The role of 5‐hydroxytryptamine as a transmitter between identified leech neurones in culture.

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Abstract
The synthesis, storage, release and synaptic actions of 5-hydroxytryptamine (5-HT or serotonin) were studied in order to characterize the synaptic connection that develops between pairs of identified neurons dissected from the CNS of the leech and maintained in culture. Experiments were made with Retzius cells (which are known to contain 5-HT in vivo) and pressure sensory neurons on which they form chemical synapses in culture. When incubated with [3H]5-HT and washed, Retzius cells in culture accumulated .apprx. 100 times more labeled 5-HT than did non-serotonergic cells, and 10 times more than Retzius cell somata acutely isolated from the animal and incubated in vitro. Chlorimipramine, a blocker of 5-HT uptake, decreased the amount of [3H]5-HT accumulated by Retzius cells and also caused a reversible increase in the amplitude of the synaptic response in the pressure sensory cell elicited by stimulation of the Retzius cell. Pressure sensory neurons in culture and in vivo responded to 5-HT focally applied by pressure ejection from a micropipette. Small pulses elicited a small, slow hyperpolarization. This response was due, at least in part, to an increase in Cl- conductance and desensitized rapidly. With larger pulses, a larger, faster non-desensitizing depolarization was elicited. These results provide evidence that 5-HT released from Retzius cells could be responsible for the chemical synaptic potentials seen in pressure sensory neurons in culture.