Further evidence that naloxone acts as an inverse opiate agonist: implications for drug dependence and withdrawal.

To test if naloxone behaved as an inverse agonist rather than as an antagonist we evaluated its responses in guinea-pig ilea with and without morphine (480 nM, 24 h). In control ilea, naloxone (100 nM) had no effect. In morphine-treated ilea, naloxone as a bolus, but not as an infusion, elicited an abstinence response. Preadministration of naloxone blocked the response to subsequent administrations. Similarly, naloxone failed to produce an abstinence response in ilea pretreated with kappa compounds (bremazocine, U50488 or xorphanol 100 nM) or with kinase inhibitors (H7 or H8 30 microM). These findings can be interpreted in the light of the two-state receptor model if naloxone behaves as an inverse agonist: Incubation with morphine increased the active state of receptors making them susceptible to the inverse agonist (naloxone); exposure to naloxone favored the inactive conformation making them insensitive to further administration of naloxone; kappa compounds behaved as antagonists preventing the response to naloxone; and kinase inhibitors interfered with the active conformation making the system insensitive to naloxone. According to this model, dependence can be viewed as an overexpression of the active receptors and withdrawal as an abrupt change from the active to the inactive state.

Effects of indorenate on food intake: a comparison with fenfluramine and amphetamine.

Indorenate (TR3369, 5-methoxytryptamine b-methylcarboxylate HCl) is a 5-HT1-like receptor agonist with hypotensive activity. Here, we describe that indorenate also decreases food intake (ED50 26.1 mg/kg) without an appreciable effect in water intake (the estimated ED50 for water was 589.8 mg/kg). The anorectic activity of indorenate was compared to the effects of amphetamine and other serotonin agonists; the effect of indorenate was smaller than those of the other compounds; however, the effect of indorenate was specific to food, whereas all the other drugs also produced significant decrements in water intake. The serotonin antagonists cinanserin, cyproheptadine, methergoline and methysergide effectively prevented the decrease in food intake produced by indorenate and fenfluramine. Haloperidol, a dopaminergic antagonist, was ineffective in preventing the effect of indorenate although it prevented the anorectic effect of amphetamine. The present results suggest the participation of serotoninergic, but not dopaminergic mechanisms, in the decrease in food intake produced by indorenate.

Effect of temperature on opioid dependence and on the abstinence response in the isolated guinea pig ileum.

This paper explores, in the isolated guinea-pig ileum, the effects of temperature on the acute development of opioid dependence and on the precipitation of the abstinence response, using as reference the effect of temperature on the response to a standard nicotine concentration. Additionally, the influence of temperature on acute morphine neurodepression was examined. Three experimental groups were included. In the first, the bath temperature was adjusted and maintained along the experimental session (2.5 h) at one of the following values: 28, 32, 36 or 40 degrees C. In the second, the different values of bath temperature were applied only during the period of morphine exposure before testing the abstinence response at 36 degrees C. In the third, all segments were initially incubated at 36 degrees C for 1 h, and afterwards, abstinence and the nicotine response were elicited at the different temperatures mentioned. In all the series, a single challenge naloxone dose (3.1, 10, 31, 100, 316, 1000 or 3160 nM) was administered after 1h of morphine and complete naloxone concentration-response curves were obtained. The abstinence response was expressed as a percentage of the nicotine reference response. All segments showed robust nicotine responses at all the experimental protocols tested indicating that, at the temperature range studied, the contractile mechanisms were impaired. This study showed that changes in bath temperature modify the magnitude of acute morphine neurodepression, and of the abstinence response but did no affect the development of acute opioid dependence. These data, along with several lines of evidence, strongly suggest that acute neurodepression, the development of opiate dependence and antagonist-precipitated abstinence are separable. Results are discussed on the basis of drug-receptor interactions.

[Demonstration of clinical bioequivalency of teldane and feliberal using a bioassay with intradermal histamine challenge in healthy volunteers]. / Demostración de la bioequivalencia clínica de Teldane y Feliberal mediante un bioensayo con reto de histamina intradérmica en voluntarios sanos.

To demonstrate the bioequivalence of two terfenadine formulations, a clinical-equivalence trial was performed. The antihistaminic's ability to prevent the response to intradermal histaminic challenge was herein assessed. Twelve healthy volunteers underwent a randomized, double blind, cross over trial with seven-day treatments. Dermal response to histamine concentrations of 0, 1, 2, 5 and 10 mcg was determined by measuring the wheal produced on the deltoid area. Measurements were made 72 hours prior to terfenadine administration, 1 hour after the first dose and 11 hours after second and last doses of each treatment. Both formulations showed the same latency, extent and duration in protective effect against histaminic challenge. This confirms the clinical equivalence of the two formulations and suggests that they have a similar bioavailability.

Cardiovascular effects of different schedules of nicotine administration on spinal rats: influence of pentobarbital.

This work examined the cardiovascular effects of different schedules of i.v. nicotine administration with respect to doses and timing in non-anesthetized and pentobarbital-treated spinal rats. For this purpose, complete nicotine dose-response curves were made for mean arterial pressure and heart rate. In non-anesthetized spinal rats, tachyphylaxis was not found for the pressor effects; moreover, consecutive doses, given at short intervals, produced additive actions. Furthermore, nicotine produced a biphasic heart rate response: an initial and brief bradycardia followed by a longer lasting tachycardia. In pentobarbital-treated rats, the sensitivity of the cardiovascular system to nicotine was decreased; in these rats, consecutive nicotine doses did show tachyphylaxis for the pressor and tachycardiac responses. The present series of experiments, using different schedules of administration for single and consecutive nicotine doses, demonstrated opposite tachyphylactic effects in non-anesthetized and in pentobarbital-treated spinal rats.

Hypothalamic monoamine oxidase activity in ovariectomized rats after sexual behavior restoration.

The effect of estradiol benzoate, progesterone and a sequential treatment with both on the activity of the enzyme monoamine-oxidase (MAO) was assessed in mitochondria from hypothalami of ovariectomized rats. A differential effect on the subtypes A and B MAO was found according to the type of treatment. Estradiol benzoate administration decreased MAO activity, mainly that of MAO-A. Progesterone alone had no effect, and sequential treatment with estradiol benzoate plus progesterone restored sexual behavior and produced a significant increase of MAO-A activity, without changes in total MAO activity. Since MAO-A is an isoform of MAO that preferentially uses norepinephrine and serotonin as substrates and MAO-B acts on phenylethylamine and benzylamine as substrates, our findings suggest that the restoration of sexual behavior after the treatment with estradiol benzoate followed by progesterone may be associated with the differential effect exerted by the hormones on MAO subtypes, rather than to the simple decrease in hypothalamic monoamine concentrations as reported in the literature.

Characterization of the analgesic effect of paracetamol and caffeine combinations in the pain-induced functional impairment model in the rat.

The analgesic activities of paracetamol (100, 178, 316 and 562 mg kg-1), caffeine (10, 18, 32 and 56 mg kg-1) and combinations of these doses were tested on a pain-induced functional impairment model in the rat. Dysfunction of the right hind limb was induced by an intra-articular injection of 30% uric acid in the knee. Drugs were given orally and the recovery of functionality over time was considered as an expression of analgesia. Paracetamol alone induced a dose-dependent analgesic effect whereas caffeine alone did not show any activity at the assayed doses. Combinations of 316 mg kg-1 paracetamol with either 10, 18, 32 or 56 mg kg-1 caffeine yielded analgesic effects significantly greater than that of paracetamol alone. The highest potentiation was observed with a paracetamol-caffeine mixture of 316-32 mg kg-1. Caffeine coadministration, however, did not significantly change paracetamol plasma levels. No potentiation was obtained with other combinations. Paracetamol plasma levels and analgesic effect observed with administration of 316 mg kg-1 paracetamol alone or 316-32 mg kg-1 of paracetamol-caffeine were fitted to the sigmoidal Emax model according to the Hill equation. The curves obtained were parallel, but that of the combination was shifted to the left. It is concluded that caffeine is able to potentiate the analgesic effect of paracetamol by a pharmacodynamic mechanism, but this only occurs at certain dose combinations.

Quantitative analysis of concentration-response curves for precipitated abstinence responses in the opiate-dependent guinea pig ileum.

Our group has introduced a theoretical model of drug-receptor-effector interactions that can account for most of the kinetic aspects of opiate dependence and abstinence. The present study analyzes the experimental concentration-response curves for precipitated abstinence responses in the isolated guinea pig ileum and compares them with the curves generated by the mathematical model. Two experimental series were included: in the first, the effects of morphine exposure time on dependence development were analyzed; in the second, the effects of morphine concentration were studied. The results show that both the exposure time and morphine concentration determine the magnitude of precipitated abstinence responses. Dependence can be detected after exposure times to opiates as short as 7.5 min. The concentration-response curves for abstinence responses precipitated by naloxone in ilea exposed to morphine for different periods showed: 1) an initial increase in Emax; 2) a parallel shift to the left in the position of the abstinence curves as dependence develops; and 3) an asymptotic limit to the displacement on the left. The results presented here fit well with theoretical predictions. Practical and theoretical implications are discussed.

Acute opioid dependence in the cardiovascular system of the spinal rat.

This article analyzes and characterizes the early development of opioid dependence in the cardiovascular system of the spinal rat. The main part of the study deals with changes in mean arterial pressure and heart rate induced by naloxone (NLX) in spinal rats pretreated with single doses of morphine. The results show that the cardiovascular system of the spinal rat is highly sensitive to the abstinence-precipitating actions of NLX after a single dose of morphine. Dose-response curves for precipitated abstinence evaluated as changes in mean arterial pressure and heart rate show a gradual increase in maximum followed by a progressive shift to the left as dependence progresses. Cardiovascular abstinence is mostly mediated by catecholaminergic systems. NLX also precipitates noncardiovascular signs of abstinence. Interestingly, morphine-free spinal rats gave some abstinence-like responses to NLX, probably because of endogenous release of opioids. The spinal rat seems to be a valuable system for a rapid quantitative pharmacological characterization of the abstinence-precipitating actions of opioid antagonists and for the study of the changes that occur during the acute development of opioid dependence.

Relationship between paracetamol plasma levels and its analgesic effect in the rat.

The relationship between plasma levels of paracetamol and its analgesic effect was studied in the rat using a model of pain-induced functional impairment (PIFI). Female Wistar rats received an intraarticular injection of 30% uric acid in the knee of the right hind limb, inducing its dysfunction. Animals then received oral paracetamol at doses of 178, 316 or 562 mg kg-1 and the recovery of functionality over time was considered as an expression of analgesia. Paracetamol plasma levels were determined by HPLC. Results showed that there is a direct relationship between paracetamol plasma levels and its analgesic effect that follows a sigmoidal model according to the Hill equation. The PIFI model appears to be a useful tool to establish pharmacokinetic/pharmacodynamic relationships for non-narcotic analgesics.

A methodological basis for improving the reliability of measurements of opiate abstinence responses in the guinea-pig ileum made dependent in vitro.

The purpose of this study was to find a neurogenic response of the isolated guinea-pig ileum that could serve as an internal standard to normalize abstinence responses, which are also neurogenic, during opiate dependence produced in vitro. The internal standard is required because of baseline variability in these responses of the ileum and a time-dependent decay in neuroeffector responsiveness with prolonged incubation. Systematic studies were made of the variability in the responses to neurogenic stimulation by 1) electrical field stimulation, 2) nicotinic stimulation, and 3) precipitation of opiate abstinence with opiate antagonists as well as studies of the time-dependent decay in responsiveness with prolonged incubation. The three neurogenic responses show covariation but the best correlation is found between the nicotinic and the abstinence responses. The nicotinic system presents a pharmacological sensitivity to specific acute opiate action and also shows an improving correlation with abstinence which develops with the progression of dependence. This correlation tends to a direct linear relation with a slope approaching 1.0. The nicotinic response of the ileum seems to be a valid internal control to normalize its abstinence responses after incubation with opiates for different intervals of time.

Super-reactivity to amphetamine toxicity induced by schedule of reinforcement.

The chronic exposure of rats to a schedule of operant water reinforcement coupled with chronically restricted access to water sensitized the animals to intermittent d-amphetamine injections (0.31-2.5 mg/kg with intervals of 12-23 days between any two injections) in such a way that this drug came to produce catastrophic losses of body weight (32.4% of control levels). In the sessions when d-amphetamine was administered, the rats were also given a total of 12 brief electric shocks. Loss of body weight was unaccompanied by parallel changes in operant behavior performance, or in food or water intake. Remarkably, in other studies with the same interventions (sham schedule sessions, water deprivation, and foot shocks), with the exception that reinforcers were never delivered, d-amphetamine did not produce catastrophic falls in body weight. This super-reactivity to d-amphetamine toxicity may be mediated by a possible stressor action of the schedule of reinforcement. Its mechanism might be analogous to the known sensitization produced by classical experimental stressor stimuli to the repeated administration of d-amphetamine.

Xorphanol.

Xorphanol is a new mixed agonist-antagonist from the morphinan class of analgesics. On the basis of animal experiments, the physical dependence liability of xorphanol is predicted to be of a low order in man. Conceptually, xorphanol is of interest since in vitro experiments have revealed anti-naloxone properties and resistance to antagonism by opioid antagonists. At the practical level, xorphanol is a well tolerated, orally active analgesic that provides effective pain relief clinically.