Effects of apomorphine on self-stimulation behavior

Functional implications of dopamine D1 vs. D2 receptors: A 'prepare and select' model of the striatal direct vs. indirect pathways

The functions of the D1- and D2-dopamine receptors in the basal ganglia have remained somewhat enigmatic, with a number of competing theories relating to the interactions of the 'direct' and 'indirect pathways'. Computational models have been good at simulating properties of the system, but are typically divorced from the underlying neural architecture. In this article we propose a new model which re-addresses response selection at the level of the basal ganglia. At the core of this response selection system the D1 DA receptor-expressing striatal pathways 'prepare' the set of possible appropriate responses. The D2DR-expressing striatal pathways then shape and 'select' from this initial response set framework. This article is part of a Special Issue entitled: Ventral Tegmentum & Dopamine.

Intracranial self-stimulation in FAST and SLOW mice: effects of alcohol and cocaine

RATIONALE

Sensitivity to the stimulant and rewarding effects of alcohol may be genetically correlated traits that predispose individuals to develop an alcohol use disorder.

OBJECTIVE

This study aimed to examine the effects of alcohol and cocaine on intracranial self-stimulation (ICSS) in FAST and SLOW mice, which were selectively bred for extremes in alcohol stimulation.

METHODS

Male FAST and SLOW mice were conditioned to respond for reinforcement by direct electrical stimulation of the medial forebrain bundle (i.e., brain stimulation reward). ICSS responses were determined immediately before and after oral gavage with water or alcohol (0.3-2.4 g/kg) or intraperitoneal injection with saline or cocaine (1.0-30.0 mg/kg). In separate FAST and SLOW mice, the locomotor effects of these treatments were measured in activity chambers.

RESULTS

Alcohol dose-dependently lowered the threshold for self-stimulation (θ (0)) and the frequency that maintained 50% of maximal responding (EF50) in FAST mice but did not significantly affect these parameters in SLOW mice. The largest effects of alcohol were after the 1.7- and 2.4-g/kg doses and were about 40% compared to water injection. Alcohol did not affect MAX response rates, but dose-dependently stimulated locomotor activity in FAST mice. Cocaine lowered thresholds equally in FAST and SLOW mice, although cocaine-stimulated locomotor activity was higher in the FAST than in the SLOW mice.

CONCLUSIONS

Selective breeding for alcohol locomotor stimulation also renders the mice more sensitive to the effects of alcohol, but not cocaine, on ICSS.

Low-dose apomorphine attenuates morphine-induced enhancement of brain stimulation reward

Thresholds for brain stimulation reward (BSR) delivered to the medial forebrain bundle-lateral hypothalamus were determined by means of a rate free psychophysical method. Lower doses of apomorphine (0.5 to 0.2 mg/kg) produced modest elevations in BSR thresholds. A 0.4 mg/kg dose of apomorphine resulted in emergence of stereotypic behaviors and the loss of stimulus control. Morphine's BSR threshold lowering effects were significantly blocked by the concurrent administration of a 0.1 mg/kg dose of apomorphine. These results support the hypothesis that presynaptic dopamine neurons are involved in the mediation of morphine's reinforcing effects and that dopamine autoreceptor agonists may be of some use in the pharmacotherapy of opiate abuse.

Measurement issues in curve-shift analysis of apomorphine effects on rewarding brain stimulation

The direct dopamine agonist apomorphine has been reported to reduce the rewarding efficacy of lateral hypothalamic (LH) self-stimulation. This effect has been claimed to support the notion that dopamine mediates the rewarding effects of LH self-stimulation. Using a standard rate-frequency curve-shift paradigm with ascending order of frequency presentation, we also found that apomorphine (0.1-0.8 mg/kg, SC) appeared to decrease LH self-stimulation reward. These apparent rightward curve shifts were exacerbated by shortening the test duration, which also produced a number of sessions in which the subjects did not respond at all. When the presentation order of stimulation frequencies was reversed, apomorphine did not produce large reward decreases. These results suggest that the previously reported effects of apomorphine on LH self-stimulation were the result of artifact, perhaps related to apomorphine-induced stereotypical behavior combined with rapid pharmacological recovery.

The preferential dopamine autoreceptor antagonist (+)-UH232 antagonizes the positive reinforcing effects of cocaine and d-amphetamine in the ICSS paradigm

The dopamine autoreceptor and D3 preferring antagonist [cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin] (+)-UH232, exerts weak stimulatory effects when tested in locomotor activity experiments using habituated animals. (+)-UH232 also blocks d-amphetamine-, cocaine-, and apomorphine-induced hyperactivity, but fails to induce catalepsy. Thus, the behavioral effects of (+)-UH232 appear to be dependent upon the baseline activity of the animal. The antagonistic properties of (+)-UH232 were studied in the intracranial self-stimulation (ICSS) technique in the rat. (+)-UH232 and haloperidol produced inhibitory effects over a wide dose range. Cocaine, GBR12909 and d-amphetamine clearly lowered ICSS thresholds, indicating stimulatory effects. (+)-UH232 antagonized the stimulatory effects of cocaine, GBR12909, and d-amphetamine, whereas haloperidol, at a dose producing an inhibition similar to (+)-UH232, was significantly weaker in antagonizing cocaine- or d-amphetamine-induced stimulation. This difference between (+)-UH232 and haloperidol with respect to stimulant-blocking ability, support the concept that the effects of (+)-UH232 are not representative of either classical DA agonists or DA antagonists.

Reward summation and the effects of dopamine D1 and D2 agonists and antagonists on fixed-interval responding for brain stimulation

The effects of dopamine D1 and D2 agonists and antagonists on fixed-interval (FI) self-stimulation were investigated using a reward-summation model, trading off frequency with train duration. The D1 antagonist SCH 23390 (0.005-0.02 mg/kg) decreased FI self-stimulation and the inhibition was reversed by increasing stimulation frequency. Moreover, amphetamine (0.5 mg/kg) reversed the inhibition by a low dose of SCH 23390 (0.005 mg/kg) but not after a higher dose inhibition could not be dissociated from a performance deficit. There was no significant interaction between low doses of spiperone and SCH 23390 when coadministered that could not be predicted from their effects when given individually. Self-stimulation was inhibited by the D1 agonist SKF 38393 (5 mg/kg). When coadministered with amphetamine, SKF 38393 partially blocked amphetamine's facilitation. The D2 agonist bromocriptine (10 mg/kg) produced an extraordinary enhancement of performance that was also evident after a lower dose (5 mg/kg) when it was combined with amphetamine. This enhancement of performance showed little extinction when stimulation was no longer available, suggesting it was a novel form of stereotypy. These results support the concept that D1 dopamine receptors play a critical role in modulating the reinforcing consequences of lateral hypothalamic stimulation. The involvement of D2 receptors on reinforcement processes remains contentious due to their effects on performance and insensitivity of responding to coincide with changes in reinforcement magnitude.

Evaluation of response perseveration of rats in the radial arm maze following reinforcing and nonreinforcing drugs

The behavioral effects of three drugs with high abuse potential (amphetamine, heroin, and nicotine) and two substances with low abuse potential (haloperidol and scopolamine) were evaluated in an eight-arm radial maze. Rats were trained to explore the maze for the food reward. Unlike most radial arm maze paradigms, a food pellet was made available every time the rat entered an arm; thus, no external restrictions were placed upon rats' exploratory pattern. Following 3 days of drug-free training, rats were injected prior to testing with one of the five drugs. Analysis of the sequences of arm entries demonstrated that the variability of the search strategy was significantly decreased by amphetamine, heroin, and nicotine. In contrast, scopolamine and haloperidol either decreased or had no effect on preservation. These data, along with previous data on ethanol and diazepam, lead to the speculation that drugs of abuse may share the common property of reducing behavioral variability.

Apomorphine and electrical self-stimulation of rat brain

The participation of dopamine neurons in reward produced by electrical stimulation of the brain was examined by measuring self-stimulation thresholds after injections of apomorphine, a direct agonist of dopamine receptors. Rats were trained to press a lever to obtain 0.3-s trains of electrical stimulation applied to lateral hypothalamic electrodes in a paradigm where the pulse frequency was decreased every eight stimulations by approximately 20%. The pulse frequency interpolated at 50% of maximum rate was taken as threshold. In a completely within-subject design, five doses of apomorphine from 0.01 to 1.00 mg/kg and the ascorbic acid vehicle were injected in a random order and thresholds were tracked at intervals of 5 min for 2 h postinjection. Low doses from 0.01 to 0.10 mg/kg caused thresholds to increase while the two higher doses, 0.30 and 1.00 mg/kg, caused thresholds to drop; the switch in the direction of the behavioural effect is thought to parallel the shift in apomorphine's action from presynaptic to predominantly postsynaptic activation of dopamine receptors as the concentration of apomorphine increases.

Schedule-controlled brain self-stimulation: has it utility for behavioral pharmacology?

We review evidence that schedule-controlled intracranial self-stimulation (ICSS) has properties in common with conventional reinforcements, such as food and water, but unlike the latter, animals will respond for ICSS for long periods of time at a near-constant rate. Schedule-controlled ICSS has proven to be more sensitive to drug-induced changes than has ICSS on a continuous reinforcement schedule, and it permits a more fine-grained analysis of the pattern of responding that results in the reinforcement. Evidence is accumulating that the schedule of ICSS itself leads to neurochemical changes in areas of the brain, such as the nucleus accumbens, in which reward processes occur. Results obtained from schedule-controlled ICSS would complement those obtained by drug self-administration studies which generally use intermittent reinforcement. A systematic examination of ICSS schedules at different brain sites would greatly facilitate our interpretation of drug effects and this would have utility for behavioral pharmacology.

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.

Major psychosis and dopamine: controversial features and some suggestions

Three problems with the dopamine hypothesis of major psychosis are pointed out: the long time-course of neuroleptic therapy; the absence of tolerance to the antipsychotic effects of neuroleptic drugs, or of a supersensitivity psychosis on drug withdrawal; and the absence of potent psychotogenic properties in the direct dopamine agonists. A resolution of these paradoxes is suggested relying on a role for dopamine in learning processes at a relatively high (cognitive) functional level. The hypothesis proposed is also used to explain the origin of some of the more distinctive psychotic symptoms.

Hypothalamic self-stimulation: the role of dopamine and possible relations to neocortical slow wave activity

Reserpine abolishes self-stimulation in rats but the behavior can be restored temporarily by treatment with D-amphetamine or L-DOPA. Apomorphine does not restore self-stimulation even though it restores spontaneous motor activity in reserpinized rats. The data indicate that dopamine plays a role in reinforcement as well as in motor function. The ability of D-amphetamine to restore self-stimulation in reserpinized rats is eliminated by concurrent treatment with atropine or scopolamine. This effect may be related to the presence of continuous large amplitude slow wave activity in the neocortex under these conditions.

Effects of apomorphine on self-stimulation responding: does the drug mimic the current?

The effects of apomorphine (APO) on self-stimulation responding were examined as a function of drug dose and stimulation current intensity. The lowest dose tested (0.02 mg/kg) significantly elevated the stimulation threshold, presumably reflecting the drug's preferential affinity for presynaptic dopamine (DA) receptors at this dose. At a dose which stimulates postsynaptic DA receptors (0.2 mg/kg), the rats responded with a pattern of behavior that resembled that obtained when non-drugged animals are given non-contingent current. However, during extinction, drugged animals were identical to non-drugged rats in their pattern of responding. These data indicate an interaction of APO with the stimulating current and suggest that APO may be mimicking only one component of the stimulation--perhaps the rewarding but not the motivating one.

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.

Rate dependent inhibition of self-stimulation by apomorphine

The effect of three doses of apomorphine 0.125, 0.25 and 0.5 were studied on self-stimulation generated by three levels of current intensity. Eight rats exhibited overall dose dependent decreases in self-stimulation obtained at the two lowest current intensities. Self-stimulation at the highest current intensity, however, was unaffected by even the highest dose level of apomorphine (0.5 mg/kg) despite typical signs of stereotypy exhibited by the rats in their home cages. Additionally, self-stimulation obtained under the 0.5 mg/kg dose of apomorphine under went extinction when reinforcement was discontinued. Thus, brain stimulation can be an effective reinforcement when an animal is given a stereotypy inducing dose of apomorphine if the current intensity is of sufficient magnitude and if the response manipulandum is not compatible with stereotypic responses. These observations appear consistent with a dopaminergic involvement in the response rather than reinforcement aspect of self-stimulation.

Effects of apomorphine on escape performance and activity in developing rat pups treated with 6-hydroxydopamine (6-OHDA)

The effects of apomorphine and excape learning were examined in normal developing rat pups and littermates preferentially depleted of brain dopamine by the intracisternal administration of 6-hydroxydopamine (6-OHDA) at 5 days of age, a treatment which resulted in a rapid and permanent reduction in brain dopamine to concentrations 12-29% of littermate controls while norepinephrine was not significantly altered. At 19 days of age both 0.1 and 1.0 mg/kg doses of apomorphine increased general motor activity in normal but not 6-OHDA treated pups (though these pups were significantly hyperactive prior to apomorphine). At 26 days only the 1.0 mg/kg dose increased motor activity in both normal and 6-OHDA pups. Exploratory activity at 30 days in both normal and 6-OHDA pups was first reduced then abolished by progressive doses of apomorphine. Stereotyped activity was increased by 0.1 and 1.0 mg/kg apomorphine at 19 days in both normal and 6-OHDA pups. By 26 days, apomorphine no longer produced intense sterotypies in normal pups, but did effect such responses in 6-OHDA treated animals. Administration of apomorphine resulted in a disruption of escape performance in a T-maze and shuttle box in normal pups only at 1.0 mg/kg but disrupted performance in 6-OHDA treated animals at both 0.1 and 1.0 mg/kg dosages. These results indicate a peak effect of apomorphine on general motor activity at three weeks of age in normal pups. Our results also suggest that apomorphine will disrupt escape learning, effects that appear to be correlated with the apomorphine induced increase in motor activity.

Intracranial self-stimulation in orbitofrontal cortex and caudate nucleus of rhesus monkey: effects of apomorphine, pimozide, and spiroperidol

Rhesus monkeys were prepared with stimulating electrodes implanted into the orbitofrontal cortex and head of the caudate nucleus under stereotaxic control. These regions of the brain contain high levels of dopamine, and intracranial self-stimulation was readily elicited from these loci in all animals tested using licking behavior as the operant response. Self-stimulation at both sites was significantly attenuated following peripheral injections of the dopamine receptor blocker spiroperidol (0.02 mg/kg). Similarly, pimozide (0.15 and 0.20 mg/kg) significantly reduced self-stimulation in the orbitofrontal cortex, but the suppression observed at caudate placements did not reach statistical significance. Licking for a reward of blackcurrant juice was unaffected by either drug. Apomorphine (0.2, 0.4 mg/kg) had a differential effect on self-stimulation. This drug significantly attenuated self-stimulation in the orbitofrontal cortex, while the same treatment tended to facilitate self-stimulation in the caudate. Apomorphine did not significantly affect responding for the fruit juice reward. The parallels between the effects of dopamine agonists and antagonists on self-stimulation in the monkey and rat suggest that dopamine influences self-stimulation of some sites in both the primate and the rat.

Inhibition-mediating dopamine receptors and the control of intracranial reward

A receptor selective agonist and antagonist of inhibition-mediating dopamine receptors (type II receptors) produced significant and dose-related alterations in bar-pressing for intracranial reward. Receptor inhibition by piribedil increased responding for reward while receptor activation by 3,4-dihydroxyphenylamino-2-imidazoline reduced responding. Inhibition-mediating receptors may therefore play a role opposite to classic excitation-mediating receptors in controlling reward.

Modulation of intracranial self-stimulation behaviour by local perfusions of dopamine, noradrenaline and serotonin within the caudate nucleus and nucleus accumbens

In order to examine the possible role of dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) in the control of intracranial self-stimulation behaviour (ICSS) a push-pull perfusion system was used to administer different consecutive doses of DA, NA and 5-HT to discrete regions within the caudate nucleus (CN) and nucleus accumbens (NAC) of rats during ICSS. Electrode placements supporting ICSS were in both the medial forebrain bundle (MFB) and the ventral mesencephalic tegmentum (VMT). Animals were allowed to determine the ICSS pulse train duration thereby permitting three measures of ICSS behaviour: (1) mean presses/min; (2) mean duration/press and (3) mean time pressed/min. Eleven electrode/cannula combinations were found to be responsive to both DA and 5-HT. The DA response profile was typified by a significant increase in mean presses/min, a significant decrease in mean duration/press and no significant change in the mean time pressed/min. The response profile for 5-HT was the converse of the DA pattern for the first two measures of ICSS, however, again there was no significant change in the mean time pressed/min. In addition, 8 electrode/cannula combinations were sensitive to NA; for 5 combinations the response pattern was similar to that of DA, however, the changes in ICSS were generally larger and of longer duration. For the remaining 3 combinations sensitive to NA there was no major change in mean presses/min but a dramatic increase in the mean duration/press and consequently, an increase in the mean time pressed/min. It is suggested that the central control of ICSS behaviour might depend, in part, upon a relative balance between DA and 5-HT systems within the CN and NAC. The operational characteristics of this balance may be subject to additional modulation by the activation of an adrenergic receptor.

Effects on hypothalamic self-stimulation of drugs influencing dopaminergic neurotransmission injected into nucleus accumbens and corpus striatum of rats

The role of the nucleus accumbens septi (ACB) and corpus striatum (CPU) in self-stimulation were investigated by injecting directly or indirectly acting stimulant drugs or a dopamine-(DA)-receptor blocking agent into each site bilaterally. d-Amphetamine (68 nmol) facilitated hypothalamic self-stimulation when injected into either side. Apomorphine (40 nmol) depressed or facilitated responding, the direction and magnitude of this effect being contingent (C = 0.52) on the effect of systemic injection (0.3 mg/kg.i.p.), and correlated with the difference between the effects of d- and l-amphetamine (0.5 mg/kg i.p.) but not with injection site. Haloperidol (6.6 nmol) in either site depressed self-stimulation. Tyramine (730 nmol), an agent believed to cause noncontingent displacement of transmitter from catecholamine terminals, depressed self-stimulation when injection into CPU, but facilitated it when injected into ACB. The site-specific effects found with tyramine but not with apomorphine may have been due to release by tyramine of transmitters other than DA.

Subcutaneous injections of apomorphine, stimulus generalization and conditioning: serious pitfalls for the examiner using apomorphine as a tool

This report shows that stimulus generalization occurs in rats conditioned by a single injection of apomorphine. The data suggest that apomorphine initially acts as an unconditioned stimulus (UCS) of an unconditioned response (UCR) that, in turn, produces stimuli which become conditioned stimuli (CS) of a conditioned response (CR) having a nature identical to that of the UCR. The study also shows that behaviour elicited by a subcutaneous injection of apomorphine depends on the part of the body selected for administration. The mentioned properties should be taken into account when apomorphine is used as a tool in studies on brain and behaviour.

Effects of apomorphine on self-stimulation behaviour in dorsal and ventral area of lateral hypothalamus in mice

Intraperitoneal injections of low doses of apomorphine, a dopaminergic receptor agonist, depressed briefly, and then enhanced self-stimulation behaviour in the dorsal area of lateral hypothalamus. In contrast, only depressant effects were observed in the ventral area. These differential effects suggest the presence of a dopaminergic componant in the dorsal hypothalamic self-stimulation system.

Intracranial reward after Lilly 110140 (fluoxetine HCl): evidence for an inhibitory role for serotonin

The 5-hydroxytryptamine (5-HT, serotonin) specific presynaptic reuptake inhibitor Lilly 110140 (fluoxetine hydroxhloride) was injected systemically in rats trained to bar-press for rewarding stimulation to the caudal portion of the medial forebrain bundle. Rates of self stimulation were reduced in proportion to drug dosage, and these reductions were partially reversible by methysergide. These findings are consistent with previous reports suggesting an inhibitory role for 5-HT in self stimulation.

Prefrontal cortex and neostriatum self-stimulation in the rat: differential effects produced by apomorphine

In a dose-response experiment, the effects of intraperitoneal injections of the dopamine receptor agonist, apomorphine (0.075, 0.15, 0.3, 0.6 and 1.2 mg/kg) were studied on self-stimulation elicited from electrodes implanted in the medial and sulcal prefrontal cortex and caudate-putamen in the rat. From the medial and sulcal prefrontal cortex electrodes, apomorphine produced a dose-related decrease of self-stimulation rate which was consistent across animals. From the caudate-putamen electrodes, on the contrary, apomorphine produced a facilitatory effect in the majority of the animals at one or more doses, however, at other doses a decreased self-stimulation rate was observed. The clear and consistent effects of apomorphine on self-stimulation of the prefrontal cortex, together with other experimental evidence in the same line, suggest that dopamine is mediating self-stimulation of this cortical area.

Catecholamines and self-stimulation: evidence suggesting a reinforcing role for noradrenaline and a motivating role for dopamine

Investigation of the role of noradrenaline (NA) and dopamine (DA) in self-stimulation showed that d-amphetamine (which releases more DA than does l-amphetamine, but not more NA) was much more effective than l-amphetamine in enhancing self-stimulation of NA sites in the locus coeruleus and near-lateral hypothalamus. In DA sites in the substantia nigra and far-lateral hypothalamus the effects of the 2 isomers were confirmed to be more nearly equal. Thymoxamine HCl (10 mg/kg IP), a specific alpha-adrenergic receptor blocker, depressed self-stimulation at all sites, but significantly more severely at DA sites. Thus the drugs most effective in influencing self-stimulation at a particular site were those acting predominantly on the unstimulated system. These findings were interpreted in terms of a hypothesis that DA and NA play complementary roles in self-stimulation and that both are essential; or, more specifically, that DA pathways, implicated in other motivational activites, contribute to a state of drive or arousal necessary for self-stimulation; while response-contingent noradrenergic activity (elicited by the electrodes directly via a transsynaptic route) mediates reinforcement. Further predictions from this hypothesis were tested as follows: (1) Direct pharmacological stimulants of adrenergic alpha-receptors should disrupt self-stimulation by acting randomly on the reinforcement system and disrupting response-reward contingencies; this was confirmed by the finding that the alpha-receptor stimulant clonidine HCl (0.05 mg/kg) depressed self-stimulation at all sites tested. (2) Drect stimulants of DA receptors should enhance self-stimulation of NA sites by augmenting dopaminergic motivational activity; but in rats with DA electrodes, noncontingent stimulation of DA receptors would also impose similar noncontingent activity on the transsynaptic noradrenergic reinforcement pathways and thus depress self-stimulation; this was confirmed by the finding that apomorphine (0.3-1.0 mg/kg) was strongly stimulant for NA electrodes but strongly depressant for DA electrodes, and that the degree and direction of these effects was highly correlated with the differential effects of d- l-amphetamine (rho = .65, p less than 0.01). Neither effect of apomorphine depended on the occurrence of motor stereotypy. These results can be interpreted in terms of 2-component models for self-stimulation, with the predominant transmitter of the drive component being identified as DA and that g the reinforcing component as NA.

Separation of inhibiting and stimulating effects of morphine on self-stimulation behaviour by intracerebral microinjections

The effects on self-stimulation behaviour of 5 mug morphine HCl applied into the ventricular system and into different areas throughout the brain were studied. Injections into the ventricular system and in areas intermediate between the posterior hypothalamus and the periaqueductal grey matter had biphasic effects: an inhibition followed by an excitation. Injections into the posterior hypothalamus resulted in increased self-stimulation whereas injections into the periaqueductal grey matter and into the locus coeruleus were only inhibiting.

Effects of various inhibitors of tyrosine hydroxylase and dopamine beta-hydroxylase on rat self-stimulation after reserpine treatment

The behavioral effects of low doses of the catecholamine (CA) synthesis inhibitor, alpha-methyl-p-tyrosine (alpha-MPT, 50 mg/kg i.p.), or the norepinephrine (NE) synthesis inhibitors (FLA-63, 15 mg/kg i.p., U-14624, 50 mg/kg i.p., or disulfiram 150 mg/kg i.p.) were studied in rats pretreated with reserpine (1 mg/kg i.p.) 24 h before. Rats were implanted either in the area ventralis tegmenti (AVT) or in the lateral hypothalamus (LH). The modifications of CA synthesis and endogenous CA levels were estimated in a parallel experiment. Reserpine treatment produced a slow decrease in self-stimulation (SS) rates during the first 12 h; SS rates were 85% of control values 24 h after reserpine treatment. Injection of alpha-MPT in reserpine-pretreated rats inhibited SS (85% decrease 3 h after administration either in AVT or LH rats), whereas dopamine beta-hydroxylase inhibition had no great effect on SS. The administration of very low doses of alpha-MPT (20 mg/kg i.p.) to rats treated with reserpine (24 h before) plus FLA-63 (1 h before) induced an important decrease in SS rates in AVT-implanted rats only. The major conclusion is that dopaminergic neurons seem to be involved in AVT and LH SS. The last experiment suggests the involvement of a balance between dopaminergic and noradrenergic neurons in AVT SS.

Effects of chemical stimulation of the mesolimbic dopamine system upon locomotor activity

The effects of local injections of drugs into terminal areas of the mesolimbic dopamine system were investigated. Bilateral administration of dopamine, but not of noradrenaline and serotonin, into the nucleus accumbens of non-pretreated rats resulted in stimulation of locomotor activity. No clear or only minor effects were seen after injections of the dopamine metabolites 3-methoxytyramine, DOPAC and HVA and after injections of media with different pH and osmolality. d-Amphetamine proved more effective than dopamine in producing locomotor stimulation, whereas both stimulant and depressant effects were observed following injection of apomorphine into the nucleus accumbens. ET 495 and the noradrenaline agonists clonidine, phenylephrine and isoprenaline did not enhance locomotor activity, but theophylline was effective. Pretreatment with haloperidol, but not with clozapine, significantly reduced the effects of dopamine and theophylline. Locomotor stimulation was also found following bilateral administration of dopamine, d-amphetamine and apomorphine into the tuberculum olfactorium, whereas noradrenaline, serotonin and ET 495 produced no, or rather depressant effects. These results provide further evidence for an important role of the mesolimbic dopamine system with respect to locomotor activity.

The effect of microinjections of amphetamine into the neostriatum and the nucleus accumbens on self-stimulation behaviour

The effect of micro-injections of dexamphetamine chloride into the neostriatum, the nucleus accumbens, the anterior hypothalamus, and the ventricular system on self-stimulation with electrodes in the ventral tegmentum was studied. Unilateral injections of 10 mug into the anterior hypothalamus produced no effect. Injections into the neostriatum tended to depress the self-stimulation rate, whereas injections into the nucleus accumbens increased the rate markedly. Bilateral injections (2 times 2.5 mug and 2 times 5 mug amph.) into the nucleus accumbens were more effective than unilateral injections and were as effective as systemic injections of 1 mg/kg amphetamine (i.p.). Bilateral injections into the neostriatum also increased the self-stimulation rate. Injections of 10 mug into the ventricular system resulted in a smaller increase which was not statistically significant. These results are discussed in relation to the involvement of the dopaminergic system in the maintenance of self-stimulation behaviour.