Comparison of naltrexone and quaternary naltrexone after systemic and intracerebroventricular administration in pigeons

Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

Role of ventral tegmental area, periaqueductal gray and parabrachial nucleus in the discriminative stimulus effects of morphine in the rat

Previous studies have produced mixed results about the role of the ventral tegmental area, periaqueductal gray and parabrachial nucleus in morphine discriminations, perhaps owing to the considerably different methodologies used. The purpose of the present study was to compare the roles of these three brain areas using the same food-reinforced discrimination protocol, to determine whether the schedule of reinforcement influenced maximal substitution produced by site-specific morphine administration and to determine whether the time course of substitution differed by site of morphine administration. Rats were trained to discriminate 3.0 mg/kg subcutaneous morphine from saline under variable interval 15-s or fixed ratio 10 schedules of food reinforcement. Rats were then implanted with one cannula aimed at the lateral ventricle (intracerebroventricular) and one aimed at the ventral tegmental area, periaqueductal gray or parabrachial nucleus. Morphine discrimination curves were obtained by subcutaneous, intracerebroventricular and intrasite routes. When administered subcutaneously, morphine was equipotent in variable interval-trained and fixed ratio-trained rats, although it was more potent in fixed ratio-trained females than fixed ratio-trained males. When administered intracerebroventricularly, morphine (0.3-10 microg) engendered a maximum average of 63% drug-appropriate responding in both variable interval-trained and fixed ratio-trained rats; females showed significantly greater drug-appropriate responding than males, again under the fixed ratio but not under the variable interval schedule. In variable interval-trained rats, intrasite infusions of morphine (0.3-10 microg) produced maximal drug-appropriate responding of approximately 57% (ventral tegmental area), 56% (periaqueductal gray) and 41% (parabrachial nucleus); mean maximal substitution was slightly (< or = 17%) greater in fixed ratio-trained rats. When injected into the ventral tegmental area or periaqueductal gray, but not the parabrachial nucleus, naloxone methiodide (2 microg) significantly decreased drug-appropriate responding following 3.0 mg/kg subcutaneous morphine, in both variable interval-trained and fixed ratio-trained rats. The time course of the discriminative stimulus effects of morphine differed among the three brain sites: intraventral tegmental area morphine produced peak drug-appropriate responding by 15 min after injection, whereas the discriminative stimulus effects of intraperiaqueductal gray and intraparabrachial nucleus morphine peaked at approximately 60 min after injection. Taken together, these results indicate that ventral tegmental area, periaqueductal gray and parabrachial nucleus each play a role in the ability of morphine to function as a discriminative stimulus, regardless of the sex of the subject or the schedule under which the subjects are responding. Ventral tegmental area and periaqueductal gray, however, appear to be more critical than parabrachial nucleus in mediating the discriminative effects of systemic morphine in rats responding under a food reinforcement procedure. The pretreatment time and, to a lesser extent, the schedule of reinforcement are additional variables that should be considered when comparing the relative roles of different brain areas in drug discrimination.

Drug discrimination in neurobiology

Areas of neurobiological interest are identified towards which drug discrimination (DD) studies have made important contributions. DD allows ligand actions to be analyzed at the whole organism level, with a neurobiological specificity that is exquisite and often unrivalled. DD analyses have thus been made of a vast array of CNS agents acting on receptors, enzymes, or ion channels, including most drugs of abuse. DD uniquely offers access to the study of subjective drug effects in animals, using a methodology that also is transposable to humans and has generated unprecedented models of pathology (e.g., chronic pain, opiate addiction). Parametric studies of such independent variables as training dose and reinforcement provide refined insights into the dynamic psychophysiological mechanisms of both drug effects and behavior. Three different mechanisms have been identified by which discriminative, and perhaps other behaviors, can come about. DD also is superbly sensitive to small, partial activation of molecular substrates; this has enabled DD analyses to pioneer the unravelling of molecular mechanisms of drug action (attributing, f.ex., LSD's particular subjective effects to an unusual, partial activation of 5-HT, and perhaps other receptors). DD has both oriented and served as a tool to conduct drug discovery research (e.g., pirenperone-risperidone, loperamide). The DD response arguably constitutes a quantal, rather than graded, variable, and as such allows a comprehension of molecular, pharmacological, and behavioral mechanisms that would have been otherwise inaccessible. Perhaps most important are the following further contributions. One is the notion that particular, different levels of receptor activation are associated with qualities of neurobiological actions that also differ and are unique, this notion arguably constituting the most significant addition to affinity and intrinsic activity since the earliest theoretical conceptions of molecular pharmacology. Another contribution consists of studies that render redundant the notion of tolerance and identify fundamental mechanisms of signal transduction; these mechanisms account for apparent tolerance, dependence, addiction, and sensitization, and appear to operate ubiquitously in a bewildering array of biological systems.

Discriminative stimulus function of ethanol: role of GABAA receptors in the nucleus accumbens

Because the subjective effects of drugs may be related to abuse potential, this study was conducted to assess the involvement of GABAA receptor systems in the nucleus accumbens (N Acc) in the discriminative stimulus effects of ethanol. Male Long-Evans rats were trained to discriminate between intraperitoneal (IP) injections of ethanol (1 g/kg) and saline under a fixed-ratio 10 schedule of sucrose (10% w/v) reinforcement. When performance during training sessions met the accuracy criteria (> 80% correct responding for five consecutive days), an ethanol generalization curve was determined. The rats were then surgically implanted with bilateral stainless-steel guide cannulae aimed at the N Acc. Intra-accumbens (IA) substitution test sessions were conducted during which the direct GABAA agonist muscimol (0.01, 0.04, 0.10, and 0.40 micrograms/microliter; IA) was administered in combination with saline (IP). The direct GABAA antagonist bicuculline (0.03, 0.10, and 0.30 micrograms/microliter; IA) was administered in combination with the training dose of ethanol (1 g/kg, ip). At 10-min postinjection, IA muscimol partially substituted for IP ethanol. However, at 15-min postinjection, muscimol (0.10 microgram/microliter; IA) fully substituted for IP ethanol. Bicuculline attenuated the discriminative stimulus properties of IP ethanol, but only at doses that significantly decreased response rate. At 10-min postinjection, muscimol (0.01 and 0.04 micrograms/microliter) potentiated (> 80% ethanol lever responding) the discriminative stimulus properties of a dose of ethanol (0.5 g/kg) that alone produced only partial generalization. These data suggest that ethanol discrimination is mediated centrally and demonstrate that infusions of the GABAA agonist muscimol in the N Acc are sufficient to produce the stimulus effects corresponding to a 1.0 g/kg training dose of ethanol. When taken together with data showing that GABAA receptor activation in the N Acc potentiates the termination of ethanol self-administration, these data suggest that ethanol's discriminative stimulus function may influence its reinforcement function.

Comparison of the discriminative stimulus function of ethanol following intracranial and systemic administration: evidence of a central mechanism

Rats were trained using a two-lever drug discrimination procedure to press one lever following systemic administration of ethanol (1.0 mg/kg, IP) and another lever following IP injections of saline. After determination of an ethanol generalization curve (0.25-1.25 g/kg, IP), rats were surgically implanted with bilateral stainless steel guide cannulae that terminated in the lateral ventricles. Following surgery, the generalization curve was redetermined and did not differ from presurgery values. Then, generalization to bilateral intracerebroventricular (ICV) injections of ethanol (600.0 and 900.0 mM, 1.0 microliter/side) were administered alone and in combination with IP injections of ethanol. The ICV ethanol injections produced partial generalization, but the combination of ICV ethanol (600.0 and 900.0 mM) with IP ethanol (0.25 and 0.50 g/kg) injections were two- to threefold more potent then IP injections alone. Response rates were unaffected by any dose of ethanol tested. These data suggest central mediation of ethanol's discriminative stimulus function due to: 1) increased potency of systemically administered ethanol by centrally administered ethanol, and 2) partial generalization between centrally and peripherally administered ethanol.

Tolerance and dependence after continuous morphine infusion from osmotic pumps measured by operant responding in rats

Food-deprived rats were trained to lever-press on a fixed interval 3-min schedule of food presentation. Using a cumulative dosing procedure, morphine and naltrexone were tested weekly for effects on rates of responding. A separate group of subjects was tested weekly for morphine analgesia using the tail-flick assay. Chronic morphine infusion (10 mg/kg/day, SC, from a 4-week osmotic pump) induced tolerance to the rate-decreasing and analgesic effects of morphine as demonstrated by a more than 4-fold increase in the morphine ED50s for both of these effects, relative to those of predependent rats. Physical dependence was evidenced in the operant procedure by increased sensitivity to the rate-decreasing effects of the opioid antagonist, naltrexone, such that the ED50 for naltrexone was 5000 times lower than it was in predependent animals. In addition, marked weight loss was observed over the 2-h naltrexone test session. The degree of tolerance was a function of both the length of chronic administration and of the maintenance dose and was of a similar magnitude for operant responding and analgesia. Further, the degree of tolerance and dependence was comparable to that observed in other studies that used different methods of chronic drug administration. Thus, osmotic pumps provide a convenient and effective alternative for chronic drug administration in behavioral studies.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.