Background & Aims
Chronic pain posits a public health crisis due to the prevalence and limited targeted treatment options. Mechanistically, there are known spinal cord pathways that encode nociception such as the spinothalamic tract that process peripheral injury leading to hyperactivity of the primary somatosensory cortex (S1) in the brain. S1 projects to other brain areas that carry pain information but there is a lack of understanding of the central circuits involved in chronic pain. One understudied candidate is the basal ganglia, specifically the striatum, whose main function is sensorimotor integration. Our studies aim to understand the circuit mechanisms of S1-striatum in the transition from acute to chronic pain states. We specifically measure plasticity changes in cell-specific spiny projection neurons, dopamine receptor 1 (D1-SPN) and dopamine receptor 2 (D2-SPN). Additionally, we also analyze real-time bulk neural activity of striatal populations in a mouse neuropathic pain model.
Methods
We used two transgenic mouse lines, D1-Cre and A2A-Cre mice to target D1- and D2-SPNs. For our electrophysiological experiments, male and female mice of both mouse lines are injected with a virus containing a light-sensitive ion channel in S1. Mice undergo sham or spare nerve injury (SNI) surgeries to create a neuropathic pain model and are evaluated for nociceptive behaviors before injury, 7 days, and 25 days post-surgery. Brain slices are then harvested to optogenetically stimulate axon terminals in the striatum to measure electrophysiological parameters. For our fiber photometry experiments, mice are injected with a cell-specific genetically encoded calcium indicator as a proxy for neural activity of D1-SPNs and D2-SPNs. Recordings of these neurons occur simultaneously while probing the paw with a brush and pinprick. This is done before SNI surgery to assess baseline calcium levels and then tested weekly post-surgery. Data are analyzed using custom scripts in Matlab and Python.
Results
Nociceptive behavior testing (Von Frey, brush, and pinprick stimulation to the contralateral hind paw) shows that SNI animals have a heightened paw withdrawal response compared to preinjury and sham conditions in both D1 and D2 males and females. For our electrophysiological experiments, the intrinsic parameters (using IV curves) of both D1 and D2 cells show there are no differences in half-width height, after-hyperpolarization, and rise time in both SNI and sham conditions. During a single pulse stimulation with blue light, there were no differences in D1 cells between sham and SNI conditions, but D2 cells in SNI mice showed a significant increase in postsynaptic amplitude compared to sham mice. During paired-pulse stimulation, D2 cells in SNI mice showed a significant decrease in the second pulse (compared to the first pulse) in contrast to sham mice. During train stimulation, D2 cells in SNI mice showed a significant decrease in postsynaptic amplitude compared to sham mice. There were no differences observed in D1 cells. In the fiber photometry experiments, D1 cell populations in SNI animals show increased activity for brush stimulation post-surgery compared to sham mice. D2 cell populations were not evaluated yet.
Conclusions
Anatomical connectivity exists between the hindlimb primary somatosensory cortex, the dorsolateral striatum, and the lumbar spinal cord. SNI animals show higher sensitivity in Von Frey, brush, and pinprick stimulation compared to both preinjury and sham controls indicating a viable model of neuropathic pain. D2 cells but not D1 cells in SNI mice show changes in electrical properties ex vivo. D2 cells in SNI mice show higher post-synaptic amplitude during single-pulse stimulation and demonstrate depressing synapses through paired-pulse and train stimulation specifically during the chronic phase of nociception. D1 cells in SNI mice show greater activity during the acute phases of nociception post-surgery compared to sham animals in vivo.
References
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