Kappa opioid receptor-mediated depression of activity evoked in convergent dorsal horn cells by thermal and non-thermal noxious stimulation

Kappa opioid receptor agonists differentially inhibit two classes of rat spinal neurons excited by colorectal distention

BACKGROUND & AIMS

Quantitative neurophysiological studies have identified the presence of at least 2 spinal neuronal populations (abrupt and sustained) that are excited by the noxious visceral stimulus colorectal distention. This study examined the effects of the kappa opioid receptor agonists fedotozine and U50488H on the activity of these neurons.

METHODS

In decerebrate, cervical spinal cord-transected male rats, the lumbosacral spinal cord was exposed by a laminectomy. Dorsal horn neurons showing excitatory responses to colorectal distention (80 mm Hg, 20 seconds) were identified using microelectrodes. Cumulative doses of fedotozine and U50488H were administered intravenously or intrathecally, and antagonists were used.

RESULTS

Intravenous fedotozine and U50488H dose-dependently inhibited the evoked activity of sustained neurons. This inhibition was partially reversed by the kappa opioid antagonist norbinaltorphimine. The same agents had insignificant effects on the evoked activity of abrupt neurons. Fedotozine inhibited spontaneous activity of both abrupt and sustained neurons. Intrathecally administered U50488H had no effect on abrupt or sustained neurons, but intrathecally administered fedotozine inhibited the evoked and spontaneous activity of both groups.

CONCLUSIONS

Kappa opioid receptor agonists acting peripherally had differential effects on 2 spinal neuronal populations responsive to colorectal distention. Fedotozine had additional inhibitory effects acting within the spinal cord.

Cross-tolerance to acute administration of mu and kappa opioid agonists at the spinal cord level in the rat

Development of tolerance and cross-tolerance after acute administration of the mu agonist morphine and the kappa agonist U-50,488H was assessed in rats, through recording of a C-fiber-evoked spinal nociceptive reflex. Rats rendered tolerant to morphine (a single dose of 1 mg/kg i.p.) showed, after a 5-hour period, tolerance to morphine and cross-tolerance to the kappa-opioid receptor agonist U-50,488H, as revealed by depressed C-reflex responsiveness. In contrast, pretreatment with U-50,488H (a single dose of 1 mg/kg i.p.) rendered tolerant the rats to U-50,488H, but the animals did not develop cross-tolerance to morphine. Results indicate that acute administration of mu and kappa ligands leads to development of unidirectional cross-tolerance in rat spinal cord. This points to limitations in using alternated mu and kappa opioid agonists to bypass the problem of development of opioid tolerance in chronic pain complaints.

Intrathecal pertussis toxin but not cyclic AMP blocks kappa opioid-induced antinociception in rat

The role of inhibitory G-proteins and cyclic AMP in spinal mechanisms of kappa opioid receptor-mediated antinociception was assayed by recording the withdrawal response latency of the rat tail following immersion into a water bath of 49 degrees C. Intrathecal administration of pertussis toxin (1 microgram/rat, five days before the behavioral evaluation) prevented the antinociceptive effect of the kappa receptor agonist U-50,488H, while administration of dibutyryl cyclic AMP (10 micrograms/rat, 17 min. after U-50,488H) did not antagonize the antinociceptive action of the kappa ligand. Results suggest that in the spinal cord the signal transduction mechanism subserving the antinociceptive effect of U-50,488H involves a Gi or Go protein, but also that cyclic AMP is not implicated in coupling Gi/Go proteins to the effector system.

Calcium channel modulators modify K opioid-induced inhibition of C-fiber-evoked spinal reflexes in rat

The role of L-type Ca2+ channels on the kappa opioid-induced depression of spinal afferent transmission was assessed in spinalized rats, through recording of the C-fiber-evoked spinal flexor reflex. Six successive i.t. doses of the K agonist U-50,488H produced a dose-dependent decrease of the C-reflex duration (ID50: 25.7 nmol), the log dose-response relationship being shifted to left by pretreatment with 5 mg/kg i.v. of the calcium channel blocker verapamil, or to right by pretreatment with .25 mg/kg i.v. of the calcium channel agonist Bay K8644. Verapamil and Bay K8644, administered i.v. after U-50,488H i.t., respectively potentiated or antagonized the depressor effect of the K ligand on the reflex. The results point to a role for Ca2+ availability as a factor involved in depression of afferent nociceptive transmission by K opioids at the spinal cord.