Opiate Binding and Effect in Ileum Preparations From Normal and Morphine Pretreated Guinea‐pigs

Abstract

1 Dose-response curves for normorphine in the absence and presence of naloxone have been obtained from myenteric plexus-longitudinal muscle strip preparations from normal and morphine pretreated guinea-pigs. In addition, the high affinity stereospecific binding of [3H1-etorphine has been measured in homogenates of the same tissue. 2 Higher concentrations of normorphine were required to produce 50% inhibition of the electrically stimulated contractions of strip preparations from morphine pretreated animals. There was also an increase in the slope of linearized dose-response curves in opiate-tolerant preparations. Maximum opiate effect was unchanged, and responses to exogenous acetylcholine were not affected by the pretreatment.3 There was a slight increase in the apparent equilibrium constant for naloxone after morphine pretreatment. 4 Tolerance to opiate effect was not accompanied by a change in the affinity or number of stereospecific binding sites for [3H1-etorphine. Hill plots of [3H1-etorphine binding in both control and morphine pretreated preparations gave slopes close to unity. 5Most of these results can be explained by the assumption that in tolerant preparations, a certain fractional opiate receptor occupation threshold must be exceeded before opiate effects become apparent. It is suggested that the tissue adapts toward a threshold equivalent to the mean receptor occupancy attained during the period of opiate drug pretreatment.

The dependence of excitatory junction potential amplitude on the external calcium concentration in narcotic tolerant mouse vas deferens

“…Schulz & Herz (1976) demonstrated a marked tolerance in the ileum from guinea-pigs which had been treated with large doses of morphine before being killed. Similar tolerance was observed in the mouse vas deferens (Cox, 1978). The mechanism of the development of tolerance and dependence is still unclear.…”

Opioid receptor reserve in normal and morphine-tolerant guinea pig ileum myenteric plexus.

“…The adaptive process has been localized to changes within the opioid-sensitive neuron. Given that the number of receptor binding sites does not change in the tolerant state in this tissue (11), our present findings indicate that opioid tolerance may result from a reduction in receptor reserve that comes about through altered activity of the coupled effector system. The molecular correlates of this change in receptor reserve are difficult to specify until the details of the opioid receptor transduction mechanisms are better understood.…”

“…This method was previously shown to induce morphine tolerance (11,13). Animals were killed on day 6, and ileum longitudinal muscle strips were set up in the organ bath, then allowed to equilibrate for 2 hr before testing.…”

“…The second was the suggestion that tolerance to an opioid could be the consequence of a reduced opioid receptor reserve (10). In binding studies neither affinity nor site number in the guinea pig ileum myenteric plexus is affected by chronic morphine administration (11). The observed reduction in sensitivity of the tolerant tissue to opioids therefore implies that a higher fractional receptor occupancy is required for a given effect, as would be the direct consequence of a reduced spare-receptor fraction.…”

The dependence of excitatory junction potential amplitude on the external calcium concentration in mouse vas deferens during narcotic withdrawal

“…Washed ileum preparations from morphine-tolerant animals showed increased sensitivity to electrical stimulation Cox, 1978;Collier et al, 1981). Since there is no change in the sensitivity of muscle to transmitter (Ehrenpreis etal., 1975;Frederickson et al, 1976;Johnson et al, 1978), it is likely that the increased response is due to an increase in the quantity of neurotransmitter released by each stimulus in tolerant preparations.…”