Sensitivity to GABA of neurons of the dorsal and ventral lateral geniculate nuclei in the rat

Modulation of visually evoked responses in units of the ventral lateral geniculate nucleus of the rat by somatic stimuli

Single unit activity was recorded from the ventral part of the lateral geniculate nucleus (vLGN) in rats anaesthetized with urethane. Most of the cells located laterally in the nucleus were excited by light. The studied vLGN neurones did not respond to electrical stimulation of the tail, but about half of them changed their response to light significantly when the light flash was paired with the electrical stimulation. When the tail stimulus preceded the light, the changes consisted in a pronounced facilitation of flash-evoked activity. When the electrical stimulus was applied after the flash in a forward conditioning paradigm, facilitations were less pronounced and responses of some neurones were suppressed. These results are in contrast to those of similar experiments on the dorsal LGN, neurones of which were mainly facilitated by the conditioning paradigm. Thus, light-evoked activity of ventral geniculate cells can be enhanced by arousal-related processes.

Laminar organization and ultrastructure of GABA-immunoreactive neurons and processes in the dorsal lateral geniculate nucleus of the tree shrew (Tupaia belangeri)

The distribution and ultrastructure of neurons and neuropil labeled by an antiserum to gamma-aminobutyric acid (GABA) were examined in the lateral geniculate nucleus (LGN) of the tree shrew (Tupaia belangeri). The LGN of this species segregates center type and cell class into three distinct pairs of laminae: a medial pair (laminae 1 and 2) containing ON-center cells, a more lateral pair (4, 5) containing OFF-center cells, and 2 laminae (3, 6) containing W-like cells. The relationship between this laminar segregation and the distribution of GABA immunoreactivity was investigated in the present study. GABA-immunoreactive neurons and neuropil were present in all six of the laminae. However, both the density of labeled cells (adjusted for neuronal density across laminae) and the density of labeled neuropil showed a medial-to-lateral gradient. The adjusted density of labeled cells was higher laterally than medially, and the density of labeled neuropil was significantly greater in the more lateral OFF-center laminae and W-like laminae than in the medial two ON-center laminae. Thus, inhibitory, GABAergic influences may modulate to different degrees the visual signals in the ON, OFF, and W pathways. Labeled cells had a mean cross-sectional area (107 microns 2) approximately one-half that of unlabeled cells (216 microns 2). They constitute 16-34% of the neurons in the LGN. At the electron microscope level, three different kinds of labeled profile were observed. Vesicle containing profiles like the F2 profiles of cat were postsynaptic to retinal terminals and presynaptic to conventional dendrites. F1 axon terminals with dense clusters of vesicles were also labeled as were some myelinated axons. Another labeled profile, which we suggest should be called an F3 process, was a large dendrite of irregular caliber with punctate groups of vesicles near the synapse. Our results suggest that GABAergic circuitry is an important part of the functional organization in the LGN of the tree shrew.

Influence of atropine microinjection into nucleus reticularis thalami on activity of lateral geniculate nucleus neurones in freely moving rats

The influence of the microinjection of atropine into the visual part of the nucleus reticularis thalami (TR) on flash-evoked unit responses of the dorsal lateral geniculate nucleus (dLGN) was investigated in freely moving rats. Atropine induced a significant prolongation and accentuation of the postexcitatory inhibitory phases in water-deprived trained rats, to which light flashes acted as conditioned stimuli and initiated a drinking behaviour. In satiated relaxed rats atropine did not change the unit activity significantly. Acetylcholine injected into TR caused a facilitation of the dLGN transmission. Since the decrease of flash-evoked excitatory response phases in thirsty, trained rats as compared to satiated ones could not be blocked by atropine, not all behaviourally dependent changes are mediated by a disinhibitory action of cholinergic fibres on TR.

Ascending, descending and local control of neuronal activity in the rat lateral geniculate nucleus

Mechanisms of control for activities of relay neurons (P-cells) in the rat dorsal lateral geniculate nucleus (LGNd) are surveyed with special reference to ascending projection arising from the locus coeruleus (LC), the local projection from the visual portion of the thalamic reticular nucleus (vTRN) and the descending projection from the visual cortex (VC). Noradrenaline released from terminals of LC neurons exerts a facilitatory influence on P-cell activity via alpha-receptors. A recurrent projection of vTRN neurons on P-cells is inhibitory, utilizing GABA as a transmitter. P-cells receive an excitatory input from corticothalamic neurons of VC. However, in many P-cells the corticofugal excitation is counterbalanced by inhibition arising in vTRN neurons which are invariably exited by the collateral branches of the corticogeniculate axons. Thus, LGNd is not a simple relay station, but various modifications of visual information are made in this nucleus.

Asymmetric and symmetric synaptic junctions in the dorsal lateral geniculate nucleus of cat and monkey

We have investigated the applicability of traditional classifications of synaptic junctions in the dorsal lateral geniculate nucleus (DLGN) of the cat and monkey. Our principal sample is restricted to synapses made by the retinal terminal (round vesicles) in DLGN and to synapses made by flattened vesicle processes that were postsynaptic to the retinal terminal. We inspected consecutive thin sections through 250 synaptic junctions that showed clear synaptic clefts in every section. The thickness of the membrane and postsynaptic density (PSD) was measured on each section and an average thickness was computed for each synaptic junction. In both species the frequency distribution for these measurements forms an uninterrupted progression from the absence of a continuous PSD through the presence of a heavy density with most synaptic contacts falling in the midrange. Twenty-two of the round vesicle profiles and 40 of the flat vesicle profiles we studied had very modest densities (13-16 nm) and exhibited a continuous PSD on some sections, but only small puffs or a complete lack of density on others. We concluded that this group which constituted 25% of the synaptic contacts we studied could not be classified as asymmetric or symmetric. As a group the round vesicle synaptic junctions exhibited a heavier PSD than the flat vesicle contacts. The difference between the mean thickness in both species was statistically significant. However, we hesitate to describe the round vesicle synapses as asymmetric and the flat vesicle contacts as symmetric because such a large proportion of the former made synaptic contacts with a PSD thickness within the range of the latter.

Neural elements containing glutamic acid decarboxylase (GAD) in the dorsal lateral geniculate nucleus of the rat; immunohistochemical studies by light and electron microscopy

Immunoreactive constituents of the dorsal lateral geniculate nucleus of adult albino rats were examined by light- and electron-microscopy, using the unlabelled antibody enzyme method, following treatment of brain slices with a purified antibody to glutamic acid decarboxylase. The neuropil of the dorsal lateral geniculate nucleus displayed a conspicuous granular immunoreactivity. In addition, the antibody was bound to a class of small neurons of characteristic morphology. These cells possessed few (commonly 2-4) sparsely branched, long dendrites from some of which immunoreactive appendages were traced. Many cells were bipolar in form, and the dendrites of some appeared to be preferentially orientated. The immunoreactive cells closely resembled intrinsic interneurons characterized in previous Golgi studies of this nucleus. By electron-microscopy, immunoreactive presynaptic elements were present both in the extraglomerular neuropil and in the synaptic glomeruli. The former were axon terminals containing flattened synaptic vesicles and making Gray type II axo-dendritic synaptic contact; they appeared to correspond to axon terminals whose origin in the thalamic reticular nucleus has been established in previous studies, but it is possible that some were axon terminals of intrinsic interneurons. The immunoreactive glomerular components also contained flattened vesicles, were presynaptic to presumptive projection cell dendrites, postsynaptic to retinal axon terminals, and participated in triplet (triadic) and other complex synaptic arrangements. They corresponded in all respects to the synaptic portions of the complex dendritic appendages of intrinsic interneurons, identified and characterized in previous studies. The finding that there are high levels of glutamic acid decarboxylase in the cell bodies, dendritic shafts and dendritic appendages of intrinsic interneurons in the dorsal lateral geniculate nucleus of the rat, and in the axon terminals of fibres projecting to this site from the thalamic reticular nucleus, allows us to conclude that the inhibitory inputs to the geniculo-cortical projection cells from both of these sources are probably mediated by gamma-aminobutyric acid.