The brain receives sensory input from diverse peripheral tissues, including the skin, the body's largest sensory organ. Using genetically encoded axonal tracers expressed from the Mrgprd locus, we identify a subpopulation of nonpeptidergic, nociceptive neurons that project exclusively to the skin, and to no other peripheral tissue examined. Surprisingly, Mrgprd(+) innervation is restricted to the epidermis and absent from specialized sensory structures. Furthermore, Mrgprd(+) fibers terminate in a specific layer of the epidermis, the stratum granulosum. This termination zone is distinct from that innervated by most CGRP(+) neurons, revealing that peptidergic and nonpeptidergic epidermal innervation is spatially segregated. The central projections deriving from these distinct epidermal innervation zones terminate in adjacent laminae in the dorsal spinal cord. Thus, afferent input from different layers of the epidermis is conveyed by topographically segregated sensory circuits, suggesting that at least some aspects of sensory information processing may be organized along labeled lines.

No abstract

CB2 cannabinoid receptor-selective agonists are promising candidates for the treatment of pain. CB2 receptor activation inhibits acute, inflammatory, and neuropathic pain responses but does not cause central nervous system (CNS) effects, consistent with the lack of CB2 receptors in the normal CNS. To date, there has been virtually no information regarding the mechanism of CB2 receptormediated inhibition of pain responses. Here, we test the hypothesis that CB2 receptor activation stimulates release from keratinocytes of the endogenous opioid ␤-endorphin, which then acts at opioid receptors on primary afferent neurons to inhibit nociception. The antinociceptive effects of the CB2 receptor-selective agonist AM1241 were prevented in rats when naloxone or antiserum to ␤-endorphin was injected in the hindpaw where the noxious thermal stimulus was applied, suggesting that ␤-endorphin is necessary for CB2 receptor-mediated antinociception. Further, AM1241 did not inhibit nociception in -opioid receptordeficient mice. Hindpaw injection of ␤-endorphin was sufficient to produce antinociception. AM1241 stimulated ␤-endorphin release from rat skin tissue and from cultured human keratinocytes. This stimulation was prevented by AM630, a CB2 cannabinoid receptorselective antagonist and was not observed in skin from CB2 cannabinoid receptor-deficient mice. These data suggest that CB2 receptor activation stimulates release from keratinocytes of ␤-endorphin, which acts at local neuronal -opioid receptors to inhibit nociception. Supporting this possibility, CB2 immunolabeling was detected on ␤-endorphin-containing keratinocytes in stratum granulosum throughout the epidermis of the hindpaw. This mechanism allows for the local release of ␤-endorphin, where CB2 receptors are present, leading to anatomical specificity of opioid effects.␤-endorphin ͉ nociception ͉ pain ͉ keratinocyte ͉ skin C B 2 cannabinoid receptor-selective agonists are very promising candidates for the treatment of pain. Activation of CB 2 cannabinoid receptors inhibits nociception to thermal and mechanical stimuli (1, 2), thermal and tactile hypersensitivity produced by peripheral inflammation (2-4), and tactile and thermal hypersensitivity produced in a neuropathic pain model (5). Experimental findings suggesting that activation of peripheral (noncentral nervous system) CB 2 receptors is necessary and sufficient to inhibit pain responses come from site-specific injections of CB 2 receptor-selective agonists and antagonists (1, 3, 4). Importantly, CB 2 cannabinoid receptor-selective agonists do not cause central nervous system (CNS) effects (1, 6), consistent with the inability to demonstrate the expression of CB 2 receptors in the normal CNS (7-10). The lack of CNS effects is an important feature of this class of drug candidates because the efficacy of current pain therapies is frequently limited by CNS side effects. However, enthusiasm for this therapeutic approach has been tempered by the lack of information regarding the mechanism underlying the inhibition of n...

SUMMARY Itch is the least well understood of all the somatic senses, and the neural circuits that underlie this sensation are poorly defined. Here we show that the atonal-related transcription factor Bhlhb5 is transiently expressed in the dorsal horn of the developing spinal cord and appears to play a role in the formation and regulation of pruritic (itch) circuits. Mice lacking Bhlhb5 develop self-inflicted skin lesions and show significantly enhanced scratching responses to pruritic agents. Through genetic fate-mapping and conditional ablation we provide evidence that the pruritic phenotype in Bhlhb5 mutants may be due to selective loss of a subset of inhibitory interneurons in the dorsal horn. Our findings suggest that Bhlhb5 is required for the survival of a specific population of inhibitory interneurons that regulate pruritis and provide evidence that the loss of inhibitory synaptic input results in abnormal itch.

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the “early Ret+” DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret+ neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.