Nalorphine's ability to substitute for morphine in a drug discrimination procedure is a function of training dose.

Rats trained to discriminate the mu agonists fentanyl or morphine from their respective vehicles generalize to the partial mu agonist nalorphine incompletely and inconsistently. Any number of factors may influence the generalization patterns obtained, one of which being the specific dose of the full opioid agonist used during training, a factor reported to influence generalization with other partial opioid agonists. To assess if training dose influences stimulus generalization to nalorphine and to support its role in the aforementioned variability across studies, in the present experiments rats were trained to discriminate either a low (5.6 mg/kg) or a high (10 mg/kg) dose of morphine from distilled water within the taste aversion baseline of drug discrimination learning. Subjects were then given a range of doses of morphine, nalorphine, methadone, or naloxone to assess the degree of substitution (if any) of these compounds for the training dose of morphine. For all subjects, morphine fully substituted for itself, and the opioid antagonist naloxone failed to substitute for the morphine cue. Rats generalized the morphine cue to nalorphine in subjects trained at the lower dose but not in subjects trained at the higher dose. Rats generalized the morphine cue to methadone in the latter group (the high dose group), indicating that the failure to generalize to nalorphine in this group was not a general inability of an opioid agonist to substitute for morphine. Naloxone blocked morphine stimulus control in all subjects and nalorphine control in the low-dose group for which nalorphine substituted for morphine, suggesting that morphine control (and the nalorphine substitution) was based on opioid activity. These results indicate that the substitution patterns of nalorphine in morphine-trained subjects are a function in part of the dose of morphine used in training and support the position that nalorphine is a partial opioid agonist with intermediate efficacy.

Nalorphine as a stimulus in drug discrimination learning: assessment of the role of mu- and kappa-receptor subtypes.

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate nalorphine from distilled water. In subsequent generalization tests, the mu-opiate agonist morphine substituted for the nalorphine stimulus in a dose-dependent manner, while the kappa-opiate agonist U50,488H and the mu-opiate antagonists naloxone and naltrexone failed to do so. That the mu-agonist morphine substituted for the nalorphine stimulus while a kappa-agonist and mu-antagonists failed to substitute indicate that the discriminative control that was established with nalorphine in the present study was mu-agonist receptor-mediated. The basis for this selective control by the mu-receptor subtype may be related to the relative salience of receptor activity in opiate-naive animals. The present results suggest that discriminative control by compounds with activity at multiple receptor sites is not uniformly mediated by specific activity at all of those sites. The specific site mediating discriminative control appears to be a function of the specific training drug.

Diprenorphine as a stimulus in drug discrimination learning.

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate diprenorphine from distilled water. In subsequent generalization tests, the opiate antagonists naltrexone and naloxone and the mixed opiate agonist/antagonist nalorphine substituted for the diprenorphine stimulus in a dose-dependent manner, while the opiate agonist morphine and the nonopiate pentobarbital failed to substitute even at the highest doses tested. That a range of opiate antagonists substituted for the diprenorphine stimulus (and an opiate agonist and a nonopiate failed to substitute) suggest that diprenorphine's antagonist properties may mediate the discrimination, presumably by blocking endogenous opiate activity. The ability of these drugs to substitute for the diprenorphine stimulus may also be a function of this receptor activity. The differences in the specific generalization patterns reported in the present assessment and those of earlier reports were discussed.

Naloxone as a stimulus in drug discrimination learning: generalization to other opiate antagonists.

Nonopiate dependent animals were trained to discriminate the opiate antagonist naloxone (1 mg/kg) from distilled water within the conditioned taste aversion baseline of drug discrimination learning. Specifically, rats injected with naloxone prior to a saccharin-LiCl pairing, and with its vehicle prior to saccharin alone, rapidly acquired the drug discrimination, avoiding saccharin following the administration of naloxone and consuming saccharin following its vehicle after only three conditioning trials. Once the discrimination was acquired, generalization tests revealed that the opiate antagonists diprenorphine and naltrexone and the mixed opiate agonist/antagonist nalorphine completely generalized to the naloxone cue at doses of 1.8, 5.6 and 18 mg/kg, respectively. That discriminative control was established with a low dose of naloxone (i.e., 1 mg/kg) and other compounds with opiate antagonist activity generalized to the naloxone cue suggest that the stimulus effects of naloxone were likely mediated through the opiate receptor. Because each of these compounds are reported to bind to the mu receptor (with varying affinities and varying degrees of selectivity), the stimulus properties of naloxone are likely mediated at this specific receptor subtype.

A demonstration of the graded nature of the generalization function of drug discrimination learning within the conditioned taste aversion procedure.

Although control of discriminative performance will often generalize to different doses of the training drug or to drugs from the same class as the training drug, the nature of this generalization is unknown. Prior work has suggested that the generalization is primarily quantal in nature with animals displaying either vehicle-appropriate or drug-appropriate responding, depending upon their detection of the drug stimulus. It has been questioned whether this quantal nature of generalization reflects a characteristic response to drug stimuli or whether such responding is a function of the specific training and testing procedures used to establish and measure drug discrimination learning. The present paper evaluated this issue by analyzing the generalization functions of individual subjects trained and tested within one specific drug discrimination procedure, i.e. the conditioned taste aversion design. Responding within this design was generally graded. It is clear that quantal responding is not a necessary outcome of drug generalization assessments and that the nature of generalization in drug discrimination learning is a function of the specific procedure utilized in training and testing the discrimination. The results of the present analysis are discussed in terms of other recent work reporting graded functions.