Journal of Neuroscience, Vol 9, 967-982, Copyright © 1989 by Society for Neuroscience
An EM study of the dorsal nucleus of the lateral lemniscus: inhibitory, commissural, synaptic connections between ascending auditory pathways
DL Oliver and A Shneiderman
Department of Anatomy, University of Connecticut Health Center, Farmington 06032.
The dorsal nucleus of the lateral lemniscus (DNLL) and its connections
constitute one of the ascending auditory pathways to the inferior
colliculus. One notable feature of this nucleus is the heavy commissural
connections between DNLL on opposite sides of the midbrain. These
commissural connections may have a significant impact on the ascending
pathway. In this study, the fine structure of DNLL in the cat and its
commissural connections were examined. Both anterograde and retrograde
transport methods were used simultaneously at the EM level. Injections of
3H-leucine mixed with WGA-HRP were made in one DNLL. After axonal
transport, EM autoradiographic methods were used to identify the
anterogradely labeled axonal endings from the opposite DNLL. In the same
location, retrogradely labeled neurons with crossed connections were
identified with HRP histochemistry. Two types of axonal endings were found
in DNLL, those with round synaptic vesicles forming asymmetrical synaptic
junctions and those with pleomorphic vesicles and symmetrical synapses.
Both types were equally common. However, only endings with pleomorphic
vesicles were labeled after injections in the contralateral DNLL. The
labeled endings from the opposite DNLL appeared to represent a homogeneous
population, even though a number of variations in the 2 types of endings
were found. Labeled endings were presynaptic to all parts of neurons in
DNLL, but a large proportion of the synapses were on cell bodies and large
dendrites. Two patterns of nuclear morphology and distribution of rough
endoplasmic reticulum were identified and may represent different cell
types. Examples of both cell types were observed to project to the
contralateral side and received labeled synaptic endings. The major finding
of this study is that the crossed connections between DNLL exhibit the
morphology associated with inhibitory function. Since neurons in DNLL are
thought to use GABA as a neurotransmitter, the crossed connections could
provide inhibitory inputs to DNLL on each side. Since some neurons receive
numerous axosomatic inputs from the contralateral DNLL and also project to
the opposite side, they may participate in direct reciprocal, inhibitory
connections between the nuclei. Crossed inhibitory connections in the DNLL
pathway may be important in regulating the flow of ascending auditory
information.
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