Tachykinin neurokinin-1 and neurokinin-3 receptor-mediated responses in guinea-pig substantia nigra: an in vitro electrophysiological study.

The effects of tachykinin receptor agonists and antagonists were investigated using intra- and extracellular recordings on spontaneously firing nigral neurons in guinea-pig brain slices. In 70 of 76 electrophysiologically identified dopaminergic neurons, a concentration-dependent increase in firing rate was induced by the selective neurokinin-3 tachykinin agonist senktide and by the natural tachykinin agonists neurokinin B and substance P, with EC50 values of 14.7, 31.2 and 12200 nM respectively. These responses were inhibited in a concentration- and time-dependent manner by the selective non-peptide neurokinin-3 receptor antagonist SR 142801 (1-100 nM; n=23), but neither by its S-enantiomer SR 142806 (100 nM; n=4) nor by selective antagonists of neurokinin-1 (SR 140333) or neurokinin-2 (SR 48968) receptors (both at 100 nM; n=3). The selective neurokinin-1 agonist [Sar9,Met(O2)11]substance P (30-100 nM; n=23) and the selective neurokinin-2 agonist [Nle10]neurokinin A(4-10)(30-100 nM; n=13) were without any effect on dopaminergic cells. In 13 of 21 electrophysiologically identified, presumably GABAergic neurons located in the pars compacta of the substantia nigra, excitatory responses were evoked concentration dependently by substance P and [Sar9,Met(O2)11]substance P, with EC50 values of 18.6 and 41.9 nM respectively. These responses were inhibited by SR 140333 (100 nM; n=3), but neither by its R-enantiomer SR 140603 nor by SR 142801 (both at 100 nM; n=3). Senktide and [Nle10]neurokinin A(4-10) (both at 30-100 nM; n=10) were without effect on these presumed GABAergic neurons. A small population (12%) of pars compacta neurons was insensitive to any of the three selective tachykinin agonists. In the nigral pars reticulata, 12 neurons were recorded which had an electrophysiological profile similar to that of presumed GABAergic neurons in the pars compacta. Of these 12 cells, seven did not respond to any of the selective tachykinin agonists tested, while five were excited by senktide in a concentration-dependent manner (EC50=98.5 nM). Although this value was significantly higher than that found for dopaminergic neurons in the pars compacta, senktide-evoked responses were inhibited by SR 142801 (100 nM; n=3). We conclude that, in the guinea-pig substantia nigra, tachykinins evoke excitatory responses in both dopaminergic and non-dopaminergic neurons; however, the sensitivity to tachykinin agonists (neurokinin-1 versus neurokinin-3) depends on both neuronal type and localization.