Receptor mediation of the stimulus properties of cholecystokinin

Drug discrimination learning: Interoceptive stimulus control of behavior and its implications for regulated and dysregulated drug intake

Drug discrimination research has generated rich evidence for the capacity of interoceptive drug stimuli to control behavior by serving as discriminative cues. Owing to its neuropharmacological specificity, drug discrimination learning has been widely used to characterize the stimulus effects and neuropharmacological underpinning of drugs. Apart from such utility, discriminative drug stimuli may help regulate drug use by disambiguating conditioned associations and post-intake outcomes. First, this review summarizes the evidence supporting interoceptive regulation of drug intake from the literature of exteroceptive discriminative control of drug-related behavior, effects of drug priming, and self-titration of drug intake. Second, an overview of interoceptive control of reward-seeking and the animal model of discriminated goal-tracking is provided to illustrate interoceptive stimulus control of the initiation and patterning of drug intake. Third, we highlight the importance of interoceptive control of aversion-avoidance in the termination of drug-use episodes and describe the animal model of discriminated taste avoidance that supports such a position. In bridging these discriminative functions of drug stimuli, we propose that interoceptive drug stimuli help regulate intake by disambiguating whether intake will be rewarding, nonrewarding, or aversive. The reflection and discussion on current theoretical formulations of interoceptive control of drug intake may further scientific advances to improve animal models to study the mechanisms by which interoceptive stimuli regulate drug intake, as well as how alterations of interoceptive processes may contribute to the transition to dysregulated drug use.

Conditioned Taste Avoidance Drug Discrimination Procedure: Assessments and Applications

In the present chapter, we summarize much of the work on the taste avoidance drug discrimination procedure, presenting the logic for its initial introduction and the extension of the procedure in the investigation of the discriminative properties of various drugs. Results from these assessments parallel those from more traditional operant and maze designs in classifying and characterizing the discriminative properties of drug. At the same time, this design reveals a procedure that is sensitive in such assessments by indexing these stimulus properties more rapidly and at lower doses than in the more traditional procedures (in some cases for drugs heretofore resistant in their detection). Importantly, much remains to be learned about the taste avoidance procedure in that the nature of such learning remains unknown and the specific parameters under which it can be established and generalized and its neurochemical and neuroanatomical bases are largely unexplored. The application of drug discrimination learning to human drug abuse continues to be an important consideration for this specific design (as well as that of drug discrimination procedures in general), and recent parallels between drug use and food intake in terms of its regulation by interoceptive stimuli suggests a possible role of the loss of stimulus control in drug escalation and addiction (with possible therapeutic implications via the modulation of these interoceptive cues).

The inability of CCK to block (or CCK antagonists to substitute for) the stimulus effects of chlordiazepoxide

To further examine the relationship between cholecystokinin (CCK) and GABA, the present study assessed the ability of the CCK-A antagonist devazepide and the CCK-B antagonist L-365,260 to substitute for the stimulus effects of chlordiazepoxide (CDP), as well as the ability of CCK-8s to block these effects, in female Long-Evans rats within the conditioned taste aversion baseline of drug discrimination learning. Both devazepide and L-365,260 failed to substitute for the discriminative stimulus properties of CDP, and CCK-8s failed to block its stimulus effects. The benzodiazepine diazepam did substitute for, and the benzodiazepine antagonist flumazenil did block, the stimulus effects of CDP. This suggests that the lack of substitution for, or antagonism of, CDP by the CCK antagonists and CCK-8s, respectively, was not due to the inability of the present design to assess such effects. Possible bases for the current findings, e.g., necessity of an anxiogenic baseline, drug and receptor specificity, as well as the dose-response nature of the interaction, were discussed. Given that a relationship between CCK and GABA has been reported in other designs, the present results suggest that such a relationship may be preparation specific.

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.

Effects of mu- and delta-opioid-receptor antagonists on the stimulus properties of cholecystokinin

Melton and Riley recently reported that the relatively selective mu-opioid-antagonist naloxone potentiated the stimulus properties of the gut peptide cholecystokinin (CCK). To assess whether such opioid potentiation is limited to activity at the mu-receptor subtype, in the present experiment the effects of the highly selective delta-antagonist naltrindole on CCK's stimulus properties were examined. Because in the initial report of naloxone's potentiation of CCK a relatively high, nonphysiologic dose of CCK (i.e., 13 micrograms/kg) was used as the training drug, in the current analysis subjects were trained to discriminate 5.6 micrograms/kg CCK from its vehicle and the assessments and comparisons of the effects of naloxone and naltrindole were based on this dose. Specifically, rats were administered 5.6 micrograms/kg CCK before saccharin-LiCl pairings and the CCK vehicle before saccharin alone. With such training, they rapidly acquired the drug discrimination, avoiding saccharin consumption when it was preceded by CCK and consuming the same saccharin solution when it was preceded by its vehicle. In subsequent generalization tests, doses of CCK that were ineffective in suppressing saccharin consumption (i.e., did not substitute for the training dose of CCK) did result in the suppression of saccharin consumption when combined with doses of the mu antagonist naloxone that alone had no effect on saccharin intake. On the other hand, the highly selective delta-opioid-receptor antagonist naltrindole was ineffective in potentiating the effects of CCK. Specifically, when naltrindole was combined with ineffective doses of CCK, subjects drank at control levels. The ability of naloxone to potentiate CCK's stimulus effects is consistent with a range of other demonstrations of the role of the mu-opioid-receptor subtype in CCK-opioid interactions, although the specific basis for the interaction remains unknown. Given recent findings on the effects of delta agonists and antagonists on CCK-induced activity, the failure of naltrindole to potentiate CCK's stimulus effects may be due to the absence of delta activity within this preparation, rather than the absence of delta mediation of CCK-opioid interactions in general.