D-Ala2,D-Leu5-enkephalin generalizes to a discriminative stimulus produced by fentanyl but not ethylketocyclazocine

Differential effects of mu-, delta- and kappa-opioid receptor agonists on the discriminative stimulus properties of cocaine in rats

The effects of selective mu-, delta- and kappa-opioid receptor agonists on the discriminative stimulus properties of cocaine were examined in rats trained to discriminate between cocaine (10 mg/kg) and saline. Cocaine produced a dose-related increase in cocaine-appropriate responses in all of the rats. In generalization tests, neither morphine (mu-opioid receptor agonist) nor N-methyl-N-7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-11-4-benzofu ranacetamide (U50,488H: kappa-opioid receptor agonist) generalized to the discriminative stimulus properties of cocaine. On the other hand, the newly synthesized non-peptide selective delta-opioid receptor agonist 2-methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha-octahydro-quinolino(2,3,3,-g)isoquinoline (TAN-67) partially generalized (56.7% cocaine-appropriate responses) to the discriminative stimulus properties of cocaine. Intracerebroventricular (i.c.v.) administration of [D-Ala2]deltorphin II (peptide delta 2-opioid receptor agonist) completely generalized, while neither [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO: mu-opioid receptor agonist) nor [D-Pen2,D-Pen5]enkephalin (DPDPE; delta 1-opioid receptor agonist) generalized to the discriminative stimulus properties of cocaine. These results suggest that the discriminative stimulus properties of cocaine may be partially mediated by delta-opioid (especially delta 2-opioid) receptors. In combination tests, pretreatment with morphine (3.0 mg/kg) and TAN-67 (3.0 and 10 mg/kg) significantly potentiated the discriminative stimulus properties cocaine. In contrast, pretreatment with U50,488H (2.0 and 4.0 mg/kg) scarcely shifted the discriminative stimulus properties of cocaine. Furthermore, the potentiating effect of 3.0 mg/kg morphine on the discriminative stimulus properties of cocaine was attenuated by 2.0 mg/kg U50,488H. In contrast, the potentiating effect of 10 mg/kg TAN-67 on the discriminative stimulus properties of cocaine was not reversed by either 2.0 or 4.0 mg/kg U50,488H. These results suggest that mu-, delta- and kappa-opioid receptor agonists modulate the discriminative stimulus properties of cocaine through different mechanisms, perhaps through different effects on the dopaminergic system.

Morphine acts in the parabrachial nucleus, a pontine viscerosensory relay, to produce discriminative stimulus effects

Morphine is known to act centrally to produce discriminative stimulus effects, but the specific neuroanatomical sites mediating this action have not been identified. We used morphine as a discriminative stimulus in a taste aversion paradigm to elucidate the neural basis of morphine's cueing properties. Rats were injected subcutaneously with 5 mg/kg morphine 15 min prior to the presentation of a 0.1% saccharin solution. After 20 min of exposure to the flavor, lithium chloride (130 mg/kg, IP) was injected. On alternate days, an injection of 0.9% physiological saline both preceded and followed the presentation of saccharin. Animals learned to consume significantly less saccharin after morphine than after saline injections. Unilateral guide cannulae were then implanted into brain areas containing relatively high densities of opiate binding sites, comprising the medial prefrontal cortex, the nucleus accumbens, the anterior dorsolateral striatum, the medial thalamus, the basolateral amygdaloid nucleus, the dorsal hippocampus, the caudal periaqueductal grey and the parabrachial nucleus. Generalization to central routes of administration was then evaluated by microinjecting morphine (2.5, 5, 10 and 20 micrograms) into these brain areas. Dose-dependent decreases in saccharin consumption similar to those of systemic morphine were produced by the administration of morphine into the parabrachial nucleus and the nucleus accumbens. Control data showed that only in the parabrachial nucleus could these effects be attributed to the cueing properties of morphine; in the nucleus accumbens, morphine administration induced unconditioned decreases in saccharin consumption. In the remaining brain areas, morphine generalized to the systemic saline condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance to the respiratory actions of opiates: withdrawal tolerance and asymmetrical cross-tolerance

Cross-tolerance to the respiratory depression induced by i.c.v. [D-Ala2,D-Leu5]enkephalin (DADLE) and by s.c., i.c.v. and i.v. morphine was studied in anesthetized rats that had been rendered tolerant to s.c. sufentanil (4 micrograms/h per 7 days). Tolerance induced by i.c.v. sufentanil was also compared during withdrawal and its absence. Tolerance was evaluated by estimation of the changes in both the maximum attained reduction of respiratory frequency (Emax) and the area under the time-course curve for the effects (AUC). In contrast to the failure to develop tolerance, as estimated with the Emax, after i.c.v. sufentanil in the absence of abstinence (tolerance index: 1.1), rats displayed significant degrees of cross-tolerance to i.c.v. morphine (3.6), i.c.v. DADLE (4.3) and s.c. morphine (6.6). No tolerance, however, was obtained to i.v. morphine. Tolerance to i.c.v. sufentanil in non-abstinent rats was not present when the Emax was considered but was when AUC was considered. During withdrawal animals showed tolerance as estimated with both the Emax and AUC. These results complement previous findings and show that the appearance of dissimilar degrees of cross-tolerance to different agonists depends not only on their intrinsic activities but also on their pharmacokinetic properties. These observations, along with the critical influence of withdrawal upon the manifestation of tolerance, support the need to distinguish between the development of the tolerance state and the degree of tolerance that can be expressed under specific conditions.

Morphine-like discriminative stimulus effects of opioid peptides: possible modulatory role of D-Ala2-D-Leu5-enkephalin (DADL) and dynorphin A (1-13)

The role of the different opioid receptors was studied in rats trained to discriminate SC injections of 3.0 mg/kg morphine from saline by tests for generalization to graded doses of morphine and receptor-selective peptides administered into the lateral cerebral ventricle. Dose-dependent morphine-like stimulus effects were produced over a wide range of doses (0.001-30 micrograms), depending upon ligand and animal, by morphine, by the mu-selective peptides DAGO[D-Ala2-NMePhe4-Gly(ol)-enkephalin] and FK33824[D-Ala2,NMePhe4-Met(O)5-(ol)-enkephalin], and by the delta-selective peptide, DADL[D-Ala2,D-Leu5)enkephalin]. The order of relative potency of these substances was: FK33824 greater than DAGO greater than morphine greater than DADL. In contrast, DPLPE[D-Pen2,L-Pen5)enkephalin], which has much greater delta receptor selectivity than does DADL, and dynorphin A(1-13) (0.1-10 micrograms), a kappa-receptor agonist, engendered choice responding appropriate for saline. When 1.0 micrograms DADL, a dose lacking morphine-like discriminative effects, was administered concurrently with SC morphine, the stimulus effects of morphine were potentiated. Concurrent administration of 10 micrograms dynorphin A(1-13) and morphine attenuated the stimulus effects of morphine inconsistently. These results support previous findings that mu-opioid receptors are of primary importance in mediating the morphine-like discriminative effects of opioid peptides. They also suggest that morphine-like discriminative effects can be modulated by other types of opioid receptors.

Discriminative stimulus properties of beta-phenylethylamine, deuterated beta-phenylethylamine, phenylethanolamine and some metabolites of phenylethylamine in rodents

The discriminative stimulus (cue) properties of phenylethylamine (PEA) were analysed in rodents in a conventional two lever FR10 operant drug discrimination task. Rats trained to discriminate phenylethylamine at 30 mg/kg showed complete dose-related generalization to PEA and to two potential PEA metabolites: phenylethanolamine (PEOH) and N-Methyl PEA (NMPEA). Only partial (50%) generalization was seen with N-Methylphenylethanolamine (NMPEOH), another potential PEA metabolite. The specificity of PEA's action as a discriminative stimulus was demonstrated by the finding that fenfluramine, a substituted phenylethylamine, failed to generalize to PEA even at high doses with marked behavioural effects which are known to have discriminative stimulus properties themselves. These data suggest that NMPEA and PEOH may be functionally important active metabolites of PEA, particularly if the major pathway of PEA metabolism to phenylacetic acid under the influence of MAO Type B is for any reason impaired. A long acting deuterium substituted form of PEA (alpha, alpha, d2 PEA), which is resistant to metabolism by MAO, produced complete dose-related generalization to the PEA cue but was more potent than PEA, due presumably to its resistance to metabolism by MAO. Deuterated PEA may therefore be a useful agent to use in future studies of the PEA cue, because the discriminability of PEA itself appears to be low due to its very rapid metabolism in vivo.

Generalization tests with intraventricularly applied pro-enkephalin B-derived peptides in rats trained to discriminate the opioid kappa receptor agonist ethylketocyclazocine

Rats were trained in a two-lever food-reinforced procedure to discriminate between the effects of saline and the opioid kappa receptor agonist ethylketocyclazocine. After acquisition of this discrimination, generalization tests with opioid peptides such as beta-endorphin, alpha-neoendorphin, dynorphin A and some dynorphin-derived peptides were conducted. The rats dose-dependently generalized the effects of intracerebroventricularly injected ethylketocyclazocine but not beta-endorphin, alpha-neoendorphin, dynorphin A1-8, dynorphin A1-13, D-Cys2-L-Cys5-dynorphin A1-13 or dynorphin A. D-Cys2-L-Cys5-dynorphin A1-13, in contrast to dynorphin A itself, dose-dependently caused analgesia and catatonia that was reversible with naloxone. Studies into the receptor preference of this derivative, using the technique of "selective tolerance", revealed that this dynorphin derivative is almost devoid of kappa-receptor activity.

Opioid agonist and antagonist behavioural effects of buprenorphine

1 The agonist and antagonist effects of a range of buprenorphine doses (0.08-20 mg/kg) were studied on the responding of pigeons under a multiple fixed-ratio, fixed-interval schedule of grain presentation. Various doses (0.02-10 mg/kg) of buprenorphine were also tested in pigeons trained to discriminate between injections of 0.05 mg/kg of fentanyl and injections of distilled water. 2 Buprenorphine, over a broad dose range (0.08-5 mg/kg), increased the rates of responding in the fixed-interval component of the multiple schedule and disrupted patterning of responding within the fixed-interval, without affecting fixed-ratio responding even at a dose of 40 mg/kg. The effects of some of the high doses on fixed-interval responding were still evident one and two days after buprenorphine injection. 3 Doses of buprenorphine which produced increases in fixed-interval responding were also effective as antagonists of the behavioural depression produced by 40 mg/kg of morphine, and were discriminated as fentanyl-like by pigeons trained to discriminate between injections of fentanyl and injections of water. 4 These results show that buprenorphine produces marked agonist and antagonist effects over an extremely broad dose range without producing behavioural depressant effects.