Partial bilateral mesencephalic lesions affect D1 but not D2 binding in both the striatum and cortex

Rodent models of Parkinson's disease: beyond the motor symptomatology

Parkinson's disease (PD) is classically characterized by motor symptoms; however, non-motor symptoms (NMS) are increasingly recognized as relevant in disease-state, given the associated alterations in mood (depression and anxiety) and cognition. Here, particularly in regards to NMS, we aimed to compare the motor, emotional and cognitive behavior of three animal models of PD that trigger dopaminergic (DAergic) degeneration on both brain hemispheres: (i) the 6-hydroxydopamine (6-OHDA, 8 or 6 μg) lesion model; (ii) the paraquat (PQ) induced model, and (iii) a genetic model based on α-synuclein overexpression (α-syn). 6-OHDA and α-syn vector were injected bilaterally in the substantia nigra pars compacta (SNpc) of adult male Wistar rats; as for PQ delivery, micro-osmotic pumps were implanted in the interscapular region. Motor deficits were observed in all models, with histological analysis of tyrosine hydroxylase positive cells in the SNpc revealing a significant loss of DAergic neurons in all animal models. In addition, the α-syn animal model also presented a reduction in exploratory activity, and the 6-OHDA and PQ animals displayed a significant increase in both depressive- and anxiety-like behavior. Interestingly, cognitive impairment (working memory) was only observed in the 6-OHDA model. Overall, these PD models are suitable for mimicking the motor symptoms associated to PD, with each encompassing other relevant NMS components of the disorder that may prove beneficial for further studies in PD.

Dopamine receptors, motor responses, and dopaminergic agonists

Dopamine receptors are widely distributed within the central nervous system with its highest expression in the striatum. Two different families of dopamine receptors have been identified. The D₁ family comprises D₁ and D5 receptors, whereas D₂, D₃, and D₄ receptors form the D₂ family. These 2 families mediate different behavior patterns that are linked to activation of specific transduction pathways. The functional relevance of dopamine receptors derives from the reduced dopamine content found in the striatum of Parkinson disease (PD) patients and the ability of dopamine and dopamine receptors to reverse the motor deficits exhibited by PD patients. During the last 2 decades dopamine receptor agonists have been used either in de novo PD patients to prevent the appearance of dyskinesias or in PD patients with motor fluctuations to reduce the number of daily "off" hours. It seems that all dopamine receptors agonists produce similar motor responses and adverse effects, but data comparing their effectiveness in the treatment of PD are not available. In this article we summarize the main characteristics of dopamine receptors, their structure, their signaling pathways, and the responses mediated by their independent activation. Here is also described the therapeutic value of the different dopamine receptor agonists in the treatment of PD.

Absence of quasi-morphine withdrawal syndrome in adenosine A2A receptor knockout mice

RATIONALE

Caffeine and other methylxanthines induce behavioral activation and anxiety responses in mice via antagonist action at A2A adenosine receptors. When combined with the opioid antagonist naloxone, methylxanthines produce a characteristic quasi-morphine withdrawal syndrome (QMWS) in opiate-naive animals.

OBJECTIVES

The aim of this study was to establish the role of A2A receptors in the quasi-morphine withdrawal syndrome induced by co-administration of caffeine and naloxone and in the behavioral effects of caffeine.

METHODS

We have used A2A receptor knockout (A(2A)R(-/-)) mice in comparison with their wild-type and heterozygous littermates to measure locomotor activity in the open field and withdrawal symptoms induced by caffeine and naloxone. Naïve wild-type and knockout mice were also examined for enkephalin and dynorphin mRNA expression by in situ hybridization and for mu-opiate receptor by ligand binding autoradiography to check for possible opiate receptor changes induced by A2A receptor inactivation.

RESULTS

Caffeine increases locomotion and anxiety in wild-type animals, but it has no psychomotor effects in A(2A)R(-/-) mice. Co-administration of caffeine (20 mg/kg) and naloxone (2 mg/kg) resulted in a severe quasi-morphine withdrawal syndrome in wild-type mice that was almost completely abolished in A(2A)R(-/-) mice. Heterozygous animals exhibited a 40% reduction in withdrawal symptoms, suggesting that there is no genetic/developmental compensation for the inactivation of one of the A(2A)R alleles. A(2A)R(-/-) and wild-type mice have similar levels of striatal mu-opioid receptors, thus the effect is not due to altered opioid receptor expression.

CONCLUSIONS

Our results demonstrate that A2A receptors are required for the induction of quasi-morphine withdrawal syndrome by co-administration of caffeine and naloxone and implicate striatal A2A receptors and mu-opiate receptors in tonic inhibition of motor activity in the striatum.