Cutaneous sensory neurons that detect noxious stimuli project to the dorsal horn of the spinal cord, while those innervating muscle stretch receptors project to the ventral horn. DRG11, a paired homeodomain transcription factor, is expressed in both the developing dorsal horn and in sensory neurons, but not in the ventral spinal cord. Mouse embryos deficient in DRG11 display abnormalities in the spatio-temporal patterning of cutaneous sensory afferent fiber projections to the dorsal, but not the ventral spinal cord, as well as defects in dorsal horn morphogenesis. These early developmental abnormalities lead, in adults, to significantly attenuated sensitivity to noxious stimuli. In contrast, locomotion and sensori-motor functions appear normal. Drg11 is thus required for the formation of spatio-temporally appropriate projections from nociceptive sensory neurons to their central targets in the dorsal horn of the spinal cord.

Substantia gelatinosa (SG, lamina II) is a spinal cord region where most unmyelinated primary afferents terminate and the central nociceptive processing begins. It is formed by several distinct groups of interneurons whose functional properties and synaptic connections are poorly understood, in part, because recordings from synaptically coupled pairs of SG neurons are quite challenging due to a very low probability of finding connected cells. Here, we describe an efficient method for identifying synaptically coupled interneurons in rat spinal cord slices and characterizing their excitatory or inhibitory function. Using tight-seal whole-cell recordings and a cell-attached stimulation technique, we routinely tested about 1500 SG interneurons, classifying 102 of them as monosynaptically connected to neurons in lamina I-III. Surprisingly, the vast majority of SG interneurons (n = 87) were excitatory and glutamatergic, while only 15 neurons were inhibitory. According to their intrinsic firing properties, these 102 SG neurons were also classified as tonic (n = 49), adapting (n = 17) or delayed-firing neurons (n = 36). All but two tonic neurons and all adapting neurons were excitatory interneurons. Of 36 delayed-firing neurons, 23 were excitatory and 13 were inhibitory. We conclude that sensory integration in the intrinsic SG neuronal network is dominated by excitatory interneurons. Such organization of neuronal circuitries in the spinal SG can be important for nociceptive encoding.

DRG11 is a paired domain transcription factor that is necessary for the assembly of the nociceptive circuitry in the spinal cord dorsal horn. It is expressed in small dorsal root ganglion (DRG) neurons and in their projection area in the spinal cord. Drg11 knockout mice exhibit structural and neurochemical defects both at the DRG and spinal superficial dorsal horn and present reduced nociceptive responses. In this study, a polyclonal antibody against DRG11 was generated and used for a detailed systematic spatiotemporal analysis of DRG11 expression during development. DRG11 is first detected at E10.5 in the spinal dorsal horn, DRG and trigeminal ganglion, where it persists until P14-21. At the cranial level, DRG11 expression is observed from E10.5 up to the same early post-natal ages in several cranial sensory ganglia and brain nuclei. These results suggest that DRG11 is required for the establishment of the first neuronal sensory relay along development. Developmental Dynamics 236:2653-2660, 2007.

No abstract

Perception of noxious events relies on activation of complex central neuronal circuits. The spinal cord dorsal horn plays a pivotal role in the process relaying to the brain various types of somatosensory input. These functions are accomplished by distinct sensory neurons specifically organized in different laminae. They differentiate during development in a spatial-temporal order due to the expression of combinatorial sets of homeodomain transcription factors. Here we demonstrate that the differential expression of the homeodomain transcription factors Prrxl1 (DRG11), Tlx3, and Lmx1b defines various subpopulations of spinal cord dorsal horn glutamatergic early born and late born neurons. Accordingly, in the superficial dorsal horn of Prrxl1 2/2 mice, the number of glutamatergic neurons is reduced by 70%, while the number of Golgi-impregnated and noxious-induced Fos immunoreactive neurons is reduced by 85%. These results suggest a crucial role for Prrxl1 in the generation of various subpopulations of nociceptive glutamatergic superficial dorsal horn neurons. Developmental Dynamics 239:1684-1694,