The cat's lateralis medialis-suprageniculate nuclear
complex (LM-Sg) in the thalamus receives input from various
brain regions such as the superior colliculus, brain stem,
and spinal cord, as well as from visual association cortex.
In a previous study, we demonstrated that LM-Sg receives
cholinergic fibers from the pedunculopontine tegmental
nucleus (PPT) and that cholinergic terminals make synaptic
contacts with the dendrites of glutamatergic projection
neurons and of GABAergic interneurons (Hoshino et al.,
1997). In this study, we investigate the distribution and
the organization of PPT terminals by means of a combined
anterograde tracer (biotinylated dextran amine, BDA) and
immunohistochemical methods. When stained by acetylcholinesterase
(AChE), the LM-Sg is not uniformly immunoreactive, but
rather is patchily labeled and shows a streaming type of
reactivity. The tissue content appears high in enzyme activity
in AChE-positive zones and is much lighter in activity
in AChE-negative zones. We compared the synaptic organization
between AChE-positive and AChE-negative portions of the
LM-Sg in separate groups of electron-microscopic material:
four types of vesicle containing profiles (RS, RL, F1,
and PSD) as well as synaptic glomeruli were observed in
this nucleus. Among these, the PSD profiles were observed
more frequently in AChE-positive portions than in AChE-negative
zones. Furthermore, the number of glomeruli was significantly
higher in AChE-positive than in AChE-negative zones. Following
the injection of BDA into PPT, labeled terminals within
LM-Sg were rather more concentrated in the AChE-positive
portion. Although the majority of PPT terminals made synaptic
contacts with dendrites in the neuropil, a few terminals
were involved in the synaptic glomeruli. The present results
show that the synaptic organization is distinctly different
between the AChE-positive and AChE-negative portions of
LM-Sg. These results suggest that the AChE-positive portions
of LM-Sg are relatively more involved in integrating information
arising from a diverse set of inputs and processing that
information within glomeruli in a complex manner than occurs
in the AChE-negative portion of LM-Sg.