CRF-evoked bradycardia in urethane-anesthetized rats is blocked by naloxone

CRF, injected IV at a dose of 6 nmol/kg, produced a fall in blood pressure and in heart rate in urethane-anesthetized rats. The CRF-bradycardia was not obtained in hypophysectomized animals, in animals pretreated with dexamethasone, or in animals pretreated with the narcotic antagonist, naloxone (1 mg/kg, IV). By contrast, the hypotensive effects of CRF were not affected by these procedures. Vagotomy or pretreatment with a low dose of N-methylnaloxone did not affect the CRF-bradycardia, indicating that the slowing of the heart was not due to parasympathetic stimulation or due to a peripherally mediated opioid chemoreflex. The results suggested that the CRF-bradycardia was mediated by the release of opioid peptides from the pituitary.

Peripheral opioid receptors influencing heart rate in rats: evidence for endogenous tolerance

Opioid peptides injected into the circulation of rats evoke a vagally mediated bradycardia. The intravenous ED50 of morphine for producing a greater than or equal to 10% fall in heart rate was determined in urethane-anesthetized rats. Hypophysectomy, or adrenalectomy plus treatment with dexamethasone (0.5 microgram/h, s.c., 1 day), procedures that remove endogenous sources of opioid peptides, increased the sensitivity of the animal to morphine bradycardia by 6-10-fold. Conversely, stressing the animals by exposure to cold (4-6 degrees C for two days) elevated the ED50 for morphine sulfate and for beta h-endorphin by about 5-fold. Dexamethasone infusions prevented the cold-induced desensitization to morphine. Intravenous administration of rat corticotropin-releasing factor (CRF) also desensitized the animals to morphine. CRF alone produced a fall in blood pressure and heart rate. The bradycardia was prevented by pretreatment with naloxone. These results indicate that the sensitivity of vagal opioid chemoreceptors is influenced by endogenous sources of opioid peptides. This phenomenon can be called 'endogenous tolerance'.

Sensitivity to morphine-evoked bradycardia in rats is modified by dynorphin (1-13), Leu- and Met-enkephalin

Intravenous injection of morphine evokes, in the rat, a vagal chemoreflex resulting in bradycardia, hypotension and transient apnea. The morphine ED50 for producing bradycardia was utilized here as an index of the sensitivity to opioid stimulation of the organism. The potency of morphine for producing bradycardia can be modified by dynorphin (1-13), Leu- and Met-enkephalin. Both dynorphin (1-13) and Leu-enkephalin administered s.c. to normal and morphine-tolerant rats, 30 min before blood vessel cannulation and testing, lowered the morphine ED50 for bradycardia. Dynorphin (1-13) was 3 times more potent than Leu-enkephalin. Under the same test conditions, Met-enkephalin, given s.c. (0.45-0.9 mumol/kg) 30 min before morphine challenge, produced only desensitization to morphine-evoked bradycardia. The effects of dynorphin (1-13) were biphasic with respect to time of injection. When dynorphin (1-13) (0.3 mumol/kg s.c.) was injected s.c. 2 hr before morphine challenge, animals were desensitized to morphine by 2.5 times. Dynorphin (1-13) (0.19-0.77 nmol/hr s.c.) co-infused with morphine (74 nmol/hr s.c.) for 2 days accelerated tolerance development to morphine bradycardia. These results confirm previous studies which suggest that opioid peptides having Leu- or Met-enkephalin at its N-terminus manifest different pharmacological properties in vivo.

An etorphine-evoked vagal reflex in rats is inhibited by naloxone, N-methylnaloxone, and SMS 201-995

Intravenous injection of opiates in rats is known to produce a rapid fall in heart rate, blood pressure, and respiratory frequency. Etorphine, a potent opiate agonist which reaches central and peripheral receptors, administered at 1.1 nmol/kg i.v., evoked these characteristic effects in the urethane-anaesthetized rat. Three opiate antagonists, with somewhat different properties, have been used to assess the site of action of etorphine-induced bradycardia, hypotension, and inhibition of respiration. The antagonists used were naloxone hydrochloride, N-methylnaloxone bromide, and SMS 201-995. Low doses of naloxone blocked the cardiovascular and respiratory effects of etorphine. N-Methylnaloxone blocked the bradycardia, hypotension and the initial phase of apnea produced by etorphine but not the subsequent slowing of respiration. SMS 201-995 blocked the bradycardia and partially antagonized the hypotension and inhibition of respiratory rate produced by etorphine. These results indicate that N-methylnaloxone and SMS 201-995 can block the peripheral receptors which mediate opiate-induced bradycardia. Naloxone blocks both the central and peripheral actions of etorphine and so more completely antagonizes the cardiovascular and respiratory effects of etorphine.

Tolerance to morphine bradycardia in the rat

Intravenous injection of opiate agonists produces in the rat a precipitous but transient fall in heart rate. This bradycardia, which may be a vagal chemoreflex, appears to originate from peripheral opiate receptors because the onset is faster after injections of morphine into the peripheral circulation than after central injections. The bradycardia is blocked by i.v. administration of tertiary and quaternary naloxone at doses which are not effective centrally. Tolerance develops to morphine bradycardia after s.c. infusions of morphine sulfate (e.g., 74 nmol/hr/rat s.c. for 2 days elevated the morphine ED50 by 22 times), but not after central infusions of morphine at doses which are sufficient to produce physical dependence and tolerance to morphine analgesia. Subcutaneously infused morphine animals are cross-tolerant to FK33,824 (Tyr-D-Ala-Gly-NMePhe-Met(O)-ol), a potent enkephalin analog, and vice versa, but are not tolerant to serotonin or phenyldiguanide. Vagal bradycardia may be a convenient index for studying the peripheral action of opioid agonists.

Inhibition of an opioid-evoked vagal reflex in rats by naloxone, SMS 201-995 and ICI 154, 129

Intravenous injection of opioid agonists in rats evokes a vagal reflex resulting in a fall in heart rate, and blood pressure. Three opioid antagonists, naloxone, SMS 201-995, and ICI 154,129 were used to assess the nature of the opioid receptors that mediate the vagal reflex. The agonists used were morphine, Tyr-Pro-NMePhe-D-Pro-NH2 (PLO17), and D-Ala2-Leu5-enkephalin (DADL). At challenge doses of morphine, PLO17, and DADL at five times the ED50 for bradycardia, the naloxone ED50 for DADL was nine times greater than that for morphine and PLO17. The pA2 value of naloxone against DADL was significantly less than that for morphine and PLO17. The antagonist properties of SMS 201-995 were similar to those of naloxone. ICI 154,129, a putative delta receptor antagonist, was not, however, selective in its antagonism of opioid bradycardia. Both SMS 201-995 and ICI 154,129, when injected alone, produced changes in heart rate and blood pressure. The cardiovascular actions of the peptide antagonists were not affected by naloxone hydrochloride at doses up to 4 mg/kg i.v.