Determination of release of endogenous dopamine from superfused substantia nigra slices

Partial depletion of dopamine in substantia nigra impairs motor performance without altering striatal dopamine neurotransmission

Previous data indicate that the release of somatodendritic dopamine in substantia nigra influences motor activity and coordination, but the relative importance of somatodendritic dopamine release vs. terminal striatal dopamine release remains to be determined. We utilized simultaneous measurement of dopamine neurotransmission by microdialysis and motor performance assessment by rotarod test to investigate the effects of local dopamine depletion in rats. The vesicular monoamine transporter inhibitor tetrabenazine (100 microm) was administered locally in substantia nigra as well as in striatum. Nigral tetrabenazine administration decreased nigral dopamine dialysate concentrations to 7% of baseline and whole-tissue dopamine content by 60%. Nigral dopamine depletion was associated with a reduction in motor performance to 73 +/- 6% of pretreatment value, but did not alter dialysate dopamine concentrations in the ipsilateral striatum. Striatal tetrabenazine administration decreased striatal dopamine dialysate concentrations to 5% of baseline and doubled the somatodendritic dopamine response to motor activity, but it was not associated with changes in motor performance or dopamine content in striatal tissue. Simultaneous treatment of substantia nigra and striatum reduced motor performance to 58 +/- 5% of the pretreatment value. The results of this study indicate that partial depletion of nigral dopamine stores can significantly impair motor functions, and that increased nigral dopamine release can counteract minor impairments of striatal dopamine transmission.

Influence of R-type (Cav2.3) and t-type (Cav3.1-3.3) antagonists on nigral somatodendritic dopamine release measured by microdialysis

The release of dopamine from soma and dendrites of dopaminergic neurons in substantia nigra has been reported to be calcium-dependent, but it remains to be determined which calcium channels mediate this effect. We have used in vivo microdialysis in rat substantia nigra and striatum to investigate the effect of Ca(v)3.1-3.3 (T-type) and Ca(v)2.3 (R-type) calcium channel antagonists on somatodendritic and terminal dopamine release. Local reverse dialysis administration of 0.1-10 microM of the Ca(v)2.3 inhibitor SNX-482, or 100 microM of mibefradil, decreased the concentrations of dopamine and its metabolites in dialysate from substantia nigra, whereas 1 microM mibefradil or 40-80 microM nickel(II) induced an increase in nigral dialysate dopamine concentrations. Dopamine concentrations in striatal dialysates were decreased only by 10 microM of SNX-482 or 100 microM of mibefradil. Nickel(II) induced an increase in striatal dialysate dopamine concentration similar to that in substantia nigra. The results indicate a role for Ca(v)2.3 (R-type) voltage sensitive calcium channels in the calcium dependency of somatodendritic dopamine release, but argue against a calcium dependency mediated substantially by Ca(v)3.1-3.3 (T-type) channels.

Evidence for different exocytosis pathways in dendritic and terminal dopamine release in vivo

Although dendritic release was first proposed in the 1970s, the mechanism of release is still subject to debate. We have used in vivo microdialysis to study the acute effects of botulinum toxin A, B and tetanus toxin injected in the substantia nigra or striatum of freely moving rats. Spontaneous and evoked dopamine release decreased in both regions after treatment with the SNAP-25 (synaptosome-associated protein of 25 kDa) cleaving protease botulinum toxin A (1000 mouse lethal doses, MLD). Tetanus toxin (4000 MLD) did not significantly change spontaneous or evoked dopamine release in striatum or in the substantia nigra. Another synaptobrevin cleaving protease, botulinum toxin B, inhibited release in the striatum by 55% but did not affect dopamine release when injected in the substantia nigra. The results indicate that both terminal and somatodendritic dopamine release need intact SNAP-25 to occur, but somatodendritic dopamine release in contrast to terminal release depends on a botulinum toxin B resistant pathway.

Effects of drugs interfering with sodium channels and calcium channels on the release of endogenous dopamine from superfused substantia nigra slices

The importance of voltage-dependent sodium channels and different types of voltage-sensitive calcium channels for depolarisation-induced release of endogenous dopamine from dendrites and cell bodies in superfused guinea pig substantia nigra slices was investigated. The stimulatory effect of veratridine (10 microM) on dopamine release was only marginally attenuated in Ca(2+)-free medium but was completely blocked by tetrodotoxin (1 microM) and by the dopamine reuptake inhibitor GBR 12909 (10 microM). Low extracellular concentration of Na+ stimulated the dopamine release. Potassium-evoked dopamine release was completely Ca(2+)-dependent, not blocked by GBR 12909 and partially blocked by tetrodotoxin. Nifedipine (20 microM), omega-conotoxin GVIA (0.5 microM), penfluridol (5 microM), and Ni2+ (20 microM) had no effect, amiloride (1 mM) attenuated and neomycin (350 microM), and omega-agatoxin IVA (1 microM) almost totally blocked the potassium-induced dopamine release. The results suggest that veratridine released dopamine mostly by reversing the dopamine transporter. High concentrations of potassium induced release of nigral dopamine by opening of voltage-sensitive calcium channels of P/Q type but not L-type, N-type and probably not T-type. The depolarisation evoked by high concentrations of potassium seems to open voltage-sensitive calcium channels both by the depolarisation induced by potassium per se and by the secondary depolarisation induced by opening of voltage-dependent sodium channels.

The effects of methylmercury on endogenous dopamine efflux from mouse striatal slices

The present study investigated the effects of in vitro methylmercury (MeHg) exposure on endogenous dopamine (DA) efflux from mouse striatal slices. MeHg produced a concentration-dependent increase in the spontaneous efflux of DA which was independent of the availability of Ca2+ in the superfusion medium. The Ca(2+)-dependent K(+)-evoked release of DA was significantly enhanced by 50 and 100 microM MeHg. This increase could not be solely accounted for by the MeHg-induced increased in spontaneous DA efflux. The K(+)-stimulated efflux of DA was enhanced by MeHg in both the presence and absence of Ca2+ in the superfusion medium, suggesting that under depolarizing conditions, DA efflux induced by MeHg has a Ca(2+)-independent component. The alterations in DA efflux occurred at concentrations of MeHg previously found in the CNS of animals exhibiting symptoms of MeHg intoxication suggesting that alterations in DA neurotransmission in the striatum may contribute to the symptoms of MeHg toxicity.

Direct monitoring of dopamine and 5-HT release in substantia nigra and ventral tegmental area in vitro

Fast-scan cyclic voltammetry with carbon fibre microelectrodes was used to detect endogenous dopamine (DA) and 5-hydroxytryptamine (5-HT) release from three distinct regions of guinea-pig mid-brain in vitro: rostral and caudal substantia nigra (SN) and the ventral tegmental area (VTA). Previous electrophysiological studies have demonstrated that cells of the caudal SN and the VTA have similar characteristics, whereas cells in the rostral SN have distinctly different properties. In the present study, we confirmed that each region has tyrosine hydroxylase-positive neurons and determined, using high-performance liquid chromatography, that DA levels were similar in rostral and caudal SN, but lower in SN than in VTA. In each region, application of veratrine, which was shown by intracellular recordings to have a reversible depolarising action, evoked a signal attributable to DA and distinguishable from that of 5-HT. Release signals were monitored every 250 ms with a spatial resolution of less than 50 microns.l DA release was calcium-dependent and was not detectable in a catecholamine-poor area such as the cerebellum, or in mid-brain tissue pre-treated with reserpine. Within the normal mid-brain, the amount of DA released was correlated with tissue content in that it was higher in the VTA than in either region of SN. It is concluded that DA released from somato-dendritic parts of mid-brain neurons exhibits site-specific variation. This is the first report of direct monitoring of DA and 5-HT release from these regions with in situ electrodes and demonstrates the utility of fast-scan cyclic voltammetry to investigate the mechanisms and possible non-classical functions of somato-dendritic DA release.

Uric acid is reduced in the substantia nigra in Parkinson's disease: effect on dopamine oxidation

Postmortem caudate and substantia nigra tissue samples from human parkinsonian patients (PD) and age-matched controls (NC) were analyzed for uric acid (UA), dopamine (DA), and ascorbic acid (AA) by HPLC/UV/ED. Uric acid and DA levels were significantly lower in the substantia nigra of PD by 54% and 85%, respectively. In the caudate, DA levels were significantly lower while UA levels were nonsignificantly reduced (0.10 < p < 0.05). Ascorbic acid levels were not significantly different from the controls in either brain region. Conditions favorable for oxidative stress were evaluated by measuring the oxidation of DA in individual brain homogenates. The rate constant for DA oxidation in control caudate was 0.34 x 10(-2) min-1 and in parkinsonian caudate was 4.20 x 10(-2) min-1. In control and parkinsonian substantia nigra DA oxidation rate constants were 2.82 x 10(-2) min-1 and 4.57 x 10(-2) min-1, respectively. Addition of UA or catalase to parkinsonian homogenate decreased the rate of DA oxidation, while addition of uricase to control homogenate increased the rate of DA oxidation. The data support the hypothesis that UA is decreased in nigrostriatal dopamine neurons in parkinsonian patients which contributes to an environment susceptible to oxidative stress, particularly through dopamine oxidation reactions.