Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 34 (28), 9310-8

Aging Decreases L-type Calcium Channel Currents and Pacemaker Firing Fidelity in Substantia Nigra Dopamine Neurons

Affiliations

Aging Decreases L-type Calcium Channel Currents and Pacemaker Firing Fidelity in Substantia Nigra Dopamine Neurons

Sarah Y Branch et al. J Neurosci.

Abstract

Substantia nigra dopamine neurons are involved in behavioral processes that include cognition, reward learning, and voluntary movement. Selective deterioration of these neurons is responsible for the motor deficits associated with Parkinson's disease (PD). Aging is the leading risk factor for PD, suggesting that adaptations occurring in dopamine neurons during normal aging may predispose individuals to the development of PD. Previous studies suggest that the unique set of ion conductances that drive spontaneous, rhythmic firing of action potentials could predispose substantia nigra dopamine neurons to selective neurodegeneration. Here we show, using patch-clamp electrophysiological recordings in brain slices, that substantia nigra dopamine neurons from mice 25-30 months of age (old) have comparable membrane capacitance and input resistance to neurons from mice 2-7 months of age (young). However, neurons from old mice exhibit slower firing rates, narrower spike widths, and more variable interspike intervals compared with neurons from young mice. Dopamine neurons from old mice also exhibit smaller L-type calcium channel currents, providing a plausible mechanism that likely contributes to the changes in impulse activity. Age-related decrements in the physiological function of dopamine neurons could contribute to the decrease in voluntary movement and other dopamine-mediated behaviors observed in aging populations. Furthermore, as pharmacological antagonism of L-type calcium channels has been proposed as a potential treatment for the early stages of PD, our results could point to a limited temporal window of opportunity for this therapeutic intervention.

Keywords: L-type calcium channels; dopamine; firing; mouse; pacemaking.

Figures

Figure 1.
Figure 1.
DA neurons from aging mice maintain their appearance and basal electrophysiological parameters. Mouse brain slices from old and young mice were examined with an upright microscope using gradient contrast optics, and DA neurons of the substantia nigra pars compacta were initially identified by location and appearance. A, Brain slices from old mice (25–30 months) showed visually intact DA neurons similar to young adult (2–7 months) mice. B, Cell membrane capacitance was measured and was not different in neurons from old mice compared with their young counterparts. C, Input resistance was also similar in DA neurons from old and young mice. Boxplot whiskers indicate maximum and minimum data points. D, In a subset of experiments, 0.2% neurobiotin was included in the intracellular solution and slices were fixed with PFA for immunohistochemistry; 70% of neurons costained for neurobiotin and TH, an indicator of catecholaminergic neurons. When whole-cell patch-clamp experiments exceeded 15 min, a nearby neuron was also briefly patch-clamped to account for the fact that false negatives occur when TH staining is attempted after long recordings (E, blue arrowheads) (Zhang et al., 2010); and in 9 of 10 remaining cases, the nearby neuron dual-stained for neurobiotin and TH (E, white arrowheads).
Figure 2.
Figure 2.
Pacemaker firing fidelity is compromised in DA neurons from aging mice. Intrinsic firing of substantia nigra DA neurons was monitored using loose cell attached extracellular recordings. A, The majority of DA neurons from young mice fired in a characteristic rhythmic pattern. B, In contrast, DA neurons from old mice (in red) often exhibited more varied firing that, in extreme cases, included gaps of several seconds with no spikes. C, D, Overall firing rates of neurons from old mice were slower than their young counterparts. Interspike intervals were calculated over 30 s of firing, and neurons from old mice exhibited significantly larger coefficients of variation of the interspike interval compared with neurons from young mice, indicative of a more variable firing pattern (E). F, Cumulative frequency analysis revealed a bimodal distribution for the coefficient of variation, with values <0.10 generally indicative of pacemaker firing. G, H, The kinetics of the extracellular waveforms were also analyzed and indicated that DA neurons from old mice exhibit narrower spike widths compared with their young counterparts. *p < 0.05. **p < 0.01. ****p < 0.0001.
Figure 3.
Figure 3.
Currents associated with HCN and SK channels are not altered in aging mice. A, The AHC, which is primarily mediated by SK channels, was calculated as the area under the curve of the outward current beginning 20 ms after a 100 ms depolarization (green shading). B, Summary data indicate that AHCs in DA neurons did not differ with the age of the mice. C, IH currents were calculated as the amplitude of the slow inward current that develops during a 1 s hyperpolarization step. D, Summary data show that IH currents in neurons from old mice were similar to currents in neurons from young mice. Boxplot whiskers indicate maximum and minimum data points.
Figure 4.
Figure 4.
L-type calcium channel-mediated currents are smaller in DA neurons from aging mice. A, Using a cesium-based intracellular solution, whole-cell patch-clamp recordings of DA neurons were obtained and slow depolarizing voltage ramps were applied over 10 s. B, Currents subtracted after bath perfusion of TTX indicated that there was no effect of age on voltage-gated sodium channel-mediated currents. C, Two sample traces of DA neurons from young mice illustrate a nimodipine-sensitive inward current. A subset of cells exhibited brief inward current deflections in the presence of TTX that were not blocked by nimodipine (e.g., bottom trace). D, Summarized data indicate a dramatic decrease in the amplitude of near-threshold nimodipine-sensitive inward currents in neurons from old versus young mice. E, Linear regression analysis indicated a significant inverse relationship between the age of the mouse and the maximum nimodipine-sensitive currents (dashed line; p = 0.0018). Linear regression analysis of only the data points from young mice indicated no significant difference in the nimodipine-sensitive currents within that subset of ages (inset, F(1,19) = 0.87, p = 0.36). Using the cell attached method, firing rates of neurons from old and young mice were examined before and after bath application of 3 μm nimodipine. F, Summarized data indicate a differential effect of nimodipine on firing rate in DA neurons from old and young mice (p < 0.0001). Pacemaking neurons from both old (G) and young (H) mice exhibited similar nimodipine-induced increases in the coefficient of variation of their interspike intervals.

Comment in

Similar articles

See all similar articles

Cited by 19 articles

See all "Cited by" articles

Publication types

Substances

Feedback