Increased drug sensitivity in the drug discrimination procedure afforded by drug versus drug training

Discriminative stimulus effects of gamma-hydroxybutyrate (GHB) in rats discriminating GHB from baclofen and diazepam

Gamma-hydroxybutyrate (GHB) is a drug of abuse with actions at GHB and GABA receptors. This study tried to increase the selectivity of the discriminative stimulus effects of GHB by training animals to discriminate GHB from compounds that share pharmacological mechanisms with GHB. In comparison with a previous GHB versus saline discrimination (group 1), rats were trained to discriminate GHB (200 mg/kg) either from saline and the GABA(B) agonist baclofen (3.2 mg/kg) (group 2) or from saline, baclofen, and the positive GABA(A) modulator diazepam (1 mg/kg) (group 3). In all groups, GHB produced more than 80% GHB-appropriate responding. Baclofen produced 84% GHB-appropriate responding in group 1 but less than 30% in groups 2 and 3. Diazepam produced 68% GHB-appropriate responding in group 1, 30% in group 2, and only 5% in group 3. The GABA(B) receptor antagonists CGP35348 [3-[aminopropyl(diethoxymethyl)phosphinic acid] and CGP52432 [3-[[[((3,4-dichlorophenyl)methyl]amino]propyl]diethoxymethyl)phosphinic acid] attenuated the discriminative stimulus effects of GHB; CGP35348 did so with similar potency in all groups, but CGP52432 was significantly less potent in groups 2 and 3 than in group 1. In all groups, the GHB antagonist NCS-382 [(2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid] partially attenuated the discriminative stimulus effects of GHB. The selective GHB receptor ligand UMB86 (4-hydroxy-4-napthylbutanoic acid sodium) tended to attenuate the discriminative stimulus effects of GHB more in group 3 than in the other groups. The finding that animals can discriminate GHB from baclofen is further evidence that the effects of GHB and baclofen are not identical. Effects that GHB does not share with baclofen may involve GHB receptors or differential interactions with GABA(B) receptors.

Drug-drug discrimination: stimulus properties of drugs of abuse upon a serotonergic-dopaminergic continuum

Ten male N/Nih rats were trained to discriminate between the interoceptive cues produced by the purportedly dopaminergically-mediated drug d-amphetamine at 0.4 mg/kg intraperitoneally administered 20 min prior to training and those produced by the purportedly serotonergically-active agent norfenfluramine at 0.7 mg/kg. Once this discrimination was successfully acquired, the rats were tested with saline and with both drugs administered simultaneously and these manipulations were seen to produce random responding; indicating roughly equivalent cueing strength. Subsequently, various drugs thought to act upon serotonergic neurons, i.e., LSD and MDMA, were tested and shown to generalize in a dose-responsive manner to the norfenfluramine-appropriate lever. In contrast, the dopaminergically-active agent methcathinone and the D3 agonist 7-OH-DPAT produced generalization on the amphetamine-appropriate lever. Results are discussed in light of the increased specificity of behavioral testing available in a drug vs. drug discriminative paradigm using two drugs with different mechanisms of action.

Functional relationships, previous history and the discrimination of a drug mixture in rats

These experiments aim to probe the role of different functional relationships between drugs, responses and reinforcers in studies on the discrimination of drug mixtures. Two-lever discriminations based on mixtures of (+)-amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) have been compared in three groups of rats (n = 8) trained to discriminate: (i) the mixture from saline, called the AND-discrimination; (ii) either the mixture, amphetamine or pentobarbitone from saline, the OR-discrimination; (iii) the mixture from either amphetamine or pentobarbitone, the AND-OR-discrimination. The rats were trained in a two-bar operant conditioning procedure with a tandem schedule of food reinforcement. The rats trained on the AND procedure acquired the discrimination more rapidly than the rats trained on the other two procedures, but after 60 training sessions, all discriminations were performed with similar (90-94%) accuracy. In rats trained under the AND and the OR procedures, there was full generalization from the mixture to the largest doses used of either amphetamine or pentobarbitone. In contrast, in rats trained under the AND-OR procedure, there was no generalization from the mixture to any dose of either drug separately. The training procedures for the OR and the AND-OR groups were changed to AND-discrimination training; the effects of the different previous histories were found to persist for a prolonged period, despite the fact that training conditions were now the same in all groups. The effects of the drugs on rates of responding were also influenced by the previous training history. Thus, using different training procedures to alter functional relationships can substantially and persistently influence the major characteristics of a discrimination based on a mixture of drugs.