Age-dependent alterations in key components of the nigrostriatal dopaminergic system and distinct motor phenotypes

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 α4β2 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-β-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.

Protocol to study projection-specific circuits in the basal ganglia of adult mice using viral vector tracing, optogenetics, and patch-clamp technique

Analysis of synaptic strength and plasticity provides functional insights of complicated neural circuits. Here, we describe steps for cell- and projection-specific optogenetic manipulation of divergent basal ganglia circuits using anterograde and retrograde viral vectors. We quantitatively analyze synaptic function of these circuits utilizing a patch-clamp technique. This protocol is applicable to probe potential circuit targets for treatment of brain diseases. For complete details on the use and execution of this protocol, please refer to Ji et al.1.

Plasticity in ventral pallidal cholinergic neuron-derived circuits contributes to comorbid chronic pain-like and depression-like behaviour in male mice

Nucleus- and cell-specific interrogation of individual basal forebrain (BF) cholinergic circuits is crucial for refining targets to treat comorbid chronic pain-like and depression-like behaviour. As the ventral pallidum (VP) in the BF regulates pain perception and emotions, we aim to address the role of VP-derived cholinergic circuits in hyperalgesia and depression-like behaviour in chronic pain mouse model. In male mice, VP cholinergic neurons innervate local non-cholinergic neurons and modulate downstream basolateral amygdala (BLA) neurons through nicotinic acetylcholine receptors. These cholinergic circuits are mobilized by pain-like stimuli and become hyperactive during persistent pain. Acute stimulation of VP cholinergic neurons and the VP-BLA cholinergic projection reduces pain threshold in naïve mice whereas inhibition of the circuits elevated pain threshold in pain-like states. Multi-day repetitive modulation of the VP-BLA cholinergic pathway regulates depression-like behaviour in persistent pain. Therefore, VP-derived cholinergic circuits are implicated in comorbid hyperalgesia and depression-like behaviour in chronic pain mouse model.

Subthalamic neurons interact with nigral dopaminergic neurons to regulate movement in mice

AIM

This study aims to address the role of the interaction between subthalamic (STN) neurons and substantia nigra pars compacta (SNc) dopaminergic (DA) neurons in movement control.

METHODS

Fiber photometry and optogenetic/chemogenetic techniques were utilized to monitor and manipulate neuronal activity, respectively. Locomotion in mice was recorded in an open field arena and on a head-fixed apparatus. A hemiparkinsonian mouse model was established by unilateral injection of 6-OHDA in the medial forebrain bundle. Whole-cell patch-clamp techniques were applied to record electrophysiological signals in STN neurons and SNc DA neurons. c-Fos-immunostaining was used to label activated neurons. A rabies virus-based retrograde tracing system was used to visualize STN neurons projecting to SNc DA neurons.

RESULTS

The activity of STN neurons was enhanced upon locomotion in an open field arena and on a head-fixed apparatus, and the enhancement was significantly attenuated in parkinsonian mice. Optogenetic stimulation of STN neurons enhanced locomotion, increased activity of SNc DA neurons, meanwhile, reduced latency to movement initiation. Combining optogenetics with patch-clamp recordings, we confirmed that STN neurons innervated SNc DA neurons through glutamatergic monosynaptic connections. Moreover, STN neurons projecting to SNc DA neurons were evenly distributed in the STN. Either 6-OHDA-lesion or chemogenetic inhibition of SNc DA neurons attenuated the enhancement of locomotion by STN stimulation.

CONCLUSION

SNc DA neurons not only affect the response of STN neurons to movement, but also contribute to the enhancement of movement by STN stimulation. This study demonstrates the role of STN-SNc interaction in movement control.

Carving the senescent phenotype by the chemical reactivity of catecholamines: An integrative review

Macromolecules damaged by covalent modifications produced by chemically reactive metabolites accumulate in the slowly renewable components of living bodies and compromise their functions. Among such metabolites, catecholamines (CA) are unique, compared with the ubiquitous oxygen, ROS, glucose and methylglyoxal, in that their high chemical reactivity is confined to a limited set of cell types, including the dopaminergic and noradrenergic neurons and their direct targets, which suffer from CA propensities for autoxidation yielding toxic quinones, and for Pictet-Spengler reactions with carbonyl-containing compounds, which yield mitochondrial toxins. The functions progressively compromised because of that include motor performance, cognition, reward-driven behaviors, emotional tuning, and the neuroendocrine control of reproduction. The phenotypic manifestations of the resulting disorders culminate in such conditions as Parkinson's and Alzheimer's diseases, hypertension, sarcopenia, and menopause. The reasons to suspect that CA play some special role in aging accumulated since early 1970-ies. Published reviews address the role of CA hazardousness in the development of specific aging-associated diseases. The present integrative review explores how the bizarre discrepancy between CA hazardousness and biological importance could have emerged in evolution, how much does the chemical reactivity of CA contribute to the senescent phenotype in mammals, and what can be done with it.