Involvement of enzymatic degradation in the inactivation of tachykinin neurotransmitters in neonatal rat spinal cord.

1. The possible involvement of enzymatic degradation in the inactivation of tachykinin neurotransmitters was examined in the spinal cord of the neonatal rat. 2. The magnitude of substance P (SP)- or neurokinin A (NKA)-evoked depolarization of a lumbar ventral root in the isolated spinal cord preparation was increased by a mixture of peptidase inhibitors, consisting of actinonin (6 microM), arphamenine B (6 microM), bestatin (10 microM), captopril (10 microM) and thiorphan (0.3 microM). The mixture augmented the response to NKA more markedly than that to SP. 3. In the isolated spinal cord-cutaneous nerve preparation, the saphenous nerve-evoked slow depolarization of the L3 ventral root was augmented by the mixture of peptidase inhibitors in the presence of naloxone (0.5 microM) but not in the presence of both naloxone and a tachykinin receptor antagonist, GR71251 (5 microM). 4. Application of capsaicin (0.5 microM) for 6 min to the spinal cord evoked an increase in the release of SP from the spinal cord. The amount of SP released was significantly augmented by the mixture of peptidase inhibitors. 5. Synaptic membrane fractions were prepared from neonatal rat spinal cords. These fractions showed degrading activities for SP and NKA and the activities were inhibited by the mixture of peptidase inhibitors. The degrading activity for NKA was higher than that for SP and the inhibitory effect of the mixture for NKA was more marked than that for SP. Although some other fractions obtained from homogenates of spinal cords showed higher degrading activities for SP, these activities were insensitive to the mixture of peptidase inhibitors. 6. Effects of individual peptidase inhibitors on the enzymatic degradation of SP and NKA by synaptic membrane fractions were examined. Thiorphan, actinonin and captopril inhibited SP degradation, while thiorphan and actinonin, but not captopril, inhibited NKA degradation. The potency of the inhibition of each peptidase inhibitor was lower than that of the mixture.7. The present results suggest that enzymatic degradation is involved in the inactivation of tachykinin neurotransmitters in the spinal cord of the neonatal rat.