NK-1 and NK-3 type tachykinin receptor mRNA expression in the rat spinal cord dorsal horn is increased during adjuvant or formalin-induced nociception

Abstract

Substance P (SP) and other related tachykinins such as neurokinin B (NKB) have been studied widely as mediators of sensory information. The release of SP into the dorsal horn of the spinal cord is increased during nociception, and SP activates nociception-specific dorsal horn neurons. The tachykinin NKB has antinociceptive effects in the spinal cord and is contained in intrinsic spinal neurons; thus, NKB may also contribute to the processing of sensory information. Both neurokinin-1 (NK-1) and neurokinin-3 (NK-3) receptors have been localized in the superficial laminae of the dorsal horn. This study investigated changes in NK-1 and NK-3 receptor mRNA expression during nociception. Following injection of either formalin or complete Freund's adjuvant (CFA) into one hindpaw, the levels of expression of NK-1 and NK-3 mRNAs in the spinal cord dorsal horn and preprotachykinin (PPT) mRNA expression in the lumbar dorsal root ganglia (DRG) were quantitated using solution hybridization-nuclease protection assays. Peptide and receptor mRNA expression levels were normalized to β-actin mRNA levels, which did not change during the treatments. Formalin (2 or 6 hr) or CFA (4 d) injection produced approximately a twofold increase in SP-encoding PPT mRNA expression in the ipsilateral lumbar DRG. Increased activity in primary afferent neurons containing SP may stimulate the production of SP precursors, providing substrate for increased SP production, release, and turnover in the dorsal horn and periphery. Formalin or CFA increased NK-1 receptor mRNA expression by about twofold in the ipsilateral dorsal horn. Activated SP-containing primary afferent neurons may provide the neuronal drive for this increase. Formalin or CFA also increased NK-3 receptor mRNA expression by about twofold in the dorsal horn. Nociception may trigger increased activity of intrinsic NKB antinociceptive systems that could act to limit or modify transmission of nociceptive information to higher brain centers. These results provide evidence for activity-induced plasticity of tachykinin receptor expression in nociceptive sensory systems during subchronic nociceptive activation.