ANTINOCICEPTION INDUCED BY EXOGENOUS AND ENDOGENOUS OPIOIDS - ROLE OFDIFFERENT BRAIN STRUCTURES

RB 101, the mixed inhibitor of the enkephalin degrading enzymes able t o cross the blood brain barrier induced similar antinociceptive effect s to exogenous opiates, but produced less tolerance and dependence aft er chronic treatment (1). In this study we have investigated the parti cipation of several brain structures in antinociception induced after exogenous opioid injection or activation of the endogenous opioid syst em with RB 101. Rats were implanted with cannulae into thalamus ventro -basal (THB), central amygdala (AMG), periaqueductal gray matter (FAG) and raphe magnus nucleus (NRM). The antinociceptive responses induced by systemic injection of morphine or RB 101 were evaluated in the tai l electrical-stimulation test after local administration of the opioid antagonist methylnaloxonium (MN) into the implanted structures. The b lockade of morphine and RB 101 antinociception was similar after MN ad ministration in the THB, FAG and NRM. However, morphine responses were more efficiently blocked than the responses induced by RB101 when MN was injected into the AMG. This result suggests a different implicatio n of AMG in endogenous and exogenous opioids induced antinociception.