RESUMEN

1. Myometrial responses to different agents acting on adrenoceptors were examined in vivo in the pro-oestrous rat. Changes in spontaneous uterine mechanical activity were recorded isometrically and evaluated in terms of amplitude and duration of uterine contractions. 2. Phenylephrine (10 micrograms kg-1) markedly increased the amplitude and duration of contractions and 40 micrograms kg-1 gave rise to tetanic contractions. 3. Administration of either nicergoline (400 micrograms kg-1) or phentolamine (1000 micrograms kg-1) to phenylephrine-primed rat uterus reduced the strength of contractions and phentolamine abolished the phenylephrine-induced uterine contracture. 4. Following blockade of alpha 2-adrenoceptors by yohimbine (1000 micrograms kg-1) and beta-adrenoceptors by propranolol (2400 micrograms kg-1), a single injection of phenylephrine (100 micrograms kg-1) increased the amplitude of uterine contractions by 30%. 5. Noradrenaline reduced the amplitude of contractions and caused elevation of the baseline level. The response of myometrium to the combination of both propranolol and noradrenaline was the establishment of uterine contracture with subsequent increase of the duration of contractions. 6. These results clearly demonstrate the involvement of alpha-adrenoceptors in the myometrial activity of the rat in vivo during pro-oestrus.

Asunto(s)

RESUMEN

Thiorphan and acetorphan, two potent inhibitors of enkephalinase (EC 3.4.24.11 membrane-metalloendopeptidase) significantly reduced the castor oil-induced diarrhea in rats when administered intravenously (or orally, for acetorphan) but not when administered intracerebroventricularly. These effects were more marked during the 90 min period following the castor oil challenge but were still significant up to 4-8 h after the latter. Acetorphan was about 6 times more potent than thiorphan. The antidiarrheal activity of both compounds was completely prevented in rats receiving naloxone subcutaneously but not intracerebroventricularly (in the case of thiorphan). In contrast to loperamide, a peripherally acting opiate receptor agonist, the enkephalinase inhibitors did not significantly reduce gastrointestinal transit as measured in the charcoal meal test. The antidiarrheal activity of enkephalinase inhibitors therefore seems attributable to protection of endogenous opioids, presumably outside the brain, and to reduction of intestinal secretion rather than transit.

Asunto(s)

RESUMEN

Acetorphan, i.e. N-[(R,S)-3-acetylmercapto-2-benzylpropanoyl]-glycine, benzyl ester, is a lipophilic derivative of Thiorphan, a potent inhibitor of "enkephalinase" (EC 3.4.24.11). On purified enkephalinase its inhibitory potency was approximately 1000 fold less than that of Thiorphan but became close to the latter (nanomolar) when it was incubated previously with cerebral membranes. After parenteral administration to mice and rats (1-10 mg/kg) extensive inhibition of cerebral enkephalinase was shown by the depressed enzyme activity in brain membranes from treated animals and the long-lasting potentiation of analgesia elicited by (D-Ala2,Met5)enkephalin (i.c.v.). This suggests that acetorphan easily enters the brain where the active Thiorphan is released. Parenteral acetorphan elicited a series of naloxone-reversible, opioid-like effects, most of which were described previously with intracerebral Thiorphan or other enkephalinase inhibitors. Antinociceptive effects were found in some tests (hot plate jump and phenylbenzoquinone-induced writhing) but not in others (hot plate licking and tail withdrawal). "Antidepressant" effect was found in the "mouse despair" test and antidiarrhoeal effect in the rat castor oil test. Acetorphan also elicited significant increases and decreases in turnover indexes of serotonin and noradrenaline, respectively, in mouse cerebral cortex. In mice chronically treated with acetorphan, the antinociceptive activity of the compound was not modified markedly and no overt withdrawal symptom could be observed after either treatment interruption or administration of naloxone.

Asunto(s)

RESUMEN

The effects of some opiates on the horizontal and vertical components of locomotor activity in mice were measured in actometers fitted with photoelectric cells counting horizontal displacements and the striding over a low partition wall. Morphine and fentanyl, used at low doses which did not modify the horizontal locomotor activity, decreased the striding over of the partition wall. Such a dissociation was not observed with either kappa or sigma agonists which affected in the same way the two components of the locomotor activity: both components were inhibited by kappa agonists and slightly stimulated by the sigma agonist SKF 10047. Although the opiate-induced inhibition of the striding over was induced by low doses of morphine, relatively high doses of naloxone were necessary for its antagonism. This apparently difficult antagonism could result in fact from the sedation which appeared when morphine was associated with naloxone.

Asunto(s)

RESUMEN

The spontaneous climbing behavior (SCB) developed by naive mice when they are introduced into small individual cylindrical cages with walls of vertical bars is inhibited in a dose-dependent manner by all the tested opiates. Only nalorphine administered over a large scale of doses never completely inhibits the SCB; its maximal effect is only about 50% of total inhibition. The efficacy of opiates on this special component of the locomotor activity has been compared with that on the horizontal displacements measured with an actometer equipped with photoelectric cells. Whereas the drugs known as specific kappa agonists depress both the horizontal locomotor activity and the SCB, most of the other tested opiates inhibit the SCB at doses which do not modify the horizontal locomotion. The low efficacy of loperamide, an opiate agent crossing the blood-brain barrier with difficulty, as well as the strong efficacy of a morphine low dose injected i.c.v. suggest that the opiate-induced SCB inhibition is centrally mediated. Finally, the involvement of mu and kappa receptors is discussed from the observed interactions with various opiate agonists, partial agonist (nalorphine) and antagonists (naloxone and diprenorphine).

Asunto(s)

RESUMEN

The intracerebroventricular injections in mice of the mu receptor agonists morphine and fentanyl induced an immobility state (the animals staying motionless with the head down on a 45 degree inclined plane) which was apparently hypertonic (catatonia ?) or at least enabled them to remain hanging on a horizontal wire with their forepaws. In similar conditions, injections of the kappa receptor agonists ketocyclazocine and bremazocine induced an immobility state which was hypotonic, in contrast with the preceding one. In a similar way to the mu agonists, Met-enkephalin or Leu-enkephalin injected i.c.v. in association with the inhibitor of enkephalinase thiorphan induced an apparently hypertonic immobility which was easily antagonized by naloxone. The association of thiorphan with bestatin (an inhibitor of aminopeptidases involved in enkephalins inactivation) produced similar results. In contrast, the hypotonic immobility induced by the kappa receptor agonists required relatively high doses of naloxone to be antagonized. The opiate antagonist MR 2266 antagonized equipotent doses of all the above mentioned agents with a similar efficacy. From these data it is suggested that enkephalins could induce an apparently tonic immobility by stimulating mu receptors and that endogenous enkephalins could be involved in a tonic mediation modulating the locomotor activity or regulating the muscular tone.

Asunto(s)

Asunto(s)

RESUMEN

New carboxyalkyl compounds derived from Phe-Leu and Phe-Ala were synthesized and checked as inhibitors of "enkephalinase", a metalloendopeptidase cleaving the Gly3-Phe4 bond of enkephalins from mouse striatal membranes. Differential recognition of both brain enkephalinase and angiotensin-converting enzyme (ACE) catalytic sites by these carboxylalkyl compounds lead to potent (KI approximately 0.5 microM), competitive and selective inhibitors of the enkephalin-degrading enzyme. The most interesting compound, N-[(RS)-2-carboxy-3-phenylpropanoyl]-L-leucine (3, KI = 0.34 microM), is 10000 times more potent on enkephalinase than on ACE activities. Intracerebroventricular (icv) injection of 3 in mice leads to a high potentiation of the analgesic effect of the exogenously administered D-Ala2-Met-enkephalin, evidencing the in vivo inhibition of enkephalinase. Moreover, icv administration of 3 alone induces a dose-dependent analgesia in mice measured on both hot-plate and writhing tests. In the former assay, the ED50 was approximately 10 micrograms per animal, slightly higher than that of thiorphan. All the antinociceptive effects were antagonized by naloxone, demonstrating the involvement of enkephalins in analgesia and their in vivo protection from enkephalinase by 3. The described compounds can be considered as first examples of a new series of analgesics and potentially psychoactive agents.

Asunto(s)

RESUMEN

Among the various cerebral enzyme activities able to hydrolyse the enkephalins into inactive fragments only two seem responsible for the metabolism of the endogenous opioid peptides: a dipeptidylcarboxypeptidase ("enkephalinase"), and a bestatin-sensitive aminopeptidase. Their inhibition by thiorphan and bestatin results in an antinociceptive effect observed in tests in which the nociceptive stimulation is probably accompanied by a concomittent release of enkephalins.

Asunto(s)

RESUMEN

In the presence of thiorphan an 'enkephalinase' inhibitor, bestatin an aminopeptidase inhibitor of bacterial origin potently inhibited the hydrolysis of [3H][Leu5]enkephalin by slices from rat striatum with an IC50 value of about 0.2 microM whereas puromycin was approximately 1000 times less potent on this preparation. In vivo bestatin or thiorphan (but not puromycin) significantly protected [3H][Met5]enkephalin administered intracerebroventricularly to mice from hydrolysis and co-administration of these two peptidase inhibitors resulted in a strong reduction in the appearance of hydrolysis products in brain. In a parallel fashion the antinociceptive activity of [Met5]enkephalin in the mouse hot-plate test was additively potentiated by bestatin and thiorphan but not by puromycin. Finally both bestatin and thiorphan themselves displayed antinociceptive properties on either the hot-plate jump test or the phenyl-benzo-quinone writhing test. It is concluded that a bestatin-sensitive aminopeptidase activity together with the 'enkephalinase' activity plays a critical role in the inactivation of both exogenous and endogenous enkephalins.

Asunto(s)

Asunto(s)

RESUMEN

Various cerebral peptidases are able to hydrolyse the enkephalins into inactive fragments. Among these enzymes enkephalin-dipeptidylcarboxypeptidase ("enkephalinase") inhibited by thiorphan and a bestatin-sensitive aminopeptidase activity seem to play a key-role as "inactivating neuropeptidases". Their inhibition, in vitro as well as in vivo, leads to a protection of endogenous enkephalins and to antinociceptive effects.

Asunto(s)