Vagal bradycardia produced by microinjections of morphine-like drugs into the nucleus ambiguus in anaesthetized dogs

Effect of three opioid-based analgesic protocols on the perioperative autonomic-mediated cardiovascular response in sheep

Few reports evaluate the clinical effects of opioids in sheep during experimental surgical procedures. Catecholamine-mediated haemodynamic changes resulting from surgical noxious stimulation are blunted by opioids. The aim of this study was to evaluate the efficacy of three opioid-based analgesic protocols in avoiding a 20% increase in heart rate (HR) and/or mean arterial blood pressure (MAP) during experimental intervertebral disk nucleotomy in sheep. Eighteen female Brogna sheep were anaesthetized with propofol and maintained with a fixed end-tidal isoflurane concentration of 1.5 ± 0.1%. Sheep were assigned to one of three groups that intravenously received methadone 0.3 mg/kg (group M), fentanyl 2 µg/kg followed by 10 µg/kg/h (group F), or buprenorphine 10 µg/kg and 30 minutes later ketamine 1 mg/kg followed by 5 mg/kg/h (group BK). Intravenous fentanyl at 2 µg/kg would have been used for rescue analgesia in case HR and/or MAP had increased. During surgery, HR and MAP values did not increase over 20% in all groups. All animals maintained the percentage change between -4 and 7% for both variables; only one sheep in group BK had an increase in MAP superior to 20% after ketamine administration before surgical stimulation. In group M, HR decreased over time and in group BK, MAP tended to increase during surgery. All the opioid-based protocols tested were able to control the cardiovascular response to noxious stimulation in sheep undergoing spinal surgery, although ketamine may have represented a confounding factor.

Involvement of Autonomic Nervous System in Antiarrhythmic Effect of Intermittent Hypobaric Hypoxia

We studied the involvement of the autonomic nervous system in the antiarrhythmic effect of intermittent hypobaric hypoxia modeled by daily placing the rats into an altitude chamber at 405 mm Hg (5000 m above sea level). The antiarrhythmic effect of hypoxia was observed on the model of acute coronary occlusion/reperfusion in vivo, but not during simulation of total ischemia/reperfusion of the isolated myocardium. Intravenous injection of ganglionic blocker hexamethonium (30 mg/kg) 15 min prior to in vivo coronary occlusion modeling abolished the antiarrhythmic effect of intermittent hypobaric hypoxia, which suggests that this effect is mediated via activation of the autonomic nervous system.

Extended infusion of dexmedetomidine to an infant at sixty times the intended rate

Dexmedetomidine is an α2 adrenergic agonist which has recently been approved in the United States for procedural sedation in adults. This report describes an infant who inadvertently received an intravenous infusion of dexmedetomidine at a rate which was 60 times greater than intended. We describe the hemodynamic, respiratory, and sedative effects of this overdose.

Avoidance and management of trigeminocardiac reflex complicating awake-craniotomy

The trigeminocardiac reflex occurs from manipulation or stimulation of peripheral branches or the central component of the trigeminal nerve and consists of bradycardia, hypotension, apnea, and increased gastric motility. The efferent limb of the response is mediated by the vagus nerve. This 65-year-old Caucasian male suffered an episode of bradycardia progressing to transient asystole during the course of an awake-craniotomy procedure for tumor resection. The cardiac rhythm changes resolved with administration of intravenous atropine, removal of the precipitating stimulus, and application of topical anesthetic on the dura of the middle cranial fossa. The trigeminocardiac response may complicate the course of a craniotomy and may place an awake, unintubated patient at increased risk for morbidity. The reflex may be prevented by anesthetizing the dura innervated by the trigeminal nerve via injection or topical application of local anesthetic. If encountered, removal of the stimulus, airway protection, and administration of vagolytic medications are measures that need to be considered.

Deep sedation with dexmedetomidine in a porcine model does not compromise the viability of free microvascular flap as depicted by microdialysis and tissue oxygen tension

BACKGROUND

Deep sedation is often necessary after major reconstructive plastic surgery in the face and neck regions to prevent sudden spontaneous movements capable of inflicting mechanical injury to the transplanted musculocutaneous flap(s). An adequate positioning may help to optimize oxygenation and perfusion of the transplanted tissues. We hypothesized that dexmedetomidine, a central alpha2-agonist and otherwise potentially ideal postoperative sedative drug, may induce vasoconstriction in denervated flaps, and thus increase the risk of tissue deterioration.

METHODS

Two symmetrical myocutaneous flaps were raised on each side of the upper abdomen in 12 anesthetized pigs. The sympathetic nerve fibers were stripped from the arteries in one of the flaps (denervated flap), while nerve fibers were kept untouched in the other (innervated flap). After simulation of ischemia and reperfusion periods, the animals were randomized to deep postoperative sedation with either propofol (n = 6) or dexmedetomidine (n = 6). Flap tissue metabolism was monitored by microdialysis and tissue-oxygen partial pressure. Glucose, lactate, and pyruvate concentrations were analyzed from the dialysate every 30 min for 4 h.

RESULTS

Mean arterial blood pressure was higher in the dexmedetomidine group (P = 0.036). Flap tissue metabolism remained stable throughout the experiment as measured by lactate-pyruvate and lactate-glucose ratios (median ranges 14.3-24.5 for lactate-pyruvate and 0.3-0.6 for lactate-glucose) and by tissue-oxygen partial pressure, and no differences were found between groups.

CONCLUSIONS

Our data suggest that dexmedetomidine, even if used for deep sedation, does not have deleterious effects on local perfusion or tissue metabolism in denervated musculocutaneous flaps.

Enkephalins and functionally specific vagal preganglionic neurons to the heart: Ultrastructural studies in the cat

In cat, distinct populations of vagal preganglionic and postganglionic neurons selectively modulate heart rate, atrioventricular conduction and left ventricular contractility, respectively. Vagal preganglionic neurons to the heart originate in the ventrolateral part of nucleus ambiguus and project to postganglionic neurons in intracardiac ganglia, including the sinoatrial (SA), atrioventricular (AV) and cranioventricular (CV) ganglia, which selectively modulate heart rate, AV conduction and left ventricular contractility, respectively. These ganglia receive projections from separate populations of vagal preganglionic neurons. The neurochemical anatomy and synaptic interactions of afferent neurons which mediate central control of these preganglionic neurons is incompletely understood. Enkephalins cause bradycardia when microinjected into nucleus ambiguus. It is not known if this effect is mediated by direct synapses of enkephalinergic terminals upon vagal preganglionic neurons to the heart. The effects of opioids in nucleus ambiguus upon AV conduction and cardiac contractility have also not been studied. We have tested the hypothesis that enkephalinergic nerve terminals synapse upon vagal preganglionic neurons projecting to the SA, AV and CV ganglia. Electron microscopy was used combining retrograde labeling from the SA, AV or CV ganglion with immunocytochemistry for enkephalins in ventrolateral nucleus ambiguus. Eight percent of axodendritic synapses upon negative chronotropic, and 12% of axodendritic synapses upon negative dromotropic vagal preganglionic neurons were enkephalinergic. Enkephalinergic axodendritic synapses were also present upon negative inotropic vagal preganglionic neurons. Thus enkephalinergic terminals in ventrolateral nucleus ambiguus can modulate not only heart rate but also atrioventricular conduction and left ventricular contractility by directly synapsing upon cardioinhibitory vagal preganglionic neurons.

Mu-opioid receptors are located postsynaptically and endomorphin-1 inhibits voltage-gated calcium currents in premotor cardiac parasympathetic neurons in the rat nucleus ambiguus

Activation of opioid receptors in the CNS evokes a dramatic decrease in heart rate which is mediated by increases in inhibitory parasympathetic activity to the heart. Injection of opiates into the nucleus ambiguus, where premotor cardiac parasympathetic nucleus ambiguus neurons are located elicits an increase in parasympathetic cardiac activity and bradycardia. However, the mechanisms responsible for altering the activity of premotor cardiac parasympathetic nucleus ambiguus neurons is unknown. This study examined at the electron microscopic level whether premotor cardiac parasympathetic nucleus ambiguus neurons possess postsynaptic opioid receptors and whether mu-opioid receptor agonists alter voltage-gated calcium currents in these neurons. Premotor cardiac parasympathetic nucleus ambiguus neurons were identified in the rat using retrograde fluorescent tracers. One series of experiments utilized dual-labeling immunocytochemical methods combined with electron microscopic analysis to determine if premotor cardiac parasympathetic nucleus ambiguus neurons contain mu-opioid receptors. In a second series of experiments whole cell patch clamp methodologies were used to determine whether activation of postsynaptic opioid receptors altered voltage-gated calcium currents in premotor cardiac parasympathetic nucleus ambiguus neurons in brainstem slices. The perikarya and 78% of the dendrites of premotor cardiac parasympathetic nucleus ambiguus neurons contain mu-opioid receptors. Voltage-gated calcium currents in premotor cardiac parasympathetic nucleus ambiguus neurons were comprised nearly entirely of omega-agatoxin-sensitive P/Q-type voltage-gated calcium currents. Activation of mu-opioid receptors inhibited these voltage-gated calcium currents and this inhibition was blocked by pretreatment with pertusis toxin. The mu-opioid receptor agonist endomorphin-1, but not the mu-opioid receptor agonist endomorphin-2, inhibited the calcium currents. In summary, mu-opioid receptors are located postsynaptically on premotor cardiac parasympathetic nucleus ambiguus neurons. The mu-opioid receptor agonist endomorphin1 inhibited the omega-agatoxin-sensitive P/Q-type voltage-gated calcium currents in premotor cardiac vagal nucleus ambiguus neurons. This inhibition is mediated via a G-protein mediated pathway which was blocked by pretreatment with pertusis toxin. It is possible that the inhibition of calcium currents may act to indirectly facilitate the activity of premotor cardiac parasympathetic nucleus ambiguus neurons by disinhibition, such as by a reduction in inhibitory calcium activated potassium currents.

The effects of propofol on heart rate, arterial pressure and adelta and C somatosympathetic reflexes in anaesthetized dogs

The effects of propofol on mean arterial pressure, heart rate and Adelta and C somatosympathetic reflexes, recorded in renal nerves, evoked by repeated individual supramaximal electrical stimuli applied to radial nerves, were observed in anaesthetized, paralysed and artificially ventilated dogs. Propofol was infused at rates from 0.4 to 2.0 mg kg-1 min-1. Mean C and Adelta reflexes were abolished at plasma concentrations (mean, SEM) of 24.3 (3.3) and 29.2 (2.6) microg mL-1 (P < 0.05), respectively, when mean arterial pressure and mean heart rate were reduced by approximately 55% (P < 0.01) and 26% (P > 0.05), respectively. Recovery of Adelta and C reflexes occurred at plasma concentrations of 13.1 (2.3) and 9.9 (1.3) microg mL-1 (P > 0.05), respectively. There was a log- arithmically linearly related fall in mean arterial pressure by 70% up to a plasma concentration approximately 97 microg mL-1 (r 2=0.7) with a 28% reduction in heart rate which was uncorrelated with the plasma concentrations (r 2=0.12). In conclusion, propofol abolished Adelta and C responses at comparable plasma concentrations and caused a major reduction in both mean arterial pressure and heart rate which is consistent with resetting of the baroreflexes. The reduction in mean arterial pressure was logarithmically, linearly correlated with a progressive increase in plasma concentrations without evidence of a ceiling effect.

Central effects of opioid agonists and naloxone on blood pressure and heart rate in normotensive and hypertensive rats

1. The central cardiovascular effects of several opioid receptor selective agonists and the nonselective opioid antagonist, naloxone, were studied in anesthetized normotensive control rats, in spontaneously hypertensive rats (SHR), and in foot-shock-stressed rats. 2. Receptor-selective agonists injected into the rostral ventrolateral medulla (RVLM), paraventricular nucleus (PVN), and dorsal hippocampus (dHip) were DAGO (mu), DADLE (delta), and U50,488H (kappa). 3. DAGO and DADLE (3 nM) decreased arterial pressure and heart rate in RVLM and PVN of all rat strains, while U-50,488H (9 nM) had only minimal effects in these areas. 4. In dHip, only DADLE (3 nM) had depressor and bradycardic effects, and then, only in SHR, with DAGO and U50,488H being ineffective in any strain, even at 9 nM. 5. Prior injection of naloxone (10 nM) into the RVLM, PVN and dHip blocked and postinjection reversed the cardiovascular effects of the agonists. Naloxone alone increased blood pressure and heart rate in all three areas, in all rat strains except SHR, suggesting a tonic depressor effect of endogenous opioids. 6. Lack of significant quantitative differences in opioid agonist and antagonist effects between normotensive and hypertensive or stressed rats argues against a role for endogenous brain opioids in experimental hypertension.

Mechanisms of chronic pain

Chronic pain differs from acute pain in that it serves no useful function, causes suffering, limits activities of daily living, and increases costs of healthcare payments, disability, and litigation fees. Pain perception begins with activation of peripheral nociceptors and conduction through myelinated A delta and unmyelinated C fibers to the dorsal root ganglion. From here, signals travel via the spinothalamic tract to the thalamus and the somatosensory cortex. Modulation of sensory input (i.e., pain) occurs at many levels. Nociceptors are also neuroeffectors, and transmission can be modulated by their cell bodies, which secrete inflammatory mediators, neuropeptides, or other pain-producing substances. Descending pathways from the hypothalamus, which has opioid-sensitive receptors and is stimulated by arousal and emotional stress, can transmit signals to the dorsal horn that modulate ascending nociceptive transmissions. Modulation to alter the perception of pain also can occur at higher centers (e.g., frontal cortex, midbrain, medulla) by opioids, anti-inflammatory agents, as well as antagonists and agonists of neurotransmitters. This article will review our current knowledge of the mechanisms involved in (1) the transduction of tissue injury or disease signals (nociception and nociceptive receptors); (2) the transmission of signals rostrally to the thalamus and higher nervous system centers (involving perception of the quality, location, and intensity of noxious signals); and (3) the modulation of ascending sensory messages at all levels (periphery, spinal cord, and higher centers).

Principles of opioid pharmacotherapy: practical implications of basic mechanisms

Opioids form the cornerstone of the pharmacologic armamentarium for the treatment of pain. Despite their long history of use, much confusion and misperception still surrounds their use. This short review will focus on pharmacodynamic and physiologic considerations in the clinical use of oral and parenteral opioids.

The effect of a cyclodextrin vehicle on the cardiovascular profile of propofol in rats

We studied the aqueous solution of propofol dissolved in hydroxypropyl-beta-cyclodextrin (HP beta CD) 20% to determine whether the cardiovascular profile differed from that measured for propofol prepared in Intralipid 10% (Diprivan). Conscious male rats were given an intravenous bolus of propofol, 5.0 mg/kg, the minimum dose that induces a loss of righting. Immediately severe bradycardia occurred which was the result of a combination of sinus arrest and atrioventricular block; a significant decrease of blood pressure resulted. A bolus of HP beta CD produced no significant changes in heart rate rhythm. The severe bradycardia produced by propofol in HP beta CD was blocked by both atropine and bilateral cervical vagotomy. Therefore, the effects of propofol in HP beta CD are cholinergic and neurally mediated. These results are consistent with the hypothesis that propofol reduces sympathetic tone prior to reduction in vagal tone, and thereby produces a period of time during which vagal tone is dominant.

What are the roles of substance P and neurokinin-1 receptors in the control of negative chronotropic or negative dromotropic vagal motoneurons? A physiological and ultrastructural analysis

Recent data indicate that there is a cardiotopic organization of negative chronotropic and negative dromotropic neurons in the nucleus ambiguus (NA). Negative dromotropic neurons are found in the rostral ventrolateral NA (rNA-VL), negative chronotropic neurons are found in the caudal ventrolateral NA (cNA-VL), and both types of neurons are found in an intermediate level of the ventrolateral NA (iNA-VL). Substance P (SP) immunoreactive nerve terminals synapse upon negative chronotropic vagal motoneurons in the iNA-VL, and SP microinjections in the NA cause bradycardia. In the present report we have attempted to: (1) define the type of tachykinin receptor which mediates the negative chronotropic effect of SP microinjections into the iNA-VL; (2) define the physiological effect of microinjections of a selective SP agonist into the rNA-VL on atrioventricular (AV) conduction: and (3) find ultrastructural evidence for synaptic interactions of SP-immunoreactive nerve terminals with negative dromotropic vagal motoneurons in the rNA-VL. Microinjections of the excitatory amino acid glutamate (Glu) into the iNA-VL to activate all local vagal preganglionic neurons caused both bradycardia and a decrease in the rate of AV conduction. Injections of the selective neurokinin-1 (NK-1) receptor agonist drug GR-73632 also caused bradycardia, however the rapid onset of agonist induced desensitization prevented an evaluation of potential effects on AV conduction in the iNA-VL. These data suggest that the SP-induced bradycardia which can be elicited from the NA is mediated, at least in part, by NK-1 receptors. Microinjections of Glu into the rNA-VL caused a decrease in AV conduction without an effect on cardiac rate. On the other hand, GR-73632 microinjections into rNA-VL did not affect AV conduction. Following injections of the beta subunit of cholera toxin conjugated to horseradish peroxidase (CTB-HRP) into the left atrial fat pad ganglion which selectively mediates changes in AV conduction, retrogradely labeled neurons were histochemically visualized in the rNA-VL. These tissues were subsequently processed for the simultaneous immunocytochemical visualization of SP, and examined by electron microscopy. Histochemically labeled neurons were large, multipolar, with abundant cytoplasm containing large masses of rough endoplasmic reticulum, and exhibited distinctive dendritic and somatic spines. Unlabeled nerve terminals were noted to form either asymmetric or symmetric synapses with dendrites, dendritic spines, and perikarya of histochemically labeled neurons. SP-immunoreactive nerve terminals were also detected in the rNA-VL. SP terminals typically contained numerous small pleomorphic vesicles, multiple large dense core vesicles, and several mitochondria, and they synapsed upon unlabeled dendritic profiles. A total of 154 SP-immunoreactive nerve terminals were observed on photomicrographs of tissues which also contained histochemically labeled profiles. None made an identifiable synapse with a retrogradely labeled profile on the sections examined. In summary, both physiological and ultrastructural data indicate that SP terminals in the iNA-VL do modify the output of negative chronotropic vagal motoneurons. This effect is mediated by NK-1 receptors. On the other hand both physiological and ultrastructural data indicate that SP terminals in the rNA-VL do not modify the output of negative dromotropic vagal motoneurons. Therefore different mechanisms (neurotransmitters or receptors) mediate the central vagal control of cardiac rate and AV conduction.

Methoctramine, a cardioselective muscarinic cholinergic antagonist, prevents fentanyl-induced bradycardia in the dog

A controlled study examining the effects of the cardioselective muscarinic cholinergic antagonist methoctramine on fentanyl-induced bradycardia was performed in six dogs. Five doses of methoctramine (6, 10, 20, 30 and 60 micrograms/kg) followed by fentanyl (20 micrograms/kg) were administered randomly on separate days. Fentanyl caused a significant reduction in heart rate from baseline values. Moreover, fentanyl produced a variety of arrhythmogenic actions indicative of vagal hyperactivity, including sinus bradycardia, second-degree atrioventricular block and ventricular and supraventricular escape beats. Administration of methoctramine 5 min before fentanyl injection prevented the bradycardic effects of fentanyl in a dose-dependent manner, with high doses of methoctramine causing sinus tachycardia. Using regression analysis, the dose of methoctramine necessary to prevent fentanyl-induced bradyarrhythmias without causing tachycardia was calculated as 14.4 micrograms/kg. The study confirmed that fentanyl administration in the conscious dog causes profound bradycardia with bradyarrhythmias. The cardioselective muscarinic antagonist agent methoctramine prevented the bradycardic effects of fentanyl.

Fentanyl pretreatment attenuates the haemodynamic response to sudden inhalation of 5% isoflurane

High concentrations of inhaled isoflurane can increase heart rate and/or arterial pressure. The purpose of this study was to determine whether fentanyl has a prophylactic effect on the isoflurane-induced circulatory response in adult patients. Thirty patients due to undergo elective surgery were randomly allocated to one of three groups of ten patients. Prior to surgery, one group inhaled 2.5% isoflurane, another inhaled 5.0% isoflurane, and the third group inhaled 5.0% isoflurane and were given fentanyl 2 micrograms.kg-1 i.v. two minutes before induction of anaesthesia. Anaesthesia was induced with thiamylal followed by vecuronium. The lungs were ventilated with 100% oxygen and either 2.5% or 5.0% isoflurane via face mask. Ventilation was continued for five minutes. Heart rate (HR) and mean arterial pressure (MAP) were recorded at two minutes before induction of anaesthesia (baseline), immediately before the induction of anaesthesia, and at three and five minutes after induction, respectively. It was found that 5.0% isoflurane caused an increase in HR compared with baseline (P < 0.01) and with the 2.5% isoflurane (P < 0.05): 2.5% isoflurane did not elicit this response. An increase was also noted in MAP, compared with the 2.5% isoflurane (P < 0.01): 2.5% isoflurane did not elicit this increase. Fentanyl pretreatment attenuated the increases in HR and in MAP that occurred with 5.0% isoflurane (P < 0.01). These results suggest that fentanyl attenuates the enhancement of both HR and MAP from face mask inhalation of a high concentration of isoflurane.

Effect of ICI197067, a kappa-opioid receptor agonist, spinally on A delta and C reflexes and intracerebrally on respiration

Intrathecal (i.t.) injection of a kappa-opioid receptor agonist, ICI197067, caused a similar dose dependent depression of A delta and C fibre mediated nociceptive reflexes in renal sympathetic nerves due to supramaximal electrical stimulation of tibial nerves in anaesthetized dogs. A total dose of 8 mg i.t. abolished these reflexes. When administered into the 4th ventricle (i.c.v.) in a total dose range from 0.1-2.5 mg ICI197067 caused no respiratory depression; a total dose of 10 mg i.c.v. abolished both phrenic nerve activity and spontaneous respiration. The ED50 ratio of ICI197067 for depression of respiration (i.c.v.) and somatosympathetic reflexes (i.t.) is approximately 1.5:1 compared with 0.3:1 for fentanyl. ICI197067 i.c.v. caused a similar reduction in arterial pressure compare to fentanyl without comparable changes in heart rate. Thus in terms of cardiorespiratory depression and blockade of A delta and C fibre pathways kappa-opioid receptor agonists may be safer and more effective for producing spinal analgesia than mu-opioid receptor agonists.

The calcium antagonist diltiazem has antiarrhythmic effects which are mediated in the brain through endogenous opioids

The purpose of this study was to examine the hypothesis that the calcium channel blocker, diltiazem, modulates catecholamine-induced arrhythmias through CNS mechanisms. Rats, that had catheters previously inserted into the lateral cerebral ventricle and femoral artery, received diltiazem, 10 or 50 micrograms/kg or the diluent, into the lateral cerebral ventricle (i.c.v.). Epinephrine was infused to produce arrhythmias. The onset of ventricular arrhythmias, premature ventricular complexes, occurred at a significantly (P less than 0.05) greater dose of epinephrine, after diltiazem, compared to the control group and in a dose-dependent manner, with the mean (+/- 1 SEM) dose of epinephrine being 198 +/- 5, 175 +/- 13 and 115 +/- 15 micrograms/kg in the groups treated with 50 and 10 micrograms/kg of diltiazem and the control groups, respectively. The development of fatal arrhythmias, mainly ventricular tachyarrhythmias, occurred at significantly (P less than 0.05) greater concentrations of epinephrine with diltiazem, 50 and 10 micrograms/kg, 225 +/- 5 and 183 +/- 13 micrograms/kg, respectively, compared to controls, 131 +/- 15 micrograms/kg. Endogenous opioids of the mu-type were implicated in this action of diltiazem, because the mu opioid antagonist naloxone, 1 mg/kg (i.v.), significantly (P less than 0.05) antagonized the antiarrhythmic effects of centrally administered diltiazem and the mu opioid agonist DAGO (i.c.v.), did not further enhance the suppression of epinephrine-induced arrhythmias, produced by diltiazem, 50 micrograms/kg. Atropine sulfate, which crosses the blood-brain barrier and atropine methylnitrate, which does not enter the brain, each at 1 mg/kg (i.v.), produced an equal and significant antagonism of the effect of diltiazem, 50 micrograms/kg, that was less than that of naloxone. The combination of naloxone plus atropine sulfate completely prevented the effect of diltiazem, 50 micrograms/kg, on arrhythmias. The antiarrythmic action of diltiazem could not be explained by alteration of the blood pressure or heart rate response to epinephrine. The results suggest that: (a) calcium channels on neurons in the CNS play an important role in the modulation of epinephrine-induced cardiac arrhythmias, (b) diltiazem can suppress arrhythmias through CNS mechanisms, (c) activation of the parasympathetic nervous system mediates some of the effect of diltiazem, but (d) the mechanism of action of diltiazem is modulated through endogenous opioids.

Alterations in Na(+)-K(+)-ATPase activity and beta-endorphin content in acute ischemic brain with and without naloxone treatment

The Na(+)-K(+)-adenosine triphosphatase (Na(+)-K(+)-ATPase) activity and beta-endorphin immunoreactivity were determined in rat brain at the acute stage of ischemia produced by unilateral occlusion of the middle cerebral artery (MCA). The effect of pretreatment with naloxone on these activities was also evaluated in the same model. After MCA occlusion, Na(+)-K(+)-ATPase activity was promptly reduced in the ischemic hemisphere and remained at a lower level than in the contralateral hemisphere during 90 minutes of ischemia. A single intraperitoneal 0.5-mg injection of naloxone prior to MCA occlusion attenuated the inactivation. On the other hand, beta-endorphin immunoreactivity was significantly increased following ischemia. The increase was marked in the ischemic hemisphere and was also observed in the contralateral hemisphere; this increase was not affected by the administration of naloxone. These results indicate the possibility that naloxone contributes to protecting the brain from ischemia through stabilizing the cellular membrane. The possible mechanism by which naloxone attenuates the inactivation of Na(+)-K(+)-ATPase in the ischemic brain is discussed in view of alterations of the central beta-endorphin system during ischemia.

Renal sympathetic nerve activity during morphine abstinence in sino-aortic baroreceptor-denervated rats

This study was undertaken to investigate the influence of the arterial baroreceptors on the response of renal sympathetic nerve activity (rSNA) during naloxone-precipitated morphine abstinence in rats. In chronically baroreceptor-denervated, morphine-dependent rats, rSNA, mean arterial pressure (MAP) and heart rate (HR) were studied before and after repeated i.v. bolus doses of naloxone (0.005-5 mg kg-1), during chloralose anaesthesia or in the conscious state. In the anaesthetized animals, naloxone doses of 0.05-5 mg kg-1 caused a pronounced inhibition of rSNA, reaching a level 61% below pre-naloxone activity. This was accompanied by increases in MAP and HR. In the conscious rats, the lower doses of naloxone elicited an initial state of increased somatomotor activity. This was paralleled by slight increases in rSNA and MAP. After 4-5 min, the behavioural excitation faded and was replaced by lethargy. The rats exhibited still signs of withdrawal in the form of piloerection, chromodacryorrhoea and defaecations. Concomitantly, rSNA returned towards the pre-naloxone level, while MAP showed a sustained increase. The higher naloxone doses exacerbated the hypertension without any further changes in rSNA or in behaviour. We conclude that the influence of the baroreceptors is of minor significance for the inhibition of rSNA during naloxone-precipitated abstinence in anaesthetized rats. In conscious, intact rats, however, the baroreceptors seem to contribute to rSNA inhibition since no significance decrease of rSNA occurred in baroreceptor-denervated rats in the present study. This is in contrast to our previous finding of a marked inhibition of rSNA in rats with intact baroreceptors.

Central haemodynamics during morphine abstinence in anaesthetized rats

Central haemodynamics were studied in one group of morphine-dependent rats, and in a non-dependent control group, before and after administration of repeated bolus doses of naloxone. Dependence was induced by s.c. morphine pellet implantations. Mean arterial pressure (MAP), heart rate (HR), cardiac output (CO) and mean transit time (MTT) were measured in the conscious state, after induction of chloralose anaesthesia and after the administration of naloxone (0.005, 0.05, 0.5 and 5 mg kg-1 i.v.). Total peripheral resistance (TPR), stroke volume (SV) and central blood volume (CBV) were subsequently calculated. The haemodynamic variables did not differ significantly in the conscious state, except for a lower SV, when compared with the non-dependent control group. However, in response to anaesthesia the dependent rats exhibited a greater fall in MAP, mainly due to a TPR decrease. Naloxone elicited a marked increase in MAP in the morphine-dependent group, which was mainly caused by an increase in TPR. Naloxone induced no significant change compared with the control group in CO and CBV, while SV increased concomitantly with a lowered HR after naloxone in the morphine-dependent group. These results suggest that the withdrawal hypertension during morphine abstinence was mainly explained by an increase in TPR, reflecting an augmented tone of the resistance vessels. The minor changes in CBV indicate that the tone of the venous capacitance vessels was largely unaffected by naloxone-induced morphine abstinence.

Depressor and bradycardic effects induced by spinal subarachnoid injection of D-Ala2-D-Leu5-enkephalin in rats

The effects of D-Ala2-D-Leu5-enkephalin (DADLE), a specific delta receptor agonist, on spinal control of cardiovascular function, were investigated by its intrathecal (i.th) injection into the spinal subarachnoid space at the T-9 level. In chloralose-anesthetized rats, DADLE (17.5, 35 and 70 nmol, i.th) caused dose-dependent hypotension and bradycardia. The mean maximal hypotension by 70 nmol of DADLE was -45 +/- 7 mmHg, with a bradycardia of -79 +/- 15 beats/min. These inhibitory cardiovascular effects were antagonized by the opiate antagonist naloxone (50 nmol, i.th.) given prior to DADLE. Intrathecal injection of DADLE also decreased splanchnic sympathetic nerve discharge (-46 +/- 5%). DADLE (70 nmol) given i.v. did not cause significant changes in mean arterial pressure (MAP) and heart rate (HR). Neither bilateral vagotomy nor pretreatment with atropine (0.2 mg/kg, i.v.) prevented the BP and HR effects of intrathecal injection of DADLE at a dose of 35 nmol. DADLE at this dose failed to produce significant alteration in the frequency of respiration and blood PaO2, PaCO2 and blood pH. In conscious rats, 140 nmol of DADLE (i.th.) did not produce any consistent changes in MAP and HR. These data suggest that intrathecal injection of DADLE inhibits central sympathetic activity, possibly at a spinal locus.

Effect of parasympathetic blockade on ventilatory and cardiac depression induced by opioids

We have previously shown that delta-opioid agonists decrease ventilation and heart rate. Because of these results and the known interactions between opioid and acetylcholine metabolism, we hypothesized that opioids induce cardiorespiratory changes via the parasympathetic nervous system. To test this hypothesis, we administered atropine sulfate (systemically) at maximal effect of D-Ala-D-Leu-enkephalin (DADLE; a preferential delta-opioid agonist), injected intracisternally, and examined its effect on cardiorespiratory function. All experiments were performed on chronically instrumented and conscious adult dogs. Mean instantaneous minute ventilation or VT/TTOT decreased and PaCO2 increased after DADLE; atropine had little effect on these changes. Naloxone, even in small doses, reversed opioid effects on VT/TTOT and PaCO2. Atropine, however, reversed the DADLE-induced depression in cardiac rate. In doses that reversed this cardiac depression, atropine had no effect on cardiorespiratory function at rest, i.e., with no prior administration of DADLE. We conclude that DADLE decreases heart rate by increasing parasympathetic activity to the heart and induces hypoventilation by a different mechanism. We speculate that the opioid-induced ventilatory depression is due to either direct opioid action on central respiratory regulation or parasympathetic non-muscarinic or non-cholinergic mediating mechanisms.

Selective tolerance of group III and IV somatosympathetic reflexes to the effects of alfentanil

The effect of alfentanil on responses in renal sympathetic nerves evoked by supramaximal electrical stimulation of the radial nerve, has been observed in 6 dogs anaesthesized with alpha-chloralose, paralysed with suxamethonium and ventilated artificially. During an initial infusion of alfentanil the responses of the late group IV (C fibre) and early group III (A delta) were abolished by mean doses of 68 micrograms kg-1 (SEM 3.2 micrograms kg-1) and 797 micrograms kg-1 (SEM 120 micrograms kg-1), respectively. Recovery was allowed to occur to approximately 50% of control values (mean time 76 +/- 14.3 min). The preparations were then conditioned with 7 incremental doses from 7.5 to 120 micrograms kg-1 (i.v.) (total dose 308.5 micrograms kg-1), administered at intervals of 10 min, and subsequently tested with large bolus doses (up to 2000 micrograms kg-1) of alfentanil. In two preparations, the responses of both group IV and group III became completely tolerant to the effects of alfentanil while in the other four the response of the group IV was still eliminated by the drug and the response of group III showed selective tolerance. The heart rate and arterial pressure were reduced by 45 and 29%, respectively during the initial infusion of alfentanil. Thereafter there were no further significant changes in the circulation until the administration of naloxone (2 mg i.v.), which restored the sympathetic responses, heart rate and arterial pressure to control values.

The influence of fentanyl on the cardiovascular effect of suxamethonium

The influence of fentanyl on the cardiovascular effect of a single dose of suxamethonium was evaluated during thiopentone N2O/O2 anaesthesia. Sixty adult patients were randomly allocated to three groups. In one group (control group) no fentanyl was given. In two groups fentanyl 0.002 mg/kg and 0.004 mg/kg, respectively, was given before induction of anaesthesia. Three minutes after the injection of suxamethonium 1 mg/kg, both heart rate (HR) and mean arterial pressure (MAP) in the two groups receiving fentanyl were significantly lower than in the control group. However, the absolute changes were small and no dose-related effect was seen. An estimation of the change in HR following suxamethonium was made using as predictors initial HR before induction of anaesthesia, age, sex, and the injection of fentanyl. The higher the initial HR, the greater was the chance of a decrease in HR following suxamethonium. Injection of fentanyl lowered the threshold HR above which a decrease was to be expected following injection of suxamethonium. This effect was most pronounced among the younger patients. It is concluded that injection of fentanyl potentiates the decrease in HR sometimes seen after a single dose of suxamethonium and that this effect is influenced by HR before induction of anaesthesia and by age of the patient.

Effects of clonidine, alpha-methyldopa and hydralazine on met-enkephalinergic neurons in cerebral nuclei of spontaneously hypertensive rats

Immunocytofluorescent and microautoradiographic methods were applied to measure met-enkephalin-like immunoreactivity (MELI) and ME receptor binding (MERB) levels in cerebral nuclei of serial brain slices from SHR which received clonidine, alpha-methyldopa and hydralazine at equivalent hypotensive doses. All three drugs increased both MELI and MERB levels in the caudal part of the n. tractus solitarii and its functionally related dorsomedullary nuclei and decreased them in the n. accumbens septi, in accordance with the correspondent change in glucose utilization rates in these nuclei as reported previously. Both CNS-active agents (not hydralazine) also increased MELI and MERB levels in the n. intercalatus and substantia grisea centralis, and they decreased them in the n. ventromedialis hypothalami. Differences in both CNS-active agents were minor. Vasodilative hydralazine alone decreased these levels both in the n. reticularis medialis and n. tegmenti ventralis, and it increased them slightly in the area lateralis hypothalami. The present studies indicate that ME neurons of these dorsomedullary and supramedullary nuclei may act as direct as well as homeostatic controls of blood pressure.

[D-Met2,Pro5]enkephalinamide activates cardioinhibitory efferents in anaesthetized dogs

[D-Met2,Pro5]enkephalinamide (DMPEA) strongly activated cardioinhibitory vagal efferents and elicited bradycardia when perfused through the cerebroventricular system of anaesthetized dogs. These effects were concentration-dependent but plateaued at an intraventricular concentration of 200 micrograms/ml. At this maximally effective concentration, the vagal discharge rate was fourfold higher and the heart rate was lowered by 28% compared to the controls. These effects were reversed by naloxone (40 micrograms/ml). There was no significant change in blood pressure. Vagal discharge rate and heart rate correlated closely and inversely with each other with r values ranging between -0.75 and -0.95. Opiate receptors in brain structures bordering the cerebroventricular system are the most likely mediators of the effects of DMPEA. The opiate receptor/endorphin system may therefore play a role in the physiological control of cardiac vagal tone and thus of heart rate.

Reduction by phentolamine of the hypotensive effect of methionine enkephalin in anaesthetized rabbits

In intact rabbits anaesthetized with pentobarbitone, methionine enkephalin (Met enkephalin, 1-1,000 micrograms kg-1 i.v.) produced a dose-dependent bradycardia and hypotension. The bradycardia and hypotension were antagonized by naloxone hydrochloride (1 mg kg-1), but not by naloxone methobromide (1.3 mg kg-1). Phentolamine (1 and 4 mg kg-1 i.v.) blocked both the hypotension and bradycardia produced by Met enkephalin. The inhibitory effect of phentolamine was not due to a simple hypotensive action of this drug per se because a similar degree of hypotension induced by nitroprusside (15 micrograms kg-1, i.v.) caused a further reduction of pressure when Met enkephalin was added. Atropine (2 mg kg-1) reduced the bradycardia but not the hypotensive response to Met enkephalin. Met enkephalin did not antagonize the vasopressor effect of exogenous noradrenaline (2-8 micrograms kg-1, i.v.). Met enkephalin had no significant effects in superfused thoracic aortic strips and in isolated perfused hearts of rabbits. It is concluded that the cardiovascular effects of Met enkephalin are more probably due to an action on the central nervous system, although a peripheral site of action cannot be completely excluded.

Central cardiovascular actions of D-Ala2-Met5-enkephalinamide in the rat: effects of naloxone and nucleus reticularis gigantocellularis lesion

In pentobarbital-anesthetized rats, intraventricular administration of D-Ala2-Met5-enkephalinamide (D-Ala, 30, 100 or 300 nmol/kg) dose-dependently elicited a reduction in arterial pressure. This D-Ala-promoted hypotension was significantly antagonized by naloxone pretreatment (3 mg/kg, i.c.v.) and attenuated by bilateral focal nucleus reticularis gigantocellularis (NRGC) lesions. At higher doses, D-Ala also produced a delayed hypertension that was not only unaffected by naloxone or NRGC lesions, but was in fact potentiated by such pretreatments. We speculate that D-Ala may produce its cardiovascular effect by activating separate subclasses of opiate receptors, possibly at different neural substrates that include the NRGC.

Depression by morphine and levorphanol of activity in sympathetic nerve fibres in anaesthetized rats

In urethane-anesthetized rats, the effects of intravenous injections of morphine, levorphanol, dextrorphan, pentazocine and naloxone were studied studied on the activity in nerve fibres of the cervical sympathetic trunk, and on mean arterial blood pressure and heart rate. Impulse frequency in sympathetic nerve fibres was recorded with tungsten microelectrodes and proved to be more sensitive to drug action than blood pressure or heart rate. Morphine 1 and 2 mg/kg dose dependently reduced sympathetic impulse frequency, blood pressure and heart rate; morphine 0.5 mg/kg was ineffective. Levorphanol 1 and 2 mg/kg dose dependently reduced sympathetic impulse frequency and blood pressure but did not affect heart rate. Dextrorphan (the dextro-isomer of levorphanol) 2 and 4 mg/kg had no effect on the parameters tested. Pentazocine 3 and 6 mg/kg did not cause a consistent change in sympathetic impulse frequency, blood pressure and heart rate. Naloxone 0.2 mg/kg abolished the depressant effects of morphine and levorphanol and, when given alone, increased sympathetic impulse frequency. Naloxone 1 mg/kg increased blood pressure but did not affect heart rate. It is concluded that morphine can reduce blood pressure and heart rate by causing opiate-specific central sympathetic depression.

Co-localization of CRF-ir perikarya and ACTH-ir fibers in rat brain

The distribution of corticotropin-releasing factor immunoreactive (CRF-ir) perikarya and ACTH-ir fibers was examined immunocytochemically, in adjacent sections, in the forebrain and brainstem of the rat. Throughout the nervous system, a remarkable concordance of localization of these neuropeptide systems was noted. Both ACTH-ir fibers and CRF-ir perikarya were conjointly distributed within discrete hypothalamic, limbic and brainstem/autonomic regions previously demonstrated to contain opiate receptors and opiocortin (beta-endorphin, beta-LPH, ACTH) fibers. In view of the demonstrated interactions of CRF with the peripheral (pituitary) opiocortin system, these data suggest the possibility of a similar relationship of CRF and opiocortin systems in the central nervous system as well.

Differential effects of clonidine on pain, arterial blood pressure, and heart rate in the cat: lack of interactions with naloxone

In cats anesthetized with alpha-chloralose and urethane, intravertebral administration of clonidine (4 and 10 micrograms/kg) dose-dependently suppressed the jaw-opening reflex, arterial blood pressure, and heart rate. For a given dose, there was a differential degree of inhibition in the order of analgesia much greater than hypotension greater than bradycardia. Naloxone injections (0.4 and 1.0 mg/kg, i.vert.) essentially failed to antagonize these effects, suggesting the lack of involvement of the opiate receptors or endogenous opioids in these processes. Furthermore, pain suppression by clonidine appeared to be independent of the vasodepression and cardioinhibition it promoted. It is possible that neural mechanisms responsible for clonidine-induced antinociception, hypotension, and bradycardia are likely to have differential sensitivities to the imidazoline compound, regardless of whether they exist in separate central sites or in subpopulations of neurons within common neural substrates.

Selective cardiorespiratory effects mediated by mu opioid receptors in the nucleus ambiguus

The respiratory and cardiovascular effects of the highly selective mu opioid agonist, D-Ala2, MePhe4, Gly- ol5 enkephalin ( DAGO ) and the relatively selective delta agonist, D-Leu5 enkephalin (DADL) were compared following injection (0.1 microliter) into the nucleus ambiguus (NA) of spontaneously-breathing and artificially-respired, pentobarbital-anesthetized rats. In non-ventilated animals, the opioids elicited dose-related (3 X 10(-11) -3 X 10(-9) M), naloxone-reversible depression of respiratory rate (RR) without altering the tidal volume. Mean arterial pressure (MAP) was unchanged at small doses and decreased at the largest dose; heart rate (HR) was unchanged. In artificially-respired animals, both peptides elicited dose-related, naloxone-reversible increases in mean arterial pressure and heart rate; DAGO was significantly more potent than DADL (P less than 0.01). Given the relative potency and selectivity of the opioids tested, these findings are consistent with the conclusion that mu receptors may selectively mediate the respiratory and cardiovascular actions of opioids in an important brain stem cardiorespiratory center in the rat. Moreover, these data indicate the importance of respiratory effects on the cardiovascular activity of centrally administered opioids.

Relief of pain in intensive care patients

Many critically ill patients suffer pain which can produce by itself undesirable effects. Consequently, pain must be carefully prevented, or at least, treated early and effectively. Basal analgesia can be provided by repeated intramuscular administration of narcotics, or rather by continuous intravenous infusion of morphine or meperidine or by a regional anesthetic procedure such as an epidural block. Computer-assisted intravenous "on demand" analgesia with Fentanyl can also be used. When pain coverage is required during transient events such as active physiotherapy or dressing changes, additional intravenous of a narcotic (1-2 mg morphine e.g.) or inhalation of nitrous oxide with oxygen are usually effective.

Increases in heart rate and blood pressure produced by injections of dermorphin into discrete hypothalamic sites

Central cardiovascular sites of action for dermorphin were determined by injecting 40 pmol of the peptide into discrete sites within the hypothalamus of halothane anesthetized rats. Blood pressure and heart rate in 101 rats were 88 +/- 1 mm Hg and 338 +/- 3 beats/min, respectively, prior to 100 nl intrahypothalamic injections of either vehicle or dermorphin. In the caudal anterior hypothalamic nucleus (A5800-5300), dermorphin, but not vehicle, increased blood pressure 8% and heart rate 26%, without changing respiration rate. The peak response was at 32 +/- 5 min, the duration greater than 90 min. Injections of naloxone (30 nmol) into the same anterior hypothalamic site, or 3 mg/kg naloxone administered i.m., completely reversed the cardiovascular actions. Similar increases in heart rate and blood pressure occurred at A6600-6300, the region between medial preoptic and anterior hypothalamic nuclei. Small increases in heart rate, but not blood pressure, resulted from dermorphin injections into the septal area, medial preoptic nucleus, paraventricular hypothalamic nucleus and the lateral ventricle, while injections in the posterior and dorsomedial hypothalamic nuclei were without effect on blood pressure and heart rate. These data provide support for anterior hypothalamic and medial preoptic sites for the cardiovascular actions of the opiate receptor agonist, dermorphin, and indicate greatest effects with this dose (primarily on heart rate) are produced at discrete sites (A6600-6300 and A5800-5300) within these nuclei.

Lateral reticular formation as a site for morphine- and clonidine-induced hypotension

The possible role of the lateral reticular formation (LRF) in morphine-induced hypotension was investigated. Morphine (5-10 micrograms) microinjected into 15 of 25 sites in the LRF of anesthetized cats reduced mean (+/- S.E.) arterial pressure by 18.4 +/- 2.1 mmHg. Morphine applied microiontophoretically on LRF neurons had a predominately inhibitory effect on LRF cells, decreasing spontaneous rate in 51%. Since the LRF has been suggested as a site of action for clonidine-induced hypotension, clonidine was also applied microiontophoretically to these neurons. Of the cells evaluated, 43% were inhibited by clonidine. Individual cells typically responded in the same manner to morphine and clonidine. An inhibition of LRF cells by morphine and clonidine is consistent with their hypotensive action and the tonic vasopressive role assigned to the LRF. Collectively, these results suggest that the LRF is a site at which morphine and clonidine may produce their hypotensive effect.

Proanaphylactic action of morphine is mediated through a central cholinergic mechanism

Intracerebroventricular (i.c.v.) or intravenous (i.v.) administration of morphine (10 micrograms or 25 mg/kg, respectively) increased the mortality rate in experimental anaphylaxis in mice. This effect was blocked by the opiate antagonist naltrexone administered systemically (5 mg/kg) or i.c.v. (10 micrograms). Moreover, the effect of morphine was blocked by i.c.v. administration of the muscarine antagonist methyl atropine (10 micrograms). The systemic administration of 5 mg/kg methyl atropine was ineffective in blocking the action of morphine. The results suggest that the proanaphylactic effect of morphine involves activation of cholinergic mechanisms in the central nervous system.

Origin of the hypotensive and sympathoinhibitory effect of morphinomimetic agents

In vagotomized dogs, lesions of the lateral reticular nucleus (LRN) region did not change arterial blood pressure and sympathetic nerve activity. The hypertensive and tachycardic responses elicited by section of both carotid sinus nerves and both vagus nerves were not altered. However the sympathoinhibitory component of the baroreflex arc was attenuated. These lesions also abolished the hypotensive and sympathoinhibitory effects of fentanyl (10 micrograms/kg i.v.). In debuffered dogs, bilateral lesions of the LRN regions attenuated the effects of fentanyl (20 micrograms/kg i.v.) on arterial blood pressure and splanchnic discharges. Microinfusion of naloxone into the LRN reduced the effects of fentanyl (10 micrograms/kg) administered intravenously. Microinfusion of [D-Ala2,Met5]enkephalinamide (0.1-1 microgram) into the LRN region produced a dose dependent decrease in arterial blood pressure and sympathetic nerve activity. Lesions of the nucleus ambiguus (NA) region also reduced the effects of fentanyl. In contrast, lesions placed in the ventral median reticular formation or above the NA were ineffective in changing the sympathoinhibitory effect of fentanyl. These results indicate that the lateral reticular formation ventral to the nucleus ambiguus including the lateral reticular nucleus region is involved in the sympathoinhibitory effect of morphinomimetic agents.

Selective effect of fentanyl on group III and IV somatosympathetic reflexes

The effect of fentanyl on sympathetic reflexes evoked by supramaximal electrical stimulation of the radial nerve, and the subsequent reversal of its effects by naloxone, have been observed in 10 dogs anaesthetized with alpha-chloralose, paralysed with suxamethonium and artificially ventilated. During infusions of 5 micrograms kg-1 min-1 the late, long-latency, sympathetic response evoked by unmyelinated fibres was abolished at a mean dose of 27 micrograms kg-1 (SD 12.6 micrograms kg-1) after which the early, short-latency response evoked by small myelinated fibres was eliminated at a mean dose of 90.3 micrograms kg-1 (SD 54.6 micrograms kg-1) so that there was no longer any response to stimulation of the radial nerve. During a subsequent infusion of naloxone (200 micrograms min-1) the late response returned to control values at a mean dose of 0.5 mg and subsequently the early response reappeared to return to control values at a total dose of 1.6 mg. In 2 preparations phrenic nerve activity was abolished after 6.1 and 17.4 micrograms kg-1 of fentanyl and returned immediately before the late response, during the infusion of naloxone. In 2 preparations, induced tolerance occurred so that the early response could not be eliminated.

Cardiovascular responses to opioid agonists injected into the nucleus of tractus solitarius of anesthetized cats

It has been proposed that various opiate receptor subtypes mediate different cardiovascular responses to centrally administered opioids. We evaluated this hypothesis in chloralose-urethane anesthetized cats by monitoring the cardiovascular and respiratory responses to relative mu [morphine, morphiceptin, D-Ala2, MePhe4, Gly-ol5 enkephalin (DAGO)] and delta [D-Ala2, D-Leu5enkephalin (DADL)] agonists microinjected (0.5 ul/kg) into the caudal region of the Nucleus of Tractus Solitarius (NTS). Dynorphin (1-13), an endogenous opioid which exhibits selective affinity towards the kappa receptor, was also tested. Dynorphin at a dose of 50 nMol/kg did not alter cardiovascular or respiratory variables. Morphine (10-54 nMol/kg) and DAGO (50 nMol/kg) had no effect on blood pressure, heart rate or respiratory rate; morphiceptin (100-320 nMol/kg) caused tachycardia only at the highest dose. DADL (10-100 nMol/kg) elicited a dose-dependent depression of blood pressure. High doses of DADL depressed heart rate and respiratory rate. The depressor effects of DADL were reversed by low doses of naloxone (0.1 mg/kg). This dose of naloxone also elicited pressor responses in cats treated with the other opioids and reversed the morphiceptin-induced tachycardia. These data indicate that opioid agonists differ with regard to their cardiovascular and respiratory effects following microinjection into the NTS of anesthetized cats, with the delta agonist DADL showing greatest activity.