Properties of a slow nonselective cation conductance modulated by neurotensin and other neurotransmitters in midbrain dopaminergic neurons.

1. A widespread mechanism of slow excitation throughout the nervous system involves overlapping changes in nonselective ion conductance and K+ conductance. We used whole cell patch-clamp recording to characterize such a nonselective conductance induced by neurotensin (NT) and other neurotransmitters in immunocytochemically identified dopaminergic neurons cultured from the rat ventral tegmental area (VTA). 2. The NT-induced inward current consisted of an initial peak and later "hump." The response was blocked reversibly by the nonpeptide NT-receptor antagonist SR48692, suggesting that it resulted from activation of NT receptors. 3. The channel was almost equally permeable to Na+ and K+, as determined from the reversal potential shift upon switching from Na+- to K(+)-containing external solution. The permeability of Cs+ was similar to that of Na+, as determined from the zero-current equation and average reversal potential in the 75 mM Na+ solution. Cl- was not significantly permeable. 4. In Ca(2+)-free external solution, the NT-induced current showed a fourfold increase in amplitude, and in high Mg2+ (20 mM) external solution, the NT-induced current showed an 80% decrease in amplitude, suggesting that external Ca2+ and Mg2+ could block the nonselective conductance. 5. The NT response was unaffected by loading the neurons with either the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or with 1 mM ca2+. The nonselective conductance was therefore not Ca2+ activated. 6. Loading the neurons with cyclic GMP or cyclic AMP (each with the phosphodiesterase inhibitor isobutyl-methylxanthine) did not affect the NT response. The NT-induced nonselective conductance was therefore not cyclic nucleotide-activated. 7. The latency of the NT response was long (> or = 185 ms, average 406 ms, 30 degrees C), indicating that NT did not induce the conductance through ligand-gated channels. Thus, NT activated a slow nonselective cation conductance. 8. Neurokinin B, a metabotropic glutamate agonist, and muscarine elicited responses similar to the NT response. The NT response could be elicited after desensitizing the responses to these other neurotransmitters, indicating receptor specificity in the activation of the nonselective conductance.