Involvement of substance P and neurokinin-1 receptors in the hyperexcitability of dorsal horn neurons during development of acute arthritis in rat's knee joint.

1. In anesthetized rats we studied the involvement of substance P and neurokinin-1 receptors in the generation and maintenance of hyperexcitability in spinal cord neurons, which develops in the course of an acute experimental inflammation in the knee. In all experiments one nociceptive neuron with knee input was identified, and the responses to mechanical stimuli and the receptive fields were monitored before and after induction of inflammation by the injections of kaolin and carrageenan into the knee joint. In 18 experiments either the specific antagonist at the neurokinin-1 receptor ionophoretically close to the neuron or intravenously during the injections of kaolin and carrageenan and in three periods of 15 min in the 95 min postkaolin (initial period of inflammation) to test their effects on the development of hyperexcitability. CP96,345 and CP96,344 were also administered after full development of inflammation to study their effects in hyperexcitable neurons. CP96,345 was ejected at currents that reduced or completely suppressed the effects of ionophoretically administered substance P but not those of neurokinin A, the agonist at neurokinin-2 receptors. 2. After the injections of kaolin and carrageenan into the knee joint, untreated control neurons (n = 8) developed hyperexcitability consisting of enhanced responses to noxious stimuli applied to the injected knee and the noninjected ankle, of an enhancement or induction of the responses to innocuous pressure applied to the joints and of an expansion of the receptive field. In eight neurons treated with ionophoretic administration of CP96,345 during the induction and initial period of inflammation, the development of hyperexcitability was not completely prevented but significantly attenuated. In comparison with the changes in the control neurons, the development of hyperexcitability was markedly reduced from the 2nd h up to 5 h postkaolin, but it was barely affected by CP96,345 within the 1st h postkaolin. Intravenous administration of CP96,345 in the initial period of inflammation produced a similar reduction of the development of hyperexcitability in another four neurons. The ionophoretic application of CP96,344 during and after induction of inflammation did not apparently impair the development of hyperexcitability (n = 6 neurons). 3. After development of inflammation and hyperexcitability, both the responses to innocuous and noxious pressure applied to the inflamed knee joint were reduced by the ionophoretic (n = 16 neurons) and intravenous administration (n = 9 neurons) of CP96,345 (tested 4.5-8 h postkaolin). Similarly, the responses to innocuous and noxious pressure applied to the noninflamed ankle were reduced by CP96,345 after inflammation had developed in the knee joint.(ABSTRACT TRUNCATED AT 400 WORDS)