Glycine uptake by lamprey spinal neurons demonstrated by light microscopic autoradiography

We have mapped the neuronal uptake of 3 H‐glycine in the spinal cords of large larval sea lampreys: Petromyzon marinus . Spinal cords were incubated in 10 −6 M 3 H‐glycine for 15 minutes. They were rinsed in lamprey solution, fixed in phosphate‐buffered 2% glutaraldehyde, and washed in phosphate buffer. They were then sectioned with a cryostat at 16‐m thickness or dehydrated, embedded in Epon, and sectioned at 1–4 μm. Sections were coated with a photographic emulsion and maintained at 4°C for 1–7 days. By sectioning horizontally, it was possible to obtain complete serial reconstructions of up to 1.5‐mm lengths of cord in 100–150 sections. The outlines of labelled cells were traced with a Nikon drawing attachment. For one Epon‐embedded spinal cord sectioned at 4 μm, tracings were superimposed to form complete maps for 0.6–1.5‐mm lengths in three representative regions of cord: rostral (gill region), caudal (dorsal fin region), and midsection. The labelled neurons were small (5–10‐μm diameter) cells distributed throughout the central gray columns. They numbered 22 cells per hemisegment in the rostral region, 33 in the midsection, and 43 in the caudal region. None of the previously identified cell types were labelled, including lateral interneurons, edge cells, giant interneurons, dorsal cells, and Müller and Mauthner axons.