of the sinus nerves. The magnitude of the activation of the reflex was estimated from the resultant decrease in spontaneous sympathetic activity recorded from the preganglionic splanchnic and a postganglionic renal nerve and from the subsequent fall in blood pressure. The effect of bilateral sinus nerve stimulation was frequency-dependent over the range of 2 to 32 shocks/sec and allowed the construction of frequency-response curves. Clonidine in low doses (1 and 3 g/kg i.v.), which caused no or only a slight depression of spontaneous sympathetic nerve activity and of the blood pressure, increased the response to bilateral sinus nerve stimulation with a resultant shift of the respective frequency-response curves to lower rates of stimulation. Clonidine also augmented the bradycardia produced by an activation of the depressor baroreceptor reflex. These results indicate a facilitation of the depressor baroreceptor reflex by clonidine.While the increase in activity of the splanchnic and renal sympathetic nerves and the rise in blood pressure due to hypothalamic stimulation were reduced by simultaneous bilateral sinus nerve stimulation or by an injection of clonidine, the evoked contractions of the nictitating membranes remained uninfluenced by both procedures. Thus, the effect of clonidine resembles an activation of the depressor baroreceptor reflex in inhibiting preferentially the activity in adrenergic vasomotor fibres. This and the clonidine-induced facilitation of the reflex suggest an intimate relationship between the action of clonidine and an activation of the central pathway of the depressor baroreceptor reflex.Preliminary results have been presented at the British-German Meeting on Pharmacology in Berlin (Haeusler, 1973 c).     

Adrenomedullin enhances baroreceptor reflex response via cAMP/PKA signaling in nucleus tractus solitarii of rats

Adrenomedullin (ADM), a 52-amino acid peptide, elicits differential cardiovascular responses when it is administered systemically or directly to the brain. We evaluated in the present study the hypothesis that ADM may modulate baroreceptor reflex (BRR) response through an ADM receptor-mediated cAMP/ protein kinase A (PKA)-dependent mechanism in the nucleus tractus solitarii (NTS), the terminal site for primary baroreceptor afferents, using Sprague-Dawley rats. Our immunoblot and immunohistochemical results showed that the two component proteins of the ADM(1) receptor complex, calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein (RAMP)-2, were uniformly distributed and highly co-localized in the NTS. Site-specific microinjection of ADM (0.02-0.2pmol) unilaterally into the NTS significantly increased BRR response and sensitivity in a time- and dose-related manner, without affecting arterial pressure and heart rate. The BRR enhancing effect of ADM was also temporally correlated with an up-regulation of PKA(beta), the active form of PKA and an increase in PKA activity. In addition, the ADM-evoked BRR enhancement or PKA activation was abolished by co-microinjection with a selective ADM(1) receptor antagonist, ADM(22-52), an adenylyl cyclase inhibitor, SQ22536, or a PKA inhibitor, Rp-8-bromo-cAMP. These results suggest that ADM enhances BRR via activation of a cAMP/PKA-dependent mechanism by acting site-specifically on ADM(1) receptors in NTS.     

Modulating effect of central adrenergic neurones on a vagally mediated cardioinhibitory reflex

Pentobarbital-anaesthetized dogs were pretreated with the β-adrenoceptor blocking agent, KÖ 592, 5 mg/kg, and blood pressure and heart rate were measured continuously. Angiotensin (ANG), 0.0125–0.3 μg/kg, i.v., produced a rise in blood pressure accompanied by a reflex fall in heart rate. The α-adrenoceptor inhibiting drugs phentolamine, 5 mg/kg, i.v., or 0.5 mg/kg, intracisternally (i.ci.), and chlorpromazine, 0.5–1 mg/kg, i.ci., significantly suppressed the reflex bradycardia of ANG, whereas 0.5 mg/kg phentolamine i.v. was ineffective. In dogs pretreated with reserpine, 2.0 mg/kg, s.c., 18 hr, and KÖ 592 (as above) ANG regularly produced a reflex bradycardia which was not influenced by phentolamine, 0.5 mg/kg, i.ci.; the reflex bradycardia was potentiated by clonidine, 1 μg/kg, i.ci., and subsequent i.ci. injection of phentolamine reduced the reflex to the pre-clonidine value. Following pretreatment with atropine at a dose of 1.0 mg/kg, the reflex bradycardia of ANG was not changed by 0.5 mg/kg phentolamine i.ci. The conclusion is drawn, that cardio-inhibiting reflexes effected by the vagus are facilitated by adrenergic neurones within the central nervous system.     

Effect of naloxone on the cardiovascular and sympathetic response to hypovolemic hypotension in the rat

Conscious rats were exposed to acute hypovolemic-hypotension by bleeding (5 ml/300 g body weight). Treatment with the opiate antagonist naloxone (7 mg/kg intra-arterial) following hemorrhage resulted in an increase of systolic blood pressure but not heart rate. Changes in plasma catecholamine levels did not differ between control and naloxone-treated animals. From these results we suggest that the ability of naloxone to improve blood pressure after hemorrhage is not due to increased sympatho-adrenomedullary activity.     

The opiate receptor binding interactions of 3H-methionine enkephalin, an opioid peptide.

3H-Methionine enkephalin binds stereospecifically with high affinity to opiate receptors in rat brain membranes. Equilibrium experiments indicate two distinct dissociation constants with KD values of 1.8 and 5.8 nM respectively. 3H-Methionine enkephalin associates and dissociates from the opiate receptor with 8--10 fold slower kinetics than 3H-opiates. Though several opiates have similar affinities for sites labeled by 3H-methionine enkephalin, 3H-dihydromorphine and 3H-naloxone, some opiates such as morphine, dihydromorphine and oxymorphone are only one tenth as potent in competing for 3H-methionine enkephalin as for 3H-dihydromorphine and 3H-naloxone binding. As with other opiate agonists, 5--10 mM sodium selectively decreases the binding of 3H-methionine enkephalin. At 26 degrees C, 0.1--1.0 mM manganese but not magnesium or calcium increases the binding of 3H-methionine enkephalin, while at 0 degrees C manganese decreases the binding of methionine enkephalin.     

The effect of spontaneous seizures on pentylenetetrazole and maximum electroshock induced seizures in the Mongolian gerbil

The sedative effect of xylazine and its interaction with antagonists of alpha-adrenoceptors or opiate receptors was examined in chicks. The duration of the sleep-like behavioral state induced by xylazine was determined by measuring the time interval during which the chicks failed to exhibit the righting reflex. In these chicks, intramuscular administration of xylazine (0.3-4.8 mg/kg) induced a loss of the righting reflex, the duration of which was dose-dependent. Both alpha-adrenoceptor antagonists of the alpha2 type, i.e. yohimbine (0.1-1.0 mg/kg) and compound 170150 (0.1-1.0 mg/kg) (a benzodioxane derivative), and opiate receptor antagonists, i.e. naloxone (0.4-1.6 mg/kg) and nalorphine (3.0-30.0 mg/kg), effectively inhibited the sedative effect of xylazine while prazosin (0.1-10.0 mg/kg), an alpha 1-adrenoceptor antagonist, failed to antagonize xylazine-induced sedation. These findings suggest that in addition to the stimulation of central alpha 2-adrenoceptors, activation of an endogenous opiate mechanism may be involved in the sedative effect of xylazine.     

Effects of pentazocine and other opiates on shock detection in the rat: Involvement of opiate and dopamine receptors

The hypothesis that the antinociceptive effects of pentazocine, a mixed agonist-antagonist opiate of the benzomorphan class, are mediated by a dual opiate-dopaminergic mechanism was tested using a two-choice procedure in which rats were required to discriminate the presence or absence of shock. The results showed that pentazocine decreased shock sensitivity and speed of responding, effects that were qualitatively similar to those of morphine. However, while the antinociceptive effect of both pentazocine and morphine could be antagonized by opiate receptor blockade, that of pentazocine, but not of morphine, could also be antagonized by dopamine receptor blockade. Observations with levorphanol and phenazocine suggested further that dopamine, as well as opiate receptor agonism may be characteristic of the benzomorphans.     

Inhibition of catecholamine (noradrenaline,dopamine) release in the locus coeruleus and the hypothalamus by baroreceptor activation: identification of the involved baroreceptors

We have previously shown that experimentally induced blood pressure changes modify the release rates of catecholamines in the hypothalamus and the locus coeruleus. The aim of the present investigation was to identify the peripheral baroreceptors and the centripetal pathways responsible for the changes of catecholamine release in these brain areas.In anaesthetized cats, push-pull cannulae were bilaterally inserted into the locus coeruleus and the posterior hypothalamus. The two brain areas were superfused simultaneously with artificial cerebrospinal fluid. Baroreceptor activation by phenylephrine-induced blood pressure elevation decreased the release rate of noradrenaline in the locus coeruleus and the release rates of noradrenaline and dopamine in the posterior hypothalamus. Similar effects were elicited by electrical stimulation of the central trunk of the transected vagus and aortic depressor nerves (vagus-ADN). Transection of the nerves abolished the effect of phenylephrine on the release of noradrenaline in the locus coeruleus. Nerve transections attenuated slightly the decreased release of noradrenaline elicited by phenylephrine in the posterior hypothalamus, while the reduced dopamine release rate was not influenced. The selective stimulation of baroreceptors in the carotid sinus by an inflatable catheter did not influence the release of catecholamines in the locus coeruleus, while release rates of noradrenaline and dopamine in the posterior hypothalamus were decreased.The simultaneous superfusion of locus coeruleus and hypothalamus revealed that, in both areas, noradrenaline release is inhibited by baroreceptor activation. Noradrenergic neurons of the posterior hypothalamus are inhibited by baroreceptor impulses conducted by the carotid sinus nerve and vagus-ADN, while the noradrenergic neurons of the locus coeruleus seem to respond to impulses transmitted by vagus-ADN. Furthermore, baroreceptor activation inhibits dopaminergic neurons in the hypothalamus but not in the locus coeruleus.     

Apomorphine-like effect of an aminotetralin on the linguomandibular reflex of the cat.

Low doses of 5,6-dihydroxy-2-dimethylaminotetralin (M-7) produced inhibition of the linguomandibular reflex in the cat. This inhibition was similar to that produced by apomorphine and was blocked by haloperidol, bulbocapnine or chlorpromazine. M-7 also had a brief but marked hypertensive effect. Gradual bradycardia also developed. These results indicate that M-7 inhibits the linguomandibular reflex by interaction at dopamine receptors, or 'apomorphine' receptors.     

The effect of enalapril on baroreceptor mediated reflex function in normotensive subjects.

The effects of enalapril, 20 mg orally, on the responses to baroreflex activation and deactivation by respectively phenylephrine and nitroglycerin were investigated in normotensive subjects on a normal sodium diet, with simultaneous measurement of plasma renin activity (PRA), converting enzyme activity (PCEA), aldosterone and catecholamines. Enalapril, 4 h after administration, lowered artificial blood pressure without modifying heart rate and plasma catecholamines. PCEA was abolished, PRA increased and plasma aldosterone decreased. Enalapril (a) displaced to the left the baroreflex set-point, (b) did not affect baroreflex sensitivity since the slopes of the RR-interval/systolic blood pressure regression lines remained unchanged during both activation and deactivation and (c) did not modify baroreflex efficacy since the maximal RR-interval responses as well as the overall RR-interval-time products to identical blood pressure variations were not modified. Thus, enalapril induced a resetting of the baroreflex, which probably accounts for the lack of reflex tachycardia observed during the drug-induced fall in blood pressure.     

Effects of clonidine on baroreceptor function in anesthetized dogs

The effects of clonidine (15–30 μg/kg i.v.) on carotid sinus and other baroreceptors were investigated in anesthetized dogs. In 14 control dogs, right carotid sinus pressure was controlled by retrograde perfusion through the common carotid artery at constant flow with femoral arterial blood. Graded reductions in heart rate and blood pressure induced by graded increases in carotid sinus pressure were prevented, whereas reflex bradycardias associated with norepinephrine pressor activity were potentiated by clonidine. Norepinephrine-induced bradycardia, although reduced, still persisted after chronic bilateral sinusectomy and these responses were also potentiated by clonidine. In contrast, clonidine did not potentiate reflex bradycardia in dogs 20 days after aortic stripping. In intact dogs, clonidine inhibited the response to bilateral carotid artery occlusion and to carotid sinus nerve stimulation. These studies suggest that clonidine can inhibit carotid sinus baroreceptor function and simultaneously potentiate other, presumably aortic, baroreceptor activity.     

Paradoxical effect of naloxone on nitrous oxide analgesia in man

The effect of naloxone on nitrous oxide analgesia in man has been investigated. The paradoxical response so obtained indicates the possibility of a dural system mediating the pain response in man. These results support previous animal experiments indicating that nitrous oxide analgesia is mediated by the opiate receptors.     

Effects of ketocyclazocine alone and in combination with naloxone on schedule-controlled responding in squirrel monkeys

A multiple, fixed interval 5 minutes, fixed ratio 30, schedule of food presentation (Mult FI FR) was used to evaluate the effects of ketocyclazocine, a kappa-receptor agonist, in four squirrel monkeys. Two monkeys were initially trained with 1-minute time-out (TO) periods between the components of the multiple schedule and two monkeys were initially trained without these TO periods. Ketocyclazocine dose-response functions were determined for each monkey under their original training conditions and then the conditions were reversed and dose-response functions were re-determined under the new conditions. Ketocyclazocine consistently decreased rates of responding during the FR component of the multiple schedule under both TO and no TO conditions. Under the FI component, ketocyclazocine's effects differed dependent upon dose, conditioning history, and the presence or absence of TO periods. Intermediate doses of ketocyclazocine (0.01-0.056 mg/kg) increased FI rates of responding under the no TO condition in monkeys originally trained under this condition; however, ketocyclazocine did not increase FI rates of responding under the no TO condition in monkeys originally trained under the TO condition. Under the TO condition, intermediate doses of ketocyclazocine did not increase FI rates of responding. High doses of ketocyclazocine (0.1 and 0.17 mg/kg) decreased FI rates of responding in all monkeys under both the TO and no TO conditions Naloxone, in doses up to those which decreased responding when given alone, failed to antagonize completely the rate decreasing effects of ketocyclazocine.     

GABAA receptor,KATP channel and L-type Ca2+ channel is associated with facilitation effect of H2S on the baroreceptor reflex in spontaneous hypertensive rats

BackgroundWe aimed to investigate whether the facilitating effect of H₂S on the baroreceptor reflex is associated with the GABA_A receptor, K_ATP channel and L-type Ca²⁺ channel pathway.MethodsSpontaneously hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats were used to investigate the facilitating effect of H₂S on the baroreceptor reflex by perfusing the isolated carotid sinus. The mechanism by which H₂S facilitated the baroreceptor reflex was determined by using Bay K8644 (an agonist of calcium channels), glibenclamide (Gli, a K_ATP channel blocker), and picrotoxin (PIC, a blocker of γ-aminobutyric acid [GABA]_A receptor).ResultsAs compared with WKY rats, SHRs showed impaired baroreceptor reflex sensitivity, as demonstrated by a right and upward shift of the functional curve for the intrasinus pressure–arterial blood pressure relation. H₂S perfusion (25, 50, or 100 μmol/L) dose-dependently ameliorated the impaired sensitivity of the baroreceptor reflex. Bay K8644 (500 nmol/L), Gli (20 μmol/L) and PIC (50 μmol/L) all prevented H₂S ameliorating the impaired baroreceptor reflex.ConclusionsH₂S facilitating the baroreceptor reflex might be associated with activating the GABA_A receptor, opening the K_ATP channel, and closing the L-type Ca²⁺ channel. These areas should provide new targets for preventing and treating hypertension.