Lever-pressing Rates Research Articles

Dissociation of hypertension and fixed interval responding in two separate strains of genetically hypertensive rat

The behaviour of spontaneously hypertensive rats (SHR) may model attention-deficit hyperactivity disorder. For example, SHR are hyperactive in an open field and show high terminal rates of responding on certain fixed-interval schedules. Open field behaviour has been dissociated from the accompanying spontaneous hypertension but fixed interval responding has not. We compared the fixed interval responding of two unrelated strains of genetically hypertensive rat, the SHR ( n=6) and the New Zealand genetically hypertensive rat (GH, n=5), with their normotensive control strains, the Wistar Kyoto (WKY, n=6) and Wistar ( n=5), respectively. Both hypertensive rat strains showed increased terminal lever-pressing rates on a multiple fixed-interval schedule (FI-EXT) compared to controls. In order to investigate the association of hypertension and the behavioural characteristics in question, an F-2 hybrid strain was obtained by cross-breeding GH and Wistar rats. When these F-2 hybrids ( n=33) were tested on the FI-EXT schedule, terminal lever-pressing rate was not correlated with blood pressure. The independent segregation of these phenotypical characteristics in the hybrids suggests independent genetic control. By contrast, other behavioural characteristics, including high lever-pressing rates during the extinction component and a tendency to emit responses in bursts, did cosegregate with terminal lever-pressing rates. Taken together, these findings suggest that the genetic loci for high blood pressure and responding on the FI-EXT schedule in these two unrelated rat strains are close but distinct.

Responding for sucrose and wheel-running reinforcement: effect of body weight manipulation.

As body weight increases, the excitatory strength of a stimulus signaling an opportunity to run should weaken to a greater degree than that of a stimulus signaling an opportunity to eat. To test this hypothesis, six male albino Wistar rats were placed in running wheels and exposed to a fixed interval 30-s schedule that produced either a drop of 15% sucrose solution or the opportunity to run for 15s as reinforcing consequences for lever pressing. Each reinforcer type was signaled by a different stimulus. The effect of varying body weight on responding maintained by these two reinforcers was investigated by systematically increasing and decreasing post-session food amounts. The initial body weight was 335 g. Body weights were increased to approximately 445 g and subsequently returned to 335 g. As body weight increased, overall and local lever-pressing rates decreased while post-reinforcement pauses lengthened. Analysis of post-reinforcement pauses and local lever-pressing rates in terms of transitions between successive reinforcers revealed that local response rates in the presence of stimuli signaling upcoming wheel and sucrose reinforcers were similarly affected. However, pausing in the presence of the stimulus signaling a wheel-running reinforcer lengthened to a greater extent than did pausing in the presence of the stimulus signaling sucrose. This result suggests that as body weight approaches ad-lib levels, the likelihood of initiation of responding to obtain an opportunity to run approaches zero and the animal "rejects" the opportunity to run in a manner similar to the rejection of less preferred food items in studies of food selectivity.

Lack of self-administration and behavioural sensitisation to morphine, but not cocaine, in mice lacking NK1 receptors

Mice lacking the NK1 receptor, the preferred receptor for substance P, demonstrate normal analgesic responses to morphine on the hot plate assay, but have been predicted, on the basis of conditioned place preference studies, to be insensitive to the rewarding properties of opiates. In this study, self-administration and the development and maintenance of locomotor sensitisation of both morphine and cocaine were investigated in mice that lacked the NK1 gene (NK1 knockout mice, NK1 −/−). Both wildtype and NK1 −/− mice learned an operant lever-press response to obtain food. When intravenous infusions of morphine (0.2 mg/kg/infusion) were substituted for the food reward, the wildtype mice initially reduced rates of lever pressing, but then increased them on the rewarded lever to obtain approx. 10 infusions per 90 min session; in contrast, NK1 −/− mice continued to operate both the rewarded, and non-rewarded levers at low rates. Additionally, NK1 −/− mice failed, following repeated administration, to sensitise to the locomotor stimulant effects of morphine (15 mg/kg, i.p.). These deficits were specific to opiates, since NK1 −/− mice responding for food or cocaine self-administration (0.65 mg/kg/infusion) did not differ from wildtypes, and they showed normal behavioural sensitisation to repeated cocaine administration (10 mg/kg, i.p.). These results demonstrate that NK1 receptors are critical for the reinforcing properties of morphine, and for adaptive responses elicited by repeated opiate administration. It is postulated that substance P and the NK1 receptor may be necessary for the development of opiate, but not cocaine addiction.

TIME OF SUPPLEMENTAL FEEDING ALTERS THE EFFECTS OF COCAINE ON LEVER PRESSING OF RATS

The present experiment assessed the effects of cocaine on the lever pressing of 4 rats maintained during 15-min sessions by a fixed-ratio 50 schedule of food reinforcement. Across phases, supplemental food was provided either immediately or 2 hr after sessions. Two rats began the experiment in the delayed-feeding condition, and 2 began the experiment in the immediate-feeding condition. Rates of lever pressing of 2 rats sometimes decreased to low levels near the ends of sessions when supplemental feeding was provided immediately, but were consistently high throughout sessions when supplemental feeding was delayed. Cocaine (1.0 to 17.0 or 30.0 mg/kg) was administered intraperitoneally 15 min prior to test sessions. In most cases, cocaine suppressed response rates at lower doses under immediate-feeding conditions. Decreases in overall response rates were correlated with dosedependent increases in the time rats spent not responding. It is suggested that delaying the time of postsession feeding increased response strength, as indicated by greater resistance to the rate-suppressive effects of cocaine.

The role of nicotinic receptor beta-2 subunits in nicotine discrimination and conditioned taste aversion

The subtypes of nicotinic receptors at which the behavioural effects of nicotine originate are not fully understood. These experiments use mice lacking the β2 subunit of nicotinic receptors to investigate its role in nicotine discrimination and conditioned taste aversion (CTA). Wild-type and mutant mice were trained either in a two-lever nicotine discrimination procedure using a tandem schedule of food reinforcement, or in a counterbalanced two-flavour CTA procedure. Rates of lever-pressing of wild-type and mutant mice did not differ. Wild-type mice acquired discrimination of nicotine (0.4 or 0.8 mg/kg) rapidly and exhibited steep dose–response curves. Mutant mice failed to acquire these nicotine discriminations and exhibited flat dose–response curves. Both wild-type and mutant mice acquired discrimination of nicotine (1.6 mg/kg) although discrimination performance was weak in the mutants. Nicotine initially reduced response rates in wild-type and mutant mice, and tolerance developed to this effect in each genotype. Both genotypes acquired discrimination of morphine (3 mg/kg) with similar degrees of accuracy, and dose–response curves for morphine discrimination in the two genotypes were indistinguishable. Nicotine produced dose-related CTA in both genotypes, but the magnitude of the effect was less in the mutants than in the wild-type controls. It is concluded that nicotinic receptors containing the β2 subunit play a major role in the discriminative stimulus and taste aversion effects of nicotine that may reflect psychological aspects of tobacco dependence. Such receptors appear to have a less crucial role in the response-rate, reducing effects of nicotine and in nicotine tolerance.

Evidence for impairment of behavioural inhibition in performance of operant tasks in tPA−/− mice

We have previously shown that mice that lack the serine protease, tissue plasminogen activator (tPA), show over-responding on the active lever during time-out periods in an I.V. cocaine self-administration task. To investigate this effect further, tPA knockout mice (tPA−/−) were tested in a number of operant paradigms for a liquid food reinforcer. tPA−/− and wild-type (WT) control mice acquired a fixed ratio (FR) and a fixed interval (FI) task equally. However, extinction from the FR schedule resulted in a significant decrease in responses on the active and inactive levers in the WT mice whilst responding on the inactive lever remained high in the tPA−/− animals. In a differential reinforcement of low rate (DRL) task, tPA−/− mice acquired the task at a slower rate than WT animals. This was characterised by high levels of responding on the active lever during the first 15 sessions in the tPA−/− mice. Burst responding on the active lever (lever press rate with an inter-response time of less than 3 s) was especially high in these animals. This behaviour pattern resulted in the animals obtaining less reinforcers than the WT controls. Acute cocaine dose-dependently shifted the pattern of behaviour on the active lever towards shorter inter-response times. However, there was no difference between the tPA−/− and WT mice in their sensitivity to cocaine on this task. Repeated administration of a low dose of cocaine did not alter performance on this task in either set of animals. When the DRL task was modified to allow the tPA−/− and WT mice an equal number of reinforced trials per session there was no difference in the ability of the animals to perform the task. This would suggest that the tPA−/− mice have a tendency to over-respond but that this can be overcome when the task is modified to allow equal opportunity to learn.

Effects of SR141716A on Ethanol and Sucrose Self‐Administration

Previous studies have demonstrated that administration of central cannabinoid receptor (CB1) ligands can produce marked effects on ingestive behaviors. However, the possible relationship to ethanol self-administration has not been fully examined. The present series of experiments was designed to characterize further the role of CB1 receptors in appetitive and consummatory behaviors related to sucrose and ethanol. To determine the relative contribution of CB1 receptors to ethanol seeking and consumption, a series of experiments was designed using the sipper-tube model. In this paradigm, the appetitive and consummatory phases of ethanol and sucrose self-administration are separated. In the appetitive phase, animals are required to complete a response requirement (16 lever presses) within 20 min. If the requirement is successfully completed, access to a sipper tube containing either sucrose or ethanol (consummatory phase) is made available for 20 min. In the ethanol condition, the CB1 receptor antagonist SR141716A (0.3-3.0 mg/kg, ip) produced dose-related decreases in the probability of response requirement completion without significantly affecting latency to first lever press or overall lever press rate. In the sucrose condition, SR141716A (0.3-3.0 mg/kg, ip) increased first lever press latency without affecting lever press rate. In the consummatory phase, SR141716A (0.3-3.0 mg/kg, ip) administration markedly decreased total intake and the total number of licks for both ethanol and sucrose. These data indicate that CB1 receptors are involved in mediating both appetitive and consummatory aspects of ingestive behaviors related to sucrose and ethanol.

Effects of SR141716A on Ethanol and Sucrose Self-Administration

Background: Previous studies have demonstrated that administration of central cannabinoid receptor (CB1) ligands can produce marked effects on ingestive behaviors. However, the possible relationship to ethanol self-administration has not been fully examined. The present series of experiments was designed to characterize further the role of CB1 receptors in appetitive and consummatory behaviors related to sucrose and ethanol. Methods: To determine the relative contribution of CB1 receptors to ethanol seeking and consumption, a series of experiments was designed using the sipper-tube model. In this paradigm, the appetitive and consummatory phases of ethanol and sucrose self-administration are separated. In the appetitive phase, animals are required to complete a response requirement (16 lever presses) within 20 min. If the requirement is successfully completed, access to a sipper tube containing either sucrose or ethanol (consummatory phase) is made available for 20 min. Results: In the ethanol condition, the CB1 receptor antagonist SR141716A (0.3–3.0 mg/kg, ip) produced dose-related decreases in the probability of response requirement completion without significantly affecting latency to first lever press or overall lever press rate. In the sucrose condition, SR141716A (0.3–3.0 mg/kg, ip) increased first lever press latency without affecting lever press rate. In the consummatory phase, SR141716A (0.3–3.0 mg/kg, ip) administration markedly decreased total intake and the total number of licks for both ethanol and sucrose. Conclusions: These data indicate that CB1 receptors are involved in mediating both appetitive and consummatory aspects of ingestive behaviors related to sucrose and ethanol.

Different behavioral functions of dopamine in the nucleus accumbens and ventrolateral striatum: a microdialysis and behavioral investigation

Three experiments were conducted to investigate the behavioral functions of dopamine in the nucleus accumbens and ventrolateral striatum. In the first experiment, dialysis probes were implanted in the nucleus accumbens or ventrolateral striatum of rats previously trained to respond on fixed interval lever pressing schedules for food reinforcement. During the dialysis test session, both schedule- and site-dependent effects on dopamine release were observed. Overall, lever pressing on a fixed interval 30-s schedule produced a greater increase in extracellular dopamine than did responding on a fixed interval 120-s schedule. The fixed interval 30-s schedule was also accompanied by a higher rate of lever pressing. Rats with nucleus accumbens probe placements showed significantly higher increases in dopamine release than rats with ventrolateral striatal placements. An additional dialysis experiment showed that baseline levels of dopamine were suppressed by 1.0 μM tetrodotoxin to a similar extent in the nucleus accumbens and ventrolateral striatum. In the third experiment, 6-hydroxydopamine was injected locally into either the nucleus accumbens or the ventrolateral striatum in order to deplete dopamine. Nucleus accumbens dopamine depletions produced only a minor decrease in operant responding, whereas rats with ventrolateral striatal dopamine depletions showed low levels of responding that differed from both the control group and from the group that had accumbens dopamine depletions. Thus, these results are somewhat paradoxical, in that the structure that showed the greatest increase in dopamine release (i.e. the nucleus accumbens) was also the terminal region at which dopamine depletions had very little effect on operant responding. Ventrolateral striatal dopamine appears to be largely permissive over lever pressing, in that normal levels of dopamine in the ventrolateral striatum are critical for responding, although dopamine levels do not fluctuate much during behavioral sessions.

Effects of High Doses of Naltrexone on Running and Responding for The Opportunity to Run in Rats: A Test of the Opiate Hypothesis

Previous research shows that low to moderate doses of opiate antagonists do not affect running whereas high doses decrease running. This decrease in running has been interpreted as a motivational effect; however, it may also be an effect of motoric impairment, malaise, or sedation. The purpose of the present study was to evaluate the effects of high doses of naltrexone on running and responding for the opportunity to run in rats. Seven male Wistar rats, trained to press levers for the opportunity to run for 30 s, were exposed to a series of tandem fixed-ratio 1 variable-interval (VI) schedules of reinforcement where the value of the VI schedule was varied. Four rats were exposed to a VI 60-, VI 30-, and VI 5-s schedule order and the remaining three were exposed to VI 5-, VI 30-, and VI 60-s schedule order. After 50 sessions, doses of 10, 20, or 40 mg/kg of naltrexone were administered prior to a session. Results showed that naltrexone significantly decreased rates of lever pressing and running and increased postreinforcement latency to respond. Within-session analyses showed that naltrexone significantly decreased responding and running during the first schedule of a session regardless of schedule value. The lack of interaction of the drug effect with the schedule of reinforcement is inconsistent with a motivational effect. Consequently, it was concluded that decreases in running caused by high doses of naltrexone are not motivational and should not be taken as evidence in support of the opiate hypothesis. After engaging in rigorous bouts of physical activity an individual sometimes experiences subjective feelings of euphoria. This phenomenon is known as "runner's high." Although widely known, the physiology underlying this phenomenon is not well understood. The endogenous opiate hypothesis attributes this phenomenon to a release

RUNNING AND RESPONDING REINFORCED BY THE OPPORTUNITY TO RUN: EFFECT OF REINFORCER DURATION

The present study investigated the effect of reinforcer duration on running and on responding reinforced by the opportunity to run. Eleven male Wistar rats responded on levers for the opportunity to run in a running wheel. Opportunities to run were programmed to occur on a tandem fixed-ratio 1 variable-interval 30-s reinforcement schedule. Reinforcer duration varied across conditions from 30 to 120 s. As reinforcer duration increased, the rates of running and lever pressing declined, and latency to lever press increased. The increase in latency to respond was consistent with findings that unconditioned inhibitory aftereffects of reinforcement increase with reinforcer magnitude. The decrease in local lever-pressing rates, however, was inconsistent with the view that response strength increases with the duration of the reinforcer. Response rate varied inversely, not directly, with reinforcer duration. Furthermore, within-session data challenge satiation, fatigue, and response deprivation as determinants of the observed changes in running and responding. In sum, the results point to the need for further research with nonappetitive forms of reinforcement.

Selective antagonism of behavioural effects of nicotine by dihydro-beta-erythroidine in rats.

The influence of the nicotine antagonist dihydro-beta-erythroidine (DH beta E) was examined on various behavioural effects of nicotine in rats. Motor activity was recorded in photocell cages whereas discriminative stimulus effects were examined using two-lever drug discrimination procedures with a tandem schedule of food reinforcement (n = 8 throughout). DH beta E (0.1-3.2 mg/kg) failed to antagonise the decreases in motor activity that nicotine (0.4-0.6 mg/kg) produced in experimentally naive rats, whereas mecamylamine (1.5 mg/kg) completely blocked this effect of nicotine. DH beta E (0.1-3.2 mg/kg) antagonised the increases in motor activity that nicotine (0.4 mg/kg) produced in rats with extensive previous exposure to both nicotine and the photocell apparatus. In rats trained to discriminate either 0.1 or 0.4 mg/kg nicotine from saline, DH beta E (0.1-3.2 mg/kg) blocked the discriminative stimulus effect of nicotine. The block of the discriminative effect could be reversed by increasing the dose of nicotine; DH beta E (1.6 mg/kg) shifted the dose-response curve for nicotine discrimination to the right by a factor of 9.4. In addition, nicotine in doses of 0.32-0.64 mg/kg decreased the overall rate of lever pressing but DH beta E (1.6 mg/kg) did not influence the dose-response curve for this effect. Thus, DH beta E potently blocked the locomotor activating and discriminative stimulus effects of nicotine at doses that did not antagonise its locomotor depressant and operant response rate-reducing effects. This selective blockade supports the involvement of different subtypes of nicotinic receptor in the mediation of diverse behavioural effects. Furthermore, the rightward shift of the dose-response curve for nicotine discrimination suggested a competitive mode of action for DH beta E.

Within-Session Changes in Adjunctive and Instrumental Responding

Rats pressed levers for food delivered by several fixed interval schedules. A drinking spout or running wheel was also available during some conditions, but not during others. The rate of lever pressing, drinking and running often changed within experimental sessions. The within-session patterns of lever pressing did not differ when drinking or running was available and when it was not. The correlation between the amount of lever pressing and the amount of drinking or running at a particular time in the session was inconsistently positive or negative. Finding within-session changes in responding for adjunctive behaviors implies that the factors that produce these changes are present for both adjunctive and instrumental behavior. Finding inconsistent correlations between instrumental responding and adjunctive behaviors questions arousal and interference from adjunctive behaviors as explanations for within-session changes in instrumental responding.

The Effect of a Change in Body Weight on Running and Responding Reinforced by the Opportunity to Tun

Previous research has shown that running increases as body weight decreases. One interpretation of this relationship is that motivation to run increases as body weight decreases. In the present study, the relationships among body weight, running, and responding for the opportunity to run were investigated. Fourteen male Wistar rats were trained to respond on levers. The reinforcing consequence was the opportunity to run for 60 seconds. In each session, subjects were exposed to a series of variable-interval schedules. Over sessions, body weight was increased from 80% to 100% of an initial ad-lib level and then returned to 80%. Results showed that running and lever-pressing rates were lower at the higher body weight whereas mean latency to respond following reinforcement was higher at the higher body weight. Analysis of between subjects data also revealed orderly relationships. At each body weight, lever-pressing rates were positively related to running rates. In addition, changes in running caused by change in body weight were positively related to changes in lever-pressing rates and negatively related to changes in latency. Finally, changes in running were not related to initial levels of running. In sum, the results were consistent with the interpretation that motivation to run varies with body weight.

Behavioral tolerance and cross-tolerance to the response rate-decreasing effects of mu opioids in rats

The present study was designed to characterize the degree of cross-tolerance between the response rate-decreasing effects of morphine and three mu opioids with varying relative intrinsic efficacies at the mu receptor, buprenorphine, butorphanol and fentanyl, and a non-opioid (+) amphetamine, in a behavioral-tolerance paradigm. Lever pressing of rats was maintained by a fixed-ratio 20-schedule of food presentation, and dose-effect curves for each drug were obtained prior to, during, and after daily administrations of morphine in separate groups of rats administered morphine either before (pre-session) or after (post-session) experimental sessions. Each of the mu opioids and the non-opioid (+)-amphetamine dose-dependently decreased response rates. In the pre-session group, daily administration of morphine shifted the morphine dose-effect curve 0.33 log unit rightward, indicating that tolerance had developed, and shifted the butorphanol dose-effect curve 0.96 log unit rightward. Daily pre-session administrations of morphine did not shift the dose-effect curves for buprenorphine, fentanyl, or (+)-amphetamine. In the post-session group, daily administration of morphine did not shift the morphine, butorphanol, buprenorphine, fentanyl, or (+)-amphetamine dose-effect curves. These data suggest that pharmacological variables, such as the drug's relative intrinsic efficacy at the mu receptor, can play a role in behavioral tolerance and cross-tolerance.

Elevations of nucleus accumbens dopamine and DOPAC levels during intravenous heroin self‐administration

Extracellular dopamine and DOPAC (3,4-dihydroxyphenylacetic acid) levels in nucleus accumbens were sampled by microdialysis and quantified with high-performance liquid chromatography during intravenous heroin self-administration sessions in rats. Dopamine levels in 10 and 20 min samples were elevated following the first injection of each session, reaching a plateau of elevation within the first two or three injections and falling back toward baseline only when drug access was terminated. Elevations were in the range of 150-300% when unit dosages of 0.05-0.2 mg/kg were given. Increasing the work requirement from FR-1 to FR-10 did not appear to alter the degree of elevation of dopamine levels, and dopamine levels fell during extinction while lever-pressing rates increased 20-fold. While animals compensated for unit dose changes between 0.05 and 0.2 mg/kg/injection, adjusting their response rate such that the same hourly drug intake and the same asymptotic dopamine levels were maintained across these conditions, at 0.4 mg/kg/injection hourly drug intake and asymptotic dopamine levels were elevated beyond the levels sustained by the lower doses. These findings confirm that self-administered doses of intravenous heroin are sufficient to activate the mesolimbic dopamine system and suggest that significant heroin "craving" can emerge when dopamine levels are still moderately elevated, long before the development of dopamine depletion associated with opiate withdrawal.

The effects of repeated treatment with 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) on the lever press responding of the rat under FI and DRL schedules of food reinforcement.

In general, the effects of 8-OH-DPAT on the body temperature of rats or in inducing the 5-HT syndrome show rapid tolerance. However, in contrast, the 8-OH-DPAT-induced increase in the activity of rats in a two-way active avoidance task only occurs after repeated administration, i.e. there is sensitisation. The present study was conducted to examine whether this developing hyperactivity may also be expressed as increased rates of lever press responding, and if so, under which conditions it occurs. Rats were trained to press levers under fixed interval 60-s (FI 60) or differential reinforcement of low rates 20-s or 72-s (DRL20, DRL72) schedules of food reinforcement. Groups of trained rats were then treated daily 5 min before testing with doses of 0.01, 0.1 and 1.0 mg/kg 8-OH-DPAT SC for 10-21 days. In all three procedures, in the first couple of days of drug treatment, 8-OH-DPAT generally suppressed lever pressing in a dose-dependent manner. Thereafter, tolerance to this effect was seen to a greater (DRL20, DRL72) or lesser (FI60) extent. Some evidence for stimulation of low rates of lever press responding was seen after 10 days treatment under FI60, but not in DRL20 or DRL72 during short 30 to 60 min long daytime tests although in the latter case, the rats responded to the stimulating effects of 0.8 mg/kg SC amphetamine administered once at the end of the experiment. However, when rats were allowed to respond under DRL72 testing for 12 h during the night, after 10 days treatment a clear stimulation of lever pressing was observed. This stimulation was not specific to lever pressing, however, since a stimulation of entries into the food tray and licking were also seen. From these results, it may be concluded that the stimulating effect of 8-OH-DPAT after repeated administration may be expressed as increased rates of lever pressing, but not under all conditions in which psychomotor stimulation by amphetamine is seen. The potential for 8-OH-DPAT and related compounds to stimulate motor responding in this way should be taken into account when interpreting the effects of these drugs in animal models of psychiatric disorders.

Rate-dependency hypothesis and the rate-decreasing effects of d-amphetamine on schedule-induced drinking

The high levels of drinking induced by intermittent food-reinforcement schedules are dose-dependently reduced by acute doses of d-amphetamine. The present study evaluated whether the effects of d-amphetamine on this schedule-induced drinking reflect the reduction of high rates of responding. Twenty-four rats were divided into six groups (n = 4) according to the interval and time durations of a multiple fixed-time (FT) fixed-interval (FI) schedule (15s, 30s, 60s, 120s, 240s and 480s). FT components were signalled by a tone and by lever withdrawal. Doses of 0.25 to 4.0mg/kg of d-amphetamine were administered i.p. 10min before test sessions. d-amphetamine produced similar dose-dependent reductions in rate of licking induced by FT and FI schedules. Rate-decreasing effects on operant lever pressing were also found after administrations of d-amphetamine. The dose-dependent decrements produced by d-amphetamine were a function of the inter-food interval length in both schedule-induced and operant behaviours. These rate-decreasing effects were rate-dependent, but d-amphetamine interacted differentially with control rates of adjunctive and operant behaviours, causing a greater suppression of the lower rates of adjunctive licking and the higher rates of operant lever pressing.

Effects of cocaine on fixed-interval behavior and schedule-induced alcohol consumption in male and female rats

Three male and three female Wistar rats pressed a lever on a fixed-interval 60-s schedule of food reinforcement while they had simultaneous access to an alcohol solution. They were challenged with different doses of cocaine hydrochloride (vehicle, 1, 3, 10, and 30 mg/kg) once lever press rates and lick rates had stabilized. Low doses of cocaine (1 and 3 mg/kg) did not systematically affect lever press rates or lick rates. The administration of 10 and 30 mg/kg cocaine dose-dependently decreased lever press rates and schedule-induced licking to a greater extent in female than in male rats. Lick rates decreased even when cocaine administration did not affect the number of pellets obtained during an experimental session. Lever press rates accelerated throughout the interreinforcement interval during control sessions. Licking was mostly limited to the first 10 s (males) or 20 s (females) after pellet presentation. Cocaine administration did not affect the distribution of lever presses and licks during the interreinforcement interval. The results of the present experiment extend previous observations that cocaine's rate-dependent effects on lever press rates may be limited to situations in which changes in lever press frequency and/or distribution negatively affect reinforcement frequency and/or physiological consequences of schedule-induced behavior.

Behavioral sensitivity of rats to extremely‐low‐frequency magnetic fields

Work in our laboratory has revealed autonomic and/or behavioral sensitivity of mice, rats, and a domestic fowl to extremely-low-frequency (ELF) or nominally static magnetic (B) fields at flux densities between 250 and 1700 microT (rms). To extend our work, an automated exposure and data-acquisition system was used with the technique of conditional suppression to assess behavioral sensitivity to time-varying B fields. Each of five rats was exposed aperiodically to a B field during 3 min warning periods that terminated in a brief electric shock. The difference between rates of lever pressing during B-field warning periods and rates during immediately antecedent, 3 min control periods was analyzed at frequencies of 7, 16, 30, 60, and 65.1 Hz. To produce equivalent induced voltages in the rat at each frequency, graded flux densities were established that ranged from 1900 microT at 7 Hz to 200 microT at 65.1 Hz. Analysis of differences in lever-pressing rates revealed that in a given session of testing the rats would increasingly suppress responding when exposed to a B field, but this trend was independent of frequency. This experiment provides evidence of behavioral sensitivity by a mammal to an ELF magnetic field.