Morphological Correlates of Triadic Circuitry in the Lateral Geniculate Nucleus of Cats and Rats

We used an in vitro slice preparation of the lateral geniculate nucleus in cats and rats to study morphological correlates of triadic circuitry in relay cells. The three triadic elements involve a retinal synapse onto a GABAergic dendritic terminal of an interneuron, a synapse from the same retinal terminal onto a relay cell dendrite, and a synapse from the same interneuron terminal onto the same relay cell dendrite. We made whole cell recordings and labeled cells with biocytin. Previous methods were used to identify triadic circuitry based on evidence that the retinal terminal activates a metabotropic glutamate receptor on the interneuronal terminal. Thus application of (±)-1-aminocyclopentane- trans-1,3-dicarboxylic acid (an agonist to that receptor) increases the rate of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded in the relay cell, and if some of this increase remains with further addition of TTX (a TTX-insensitive response), a triad is indicated. We quantified the extent of the TTX-insensitive response and sought morphological correlates. In both rats and cats, this response correlated (negatively) with the number of primary dendrites and (positively) with polarity of the dendritic arbor. There was no correlation with cell size. Curiously, in cats, this response correlated with the presence of appendages at primary dendritic branches, but there was no such correlation in rats. These observations in cats map onto the X/Y classification, with X cells having triads, but it is not clear from our results if a comparable classification exists for rats.

Distribution of GABA-like immunoreactive neurons and fibers in the central visual nuclei and retina of frog, Rana pipiens

Immunocytochemistry was used to study the distribution of gamma-aminobutyric acid (GABA) throughout the central visual nuclei and retina in Rana pipiens. In the retina, GABA immunoreactivity (both somata and fibers) was observed in all layers except the outer nuclear layer (ONL). Contrary to earlier reports, about 30% of total neurons within ganglion cell layer (GCL) expressed GABA immunoreactivity. Double-labeling studies indicated that about half of the GABA-containing perikarya in the GCL were retinal ganglion cells (RGCs). In the diencephalon, intensely labeled GABA-immunoreactive neurons and nerve fibers were observed within the neuropil of Bellonci (nB) and corpus geniculatum (CG), while only immunoreactive puncta were found in the rostral visual nucleus (RVN). In the pretectal region, the posterior thalamic nucleus (nPT) contained the most intensely labeled GABA immunoreactive perikarya and nerve fibers in the entire brain. Lightly immunoreactive perikarya were also found in the large-celled nucleus lentiformis mesencephali (nLM), as well as in the pretectal gray, which contains neurons postsynaptic to the retinal terminal zones within nLM. In the optic tectum (OT), both immunoreactive perikarya and fibers were found within superficial layers 8 and 9, whereas only densely packed immunoreactive perikarya were evident in the deep tectal layers (i.e. 2, 4, 6). The nucleus of the basal optic root (nBOR) contained a small number of lightly labeled GABA-immunoreactive perikarya, mostly located in the dorsal half of the nucleus. A large number of perikarya within the nucleus isthmi (NI) were also immunostained.