Opiate receptors, neuropeptides in CNS and CSF of two Macaca species with different responsiveness to opiates

Of two related Macaca species, the rhesus (M. mulatta), acquires opiate tolerance and dependence more readily than the cynomolgus (M. fascicularis). In the cynomolgus, mu-opiate receptors were significantly fewer in the caudate nucleus and globus pallidus; delta-sites were fewer in the thalamus. kappa-Sites showed no species difference. The levels of [Met5]enkephalin, substance P and dynorphin B in various brain areas were comparable. On the other hand, receptor-assayed endorphin activity was higher in CSF of cynomolgus than rhesus monkeys.