Amphetamine and apomorphine responses in the rat following 6-OHDA lesions of the nucleus accumbens septi and corpus striatum

Eight mug of 6-hydroxydopamine (6-OHDA) injected bilaterally into the nucleus accumbens septi (NAS) or the caudate nucleus of the rat resulted in 79% and 50% depletion of endogenous dopamine (DA) at these respective sites. Fourteen days after the injection a low dose of amphetamine failed to induce the characteristic locomotor response in the NAS-lesioned rats but did so in the caudate-lesioned animals. By contrast the caudate lesion, but not the NAS lesions, abolished intense forms of stereotyped behaviour induced by higher doses of amphetamine. Both lesioned groups exhibited supersensitivity to the dopamine agonist, apomorphine; the NAS group showed enhanced locomotor activity and the caudate group enhanced stereotyped behaviour. The block of amphetamine locomotion and the enhanced response to apomorphine were maximal around 14 days after the operation and gradually attenuated up to 90 days. Theer is evidence that remaining DA levels in the NAS are greater at 90 than at 14 days postoperatively. Thus recovery of behavioural effects correlated with an increase in the remaining levels of DA in the NAS.

Cognitive deficit caused by regional depletion of dopamine in prefrontal cortex of rhesus monkey

Depletion of dopamine in a circumscribed area of association cortex in rhesus monkeys produces an impairment in spatial delayed alternation performance nearly as severe as that caused by surgical ablation of the same area. This behavioral deficit can be pharmacologically reversed with dopamine agonists such as L-dopa and apomorphine. These data provide direct evidence that dopamine plays an important role in a specific cortical function.

Antischizophrenic drugs: chronic treatment elevates dopamine receptor binding in brain

Chronic treatment of rats with the neuroleptic drugs haloperidol, fluphenazine, and reserpine elicits a 20 to 25% increase in striatal dopamine receptor binding assayed with [3H]haloperidol. This increase in receptor sites may account for behavioral supersensitivity to dopamine receptor stimulants in such animals and for tardive dyskinesia in patients treated with these drugs.

Intracellular and extracellular electrophysiology of nigral dopaminergic neurons--1. Identification and characterization

Intracellular recordings were obtained from directly identified rat nigral dopamine cells in vivo. This identification was based on an increase in glyoxylic acid-induced catecholamine fluorescence in the impaled dopamine neurons. One of three compounds was injected intracellularly into each cell to produce the heightened fluorescence: (1) L-DOPA, to increase the intracellular dopamine content by precursor loading; (2) tetrahydrobiopterin, a cofactor for tyrosine hydroxylase, to increase intracellular dopamine concentration through activation of the rate-limiting enzyme for dopamine synthesis and (3) colchicine, to arrest intraneuronal transport and thus allow the build-up of dopamine synthesizing enzymes and dopamine in the soma. In addition, dopamine cells were antidromically activated from the caudate nucleus and collision with a directly elicited action potential was demonstrated. Identified dopamine neurons were shown to possess an input resistance of 31.2 +/- 7.4 M omega (means +/- SD) and a time constant of 12.1 +/- 3.2 ms. The action potentials were of long duration (2.75 +/- 0.5 ms) with a marked break between the initial segment and the somatodendritic spike components. The initial segment was the only component commonly elicited during antidromic activation. Spontaneously occurring action potentials were usually preceded by a slow, pacemaker-like depolarization. Burst firing by summation of depolarizing afterpotentials was observed to occur spontaneously, but could not be triggered by short depolarizing current pulses. Intravenously administered apomorphine demonstrated the same inhibitory effect on cell firing that was previously reported to occur when recording extracellularly from identified dopaminergic neurons. The determination of the electrophysiological characteristics of a population of cells directly identified as containing a specific neurotransmitter (in this case, dopamine) may allow one to construct better models of a system's functioning. Thus, the high input resistance and long time constant of dopamine-containing cells, combined with their burst/pause firing mode, may be important functionally with respect to a possible modulatory effect of dopamine in postsynaptic target areas.

Selective 6OHDA-induced destruction of mesolimbic dopamine neurons: abolition of psychostimulant-induced locomotor activity in rats

Selective large scale destruction of mesolimbic dopamine-containing terminals is produced by bilateral injection of 8 mug of 6-hydroxydopamine (6OHDA) into the nucleus accumbens septi (NAS) of rats pretreated with pargyline and desipramine (DMI). The DMI prevents the destruction of the noradrenergic innervation of the forebrain normally produced by the NAS 6OHDA lesion, without affecting the destruction of dopamine-containing neurons. The locomotor stimulation produced by the psychostimulants d-amphetamine (1.5 mg/kg) and cocaine (20 mg/kg) is blocked in rats with selective destruction of the mesolimbic dopamine system. In contrast the locomotor stimulation produced by the directly acting dopamine agonist apomorphine (1.0 mg/kg) is enhanced, which may indicate supersensitivity of the denervated dopamine receptors. These results lend further support to the view that psychostimulant-induced locomotr stimulation in rats results from effects on mesolimbic dopamine neurons. In addition, the protection by DMI of noradrenergic neurons from the toxic effects of 6OHDA is evidence that 6OHDA, as used here, destroys catecholamine neurons mainly by an uptake-dependent specific mechanism.

Dopamine receptor binding enhancement accompanies lesion-induced behavioral supersensitivity

The binding of [3H]haloperidol to rat striatal dopamine receptors increases after lesion (made by injection of 6-hydroxydopamine) of the nigrostriatal dopamine pathway in those rats which are behaviorally supersensitive, as reflected by apomorphine-induced contralateral rotations. The enhanced binding is associated with an increased number of receptor sites with no change in their affinity.

Distinct functions of the two isoforms of dopamine D2 receptors

Signalling through dopamine D2 receptors governs physiological functions related to locomotion, hormone production and drug abuse. D2 receptors are also known targets of antipsychotic drugs that are used to treat neuropsychiatric disorders such as schizophrenia. By a mechanism of alternative splicing, the D2 receptor gene encodes two molecularly distinct isoforms, D2S and D2L, previously thought to have the same function. Here we show that these receptors have distinct functions in vivo; D2L acts mainly at postsynaptic sites and D2S serves presynaptic autoreceptor functions. The cataleptic effects of the widely used antipsychotic haloperidol are absent in D2L-deficient mice. This suggests that D2L is targeted by haloperidol, with implications for treatment of neuropsychiatric disorders. The absence of D2L reveals that D2S inhibits D1 receptor-mediated functions, uncovering a circuit of signalling interference between dopamine receptors.

Characterization of behavioral and neurodegenerative changes following partial lesions of the nigrostriatal dopamine system induced by intrastriatal 6-hydroxydopamine in the rat

Partial lesions of the nigrostriatal dopamine system have been investigated with respect to their ability to induce consistent long-lasting deficits in movement initiation and skilled forelimb use. In eight different lesion groups 6-hydroxydopamine (6-OHDA) was injected at one, two, three, or four sites into the lateral sector of the right striatum, in a total dose of 20-30 microgram. Impairments in movement initiation in a forelimb stepping test, and in skilled paw use in a paw-reaching test, was seen only in animals where the severity of the lesion exceeded a critical threshold, which was different for the different tests used: single (1 x 20 microgram) or two-site (2 x 10 microgram) injections into the striatum had only small affects on forelimb stepping, no effect on skilled paw use. More pronounced deficits were obtained in animals where the same total dose of 6-OHDA was distributed over three or four sites along the rostro-caudal extent of the lateral striatum or where the injections were made close to the junction of the globus pallidus. The results show that a 60-70% reduction in tyrosine hydroxylase (TH)-positive fiber density in the lateral striatum, accompanied by a 50-60% reduction in TH-positive cells in substantia nigra (SN), is sufficient for the induction of significant impairment in initiation of stepping. Impaired skilled paw-use, on the other hand, was obtained only with a four-site (4 x 7 microgram) lesion, which induced 80-95% reduction in TH fiber density throughout the rostrocaudal extent of the lateral striatum and a 75% loss of TH-positive neurons in SN. Drug-induced rotation, by contrast, was observed also in animals with more restricted presymptomatic lesions. The results indicate that the four-site intrastriatal 6-OHDA lesion may be a relevant model of the neuropathology seen in parkinsonian patients in a manifest symptomatic stage of the disease and may be particularly useful experimentally since it leaves a significant portion of the nigrostriatal projection intact which can serve as a substrate for regeneration and functional recovery in response to growth promoting and neuroprotective agents.

Dopamine-sensitive adenylate cyclase in caudate nucleus of rat brain, and its similarity to the "dopamine receptor"

An adenylate cyclase that is activated specifically by low concentrations of dopamine has been demonstrated in homogenates of caudate nucleus of rat brain. A half-maximal increase in the activity of the enzyme occurred in the presence of 4 muM dopamine. Concentrations of dopamine as low as 0.3 muM stimulated the activity of the enzyme. The adenylate cyclase activity of the homogenates was also stimulated by low concentrations of apomorphine, a substance known to mimic the physiological and pharmacological effects of dopamine. The stimulatory effect of dopamine was blocked by low concentrations of either haloperidol or chlorpromazine, agents known to block the actions of dopamine in mammalian brain. The results suggest that dopamine-sensitive adenylate cyclase may be the receptor for dopamine in mammalian brain. The isolation of this enzyme from caudate nucleus should facilitate the search for new therapeutic agents useful in the treatment of extrapyramidal diseases.

The pharmacological and anatomical substrates of the amphetamine response in the rat

Bilateral 6-hydroxydopamine microinjections into the substantia nigra abolished both the locomotor and stereotyped responses to d-amphetamine in adult rats. The lesions resulted in a depletion of over 99 per cent of striatal tyrosine hydroxylase activity (indicating a near total lesion of the nigro-striatal dopamine pathway) as well as severe noradrenaline depletions. However, lesion of the dorsal or ventral noradrenergic pathways resulted in similar noradrenaline depletions but with no effect on striatal tyrosine hydroxylase levels and without the concomitant blockage of the amphetamine response. The substantia nigra lesioned rats were behaviourally supersensitive to apomorphine and L-DOPA and did not show a locomotor response to cocaine. The substantia nigra lesioned rats were not aphagic or adipsic. It was concluded that both the locomotor and stereotyped responses induced by amphetamine are dependent on the functional integrity of the nigro-striatal dopamine pathway.

Dopaminergic stimulation disrupts sensorimotor gating in the rat

Prepulse inhibition is a cross-species phenomenon in which reflex responses to discrete sensory events are modified by weak prestimulation. In experiments designed to investigate the neuropharmacological mechanism of this form of information processing, and its relevance to schizophrenic psychopathology, apomorphine (0.125-4.0 mg/kg) and d-amphetamine (0.5-4.0 mg/kg) were administered to rats in an attempt to modify prepulse inhibition of the acoustic startle response. Rats were presented with 40 ms, 118 dB[A] acoustic pulses which were intermittently preceded by a weak 80 dB[A] acoustic prepulse. Both apomorphine and d-amphetamine induced a significant loss of prepulse inhibition, as reflected by increased pulse-preceded-by-prepulse versus pulse-alone startle magnitudes. Haloperidol (0.1 mg/kg), a specific D2 dopamine receptor antagonist, prevented the effects of 2.0 mg/kg apomorphine on prepulse inhibition, while having little effect by itself. An additional study investigated the effects of chronic intermittent administration of 2.5 mg/kg d-amphetamine. Rats given amphetamine for 8 consecutive days also displayed a loss of prepulse inhibition, with no evidence of tolerance. Finally, prepulse inhibition was examined under high- and low-intensity startle stimulus conditions; apomorphine (1.0 mg/kg) induced a loss of prepulse inhibition under both intensity conditions in approximately equal proportion. The results of these studies suggest a connection between sensorimotor gating, as measured by prepulse inhibition, and dopaminergic overactivity, supporting suggestions that information processing deficits in schizophrenia may be responsible for some psychotic symptoms and their effective treatment by antipsychotic D2 dopamine antagonists.

Periadolescence: age-dependent behavior and psychopharmacological responsivity in rats

The behavior and psychopharmacological sensitivity of periadolescent rats are examined in this review. Periadolescent rats are hyperactive and engage in more conspecific play behavior than younger or older rats. When compared with other-aged rats, periadolescents exhibit enhanced performance in simple active-avoidance learning tasks, but perform poorly in more complex appetitive and avoidance learning tasks in which increases in locomotor activity do not improve performance, perhaps as a result of age-specific alterations in selective attention or stimulus processing. Such behavioral "anomalies" of periadolescent animals observed in traditional laboratory situations may be in some way adaptive when considered in the context of the animals' natural habitat. In terms of psychopharmacological responsiveness, periadolescent rats, when compared with younger or older animals, are less sensitive to catecholaminergic agonists but are more responsive to the catecholaminergic antagonist haloperidol. This pattern of psychopharmacological sensitivity suggests that the catecholaminergic systems may be temporarily hyposensitive during the periadolescent period. Evidence is presented that a negative feedback system in the form of dopamine autoreceptors may become functionally mature in mesolimbic brain regions during the periadolescent period. The possibility is presented that maturation of these self-inhibitory autoreceptors might result in a temporary decrease in the efficacy of mesolimbic dopamine projections, perhaps contributing to the psychopharmacological and behavioral characteristics of periadolescent animals. In support of this suggestion, evidence is reviewed indicating that the behavior of adult animals with lesions of the ventral tegmental area, a region containing cell bodies from which these mesolimbic dopaminergic projections originate, resembles that of periadolescent rats.

Dopamine neurons encode the better option in rats deciding between differently delayed or sized rewards

The dopamine system is thought to be involved in making decisions about reward. Here we recorded from the ventral tegmental area in rats learning to choose between differently delayed and sized rewards. As expected, the activity of many putative dopamine neurons reflected reward prediction errors, changing when the value of the reward increased or decreased unexpectedly. During learning, neural responses to reward in these neurons waned and responses to cues that predicted reward emerged. Notably, this cue-evoked activity varied with size and delay. Moreover, when rats were given a choice between two differently valued outcomes, the activity of the neurons initially reflected the more valuable option, even when it was not subsequently selected.

Dopamine"autoreceptors": pharmacological characterization by microiontophoretic single cell recording studies

The effects on the firing of single dopamine (DA) neurons in the substantia nigra (and adjacent ventral tegmental area) of a representative group of catecholamine agonists and antagonists were studied in rats using single cell recording and microiontophoretic techniques. Microiontophoretic application of DA or the DA agonist apomorphine depressed the firing of these cells; the DA antagonist trifluoperazine blocked this effect. However, the alpha-agonist clonidine had no depressant effect and the beta-agonist isoproteronol had only a weak depressant action on DA neurons. Furthermore, the alpha-antagonist piperoxane and the beta-antagonist sotolol were completely ineffective in blocking the depressant effects of DA. These results show that DA-sensitive receptors on the soma of DA neurons are pharmacologically distinct from alpha or beta adrenoreceptors. Because of their location and selective responsiveness to DA agonists, the catecholamine receptors on the soma of DA neurons appear best classified as DA "autoreceptors".

Reinnervation of the denervated striatum by substantia nigra transplants: functional consequences as revealed by pharmacological and sensorimotor testing

Embryonic substantia nigra (SN) was transplanted to the neostriatum in adult rats subjected to a unilateral or serial bilateral destruction of the nigrostriatal dopamine (DA) pathway. The survival of the graft and the growth of DA-containing fibers from the graft into the host brain was studied by fluorescence histochemistry and micro-fluorometry. The motor asymmetry, and the contralateral 'sensory neglect' induced by a unilateral destruction of the nigrostriatal DA pathway, were monitored in the transplanted rats and in non-transplanted controls through repeated measurements of the amphetamine-and apomorphine-induced rotational behaviour, and through analysis of the rats performance in a number of sensorimotor tests. Finally, the development of aphagia and adipsia after a serial bilateral destruction of both nigrostriatal DA pathways were followed in rats bearing bilateral SN transplants and in lesioned non-transplanted controls. The results show that large parts of the dorsal neostriatum can be reinnervated by DA-containing axons from the intracortical transplant and that this new DA input can fully compensate for the amphetamine-induced motor assymmetry that resulted from the initial destruction of the innate nigrostriatal DA pathway. The fluorescence microscopical observations provide strong evidence that the compensation of the amphetamine-induced rotational response was specifically related to the re-establishment of a new DA input to the denervated neostriatum, and that the degree of rotational compensation was well correlated to the magnitude of ingrowth into the neostriatum. Subsequent surgical removal of the SN transplant reinstated the initial rotational behaviour. In sharp contrast to the marked compensation in motor asymmetry, the transplanted rats showed no tendency to recover in their sensorimotor performance. Thus, while the sensorimotor deficit had recovered in the control group a marked contralateral 'sensory neglect' remained in the transplanted animals. Furthermore, removal of the transplant produced a significant improvement in their sensorimotor performance within 3 days. In the bilaterally transplanted animals the presence of the transplants did not prevent the development of severe adipsia, aphagia and akinesia following the destruction of the remaining contralateral nigrostriatal pathway. In fact, the recovery from the consummatory deficits tended to be better in the lesioned control rats than in the transplanted ones. It is concluded that SN transplants reinnervation the dorsal part of the neostriatum are able to replace the innate SN in normalizing some aspects of the rats motor behaviour, while their sensorimotor deficits and deficits in consummatory behaviour were unaffected. It is suggested that this dissociation of transplant-induced recovery is due to the failure of the SN transplants to reinnervate those parts of the neostriatum which are most directly implicated in sensorimotor and consummatory behaviour.