Profils d'expression différents des canaux Ca2+ somatiques dans les neurones dopaminergiques du mésencéphale

peer reviewedDopaminergic (DA) neurons located in the ventral midbrain continuously generate a slow endogenous pacemaker activity, the mechanism of which is still debated. It has been suggested that, in the substantia nigra pars compacta (SNc), the pacemaking relies more on Ca2+ channels and that the density of L type Ca2+ channels is higher in these DA neurons than in those located in the ventral tegmental area (VTA). This might lead to a higher Ca2+ load in SNc DA neurons, and explain their higher susceptibility to degeneration. However, direct evidence for this hypothesis is lacking. We found that the L-type current and channel density is indeed higher in the somata of rat SNc DA neurons, and that this current undergoes less inactivation in this region. Non stationary fluctuation analysis (NSFA) measurements showed a much higher number of L-type channels in the soma of SNc DA neurons, as well as a smaller single channel conductance, pointing to a possible different molecular identity of L-type channels in DA neurons from the two areas. A major consequence of this is that pacemaking and even more so bursting are associated with a larger Ca2+ entry through L-type channels in SN DA neurons than in their VTA counterparts. Our results establish a molecular and functional difference between two populations of midbrain DA neurons that may contribute to their differential sensitivity to neurodegeneration.Caractérisation des canaux calciques somatiques au sein des neurones dopaminergiques mésencéphalique

Modulation of NMDA receptor mediated excitatory synaptic currents in dopamine neurons of the substantia nigra.

Dopamine (DA) neurons of the substantia nigra pars compacta (SNc) exhibit two main firing modes, spontaneous single action potential (AP) firing and bursting. The amount of DA released by these neurons in target areas depends on the presynaptic AP firing pattern and is essential to modulate several aspects of behavior such as the control of movement. Bursting activity is mediated by excitatory afferents and specifically by the activation of NMDA receptors (NMDARs). However, the level of activation of NMDARs at these synapses during spontaneous synaptic activity is unknown. We assessed the occupancy of the glycine binding sites of the NMDAR by testing the effects of coagonists and by blocking the uptake of glycine on spontaneous excitatory postsynaptic currents (sEPSCs) in DA neurons of the SNc. Parasagittal midbrain slices (300 – 350 µm thickness) were cut from the brains 16- to 26- days old Wistar rats using a vibratome. Whole-cell recordings were made using warm (~35C°) standard artificial cerebrospinal fluid. sEPSCs were pharmacologically isolated and recorded at +40 and -70 mV. A large NMDAR-sEPSC component was present at +40 mV as revealed by the application of 50 µM D-AP5 (n=8, P < 0.001). At -70 mV, D-AP5 had no effect on the EPSC, as expected (n=7, P = 0.52). The mean amplitude of the average NMDAR sEPSC measured at +40 mV increased significantly from 8.6 ± 0.8 pA in control conditions to 10.9 ± 1.0 pA during bath application of 300 μM glycine (n=10, P < 0.001). We also investigated the effect of D-serine, the other coagonist at the NMDAR glycine site. The mean NMDAR EPSC amplitude measured at +40 mV showed a tendency toward larger values, from 6.8 ± 0.8 pA in control conditions to 8.8 ± 1.1 pA in the presence of 100 µM D-serine (n=4). Blockade of the glycine transporter-1 (GlyT1) significantly increased the mean amplitude of NMDAR sEPSCs measured at +40 mV from 7.1 ± 0.5 pA in control conditions to 10.5 ± 0.8 pA in the presence of 5 μM NFPS (n=8, P < 0.001). At -70 mV, NFPS had no effect on the AMPAR-sEPSC component, as expected (n=8, P = 0.23). In conclusion, our results indicate that the NMDAR glycine sites are not saturated by glycine and D-serine during sEPSCs in control conditions.U.N002.13; T.N0015.1

La sous-saturation du site de la glycine des récepteurs NMDA permet la régulation de l'activité en bouffée dans les neurones à dopamine de la substance noire compacte de rat juvénile

The activation of N-methyl-D-aspartate receptors (NMDARs) in substantia nigra pars compacta (SNc) dopamine (DA) cells is central to generate the bursting activity, a phasic signal linked to DA related behaviors via the change in postsynaptic DA release. NMDARs are recruited during excitatory synaptic transmission by glutamate release but the glycine site level of occupancy of these receptors during basal action potential-dependent activity is not known for SNc DA neurons. We explored NMDARdependent signals during exogenous applications of co-agonists in midbrain slices from juvenile rats. We found that both glycine and D-serine strengthened the NMDAR-dependent component of excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner. EPSCs were also increased by endogenous glycine via the blockade of the glycine transport. The glycine site of NMDARs contributing to synaptic transmission is therefore subsaturated. The behaviorally relevant burst firing was more sensitive to exogenous D-serine and endogenous glycine than to exogenous glycine. The mechanisms regulating the availability of the co-agonists exert consequently a critical influence on the excitability of DA neurons via NMDARs. The modulation of the phasic firing in DA neurons by ambient NMDAR co-agonists may be important for nigral information processing and downstream motor-related behavior.Physiology of dopaminergic neuron