Effects of caffeine and PD 116,600 on the differential-reinforcement-of-low rate 72-S (DRL 72-S) schedule of reinforcement

Acute but not long-lasting antidepressant-like effect of psilocybin in differential reinforcement of low-rate 72 schedule in rats

BACKGROUND

In clinical studies, psychedelics including psilocybin and D-lysergic acid diethylamide (LSD) demonstrate rapid and persistent antidepressant effects. Since the effective treatment with psychedelics is usually provided with psychotherapy, it is debatable whether their prolonged efficacy can be observed in infrahuman species. Preclinical reports on psychedelics' effects most often address their acute actions, and different tests and models provide inconsistent results. The goal of this study was to examine whether the treatment with psilocybin and/or LSD would demonstrate immediate and/or sustained antidepressant-like effects in the differential reinforcement of low-rate responding (DRL) schedule in rats. In contrast to the antidepressant screening tools, the DRL 72s test is known to detect antidepressants with high predictive validity as it differentiates clinically effective antidepressants from other psychoactive drugs in non-stressed animals.

METHODS

Adult male Sprague Dawley rats were injected over three consecutive days with psilocybin (1 mg/kg), LSD (0.08 mg/kg), or saline and then tested in DRL 72s for the following 4 weeks.

RESULTS

Treatment with psilocybin but not LSD demonstrated an immediate antidepressant-like effect, manifested as an increased number of reinforced presses and response efficiency. By contrast, neither of the drugs showed a long-term (up to 4 weeks following administration) antidepressant-like effect.

CONCLUSIONS

Using DRL 72s schedule of reinforcement, we demonstrated the acute antidepressant-like effect of psilocybin but not of LSD, and failed to detect their persistent antidepressant-like efficacy. The present study suggests that the detection of long-lasting antidepressant-like activity in rats could be challenging and may require entirely novel behavioral methods.

Stimulant effects of adenosine antagonists on operant behavior: differential actions of selective A2A and A1 antagonists

RATIONALE

Adenosine A(2A) antagonists can reverse many of the behavioral effects of dopamine antagonists, including actions on instrumental behavior. However, little is known about the effects of selective adenosine antagonists on operant behavior when these drugs are administered alone.

OBJECTIVE

The present studies were undertaken to investigate the potential for rate-dependent stimulant effects of both selective and nonselective adenosine antagonists.

METHODS

Six drugs were tested: two nonselective adenosine antagonists (caffeine and theophylline), two adenosine A(1) antagonists (DPCPX and CPT), and two adenosine A(2A) antagonists (istradefylline (KW6002) and MSX-3). Two schedules of reinforcement were employed; a fixed interval 240-s (FI-240 sec) schedule was used to generate low baseline rates of responding and a fixed ratio 20 (FR20) schedule generated high rates.

RESULTS

Caffeine and theophylline produced rate-dependent effects on lever pressing, increasing responding on the FI-240 sec schedule but decreasing responding on the FR20 schedule. The A(2A) antagonists MSX-3 and istradefylline increased FI-240 sec lever pressing but did not suppress FR20 lever pressing in the dose range tested. In fact, there was a tendency for istradefylline to increase FR20 responding at a moderate dose. A(1) antagonists failed to increase lever pressing rate, but DPCPX decreased FR20 responding at higher doses.

CONCLUSIONS

These results suggest that adenosine A(2A) antagonists enhance operant response rates, but A(1) antagonists do not. The involvement of adenosine A(2A) receptors in regulating aspects of instrumental response output and behavioral activation may have implications for the treatment of effort-related psychiatric dysfunctions, such as psychomotor slowing and anergia in depression.

Activation of adenosine(1) (A(1)) receptors suppresses head shakes induced by a serotonergic hallucinogen in rats

Modulation of glutamatergic neurotransmission by metabotropic glutamate2/3 (mGlu2/3) receptor agonists effectively treats seemingly diverse neuropsychiatric illness such as generalized anxiety disorder and schizophrenia. Activation of adenosine A(1) heteroceptors, like mGlu2 autoreceptors, decreases glutamate release in the medial prefrontal cortex (mPFC) and other limbic brain regions. Previously, we have reported electrophysiological, neurochemical and behavioral evidence for interactions between the 5-hydroxytryptamine(2A) (5-HT(2A)) and mGlu2/3 receptors in the mPFC. The present studies were designed to investigate the effects in rats of adenosine A(1) receptor activation/blockade on a behavior modulated by 5-HT(2A) receptor activation/blockade in the mPFC: head shakes induced in the rat by phenethylamine hallucinogens. An adenosine A(1) receptor agonist, N(6)-cyclohexyladenosine (CHA) suppressed head shakes induced by activation of 5-HT(2A) receptors with the phenethylamine hallucinogen (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI). An adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), enhanced DOI-induced head shakes and blocked the suppressant action of an adenosine A(1) receptor agonist on DOI-induced head shakes. Thus, the pattern of activity for an agonist and antagonist at the adenosine A1 receptor with respect to modulating DOI-induced head shakes is similar to the pattern observed with mGlu2/3 receptor agonists and antagonists. These novel observations with an adenosine A(1) receptor agonist suggest that this pharmacological action could contribute to antipsychotic effects in addition to thymoleptic effects.

Antidepressant effects assessed using behavior maintained under a differential-reinforcement-of-low-rate (DRL) operant schedule

Behavior maintained under a differential-reinforcement-of-low-rate (DRL) 72-s operant schedule, which reinforces responses with interresponse times greater than 72 s, exhibits a rather unique sensitivity to antidepressant drugs. Antidepressants from a number of pharmacological classes, including tricyclic antidepressants, selective serotonin or norepinephrine reuptake inhibitors, monoamine oxidase inhibitors, as well as a number of atypical antidepressants and putative antidepressants, reduce response rate and increase reinforcement rate of rats under this schedule. These effects are observed acutely but persist or are augmented with repeated treatment. By contrast, drugs from a number of other psychotherapeutic classes do not, in general, produce similar effects. This includes anxiolytic, sedative, stimulant, opioid, antihistaminic, and anticholinergic drugs, which can produce false positive results in some preclinical tests for antidepressant efficacy. There are conflicting data regarding the utility of DRL behavior for discriminating the effects of antidepressant and antipsychotic drugs. This results in part from methodological differences among studies, but likely also reflects the overlap between the neuropharmacological and clinical effects of some antipsychotic and antidepressant drugs. DRL behavior also has proven useful for identifying neurochemical and neuroanatomical mediators of antidepressant effects on behavior. Consistent with clinical data, it appears that activation of noradrenergic or serotonergic systems provides for parallel means of producing antidepressant-like effects on DRL behavior. Finally, the results of studies using DRL behavior highlight important roles for central beta-1 adrenergic receptors, as well as 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT2C receptors, in the mediation of antidepressant-like behavioral effects.

Caffeine has similar pharmacokinetics and behavioral effects via the i.p. and p.o. routes of administration

Caffeine administered intraperitoneally (i.p.) or orally (p.o.) decreased the reinforcement rate and increased the nonreinforced response rate in a dose-related fashion under a differential reinforcement of low rate schedule (DRL 45-s) in 3-h sessions. These effects were similar following both routes of caffeine administration. The parallel pharmacokinetics for i.p. and p.o. caffeine were each determined and related to the respective effects of caffeine on reinforcement rate. Serum caffeine concentrations were similar across the session after the absorption phase for a given dose. Consequently, the effect remained in approximately the same range within a dose, and no single dose possessed a full concentration-effect relation for the two routes. The effects of i.p. and p.o. caffeine on reinforcement rate plateaued at doses higher than 40 mg/kg, which produced a serum caffeine concentration of approximately 25 microg/ml regardless of the route of administration. The EC50 values were 7.34 and 9.93 microg/ml for i.p. and p.o. caffeine, respectively. This study as well as our previous studies demonstrated that the i.p. route is dependable for studying caffeine dose response relations but not for studying other drugs (e.g., midazolam). The possible mechanism accounting for this difference is discussed.