IP3- and cAMP-induced responses in isolated olfactory receptor neurons from the channel catfish.

Olfactory receptor neurons enzymatically dissociated from channel catfish olfactory epithelium were depolarized transiently following dialysis of IP3 or cAMP (added to the patch pipette) into the cytoplasm. Voltage and current responses to IP3 were blocked by ruthenium red, a blocker of an IP3-gated Ca(2+)-release channel in sarcoplasmic reticulum. In contrast, the responses to cAMP were not blocked by extracellularly applied ruthenium red, nor by L-cis-diltiazem or amiloride and two of its derivatives. The current elicited by cytoplasmic IP3 in neurons under voltage clamp displayed a voltage dependence different from that of the cAMP response which showed marked outward rectification. A sustained depolarization was caused by increased cytoplasmic IP3 or cAMP when the buffering capacity for Ca2+ of the pipette solution was increased, when extracellular Ca2+ was removed or after addition of 20-200 nM charybdotoxin to the bathing solution, indicating that the repolarization was caused by an increase in [Cai] that opened Ca(2+)-activated K+ channels. The results suggest that different conductances modulated by either IP3 or cAMP are involved in mediating olfactory transduction in catfish olfactory receptor neurons and that Ca(2+)-activated K+ channels contribute to the termination of the IP3 and cAMP responses.