David Pollock

ACUTE EFFECTS OF MORPHINE AND OPIOID PEPTIDES ON THE MOTILITY AND RESPONSES OF RAT COLON TO ELECTRICAL STIMULATION

1 Morphine and leucine‐ and methionine‐enkephalins inhibited the contractile response of the pithed rat colon to electrical stimulation of the spinal motor outflows and inhibited motor responses of the isolated colon to field stimulation. 2 Morphine and the opioid peptides also had an excitatory action in the colon. In the pithed rat, opiates caused regular fluctuations in intracolonic pressure and in the isolated colon, caused regular waves of contraction. This excitatory response was produced by low concentrations of the enkephalins (2 × 10−8 m, 2 × 10−9 m), was stereospecific and was antagonized by naloxone. 3 Opiate‐induced contractions in the isolated colon were inhibited by catecholamines, adenine nucleotides and by phosphodiesterase inhibitors. These contractions were unaffected by ergotamine and tolazoline, or by propranolol. 4 The excitatory action of opiates in the isolated colon was not antagonized and usually was potentiated by atropine, (+)‐tubocurarine and hexamethonium. In the absence of opiates, these drugs also produced similar waves of contraction, which were unaffected by naloxone. 5 Opiate‐induced contractions occurred in colon rendered unresponsive to 5‐hydroxytryptamine (5‐HT) and these contractions were potentiated by the 5‐HT antagonist, lysergic acid diethylamide, which, when administered alone, caused similar contractions. The 5‐HT antagonist, cyproheptadine, inhibited opiate‐induced contractions but was non‐specific, since it also inhibited responses of the colon to carbachol and KCl. 6 Opiate‐induced contractions were unaffected by procaine and were potentiated by tetrodotoxin. Both of these drugs, when administered alone, produced waves of contractions, which were similar to those produced by opiates but were unaffected by naloxone. 7 Contractions produced in the isolated colon either by opiates, atropine or (+)‐tubocurarine, or any combination of these drugs, were inhibited by field stimulation applied at the peak of a wave of contraction. This inhibitory response to field stimulation occurred at low frequencies of stimulation (< 10 Hz), and persisted in colon from rats pretreated with reserpine to deplete, or 6‐hydroxydopamine to destroy, adrenergic nerve endings. It was unaffected by guanethidine but abolished by tetrodotoxin. 8 The implications of these results are considered and it is concluded that the excitatory action of opiates in the rat colon is probably not mediated by the release of acetylcholine or 5‐HT but instead, may be due either to a direct action on smooth muscle or to a presynaptic inhibitory action at a ganglionic site in a non‐adrenergic inhibitory mechanism, which normally suppresses myogenic activity.

Oxyhaemoglobin blocks non-adrenergic non-cholinergic inhibition in the bovine retractor penis muscle.

Abstract The bovine retractor penis muscle is innervated by motor nerves which are adrenergic, and by inhibitory nerves whose transmitter is unknown. Relaxations in response to inhibitory nerve stimulation, or to the inhibitory factor extracted from the retractor penis, are blocked by oxyhaemoglobin.

The effects of drugs on the sensitivity of the rat anococcygeus muscle to agonists

1 The effects of cocaine, 6‐hydroxydopamine (6‐OHDA), reserpine and 17‐β‐oestradiol on the sensitivity of the rat anococcygeus muscle to noradrenaline (NA), acetylcholine (ACh) and KCl were investigated. 2 Cocaine (10−5m) increased the sensitivity of the anococcygeus to NA (100‐fold) but not to ACh or KCl. 3 6‐OHDA treatment, whether discontinuous over 6 days or continuous for 12 days, also produced a specific increase in sensitivity to NA (50‐fold). 4 Reserpine ((1 mg/kg)/day) produced a small increase in sensitivity of the muscle to NA and ACh but not to KCl, after 6 days and 12 days treatment. 5 17‐β‐Oestradiol (10–5m) had no effect on the sensitivity of the anococcygeus to NA or ACh, but reduced the sensitivity to KCl and the possible mechanism of this effect is discussed. 6 Cocaine increased, while reserpine, 6‐OHDA and 17‐β‐oestradiol decreased, the maximum response of the muscle to KCl. The response to KCl was shown to consist of at least three components; a direct action on the muscle, an effect due to release of NA, and an inhibitory action, probably due to release of the unknown inhibitory transmitter.

The effects of drugs or denervation on thymidine uptake into rat regenerating liver

Abstract Thymidine incorporation into regenerating liver was unaffected by the α-receptor antagonist tolazoline but was inhibited by the β-receptor antagonist propanolol, by the adrenergic neurone blockers guanethidine and reserpine and by degenerative section of the hepatic adrenergic nerves. Paradoxically 6-hydroxydopamine stimulated thymidine incorporation into regenerating liver. It is concluded that catecholamines, derived at least partly from hepatic adrenergic nerves and acting on β-receptors, facilitate thymidine incorporation into regenerating liver.

Impairment of relaxations to acetylcholine and nitric oxide by a phorbol ester in rat isolated aorta.

1 This study compared the abilities of acetylcholine (ACh) (endothelium‐dependent) and nitric oxide (NO) (endothelium‐independent and which may be the active component of the endothelium‐derived relaxing factor) to relax rat isolated aortic rings contracted with equi‐effective concentrations of noradrenaline (NA) or phorbol 12‐myristate 13‐acetate (PMA). 2 ACh and NO induced concentration‐dependent relaxations of aortic rings contracted with NA (EC70 value: 0.2 μm). However, relaxations to both ACh and NO were markedly reduced in rings contracted with PMA (EC80 value: 0.5 μm). NO‐induced relaxations of tissues were not affected by removal of the endothelium, but ACh‐induced relaxations were confirmed to be endothelium‐dependent. 3 ACh (10 μm) induced a 10 fold increase in guanosine 3′:5′‐cyclic monophosphate (cyclic GMP) levels above control values in aortic rings contracted with NA (0.2 μm), but did not affect cyclic GMP levels in rings contracted with PMA (0.5 μm). 4 NO (3 μm) induced a 100 fold increase in cyclic GMP levels above control values in aortic rings contracted with NA (0.2 μm), but only an 11 fold increase in tissues contracted with PMA (0.5 μm). 5 It is concluded that the action (s) of EDRF (NO) are impaired in the presence of PMA by a mechanism that may involve the stimulation of protein kinase C in vascular smooth muscle cells.

Morphine-induced changes in the sensitivity of the isolated colon and vas deferens of the rat

Abstract Prolonged (25-day) morphine treatment with increasing doses (4–400 mg/kg) followed by withdrawal, significantly increased the maximum tension developed in response to agonists in both the isolated rat vas deferens (noradrenaline, furmethide and 5-hydroxytryptamine) and colon (furmethide and acetylcholine) with no accompanying leftwards displacement of the conventional dose-response curves. In comparison, reserpine pretreatment (1 mg/kg for 5 days) significantly displaced the conventional dose-response curves to noradrenaline, furmethide and 5-hydroxytryptamine to the left and decreased the mean ED 50 values. The enhanced adaptive sensitivity changes probably arise postsynaptically in the effector organ subsequent to receptor interaction.

Reduction in the cholinesterase activity of the rat anococcygeus muscle produced by corticosterone.

1 Administration of corticosterone caused a 47% reduction in the cholinesterase (ChE) activity of homogenates of the rat anococcygeus muscle. 2 ChE activity was also reduced by morphine withdrawal and this effect was abolished by the corticosteroid synthesis inhibitor, metyrapone. 3 A single dose of reserpine reduced ChE activity in normal but not in adrenalectomized rats. 4 ChE activity was increased by adrenalectomy or by metyrapone treatment. 5 The mechanism of the corticosteroid‐induced reduction in ChE activity is discussed. 6 The reduction in ChE activity produced by corticosterone, morphine withdrawal, or a single dose of reserpine might explain the leftward shift of the dose‐% response curve to acetylcholine produced by these procedures in the isolated anococcygeus muscle of the rat.

The absence of cocaine- and 6-hydroxydopamine-induced supersensitivity to oxymetazoline in the rat anococcygeus muscle

Neither cocaine nor 6-hydroxydopamine potentiated the responses of the anococcygeus muscl_ to oxymetazoline, suggesting that the specific supersensitivity to noradrenaline produced by these drugs is due to abolition of neuronal uptake mechanisms.

Clonidine and morphine increase [3H]-noradrenaline overflow in mouse vas deferens

1 Field stimulation of mouse isolated vas deferens produced a biphasic contraction that consisted of an initial brief non‐adrenergic, non‐cholinergic (NANC) twitch, followed by a more prolonged noradrenergic component. 2 Field stimulation of vasa, previously loaded with [3H]‐noradrenaline ([3H]‐NA), increased the amount of radioactivity in the Krebs bathing solution; 77% of this radioactivity was derived from [3H]‐NA. 3 Tetrodotoxin (3 × 10−6 m) abolished the biphasic motor response to field stimulation and the accompanying increased overflow of [3H]‐NA. 4 Morphine (10−7‐10−5 m) inhibited the initial NANC component but potentiated the secondary noradrenergic component of the motor response to field stimulation. Morphine also increased the field stimulation‐induced overflow of radioactivity. Naloxone (10−6 m) antagonized the effects of morphine on the motor response and also on the overflow of radioactivity. 5 Clonidine (10−9‐10−7 m) inhibited the initial NANC component but potentiated the secondary noradrenergic component of the motor response to field stimulation. Clonidine also increased the field stimulation‐induced overflow of radioactivity. 6 The ability of morphine (10−7 m) and of clonidine (10−9 m) to potentiate the field stimulation‐induced overflow of radioactivity persisted in the presence of a combination of tranylcypromine (10−5 m), desmethylimipramine (10−5 m) and 17‐β‐oestradiol (10−5 m). 7 The inhibition of the initial NANC component of the motor response to field stimulation produced by morphine and clonidine may be related to the ability of these drugs to potentiate both the secondary noradrenergic component and the overflow of radioactivity, if the NANC transmitter is involved in regulating NA release.

A comparison of the effects of morphine withdrawal, thyroxine or thyroidectomy on the sensitivity of the anococcygeus muscle to agonists and on serum thyroxine levels in the rat

Abstract The effects of morphine withdrawal, thyroxine or thyroidectomy on the plasma levels of thyroxine and on the sensitivity of the anococcygeus muscle to acetylcholine and noradrenaline were investigated in the rat. Morphine withdrawal or thyroidectomy increased the maximum response of both agonists and morphine withdrawal increased the pD 2 value for acetylcholine. Thyroxine did not affect the maximum responses but increased the pD 2 values for both agonists. Plasma thyroxine levels were raised by thyroxine administration and were reduced by thyroidectomy and also during morphine administration and withdrawal.