A study was made of the synthesis of acetylcholine (ACh) and other transmitters by the cell bodies of functionally identified neurons in leech segmental ganglia. 2. Choline acetyltransferase, the synthetic enzyme for ACh, was detected in excitatory motoneurons but not in mechanosensory cells or Retzius cells. The ability of motoneurons to synthesize ACh was also demonstrated by their accumulation of [3H]ACh following incubation of segmental ganglia with [3H]choline. [3H]ACh was not detected in the other cell types. When eserine was included in [3H]choline incubations, the amount of [3H]ACh in motoneurons increased severalfold and small amounts of [3H]ACh (1% that in motor cells) appeared in extracts of sensory and Retzius cells. 3. In addition to [3H]ACh segmental ganglia synthesized [3H]5-HT, [3H]gamma-aminobutyric acid, [3H]dopamine, and [3H]octopamine from exogenous, labeled precursors. None of these labeled transmitters was detected in identified neurons except [3H]5-HT, which was found in Retzius cells. 4. These results provide biochemical evidence that excitatory motoneurons in the leech are cholinergic, but leave open the identity of the sensory transmitter(s).
