Endogenous Calcium Buffering Capacity of Substantia 14 Nigral Dopamine Neurons

42 43 Dopamine (DA) containing cells from the substantia nigra pars compacta (SNc) play a 44 major role in the initiation of movement. Loss of these cells results in Parkinson’s 45 disease (PD). Changes in [Ca]i elicit several events in DA cells, including spike 46 afterhyperpolarizations (AHPs) and subthreshold oscillations underlying autonomous 47 firing. Continuous Ca load due to Ca-dependent rhythmicity has been proposed to 48 cause death of DA cells in PD and normal aging (Surmeier, 2007). Because of the 49 physiological and pathophysiological importance of [Ca]i in DA cells, we characterized 50 their intrinsic Ca-buffering capacity (Ks) in brain slices. We introduced a fluorescent 51 Ca-sensitive exogenous buffer (200 μM fura-2) and cells were tracked from break-in 52 until steady state by stimulating with a single action potential (AP) every 30s and 53 measuring the Ca-transient from the proximal dendrite. DA neurons filled 54 exponentially with a τ of ~5-6 min. [Ca]i was assumed to equilibrate between the 55 endogenous Ca buffer and the exogenous Ca indicator buffer. Intrinsic buffering was 56 estimated by extrapolating from the linear relationships between the amplitude or time 57 constant of the Ca transients vs. [fura-2]. Extrapolated Ca-transients in the absence of 58 fura-2 had mean peak amplitudes of 293.7 ± 65.3 nM and τ = 124 ± 13 ms (P13-P17). 59 Intrinsic buffering increased with age in DA neurons. For cells from animals P13-P17, 60 ΚS was estimated to be ~110 (n = 20). In older animals (P25-P32), the estimate was ~179 61 (n = 10). These relatively low values may reflect the need for rapid Ca signaling, e.g., 62 to allow activation of sK channels, which shape autonomous oscillations and burst firing. 63 Low intrinsic buffering may also make DA cells vulnerable to Ca-dependent pathology. 64 65