The cytoarchitecture of GABAergic neurons in rat spinal cord

Glutamic acid decarboxylase (GAD), the enzyme that synthesizes the transmitter gamma-aminobutyric acid (GABA), has previously been localized within synaptic terminals in rat spinal cord by immunocytochemistry. In the present study, GAD was localized within the somata and dendrites of GABA neurons following colchicine injections into rat lumbar spinal cord. All regions of the spinal gray matter contained GAD-positive somata except the motoneuron pools (lamina IX). GAD-positive somata also were observed in the ependymal layer and in the dorsolateral funiculus. Small GAD-positive somata, averaging 9 X 13 micrometer in size, were located in laminae I-III, and the size of GAD-positive somata increased for cells located in progressively more ventral laminae, reaching a maximum in lamina VII where somal size averaged 12 X 19 micrometer. Lamina I contained two classes of GAD-positive cell bodies; lenticular shaped, intermediate size neurons that were reminiscent of stalked cells, and a smaller cell type that was elongated in the sagittal plane. GAD-positive somata in laminae II and III had the size and position of islet cells. In laminae IV-VI, GAD-positive somal profiles averaged 12 X 17 micrometer in size. Lamina IV neurons were concentrated along laminar edges, while those in laminae V and VI were distributed more homogeneously. In lamina VIII, GAD-positive cell bodies appeared in groups of 3 or 4 and were smaller than those in lamina VII. Lamina X contained GAD-positive somal profiles averaging 12 X 16 micrometer in size. In the ependymal layer, there were two types of cerebrospinal fluid (CSF)-contacting neurons that contained GAD; one spherical and the other elongated. Both types sent extensions into the central canal where these processes expanded into 4-5 micrometer-wide end bulbs. CSF-contacting cells with sizes and shapes similar to the GAD-positive ones were seen to receive synapses in electron micrographs. The widespread distribution of GABA neurons in spinal cord was suggestive of diverse functions for these cells, encompassing conventional synaptic roles and, perhaps, an involvement in hormonally modulated communication via GABAergic, CSF-contacting neurons.