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The loss of dopaminergic neurons in the substantia nigra compacta followed by striatal dopamine depletion is a hallmark of Parkinson's disease. After dopamine depletion, dopaminergic D2receptor (D2R)-class supersensitivity develops in striatal neurons. The supersensitivity results in an enhanced modulation of Ca2+currents by D2R-class receptors. However, the relative contribution of D2R, D3R, and D4R types to the supersensitivity, as well as the mechanisms involved, have not been elucidated. In this study, whole cell voltage-clamp recordings were performed to study Ca2+current modulation in acutely dissociated striatal neurons obtained from rodents with unilateral 6-hydroxydopamine lesions in the substantia nigra compacta. Selective antagonists for D2R, D3R, and D4R types were used to identify whether the modulation by one of these receptors experiences a selective change after dopaminergic denervation. It was found that D3R-mediated modulation was particularly enhanced. Increased modulation targeted CaV2.1 (P/Q) Ca2+channels via the depletion of phosphatidylinositol 4,5-bisphosphate, an intracellular signaling cascade hard to detect in control neurons and hypothesized as being amplified by dopamine depletion. An imbalance in the striatal expression of D3R and its splice variant, D3nf, accompanied enhanced D3R activity. Because CaV2.1 Ca2+channels mediate synaptic GABA release from the terminals of striatal neurons, reinforcement of their inhibition by D3R may explain in part the profound decrease in synaptic strength in the connections among striatal projection neurons observed in the dopamine-depleted striatum.

Familial hemiplegic migraine type-1 FHM-1 is caused by missense mutations in the CACNA1A gene that encodes the α1A pore-forming subunit of CaV2.1 Ca2+ channels. We used knock-in (KI) transgenic mice harboring the pathogenic FHM-1 mutation R192Q to study neurotransmission at the calyx of Held synapse and cortical layer 2/3 pyramidal cells (PCs). Using whole cell patch-clamp recordings in brain stem slices, we confirmed that KI CaV2.1 Ca2+ channels activated at more hyperpolarizing potentials. However, calyceal presynaptic calcium currents (IpCa) evoked by presynaptic action potentials (APs) were similar in amplitude, kinetic parameters, and neurotransmitter release. CaV2.1 Ca2+ channels in cortical layer 2/3 PCs from KI mice also showed a negative shift in their activation voltage. PCs had APs with longer durations and smaller amplitudes than the calyx of Held. AP-evoked Ca2+ currents (I Ca) from PCs were larger in KI compared with wild-type (WT) mice. In contrast, when ICa was evoked in PCs by calyx of Held AP waveforms, we observed no amplitude differences between WT and KI mice. In the same way, Ca2+ currents evoked at the presynaptic terminals (IpCa)of the calyx of Held by the AP waveforms of the PCs had larger amplitudes in R192Q KI mice that in WT. These results suggest that longer time courses of pyramidal APs were a key factor for the expression of a synaptic gain of function in the KI mice. In addition, our results indicate that consequences of FHM-1 mutations might vary according to the shape of APs in charge of triggering synaptic transmission (neurons in the calyx of Held vs. excitatory/inhibitory neurons in the cortex), adding to the complexity of the pathophysiology of migraine. Fil: González Inchauspe, Carlota María Fabiola. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Urbano Suarez, Francisco Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Di Guilmi, Mariano Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Forsythe, Ian D.. University of Leicester; Reino Unido Fil: Ferrari, Michel D.. Leiden University; Países Bajos Fil: Maagdenberg, Arn M. J. M. van den. Leiden University; Países Bajos Fil: Uchitel, Osvaldo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina

Besides a reduction of L-type Ca2+-currents (CaV1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through CaV2.1 (P/Q) and CaV2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.

CaV2.1 Ca2+ channels have a dominant and specific role in initiating fast synaptic transmission at central excitatory synapses, through a close association between release sites and calcium sensors. Familial hemiplegic migraine type-1 (FHM-1) is an autosomal-dominant subtype of migraine with aura, caused by missense mutations in the CACNA1A gene that encodes the α1A pore-forming subunit of CaV2.1 channel. We used knock-in (KI) transgenic mice harbouring the FHM-1 mutation R192Q to study the consequences of this mutation in neurotransmission at the giant synapse of the auditory system formed by the presynaptic calyx of Held terminal and the postsynaptic neurons of the Medial Nucleus of the Trapezoid Body (MNTB). Although synaptic transmission seems unaffected by low frequency stimulation in physiological Ca2+ concentration, we observed that with low Ca2+concentrations (< 1 mM) excitatory postsynaptic currents (EPSCs) showed increased amplitudes in R192Q KI mice compared to WT, meaning significant differences in the non-linear calcium-dependence of nerve-evoked transmitter release. In addition, when EPSCs were evoked by broadened presynaptic action potentials (achieved by inhibition of K+channels) via Cav2.1 triggered exocytosis, the R192Q KI mice exhibited further enhancement of EPSC amplitude and charge compared with WT mice. Repetitive stimulation of afferent axons to the MNTB at different frequencies caused short term depression of EPSCs that recovered significantly faster in R192Q KI than in WT mice. Faster recovery in R192Q KI mice was prevented by the calcium chelator EGTA-AM, pointing to enlarged residual calcium as a key factor in accelerating the replenishment of synaptic vesicles. Fil: González Inchauspe, Carlota María Fabiola. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Urbano Suarez, Francisco Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Di Guilmi, Mariano Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Ferrari, Michel D.. Leiden University Medical Centre; Países Bajos Fil: Van den Maagdenberg, Arn M. J. M.. Leiden University Medical Centre; Países Bajos Fil: Forsythe, Ian. University of Leicester; Reino Unido Fil: Uchitel, Osvaldo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina