Distribution of receptors for acetylcholine and 5‐hydroxytryptamine on identified leech neurones growing in culture.

The spatial distribution of receptors on identified leech neurones removed from the C.N.S. and grown in culture has been studied by applying acetylcholine (ACh) and 5‐hydroxytryptamine (5‐HT) ionophoretically and by pressure. Two cells were selected: a neurone called anterior Pagoda (Ap), that shows responses to ACh, and the pressure sensory neurone (P cell), upon which 5‐HT synapses form in culture. ACh receptors of Ap neurones in culture had properties similar to those of their counterparts in situ. Thus, ACh responses of Ap cells were mediated by Cl‐ and were blocked by curare and alpha‐bungarotoxin. The cell bodies of these neurones in culture had low (10 mV/nC) and uniform sensitivity to ACh over the surface of the soma. When a sprout grew out from the Ap cell, a region of increased sensitivity appeared at its base, with a gradient of sensitivity decreasing toward the tip of the neurite. Characteristically, the base was 3‐5 times more sensitive to ACh than the soma or the growth cone. Cells with multipolar processes developed a similar pattern of sensitivity for each sprout. P sensory neurones in culture showed similar distributions of sensitivity to 5‐HT and ACh. Experiments made with voltage clamp suggested that the non‐uniform responses to transmitter represent true differences in sensitivity. Together these findings suggest that the receptors for ACh and 5‐HT have a greater density at the base of each neurite compared to that of the soma and the tip.