Morphine dependence in the isolated guinea-pig ileum and its modification by p-chlorophenylalanine

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.

Naloxone-precipitated contractions of the guinea pig ileum made tolerant to a synthetic opioid peptide: evidence against the involvement of capsaicin-sensitive neurones

In segments of guinea pig ileum made tolerant-dependent to the synthetic opioid peptide [D-Met2,Pro5]enkephalinamide, naloxone (1 microM) caused half-maximal longitudinal contractions. These were strongly reduced by hyoscine or desensitization to serotonin but not by hexamethonium, desensitization to substance P or pretreatment with capsaicin. Contractions of similar size evoked by capsaicin (0.3 microM) in the presence of naloxone were abolished by capsaicin pretreatment, reduced by substance P desensitization or hyoscine but not by desensitization to serotonin. Thus, the two types of contraction clearly have different mechanisms; enteric capsaicin-sensitive neurones or substance P-containing motor neurones seem not to be involved in the naloxone-precipitated contraction while serotonin may play a role in this response.

Kindling-like convulsive activities in the isolated ileum of the guinea pig. I. Electrical stimulation

A kindling-like convulsive activity model produced by subthreshold, iterative electrical stimulation of the isolated male guinea pig ileum is described. In this model, the mechanical activity of the longitudinal muscular fibers of the ileum was systematically quantified in terms of the basic frequency, tonus, amplitude, and transient amplitude increments (A, n spikes/20 min) of "normal" contractions and presence of high-amplitude, paroxysmal, "epileptiform" contractures (B, n spikes/20 min). Changes in these parameters were statistically compared through consecutive stages of the same stimulated male ileum (I = initial activation, II = initial inhibition. IIIA = late activation, and IV = late inhibition) and equivalent stages of stimulated female and nonstimulated male ilea where "kindling-like" activities were only occasionally observed. Basic tonus, amplitude, and number of A spikes showed significant changes through consecutive stimulated male ileum "kindling" stages: increased from baseline to stage I, decreased from I to II, increased from II to III and from III to IIIA, and decreased from IIIA to IV. The number of B spikes significantly increased from II to III, III to IIIA, and IIIA to IV. No significant changes in baseline frequency were found through all stages nor in tonus, amplitude, A and B spikes between stage IV and the self-sustained activity observed 120 min after stimulation. In addition, there were significant correlations between B spikes vs. basic tonus and amplitude and A spikes when the stimulated male ileum shifted from stage II to IIIA (positive correlations) and from IIIA to IV (negative correlations). Basic tonus during stage I, basic amplitude during IIIA, A spikes during self-sustained activity and B spikes during III, IIIA, IV, and self-sustained activity were in the stimulated male ileum larger than those in both the stimulated female and the nonstimulated male ilea. Tonus and A spikes during III and IIIA in the stimulated male ileum were larger than in the nonstimulated, whereas tonus during IV and self-sustaining activity and amplitude during II in the stimulated male ileum were smaller than in the female stimulated ileum.

Kindling-like phenomenon in the isolated ileum of the guinea-pig

Repeated bursts of low voltage electrical stimulation of the isolated ileum of the guinea-pig gradually leads to the development and progressive intensification of the tissue basal activity, culminating in spontaneous, sudden strong contractions of the preparation, which persist for several hours after the stimulation has been discontinued. The magnitude of these alterations are determined by the parameters of the stimulation, mainly by the number of electrical stimulations, the frequency of stimulation, and the interstimulus interval. Maximal alterations are obtained with periods of stimulation of 20 Hz for 10 sec, pulses of 3.0 msec, repeated every 20 min for 15 times. Phenytoin, flunitrazepam, diazepam, phenobarbital and carbamazepine effectively inhibited the fully developed phenomenon in the tissues. The effect described in this report may be related to kindling in the brain.

Model of opiate dependence in the guinea-pig isolated ileum

1 Segments of ileum, incubated for 2-24 h at 22 degrees C with normorphine (0.01 - 1.0 muM), in the presence of hexamethonium, contracted when challenged with naloxone (0.03 muM). No response to this dose of naloxone was induced either by incubation in control solution without opiate for 2-24 h or by exposure of the preparation to opiate for 30 min at 37 degrees C.2 When segments were incubated for 24 h, the size of the response to naloxone was directly related both to the normorphine concentration in the incubation fluid (0.01 to 0.1 muM), and to the concentration of naloxone applied (0.03 to 0.1 muM).3 A spontaneous withdrawal contracture was elicited in ilea that had been incubated with normorphine (1.0 muM), when the normorphine-containing bathing fluid was exchanged for one without opiate.4 Normorphine restored to resting level the tension of the withdrawal contracture, whether it had been elicited spontaneously or by naloxone challenge.5 Addition of naloxone (1.0 muM) to normorphine (1.0 muM) in the incubation fluid abolished the withdrawal contracture to subsequent challenge with naloxone.6 Naloxone elicited a contracture from segments incubated for 24 h at 22 degrees C with levorphanol (0.1 muM) but not from those incubated with dextrorphan.7 Application of (+)-naloxone (0.03 muM) to segments previously incubated with normorphine (0.1 muM) did not elicit a contracture.8 The contracture elicited by naloxone in preparations incubated with morphine (10 muM) was associated with a reduction in sensitivity to the acute inhibitory effect of morphine on the electrically-evoked response.9 Addition of hyoscine (0.5 muM) immediately after challenge with naloxone restored the tension of the withdrawal contracture to resting level.10 Tetrodotoxin (3.0 muM) given before challenge, prevented naloxone from eliciting a withdrawal contracture.11 The inclusion of 5-hydroxytryptamine (10 muM) with morphine (10 muM) inhibited the induction of tolerance to morphine.12 These experiments, together with those described earlier, indicate that incubation with opiate induces a dependence in the final cholinergic motor neurones of the myenteric plexus, manifested as a contracture of the longitudinal muscle on removal of opiate or administration of an antagonist. This dependence is associated with tolerance, expressed as a decrease in sensitivity to inhibition by morphine of the electrically-evoked contracture. Tolerance and dependence are induced and withdrawal precipitated through specific and stereospecific opiate receptors.

Pharmacological characterization of opiate physical dependence in the isolated ileum of the guinea-pig

1 Physical dependence was produced in ilea from naive guinea-pigs by exposure of the tissue to different opiates for logarithmically-spaced periods of time (20-320 min). The responsiveness of the tissue to naloxone, as indicated by a strong contracture of the ileum, was enhanced in contrast to that found in intestines not exposed to opiates. 2 The dose-response curves to naloxone obtained in tissues individually exposed to different opiates showed that their relative potency in increasing sensitivity to naloxone was as follows: levorphan greater than morphine greater than Met-enkephalin greater than nalorphine greater than pentazocine. 3 The naloxone-induced response was dose-dependent and was directly related to the opiate concentration and length of exposure. 4 Dextrorphan the inactive isomer of levorphan, did not increase the responsiveness of the tissues to the narcotic antagonist, indicating that the phenomenon is stereospecific. 5 The naloxone-induced contraction in ilea exposed for 320 min to morphine (1 x 10(-6)M) was not prevented or suppressed by the administration of a large dose of morphine (1 x 10(-5)M) before or immediately after the naloxone challenge. 6 The evidence presented here shows that a phenomenon resembling in vivo opiate physical dependence can be acutely produced in vitro with pharmacological characteristics similar to other naloxone-induced abstinence effects.

An electromyographic method for the assessment of naloxone-induced abstinence in morphine-dependent rats

The electromyographic (EMG) activities of suprahyoideal muscle were recorded to measure naloxone-precipitated abstinence signs in morphine-dependent rats anesthetized with urethane (1 g/kg). Rats were rendered dependent on morphine by implanting 2 morphine pellets (75 mg each) and abstinence signs were induced by intravenous injections of various doses of naloxone at different times after pellet implantation. Three precipitated abstinence signs, a) myoclonic twitch activity (MTA), b) mastication, and c) body shakes were observed on EMG recordings after the injection of naloxone. Of these symptoms, only the MTA induced by naloxone (10 microgram/kg) occurred 4h after pellet implantation and its sensitivity to naloxone increased with prolonged pellet implantation. Both mastication and precipitated shakes could be induced at 24h. However, higher doses of naloxone were required to produce the shakes than is required to induce mastication. There appears to be a positive correlation between the intensity of naloxone-induced MTA and the degree of physical dependence on morphine. Since the MTA and mastication can be induced by low doses of naloxone in morphine-dependent rats, we suggest that these two parameters may be used to detect morphine abstinence signs in this species.