|
SYNAPTIC ORGANIZATION OF PULVINAR PROJECTING PORTIONS
OF THE CAT THALAMIC RETICULAR NUCLEUS. |
|
|
|
|
Intrinsic connections between GABAergic cells in the
thalamic reticular nucleus (TRN) may play an important role in synchronizing
the activity of the TRN and its target nuclei in the dorsal thalamus. We
recently examined the synaptic circuitry of the cat perigeniculate nucleus
(PGN), which projects specifically to the dorsal lateral geniculate nucleus (dLGN),
and were surprised to find very few GABAergic contacts on PGN cells. To
determine whether this reflects the general organization of the visual sector
of the TRN, or is a unique feature of the PGN, we have begun to examine the
synaptic organization of portions of the TRN that project to the cat pulvinar
nucleus. We injected tracers into the pulvinar nucleus to label
pulvinar-projecting TRN cells (PP-TRN). We then characterized the synaptic
contacts on PP-TRN cells labeled by retrograde transport in tissue stained
for GABA using postembedding immunocytochemistry. In contrast to our previous
study, where GABAergic contacts made up as little as 3% of the input to PGN
cells, we found that PP-TRN cells received up to 42% of their input from
GABAergic terminals. Most of these profiles contained densely packed synaptic
vesicles characteristic of axon terminals, although occasional GABAergic
dendrodendritic contacts were also observed. Most other contacts in the
PP-TRN were made by small non-GABAergic profiles (42%, presumed
corticothalamic terminals). In contrast to the PGN, where contacts made by
large non-GABAergic terminals (presumed thalamocortical collaterals) provide
the majority of the input to PGN cells, large non-GABAergic profiles were not
observed in the PP-TRN. These results suggest that connections between
GABAergic cells are more common in the main body of the TRN than in the PGN,
and thalamocortical collateral projections may be more prevalent in the PGN,
or may display distinct morphologies in the PGN and TRN. |