Serotonergic mediation of fenfluramine discriminative stimuli in Fawn-Hooded rats

Impure but not inactive: Behavioral pharmacology of dibenzylpiperazine, a common by-product of benzylpiperazine synthesis

BACKGROUND

Substituted piperazines comprise a substantial proportion of the novel psychoactive substance market. Among the most widely abused piperazine compounds are meta-chlorophenylpiperazine (mCPP), tri-fluoromethylphenylpiperazine (TFMPP), and, especially, benzylpiperazine (BZP), which are commonly incorporated, either alone or in combination, in illicit "party pills" or "ecstasy" formulations. Illicit synthesis of BZP often results in production of an impure by-product dibenzylpiperazine (DBZP), which frequently appears alongside BZP in these formulations; however, despite its ubiquity, little information exists regarding the abuse liability of DBZP.

AIMS

The current study aimed to evaluate the abuse-related behavioral pharmacology of DBZP.

METHODS

DBZP, mCPP, and TFMPP were tested in parallel in mice in locomotor activity and conditioned place preference assays, and in a drug discrimination assay with rats trained to discriminate either methamphetamine, cocaine, (±)-3,4-methylenedioxymethamphetamine (MDMA), or -2,5-dimethoxy-4-methylamphetamine(DOM).

RESULTS

Each of the compounds tested produced dose-dependent decreases in locomotor activity. DBZP substituted fully for methamphetamine, produced subthreshold drug-appropriate responding for cocaine and MDMA, and failed to substitute for DOM. Conversely, TFMPP and mCPP only produced subthreshold drug-appropriate responding for methamphetamine and MDMA, respectively, and both compounds failed to substitute for cocaine or DOM. None of the compounds tested produced a place preference. DBZP produced convulsions in rats at the highest dose tested.

CONCLUSIONS

These data indicate that DBZP is more similar to BZP, albeit with lower potency and efficacy, than its serotonergic piperazine counterparts, and is a behaviorally-active compound with some abuse liability and potential for adverse health effects.

Reinforcement schedule effects in rats trained to discriminate 3,4-methylenedioxymethamphetamine (MDMA) or cocaine

RATIONALE

Relatively few studies have compared the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine, and findings from different laboratories are somewhat inconsistent. One possible reason for discrepant results may be the use of different reinforcement schedules during discrimination training.

OBJECTIVE

The present study compared fixed ratio (FR) 20 and variable interval (VI) 15-s reinforcement schedules to determine their influence on discrimination acquisition, response rates, frequency of reinforcements, and stimulus generalization in rats trained to discriminate cocaine or MDMA.

MATERIALS AND METHODS

Thirty-two male Sprague-Dawley rats were trained to discriminate cocaine (10 mg/kg; n=16) or MDMA (1.5 mg/kg; n=16) from saline under either a FR 20 or a VI 15-s schedule of food reinforcement. Stimulus generalization tests were conducted with a range of doses of cocaine, MDMA, d-amphetamine, and lysergic acid diethylamide in all four training groups.

RESULTS

The FR 20 schedule facilitated more rapid discrimination acquisition compared to the VI 15-s schedule and established differential response rates and frequency of reinforcement under drug and vehicle conditions. However, reinforcement schedule had little influence on stimulus generalization between MDMA and cocaine. Cocaine produced partial substitution for MDMA in both training groups (FR 20, 51%; VI 15-s, 58%). Likewise, MDMA produced only partial substitution for cocaine in both training groups (FR 20, 40%; VI 15-s, 72%).

CONCLUSIONS

The present findings suggest that the number of sessions required to establish discriminative stimulus control varies with different reinforcement schedules. Nevertheless, training schedules alone do not appear to have significant effects on stimulus generalization between MDMA and cocaine.

Discriminative stimulus effects of m-chlorophenylpiperazine as a model of the role of serotonin receptors in anxiety

Serotonin is known to play a role in anxiety. The roles of serotonin reuptake and 5-HT1A receptors have been well characterized, but the contribution of other serotonin receptor subtypes is not as clear. 1-(3-Chlorophenyl)-piperazine (mCPP), which binds non-selectively to a wide range of serotonin receptors, has often been used to produce anxiety in humans and in animal models. Because functional assays indicate that mCPP is significantly more potent at 5-HT2C receptors, it may serve as a tool to investigate the contribution of 5-HT2C receptors to anxiety. This paper reviews the results of behavioral tests using mCPP, including the drug discrimination assay, to model anxiety. Although the discriminative stimulus effects of mCPP do not seem to be a useful screen for general anxiolytics, they do seem to be useful for characterization of the contribution of 5-HT1B and 5-HT2C receptors to the mediation of anxiety-like behaviors.

Discriminative stimulus properties of (+/-)-fenfluramine: the role of 5-HT2 receptor subtypes

The role of serotonin 5-HT2 receptors (5-HT2R) in the discriminative stimulus effects of fenfluramine was investigated. Male Sprague-Dawley rats were trained to discriminate (+/-)-fenfluramine (2 mg/kg ip) from saline using a 2-lever, water-reinforced paradigm. Drug-lever responding after fenfluramine was dose-dependent. The 5-HT(2C/1B)R agonist mCPP and the 5-HT(2C)R agonist MK 212 fully substituted, whereas the 5-HT(2A/2C)R agonist DOI partially substituted, for the training drug. The 5-HT(2B)R agonist BW 723C86 engendered saline-lever responding. The 5-HT(2C/2B)R antagonist SB 206553 completely antagonized the fenfluramine discrimination a well as the full substitutions of mCPP and MK 212 and the partial substitution of DOI. The selective 5-HT(2A)R antagonist M100907 partially suppressed the stimulus effects of fenfluramine, mCPP, and MK 212 and almost fully attenuated the partial substitution of DOI. RS 102221, a selective 5-HT(2C)R antagonist that does not cross the blood-brain barrier, did not alter the fenfluramine cue. Results demonstrate that the discriminative stimulus effects of fenfluramine are centrally mediated by 5-HT(2C)R and to some extent by 5-HT(2A)R.

Interleukin 1alpha alters hippocampal serotonin and norepinephrine release during open-field behavior in Sprague-Dawley animals: differences from the Fawn-Hooded animal model of depression

Detection of two biogenic amine neurotransmitters, serotonin (5-HT) and norepinephrine (NE) within the CA1 region of the hippocampus (HPC) of behaving male laboratory animals (Rattus norvegicus), was performed with miniature carbon sensors (BRODERICK PROBES) and in vivo semidifferential microvoltammetry after acute administration of the soluble immune factor, human recombinant, interleukin (IL) 1alpha (10 and 100 ng/kg i.p.). Two animal models were compared, i.e., (a) the Sprague-Dawley (SD) model, a strain neither biochemically nor immune-challenged and (b) the Fawn-Hooded (FH) model, a biochemically (5-HT-deficient) and immune-challenged animal. Open-field behaviors, locomotion (ambulations) and stereotypy (fine movements of sniffing and grooming) were monitored with infrared photobeams while 5-HT and NE were selectively and separately detected within seconds in real time. Subchronic studies were performed in the same animals 24 h later at which time no further drug was administered. Results from acute treatment studies showed that IL-1alpha altered HPC monoamines and behavior viz-a-viz habituation values (baseline) in the SD strain differently from those in the FH strain as follows: (1) although 5-HT release was significantly increased within CA1 region of HPC in both SD and FH strains (P<.0001), the extent of the HPC 5-HT increase in the 5-HT-deficient FH strain was significantly less than that of the SD strain at both doses (P<.0001). The subchronic studies showed that 5-HT release within the HPC in the SD strain significantly increased (135%) over drug treatment values (P<.001), whereas HPC 5-HT release in the FH strain remained the same as that seen in the acute drug treatment studies; the difference between strains for the subchronic study was also statistically significant (P<.01). (2) IL-1alpha significantly decreased HPC NE release in the SD strain (P<.0004) while IL-1alpha decreased HPC NE release in the FH strain only at the 10-ng/kg dose (P<.001); at the 100-ng/kg dose in the FH strain, NE rebounded towards baseline and increased 15% above baseline reaching statistical significance (P<.05). Subchronic studies in the SD strain showed a further decreased NE signal to 38% below baseline (P<.0001), whereas subchronic studies in the FH strain showed a significant increase in NE release (P<.02). The difference between strains in the subchronic NE studies was significant (P<.001). (3) Ambulations were increased after IL-1alpha administration in acute studies in both the SD and the FH strains, but the increase did not reach statistical significance, whereas in the subchronic studies, both strains exhibited significant increases as revealed by post hoc analyses (P<.05). There was a statistically significant difference between strains in acute studies (P<.001), whereas no significant differences between models were seen in ambulation behavior in subchronic studies. (4) Fine movements increased over baseline after IL-1alpha administration in both animal models in acute studies, however, results did not reach statistical significance, likely due to the episodic effect of IL-1alpha on movement behavior in both the SD and the FH strains. However, the SD strain showed a significant increase in fine movement behavior during the subchronic studies (P<.02). Significant differences in fine movements between animal models were not observed either acutely or in subchronic studies. In summary, the data show that immune modulation by IL-1alpha affects HPC neurochemistry and behavior in SD versus FH animal models differently and/or to different degrees. The data show that while the FH animal model is subsensitive to 5-HT agonists, 5-HT function can be stimulated. Comparison of genetically diverse animal models provides a reliable means to identify and discern cytokine-induced depressive versus stressor properties. Selective sensor technology provides a powerful tool as movement behavior is monitored and interpreted as a function of monoamine neurotransmission.

MDMA stimulus generalization to the 5-HT(1A) serotonin agonist 8-hydroxy-2- (di-n-propylamino)tetralin

The abused substance N-methyl-1-(3, 4-methylenedioxyphenyl)-2-aminopropane, or MDMA, serves as a training drug in animals. Because the 5-HT(1A) receptor antagonist NAN-190 has been shown to partially antagonize the MDMA stimulus, and because NAN-190 binds at several different types of receptors, in the present study we examined other agents (e.g., adrenergic, dopaminergic, sigma) in tests of stimulus generalization and stimulus antagonism to determine their influence on the MDMA stimulus. Each of these agents (i.e., clenbuterol, S(-)propranolol, R(+)SCH-23390, amantadine, NANM) was without effect on MDMA-appropriate responding. The finding that NAN-190 behaves as a 5-HT(1A) partial agonist in some studies prompted examination of the 5-HT(1A) receptor agonist 8-OH DPAT and its optical isomers. MDMA-stimulus generalization occurred to racemic 8-OH DPAT (ED(50) = 0.3 mg/kg), R(+)8-OH DPAT (ED(50) = 0.2 mg/kg), and to the 5-HT(1A) receptor partial agonist S(-)8-OH DPAT (ED(50) = 0.4 mg/kg). The results suggest that the MDMA stimulus might possess a 5-HT(1A) component of action. Furthermore, because 8-OH DPAT is known to enhance the stimulus effects of hallucinogens as discriminative stimuli, and because MDMA reportedly enhances the effects of hallucinogenic agents in humans ("flipping," "candy flipping"), this latter MDMA-induced phenomenon might involve a 5-HT(1A) mechanism.