Louis Stinus

Interactions between d-Ala-Met-enkephalin, A10 dopaminergic neurones, and spontaneous behaviour in the rat

We have investigated the interaction between opioid peptides and dopaminergic A10 (DA-A10) neurones in the ventral tegmental area (VTA). The behavioural consequences of VTA infusion of d-Ala-Met-enkephalinamide (DALA) were analyzed. DALA elicited a dose-dependent increase in locomotor activity measured in photocell cages and the circular corridor. Observations in the open field and in a hole box revealed that DALA-induced behavioural stimulation was characterized by enhancement of locomotion, rearing, and number of hole visits, while grooming time and duration of hole visits were decreased. DALA-induced stimulation was reserved by naloxone, and was completely blocked by 6-OHDA destruction of DA-A10 terminals. d-Amphetamine-induced behavioural activation was potentiated by simultaneous VTA infusion of DALA which indicates that the behavioural response to DALA is dependent on DA-A10 neuronal activity. It is postulated that stimulation of opiate receptors exerts a presynaptic inhibition of an inhibitory input to DA-A10 neurones (eg. GABA or dendritic DA), thus releasing dopaminergic activity. In contrast to the acute effect, the d-amphetamine response was strongly attenuated 4 h, 1 and 6 days after VTA infusion of DALA, and returned to normal only at 14 days. This long-lasting modification may reflect decreased activity of opioid neurones, releasing the inhibition of DA-A10 neurones. Our findings suggest that endogenous opioid peptides may exert a modulatory influence on the dopaminergic mesocorticolimbic system.

Disappearance of hoarding behavior after 6-hydroxydopamine lesions of the mesolimbic dopamine neurons and its reinstatement with L-dopa

The consequences of 6-hydroxydopamine lesions of the mesolimbic dopamine system on hoarding behavior were investigated in the rat. Specific lesions of this system, at the level of either the ventral tegmental area or the nucleus accumbens, resulted in abolition or severe reduction of hoarding activity. Similar lesions of the forebrain noradrenaline neurons did not affect hoarding. In further experiments, amphetamine and apomorphine locomotor responses, spontaneous motor behavior, food intake and eating patterns, and the existence of any regulatory deficits were examined. A subtle disorganization of eating patterns was found in animals with mesolimbic-dopamine lesions. It was determined that the hoarding deficit could not be due to motor, ingestive, or regulatory impairments. In a final experiment, it was demonstrated that hoarding behavior can be restored to control levels in dopamine-lesion rats by prior treatment with the catecholamine presursor L-dopa. These findings suggest that hoarding activity is mediated by mesolimbic dopamine neurons, and it is hypothesized that this system is necessary for the facilitation of certain types of foraging responses under a high level of arousal.

Influence of ventral mesencephalic lesions on various spontaneous and conditioned behaviors in the rat

Abstract In the first set of experiments, we used groups of rats with either bilateral or unilateral ventral mesencephalic lesions and a group of controls in the following tests: diurnal exploratory activity, nocturnal activity, Skinner box and avoidance conditioning. Both groups of lesioned rats were found to be hyperactive in both types of activity tests, but those with unilateral lesions showed hyperactivity to a lesser extent. Neither of the experimental groups showed any evidence of extinction in the Skinner box. Rats with bilateral lesions learn a conditioned avoidance response more easily than the control animals but they also tend to show inadaptive behavior during the course of the testing procedure. In a second experiment, stimulating and recording electrodes were implanted in the lateral hypothalamus in certain of the animals of the three previous groups, having lesions as well. Such lesions were found to significantly decrease the threshold of arousal to central hypothalamic stimulation. From the results of our lesioning and testing procedures, a syndrome has been defined. However, the complex structure of the region that was destroyed does not permit a clear interpretation of the results.

Is the dopaminergic mesocorticolimbic system necessary for intracranial self-stimulation? Biochemical and behavioral studies from A10 cell bodies and terminals.

The role of mesocortical and mesolimbic A10-dopaminergic neurons was tested in a self-stimulation paradigm. In a first experiment, we showed a significant activation of the A10 neurons as a result of bilateral self-stimulation of A10 cell bodies in the ventral tegmental area. This activation as measured by the ratio between endogenous dihydroxyphenylacetic acid and dopamine, was greater in the medical prefrontal cortex than in the nucleus accumbens. In a second experiment, we showed that self-stimulation obtained from A10 terminal regions in the medial prefrontal cortex and in the nucleus accumbens was not reduced by near total destruction of A10 cell bodies. In contrast we observed an increase in self-stimulation from the nucleus accumbens. On the basis of these results it may be stated that the dopamine of A10 neurons is not critical in maintaining self-stimulation from medial prefrontal cortex and from nucleus accumbens. While a role for dopamine in self-stimulation cannot be excluded, there are most likely other noncatecholaminergic systems involved.

Radiofrequency lesion of the ventral mesencephalic tegmentum: Neurological and behavioral considerations

Abstract Radiofrequency ablation of the ventral mesencephalic tegmentum provoked hyperactivity and hyper-reactivity in rats. The critical lesion zone for the appearance of hyperactivity surrounds the nucleus interpeduncularis, a region occupied by fibers corresponding to the fasciculus medialis prosencephali and by the A10 group of mesolimbic dopaminergic cells. The behavioral syndrome appeared 8 days after the lesions and persisted throughout the experiment (210 days), whereas only minor sleep disturbances were observed in animals with lesions. A role of the A10 mesolimbic dopaminergic system in the behavioral effects of these lesions in rats is discussed.

Short and long-term changes in dopamine and serotonin receptor binding sites in amphetamine-sensitized rats: a quantitative autoradiographic study

The biochemical changes in DA and 5HT systems were investigated in amphetamine (AMPH)-sensitized rats, 1 and 15 days after cessation of treatment (5 mg/kg AMPH, i.p., twice a day for 6 days). At both times, AMPH-treated rats exhibited behavioral sensitization, as revealed by an enhancement of the stereotypic response to a challenge dose of 2 mg/Kg, ip. AMPH. Basal dopamine (DA) and serotonin (5-HT) metabolism was not significantly modified in different brain areas of AMPH-sensitized rats. Quantitative autoradiographic analysis of DA and serotonin 5-HT receptor subtypes was performed in the following brain regions: medial prefrontal cortex, nucleus accumbens, striatum, substantia nigra, ventral tegmental area, dorsal and median raphe nuclei. A significant increase of [3H]SCH 23390 binding to D1 DA receptors was observed in the substantia nigra pars reticulata 1 day but not 15 days after the cessation of AMPH treatment, whereas [3H]8-OH-DPAT binding to 5-HT1A sites was found to be significantly enhanced in the dorsal raphe nucleus at both time points. No change in D2 DA nor in 5-HT1B or 5-HT2A receptors was found in any of the brain structures examined at either time point. The obtained results suggest that DA and 5-HT systems are differently and time-dependently involved in AMPH-induced behavioral sensitization.

Increased sensitivity to (+)amphetamine self-administered by rats following meso-cortico-limbic dopamine neurone destruction.

DRUG self-administration (SA) in laboratory animals shows some of the characteristics of intracranial self-stimulation and of other natural reinforcers (for review see ref. 1). Animals will self-administer many of the same drugs abused by human subjects. Recent evidence has shown that (+)amphetamine reinforces behaviour in the same manner as conventional reinforcers1–3 that it acts mainly on dopamine (DA) transmission, and that locomotor activation and stereotypy are mediated by the mesolimbic–mesocortical (A10 group) and nigro-striatal (A9 group) DA systems, respectively4,5. Using various specific noradrenergic and dopaminergic blocking agents in a (+)amphetamine SA retention paradigm, DA transmission blockade was shown to produce effects similar to reward reduction and reward termination3,6,7. DA transmission in the frontal-mesolimbic system has been implicated in brain stimulation reward8, and the link between self-administration of (+)amphetamine and reward mechanisms, both of which depend on DA transmission, may be related to the craving and early dependence that certain subjects display for addictive drugs. However, the mechanisms which lead to addiction are still unclear, and it is possible that in drug addicts the particularly strong and rapid addicting effect of the drug could be due to a pre-existing imbalance in some homeostatic mechanism. Thus, the use of healthy animals in a steady-state paradigm is not necessarily a good model for the study of the psychological or neurobiological vulnerability to drug addiction acquisition. Because the meso-cortico-limbic DA system may be an essential target for psychostimulant drugs, the lesion of this system could induce disruption of SA behaviour.

Self-stimulation and catecholamines. II. Blockade of self-stimulation by treatment with alpha-methylparatyrosine and the reinstatement by catecholamine precursor administration

The behavioral effects of catecholamine precursors, l-3,4-dihydroxyphenylalanine (l-DOPA) and dl-threo-3,4-dihydroxyphenylserine (DOPS), in association with an inhibitor of peripheral decar☐ylases (Ro 04-4602), were studied in rats pretreated with α-methylparatyrosine (α-MpT), an inhibitor of catecholamine synthesis at the level of tyrosine hydroxylase. Rats were implanted either in the lateral hypothalamus (LH) or in the area ventralis tegmenti (AVT). The changes in brain catecholamine levels induced by these treatments were estimated in a parallel experiment. l-DOPA treatment reinstated the self-stimulation behavior, previously inhibited by α-MpT, in rats implanted in the AVT or LH. After self-stimulation behavior had been inhibited by α-MpT, treatment with DOPS reinstated the rates of self-stimulation only in the AVT group; this norepinephrine precursor did not reinstate LH self-stimulation. Biochemical analysis showed thatl-DOPA reinstated normal endogenous norepinephrine and induced an increase of endogenous dopamine exceeding the control value in rats pretreated with α-MpT, whereas DOPS did not modify endogenous norepinephrine. The major conclusions are: (1) The dopaminergic system originating from the A10 group does not seem to be involved in self-stimulation elicited from the AVT. (2) The failure of DOPS to restore self-stimulation elicited from the LH suggests that noradrenergic neurons in this area are not directly involved in LH self-stimulation; however, it cannot be excluded that DOPS is not homogeneously distributed in the brain. (3) These results demonstrate the heterogeneity of the reward system.

Effects of morphine and naloxone on behaviour in the hot plate test: an ethopharmacological study in the rat

The objectives of this study were: i) to analyse the effects of morphine and naloxone on the rats behaviour in the hot plate test using an ethological approach, and ii) to compare the effectiveness of repeated versus single test paradigms. Animals received either morphine (0, 3, 6 or 9 mg/kg SC) or naloxone (0, 0.01, 0.1 or 1 mg/kg SC). For repeated hot plate measures, rats were tested before and 60, 120, 180 and 240 min following morphine treatment, as well as 30, 60, 90 and 120 min after naloxone injection. For the single test schedule, rats were tested only once 60 min after morphine or 30 min after naloxone administration, or at 60, 120, 180, 240 and 300 min after 9 mg/kg morphine treatment. Behaviour was videotaped and analysed by an ethogram and ethological techniques. A cluster analysis revealed that the most frequently displayed patterns could be categorised into exploratory sniffing reactions (walk-sniff, immobile-sniff) and noxious-evoked elements, including primary (paw-licking, stamping), escape (jumping, leaning posture) and independent (hindleg-withdrawal) patterns. During repeated tests, morphine treatment induced: i) a maximum hypoalgesic effect 60 min post-injection (noxious-evoked patterns were significantly reduced), and ii) an unexpected “thermal hyperreactivity rebound effect” after 120 min (paw-licking and hindleg-withdrawal were enhanced), although changes in hindpaw-licking are more indicative of a hyperalgesic rebound effect. Most changes were quite similar during the single test schedule at 60 and 120 min after morphine injection. With regard to naloxone treatment, jumping latency was significantly decreased during the repeated test schedule, but not on single exposure to the plate. Other elements were facilitated, however, in the single test (stamping, leaning posture, hindleg-withdrawal). The results indicated that both repeated and single tests paradigms are of value for testing the effects of morphine and naloxone on rats. However, under our conditions the single test paradigm gave a better picture of the overall effects of the drug. Learning as well as habituation and sensitization may mask certain effects during repeated tests. In conclusion, an ethological analysis of the rats behaviour in the hot plate test following administration of morphine and naloxone has been validated in this study.

GABAergic mechanisms within the ventral tegmental area: Involvement of dopaminergic (A 10) and non-dopaminergic neurones

The spontaneous activity of rats was measured after activation or inhibition of GABA activity in the ventral tegmental area of the midbrain (VTA). Six hours after bilateral injection of ethanolamine-o-sulphate (GABA agonist) into the VTA, the behavioural activation induced either by d-amphetamine (amph) or by bilateral VTA infusion of a long-lasting enkephalin analogue was completely blocked. Bilateral infusion of picrotoxin (GABA antagonist) into the VTA elicited a short-lived (40 min) dose-dependent behavioural activation which was not reduced either by prior specific lesion of the meso-cortico-limbic dopaminergic neurones or by administration of the opiate antagonist naloxone. Moreover, the simultaneous administration of picrotoxin and amph induced complex changes in behaviour which consisted of additive effects during the first 40 min, followed by an inhibition of the activating effect of amph. Our findings indicate that GABA-mediated inhibition involves both dopaminergic and non-dopaminergic neurones within the VTA, and possible implications for human pathology are discussed.