Antinociception induced by exogenous and endogenous opioids: Role of different brain structures

Antinociception induced by exogenous and endogenous opioids: Role of different brain structures

309 A N T I N O C I C E P T I O N INDUCED BY E X O G E N O U S AND OPIOIDS : ROLE OF DIFFERENT BRAIN STRUCTURES. ENDOGENOUS Valverde, 0., Coric, P.,...

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309 A N T I N O C I C E P T I O N INDUCED BY E X O G E N O U S AND OPIOIDS : ROLE OF DIFFERENT BRAIN STRUCTURES.

ENDOGENOUS

Valverde, 0., Coric, P., Roques, B.P. and Maldonado, R. Pharmacochimie MolEculaire et Structurale. INSERM U 266-CNRS URA 1500, Facult6 de Pharmacie, 4, Av. de l'Observatoire. 75270 Paris Cedex 06. S u m m a r y . RB 101, the mixed inhibitor of the enkephalin degrading enzymes able to cross the blood brain barrier induced similar antinociceptive effects to exogenous opiates, but produced less tolerance and dependence after chronic treatment (1). In this study we have investigated the participation of several brain structures in antinociception induced after exogenous opioid injection or activation of the endogenous opioid system with RB 101. Rats were implanted with cannulae into thalamus ventro-basal (THB), central amygdala (AMG), periaqueductal gray matter (PAG) and raphe magnus nucleus (NRM). The antinociceptive responses induced by systemic injection of morphine or RB 101 were evaluated in the tail electrical-stimulation test after local administration of the opioid antagonist methylnaloxonium (MN) into the implanted structures. The blockade of morphine and RB 101 antinociception was similar after MN administration in the THB, PAG and NRM. However, morphine responses were more efficiently blocked than the responses induced by RB 101 when MN was injected into the AMG. This result suggests a different implication of AMG in endogenous and exogenous opioids induced antinociception. Methods. Male Sprague-Dawley rats were anaesthetized with chloral hydrate, and unilateral (NRM) or bilateral (THB, AMG and PAG) stainless steel cannula guide were implanted and secured to the skull. The coordinates (2) were THB = A - 3.2 mm from bregma, L + 3.2, V - 6.1 from skull; AMG = A - 2.5 from bregma, L + 4.2, V -8.1 from skull; NRM = A - 0.7 from interaural line, L 0, V 10.2 from skull. Central injections (MN or saline) were performed by an infusion pump in a constant volume of 0.5 gl followed by a 1 min diffusion period. For tail electrical-stimulation test, rats were placed in a horizontal aerated plexiglas cylinder, and two electrodes were implanted s.c. at the base of the tail and connected to a stimulator, which delivered the current (10 msec rectangular pulse, 60 cycles/sec) for 2 sec. The intensity of the stimulation was increased by 0.5 V until a response was observed. As described (3), threshold corresponding to three reactions were measured: 1) regional reflexes (tail withdrawal and hind limb movements), 2) vocalization during the electrical stimulation, 3) vocalization occuring briefly after the nociceptive stimulus had ceased (post-discharge vocalization). Morphine (9 mg/kg), RB 101 (20 mg/kg) or vehicle were administered i.v., 10 min before test. MN (0.3 and 1 p.g) or vehicle were injected into the brain structures 15 min before test. Correct implantation of cannula was verified by a histological procedure. Individual group comparisons were made using a one-way ANOVA. Post-hoc individual treatment effects were analyzed using a Newman-Keuls test. Results.

Talamus ventro-ba~l: MN injected into THB blocked morphine effects on vocalization (0.3 and llag, p < 0.01), but only sligthly reduced the effects induced on motor response (N.S.) and on vocalization post-discharge (1 I-tg, p < 0.05). MN blocked RB 101 effect on motor response (0.3 gg, p < 0.05) and on vocalization (0.3 and ll.tg, p < 0.01). Effect induced on vocalization post-discharge by RB 101 was slightly but not significantly reduced after the administration of MN. Periaoueductal erav matter: Morphine-induced increases on the three nociceptive threshold were completely blockecl after MN injection into the PAG (0.3 and 1 I.tg, p < 0.01). Similarly, the antinociceptive effects induced by RB 101 on the three thresholds were also completely antagonized by MN (0.3 and 1 lag, p < 0.01). Ranhe maenus nucleus: Morphine and RB 101 antinociception on motor responses and vocalization during discharge were blocked by local administration of MN (0.3 and 1 gg, p < 0.01). The effects of morphine and RB 101 on vocalization after discharge were reversed by MN at the dose of 1 lag (p < 0.01) and only partially reduced at the dose of 0.3 gg. Central amygdala: MN injected into AMG antagonized morphine analgesia on motor response (0.3 and 1 lag, p < 0.05), vocalization (1 lag, p < 0.01) and vocalization post-discharge (1 lag, p < 0.01). In the case of RB 101, the effect induced on the motor response was reversed by MN only partially (N.S.) and at the highest dose (1 gg). The effects induced by RB 101 on vocalization and vocalization post-discharge were slighly reduced after MN administration at the dose of 1 lag (p <

0.05).

310 Conclusion. The administration of MN in the different structures investigated was able to block morphine and RB 101-induced analgesia with a different magnitude depending on the structure and the antinociceptive threshold studied. Thus, MN injected PAG and NRM, strongly antagonized the antinociceptive responses of exogenous and endogenous opioids on the three thresholds, with a similar intensity. In the case of THB, MN did not modify and slighly decreased the effect induced by RB 101 and morphine respectively on the vocal post-discharge response. One possible explanation may be the participation of other superior structures, such as cortex on the elaboration of the vocalization postdischarge threshold. Another possibility could be that the thalamus is a relative large area and MN, considering its very low diffusion, probably did not reach a enough large surface to inhibit the antinociceptive responses. Finally, the effects of MN injection in AMG were stronger on morphine responses than on RB 101 antinociception, suggesting a different implication of AMG in endogenous and exogenous opioid induced antinociception. References; 1.- Noble et al., Eur. J. Pharmacol. 223: 83-89, 1992. 2.- Paxinos and Watson. The rat brain. Academic Press. New York. 1986. 3.- Carroll and Lim. Arch. lnt.Pharmacodyn. 125: 383-403, 1960.