Morphine sensitization in the pentylenetetrazole-induced clonic seizure threshold in mice: Role of nitric oxide and μ receptors

Abstract

Behavioral sensitization occurs after repeated administration of μ-opioid receptor agonists following a drug-free period. It seems that the changes in dopaminergic systems induced by μ-opioid receptor agonists play a crucial role in behavioral sensitization to opioids. Nitric oxide also plays a role in some behavioral effects of morphine, including sensitization to the locomotor-stimulating effect. This study investigated whether morphine sensitization appears in seizure threshold and the possible role of μ-opioid receptor and nitric oxide in this sensitization. Sensitization was produced by daily injections of morphine (0.1, 0.5, 1, 5, 15, or 30 mg/kg), followed by a 10-day washout period. Then the challenge test was performed using morphine (0.1, 0.5, 1, 5, 15, or 30 mg/kg) in different groups. To assess clonic seizure threshold, pentylenetetrazole (PTZ) was administered intravenously. Subcutaneous administration of morphine (0.1 and 0.5 mg/kg) induced sensitization in PTZ-induced clonic seizures in mice. Intraperitoneal administration of L-NAME (20 mg/kg), a nonselective inhibitor of nitric oxide synthase, or naltrexone (10 mg/kg), an opioid receptor antagonist, along with morphine inhibited morphine-induced sensitization in PTZ-induced seizure threshold. In conclusion, at low doses, morphine induces sensitization in PTZ-induced clonic seizures in mice probably as a result of the interaction with μ-receptors and nitric oxide.

Introduction

Continuous exposure to morphine in rodents leads to the development of tolerance to most of its pharmacological effects, whereas intermittent exposure produces a subsequent augmentation of the motor-stimulating effects of this drug termed behavioral sensitization. Morphine sensitization has been reported using various pretreatment protocols and various doses [1], [2], [3], and it has been suggested that the temporal pattern of the drug treatment and posttreatment periods may be critical to the development of long-lasting behavioral sensitization and associated neuronal adaptations [3], [4].

Sensitization is characterized by a progressive augmentation of behavioral effects elicited by the repeated administration of drugs. Although repeated administration of morphine at intervals shorter than 12 hours may induce tolerance, repeated administration at interdose intervals of 24 hours elicited sensitization to the locomotor-stimulating effect [5]. In experimental studies, sensitization was observed after repeated administration of opioids [6], cocaine [7], amphetamine [8], ethanol [9], and nicotine [10]. Sensitization can last weeks to months after cessation of drug treatment [11], and it might contribute to a rapid relapse of drug abuse on reexposure to a drug. Behavioral sensitization is considered to be linked to central aspects of the development of drug addiction, such as drug craving and the persistence of compulsive drug-seeking behavior [12]. It has been argued that behavioral sensitization may be responsible for the development of addiction and especially for the high rate of relapse among drug addicts even after very long periods of abstinence [13], [14]. Therefore, investigation of morphine behavioral sensitization may help to better understand the pathophysiology of opioid-associated disorders, and a study of intervention in its behavioral sensitization may provide new strategies for the treatment of drug addiction.

Although it has been assumed that expression of opiate sensitization is related to the ability of the opiate to change dopamine responsiveness in the nucleus accumbens shell and core and in the dorsal caudate putamen [15], [16], it cannot be ruled out that neurochemical adaptations present in morphine sensitization are coupled to the stimulation of μ-opioid receptors, these receptors being the most involved in morphine sensitization [17], [18]. In morphine-sensitized animals μ-opioid receptor autoradiography revealed a significant increase in the caudate putamen, nucleus accumbens shell, prefrontal and frontal cortex, medial thalamus, hypothalamus, and central gray matter [19]. Furthermore, nitric oxide also mediates some of the behavioral effects of morphine. Morphine releases nitric oxide (NO) [20], and inhibitors of NO synthase (NOS) decrease morphine tolerance [21] and/or inhibit the development and expression of morphine withdrawal syndrome [22]. It has also been reported that N^G-nitro-L-arginine methyl ester (L-NAME) attenuates the development of sensitization to the locomotor-stimulating effect of morphine [23], [24] and cocaine [24].

Opiates exert differential effects on seizure threshold based on their dose and the animal model used [25]. Thus, in chemical models of seizures, morphine generally induces an anticonvulsant effect at lower doses, but is proconvulsant at higher doses [26], [27]. Moreover, endogenous opioids protect against seizures induced by electroconvulsive shock [28], [29], and have been implicated in the anticonvulsant properties of acute stress [30]. Both the anti- and proconvulsant effects of morphine in the chemical and electrical models of seizures can be reversed by μ-opioid receptor antagonists [26], [31].

This study investigated if the morphine sensitization affects seizure threshold. We also investigated the influence of an opioid receptor antagonist or a nitric oxide synthase inhibitor on seizure threshold in morphine-sensitized animals.