The response properties of postreceptoral sensory neurones
are determined by the properties of their input neurones, by
intrinsic membrane properties, and by the properties of
neurotransmitter receptors on the soma and dendritic tree. We
previously showed that inhibitory neurotransmitter
(GABAA and glycine) receptors on a well-characterised
sensory neurone, the parasol ganglion cell in the primate retina,
are segregated towards the distal part of the dendritic tree.
Here we studied the distribution of excitatory ionotropic glutamate
receptor subunits on the dendrites of parasol cells in the retina
of a New World monkey, the marmoset, Callithrix jacchus.
Individual ganglion cells were intracellularly injected in an
in vitro retinal wholemount preparation. Ionotropic
glutamate receptor subunits, including AMPA (GluR1-4), kainate
(GluR6/7), NMDA (NR1C2′) subunits, and the orphan receptors
δ1 and δ2 were visualized with immunocytochemical methods.
Immunoreactive puncta that colocalized with the dendrites of
ganglion cells were analyzed using standard and/or confocal
light microscopy. Colocalized puncta were present on parasol
dendrites for all subunits studied, but their density was much
lower (approximately 1/5) than previously reported for inhibitory
(GABA and glycine) receptors. Segregation of the glutamate receptor
clusters (GluR1, GluR6/7 subunits) to the peripheral dendrites
was less marked than that shown for GABA and glycine receptor
clusters. No sign of segregation of colocalized puncta to the
peripheral part of the dendritic field was seen with antibodies
to the GluR2, GluR2/3, GluR4, δ1/2, or NR1C2′ subunits.
The results suggest that although there is diverse expression
of glutamate receptor subtypes, the glutamatergic synapses form
only a small proportion of the total synaptic input to primate
ganglion cells. They further suggest that the processes which
control distribution of excitatory and inhibitory synapses on
the dendritic field of ganglion cells are, at least to some