Muscarinic feedback inhibition of acetylcholine release from the myenteric plexus in the guinea pig ileum and its status after chronic exposure to morphine

Atropine (10−8–10−6 M) produced a dose-related increase in the release of ACh from the electrically stimulated longitudinal muscle – myenteric plexus preparation. This increase in release was frequency dependent, being greater at low frequencies of stimulation where the pulse release of ACh was greater than at high frequencies. The muscarinic agonist oxotremorine (up to 100 μM) did not inhibit ACh release although 100 μM oxotremorine did antagonize the ability of atropine to increase release. The effect of chronic exposure to morphine, both in vivo and in vitro, on the reactivity of muscarinic receptors mediating this increase was examined. Neither the control rate of ACh release nor the dose–response curve for atropine were altered by (a) an 8-day morphine injection schedule, and (b) exposure of the tissue to morphine in the bath for 90 min. The results of this study support the existence of a muscarinic feedback mechanism which controls ACh release from the myenteric plexus. Chronic exposure to morphine does not produce specific changes in the muscarinic receptor mediating this inhibition. </jats:p

Interactions of methylxanthines, nonxanthine phosphodiesterase inhibitors, and calcium with morphine in the guinea pig myenteric plexus

In an earlier study, theophylline was shown to antagonize the morphine-induced inhibition of electrically induced contractions of the longitudinal muscle – myenteric plexus preparation from the guinea pig ileum. In the present study, acetylcholine (ACh) released from the myenteric plexus was measured directly using a radioenzymatic assay. Theophylline antagonized the morphine-induced inhibition of ACh release. A similar antagonism was also observed with caffeine and 3-isobutyl-1-methylxanthine (IBMX). All three methylxanthines also increased ACh release. The nonxanthine phosphodiesterase (PDE) inhibitors 4-(3-butoxy-4-methoxy)-2-imidazolidinone (Ro 20-1724) and 1-ethyl-4-isopropylidenehydrazino-1H-pyrozolo(3,4-b)-pyridine-5-carboxylate, ethylester, HCl (SQ 20,009) generally did not antagonize the morphine-induced inhibition of ACh release. The PDE inhibitor SQ 20,009, but not Ro 20-1724, enhanced the release of ACh. Both high calcium concentration and the divalent cation ionophore A23187 antagonized the inhibitory action of morphine on ACh release. These observations suggest that alteration in calcium fluxes rather than the inhibition of PDE mediate the methylxanthine-induced antagonism of morphine in this preparation. </jats:p

The K⁺-evoked release of [³H]acetylcholine from slices of rat globus pallidus: modulation by δ-opioid receptors

Previous investigations have shown that the activation of δ-opioid receptors depresses the release of acetylcholine (ACh) in the rat caudate putamen. This finding raised the possibility that the release of ACh is similarly modulated in the globus pallidus, a region containing a distinct population of cholinergic neurons and enriched in enkephalinergic nerve terminals. In the present study the pallidal release of ACh was characterized and the effects of δ-opioid receptor activation on this release were examined. The results show that this release is stimulated by high K+ in a concentration- and Ca2+-dependent manner. D-Pen2,L-Pen5-enkephalin (0.1 – 10 μM), a selective δ-opioid receptor agonist, produced a dose-related inhibition of the 25 mM K+-evoked tritium release. The maximal inhibitory effect, representing a 34% decrease in the K+-induced tritium release, was observed at a concentration of 1 μM. This opioid effect was attenuated by the selective δ-opioid receptor antagonist, ICI 174864 (1 μM). These findings support the role of a δ-opioid receptor in the modulation of ACh release in the rat globus pallidus.Key words: globus pallidus, acetylcholine, enkephalin, release. </jats:p

Effects of a κ-receptor agonist, U-50,488H, on the release of endogenous brain dopamine

The potential interaction between κ-opiate receptors and dopamine activity was examined in this study by monitoring the effect of U-50,488H on the release of endogenous dopamine from rat striatal slices in both the absence and presence of 10 μM nomifensine, a potent dopamine uptake inhibitor. Basal dopamine release was increased 10-fold in the presence of nomifensine, and the normally steady base line was observed to increase gradually under these conditions. U-50,488H, a potent κ-agonist, enhanced the spontaneous release of dopamine, but only at relatively high concentrations (40.0 μM) and only in the absence of nomifensine. Likewise, nomifensine and U-50,488H (40.0 μM) each significantly inhibited the synaptosomal uptake of [3H]dopamine. As with basal release, nomifensine markedly enhanced the potassium-evoked release of dopamine, and this evoked release was significantly attenuated by U-50,488H (0.4 and 40.0 μM) in both the absence and presence of nomifensine. This opiate-mediated inhibition of evoked dopamine release was antagonized in a time-dependent manner by the putative κ-antagonist, WIN 44,441-3, suggesting that striatal κ-receptor activation modulates dopamine release. </jats:p

Depression of potassium-evoked striatal acetylcholine release by δ-receptor activation: inhibition by cholinoactive agents

To examine the role of δ-opioid receptors in the modulation of striatal acetylcholine (ACh) release, the action of D-Pen2,L-Pen5-enkephalin, a selective δ-opioid receptor agonist, was tested on [3H]ACh release from slices of the rat caudate–putamen. Slices, incubated with [3H]choline, were superfused with a physiological buffer and stimulated twice by exposure to a high potassium (K+) concentration. In the absence of a cholinesterase inhibitor, 1 μM D-Pen2,L-Pen5-enkephalin produced a 46 and 35% decrease in the release of [3H]ACh evoked by 15 and 25 mM K+, respectively. The depressant action of the enkephalin analogue was concentration dependent, with a maximal effect on K+-evoked [3H]ACh release occurring at 1.0 μM, and was completely blocked in the presence of the δ-opioid receptor selective antagonist, ICI 174864 (1 μM). In the presence of the cholinesterase inhibitors physostigmine (10 μM) and neostigmine (10 μM), or the muscarinic receptor agonist oxotremorine (10 μM), D-Pen2,L-Pen5-enkephalin did not depress the K+-evoked release of [3H]ACh. Atropine (1 μM) blocked the inhibitory effect of physostigmine on the depressant action of D-Pen2,L-Pen5-enkephalin. The results of this study indicate that δ-opioid receptor activation is associated with an inhibition of striatal ACh release, but this opioid–cholinergic interaction is not apparent under conditions of presynaptic muscarinic receptor activation. </jats:p