Involvement of potentially distinct neurotensin receptors in neurotensin-induced stimulation of striatal [3H]dopamine release evoked by KCl versus electrical depolarization

We intended to determine whether the effect of neurotensin (NT) on K+ and electrically evoked 3H]dopamine (DA) release from rat and guinea-pig striatal slices involved different mechanisms and/or receptors. In the two species, NT and three NT agonists were found to exhibit different relative potencies to enhance K+- and electrically-evoked 3H]DA release. NT(1-13) increased 3H]DA release with EC50 values in the nanomolar range and Emax values in the range of 100% of control. NT(8-13) and Eisai hexapeptide were both as active as NT(1-13) under K+ depolarization, but did not exceed 40% of the NT(1-13) effect under electrical depolarization. In rats, when 3H]DA release was stimulated with two successive K+ depolarizations, in the presence of NT(1-13), the NT effect during the second exposure to K+ was drastically decreased, suggesting that the NT receptor was desensitized. The desensitization process was essentially observed on Emax values, EC50 values being weakly affected. Similar results were obtained in guinea pig. In contrast, with two electrical depolarizations or with two different depolarizations (K+ followed by electrical), the NT effect during the second depolarization was not significantly affected. Concerning NT antagonists, SR 48692 antagonized with IC50 values in the nanomolar range the NT(1-13) stimulated K+-evoked 3H]DA release but did not affect, up to 10(-6) M, the NT(1-13) enhancement of electrically stimulated 3H]DA release. On the contrary, SR 142948A antagonized the NT(1-13) effect on K+- and electrically-evoked 3H]DA release. In conclusion, these results suggest the possible existence of potentially distinct neurotensin receptors differentially involved in the control exerted by NT on DA release under KCl vs electrical depolarization.