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A kainate receptor is an ionotropic receptor activated by the neurotransmitter glutamate. Activation opens an ion channel, coupling chemical recognition to an electrical response. The family is named after kainic acid, an experimental agonist. Kainate receptors and AMPA receptors are distinct receptor families even though some experimental agonists can activate both, making pharmacological responses alone difficult to interpret.^[ref]

A genetic experiment helped isolate the kainate component. C. Mulle and colleagues generated mice lacking the subunit then called GluR6. Neurons in the hippocampal CA3 region became much less sensitive to kainic acid; a postsynaptic kainate current evoked by a train of mossy-fibre stimulation was absent in the mutants. This linked a molecular subunit to a synaptically evoked response, rather than merely to a response produced by bathing tissue in an agonist.^[ref] The subunit's current name is GluK2.^[ref]

For example, a current produced by an agonist that also activates AMPA receptors cannot be assigned wholly to kainate receptors. Comparing selective perturbations with synaptic stimulation supplies stronger evidence for the receptor responsible. In another experimental setting, paired recordings in ground-squirrel retina showed that AMPA receptors and kainate receptors could route cone signals into OFF retinal bipolar cells with different temporal filtering properties.^[ref]

An OFF retinal bipolar cell is a retinal bipolar cell whose central receptive response shows depolarization for an appropriate decrease in illumination. Its input synapse preserves the sign of a cone photoreceptor's voltage response through excitatory AMPA receptors or kainate receptors, rather than the sign-inverting arrangement of an ON retinal bipolar cell.^[ref]

In a 2000 experiment, simultaneous recordings from connected cone cells and OFF retinal bipolar cells, together with receptor-selective manipulations, distinguished receptor mechanisms in different morphological OFF types. Differences in recovery from receptor desensitization filtered the same cone signal into different temporal components.^[ref]

Thus two OFF types can agree on whether a light decrement excites them while differing in response timing. Response sign, temporal filtering and neuron identity are related measurements, not interchangeable labels. The result from the studied retina should not be taken as an identical receptor assignment for every OFF subtype in every species.

S. H. DeVries investigated how parallel pathways in the retina extract different temporal components from a shared cone cell signal. In a 2000 study, simultaneous recordings from synaptically connected cone cells and OFF retinal bipolar cells linked distinct bipolar-cell types to different AMPA receptors or kainate receptors.^[ref]

The glutamate signal released by one cone cell can therefore enter several postsynaptic channels with different response dynamics. The study combined paired measurements with receptor-selective pharmacology in ground-squirrel retina, making it possible to compare the input and output at an identified connection.^[ref]

The motivating question is how the same changing light signal can support both a response to a rapid transition and a response to a more sustained change. Different receptor kinetics supply one experimentally testable part of that explanation. A common transmitter does not require identical temporal filtering in every receiving neuron; neither does a result in these cell types establish a universal receptor assignment across every species and retina.