Chronic hyperactivation of midbrain dopamine neurons causes preferential dopamine neuron degeneration.

Parkinson's disease (PD) is characterized by the death of substantia nigra (SNc) dopamine (DA) neurons, but the pathophysiological mechanisms that precede and drive their death remain unknown. The activity of DA neurons is likely altered in PD, but we understand little about if or how chronic changes in activity may contribute to degeneration. To address this question, we developed a chemogenetic (DREADD) mouse model to chronically increase DA neuron activity, and confirmed this increase using ex vivo electrophysiology. Chronic hyperactivation of DA neurons resulted in prolonged increases in locomotor activity during the light cycle and decreases during the dark cycle, consistent with chronic changes in DA release and circadian disturbances. We also observed early, preferential degeneration of SNc projections, recapitulating the PD hallmarks of selective vulnerability of SNc axons and the comparative resilience of ventral tegmental area axons. This was followed by eventual loss of midbrain DA neurons. Continuous DREADD activation resulted in a sustained increase in baseline calcium levels, supporting an important role for increased calcium in the neurodegeneration process. Finally, spatial transcriptomics from DREADD mice examining midbrain DA neurons and striatal targets, and cross-validation with human patient samples, provided insights into potential mechanisms of hyperactivity-induced toxicity and PD. Our results thus reveal the preferential vulnerability of SNc DA neurons to increased neural activity, and support a potential role for increased neural activity in driving degeneration in PD.

Cholinergic Transmission at Muscarinic Synapses in the Striatum Is Driven Equally by Cortical and Thalamic Inputs.

The release of acetylcholine from cholinergic interneurons (ChIs) directly modulates striatal output via muscarinic receptors on medium spiny neurons (MSNs). While thalamic inputs provide strong excitatory input to ChIs, cortical inputs primarily regulate MSN firing. Here, we found that, while thalamic inputs do drive ChI firing, a subset of ChIs responds robustly to stimulation of cortical inputs as well. To examine how input-evoked changes in ChI firing patterns drive acetylcholine release at cholinergic synapses onto MSNs, muscarinic M4-receptor-mediated synaptic events were measured in MSNs overexpressing G-protein gated potassium channels (GIRK2). Stimulation of both cortical and thalamic inputs was sufficient to equally drive muscarinic synaptic events in MSNs, resulting from the broad synaptic innervation of the stimulus-activated ChI population across many MSNs. Taken together, this indicates an underappreciated role for the extensive cholinergic network, in which small populations of ChIs can drive substantial changes in post-synaptic receptor activity across the striatum.

Revealing a Role for NMDA Receptors in Regulating STN Inputs following the Loss of Dopamine.

In this issue of Neuron, Chu et al. (2017) show that dopamine depletion using a 6-OHDA model causes a decrease in hyperdirect inputs from the motor cortex directly to the STN and that rescuing this loss alleviates Parkinsonian symptoms.

Nicotinic and opioid receptor regulation of striatal dopamine D2-receptor mediated transmission.

In addition to dopamine neuron firing, cholinergic interneurons (ChIs) regulate dopamine release in the striatum via presynaptic nicotinic receptors (nAChRs) on dopamine axon terminals. Synchronous activity of ChIs is necessary to evoke dopamine release through this pathway. The frequency-dependence of disynaptic nicotinic modulation has led to the hypothesis that nAChRs act as a high-pass filter in the dopaminergic microcircuit. Here, we used optogenetics to selectively stimulate either ChIs or dopamine terminals directly in the striatum. To measure the functional consequence of dopamine release, D2-receptor synaptic activity was assessed via virally overexpressed potassium channels (GIRK2) in medium spiny neurons (MSNs). We found that nicotinic-mediated dopamine release was blunted at higher frequencies because nAChRs exhibit prolonged desensitization after a single pulse of synchronous ChI activity. However, when dopamine neurons alone were stimulated, nAChRs had no effect at any frequency. We further assessed how opioid receptors modulate these two mechanisms of release. Bath application of the κ opioid receptor agonist U69593 decreased D2-receptor activation through both pathways, whereas the µ opioid receptor agonist DAMGO decreased D2-receptor activity only as a result of cholinergic-mediated dopamine release. Thus the release of dopamine can be independently modulated when driven by either dopamine neurons or cholinergic interneurons.

Spontaneous Synaptic Activation of Muscarinic Receptors by Striatal Cholinergic Neuron Firing.

Cholinergic interneurons (CHIs) play a major role in motor and learning functions of the striatum. As acetylcholine does not directly evoke postsynaptic events at most striatal synapses, it remains unclear how postsynaptic cholinergic receptors encode the firing patterns of CHIs in the striatum. To examine the dynamics of acetylcholine release, we used optogenetics and paired recordings from CHIs and medium spiny neurons (MSNs) virally overexpressing G-protein-activated inwardly rectifying potassium (GIRK) channels. Due to the efficient coupling between endogenous muscarinic receptors and GIRK channels, we found that firing of individual CHIs resulted in monosynaptic spontaneous inhibitory post-synaptic currents (IPSCs) in MSNs. Paired CHI-MSN recordings revealed that the high probability of acetylcholine release at these synapses allowed muscarinic receptors to faithfully encode physiological activity patterns from individual CHIs without failure. These results indicate that muscarinic receptors in striatal output neurons reliably decode CHI firing.

Phasic dopamine release drives rapid activation of striatal D2-receptors.

Striatal dopamine transmission underlies numerous goal-directed behaviors. Medium spiny neurons (MSNs) are a major target of dopamine in the striatum. However, as dopamine does not directly evoke a synaptic event in MSNs, the time course of dopamine signaling in these cells remains unclear. To examine how dopamine release activates D2-receptors on MSNs, G protein activated inwardly rectifying potassium (GIRK2; Kir 3.2) channels were virally overexpressed in the striatum, and the resulting outward currents were used as a sensor of D2-receptor activation. Electrical and optogenetic stimulation of dopamine terminals evoked robust D2-receptor inhibitory postsynaptic currents (IPSCs) in GIRK2-expressing MSNs that occurred in under a second. Evoked D2-IPSCs could be driven by repetitive stimulation and were not occluded by background dopamine tone. Together, the results indicate that D2-receptors on MSNs exhibit functional low affinity and suggest that striatal D2-receptors can encode both tonic and phasic dopamine signals.

Species differences in somatodendritic dopamine transmission determine D2-autoreceptor-mediated inhibition of ventral tegmental area neuron firing.

The somatodendritic release of dopamine within the ventral tegmental area (VTA) and substantia nigra pars compacta activates inhibitory postsynaptic D2-receptors on dopaminergic neurons. The proposed mechanisms that regulate this form of transmission differ between electrochemical studies using rats and guinea pigs and electrophysiological studies using mice. This study examines the release and resulting dopamine D2-autoreceptor-mediated IPSCs (D2-IPSCs) in the VTA of mouse, rat, and guinea pig. Robust D2-IPSCs were observed in all recordings from neurons in slices taken from mouse, whereas D2-IPSCs in rat and guinea pig were observed less frequently and were significantly smaller in amplitude. In slices taken from guinea pig, dopamine release was more persistent under conditions of reduced extracellular calcium. The decline in the concentration of dopamine was also prolonged and not as sensitive to inhibition of reuptake by cocaine. This resulted in an increased duration of D2-IPSCs in the guinea pig. Therefore, unlike the mouse or the rat, the time course of dopamine in the extracellular space of the guinea pig determined the duration the D2-IPSC. Functionally, differences in D2-IPSCs resulted in inhibition of dopamine neuron firing only in slices from mouse. The results suggest that the mechanisms and functional consequences of somatodendritic dopamine transmission in the VTA vary among species. This highlights the complexity that underlies dopamine-dependent transmission in one brain area. Differences in somatodendritic transmission would be expected in vivo to affect the downstream activity of the mesocorticolimbic dopamine system and subsequent terminal release.