SYNAPTIC CONDUCTANCES OF RETINAL BIPOLAR CELLS

Excitatory synaptic transmission in many pathways of the vertebrate CNS is
due to presynaptic release of an acidic amino acid, probably L-glutamate,
that binds to postsynaptic receptors directly linked to ion channels. In
addition to these tightly coupled receptor/channels, other classes of
glutamate receptors are apparently coupled to their effectors by indirect
mechanisms. L-glutamate and quisqualate stimulate IP3 turnover in both
Xenopus oocytes injected with brain mRNA and in hippocampal neurons. At
presynaptic receptors activated specifically by L-2-amino-4-
phosphonobutyrate (L-APB), L-glutamate inhibits excitatory transmission,
probably by decreasing voltage-dependent calcium currents (see Preliminary
Results). Finally, in the retina, ON bipolar cells are hyperpolarized by
L-glutamate released by photoreceptors by acting at a receptor specifically
activated by L-APB. Dialysis of ON bipolar cells results in the loss of
this response (see Results). The proposed research will attempt to determine the mechanisms by which the
effects of L-glutamate at APB receptors are produced in selected pathways
in the brain and retina. Electrophysiological techniques which measure
synaptic activity in populations of neurons and at single synapses as well
as molecular processes of single ion channels will be used. The actions of
L-APB on synaptic transmission will be characterized in neuronal cultures
derived from hippocampus and entorhinal cortex and in the hippocampal
slice. The molecular pathways altered by this receptor will be determined
by directly monitoring its effects on voltage-dependent currents and by
attempting to mimic its effects with exogenous compounds that alter second
messenger systems and the functioning of GTP binding proteins. The only neuron in the CNS that has clearly been shown to have a direct
electrical response to L-APB is the ON bipolar cell of the neural retina.
This neuron is hyperpolarized by both L-glutamate and L-APB, the end effect
of which is a decrease in transmitter release. When these neurons are
dialyzed by whole cell patch pipets, the response to the agonists "wash
out" suggesting that the response is mediated by indirect coupling of the
receptor to the conductance. The conductance changes induced by glutamate
and L-APB will be studied in slices of retina using whole cell and single
channel recording to determine the intracellular mechanisms which couple
this receptor to the hyperpolarization.