enkephalin--ala(2)-mephe(4)-gly(5)- and Spinal-Cord-Injuries

Chronic pain caused by spinal cord injury (SCI) is notoriously resistant to treatment, particularly by opioids. After SCI, DRG neurons show hyperactivity and chronic depolarization of resting membrane potential (RMP) that is maintained by cAMP signaling through PKA and EPAC. Importantly, SCI also reduces the negative regulation by Gαi of adenylyl cyclase and its production of cAMP, independent of alterations in G protein-coupled receptors and/or G proteins. Opioid reduction of pain depends on coupling of opioid receptors to Gαi/o family members. Combining high-content imaging and cluster analysis, we show that in male rats SCI decreases opioid responsiveness

Topics: Animals; Cells, Cultured; Chronic Pain; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Ganglia, Spinal; HEK293 Cells; Humans; Male; Membrane Potentials; Nociceptors; Proto-Oncogene Proteins c-raf; Rats, Sprague-Dawley; Receptors, Opioid, mu; Signal Transduction; Spinal Cord Injuries

The current study aimed to evaluate the contribution(s) of specific opioid receptor systems to the analgesic and detrimental effects of morphine, observed after spinal cord injury in prior studies.. We used specific opioid receptor agonists to assess the effects of μ- (DAMGO), δ- (DPDPE) and κ- (GR89696) opioid receptor activation on locomotor (Basso, Beattie and Bresnahan scale, tapered beam and ladder tests) and sensory (girdle, tactile and tail-flick tests) recovery in a rodent contusion model (T12). We also tested the contribution of non-classic opioid binding using [+]- morphine.. First, a dose-response curve for analgesic efficacy was generated for each opioid agonist. Baseline locomotor and sensory reactivity was assessed 24 h after injury. Subjects were then treated with an intrathecal dose of a specific agonist and re-tested after 30 min. To evaluate the effects on recovery, subjects were treated with a single dose of an agonist and both locomotor and sensory function were monitored for 21 days.. All agonists for the classic opioid receptors, but not the [+]- morphine enantiomer, produced antinociception at a concentration equivalent to a dose of morphine previously shown to produce strong analgesic effects (0.32 μmol). DAMGO and [+]- morphine did not affect long-term recovery. GR89696, however, significantly undermined the recovery of locomotor function at all doses tested.. On the basis of these data, we hypothesize that the analgesic efficacy of morphine is primarily mediated by binding to the classic μ-opioid receptor. Conversely, the adverse effects of morphine may be linked to activation of the κ-opioid receptor. Ultimately, elucidating the molecular mechanisms underlying the effects of morphine is imperative to develop safe and effective pharmacological interventions in a clinical setting.. USA.. Grant DA31197 to MA Hook and the NIDA Drug Supply Program.

Topics: Analgesics, Opioid; Analysis of Variance; Animals; Body Weight; Disease Models, Animal; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Locomotion; Male; Morphine; Pain Measurement; Piperazines; Pyrrolidines; Rats; Rats, Sprague-Dawley; Recovery of Function; Severity of Illness Index; Spinal Cord Injuries

Spinal cord neurons can support a simple form of instrumental learning. In this paradigm, rats completely transected at the second thoracic vertebra learn to minimize shock exposure by maintaining a hindlimb in a flexed position. Prior exposure to uncontrollable shock (shock independent of leg position) disrupts this learning. This learning deficit lasts for at least 24h and depends on the NMDA receptor. Intrathecal application of an opioid antagonist blocks the expression, but not the induction, of the learning deficit. A comparison of selective opioid antagonists implicated the kappa-opioid receptor. The present experiments further explore how opioids affect spinal instrumental learning using selective opioid agonists. Male Sprague-Dawley rats were given an intrathecal injection (30 nmol) of a kappa-1 (U69593), a kappa-2 (GR89696), a mu (DAMGO), or a delta opioid receptor agonist (DPDPE) 10 min prior to instrumental testing. Only the kappa-2 opioid receptor agonist GR89696 inhibited acquisition (Experiment 1). GR89696 inhibited learning in a dose-dependent fashion (Experiment 2), but had no effect on instrumental performance in previously trained subjects (Experiment 3). Pretreatment with an opioid antagonist (naltrexone) blocked the GR89696-induced learning deficit (Experiment 4). Administration of GR89696 did not produce a lasting impairment (Experiment 5) and a moderate dose of GR89696 (6 nmol) reduced the adverse consequences of uncontrollable nociceptive stimulation (Experiment 6). The results suggest that a kappa-2 opioid agonist inhibits neural modifications within the spinal cord.

Topics: Adaptation, Physiological; Animals; Association Learning; Benzeneacetamides; Conditioning, Operant; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Male; Neuronal Plasticity; Neurons; Neurotransmitter Agents; Piperazines; Pyrrolidines; Rats; Rats, Sprague-Dawley; Receptors, Opioid, kappa; Spinal Cord; Spinal Cord Injuries; Thoracic Vertebrae

We examined the effects of intrathecal (i.t.) selective opioid receptor agonists in alleviating mechanical and cold allodynia in spinally injured rats. Both DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a mu-opioid receptor agonist) and DPDPE ([D-Phe2,D-Phe5]-enkephalin, a delta-opioid receptor agonist) dose-dependently relieved the chronic allodynia-like behavior at doses selective for their respective receptors. The anti-allodynic effect of DAMGO and DPDPE was reversed by the selective mu- and delta-opioid receptor antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) and naltrindole, respectively. In contrast, the selective kappa-opioid receptor agonist U50488H did not alleviate the allodynia-like behavior, but rather enhanced it. The anti-nociceptive and anti-allodynic effect of i.t. DAMGO was blocked by U50488H. Thus, activation of spinal mu- and delta-, but not kappa-opioid receptors produced anti-allodynic effect in this model of central pain. Drugs which act selectively on opioid receptor subtypes may be useful in managing chronic central pain of spinal cord origin.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics, Non-Narcotic; Analgesics, Opioid; Animals; Behavior, Animal; Chronic Disease; Drug Interactions; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Enkephalins; Female; Hypesthesia; Injections, Spinal; Nociceptors; Rats; Rats, Sprague-Dawley; Receptors, Opioid; Spinal Cord Injuries

Recent studies with dynorphin (an endogenous ligand for the kappa-opiate receptor) and receptor-selective opiate antagonists have indicated a role for the kappa-receptor in the pathophysiology of spinal cord injury. However, no studies have specifically examined opiate receptor binding in relation to spinal injury. In the present experiments, opiate receptor binding was measured in spinal cord after traumatic injury in rats using the selective radioligands [3H] [D-Ala2, D-Leu5]enkephalin (delta-receptor agonist); [3H] [D-Ala2,MePhe4,Gly-(ol)5]enkephalin (mu-receptor agonist); and [3H]ethylketocyclazocine (kappa-receptor agonist). The specific binding of ethylketocyclazocine, but not the other agonists, showed a significant, time-dependent, and localized increase at the injury site. Since dynorphin, which has been implicated as an injury factor after spinal trauma, shows similar localized increases after spinal injury, the present data are consistent with the hypothesis that up-regulation of the kappa-receptor after injury may contribute to the subsequent secondary injury process.

Topics: Animals; Cyclazocine; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Enkephalins; Ethylketocyclazocine; Male; Rats; Rats, Inbred Strains; Receptors, Opioid; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Spinal Cord; Spinal Cord Injuries