Specificity of axonal transport in C2, a histaminergic neuron of Aplysia californica.

Several labeled neurotransmitter substances or precursors ( [3H]histamine, [3H]serotonin, and [3H]choline) were injected with pressure into the cell body of C2, an identified putative histaminergic neuron in the cerebral ganglion of Aplysia in order to study the selectivity of axonal transport. We examined transport of these substances along the posterior lip nerve which contains one of the 3 extraganglionic axons of C2. The distribution of radioactivity along this axon indicated that some of the [3H]histamine is moved by fast transport, and some by diffusion. The velocity at which [3H]histamine moved along the axon was estimated at approximately 50 mm per day at 23 degrees C. The movement was slower at lower temperatures, and was partially inhibited by colchicine. Transport was selective: 6 h after injection of [3H]histamine into the cell body of C2, most of the radioactivity that appeared in axons was in the form of histamine. In contrast, equal amounts of the labeled amine and its metabolite, gamma-glutamylhistamine, were found in the cell body. Fast transport was not observed when [3H]serotonin or [3H]choline were injected. Subcellular fractionation experiments after injection showed that [3H]histamine was enriched in particulate fractions; [3H]gamma-glutamylhistamine was recovered only in the soluble fraction. Packaging of [3H]histamine in C2 was not affected by treatment with reserpine; in contrast, subcellular fractionation experiments indicated that reserpine blocks the uptake of [3H]histamine into vesicles in the giant cerebral neuron, an identified serotonergic Aplysia cell.