Differentiation of 5-HT1A receptor ligands by drug discrimination

The effects of chronic neuropathic pain states on the discriminative stimulus effect of fentanyl and other MOR agonists

Pleasant subjective effects of drugs (e.g., euphoria) have been demonstrated to contribute to their abuse potential. In humans, there is some evidence that acute pain states may decrease the positive subjective effects of opioids; however, no studies have directly tested the impact of a long-lasting pain state. Therefore, the goal of this study was to directly evaluate the discriminative stimulus of mu opioid receptor (MOR) agonist, fentanyl, or the non-opioid drug of abuse, cocaine, in the presence or absence of spared nerve injury (SNI) induced chronic neuropathic pain. Prior to surgery, MOR agonists (fentanyl, morphine, nalbuphine) dose-dependently increased % fentanyl-like responding, as expected; surprisingly, after surgery, we saw small, significant rightward shifts in the fentanyl and morphine dose response curves in both sham and SNI groups suggesting that the observed shifts were not due to chronic pain. In both sham and SNI groups, there was an increase in the generalization of nalbuphine to the fentanyl-discriminative stimulus. There was no change in the discriminative stimulus of cocaine (or amphetamine substitutions) over 4 months of SNI-induced chronic neuropathic pain or sham states, suggesting that the SNI model failed to alter the discriminative stimuli of fentanyl and cocaine. Following induction of chronic neuropathic pain, there was an observed increase in quinpirole-induced generalization to the cocaine discriminative stimulus. In the future, studies should directly examine the abuse potential of low efficacy MOR agonists and dopaminergic agonists in the presence and absence of chronic pain states.

Differentiation between low- and high-efficacy CB1 receptor agonists using a drug discrimination protocol for rats

RATIONALE

The "subjective high" from marijuana ingestion is likely due to Δ(9)-tetrahydrocannabinol (THC) activating the central cannabinoid receptor type 1 (CB1R) of the endocannabinoid signaling system. THC is a weak partial agonist according to in vitro assays, yet THC mimics the behavioral effects induced by more efficacious cannabinergics. This distinction may be important for understanding similarities and differences in the dose-effect spectra produced by marijuana/THC and designer cannabimimetics ("synthetic marijuana").

OBJECTIVE

We evaluated if drug discrimination is able to functionally detect/differentiate between a full, high-efficacy CB1R agonist [(±)AM5983] and the low-efficacy agonist THC in vivo.

MATERIALS AND METHODS

Rats were trained to discriminate between four different doses of AM5983 (0.10 to 0.56 mg/kg), and vehicle and dose generalization curves were determined for both ligands at all four training doses of AM5983. The high-efficacy WIN55,212-2 and the lower-efficacy (R)-(+)-methanandamide were examined at some AM5983 training conditions. Antagonism tests involved rimonabant and WIN55,212-2 and AM5983. The separate (S)- and (R)-isomers of (±)AM5983 were tested at one AM5983 training dose (0.30 mg/kg). The in vitro cyclic adenosine monophosphate (cAMP) assay examined AM5983 and the known CB1R agonist CP55,940.

RESULTS

Dose generalization ed50 values increased as a function of the training dose of AM5983, but more so for the partial agonists. The order of potency was (R)-isomer > (±)AM5983 > (S)-isomer and AM5983 > WIN55,212-2 ≥ THC > (R)-(+)-methanandamide. Surmountable antagonism of AM5983 and WIN55,212-2 occurred with rimonabant. The cAMP assay confirmed the cannabinergic nature of AM5983 and CP55,940.

CONCLUSIONS

Drug discrimination using different training doses of a high-efficacy, full CB1R agonist differentiated between low- and high-efficacy CB1R agonists.

Efficacy of buspirone for attenuating cocaine and methamphetamine reinstatement in rats

BACKGROUND

There are no approved pharmacotherapies for preventing psychomotor stimulant relapse. The operant reinstatement model has been suggested as a screen for identifying candidate medications. The present study examined if the anxiolytic buspirone could attenuate reinstatement of extinguished responding in Long-Evans rats that previously self-administered intravenous cocaine or methamphetamine.

METHODS

Rats were trained in 2-h daily sessions to self-administer 0.5mg/kg cocaine or 0.1mg/kg methamphetamine infusions followed by 12 days of instrumental extinction. Reinstatement was evoked by 17mg/kg i.p. cocaine primes or response-contingent cocaine-paired cues in cocaine-reinforced rats, and by 1mg/kg i.p. methamphetamine primes or response-contingent methamphetamine-paired cues in methamphetamine-reinforced rats.

RESULTS

Buspirone (1 and 3mg/kg) significantly (p<0.05) attenuated cocaine cue but not cocaine prime reinstatement. Buspirone (1 and 3mg/kg) also significantly attenuated methamphetamine cue reinstatement. Buspirone (3mg/kg) significantly attenuated methamphetamine prime reinstatement. During all reinstatement tests, 3mg/kg buspirone reduced levels of inactive lever pressing relative to those of vehicle, significantly so during the cocaine cue-induced reinstatement tests.

CONCLUSIONS

Given the complexity of buspirone's neuropharmacology consisting of serotonin 5-HT1A receptor partial agonist activity, and dopamine D2, D3 and D4 receptor antagonist effects, it is uncertain which of these activities or their combination is responsible for the present results. Overall, these results suggest that buspirone may reduce the likelihood of relapse to cocaine and methamphetamine use under some conditions, although this speculation must be interpreted with caution given buspirone's similar potency to attenuate inactive-lever responding.

Role of training dose in drug discrimination: a review

Drug discrimination has been an important technique in behavioural pharmacology for at least 40 years. The characteristics of drug-produced discriminative stimuli are influenced by behavioural and pharmacological variables, including the doses used to establish discriminations. This review covers studies on the effects of varying the training dose of a drug in a search for general principles that are applicable across different drug classes and methodological approaches. With respect to quantitative changes, relationships between training dose and the rate of acquisition or magnitude of stimulus control were found for most drug classes. Acquisition accelerated with dose up to a point beyond which drug-induced impairments of performance had a deleterious impact. Sensitivity to the training drug as measured by ED(50) values typically increased when the training dose was reduced. Qualitative changes were more complex and appeared to fall into three categories: (a) changes in profiles of generalization between partial and full agonists; (b) reduced specificity of some discriminations at small training doses; and (c) changes in the relative salience of actions mediated through different neurotransmitter systems or from central and peripheral sites. Three-lever discrimination procedures incorporating 'drug versus drug' or 'dose versus dose' contingencies enabled detection of more subtle differences than the simple 'drug versus no drug' approach when applied to the opioid, hallucinogen and barbiturate classes of drugs. These conclusions have implications for the interpretation of data from studies that use either within-subject or between-subject designs for studying the discriminative stimulus effects of drugs.

Evaluation of the effects of α2 adrenoceptor antagonism with the D2 receptor antagonist raclopride on conditioned avoidance responding in rats

The α2 adrenoceptor antagonist idazoxan, when combined with a subeffective dose of the D2 receptor antagonist raclopride or other D2 receptor antagonists, produces inhibition of conditioned avoidance responding (CAR) in rats, an effect predictive of antipsychotic effects. In other models, this treatment combination indicates putative atypical antipsychotic effects as well, and has led to a α2/D2 receptor hypothesis for atypicality. However, this hypothesis would be better supported if other α2 adrenoceptor antagonists were investigated and the role of the alternative mechanisms, particularly 5-HT1A receptor agonism, for the behavioral effects of idazoxan were evaluated. This study sought to further test the α2/D2 receptor hypothesis by assessing the effects of α2, D2 and 5-HT1A receptor ligands on CAR in rats. Raclopride significantly reduced CAR. Administration of idazoxan or the α2 adrenoceptor antagonist yohimbine with a subeffective dose of raclopride also significantly reduced CAR. Pretreatment with the 5-HT1A receptor antagonist WAY100635 failed to significantly reverse the inhibition of CAR produced by the idazoxan and raclopride treatment combination. To the extent that 5-HT1A receptor antagonism failed to block the effects of idazoxan in combination with raclopride on CAR, α2 adrenoceptor antagonism alone appears to potentiate the putative antipsychotic effects produced through D2 receptor antagonism.

Behavioral effects of 8-OH-DPAT injections into pontine and mesencephalic areas containing 5-HT-immunoreactive perikarya in the pigeon

This study examined the distribution of 5-HT-immunoreactive perikarya (5-HT-IRp) and the effects of local injections of 8-OH-DPAT into 5-HT-IRp-containing pontine and mesencephalic regions on feeding and drinking behaviors in free-feeding pigeons. When infused into the midline 5-HT-IRp-containing areas, 8-OH-DPAT (6.1 nmol) reliably elicited drinking and, to a lesser extent, feeding responses during the first hour after injection. These responses were significantly higher than the ingestive indexes observed (1) after vehicle (ascorbic acid 0.1%, 200 nl) injections at the same sites and (2) after 8-OH-DPAT injections into adjacent sites devoid of 5-HT-IRp. Increases in drinking were proportionally higher than those observed in feeding and a significant negative correlation was observed between water and food after midline 8-OH-DPAT injections. Similar dipsogenic responses were observed after injections of different 8-OH-DPAT doses (0.6, 2.0, and 6.1 nmol). Pretreatment with local injections of p-MPPI (an antagonist of 5-HT1A receptors) attenuated the ingestive responses evoked by 8-OH-DPAT injections. Injections of 8-OH-DPAT into lateral 5-HT-IRp-containing sites evoked only inconsistent and weak ingestive responses. These results indicate that 5-HT1A receptor-mediated circuits located in the midline superior raphe system of the pigeon may play an important role in mechanisms controlling water intake, similar to that observed in mammals.

Pindolol excites dopaminergic and adrenergic neurons, and inhibits serotonergic neurons, by activation of 5-HT1A receptors

Pindolol accelerates the clinical actions of selective serotonin reuptake inhibitors (SSRIs) in man, and modulates extracellular levels of monoamines in corticolimbic structures in rats. Herein, we examined its influence upon electrical activity of serotonergic, dopaminergic and adrenergic perikarya in the dorsal raphe nucleus (DRN), ventral tegmental area (VTA) and locus coeruleus (LC) of anaesthetized rats. In analogy to the serotonin1A (5-HT1A) agonist, 8-OH-DPAT (-100%), pindolol dose-dependently (0.063- 1.0 mg/kg) decreased (-70%) the firing rate of serotonergic neurons. The inhibitory action of pindolol was abolished by the selective 5-HT1A antagonist, WAY-100,635 (0.031 mg/kg). In contrast, 8-OH-DPAT (+26%) and pindolol (0.25-4.0 mg/kg, +60%) dose-dependently increased the firing rate of dopaminergic cells. Of 57 neurons recorded (pindolol, 2.0 mg/kg), 36 (63%) were excited, 11 (19%) were unaffected and 10 (18%) were inhibited. This variable influence could be attributed to regularly firing neurons in the parabrachial subdivision, inasmuch as all neurons in the paranigral subnucleus were excited. The facilitation of firing by pindolol was accompanied by an increase in burst firing throughout the VTA. Both the increases in burst firing and in firing rate were reversed by WAY-100,635 (0.031 mg/kg). Finally, the electrical activity of adrenergic neurons was dose-dependently enhanced by 8-OH-DPAT and pindolol (+99% and +83%, respectively). WAY-100,635 reversed this excitation and, itself, inhibited the activity of adrenergic neurons. In conclusion, via engagement of 5-HT1A receptors, pindolol inhibits serotonergic, and activates dopaminergic and adrenergic, neurons in anaesthetized rats. Such actions may contribute to its influence upon mood, both alone and in association with antidepressant agents.

Discriminative stimulus properties of eltoprazine in the pigeon

Twelve pigeons were successfully (ED50 = 2.4 mg/kg p.o.) trained to discriminate the 5-HT(1A/B) receptor agonist eltoprazine (5.0 mg/kg p.o.) from its vehicle in a fixed-ratio (FR)30 two-key operant drug discrimination procedure. Tests for generalization and antagonism showed that 5-HT1A receptor agonists, such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) (66.7%), flesinoxan (72.7%), buspirone (58.3%), and ipsapirone (36.4%) only partially substituted for the eltoprazine cue. Compounds with mixed agonistic action at 5-HT1 receptors, completely (> or = 80%) [(eltoprazine; TFMPP (1-(3-trifluoromethylphenyl) piperazine (ED50 = 7.68 mg/kg) and RU 24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl-1H-indole) (ED50 = 15.8 mg/kg)] substituted for eltoprazine; whereas m-CPP (1-(3-chlorophenyl)piperazine) did not. The selective 5-HT reuptake inhibitor fluvoxamine partially (44%) substituted for the eltoprazine cue. The 5-HT1A receptor antagonist NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phtalimido)butyl]piperazine) fully blocked the eltoprazine cue. Both (+/-)-pindolol and (+/-)-propranolol showed partial antagonism of the eltoprazine cue (66.7 and 50.0%, respectively). (+/-)-Pindolol also showed partial substitution (50%) for the eltoprazine cue, but NAN-190 and (+/-)propranolol did not. It is concluded that the discriminatory stimulus properties of eltoprazine in the pigeon are mediated by 5-HT1A and 5-HT1B receptors.

Discriminative stimulus properties of the 5-HT1A receptor agonist BAY x 3702 in the rat

The aminomethylchroman derivative BAY x 3702 (R-(-)-2-¿4-[(chroman-2-ylmethyl)-amino]-butyl¿-1,1-dioxo-benzo[d] isothiazolone HCl) has recently been characterized as a relatively selective, high affinity 5-HT1A receptor agonist with neuroprotective, anxiolytic- and antidepressant-like effects in animal models. It was the aim of the present study to further confirm its receptor binding profile in an in vivo assay. Rats were trained to discriminate BAY x 3702 (0.1 mg/kg, i.p.) from vehicle in a standard two-lever fixed ratio 10 food-reinforced procedure. All rats learned the discrimination, the median number of sessions to reach criterion being 38 (range: 22-58 sessions). Generalization tests with BAY x 3702 showed dose-dependent and complete generalization after different routes of administration; the ED50 values being: 0.030, 0.007 and 0.36 mg/kg, after i.p., i.v. and p.o. administration, respectively. Assessment of the duration of action after administration of 0.1 mg/kg BAY x 3702, i.p., resulted in a T1/2 of 65 min. Dose-dependent and complete generalization was also obtained with the 5-HT1A receptor agonists 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin, ED50 in mg/kg, i.p.: 0.086), flesinoxan (0.30), SR 57746A ((4-(3-trifluoromethylphenyl)-N-(2-(naphth-2-yl)ethyl)-1,2,3,6-tet rahydropyridine HCl, 1.0), the (+)-enantiomer of BAY x 3702 (1.3) and ipsapirone (1.8); the ED50 values being closely correlated with their respective affinities for the 5-HT1A receptor. Pretreatment with the selective 5-HT1A receptor antagonist WAY-100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N(2-pyridinyl) cyclohexane carboxamide trihydrochloride) dose-dependently and completely blocked the discriminative effects of 0.1 mg/kg BAY x 3702 (ID50: 0.013 mg/kg, i.p.). WAY-100635, prazosin, idazoxan, raclopride, paroxetine, (-)-BAY k 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoro-methyl-phenyl)-p yridine-5-carboxylate), ethanol, and the putative neuroprotectants MK-801 ((+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imin e), CNS 1102 (N-(1-naphthyl)-N'-(3-ethylphenyl)-N'-methyl-guanidine), CGS 19755 (cis-4-(phosphonomethyl) piperidine-2-carboxylic acid) and nimodipine did not induce more than 20% generalization. It is concluded that the BAY x 3702 cue is mediated by its agonistic activity at 5-HT1A receptors.

Selective serotonin reuptake inhibitors potentiate 8-OH-DPAT-induced stimulus control in the pigeon

The effects of two selective serotonin reuptake inhibitors, fluoxetine and citalopram, and a nonselective monoamine reuptake inhibitor, imipramine, were characterized in pigeons that had been trained to discriminate 0.64 mg/kg of 8-hydroxy-(2-di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), a 5-HT1A receptor agonist, from saline. Neither fluoxetine, citalopram, nor imipramine generalized to the 8-OH-DPAT-induced stimulus cue. However, when administered in addition to 8-OH-DPAT, both fluoxetine (10 mg/kg) and citalopram (10 mg/kg) lowered the ED50 for generalization of 8-OH-DPAT from 0.16 mg/kg (8-OH-DPAT by itself) to 0.05 mg/kg (fluoxetine + 8-OH-DPAT) and 0.06 mg/kg (citalopram + 8-OH-DPAT). Under similar conditions, imipramine (1 mg/kg) had no effect on the generalization curve for 8-OH-DPAT. The data support the hypothesis that activation of the 5-HT1A receptor may be relevant to the mechanism of action of serotonin reuptake inhibitors.