Kiyoshi Matsuno

Involvement of σ1 receptor in (+)-N-allylnormetazocine-stimulated hippocampal cholinergic functions in rats

The effects of the stereoisomers of N-allylnormetazocine (SKF-10,047) on the hippocampal cholinergic functions were compared in rats. A putative sigma 1 receptor agonist, (+)-SKF-10,047, elicited an increase of hippocampal extracellular acetylcholine level and anti-amnesic effect against scopolamine-induced memory dysfunctions in rats. These phenomena were not produced by (-)-SKF-10,047, and were reversed by haloperidol, a putative sigma 1 receptor antagonist. Such stereoselectivity and antagonism imply an involvement of sigma 1 receptors in these (+)-SKF-10,047-stimulated hippocampal cholinergic functions.

Antinociceptive activity induced by tizanidine and α2-adrenoreceptors

Abstract Tizanidine [5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiadiazole] is able to increase the pain threshold in the tail-flick test in mice. The effect of tizanidine was investigated after treatment of mice with drugs influencing central monoaminergic and GABAergic mechanisms. A drug that inhibits the synthesis and storage of monoamines and drugs that cause specific lesions of monoaminergic neurons had no consistent effect on the antinociceptive action of tizanidine. The action of tizanidine was antagonized by the α 2 -adrenoreceptor antagonist, yohimbine, but not by the α 1 antagonist prazosin, nor by dopamine, serotonin and GABA receptor antagonists. These results indicate that the antinociceptive action induced by tizanidine may be mediated by α 2 -adrenoreceptors.

Antinociceptive action of tizanidine in mice and rats

SummaryThe antinociceptive action of tizanidine [5-chloro-(2-imidazolin-2-yl-amino)-2,1,3-benzothiadiazole], a centrally acting muscle relaxant, was evaluated after subcutaneous or peroral administration in mice and rats. Tizanidine strongly inhibited the writhing response induced by acetic acid, phenyl-p-benzoquinone and acetylcholine in mice, and its potency was found to be greater than that of morphine. Tizanidine showed antinociceptive action like morphine not only in tail pressure and electrical stimulation tests in mice but also in tail-flick tests in mice and rats. The antinociceptive action of tizanidine was unaffected by pretreatment with naloxone. These findings suggest that tizanidine develops relatively strong antinociceptive action by a nonopioid mechanism.

Comparison of α-adrenoceptor involvement in the antinociceptive action of tizanidine and clonidine in the mouse

The effect of drugs that influence the opioidergic and monoaminergic neuronal systems on the antinociceptive action of tizanidine [5-chloro-4-(2-imidazolin-2-yl-amino)-2,1, 3-benzothiodiazole] was compared with their effect on the action of clonidine. The potency of the clonidine-induced antinociceptive action was 1.83 and 7.75 times greater than that of tizanidine in the tail-flick and acetic acid-induced writhing tests, respectively. The action of tizanidine and clonidine was completely antagonized by pretreatment with yohimbine, an alpha 2-adrenoceptor blocker, but not by prazosin, an alpha 1-adrenoceptor blocker. Other alpha-adrenoceptor blockers, phenoxybenzamine and phentolamine, also attenuated the action of tizanidine and clonidine but the potency of these drugs was less than that of yohimbine. An opioid antagonist (naloxone), drugs influencing the serotonergic neuronal system (p-chlorophenylalanine, 5,6-dihydroxytryptamine, cyproheptadine), and drugs influencing the catecholaminergic system (alpha-methyl-p-tyrosine, diethyl-dithiocarbamate, 6-hydroxydopamine, haloperidol) showed no effect on the action of tizanidine and clonidine. From these results, it appears that alpha 2-adrenoceptors might be of importance in mediating the tizanidine and clonidine antinociceptive action in the tail-flick test.

Electric footshock-induced changes in behavior and opioid receptor function

The present electric shock (ES) schedule produced significant behavioral changes, such as analgesia and motor suppression, and functional changes in binding capacities for opioid agonist and antagonist. In the naloxone (5 mg/kg, SC 15 min before ES application) pretreated rats, these behavioral and biochemical changes were blocked. In addition, when preincubation (37 degrees C, 30 min) was not carried out in the process of preparation of synaptic membrane, the ES-induced functional changes in hibh affinity binding sites were not observed. Moreover, the present data indicated that preincubation may produce the destruction of [3H]-D-ala2,L-met5-enkephalinamide ([3H]-DAMEA) specific binding sites with the forced dissociation of endogenous delta-type opioid peptides from delta opioid receptors. In addition, the significant decrease of [3H]-DAMEA specific binding in the ES membrane suggested that delta-type opioid peptides were released more than steady state level by ES application and bound to the delta opioid receptors. Therefore, these results suggest that ES-induced behavioral and biochemical changes were mediated by opioid peptides which were released by ES application. In addition, the ES-induced analgesia may be mediated by high affinity delta opioid receptor.

Involvement of spinal and supraspinal structures in tizanidine-induced antinociceptive action

The site of action of tizanidine (5-chloro-4-(2-imidazolin-2-yl-amino)2,1,3-benzothiadiazole) was investigated in a tail-flick test in comparison with clonidine and morphine. Tizanidine and clonidine produced a profound and linear dose-dependent antinociceptive action by i.c.v. administration. The ED50 values for tizanidine, clonidine and morphine were increased 2-4-fold 1 or 3 days after spinalization at C5-C6. These results indicate that the antinociceptive activity of tizanidine arises mainly from activity at the supraspinal level, similar to clonidine and morphine, but these drugs act also at the spinal level at high doses.

Comparison of tizanidine and morphine with regard to tolerance-developing ability to antinociceptive action.

The antinociceptive, tolerance-developing and anti-withdrawal activities of tizanidine [5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzo-thiadiazole] were investigated by comparing its effects with those of morphine and clonidine in tail-flick-, hot plate-, acetic acid-induced writhing-, and naloxone-precipitated withdrawal jumping-tests. The antinociceptive action of tizanidine was not altered by naloxone, while that of morphine was antagonized. Tolerance to the tizanidine-induced antinociceptive action and to motor incoordination was developed by successive administration of tizanidine. In the tizanidine-tolerant mice, the antinociceptive action of morphine was significantly decreased, but not sleeping time induced by pentobarbital. The action of tizanidine was not modified in the morphine-tolerant mice. Tizanidine failed to induce morphine-withdrawal jumping and to inhibit naloxone-precipitated withdrawal jumping in the morphine-dependent mice. Cross tolerance to the antinociceptive action induced by tizanidine and clonidine was developed. These results suggest that alpha 2-adrenoreceptors may be involved in the action mechanism of tizanidine, but not opioid receptors. Functional tolerance to tizanidine action may be developed by successive administration of tizanidine.

Functional alteration of opioid receptor subtypes in the mice exhibited conditioned suppression in motility

Mice exhibit a marked suppression of motility (conditioned suppression) when placed in the same environment in which they had previously received the electric footshock. The present study was designed to investigate the functional change of opioid receptor subtypes in the conditioned suppression group using an opioid binding assay technique. In the synaptic membrane of the conditioned suppression group, the binding capacities of [3H]naloxone at high and low affinity binding sites and of [3H]phencyclidine at high affinity binding site were significantly increased compared to those of the control group. On the other hand, the binding capacity of [3H]ethylketocyclazocine at both affinity binding sites in the conditioned suppression group was not changed. These results suggest that the binding function of different opioid receptor subtypes may be altered differently by stress.

Conformational changes of opioid receptor induced by the electric footshock

The present electric shock (ES) schedule followed in these experiments produced different functional changes in endogenous putative opioid agonist- and antagonist-type receptors, depending on the type of receptor: the amount of antagonist binding was increased by ES application, while the amount of agonist binding was decreased. In order to elucidate the mechanism of these changes, we investigated whether ES application was able to affect sulfhydryl-groups and phospholipids of endogenous opioid receptors. In comparison to the control membrane, the increased antagonist binding sites of the ES membrane were liable to be inactivated by the sulfhydryl-modifying reagents, N-ethylmaleimide (NEM) and iodoacetamide. However, in the presence of 100 mM Na, the antagonist binding sites of both the control and ES membranes were inactivated by NEM in the same manner. On the other hand, the agonist binding sites of both membranes were similarly inactivated by NEM regardless of the absence or presence of 100 mM Na. Another sulfhydryl-modifying reagent, such as p-chloromercuriphenylsulfonic acid, did not produce any difference between the control and ES membranes. The increased antagonist binding sites of the ES membrane were also liable to be inactivated by phospholipase A2. These results suggest that the present ES schedule followed in these experiments produces conformational changes in endogenous opioid receptors in the rat brain. As a result of these conformational changes, the amount of binding in the antagonist sites may be increased, while the amount of binding in the agonist sites may be decreased. However, the increased antagonist binding sites may be liable to be inactivated by NEM, iodoacetamide and phospholipase A2.