Rehabilitating the dendritic spine

shown that the varicosities contain numerous small vesicles as well as a few large vesicles in both of which the neurotransmitter (serotonin) is stored. It has also been revealed by TEM that the axons are probably tethered to the ventricle surface at the site of each varicosity by desmosome-like junctions between the axons and ependymal cells 2. However, typical synapses have not been observed. By SEM the axons can be seen traversing the ventricle surface, sometimes crossing each other's path (Fig. IC) or appearing to converge together. Experiments using [~2~I]tetanus toxin as a tracer, have demonstrated that their cell bodies are located in the dorsal raphe nucleus (B7 group of serotoninergic neurons). Autoradiographical studies have shown that supraependymal nerves become selectively labelled with [SH]reserpine 4 and [3H]serotonin3 in vivo; the former binds specifically to amine-storing organelles and the latter is accumulated by the aminepump which is a physiological mechanism for inactivating the released serotonin by removing it from the vicinity of the target cell. Indeed, we have successfully used its serotonin-specific uptake mechanism to autoradiographically evaluate uptake inhibitors being developed as potential antidepressants. In this regard it is perhaps surprising to find such an uptake in this non-synaptic nerve plexus unless the transmitter were released to act locally e. Current research, focusing on the localization of serotonin receptors in the cerebroventricular system, may reveal the function of these nerves which for the time being must remain a mystery.