On the role of galanin in mediating spinal flexor reflex excitability in inflammation

The effects of exogenous and endogenous galanin on spinal flexor reflex excitability was evaluated in rats one to eight days after the induction of inflammation by subcutaneous injection of carrageenan into the sural nerve innervation area. In normal rats, electrical stimulation of C-fibres in the sural nerve elicited a brisk reflex discharge. Conditioning stimulation of C-fibres (1/s) generated a gradual increase in reflex magnitude (wind-up), which was followed by a period of reflex hyperexcitability. Intrathecal galanin dose-dependently blocked reflex hyperexcitability induced by C-fibre conditioning stimulation whereas i.t. M-35, a high-affinity galanin receptor antagonist, moderately potentiated this effect. At one to three days after the injection of carrageenen, when inflammation was at its peak, the magnitude of the reflex was significantly increased and discharge duration became prolonged. However, wind-up and reflex hyperexcitability were significantly reduced. Furthermore, reduced reflex excitability during conditioning stimulation ("wind-down") and depression of the reflex were sometimes present, which are rarely observed in normal rats. Intrathecal galanin reduced hyperexcitability during inflammation, although its potency was weaker than in normals. However, the galanin receptor antagonist M-35 strongly enhanced wind-up and reflex hyperexcitability, similarly as in normal rats. The baseline flexor reflex, wind-up and C-fibre conditioning stimulation-induced facilitation were normalized four to eight days after carrageenan injection when signs of inflammation were diminishing. Interestingly, intrathecal galanin and M-35 failed to influence spinal excitability. The results suggest a complex functional plasticity in the role of endogenous galanin in mediating spinal excitability during inflammation. There appears to be an enhanced endogenous inhibitory control by galanin on C-afferent input during the peak of inflammation, which may explain the relative ineffectiveness of exogenous galanin. During the recovery phase there may be a reduction in galanin receptors, which may impair the action of endogenous and exogenous galanin. These results further support the notion that galanin is an endogenous inhibitory peptide in nociception.