Understanding the Regional Specificity of Neurons in the Trigeminal Subnucleus Caudalis and Spinal Dorsal Horn

Abstract

Neurons in the trigeminal subnucleus caudalis (Vc) and spinal cord dorsal horn (SDH) play important roles in modulating and relaying pain signals to the higher centers of the central nervous system (CNS). Morphologically, many aspects including a laminated structure, cytoarchitecture and cellular elements in these two central regions are very similar [1 ,2, 3, 4]. Most nociceptive afferents with a small-diameter terminate in the superficial laminae of Vc and SDH. Functionally, sensory neurons in Vc and SDH respectively receive nociceptive inputs from the orofacial and other somatic regions, and convey the inputs to higher brain centers [1, 2, 3, 4, 5]. According to the mechanoreceptive properties, sensory neurons in both the Vc and SDH are classified into three types. They are low-threshold mechanoreceptive (LTM), wide-dynamic range (WDR) and nociceptive-specific neurons (NS) [1, 2, 3, 4, 5]. Because of these similarities Vc has been considered as analogous to SDH [4, 6]. However, some differences between Vc and SDH have also been reported. For example, the distribution pattern of substance P (SP) and calcitonin gene related peptide (CGRP) in Vc of adult animals is different from that in SDH. In SDH there is no dual representation of peripheral regions, which can be found in rostral and caudal Vc. A transitional zone found between Vc and trigeminal subnucleus interpolaris plays a special role in sensorimotor function. However, no such region is found in the spinal system [5, 7, 8]. CNS neurons are regionalized. They are organized as groups through their circuitry. Although it is known that differences in functions among these groups in the adult CNS are predetermined during the development of the neuroepithelium [9, 10, 11, 12, 13] , understanding how the regional specificity is developed in the CNS is still a big challenge in neuroscience research. It has recently been found that ninhydrin-reacting small molecules released locally are involved in the region-specific regulation of neuronal development in cultured Vc and SDH neurons [14, 15]. In this review we focused on present knowledge about the region-specific regulation of neuronal development in these two CNS regions associated with the transmission of nociceptive signals.