The roles of functionally defined populations of lamina I projection neurons

Neuropathic pain is a distressing condition that commonly occurs following disease or injury to nerves, and affects nearly 10% of the population. Treatment is often inadequate, and a major reason for this is our limited understanding of the nerve circuits in the spinal cord that are involved in this condition. Previous studies have identified a group of nerve cells in the superficial part of the spinal cord, known as lamina I projection neurons, which carry sensory information related to pain and skin temperature directly to the brain. It has also been shown that these cells have a role in neuropathic pain. However, the projection neurons are functionally heterogeneous, and this has made it difficult to understand the nerve circuits with which they interact, and to define their roles in different types of pain. In a recent study, we showed that two largely separate populations, which account for ~85% of lamina I projection neurons, could be identified in the mouse based on the presence of two different proteins. These proteins are encoded by genes known as Tacr1 and Gpr83. We found that the Tacr1 and Gpr83 cells differed in their responses to painful or thermal stimuli applied to the skin, implying that they are responsible for different aspects of pain and temperature sensation.

That study forms the starting point for the current project. We will initially identify specific populations of lamina I projection neurons by examining responses to skin stimulation not only for cells that express each of these receptors, but also for the smaller populations that express both receptors, or neither receptor. Previous studies in other species have suggested that different types of projection neuron have specific morphological features, and we will determine whether this is true for projection neurons in the mouse. We will then test the prediction that these different types of projection neuron are involved in different nerve circuits within the spinal cord. To do this, we will first investigate their inputs from another class of cell, known as excitatory interneurons. We will also test their responses to a class of drug known as opioids (which includes morphine) that can powerfully suppress pain. Blocking activity in a different group of spinal cord cells, known as inhibitory interneurons, leads to exaggerated sensations and this mechanism is thought to contribute to neuropathic pain. It has been shown that some lamina I projection neurons develop novel responses to brushing of hairs on the skin when this inhibition is blocked, and this is probably a correlate of the touch-evoked pain often seen in patients with nerve damage. We will test which classes of projection neuron show this type of response. We will then use an in vivo experimental approach that will allow us to silence the Tacr1 or Gpr83 cells individually, or to silence both populations simultaneously. We will initially test two different methods, involving either a cell-specific toxic protein, or a light-sensitive protein that will block activity in specific cells. We will determine which of these is more effective, and then apply it to a mouse model of nerve injury and test whether silencing each of these populations suppresses different aspects of neuropathic pain, such as spontaneous pain, or either mechanical or thermal hypersensitivity. Finally, we will examine a specific functional population of lamina I projection neurons that respond exclusively to cooling of the skin, rather than painful stimuli. We will test the prediction that these are the cells that lack both Tacr1 and Gpr83 receptors, and we will investigate the nerve circuits to which they contribute.

The project will provide valuable information about how the spinal cord processes sensory information perceived as pain and temperature, as well as the cells and circuits that underlie neuropathic pain. This is vital for the development of new therapies for treating this distressing condition.

Neuropathic pain is common, potentially debilitating and difficult to treat. Our limited understanding of CNS pain pathways has been a major impediment to developing new therapies. Projection cells in lamina I of the spinal cord convey pain and temperature information to the brain and contribute to symptoms of neuropathic pain. These cells are heterogeneous, and several studies have tried to assign them to functional classes. We have recently shown that projection cells belonging to two largely non-overlapping populations, defined by expression of the receptors Tacr1 and Gpr83, differ in their responses to noxious and thermal stimuli, implying that they underlie different aspects of pain and temperature sensation. Building on these findings, we will first provide a comprehensive functional classification of lamina I projection neurons by defining physiological and morphological properties of cells with each of these receptors, as well as those that express both or neither receptor. We will test the prediction that these populations differ in the local neuronal circuits that they engage, focussing on the roles of different types of excitatory interneuron, as well as the involvement of opioid receptors, which modulate nociceptive information. We will also identify those cells that acquire novel responses to brushing of the skin when inhibition is blocked, and are thought to underlie mechanical allodynia after nerve injury. We will then use synaptic silencing to test the prediction that these projection cell populations contribute to different symptoms of neuropathic pain. Finally, we will test whether a well-known type of lamina I projection cell, those responding only to skin cooling, correspond to the cells that lack Tacr1 and Gpr83, and we will define their location in dorsal horn circuits. The project will provide valuable information about spinal cord circuits involved in neuropathic pain, and should assist in the development of new treatments for this condition.