Organisation of receptors for neurotransmitters on Aplysia neurones

Abstract

THE application of acetylcholine (ACh) to neurones of Aplysia has shown the existence of three different responses, mediated by conductance increases to Na⁺, Cl⁻ and K⁺ respectively¹. These neurones also have receptors for various other putative neurotransmitters, including serotonin², dopamine³, octopamine^4,5, phenylethanolamine⁵, γ-aminobutyric acid^6,7, glutamic acid^6,7, aspartic acid⁷ and histamine^5,8. With one possible exception there are at least three types of response to each of these substances resulting from conductance increases to Na⁺, Cl⁻ and K⁺, respectively (refs 5–7 and our unpublished observations). These facts suggest that specific receptors (neurotransmitter-bindmg sites) and ionophores for Na⁺, Cl⁻ and K⁺ might be building blocks which can be assembled in any combination. If so there should be common properties of the receptors for one transmitter mediating the three different ionic responses, as well as similarities in the responses to different transmitters when the conductance change is to the same ion. In this report we compare the three dopamine and ACh responses as a test of this hypothesis.