Behavioral manipulation of rats causes alterations in opiate receptor occupancy

Abstract

The notion that about half of the opiate receptors of rat brain are associated with endogenous ligand even after extensive washing of brain membranes at 0 °C explains a number of observations: if brain membranes are subjected to a preliminary incubation at 37 °C, subsequent binding of [³H] opiates is approximately doubled, the assumption being that the endogenous ligand, by dissociating much more rapidly at the higher incubation temperature, makes additional opiate receptor sites available for binding (Pasternak et al., 1975a, b). If sodium chloride is included in the preincubation medium—even at 0 °C—the subsequent binding of [³H] opiates is similarly increased (Simantov et al., 1976). The agonist dihydromorphine, has been shown to dissociate from opiate receptors about three times more rapidly in the presence of sodium ion at 0 °C (Pert and Snyder, 1974), and the assumption is that sodium ion also speeds up the dissociation of the endogenous opiate agonist. In other experiments, membrane preparations from rats injected with low doses of opiate antagonists or high doses of opiate agonists show almost twice as much [³H] opiate binding after extensive washing at 0 °C than saline-injected controls. If the assay is conducted in the presence of sodium, however, the control membrane preparations become elevated to the level of the membranes from opiate-injected rats so that a significant difference between the two is no longer apparent (Pert and Snyder, 1976). These findings can be explained if the opiate alkaloids displace some of the endogenous ligand from opiate receptors in vivo and the remaining endogenous ligand (except in the presence of sodium ion) dissociates much more slowly from opiate receptors during membrane preparation than the opiate alkaloids.