Unilateral intraplantar injection of Freund's complete adjuvant (FCA) into 1 hind paw of rats was used as a model of peripheral inflammation and persistent pain in order to examine time course effects of a continuous barrage of nociceptive input on the second-messenger transducing systems in the spinal cord. cAMP, cGMP and inositol 1,4,5-trisphosphate (insP(3)) were extracted from the lumbosacral cord at days 1, 7, 14, 21 and 42 following FCA injection and quantified by either radioreceptorassay (RRA) or radioimmunoassay (RIA). The lumbosacral contents of cAMP and cGMP when quantified in whole lumbosacral cord segment were not significantly changed by FCA treatment at all time points. InsP(3) accumulation was significantly increased on days 14, 21 and 42 following FCA injection relative to sham-treated time-matched controls. However, cGMP and insP(3) contents were significantly increased in the left longitudinal half of the lumbar enlargement ipsilateral to the injected paw on day 21 following FCA treatment, but not in the sham-treated time-matched controls. With [H-3]insP(3) as a ligand, Scatchard (Rosenthal) analyses of the concentration-dependent saturation curves showed that the densities (B-max) of insP(3) receptors (insP(3)R) were significantly increased throughout the time course of adjuvant-induced peripheral inflammation. The binding affinities (K-D) for insP(3)R were significantly decreased on days 7, 14 and 21 following FCA injection corresponding to the times of most stable and peak inflammation. InsP(3)R from the cerebelli of the same rats as used in the lumbosacral insP(3)R characterization was used as a positive control in this study and did not show any change in both B-max and K-D as a result of FCA treatment, thus demonstrating that the changes in lumbosacral insP(3)R characteristics might be specific to the nociceptive sensory pathway such as the spinal cord. Thus it appears that sustained afferent nociceptive input induced by FCA injection increased the accumulation of cGMP, insP(3) and insP(3)R density in the spinal cord through increased neuronal activities of functional receptors coupled to major classes of chemical mediators of nociception including neuropeptides and excitatory aminoacids. Changes in insP(3) accumulation in the lumbosacral cord following FCA injection were significantly correlated with changes in insP(3)R density. Changes in the ratios of lumbosacral insP(3) contents and insP(3)R density were also significantly correlated with changes in body weight and hind paw size induced by FCA injection. Thus, a relationship is established between modulation in the spinal cord insP(3)/insP(3)R system, an intracellular calcium-mobilizing transduction system, and modifiable systemic variables induced by peripheral inflammation.
