Very low concentrations of acetylcholine and GABA modulate transmitter responses

Acetylcholine and GABA, at extremely low concentrations (10(-14) to 10(-8) M) which do not induce electrical responses, each modulate the chloride-dependent responses of Aplysia neurons to higher concentrations of the same transmitters. Low concentrations of acetylcholine cause a reduction of the response to both acetylcholine and GABA, while low concentrations of GABA cause a facilitation of responses to both acetylcholine and GABA. These effects have a clear threshold, are slow in onset, reversible with prolonged washing, and blocked by cooling.