Somatodendritic dopamine release: recent mechanistic insights

Abstract

Dopamine (DA) is a key transmitter in motor, reward and cogitative pathways, with DA dysfunction implicated in disorders including Parkinson's disease and addiction. Located in midbrain, DA neurons of the substantia nigra pars compacta project via the medial forebrain bundle to the dorsal striatum (caudate putamen), and DA neurons in the adjacent ventral tegmental area project to the ventral striatum (nucleus accumbens) and prefrontal cortex. In addition to classical vesicular release from axons, midbrain DA neurons exhibit DA release from their cell bodies and dendrites. Somatodendritic DA release leads to activation of D2 DA autoreceptors on DA neurons that inhibit their firing via G-protein-coupled inwardly rectifying K(+) channels. This helps determine patterns of DA signalling at distant axonal release sites. Somatodendritically released DA also acts via volume transmission to extrasynaptic receptors that modulate local transmitter release and neuronal activity in the midbrain. Thus, somatodendritic release is a pivotal intrinsic feature of DA neurons that must be well defined in order to fully understand the physiology and pathophysiology of DA pathways. Here, we review recent mechanistic aspects of somatodendritic DA release, with particular emphasis on the Ca(2+) dependence of release and the potential role of exocytotic proteins.

Keywords: exocytosis; midbrain slices; voltammetry; volume transmission.

Figure 1.

Tyrosine hydroxylase immunoreactive (TH-ir) cell bodies and processes in midbrain. (a) Low magnification view of SNc, SNr and VTA revealed by TH immunostaining. The SNc and more medially located VTA contain a mix of TH-ir somata and dendrites, whereas SNr is nearly free of TH-ir cell bodies, but contains abundant TH-ir dendrites. (b) Higher magnification view of the portion of SNc and SNr outlined in (a) to show the mixture of somata and dendrites in the SNc and primarily ventrally projecting dendrites in the SNr. Scale bars, 50 µm (adapted from [27]).

Figure 2.

Somatodendritic DA release in the SNc and VTA monitored using voltammetry and electrophysiology. (a) Left panel: average evoked increases in extracellular DA concentration ([DA]_o) during pulse-train stimulation (30 pulses, 10 Hz) recorded using FCV with carbon-fibre microelectrodes in the SNc and VTA in midbrain slices. Maximal evoked [DA]_o was 0.81 ± 0.08 µM (n = 13) in the SNc, and 0.61 ± 0.06 µM (n = 16) in the VTA. Right panel: evoked responses detected using FCV were identified as DA by the characteristic peak potentials, which were the same as for a solution of DA (1 µM); typical oxidation and reduction peak potentials are 0.60 V and −0.25 V versus Ag/AgCl, respectively (adapted from Chen et al. [22]). (b) Left panel: averaged increases in [DA]_o evoked by five pulses at 40 Hz monitored using FCV (n = 7–10 traces) in the VTA in midbrain slices from mouse, rat and guinea pig; responses were recorded in 2.5 mM [Ca²⁺]_o (black) or in 0.5 mM [Ca²⁺]_o (grey). Right panel: representative D2-IPSCs (DA-dependent inhibitory postsynaptic currents) obtained using voltage-clamp recording of VTA DA neurons from mouse, rat and guinea pig recorded in 2.5 mM (black) or 0.5 mM [Ca²⁺]_o (grey) (adapted from Courtney et al. [69]).

Figure 3.

Ca²⁺ dependence of DA release in nigrostriatal (a) and mesolimbic (b) pathways. Normalized single-pulse evoked increases in [DA]_o, where release in 1.5 mM [Ca²⁺]_o is 100%. Hill curves are fit for axonal (blue lines) and somatodendritic release (red lines). Ca²⁺ sensitivity is indicated by the [Ca²⁺]_o required for half maximal release (EC₅₀, dotted lines). Notably, two distinct Hill analyses are needed to fit data from the VTA ([Ca²⁺]_o < 1.5 mM versus [Ca²⁺]_o > 1.0 mM) revealing both axonal and somatodendritic release components. Hill coefficients indicate cooperativity of Ca²⁺ in DA release. Data are means without error bars for clarity (n = 5–7 per point) (adapted from Chen et al. [32]).

Figure 4.

Role of intracellular Ca²⁺ stores in somatodendritic DA release in SNc. (a) Left panel: Colocalization of TH and SERCA2 (overlap in merge images is yellow) in SNc DA neurons is seen in the soma and proximal dendrites (first three images), but not in distal dendrites (far right image). Right panel: a decrease in evoked [DA]_o (30 pulses, 10 Hz) detected by FCV when SERCA is inhibited with cyclopiazonic acid (CPA, 30 µM) demonstrates amplification of release by ER Ca²⁺ stores. (b) Upper panel: colocalization of TH and RyRs shows a mixture of large puncta located near the soma surface and smaller puncta within the cytoplasm (first three images). No staining is seen in SNr (far right image), implying low levels in distal dendrites. Lower panel: blockade of RyRs with dantrolene (Dant, 10 µM) decreases evoked [DA]_o in physiological [Ca²⁺]_o. (c) Upper panel: colocalization of TH and IP₃Rs in the cell soma which extends down a proximal dendrite (first three images) but with minimal IP₃R expression in distal dendrites (far right image). Lower panel: blockade of IP₃Rs with 2-aminoethoxydiphenyl borate (2-APB, 100 µM) decreases evoked [DA]_o confirming involvement of IP₃R-gated ER Ca²⁺ stores in facilitating somatodendritic DA release. Scale bar in all panels is 10 µm. (**p < 0.01, ***p < 0.001) (adapted from Patel et al. [25]).

Figure 5.

Identification of unusual SNARE proteins for exocytosis in SNc DA neurons. Immunostaining with TH (red) and SNARE proteins (green) shows that SNc DA somata lack the vesicular protein, synaptobrevin 1 and the plasma membrane protein, syntaxin 1a, that are conventionally used at glutamate synapses. However, DA somata and proximal dendrites do express conventional SNAP-25 as well as unconventional isotypes of exocytotic proteins, e.g. the vesicular protein, synaptobrevin 2 and the plasma membrane protein, syntaxin 3b (left-side image for each protein). SNAP-25, synaptobrevin 2 and syntaxin 3b are also present in distal dendrites within the SNr (right side images for each protein). Colocalization with TH is seen in yellow. Scale bar in all panels is 10 µm (adapted from Witkovsky et al. [27]).

Figure 6.

Activation of mGluR1 in SNc by endogenous glutamate enhances somatodendritic DA release. (a) Colocalization of TH and mGluR1α (overlap in merge images is yellow); mGluR1α puncta colocalize with TH in somata and proximal dendrites in SNc and distal dendrites in SNr. Arrows show dendritic colocalization. Scale bar, 10 µm. (b) Antagonizing mGluR1s with CPCCOEt (100 µM) decreases [DA]_o in SNc evoked by pulse-train stimulation (30 pulses, 10 Hz). Release was monitored using FCV (n = 9, ***p < 0.001) (adapted from Patel et al. [25]).