Activity Of Nigrostriatal Dopaminergic Neurons Research Articles

Involvement of vanilloid-like receptors in the effects of anandamide on motor behavior and nigrostriatal dopaminergic activity: in vivo and in vitro evidence

The administration of the endocannabinoid anandamide to rats produces hypokinesia in parallel to a decrease in the activity of nigrostriatal dopaminergic neurons. It was earlier hypothesized that this effect was mediated through the activation of CB 1 receptors, although these receptors have not been found in dopaminergic neurons, but in striatal projection neurons connected with them. However, two recent discoveries: (i) that anandamide is also able to activate vanilloid VR 1 receptors, and (ii) that VR 1 receptors are located on nigrostriatal dopaminergic neurons, allow to re-evaluate this hypothesis and suggest that the activation of vanilloid-like receptors rather than CB 1 receptors might be responsible of anandamide-induced hypokinesia and decreased nigrostriatal dopaminergic activity. To validate this new hypothesis, we carried out two different experiments. First, we explored whether the inhibitory effects of anandamide on motor activity and dopaminergic transmission were reversed by capsazepine, an antagonist for vanilloid-like receptors. Our data demonstrated that anandamide reduced ambulation, stereotypies and exploration, measured in the open-field test, whereas it increased the time spent in inactivity. All these effects were completely reversed by capsazepine, which had no effect by itself. Anandamide also caused a significant decrease in nigrostriatal dopaminergic activity, reflected by a reduction in DOPAC contents in the caudate–putamen, which was also reversed by capsazepine. As a second objective, we explored whether anandamide is able to directly influence nigrostriatal dopaminergic function by examining its effects on in vitro dopamine (DA) release using perifused striatal fragments. Our data confirmed that anandamide significantly decreased K +-stimulated dopamine release from nigrostriatal terminals and that this effect was vanilloid-like receptor-mediated since it was prevented by capsazepine. This in vitro inhibitory effect was not seen with a classic cannabinoid agonist that does not bind vanilloid-like receptors. In summary, anandamide behaves as a hypokinetic substance, thus producing motor depression in the open-field test, presumably related to a decrease in nigrostriatal dopaminergic activity. These effects were completely reversed by the vanilloid-like receptor antagonist capsazepine, thus indicating a role of these receptors, which are located on dopaminergic neurons, in mediating hypokinetic effects of anandamide. In vitro studies, using perifused striatal fragments, support this vanilloid-like receptor-mediated direct action, which would not be available for classic cannabinoid agonists.

Time-course of the effects of anandamide, the putative endogenous cannabinoid receptor ligand, on extrapyramidal function

We have recently described the dose-response effect of anandamide (AEA), the N-amide derivative of arachidonic acid that acts as an endogenous ligand for the cannabinoid receptor, on extrapyramidal function. The present study has been designed to examine the time-course of this effect. To this end, adult male rats were submitted to an acute i.p. injection of AEA, Δ 9-tetrahydrocannabinol (THC) or vehicle and examined at different times after drug administration. Animals were tested in an open-field test, then sacrificed and their striata used for analyses of dopaminergic indices. Results were as follows. The administration of AEA or THC produced the expected inhibition of motor behavior. Thus, the administration of AEA decreased the ambulation and the frequency of stereotypic movements (in particular, the number of rears) and increased the time spent by the rats in inactivity. These effects were evident at 10 and 30 min after the administration of the cannabinoid agonist, but mostly disappeared at 60 min. Interestingly, motor inhibition was observed again around 2 or 3 h after the administration of AEA. This was a small but persistent effect (decreased ambulation followed by increased inactivity), because it was observed until at least 6 h after AEA administration. The other cannabimimetic, THC, was always able of decreasing the ambulation and the frequency of rearing and grooming behavior, and of increasing the time spent in inactivity. This effect was usually something more marked than the effect of AEA, but the most characteristic fact was its persistence at all times studies, even at 6 h after administration. These motor disturbances were accompanied by changes in the activity of nigrostriatal dopaminergic neurons. Thus, the administration of AEA decreased the activity of tyrosine hydroxylase (TH) in the striatum at 10 and 30 min after treatment, suggesting a decreased nigrostriatal activity parallel to the motor deficit observed at these times. This was followed by an increase in TH activity and dopamine and l-3,4-dihydroxyphenylacetic acid contents at 60 min after treatment, which would likely reflect a compensatory stimulation of these neurons, whereas restoration of control values was found at 180 min after AEA administration, suggesting that the motor deficit observed at this time was not dependent on dopaminergic influence. Paradoxically, the administration of THC only produced changes in dopaminergic activity at 60 min after treatment, similar to those seen with AEA, but was ineffective at the other times. In summary, AEA inhibits motor behavior in parallel to reductions in the activity of nigrostriatal dopaminergic neurons. However, this effect was of short duration, disappearing at 60 min after treatment, as compared with the inhibitory effect of THC on motor behavior which was observed at all times studied. Interestingly, a new AEA-induced inhibition of motor behavior, which was not accompanied by dopaminergic changes, appeared at longer times although its meaning remains to be determined.

The endogenous cannabinoid receptor ligand, anandamide, inhibits the motor behavior: role of nigrostriatal dopaminergic neurons

The present study has been designed to test whether the recently described endogenous ligand for the cannabinoid receptor, arachidonylethanolamide, termed anandamide, can mimic the effects produced by exogenous cannabinoids on motor behavior and to test possible neurochemical substrates for this potential effect. To this end, adult male rats were submitted to an acute i.p. injection of anandamide, Δ 9-tetrahydrocannabinol (THC) or vehicle. Animals were behaviorally tested ten minutes after injection of the drug and, then, sacrificed and their brains used for dopaminergic analyses. Ambulation was not significantly affected by the treatment with either THC or anandamide, but a very pronounced increase was observed in the time spent in inactivity in rats treated with either THC or anandamide. This was accompanied by a marked decrease in the frequency of spontaneous non-ambulatory activities, such as grooming and rearing, although only the administration of THC decreased shaking behavior. The anandamide-induced decrease in grooming was dose-dependent, but the decrease in rearing was higher with the dose of 3 mg/kg than with the dose of 10 mg/kg. The administration of anandamide also caused a dose-dependent decrease in the activity of tyrosine hydroxylase and in the ratio between the number of D1 and D2 receptors in the striatum. Moreover, the administration of 3 mg/kg of anandamide significantly decreased the contents of dopamine and L-3,4-dihydroxyphenylacetic acid in the striatum although lesser and higher doses were less effective. THC only tended to decrease these parameters. No changes were seen in dopaminergic activity in the limbic forebrain after either cannabimimetics. In summary, anandamide, as well as THC, decreases motor behavior. This effect was paralleled by a reduction in the activity of nigrostriatal dopaminergic neurons. However, subtle differences in the behavioral and neurochemical effects between anandamide and THC could be observed.

Activation of mesolimbic dopaminergic neurons following central administration of histamine is mediated by H1 receptors

The effect of intracerebroventricular administration of histamine on the activity of mesolimbic and nigrostriatal dopaminergic (DA) neurons was determined in male rats. The activity of these neurons was estimated by measuring: (1) the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor, and (2) the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens and striatum, which contain the terminals of these neurons. Central administration of histamine increased both DOPA accumulation and DOPAC concentrations in the nucleus accumbens, but was without effect in the striatum. The increase in DOPAC concentrations in the nucleus accumbens occurred within 10 min and was sustained for at least 120 min. The H1 antagonist mepyramine blocked whereas the H2 antagonist zolantidine did not affect histamine-induced increases in DOPAC concentrations in the nucleus accumbens. Neither mepyramine nor zolantidine affected basal DOPAC concentrations in the nucleus accumbens. These results indicate that central administration of histamine stimulates mesolimbic DA neurons through an action at the H1 receptor, but has no effect upon the activity of nigrostriatal DA neurons.

The activity of catecholamine synthesis in the hypothalamus of female normotensive Wistar Kyoto and spontaneously hypertensive rats

The activity of catecholamine synthesis in the hypothalamus, as determined by the rate of 3, 4-dihydroxyphenylalanine (DOPA) accumulation after the administration of a DOPA decar☐ylase inhibitor, was compared among Wistar, spontaneously hypertensive (SH), and genetically matched normotensive Wistar Kyoto (WKY) rats. DOPA accumulation in the median eminence, an index of the activity of tuberoinfundibular dopaminergic neurons, was greater in SH rats than Wistar and WKY rats while DOPA accumulation in the medial preoptic area was smaller in Wistar rats than SH and WKY rats. No strain difference was found in DOPA accumulation in the corpus striatum, which represents the activity of nigrostriatal dopaminergic neurons. These results suggest that there are differences in catecholamine synthesis in the hypothalamus not only between SH and WKY rats but also between WKY and Wistar rats.

Effect of electrical stimulation of the arcuate nucleus on neurochemical estimates of tuberoinfundibular and tuberohypophysical dopaminergic neuronal activities

The activity of nigrostriatal dopaminergic neurons has been estimated biochemically by measuring the rates of dopamine (DA) synthesis (accumulation of dihydroxyphenylalanine (DOPA) after NSD 1015) and turnover (decline of DA concentrations after α-methyltyrosine) in the striatum. It has been assumed that the activities of tuberinfundibular dompaminergic (TIDA) and tuberohypophysial dopaminergic (THDA) neurons can also be estimated by making the same measurements in the terminals of these neurons in the median eminence and the posterior pituitary, respectively. In the present study, this assumption was tested directly by measuring the rates of DA synthesis and turnover in the median eminence and posterior pituitary following electrical stimulation of TIDA and THDA cell bodies in the arcuate nucleus. Electrical stimulation of the arcuate nucleus increased the rate of DOPA accumulation and the α-methyltyrosine-induced decline of DA concentrations in the median eminence and in the neural and intermediate lobes of the posterior pituitary. γ-Butyrolactone (GBL), an anesthetic that selectivelly inhibits DA impusle flow, reduced by the rates of DA synthesis and turnover in the median eminence. GBL also increased prolactin secretion which is tonically inhibited by DA released from TIDA neurons. Serum prolactin levels were significantly decreased by arcuate nucleus stimulation in GBL-anesthetized rats. These results indicate that the rates of DA synthesis and turnover within the median eminence and posterior pituitary reflect the activities of TIDA and THDA neurons, respectively.

Effect of superficial radial nerve stimulation on the activity of nigro-striatal dopaminergic neurons in the cat: role of cutaneous sensory input.

The release of 3H-dopamine (DA) continuously synthesized from 3H-tyrosine was measured in the caudate nucleus (CN) and in the substantia nigra (SN) in both sides of the brain during electrical stimulation of the superficial radial nerve in cats lightly anaesthetized with halothane. Use of appropriate electrophysiologically controlled stimulation led to selective activation of low threshold afferent fibers whereas high stimulation activated all cutaneous afferents. Results showed that low threshold fiber activation induced a decreased dopaminergic activity in CN contralateral to nerve stimulation and a concomitant increase in dopaminergic activity on the ipsilateral side. Stimulation of group I and threshold stimulation of group II afferent fibers induced changes in the release of 3H-DA mainly on the contralateral CN and SN and in the ipsilateral CH. High stimulation was followed by a general increase of the neurotransmitter release in the four structures. This shows that the nigro-striatal dopaminergic neurons are mainly--if not exclusively--controlled by cutaneous sensory inputs. This control, primarily inhibitory in the side contralateral to the stimulation, seems rather non-specific when high threshold cutaneous fibers are also activated. Such activations could contribute to reestablish sufficient release of DA when the dopaminergic function is impaired as in Parkinson's disease.

The striatonigral substance P pathway and dopaminergic mechanisms.

The biosynthesis of 35S-labelled substance P (SP) has been shown in the rat striatum after continuous delivery of [35S]methionine through a push-pull cannula. In vitro and in vivo studies of SP release have demonstrated that release of the peptide depends on nerve activity and is regulated by presynaptic influences. When the 125I-labelled Bolton and Hunter SP derivative was used, specific binding was observed on mesencephalic cells of the mouse embryo in primary cultures and various characteristics of SP receptors could be determined. Finally, the effects of facilitation or prevention of SP transmission in the substantia nigra on the activity of nigrostriatal dopaminergic neurons have been studied in vivo in the cat by measuring dopamine released from nerve terminals and dendrites. The results obtained indicate that the striatonigral SP neurons exert a tonic facilitatory influence on the dopaminergic neurons.