enkephalin--leucine-2-alanine and Opioid-Related-Disorders

Topics: Animals; Aorta, Thoracic; Cocaine; Cyclic AMP; Electric Stimulation; Guinea Pigs; Humans; Ileum; In Vitro Techniques; Male; Mice; Muscle, Smooth; Opioid-Related Disorders; Rats; Receptors, Dopamine; Receptors, Opioid; Receptors, Serotonin; Substance-Related Disorders

At low (< nM) concentrations, mu, delta or kappa opioid peptides as well as morphine and other opioid alkaloids elicit dose-dependent excitatory prolongation of the calcium-dependent component of the action potential duration (APD) of many mouse sensory dorsal root ganglion (DRG) neurons, whereas application of the same opioids at higher (uM) concentrations results in inhibitory shortening of the APD. These bimodal opioid excitatory/inhibitory effects on DRG neurons are blocked by naloxone. In contrast to bimodally acting opioids, the opioid alkaloids, etorphine and dihydroetorphine (thebaine-oripavine derivatives) uniquely elicited only dose-dependent, naloxone-reversible inhibitory effects on sensory neurons in DRG-spinal cord explants, even at concentrations as low as 1 pM, and showed no excitatory effects at lower concentrations. These remarkably potent inhibitory opioid receptor agonists also act as antagonists at excitatory opioid receptors since pretreatment of DRG neurons with subthreshold concentrations (< pM) blocked excitatory APD prolongation by nM morphine (or other opioids) and unmasked inhibitory APD shortening which generally requires much higher concentrations. Furthermore, acute application of pM-nM etorphine to chronic microM morphine- or D-Ala2-D-Leu5 enkephalin (DADLE)-treated DRG neurons blocked the nM naloxone-precipitated APD prolongation that generally occurs in DRG cells sensitized by bimodally acting opioids. In the presence of pM etorphine, chronic treatment of DRG neurons with microM morphine or DADLE no longer resulted in development of tolerance/dependence effects, as previously observed after similar chronic opioid treatment in the presence of cholera toxin-B subunit. These in vitro studies may clarify the mechanisms underlying the potent analgesic effects of etorphine and dihydroetorphine in vivo and to guide the use of these and other excitatory opioid receptor antagonists in attenuating development of opiate dependence/addiction.

Topics: Analgesics; Animals; Cells, Cultured; Diprenorphine; Drug Tolerance; Electrophysiology; Enkephalin, Leucine-2-Alanine; Etorphine; Ganglia, Spinal; Mice; Morphine; Naloxone; Narcotic Antagonists; Narcotics; Neurons, Afferent; Opioid-Related Disorders

This study investigated the antinociceptive properties of two alkylating derivatives of morphinone, 14 beta-(thioglycolamido)-7,8- dihydromorphinone (TAMO) and 14 beta-(bromoacetamido)-7,8-dihydromorphinone (H2BAMO) in the mouse tail-flick assay. Intracerebroventricular administration of either TAMO or H2BAMO produced short-term antinociception. Both TAMO and H2BAMO were 11.6-fold more potent than an i.c.v. administration of morphine. These effects were antagonized by the mu-selective antagonist, beta-funaltrexamine, but not by the delta-selective antagonist, N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH. TAMO pretreatment from 8 to 48 hr produced a time-related, dose-dependent antagonism of morphine-induced antinociception without showing any agonistic effect. Pretreatment with TAMO for 24 hr antagonized antinociception produced by both H2BAMO and morphine, as well as TAMO itself, but not that of the delta-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) or U50,488, a kappa-selective agonist. In order to distinguish this antagonistic effect from cross-tolerance between TAMO and morphine, two mu agonists, [D-Ala2,N(Me)Phe4,Gly-ol]enkephalin (DAMGO) and H2BAMO, were chosen for comparison. A single i.c.v. pretreatment of DAMGO or H2BAMO, at a dose that had equivalent analgesic effects as TAMO, attenuated morphine-induced antinociception, reaching a maximal effect at the time of the disappearance of agonistic effects of DAMGO and H2BAMO and lasting up to 24 hr. Additionally, a 16-hr pretreatment with TAMO, but not DAMGO or H2BAMO, reduced the development of physical dependence to morphine at 24 hr after morphine pellet implantation. Therefore, this study demonstrated that both TAMO and H2BAMO act as mu opioid agonists to produce short-term antinociception.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Analgesics; Animals; Drug Tolerance; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Leucine-2-Alanine; Enkephalins; Hydromorphone; Male; Mice; Mice, Inbred ICR; Opioid-Related Disorders; Receptors, Opioid; Receptors, Opioid, mu

This study describes a novel action of morphine on adenylate cyclase activity in the rat locus coeruleus (LC). We have previously shown that acute in vitro morphine inhibits adenylate cyclase activity in isolated LC membranes, whereas chronic in vivo morphine treatment increases enzyme activity in this brain region. We now report that acute in vivo morphine treatment produces a 25-30% decrease in adenylate cyclase activity in the LC, which persists in in vitro assays in the absence of opiates. This in vivo effect is clearly distinct from the acute inhibition of adenylate cyclase observed during exposure of isolated LC membranes to opiates in vitro. The in vivo effect was not reversed by the inclusion of naloxone, an opiate receptor antagonist, in the assay, and acute in vitro opiate inhibition of the enzyme was the same in LC membranes isolated from control and morphine-treated rats. Thus, the in vivo effect does not appear to be due to residual morphine retained in the membrane preparation. This persistent decrease in adenylate cyclase was found to occur in a dose-dependent manner and to be mediated through the actions of morphine at opiate receptors, inasmuch as the inhibition was prevented by concomitant in vivo administration of naltrexone, a long-acting opiate receptor antagonist. This effect was also specific to the LC, in that it was not observed in the other brain regions examined, which included the dorsal raphe, neostriatum, and frontal cortex. Acute in vivo clonidine, an alpha 2-adrenergic receptor agonist known to have actions in the LC similar to those of morphine, produced a similar persistent decrease in adenylate cyclase activity in this brain region. In contrast, other drugs with different actions on the LC failed to produce this effect. This decrease in adenylate cyclase activity induced by acute in vivo morphine or clonidine, which persists in isolated membranes after the removal of the drugs, may be an early step in the sequence of events that leads to the development of opiate or clonidine addiction in the LC.

Topics: Adenylyl Cyclases; Animals; Clonidine; Dose-Response Relationship, Drug; Drug Tolerance; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; GTP-Binding Proteins; Locus Coeruleus; Male; Morphine; Naloxone; Opioid-Related Disorders; Rats; Rats, Inbred Strains