Impaired pain in mice lacking first-order posterior medial thalamic neurons.

ABSTRACT: The thalamus plays an important role in sensory and motor information processing by mediating communication between the periphery and the cerebral cortex. Alterations in thalamic development have profound consequences on sensory and motor function. In this study, we investigated a mouse model in which thalamic nuclei formation is disrupted because of the absence of Sonic hedgehog (Shh) expression from 2 key signaling centers that are required for embryonic forebrain development. The resulting defects observed in distinct thalamic sensory nuclei in Shh mutant embryos persisted into adulthood prompting us to examine their effect on behavioral responses to somatosensory stimulation. Our findings reveal a role for first-order posterior medial thalamic neurons and their projections to layer 4 of the secondary somatosensory cortex in the transmission of nociceptive information. Together, these results establish a connection between a neurodevelopmental lesion in the thalamus and a modality-specific disruption in pain perception.

Taking a deep breath: How a brainstem pathway integrates pain and breathing.

In this issue of Neuron, Liu et al. (2022) shed light on the neural circuits supporting pain- and anxiety-induced elevated breathing rhythms. They reveal PBL core-Oprm1 neurons projecting onto the CeA and shell-Oprm1 neurons projecting onto the preBötC as differential regulators of these behaviors.

Using high-speed videography for objective and reproducible pain measurement on a mouse pain scale.

Mouse models are essential for studying pain neurobiology and testing pain therapeutics. The reliance on assays that only measure the presence, absence, or frequency of a reflex have limited the reliability of preclinical pain studies. Our high-speed videography protocol overcomes this by projecting the discrete sub-second kinematic behavioral features induced by hind paw stimulation onto a "mouse pain scale." This provides a more objective and robust pain measurement in mice by quantifying the quality of the stimulus-induced hind paw reflex. For complete details on the use and execution of this protocol, please refer to Abdus-Saboor et al. (2019).

A machine-vision approach for automated pain measurement at millisecond timescales.

Objective and automatic measurement of pain in mice remains a barrier for discovery in neuroscience. Here, we capture paw kinematics during pain behavior in mice with high-speed videography and automated paw tracking with machine and deep learning approaches. Our statistical software platform, PAWS (Pain Assessment at Withdrawal Speeds), uses a univariate projection of paw position over time to automatically quantify seven behavioral features that are combined into a single, univariate pain score. Automated paw tracking combined with PAWS reveals a behaviorally divergent mouse strain that displays hypersensitivity to mechanical stimuli. To demonstrate the efficacy of PAWS for detecting spinally versus centrally mediated behavioral responses, we chemogenetically activated nociceptive neurons in the amygdala, which further separated the pain-related behavioral features and the resulting pain score. Taken together, this automated pain quantification approach will increase objectivity in collecting rigorous behavioral data, and it is compatible with other neural circuit dissection tools for determining the mouse pain state.

Development of a Mouse Pain Scale Using Sub-second Behavioral Mapping and Statistical Modeling.

Rodents are the main model systems for pain research, but determining their pain state is challenging. To develop an objective method to assess pain sensation in mice, we adopt high-speed videography to capture sub-second behavioral features following hind paw stimulation with both noxious and innocuous stimuli and identify several differentiating parameters indicating the affective and reflexive aspects of nociception. Using statistical modeling and machine learning, we integrate these parameters into a single index and create a "mouse pain scale," which allows us to assess pain sensation in a graded manner for each withdrawal. We demonstrate the utility of this method by determining sensations triggered by three different von Frey hairs and optogenetic activation of two different nociceptor populations. Our behavior-based "pain scale" approach will help improve the rigor and reproducibility of using withdrawal reflex assays to assess pain sensation in mice.

Sparse genetic tracing reveals regionally specific functional organization of mammalian nociceptors.

The human distal limbs have a high spatial acuity for noxious stimuli but a low density of pain-sensing neurites. To elucidate mechanisms underlying regional differences in processing nociception, we sparsely traced non-peptidergic nociceptors across the body using a newly generated MrgprdCreERT2 mouse line. We found that mouse plantar paw skin is also innervated by a low density of Mrgprd+ nociceptors, while individual arbors in different locations are comparable in size. Surprisingly, the central arbors of plantar paw and trunk innervating nociceptors have distinct morphologies in the spinal cord. This regional difference is well correlated with a heightened signal transmission for plantar paw circuits, as revealed by both spinal cord slice recordings and behavior assays. Taken together, our results elucidate a novel somatotopic functional organization of the mammalian pain system and suggest that regional central arbor structure could facilitate the "enlarged representation" of plantar paw regions in the CNS.