Bulbospinal and intraspinal thyrotropin releasing hormone systems: modulation of spinal cord pain transmission

Recent research indicates that pain processing by spinal nociceptive neurons is modulated by: 1) descending bulbospinal pathways originating in the rostral ventral medulla (RMV) and 2) short intraspinal peptide systems located in the dorsal horn. Immunohistochemical studies have identified both bulbospinal and intraspinal thyrotropin releasing hormone (TRH) systems, however, the role of these two TRH systems in pain modulation has not been defined. The purpose of the present study was to explore the role of the bulbospinal and intraspinal TRH systems in pain modulation. Three TRH mediated neural network were examined in electrophysiologic experiments; each model predicting specific testable outcomes. Model A represented a direct TRH projection from RVM to spinal cord. This model predicted that the effect of RVM stimulation and TRH micropressure administration on dorsal horn unit activity would be in the same direction, either both inhibitory effects or both excitatory. Of 44 dorsal horn units inhibited by RVM stimulation, 82% were excited by TRH, therefore, Model A was rejected. Model B interposed an intraspinal neuron between the bulbospinal TRH projection and the dorsal horn nociceptive unit. This model predicted that desensitization of spinal TRH receptor systems should block the effect of RVM stimulation on dorsal horn neuron. In TRH desensitization experiments, no attenuation or blockade of RVM stimulation was observed, therefore, the effect of RVM stimulation did not appear mediated by TRH and Model B was rejected.(ABSTRACT TRUNCATED AT 250 WORDS)