Classification of H₂O₂as a neuromodulator that regulates striatal dopamine release on a subsecond time scale.

The striatum is the largest component of the basal ganglia and receives dense dopamine (DA) innervation from midbrain DA neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA).1 The primary targets of these DA afferents are medium spiny neurons (MSNs) that constitute over 90% of the striatal neuronal population and are the main output neurons of this region. Axons arising from midbrain DA neurons form symmetric synapses on MSNs, primarily on spine necks, adjacent to glutamatergic input to spine heads.2,3 This architectural arrangement enables DA to modulate glutamate-induced MSN excitability with differential outcomes according to whether the target MSN also expresses DA D1 receptors (D1Rs) or DA D2 receptors (D2Rs); DA enhances MSN excitability and up-state MSN spiking via D1Rs and decreases these via D2Rs.4 Thus, converging glutamatergic and dopaminergic afferents control striatal output at the level of individual spines to regulate motor and cognitive function.4−6 It is relevant to note that D1Rs and D2Rs are predominantly extrasynaptic,7 so that rapid electrochemical measurements of extracellular DA concentration ([DA]o) can provide a direct index of DA transmission (e.g., 8–10). The crucial role that DA plays in normal motor behavior has been illuminated by the findings that disturbances in DA function contribute to basal-ganglia circuit dysfunction and that DA depletion leads to the motor deficits of Parkinson’s disease.11−13 Thus, elucidating factors that regulate DA release in the striatum is of relevance for understanding the role of DA in normal brain function, as well as for providing therapeutic insight for neuropathological conditions like Parkinson’s disease. Our laboratory examines the regulation of striatal DA release using fast-scan cyclic voltammetry (FCV) with carbon-fiber microelectrodes. Because FCV detects dynamic changes in [DA]o with high temporal and spatial resolution, this technique is ideally suited to determine factors that influence the amplitude, duration, and sphere of influence of DA signaling.14−17 Basic factors that we have examined include the pattern of stimulation, Ca2+-dependence of DA release, inhibition of DA release by presynaptic DA autoreceptors, and DA clearance by the DA transporter (DAT); we have also examined the effect of a number of local neurotransmitters and neuromodulators that can act on DA axons either directly or indirectly to alter evoked [DA]o (for review, see ref (10)). One unconventional and under-appreciated modulator of striatal DA release is hydrogen peroxide (H2O2). H2O2 is most commonly known as a reactive oxygen species (ROS) that is formed from cellular oxidative metabolism and is often viewed as being toxic. However, H2O2 is not a free radical and, consequently, is not as reactive as other ROS.18 In fact, increasing evidence indicates that H2O2 plays an important role in normal cellular signaling processes, as well as in the modulation of transmitter release.19 Here we review the evidence that endogenous H2O2 fulfills criteria for classification as a neuromodulator through its actions on striatal DA release. We would note, however, that actions of H2O2 are not limited to DA axons, and possibly not even to the brain, as summarized in Concluding Remarks. Criteria often used to classify a molecule as a transmitter or modulator include the following: (1) same response elicited by the exogenous and endogenous substance; (2) synthesis of the substance within a releasing cell; (3) release in response to depolarization or receptor activation; (4) receptor/sensor for the substance at a target site; and (5) mechanism for deactivation. Using a combination of FCV to detect evoked [DA]o in striatal slices, whole-cell recording of striatal MSN activity, and fluorescence imaging to monitor H2O2 generation, we have shown that H2O2 is generated downstream from glutamatergic AMPA receptor (AMPAR) activation in MSNs, and then diffuses to DA axons where it causes transient suppression of striatal DA release through activation of ATP-sensitive K+ (KATP) channels on DA axons. Through this regulatory pathway, H2O2 acts as a key intermediate for DA modulation by glutamate and GABA, as well as other neuromodulators including the cannabinoids.