Variable interval schedules of timeout from avoidance: effects of ethanol, naltrexone, and CGS 8216

Four rats were trained on concurrent schedules of shock avoidance and timeout from avoidance, where responses on one lever postponed shock and responses on another lever occasionally (VI 45 sec schedule) produced a 2-min timeout during which the avoidance schedule was suspended. These procedures maintained stable rates of responding on both levers, providing a baseline for studying the effects of drugs on behavior under different types of aversive control (shock avoidance and timeout from avoidance). In the first experiment the effects of ethanol (0.5, 1.0, 1.5, and 2.0 g/kg) and an opiate antagonist, naltrexone (1 mg/kg) were assessed alone and in combination. Ethanol produced a dose-dependent decrease in avoidance characterized by increased shock rates and decreased response rates. At the same time, however, responding on the timeout lever generally increased relative to avoidance lever rates. All of these effects were largely confined to the early parts of the 2-hr session, when blood-ethanol levels were relatively high. Naltrexone had no effect on performances and did not interact with ethanol. In a second experiment, the effects of the benzodiazepine antagonist CGS 8216 were studied alone, and in combination with ethanol. CGS 8216 (5 mg/kg) generally disrupted both avoidance and timeout responding but did not reverse ethanol actions.     

Attenuation of the biphasic effects of ethanol on avoidance extinction by RO 15-4513 in rats

Ethanol had biphasic effects on jump-up avoidance extinction with low doses (1 g/kg) increasing, and high doses (2.5 g/kg) decreasing number of trials to extinction criterion. In Experiment 1 these doses of ethanol were studied alone, and in combination with RO 15-4513 (0.3, 3 or 6 mg/kg). The stimulation of responding produced by low ethanol doses was reversed by 3 and 6 mg/kg doses of RO 15-4513 which had intrinsic suppressive effects, but the depressed responding produced by higher ethanol doses was not attenuated by RO 15-4513. Experiment 2 analysed the interaction between ethanol and benzodiazepine antagonists RO 15-1788 and CGS 8216. RO 15-1788 did not have intrinsic action and did not interact with ethanol. CGS 8216 showed an intrinsic suppressive action much like RO 15-4513, and also reversed the stimulation produced by low dose ethanol, but not the effects of the high dose. Experiment 3 showed that the benzodiazepine agonist, chlordiazepoxide, had effects much like low dose ethanol which were reversed by CGS 8216 and RO 15-4513. The major conclusions were that RO 15-4513 and CGS 8216 possess inverse agonist properties which may cancel out the effects of alcohol under certain circumstances.     

Interactions of diazepam and pentobarbital with RO 15-4513 on intracranial self-stimulation discrimination behavior in rats

Rats implanted with electrodes in the lateral hypothalamus were trained in a discrete trial procedure to make a differential response (right or left lever press) in the presence or absence of brain stimulation. When a high level of accuracy (95% correct) was attained in the discrimination, testing with vehicle, RO 15-4513, diazepam (1.0-10 mg/kg), diazepam plus RO 15-4513 (1.0 mg/kg), pentobarbital (1.0-17.5 mg/kg) and pentobarbital plus RO 15-4513 began. Diazepam, at 10 mg/kg, disrupted the discrimination behavior, and it also decreased the total number of lever-presses and increased the time to complete the session. These effects were blocked by the coadministration of 1.0 mg/kg RO 15-4513. Pentobarbital produced effects similar to those of diazepam, but these effects were only reversed to a limited extent by RO 15-4513. By itself, however, RO 15-4513 also decreased the total number of lever presses and increased the time to complete the session. Results were consistent with our previous findings with alcohol and RO 15-4513, and supported the notion that diazepam and alcohol have some similar effects at the GABA-benzodiazepine receptor complex.     

Variable-ratio schedules of timeout from avoidance: effects of anxiolytic drugs

Concurrent performances in rats were studied under conditions where responses on one lever postponed shock on an unsignaled avoidance schedule, and responses on another level produced periods of signaled timeout from avoidance on a variable-ratio schedule. This procedure resulted in relatively high rates of responding on the timeout lever, and provided a baseline which permitted simultaneous evaluation of drug effects on two different types of negative reinforcement (shock postponement vs timeout). Chlordiazepoxide and ethanol selectively increased responding on the timeout lever at low doses, while higher doses decreased responding on both levers. Two 5-HT(1A) agonists, buspirone and 8-OH-DPAT, had different effects. Buspirone decreased responding across all effective doses, but 8-OH-DPAT increased responding on both the timeout and avoidance levers, with greater increases noted in responding maintained by timeout. These results replicate and extend previous findings, and support the notion that traditional anxiolytic drugs like chlordiazepoxide and ethanol may increase the reinforcing properties of escape from an avoidance schedule. Differences between the behavioral effects of buspirone and 8-OH-DPAT may reflect differential activity at the 5-HT(1A) receptor or the dopaminergic properties of buspirone.     

The effects of Ro 15-4513 on the behavioral actions of ethanol in an operant reaction time task and a conflict test

Ro 15-4513, an analogue of the benzodiazepine receptor antagonist Ro 15-1788, has been reported to selectively block the anxiolytic and intoxicating properties of ethanol in rats. To examine the specificity and selectivity of this ethanol antagonism, the effects of Ro 15-4513 were tested on the actions of ethanol in an operant reaction time and conflict test in rats. The operant reaction time task involved holding down a lever for 0.25-2.0 seconds to obtain food, and animals treated with 1 g/kg of ethanol showed a significant disruption in performance. This disruptive effect was reversed by Ro 15-4513 in doses of 1.5-5.0 mg/kg. Ro 15-4513 was also tested in an operant conflict paradigm sensitive to alcohol effects. Ro 15-4513 (0, 1.5, 3.0, 6.0 mg/kg) produced a significant decrease in both punished and unpunished responding in the conflict test. Ethanol (0.75 g/kg), pentobarbital (5 mg/kg) and chlordiazepoxide (5 mg/kg) all produced a significant release of punished responding that was blocked by pretreatment with 6.0 mg/kg Ro 15-4513, but again Ro 15-4513 suppressed responding on its own at this dose. These results suggest that Ro 15-4513 has inverse agonist properties that may explain its ethanol-antagonist action.     

Interactions between RO 15-4513 and ethanol on brain self-stimulation and locomotor activity in rats

Experiments were conducted to elucidate the behavioral effects of RO 15-4513, a putative alcohol antagonist, when administered alone or in combination with alcohol. Two groups of animals were trained to lever-press for brain self-stimulation (ICSS) on either a fixed ratio:15 or a fixed interval:15 second schedule of reinforcement. RO 15-4513 (0.1-3.0 mg/kg) reduced the rate of lever-pressing for ICSS in both groups. RO 15-4513 (1.0 mg/kg) further reduced rates when combined with alcohol (0.1-1.7 g/kg), and this effect was especially marked in the fixed ratio paradigm. Other groups of animals were tested in a locomotor activity apparatus. In contrast to the depression of lever-pressing in the ICSS experiments, RO 15-4513 produced a graded increase in locomotor activity. When combined with alcohol (0.1-1.7 g/kg), 1.0 mg/kg RO 15-4513 also increased locomotor activity. Thus, the depression in schedule-controlled behavior was not associated with a generalized behavioral depression. These results demonstrated that RO 15-4513 has potent behavioral effects of its own that are consistent with its classification as an anxiogenic compound.     

Variable-interval schedules of timeout from avoidance: effects of anxiolytic and antipsychotic drugs in rats

Concurrent performances were studied in rats under conditions where responses on one lever postponed shock on a Sidman avoidance schedule and responses on another lever produced periods of signaled timeout from avoidance on a variable-interval schedule. Chlorpromazine decreased rates of responding on both the timeout and avoidance levels to about the same extent. The effects of chlordiazepoxide and CGS 9896 depended upon the event maintaining responding. Both drugs increased responding on the timeout lever at doses that concurrently decreased responding on the avoidance lever. Thus, the novel anxiolytic CGS 9896 produced effects that closely resembled those of the benzodiazepine anxiolytic, chlordiazepoxide. Like chlorpromazine, buspirone decreased both avoidance and timeout responding. Despite the documented anxiolytic properties of buspirone, its actions here were unlike those of the other anxiolytic drugs tested. Nonetheless, the differentiation between drugs obtained with the timeout from avoidance procedure indicates its utility for behavioral pharmacology.     

RO15-4513 antagonizes the anxiolytic effects of ethanol in a nonshock conflict task at doses devoid of anxiogenic activity.

RO15-4513 is a partial benzodiazepine inverse agonist that has been reported to antagonize some of the biochemical and neurobehavioral actions of ethanol. However, whether this antagonistic action of RO15-4513 is dependent on the drug exerting its intrinsic (inverse agonist) properties is unclear at present. The purpose of the present study was to examine whether RO15-4513 was capable of antagonizing the anxiolytic effects of ethanol in a nonshock conflict task at doses that, by themselves, do not reveal the compound's intrinsic anxiogenic properties. The consummatory conflict task employed (negative contrast) involves quantifying how animals respond to an abrupt, unexpected reduction in reward (sucrose solution), and is particularly sensitive to the effects of anxiolytic agents, including ethanol. As previously demonstrated, depressed consummatory behavior engendered by reward reduction was significantly alleviated by ethanol (0.75 g/kg). This anxiolytic effect of ethanol, however, was antagonized dose dependently by RO15-4513 (0.1875-3.0 mg/kg). Only the highest dose of RO15-4513 (3.0 mg/kg) showed evidence of further response suppression. Lower doses of RO15-4513 tested did not exert an anxiogenic effect when given alone. Thus the antagonism of EtOH's anxiolytic (contrast-reducing) effects occurred at doses of RO15-4513 (0.375-1.5 mg/kg) that did not exhibit any intrinsic anxiogenic activity. As such, these results suggest that RO15-4513 interacts with the anxiolytic effects of ethanol in a nonadditive fashion in this test situation.     

Ethanol-induced locomotor stimulation in C57BL/6 mice following RO15-4513 administration

The purpose of this study was to examine the effects of two partial benzodiazepine inverse agonists, RO15-4513 and FG-7142, alone and in combination with ethanol on locomotor activity in C57BL/6 mice. When administered alone, 1.5 g/kg ethanol did not significantly influence activity, confirming previous reports indicating this mouse strain is relatively insensitive to the excitatory properties of ethanol. RO15-4513 treatment also did not significantly influence locomotor activity when administered alone. However, coadministration of RO15-4513 (1.5–6 mg/kg) and ethanol markedly increased locomotor activity. Moreover, the unmasking of ethanol's stimulant action by RO15-4513 (6 mg/kg) was completely reversed by pretreatment with the benzodiazepine receptor antagonist RO15-1788. In contrast, FG-7142 (10–20 mg/kg) increased activity to the same extent in both saline and ethanol-injected mice. This effect was blocked by RO15-1788 pretreatment as well. Neither RO15-4513, FG-7142, nor RO15-1788 significantly influenced blood ethanol concentrations. It is suggested that RO15-4513 unmasked the stimulant effects of ethanol by virtue of its ability to antagonize the depressant properties of ethanol in C57BL/6 mice.     

Oral ethanol reinforcement in the rat: effect of the partial inverse benzodiazepine agonist RO15-4513

The partial inverse benzodiazepine agonist RO15-4513 has been found to reverse the sedating and anti-conflict effects of acute ethanol administration. In non-food or fluid-deprived rats, orally self-administering 10% ethanol in an operant situation, RO15-4513 resulted in a dose-dependent suppression on ethanol intake. Doses of 0.3, 1.0 and 3.0 mg/kg suppressed responding from approximately 25% to 60% respectively. A dose of 0.1 mg/kg had no significant effect upon responding. These findings were discussed in terms of the potential independence of brain mechanisms related to ethanol reinforcement and sedation.     

Ethanol and Ro 15-4513: behaviour maintained by operant procedures (DRL-72s and PTZ-drug discrimination) in rats

The proposed amethystic imidazobenzodiazepine Ro 15-4513 and ethanol (ETOH) were examined in rats using two operant procedures, differential reinforcement of low rates of responding (DRL), and drug discrimination learning (DDL). In the first bar-pressing responses occurring 72 s or longer after the last reinforcement were rewarded; responses occurring earlier reset the time schedule. In the latter, animals were trained to discriminate between the effects of the analeptic pentylenetetrazole (PTZ) and the non-drug condition; the schedule of reinforcement was FR-10. Water was the reinforcer. A dose of 1000 mg/kg ETOH decreased the rate of bar pressing in the DRL experiment; doses of 300 and 560 mg/kg ETOH did not. The decrease was not attenuated by Ro 15-4513. No significant deviations from baseline responding occurred with Ro 15-4513 (1, 3 10 mg/kg). The number of reinforcements increased significantly after ETOH (1000 mg/kg), but not after Ro 15-4513. Combinations of the two agents produced increases in the number of reinforcements. Changes in DRL behaviour induced by diazepam (1 and 10 mg/kg) were normalized by 3 mg/kg Ro 15-4513. In DDL, Ro 15-4513 (10 mg/kg) substituted for PTZ; ETOH did not. Diazepam and Ro 15-1788 attenuated the response generalization from Ro 15-4513 to PTZ; ETOH did not. There was a dose-related increase in the time to complete the DDL tests after ETOH treatment; addition of Ro 15-4513 increased the time further. In conclusion, antagonism between Ro 15-4513 and ETOH did not occur in the present studies; data are furthermore consistent with the view that Ro 15-4513 acts as a partial inverse benzodiazepine agonist.     

RO 15-4513 does not protect rats against the lethal effects of ethanol

In two separate research centres the ability of RO 15-4513 to protect rats against the lethal effects of ethanol (7.5 and 15 g/kg) was investigated. In neither study did RO 15-4513 offer protection against ethanol-induced lethality or the loss of righting reflex caused by these doses of ethanol. These data fail to replicate the results of an earlier report and suggest that RO 15-4513 is unlikely to be clinically useful treating acute severe ethanol toxicity.     

Effects of Ro 15-4513, fluoxetine and desipramine on the intake of ethanol, water and food by the alcohol-preferring (P) and -nonpreferring (NP) lines of rats

The effects of the IP administration of RO 15-4513 (1,2 and 4 mg/kg), fluoxetine (5 and 10 mg/kg) and desipramine (5 and 10 mg/kg) on the intake of 10% ethanol, H₂O and food were determined in the selectively bred alcohol-preferring (P) and -nonpreferring (NP) lines of rats with daily access to fluids being limited to single 2-hour sessions. The imidazobenzodiazepine Ro 15-4513 (a partial inverse benzodiazepine agonist) significantly reduced the intake of 10% ethanol by the P rats to 50–60% of control levels in the first 30 minutes without altering food or H₂O intake. The attenuating actions of 2 mg/kg Ro 15-4513 on ethanol intake could be completely blocked by the central benzodiazepine receptor antagonist Ro 15-1788 (10 mg/kg). Ro 15-1788, by itself, produced no effects on alcohol and H₂O consumption. The 5 mg/kg dose of fluoxetine significantly reduced 10% ethanol intake by the P rats to 20% of control values without altering either H₂O or food consumption. The 10 mg/kg dose of fluoxetine further reduced ethanol intake by the P rats, but this dose also reduced daily food intake to approximately 70% of normal. Desipramine at both doses significantly (p<0.05) reduced both ethanol and food uptake by the P rats and had a tendency to reduce H₂O consumption as well. In general, the three drugs had effects in the NP rats similar to those observed for the P group, although the effects on 10% ethanol intake were difficult to compare because of the low, variable intake of alcohol by the NP group. The data are consistent with the involvement of serotonin and the GABA-benzodiazepine receptor complex in alcohol drinking behavior.     

Chronic ethanol administration increases the binding of the benzodiazepine inverse agonist and alcohol antagonist [3H]RO15-4513 in rat brain

Chronic ethanol treatment which produced intoxication and physical dependence in rats, produced an increase in the specific binding of ethanol antagonist [3H]RO15-4513 in rat brain cerebral cortex and cerebellum, but not in hippocampus and striatum. The increase in both the regions was due to an increase in the number (Bmax) of receptor sites. These results suggest that the RO15-4513 binding sites on the oligomeric GABA receptor complex are altered following chronic ethanol administration, and support the notion of a unique role of RO15-4513 as an ethanol antagonist.     

Does RO 15-4513 reverse the anxiolytic effects of ethanol by its intrinsic properties?

In order to better understand the antagonistic effects of the partial inverse agonist of benzodiazepine receptors, RO 15-4513, against the disinhibitory action of ethanol, we examined the effects of RO 15-4513 at a dose (2.0 mg/kg) that did not alter locomotor activity, given alone or in combination with ethanol, on the behavior of mice confronted with the light/dark choice procedure and the staircase test. At this dose, RO 15-4513 given alone was found to have slight anxiogenic properties and when given in combination with ethanol, to completely reverse the disinhibitory effects of ethanol. Since we previously observed postictal depression after higher doses of RO 15-4513 given alone and antagonistic effects of these same doses on the action of ethanol, it can be suggested that the antagonistic effects of RO 15-4513 against ethanol are due to its anxiogenic or depressive properties depending on doses. However, this hypothesis can only be regarded as being in early stages of development at the present time since these results do not parallel with those of several other studies and the question whether the antagonistic action of RO 15-4513 against ethanol is additive or interactive remains open.     

The psychopharmacology of impulsive behaviour in rats VIII: effects of amphetamine,methylphenidate, and other drugs on responding maintained by a fixed consecutive number avoidance schedule

Rationale Impulsive behaviour is a component of psychiatric disorders such as bipolar disorder, attention deficit hyperactivity disorder (ADHD), or personality disorders. Most experimental studies on impulsive behaviour punish impulsive choices by loss or delay of reward. In the present study, impulsive behaviour was punished by an explicitly aversive stimulus, using a novel fixed consecutive number (FCN) schedule of electric shock avoidance.Objectives This study was conducted to demonstrate stable performance using the FCN avoidance procedure, and examine the effects of drugs previously shown to affect impulsive behaviour using a conventional FCN schedule.Methods First, rats were trained in the appetitive FCN procedure. Pressing the right lever in an operant conditioning chamber after having pressed the left lever at least six times delivered a food pellet (FCN6). Responses on the right lever before completing this ratio resulted in a time out and restarted the ratio. The rats were then switched to FCN avoidance. Responses on the right lever made before completion of the ratio also resulted in food delivery, but were accompanied by an electric shock.Results Chlordiazepoxide (10.0 mg/kg), ethanol (1.0 g/kg), and haloperidol (0.1 mg/kg) increased impulsive behaviour by reducing the number of left lever responses made before the right lever was pressed. Imipramine (1.0–10.0 mg/kg) and desipramine (1.0–10.0 mg/kg) had no effect on impulsive choice. Amphetamine (0.4 and 0.8 mg/kg) and methylphenidate (6.0 mg/kg) significantly increased the mean chain length, and the proportion of very long chains, indicative of reduced impulsivity, although this did not improve efficiency.Conclusions The increases in impulsivity produced by chlordiazepoxide, ethanol, and haloperidol were similar to those under appetitive FCN schedules. In contrast, amphetamine and methylphenidate, by reducing impulsivity in the FCN avoidance, induced effects opposite to those observed in an appetitive FCN procedure. These results suggest that the therapeutic actions of stimulants, to reduce impulsive behaviour in ADHD, may arise in part by increasing the control of behaviour by aversive stimuli.     

Ethanol suppression of schedule-controlled responding: interactions with Ro 15-4513, Ro 15-1788 and CGS 8216

Rats (N = 14) were trained to respond under a five seconds differential reinforcement of low rate (DRL 5') schedule and under a fixed ratio 10 (FR10) schedule of reinforcement. Ro 15-1788 did not influence the number of responses in the DRL 5' schedule, but increased responding in the FR10 schedule. Ethanol (ETOH, 1250 mg/kg) and CGS 8216 (5 mg/kg) suppressed responding in both schedules and these effects were not antagonized by Ro 15-1788. The response suppressing effects of ETOH in both schedules were not influenced by CGS 8216. These results indicate that the response suppressing effects of ETOH and CGS 8216 are not mediated by the BDZ receptor. Ro 15-4513 suppressed responding strongly in the FR10 schedule. The response suppressing effects of Ro 15-4513 were additive with the response suppressing effects of ETOH. In rats (N = 11) trained to respond under a variable interval 40 seconds-fixed ratio 10 (VI 40'-FR10) schedule Ro 15-4513 dose-dependently suppressed responding. These results indicate that Ro 15-4513 has sedative effects and is not able to antagonize all the behavioral actions of ETOH.     

Interaction of RO 15-4513 and ethanol on the behaviour of mice: antagonistic or additive effects?

Opposite effects were observed of ethanol on the behaviour of mice in the two chambered light/dark test. At a low dose, it had anxiogenic effects, while it produced anxiolytic effects at a higher dose. Selective antagonistic actions of RO 15-4513 against the behavioural effects of ethanol have been reported by others without intrinsic actions. In contrast, we found intrinsic depressive properties of RO 15-4513. This drug reduced locomotion in a running wheel test. We suggest that RO 15-4513 reversed certain effects of ethanol in an additive, rather than interactive, manner. In addition, RO 15-4513 did not block the sedation produced by a high dose of ethanol. Since RO 15-4513 revealed proconvulsant properties, it is proposed that the depressive effects of this drug could be related to its proconvulsive activity.     

Ro 15-4513 selectively attenuates ethanol,but not sucrose,reinforced responding in a concurrent access procedure: comparison to other drugs

The experiments described in this report used a concurrent access procedure to study ethanol reinforcement. Rats were trained to lever press for a 10% sucrose solution and a 10% ethanol/10% sucrose mixture, and both reinforcers were available on variable-interval 5-s schedules. In baseline and vehicle injection sessions, the animals distributed their responding between both solutions. When injected with the partial inverse benzodiazepine agonist Ro 15-4513 (3, 9, and 18 mg/kg), responding for the ethanol solution decreased while responding for sucrose remained intact. Ethanol injections (0.5 and 1.0 g/kg) engendered a similar profile. Chlordiazepoxide led to an increase in ethanol mix responding at 2 mg/kg and a decrease in ethanol mix responding at higher doses; no dose affected sucrose responding. Morphine (0.5–16 mg/kg) decreased responding for both the ethanol mix and sucrose solutions, more or less simultaneously. Naloxone (0.125–20 mg/kg) selectively reduced ethanol mix responding at low doses, and decreased responding for both reinforcers at high doses. In another group of animals, isocaloric alternatives were concurrently available: 10% ethanol/0.25% saccharin versus 14% sucrose. Injections of Ro 15-4513 and chlordiazepoxide produced similar results as in the first group of rats: an increase in ethanol mix responding with low dose chlordiazepoxide, and a decrease in ethanol mix responding with Ro 15-4513. However, naloxone injections did not selectively affect responding for either of the reinforcers when they were isocaloric. These results are discussed in terms of ethanol's neuropharmacological actions.     

Opioid drugs and timeout from avoidance

The effects of μ agonists fentanyl, methadone and morphine and kappa agonist U50,488 on behavior maintained by negative reinforcement were determined. Rats were trained on concurrent schedules in which pressing one lever postponed shock on a Sidman avoidance schedule and pressing the other lever produced signaled periods of timeout from avoidance on variable-ratio schedules. All of the μ agonists decreased responding maintained by timeout from avoidance at doses that increased or did not affect avoidance rates. The kappa agonist U50,488 decreased response rates in some rats, but increased responding in others. In no case was a selective reduction in responding on the timeout lever produced by U50,488. Thus, the previously reported selective decreases in timeout responding by morphine are also produced by μ agonists fentanyl and methadone, but not by kappa agonist U50, 488.