Abstract
WHEN guinea pigs are exposed continuously to high morphine concentrations, tolerance and physical dependence develop within a few days in both the central nervous system (CNS) and the myenteric plexus. In the plexus morphine tolerance is associated with subsensitivity to the inhibitory catecholamines and supersensitivity to the excitatory neuro-transmitter serotonin (5-hydroxytryptamine, 5-HT)1,2. We have investigated the following questions about these phenomena, (a) After opiate withdrawal in animals made tolerant and dependent, would the altered sensitivities to neurotransmitters in the plexus return to normal? (b) If so, would loss of tolerance to morphine in the plexus proceed in parallel with loss of subsensitivity to catecholamines and loss of supersensitivity to 5-HT? (c) Would such reversion to normal in the plexus be concurrent with disappearance of CNS indications of tolerance and dependence?
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