NEUROTENSIN AND SUBSTANCE-P INHIBIT LOW-VOLTAGE-ACTIVATED AND HIGH-VOLTAGE-ACTIVATED CA2+ CHANNELS IN CULTURED NEWBORN RAT NUCLEUS BASALIS NEURONS

Inhibition of Ca2+ currents by the excitatory neurotransmitters neurot ensin and substance P was investigated in cultured nucleus basalis neu rons with the use of the whole cell. patch-clamp technique. The whole cell Ca2+ current, elicited from a holding potential of -80 mV by a st ep pulse to 0 mV and measured at 100 ms, was inhibited 67.9% by neurot ensin and 57.6% by substance P. Low-voltage-activated (LVA) Ca2+ curre nt, elicited by a step pulse to -40 mV from a holding potential of -90 mV, was inhibited by both neurotensin (26.2%) and substance P (24.1%) . High-voltage-activated Ca2+ currents were separated with the use of the Ca2+ channel antagonists. Nimodipine (3 mu M) inhibited 24.2% of t he whole cell Ca2+ current elicited by a step to 0 or +10 mV and measu red at 100 ms. Under the same conditions, omega-conotoxin (omega-CgTx) -GVIA (0.5 mu M) inhibited 46.4%, omega-CgTx-GVIA + nimodipine 58.7%, and omega-CgTx-MVIIC (5 mu M) + nimodipine 75.7% of the current. omega -Agatoxin (omega-Aga)-IVA (100 nM) did not produce any effect. Neurote nsin inhibition of the whole cell Ca2+ current was attenuated by each of these treatments except for the omega-Aga-IVA treatment, which did not change the neurotensin effect. In contrast, neither the omega-Aga- IVA nor the nimodipine treatment had any effect on the substance-P-ind uced inhibition; the rest of the treatments attenuated the substance-P -induced response. Thus the data indicate that nucleus basalis neurons express LVA as well as L-, N-, and Q-type, but not the P-type, Ca2+ c urrents. N- and Q-type HVA Ca2+ currents, as well as LVA Ca2+ currents , are inhibited by both neurotensin and substance P. In contrast, L-ty pe current is inhibited by neurotensin but not by substance P. In addi tion, a fraction of the total whole cell current was resistant to all Ca2+ channel antagonists and thus may correspond to the R-type Ca2+ cu rrent. This residual current was inhibited by both neurotensin and sub stance P. The inhibition of the whole cell Ca2+ current produced by bo th neurotransmitters was voltage independent, because a large depolari zation (+70 mV) was not able to relieve either effect. In cells loaded with 0.1 mM guanosine 5'-[gamma-thio]triphosphate, response to both n eurotensin and substance P became irreversible, indicating that the ef fects of both neurotransmitters were mediated through G proteins. Howe ver, pertussis toxin did not affect either the neurotensin or the subs tance P response.