Differential Somatic Ca2+ Channel Profile in Midbrain Dopaminergic Neurons

Dopaminergic (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 Ca(2+) channels and that the density of L-type Ca(2+) channels is higher in these DA neurons than in those located in the ventral tegmental area (VTA). This might lead to a higher Ca(2+) 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 are indeed higher in the somata of rat SNc DA neurons and that this current undergoes less inactivation in this region. Nonstationary fluctuation analysis 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 Ca(2+) entry through L-type channels in SNc 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.

SIGNIFICANCE STATEMENT

Dopamine neurons from the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) are involved in various brain functions, such as movement initiation and goal directed behavior, respectively. This work shows that, although both neurons fire in a similar regular and slow pacemaker mode, this firing activity is supported by different calcium channel landscapes. Indeed, the L-type calcium current is larger in the soma of dopamine neurons of the SNc, leading to a higher charge transfer through L-type channels during pacemaking and bursting. Therefore, these neurons may be physiologically exposed to a larger stress than their neighbors from the VTA.

Activity deprivation reduces miniature IPSC amplitude by decreasing the number of postsynaptic GABA(A) receptors clustered at neocortical synapses

Maintaining the proper balance between excitation and inhibition is necessary to prevent cortical circuits from either falling silent or generating epileptiform activity. One mechanism through which cortical networks maintain this balance is through the activity-dependent regulation of inhibition, but whether this is achieved primarily through changes in synapse number or synaptic strength is not clear. Previously, we found that 2 d of activity deprivation increased the amplitude of miniature EPSCs (mEPSCs) onto cultured visual cortical pyramidal neurons. Here we find that this same manipulation decreases the amplitude of mIPSCs. This occurs with no change in single-channel conductance but is accompanied by a reduction in the average number of channels open during the mIPSC peak and a reduction in the intensity of staining for GABA(A) receptors (GABA(A)Rs) at postsynaptic sites. In addition, the number of synaptic sites that express detectable levels of GABA(A)Rs was decreased by approximately 50% after activity blockade, although there was no reduction in the total number of presynaptic contacts. These data suggest that activity deprivation reduces cortical inhibition by reducing both the number of GABA(A)Rs clustered at synaptic sites and the number of functional inhibitory synapses. Because excitatory and inhibitory synaptic currents are regulated in opposite directions by activity blockade, these data suggest that the balance between excitation and inhibition is dynamically regulated by ongoing activity.

Distance-dependent increase in AMPA receptor number in the dendrites of adult hippocampal CA1 pyramidal neurons

The Schaffer collateral pathway provides hippocampal CA1 pyramidal cells with a fairly homogeneous excitatory synaptic input that is spread out across several hundred micrometers of their apical dendritic arborizations. A progressive increase in synaptic conductance, with distance from the soma, has been reported to reduce the location dependence that should result from this arrangement. The excitatory synaptic contacts within this pathway primarily use AMPA- and NMDA-type glutamate receptors. To investigate the underlying mechanism of the increased distal excitatory postsynaptic conductance, we used outside-out patches and a fast application system to characterize the properties and distribution of synaptic glutamate receptors across the range of apical dendrites receiving Schaffer collateral input. We observed an approximately twofold increase in AMPA-mediated current amplitude (0.3-0.6 nA) in the range of CA1 apical dendrites that receive a uniform density of Schaffer collateral input (approximately 100-250 micrometer from soma). NMDA-mediated current amplitude, however, remained unchanged. We analyzed the current kinetics, agonist affinity, single-channel conductance, maximum open probability, and reversal potential of AMPA receptors and did not find any differences. Instead, the number of AMPA receptors present in our patches increased approximately twofold. These data suggest that an increase in the number of AMPA receptors present at distal synapses may play an important role in the distance-dependent scaling of Schaffer collateral synapses.