α4 nicotinic receptors on GABAergic neurons mediate a cholinergic analgesic circuit in the substantia nigra pars reticulata.

Nicotinic acetylcholine receptors (nAChRs) regulate pain pathways with various outcomes depending on receptor subtypes, neuron types, and locations. But it remains unknown whether α4ß2 nAChRs abundantly expressed in the substantia nigra pars reticulata (SNr) have potential to mitigate hyperalgesia in pain states. We observed that injection of nAChR antagonists into the SNr reduced pain thresholds in naïve mice, whereas injection of nAChR agonists into the SNr relieved hyperalgesia in mice, subjected to capsaicin injection into the lower hind leg, spinal nerve injury, chronic constriction injury, or chronic nicotine exposure. The analgesic effects of nAChR agonists were mimicked by optogenetic stimulation of cholinergic inputs from the pedunculopontine nucleus (PPN) to the SNr, but attenuated upon downregulation of α4 nAChRs on SNr GABAergic neurons and injection of dihydro-ß-erythroidine into the SNr. Chronic nicotine-induced hyperalgesia depended on α4 nAChRs in SNr GABAergic neurons and was associated with the reduction of ACh release in the SNr. Either activation of α4 nAChRs in the SNr or optogenetic stimulation of the PPN-SNr cholinergic projection mitigated chronic nicotine-induced hyperalgesia. Interestingly, mechanical stimulation-induced ACh release was significantly attenuated in mice subjected to either capsaicin injection into the lower hind leg or SNI. These results suggest that α4 nAChRs on GABAergic neurons mediate a cholinergic analgesic circuit in the SNr, and these receptors may be effective therapeutic targets to relieve hyperalgesia in acute and chronic pain, and chronic nicotine exposure.

Differential modulation of subthalamic projection neurons by short-term and long-term electrical stimulation in physiological and parkinsonian conditions.

The subthalamic nucleus (STN) is one of the best targets for therapeutic deep brain stimulation (DBS) to control motor symptoms in Parkinson's disease. However, the precise circuitry underlying the effects of STN-DBS remains unclear. To understand how electrical stimulation affects STN projection neurons, we used a retrograde viral vector (AAV-retro-hSyn-eGFP) to label STN neurons projecting to the substantia nigra pars reticulata (SNr) (STN-SNr neurons) or the globus pallidus interna (GPi) (STN-GPi neurons) in mice, and performed whole-cell patch-clamp recordings from these projection neurons in ex vivo brain slices. We found that STN-SNr neurons exhibited stronger responses to depolarizing stimulation than STN-GPi neurons. In most STN-SNr and STN-GPi neurons, inhibitory synaptic inputs predominated over excitatory inputs and electrical stimulation at 20-130 Hz inhibited these neurons in the short term; its longer-term effects varied. 6-OHDA lesion of the nigrostriatal dopaminergic pathway significantly reduced inhibitory synaptic inputs in STN-GPi neurons, but did not change synaptic inputs in STN-SNr neurons; it enhanced short-term electrical-stimulation-induced inhibition in STN-SNr neurons but reversed the effect of short-term electrical stimulation on the firing rate in STN-GPi neurons from inhibitory to excitatory; in both STN-SNr and STN-GPi neurons, it increased the inhibition but attenuated the enhancement of firing rate induced by long-term electrical stimulation. Our results suggest that STN-SNr and STN-GPi neurons differ in their synaptic inputs, their responses to electrical stimulation, and their modification under parkinsonian conditions; STN-GPi neurons may play important roles in both the pathophysiology and therapeutic treatment of Parkinson's disease.

D2 receptor activation relieves pain hypersensitivity by inhibiting superficial dorsal horn neurons in parkinsonian mice.

Chronic pain is a common and undertreated nonmotor symptom in Parkinson's disease (PD). Although chronic pain is improved by L-dopa in some PD patients, the underlying mechanisms remain unclear. In this study, we established PD mice by unilateral microinjection of 6-OHDA in the medial forebrain bundle to investigate the contribution of spinal cord dopamine receptors to parkinsonian pain hypersensitivity. The von Frey filament tests and thermal pain tests revealed that these PD mice displayed decreased nociceptive thresholds in both hindpaws; intrathecal injection of L-dopa or apomorphine significantly increased the mechanical and thermal nociceptive thresholds, and the analgesic effect was mimicked by ropinirole (a D2 receptor agonist), but not SKF38393 (a D1/D5 receptor agonist), and blocked by sulpiride (a D2 receptor antagonist), but not SKF83566 (a D1/D5 receptor antagonist). Whole-cell recordings in lumber spinal cord slices showed that superficial dorsal horn (SDH) neurons in PD mice exhibited hyperexcitability, including more depolarized resting membrane potentials and more action potentials evoked by depolarizing current steps, which were mitigated by ropinirole. Furthermore, ropinirole inhibited the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in SDH neurons more strongly in PD mice than in control mice. However, sulpiride caused less disinhibition of sEPSCs in PD mice than in control mice. Taken together, our data reveal that pain hypersensitivity in PD mice is associated with hyperexcitability of SDH neurons, and both events are reversed by activation of spinal D2 receptors. Therefore, spinal D2 receptors can be promising therapeutic targets for the treatment of PD pain.