Effects of phencyclidine, haloperidol, and naloxone on fixed-interval performance in rats

Comparison of antidepressant-like and abuse-related effects of phencyclidine in rats

Preclinical Research N-methyl-D-aspartate (NMDA) receptor antagonists, such as ketamine, have emerged as novel candidate treatments for major depressive disorder, but abuse potential of these agents is a concern. The NMDA antagonist phencyclidine has known abuse liability but undefined efficacy as an antidepressant. To further evaluate the relationship between antidepressant-like and abuse-related effects of NMDA antagonists, this study evaluated the effects of phencyclidine (1.0-10.0 mg/kg) in male Sprague-Dawley rats responding under two procedures that have been used to assess antidepressant-like effects (differential-reinforcement-of-low-rate [DRL] 72 s schedule of food reinforcement; n = 9) and abuse-related drug effects (intracranial self-stimulation [ICSS]; n = 6). Under the DRL 72 s schedule, phencyclidine (10.0 mg/kg) increased reinforcers and decreased responses without shifting the peak location of the interresponse time (IRT) distribution. Ketamine (10.0 mg/kg) also increased reinforcers and decreased responses, but unlike phencyclidine, it produced a rightward shift in the peak location of the IRT distribution. The 10.0 mg/kg phencyclidine dose that decreased DRL 72 s responding also decreased rates of ICSS for 50 min after its administration; however, abuse-related ICSS facilitation was observed at later times (100-300 min) or after a lower phencyclidine dose (3.2 mg/kg). These results suggest that phencyclidine produces weaker antidepressant-like effects, but stronger abuse-related effects than ketamine in these procedures.

Effects of phencyclidine on schedule-controlled responding following neurotoxic lesions of the striatum

The effects of phencyclidine on an operant task were evaluated prior to and after neurotoxic lesions of the striatum in rats. Subjects were trained to respond on a fixed-interval 90-second schedule for water presentation. The degree to which phencyclidine disrupted responding was first evaluated (dose range 1.0-4.0 mg/kg). The subjects were then divided into three matched groups and received bilateral intraventricular injections of 6-hydroxydopamine (6-OHDA) (100 microg), kainic acid (0.25 microg), or vehicle delivered stereotaxically. 6-OHDA was used to destroy the presynaptic neurons of the nigro-striatal pathway and kainic acid was employed to destroy the postsynaptic neurons whose cell bodies are located in the striatum. Following recovery, the phencyclidine dose-response curve was repeated in the fixed-interval paradigm. It was observed that 6-OHDA-induced damage resulted in a rightward shift of the dose-response curve indicating tolerance to phencyclidine and caused a significant depletion of striatal dopamine and gamma-aminobutyric acid (GABA). Kainic acid-induced damage resulted in a leftward shift in the dose-response curve indicating sensitivity to the schedule-disruptive effects of phencyclidine and produced a significant GABA depletion. The vehicle-treated rats exhibited no shift in their sensitivity to phencyclidine. These observations indicate that the effects of phencyclidine are mediated, at least in part, by striatal dopaminergic neurons.

Interaction of sigma and PCP-like drugs on operant behaviors in the rat

Many PCP-like drugs interact with at least two types of binding sites in the CNS, one of which is linked to excitatory amino acid transmission and the other with an unknown function. The present experiments were designed to further clarify the mechanism of action of drugs in this class. Assessment was made of the effects of PCP, MK-801, (+)-pentazocine, (+)- and (-)-N-allyl-normetazocine (NANM), (+)-amphetamine and BMY-14802 in rats responding under a multiple timeout 600 s (TO), differential reinforcement of low rates 10 s (DRL), fixed ratio (FR) 10 schedule of reinforcement. The effects of the PCP-receptor selective drug MK-801 were compared to those of the mixed sigma/PCP drug (+)-NANM after each were combined with doses of (+)-pentazocine, PCP, BMY-14802, and (+)-amphetamine. MK-801 was also tested in combination with (+)-NANM, as was PCP with BMY-14802. When administered alone, MK-801, PCP, (+)-NANM, (+)-pentazocine, and (+)-amphetamine increased rates of responding under the DRL component of the multiple schedule. The drugs tested generally produced decreases in rates of responding under the FR component. (+)-Pentazocine and BMY-14802 did not modify the effects of (+)-NANM or of MK-801. PCP enhanced the effects of MK-801 and (+)-NANM, and (+)-amphetamine enhanced the effects of MK-801 but not of (+)-NANM. BMY-14802 attenuated the effects of PCP. Taken together, these data suggest similarities as well as some differences in the pharmacologic activities of MK-801 and (+)-NANM and PCP.

Effects of buspirone on fixed interval responding in rats

Buspirone is a novel anxiolytic which does not share the muscle relaxant, anticonvulsant and sedative properties of classical anxiolytics such as the benzodiazepines. Its effects in different animal models of anxiety are also variable. The present experiments investigated the effects of buspirone on a fixed interval 60 s schedule of reinforcement (FI). In experiment 1, four doses of buspirone (10, 3.3, 1.1 and 0.3 mg/kg, i.p.) and two doses of chlordiazepoxide (5 and 20 mg/kg, i.p.) were administered to separate groups of rats throughout acquisition of the FI task. In experiment 2, four doses of buspirone (1.1, 0.3, 0.1 and 0.03 mg/kg, i.p.) and a single dose of chlordiazepoxide (5 mg/kg, i.p.) were used. Chlordiazepoxide generally released responding. At higher doses (1.1 mg/kg and above) buspirone suppressed responding in the later parts of the FI interval. The effects of lower doses were variable but included some response release in the later parts of the FI interval. At no dose did buspirone release responding at the beginning of the FI interval. The experiments show that buspirone differs qualitatively as well as quantitatively from chlordiazepoxide and that current animal models based on behavioural inhibition may need to be used with considerable care if detection of novel anxiolytics is to be ensured.

Interactions of naloxone with morphine, amphetamine and phencyclidine on fixed interval responding for intracranial self-stimulation in rats

Rats were implanted with stimulating electrodes aimed at the medial forebrain bundle-lateral hypothalamus (MFB-LH) and were trained to lever-press for brain self-stimulation on a fixed interval: 60 s schedule of reinforcement. The effects of graded doses of naloxone (0.1-30 mg/kg), morphine (0.3-5.6 mg/kg), naloxone plus morphine, d-amphetamine (0.03-1.0 mg/kg), naloxone plus d-amphetamine, phencyclidine (0.3-5.6 mg/kg), and naloxone plus phencyclidine were tested. Naloxone produced a significant decrease in rates at 30 mg/kg. Naloxone (0.1-1.0 mg/kg) plus morphine blocked the dose-dependent decrease produced by morphine alone. In contrast, naloxone (1.0-10 mg/kg) plus d-amphetamine attenuated the graded increase in response rates produced by d-amphetamine. Naloxone (1.0-10 mg/kg) plus phencyclidine did not reliably change the increase in response rates produced by phencyclidine alone. The use of the fixed interval schedule of brain self-stimulation to study these drug interactions is novel, and further demonstrates that the highly reinforcing aspects of brain stimulation, known to be influenced by dopamine, may also be modulated by the endogenous opiate system.

Neurotensin, substance P, neurokinin-alpha, and enkephalin: injection into ventral tegmental area in the rat produces differential effects on operant responding

The neuropeptides neurotensin, substance P, neurokinin-alpha (substance K), and met-enkephalin are present endogenously in the ventral tegmental area (VTA), site of the A10 dopaminergic (DA) cell bodies. In the present study these four peptides were injected bilaterally into the VTA in the rat, and the effects on operant behavior were assessed. Cannulae aimed at the VTA were implanted in four groups of animals, which had been trained to bar-press for food reward on a fixed-interval, 40-s schedule. A fifth group, in which the effects of systemically administered amphetamine were assessed, was also tested. Response rate across the interval was measured, and the index of quarter-life was taken as an indication of the temporal pattern of responding. In addition, a rate-dependency analysis was carried out for all data. Neurotensin (NT, 0.0175, 0.175, 0.5 micrograms in 1 microliter) dose-dependently decreased response rates without affecting quarter-life, and reduced the number of reinforcements obtained. Substance P (SP, 0.1, 1.0, 3.0 micrograms) did not affect responding, and neurokinin-alpha (NKA, 0.1, 1.0, 3.0 micrograms) induced a small increase in responding. Quarter-life was not affected by SP or NKA, but responding on the non-reinforced lever was significantly increased by both peptides. d-Ala-met-enkephalin (DALA, 0.01, 0.1, 1.0 micrograms) induced a dose-dependent increase in responding which was also rate-dependent, and reduced quarter-life. DALA effects were similar to the classic pattern of responding observed after systemic amphetamine. These results suggest that although all these peptides elicit behavioral activation and may affect DA neuronal activity, the behavioral responses can be differentiated with respect to operant behavior.

Training-dependent decay in performance produced by the neuroleptic cis(Z)-flupentixol on spatial navigation by rats in a swimming pool

Rats were trained on place or cue spatial navigation tasks in a swimming pool and then given the neuroleptic, alpha-flupentixol. Initial experiments showed that regardless of testing schedule, including blocks of trials given concurrently or separated by 7 or 30 days, drugged rats showed a trial-by-trial decay in latency and accuracy of responding although they continued to swim. The rate of decay increased with increases in drug dosage. Further experiments showed that: 1) Performance decay was specifically related to conditioned components of the test environment. Animals required to swim in a different test, or to struggle, showed less decay than rats exposed to the test platform only or required to perform all aspects of the task. 2) Decay was not due to nonspecific effects of neuroleptic treatment because rats injected and replaced in their home cage, and then subsequently reinjected and tested performed like rats treated and tested for the first time. 3) A trial-dependent decay of performance was also obtained in hippocampectomized and decorticate rats, suggesting that at least part of the major action of the drug is on subcortical systems. The results are discussed with respect to hypotheses of neuroleptic action and with respect to their possible relevance to experience-dependent changes in animal analogues of Parkinson's disease. Finally, it is suggested that behavior may be organized in subsystems, which when active, become selectively sensitive to neuroleptics.

Blockade by neuroleptics of water intake and operant responding for water in the rat: anhedonia, motor deficit, or both?

Four different neuroleptic drugs, haloperidol, metoclopramide, sulpiride and cis-flupenthixol, were tested for their ability to attenuate an operant lever-pressing response with water as reward and the corresponding consummatory act, i.e., the non-conditioned water intake. All four neuroleptic drugs tested more potently attenuated the operant lever-pressing response than the consummatory water intake, just as the conditioned avoidance response previously has been found to be more potently attenuated than the non-conditioned escape reaction. The results suggest that a certain class of learned behaviors, labelled operant or instrumental behaviors, are more susceptible to the attenuating effects of neuroleptic drugs than the class of behaviors labelled non-conditioned consummatory acts. It was further concluded that the attenuation of the lever-pressing response could be explained by a decreased ability of the animals to initiate or perform the required operant response (i.e., a motor deficit) while the attenuated water intake caused by higher doses of the neuroleptics could be interpreted as a motivational effect (e.g., "anhedonia"). When studying the effects of the neuroleptic drugs it is therefore of great importance to know whether the behavior measured in the particular experimental design used is operant or consummatory. The implications of the findings are discussed.

The interaction of d-amphetamine and naloxone differs for rats trained on separate fixed-interval or fixed-ratio schedules of reinforcement

The effects of d-amphetamine and naloxone were investigated using two groups of rats trained on either an FR30 or F12 schedule of reinforcement. Amphetamine (0.1-1.0 mg/kg), and naloxone (1.0 and 10 mg/kg) administered separately reduced responding on the FR procedure in a dose-dependent manner. The combined administration of naloxone with amphetamine had an additive suppressive effect on responding. The same doses of amphetamine and naloxone, when given separately, did not significantly depress responding in the FI procedures. However, naloxone/amphetamine combinations produced a marked inhibition of lever-pressing. Naloxone did not alter the characteristic pattern of responding engendered by amphetamine in this schedule, as measured by the quarter-life and Index of Curvature. It appears that the type of procedure used is a critical factor in demonstrating the effects of naloxone on behavior, and the nature of naloxone/amphetamine interactions.

Potentiation of disruptive effects of dextromethorphan by naloxone on fixed-interval performance in rats

A centrally acting antitussive agent dextromethorphan (DM) was tested to determine its possible interaction with naloxone in rats responding under a fixed-interval schedule of positive reinforcement. A sugar sweetened milk reward was used as a positive reinforcer. Under the same experimental conditions the effects of morphine alone and in combination with naloxone were also determined. Low dose DM (10 mg/kg) produced a slight increase, while higher doses (20-40 mg/kg) produced dose-dependent decreases in response rate. Morphine (0.3, 1.0 and 3.0 mg/kg) produced dose-dependent decreases in response rate. When doses of naloxone (0.1-1.0 mg/kg) were administered after the injection of DM the rate-decreasing effects of DM were potentiated even after the rate-increasing dose of naloxone (0.1 mg/kg) was used. When a dose of naloxone (0.1 mg/kg) was administered after the injection of morphine the rate-decreasing effects of morphine were markedly antagonized, i.e., the morphine dose-response curve was shifted to the right. The observed potentiation of DM disruption by naloxone on fixed-interval performance in rats is consistent with findings showing that naloxone potentiates the disruptive behavioral effects of a number of drugs that are psychotomimetic in man.

Naloxone and fluid consumption in rats: dose-response relationships for 15 days

Rats were given daily intraperitoneal injections of 10.0, 1.0, 0.1, 0.01, 0.001 or 0.0 mg/kg naloxone for 15 days. Each day after the injections, animals were allowed access to a 20% sucrose solution for two hours and to tap water for the subsequent 10 hours. Consumption of the sucrose solution by the group that received 1.0 mg/kg was reliably decreased on Day 1 and 2, reflecting the suppressive effect of naloxone at that dose. By Day 3 until the end of the experiment, however, the suppression was no longer significant, suggesting that tolerance had developed. A similar effect was seen with the group given the highest dose, 10.0 mg/kg; although drinking was significantly less than the control in each of the 15 sessions, this group showed a trend to increase intake over the days of the experiment, thus also indicating possible tolerance to the effect of naloxone. Drinking patterns of the other groups did not differ statistically from the control. Thus, the low doses had no ability to suppress consumption, and the lowest dose that did lower it soon lost that ability; the highest dose continued to suppress drinking throughout the study but with decreasing efficacy. High performance liquid chromatography (HPLC) demonstrated that the naloxone remained intact over the 15 days of the experiment, supporting the suggestion that tolerance to naloxone might have developed.

Endogenous opiates: 1984

This paper is the seventh in an annual series of reviews of research involving the endogenous opiate peptides, each installment being restricted to work published during the previous year. As in the past three years, the review this year is limited to non-analgesic and behavioral studies of the opiate peptides. The specific topics this year include: stress, tolerance and dependence, consummatory responses, gastric and renal activity, alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, seizures and neurological disorders, activity, and miscellaneous other topics.

Interactions of naloxone and haloperidol with phencyclidine: effects on milk intake

Phencyclidine (PCP), naloxone and haloperidol were administered alone and in combination to rats trained to drink sweetened-condensed milk during a 20 min daily session. PCP (1.0-16.0 mg/kg) produced a dose-dependent decrease in milk intake. All doses of naloxone (0.1-16.0 mg/kg) produced approximately a 30% decrease in milk intake. Haloperidol (0.125 mg/kg) had virtually no effect on milk intake. When a dose of naloxone which reduced milk intake by approximately 30% (8.0 mg/kg) was administered as a pretreatment to the PCP, the PCP curve was shifted to the left (lowered) to that degree. When haloperidol (0.125 mg/kg) was administered as a pretreatment to the PCP, the PCP dose-response curve was shifted 1.5 fold to the right. These interactions are similar to those observed in other behavioral paradigms and are discussed in reference to PCP's actions as an indirect dopaminergic agonist.