Effects of various catecholamine receptor antagonists, muscle relaxation and physical hindrance on shuttlebox self-stimulation

Tobacco addiction and the dysregulation of brain stress systems

Tobacco is a highly addictive drug and is one of the most widely abused drugs in the world. The first part of this review explores the role of stressors and stress-associated psychiatric disorders in the initiation of smoking, the maintenance of smoking, and relapse after a period of abstinence. The reviewed studies indicate that stressors facilitate the initiation of smoking, decrease the motivation to quit, and increase the risk for relapse. Furthermore, people with depression or an anxiety disorder are more likely to smoke than people without these disorders. The second part of this review describes animal studies that investigated the role of brain stress systems in nicotine addiction. These studies indicate that corticotropin-releasing factor, Neuropeptide Y, the hypocretins, and norepinephrine play a pivotal role in nicotine addiction. In conclusion, the reviewed studies indicate that smoking briefly decreases subjective stress levels but also leads to a further dysregulation of brain stress systems. Drugs that decrease the activity of brain stress systems may diminish nicotine withdrawal and improve smoking cessation rates.

Effects of prazosin, clonidine, and propranolol on the elevations in brain reward thresholds and somatic signs associated with nicotine withdrawal in rats

RATIONALE

Tobacco withdrawal is characterized by a negative mood state and relatively mild somatic symptoms. Increased noradrenergic transmission has been reported to play an important role in opioid withdrawal, but little is known about the role of noradrenergic transmission in nicotine withdrawal.

OBJECTIVES

The aim of these experiments was to investigate the effects of prazosin, clonidine, and propranolol on the negative mood state and somatic signs associated with nicotine withdrawal in rats.

METHODS

A discrete-trial intracranial self-stimulation procedure was used to assess the negative affective state of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function.

RESULTS

In all the experiments, the nicotinic acetylcholine receptor antagonist mecamylamine (3 mg/kg) elevated the brain reward thresholds of the nicotine-treated rats and did not affect those of the control rats. The α1-adrenergic receptor antagonist prazosin (0.0625 and 0.125 mg/kg) dose-dependently attenuated the elevations in brain reward thresholds associated with precipitated nicotine withdrawal. The α2-adrenergic receptor agonist clonidine (10-40 μg/kg) and the nonselective β-adrenergic receptor antagonist propranolol (2.5-10 mg/kg) did not attenuate the elevations in brain reward thresholds associated with nicotine withdrawal. Furthermore, mecamylamine (2 mg/kg) induced more somatic signs in the nicotine-treated rats than in the control rats. Clonidine and propranolol, but not prazosin, decreased the total number of somatic signs associated with nicotine withdrawal.

CONCLUSION

Blockade of α1-adrenergic receptors attenuates the deficit in brain reward function associated with nicotine withdrawal. Antagonism of β-adrenergic receptors or stimulation of α2-adrenergic receptors attenuates the somatic symptoms of nicotine withdrawal.

Lobeline attenuates progressive ratio breakpoint scores for intracranial self-stimulation in rats

The alkaloid lobeline inhibits the function of vesicular monoamine and dopamine transporters and diminishes the behavioral and neurochemical effects of nicotine and amphetamines. In the present study, we examined the interaction of systemic administration of lobeline on breakpoint scores on a progressive ratio (PR) schedule of intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB). Rats were run in two 30 min sessions, separated by a 10 min timeout period. At the end of the first session, each rat was injected with either 0, 0.5, 1.0 or 2.0 mg/kg (i.p.) lobeline. Positive controls known to suppress and to augment ICSS responding included the adrenergic antagonist prazosin (0, 0.5 and 2.0 mg/kg, i.p.) and the psychostimulant cocaine (0, 1.25, and 5.0 mg/kg, i.p.). Analyses of changes in average PR breakpoint scores between the 2 sessions revealed that lobeline significantly suppressed PR scores at doses of 0.5, 1.0 and 2.0 mg/kg, as did 0.5 mg/kg and 2.0 mg/kg prazosin. These changes are unlikely to reflect motoric effects of these drugs inasmuch as neither lobeline nor prazosin alter locomotion at these doses. In contrast, PR breakpoint scores were significantly increased at 5.0 mg/kg cocaine, a dose that is sufficient to elevate locomotion in the rat. These results are consistent with the view that lobeline modulates brain reinforcement processes.

Role of alpha(1)-adrenoceptors of the locus coeruleus in self-stimulation of the medial forebrain bundle

The present experiments were undertaken to clarify the role of central alpha(1)-adrenoceptors in reward processes. Rats, trained to self-stimulate via electrodes in the medial forebrain bundle of the lateral hypothalamus, were administered alpha(1)-selective drugs near the locus coeruleus (LC), a site of a dense concentration of alpha(1)-receptors. Effects on reward potency were assessed from shifts in rate-frequency curves while effects on motor response capacity were judged from changes in the maximal rates of responding. It was found that local blockade of LC alpha(1)-receptors with terazosin produced a significant dose-dependent and site-dependent rightward shift of 0.08 log units and a significant decrease of 16.3% in the maximum response rate. Both effects were completely reversed by coadministration of the alpha(1)-agonist, phenylephrine and were not attributable to terazosin's weak action at alpha(2)-adrenoceptors. It is concluded that LC alpha(1)-adrenoceptors are involved both in reward/motivational processes and operant response elaboration which are postulated to work together to facilitate goal attainment.

Central alpha1-adrenergic system in behavioral activity and depression

Central alpha(1)-adrenoceptors are activated by norepinephrine (NE), epinephrine (EPI) and possibly dopamine (DA), and function in two fundamental and opposed types of behavior: (1) positively motivated exploratory and approach activities, and (2) stress reactions and behavioral inhibition. Brain microinjection studies have revealed that the positive-linked receptors are located in eight to nine brain regions spanning the neuraxis including the secondary motor cortex, piriform cortex, nucleus accumbens, preoptic area, lateral hypothalamic area, vermis cerebellum, locus coeruleus, dorsal raphe and possibly the C1 nucleus of the ventrolateral medulla, whereas the stress-linked receptors are present in at least three areas including the paraventricular nucleus of the hypothalamus, central nucleus of the amygdala and bed nucleus of the stria terminalis. Recent studies utilizing c-fos expression and mitogen-activated protein kinase activation have shown that various diverse models of depression in mice produce decreases in positive region-neural activity elicited by motivating stimuli along with increases in neural activity of stress areas. Both types of change are attenuated by various antidepressant agents. This has suggested that the balance of the two networks determines whether an animal displays depressive behavior. A central unresolved question concerns how the alpha(1)-receptors in the positive-activity and stress systems are differentially activated during the appropriate behavioral conditions and to what extent this is related to differences in endogenous ligands or receptor subtype distributions.

Role of locus coeruleus ?1-adrenoceptors in motor activity in rats

The question of whether or not the locus coeruleus (LC) participates in the control of motor activity has been controversial due to difficulties in demonstrating permanent motor deficits after neurotoxic lesions of this nucleus or of the dorsal noradrenergic bundle (DNB). In the present experiments it was shown in rats that acute local blockade (with terazosin) or stimulation (with phenylephrine) of LC alpha(1)-adrenoceptors respectively blocked or stimulated exploratory behavior in a novel cage and the home cage. Moreover, previous lesion of the DNB by i.p. DSP4 abolished the behavioral changes to local LC alpha(1)-receptor manipulation but did not affect motor activity in the novel or home cage by itself. These findings are consistent with the hypothesis that the intact LC does contribute to motor activity control, exerted in part by its alpha(1)-receptors; however, the permanent loss of this nucleus is compensated for by remaining CNS motor structures.

Electrical self-stimulation in the central amygdaloid nucleus after ibotenic acid lesion of the lateral hypothalamus

This experiment was carried out in order to investigate the involvement of lateral hypothalamus (LH) in electrical self-stimulation of the central amygdaloid nucleus (CeA). Adult male Sprague-Dawley rats were bilaterally implanted with a guide cannula situated above each LH and with two electrodes in the CeA. Self-stimulation was subsequently obtained separately from both right and left electrodes. The LH was then lesioned unilaterally by ibotenic acid (IBO) injection. Eight days later, the effect of this unilateral lesion on self-stimulation of the ipsilateral and contralateral CeA was tested. Then the neurons of the remaining non-lesioned LH side were lesioned with IBO and self-stimulation was tested 15 days after the second lesion. Both unilateral as well as bilateral lesions of LH produced a significant decrease in CeA self-stimulation rates but had no significant effect on the reward effectiveness. The unilateral lesions did not produce any modification of the rate-intensity function in the contralateral CeA. This lesion-induced depression in performance was reversed by treatment with phenobarbital. These results provide clear evidence that the rewarding effects of CeA electrical stimulation do not result from the activation of the LH outputs and that the apparent decrease in CeA self-stimulation may result from the LH lesion-induced increase in the frequency of epileptiform manifestations that occur following amygdaloid stimulation.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

Reward summation and the effects of pimozide, clonidine, and amphetamine on fixed-interval responding for brain stimulation

Two models of reward summation were examined in 16 rats lever pressing for intracranial stimulation under fixed-interval (FI) reinforcement. The first model examined rate-frequency functions and the second model traded off frequency and train duration. The second model was selected to assess the effects of three drugs on reward summation. Both clonidine and pimozide inhibited FI self-stimulation, but pimozide's effect could not be distinguished from a performance deficit. Two amphetamine isomers facilitated self-stimulation in a manner suggesting enhanced reinforcement. The dextro isomer was four times more effective than the levo isomer to facilitate self-stimulation. This study shows that the combination of the FI schedule with a reward-summation model is well suited for evaluating the effects of drugs on self-stimulation. The advantages of this model are that inter-reinforcement intervals are separated, which minimizes priming and stimulation aftereffects, and more responding does not increase stimulation availability, thus eliminating rate-dependency effects.

Regional neuroleptic microinjections indicate a role for nucleus accumbens in lateral hypothalamic self-stimulation reward

Bilateral microinjections of the neuroleptic, cis-flupenthixol, were made into 56 forebrain targets distributed across various dopamine (DA) terminal fields in the forebrain. Drug effects on medial forebrain bundle (MFB) stimulation-produced reward were assessed with a rate-frequency procedure implemented in a runway paradigm in a discrete-trial fashion. This method generated independent measures of drug-induced changes in the MFB stimulation reward and operant motor/performance capacity. Control experiments were run with the inactive isomer, trans-flupenthixol. Results indicate a major role for accumbens DA in MFB reward, but not for the DA in caudate and medial frontal cortex. Few drug-induced motor/performance deficits were found at any site. In 14 selected subjects, 6-OHDA-induced chronic DA lesions were made at the same site as neuroleptic microinjection. These results confirmed the reward effects of acute DA receptor blockade, but produced a greater associated motor/performance impairment. Both behavioral effects of the lesion recovered within 2 weeks in many, but not all subjects.

Antagonism of selected ethanol-enhanced brain stimulation properties by Ro 15-4513

Low doses of ethanol increase responding for brain stimulation. Recently, other intoxicating effects of ethanol have been reversed by the imidazobenzodiazepine, Ro 15-4513. Possibly, Ro 15-4513 blockade also acts on reward-enhancing properties of ethanol. Rats trained to alternately shuttle between nose poke and lever operanda for rewarding stimulation to the medial forebrain bundle, were tested following intragastric intubations of ethanol (18%, 1.35 g/kg), Ro 15-4513 (3 mg/kg in 18% ethanol), or vehicle. Ro 15-4513 reversed ethanol-enhanced effects on reinforced responses. Because Ro 15-4513 did not completely block instrumental responding for brain stimulation, we conclude its effects on ethanol were not acting on the same reward substrate as the current and consummatory response.

Comparison of deficits in electrical self-stimulation after ibotenic acid lesion of the lateral hypothalamus and the medial prefrontal cortex

The aim of the present study was to compare the self-stimulation deficit produced by a unilateral injection of the neurotoxin, ibotenic acid, in the lateral hypothalamus (LH) to the deficit produced by the same unilateral injection in the medial prefrontal cortex (MPC). Four groups of adult male Sprague-Dawley rats were used: in two control groups, electrodes were bilaterally implanted in the LH (5 rats) or in the MPC (6 rats) and self-stimulation (ICSS) was obtained separately with the right and left electrodes. In the two experimental groups the intrinsic neurons of the LH (8 rats) or of the MPC (10 rats) were destroyed unilaterally by local injection of ibotenic acid (4 micrograms in 0.5 microliter); the other side served as the sham-lesioned control. Ten days later ICSS electrodes were implanted bilaterally, one in the lesioned area, the other in the contralateral region. As in the case of the control rats, ICSS was determined separately for each electrode, first by a rate dependent test (nose-poke) then by a 'rate-free' test (shuttle-box). In the LH and MPC control rats, ICSS responses were the same with stimulation on either side. In the LH-lesioned rats, the ICSS rates measured with the nose-poke test were significantly decreased with stimulation on the lesioned side, whereas rates with stimulation of the non-lesioned LH were normal. Likewise, while shuttle responses with stimulation of the non-lesioned LH were normal, the OFF-time was increased and the ON-time was decreased with stimulation of the lesioned LH. In the MPC-lesioned rats, ICSS (nose-poke) was totally suppressed and the shuttle responses were disorganized since neither the ON- nor the OFF-times changed in response to increasing current intensities. Nose-poke responses with stimulation of the non-lesioned MPC were just about normal. These results show that in the two brain regions studied local neurons are involved in ICSS. The difference in the magnitude of the deficit observed suggests, that the neuronal circuits involved in MPC self-stimulation are poorly represented whereas in the LH many neuronal circuits involved in these mechanisms overlap.

Anxiety, anxiolytics and brain stimulation reinforcement

The premise of this review is that neuronal substrates of anxiety are amenable to investigation using brain stimulation techniques. Anxiolytics such as meprobamate and the benzodiazepines may enhance intracranial self-stimulation (ICSS) behavior. Although demonstrated by numerous investigators, this effect shows considerable variability between and within laboratories. Some of this variability is explained by sedative/muscle relaxant effects, which are dissociable from drug-induced increases in ICSS and which may mask these increases. The anticonvulsant actions of anxiolytic drugs are unlikely to account for the increases in ICSS. Rather, anxiolytics appear to increase ICSS by attenuating concurrent aversive properties of stimulation. Consistent with this explanation, anxiolytic drugs attenuate escape from aversive dorsal tegmental stimulation. The neuronal substrates of this centrally mediated escape behavior differ from those mediating footshock-induced escape. Barbiturates also enhance ICSS, possibly due in part to an excitatory component that is not involved in benzodiazepine action. Inverse benzodiazepine agonists attenuate ICSS behavior in a manner that cannot be explained by nonspecific performance impairment. These substances, however, may not necessarily enhance stimulation-induced aversiveness. A strategy is proposed to integrate brain stimulation studies with molecular approaches to anxiety. Specifically, stimulation of sites associated with fear induction or fear reduction may selectively alter the release of endogeneous anxiogens or anxiolytic substances.

Self-regulation of ICSS duration: effects of anxiogenic substances, benzodiazepine antagonists and antidepressants

A variety of anxiogenic substances, benzodiazepine antagonists and antidepressants were tested in a shuttlebox task in which rats interrupted infrared beams to initiate (ON latency) and terminate (OFF latency) continuous rewarding brain stimulation. It was hypothesized that substances exhibiting anxiogenic activity in animals (pentylenetetrazol and beta-CCM) would selectively reduce the OFF latency, since anxiolytic drugs increase this latency. beta-CCM, however, did not alter the OFF latency, but instead lengthened the ON latency. Pentylenetetrazol showed a similar, though not significant, trend. Ro 15-1788 did not alter ON latencies, but selectively lengthened the OFF latency at a high dose, consistent with previously reported anxiolytic activity at such doses. In contrast, CGS 8216 lengthened the ON latency selectively. Thus, Ro 15-1788 was differentiated from other drugs that antagonize benzodiazepines. Caffeine and dopamine uptake-blocking antidepressants (amineptine and nomifensine) preferentially decreased ON latencies, while non-dopamine-blocking antidepressants (viloxazine and CGS 7525A) lengthened both latencies nonspecifically. In conclusion, the OFF latency (but not the ON latency) appears refractory to reduction by various classes of psychotropic agents.

Comparative effects of beta 2-adrenoceptor agonists on intracranial self-stimulation, Sidman avoidance, and motor activity in rats

The effects of beta-adrenoceptor agonists were compared in various operant behavioral tasks, particularly intracranial self-stimulation (ICSS). Clenbuterol, salbutamol, and terbutaline all reduced responding by rats that lever-pressed for low stimulation intensities. The effects of clenbuterol in this test were completely reversed by propranolol, and those of salbutamol were partly reversed. Intermediate doses of clenbuterol and salbutamol slowed the initiation of rewarding brain stimulation in a shuttlebox but had little or no effect on the termination latencies. However, higher doses of both drugs lengthened the termination latencies. Motor activity was reduced at doses that attenuated ICSS responding. Complete tolerance occurred within 4 days to the effects of clenbuterol and salbutamol on lever-pressing ICSS and to the effects of clenbuterol on motor activity. The apparent performance deficits induced by these drugs were overcome by more intense motivation. For example, even at high doses, clenbuterol reduced ICSS lever-pressing only partially when animals bar-pressed for high rather than low stimulation intensities. Furthermore, all three drugs failed to alter Sidman avoidance responding at doses up to 100 times those that attenuated ICSS responding. It is concluded that although beta-adrenoceptor agonists cause apparent sedation in rats, this sedation is limited and shows rapid tolerance.

Avoidance and ICSS behavioral models dissociate TL-99 and 3-PPP from dopamine receptor antagonists

The behavioral effects of the putative dopamine autoreceptor agonists, TL-99 and 3-PPP, were explored in animal procedures that reveal highly characteristic effects of neuroleptics currently in clinical use. Sidman avoidance responding in rats was not altered appreciably by doses up to 10 mg/kg TL-99 or 30 mg/kg 3-PPP. Higher doses of TL-99 attenuated Sidman avoidance performance in squirrel monkeys, although 3-PPP had no effect. Lever pressing for intracranial self-stimulation (ICSS) was attenuated in a dose-related fashion by TL-99 and 3-PPP, with relatively shallow dose-response relationships. A low dose of haloperidol (0.03 mg/kg) partly reversed the effects of 3-PPP (3 mg/kg) on lever pressing ICSS, but not those of TL-99 (3 mg/kg). Yohimbine (3 mg/kg) failed to alter the effects of TL-99 at a dose that abolished the suppressant effect of clonidine on ICSS. Analysis of within-session ICSS response decrement patterns indicated that TL-99 reduced ICSS to a greater extent towards the end of the session than during the first 5 min. No such within-session trend was produced by 3-PPP, suggesting that 3-PPP attenuates ICSS by virtue of a performance deficit. Similar conclusions were reached using a shuttlebox task that involved self-regulation of ICSS duration by rats. Therefore, the clinical profile of neuroleptics is unlikely to be mimicked precisely by 3-PPP or TL-99. Clinical trials of DA autoreceptor agonists for antipsychotic efficacy will indicate whether or not avoidance and ICSS behaviors are relevant to the detection of the intrinsic antipsychotic activity of drugs.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Dopaminergic and noradrenergic inhibition of hypothalamic self-stimulation: differentiation of reward and performance effects

Dopaminergic and noradrenergic inhibition of lateral hypothalamic self-stimulation was investigated in a new signalled, discrete-trials shuttle-box paradigm. The differential inhibitory effects of drugs and stimulation frequency reductions within small blocks of trials differentiate reward inhibition from a variety of performance deficits. They further differentiate among the deficits produced by fatigue, sedation, dyskinesias, akinesia and sensory disruption. Pimozide's selective inhibition of the first response within each block of trials shows that its inhibition of self-stimulation is not due to either an inhibition of reward or to a general performance deficit. Instead, it suggests that pimozide specifically inhibits the initiation of motor responding. Pimozide-induced akinesia appears to be partly reversible by hypothalamic stimulation. Thus the pimozide data do not support a role for dopamine in mediating brain-stimulation reward. Since the inhibitory effects of clonidine were very similar to those of pimozide, it is suggested that clonidine also produces a stimulation-reversible akinesia. Thus the clonidine data do not support a role for noradrenaline in mediating brain-stimulation reward. LU 5-003, which selectively inhibits the presynaptic reuptake of noradrenaline, inhibited self-stimulation in almost exactly the same way as did reducing reward by reducing stimulation frequency. These data do support a primary role for noradrenaline in mediating brain-stimulation reward. However, it is suggested that LU 5-003 inhibits self-stimulation, not by inhibiting reward, but by enhancing reward and making the electrical stimulation superfluous.

Anxiolytic drugs selectively increase preferred duration of rewarding brain stimulation in a shuttlebox test

In the shuttlebox self-stimulation test described by Atrens, rewarding brain stimulation is independently initiated and terminated by rats. I has been hypothesized that gradually accumulating aversiveness of stimulation motivates the rat to terminate the rewarding stimulation train. In agreement with this view, optimal doses of the known anxiolytics, pentobarbital (5 and 10 mg/kg) diazepam (1 and 2.5 mg/kg), chlordiazepoxide (3 and 5.4 mg/kg) and CL 218,872 (3, 10 and 30 mg/kg) preferentially increased the latency to terminate stimulation (the OFF latency), as compared with the latency to initiate stimulation (the ON latency). Higher doses increased both latencies in a nonselective fashion, suggesting nonselective performance impairment. The shuttlebox self-stimulation test constitutes a potentially useful measure of experimental approach-avoidance conflict, with the OFF latency indicating anticonflict activity and the ON latency providing a control for performance variables.

Differential effects of d-amphetamine, pipradrol and bupropion on shuttlebox self-stimulation

The shuttlebox self-stimulation test is claimed by Atrens to differentiate drug effects on brain stimulation reward from those on performance variables. Thus, for example, drug-induced enhancement of the reward value of stimulation should be reflected in a selective reduction of the latency to initiate stimulation (the ON latency), as compared with the latency to terminate stimulation (the OFF latency). The effects of the psychostimulant drugs, d-amphetamine and pipradrol, and the antidepressant, bupropion, were evaluated in this procedure as well as in a bar-pressing test of self-stimulation. Pipradrol (3 and 10 mg/kg) and bupropion (54 mg/kg) reduced ON latencies by 40% or more but failed to shorten OFF latencies, indicating that performance variables were not involved in the ON latency decrements. Although d-amphetamine (0.3 and 1.0 mg/kg) shortened ON latencies, the 1.0 mg/kg dose also reduced OFF latencies. Drug doses that reduced ON latencies also increased bar-pressing self-stimulation. The shuttlebox self-stimulation test appears to be capable of discriminating drug-induced enhancement in brain stimulation reward from performance variables.