Role of Kainate Receptors in Synaptic Transmission

Pre-synaptic receptors

Over recent years, it has become clear that a major (even the major) functional locus of kainate receptors is in, or near, the pre-synaptic terminal, in particular in the hippocampus. Activation of kainate receptors in have been shown to regulate glutamate release (Chittalallu et al, 1996) and to both depress and factilitate transmission in different synapses at different stages of development.

Mossy Fibre-CA3 synapses

Pre-synaptic kainate receptors in the hippocampus facilitate both AMPA and NMDA receptor-mediated transmission at mossy fibre-CA3 synapses (Lauri et al, 2001). At low concentrations, kainate is known to facilitate mossy fibre transmission. A similar effect can be evoked by the delivery of 5 shocks @ between 25 and 100 Hz. This 'frequency facilitation' is inhibited in a concentration dependent manner by LY382884, a kainate receptor antagonist that is selective for GLUK1, and is dependent Ca2+-dependent Ca2+ release from intracellular stores, triggered by a Ca2+ influx through such GLUK1-containing kainate receptors (Lauri et al, 2003). A similar Ca2+ dependence  has been seen on kainate receptor-mediated regulation of inhibitory synaptic transmission in the medial prefrontal cortex (Mathew & Hablitz, 2008). It may also be part of the induction mechanism for mossy fibre LTP.

effect of ACET on calcium level in pre-synaptic mossy fibre boutonsACET reduces Ca 2+ influx into mossy fibre giant boutons. Individual dendate gyrus granule cells were patched and loaded with a mixture of Alexa Fluor 594 (red dye) and Fluo-4 (Ca 2+ sensitive dye) to identify boutons and to measure Ca 2+ concentrations in response to electrical stimulation of the individual cell. Treatement with ACET reduces Ca 2+ by ~20%. Mounted on the internet by permission of Dargan et al (2009) Neuropharmacology 56, 121-130 © Elsevier Ltd

There has been much controversy over recent years regarding the involvement of GluK1 containing receptors in mossy fibre function. However, further support for this has recently come from studies with ACET, the most potent and selective GluK1 antagonist developed to date. Work here has directly demonstrated that kainate receptors sensitive to ACET do indeed regulate Ca2+ signalling in giant mossy fibre boutons. This has been achieved using multi-photon imaging in acute brain slices (Dargan et al, 2009). Granule cells in the dentate gyrus were filled with red morphological dye (Alexa Fluor 594) to identify giant pre-synaptic boutons and a Ca2+ sensitive dye (Fluo-4) to measure Ca2+ fluxes. Scanning repeatably across a single line allows for very rapid measurement of the Ca2+ signal, giving very high temporal resolution. When a train of 5 stimuli is given, a rise in Ca2+ is seen in response to each individual stimulus. In the presence of ACET, the amplitude of the Ca2+ response to the same stimulus is reduced.

Like at CA1-schaffer collateral synapses, kainate receptors in neonates mediate a tonic inhibition of neurotransmitter release via a high affinity, pre-synaptic receptor (Lauri et al, 2005). The receptor is activated by ambient levels of glutame in the surrounding tissue. This effect was blocked by the use of pertussis toxin (PTX) and by blocking PKC - suggesting a metabotropic mode of action via PTX-sensitive G-proteins. This receptor is lost during the first postnatal week, a temporal profile similar to the loss of kainate receptor function in thalamocortical synapses.

CA1 - Schaffer Collateral Synapses

The GluK1 subunit antagonist ATPA has been shown to depress excitatory synaptic transmission at both CA1 and CA3 synapses (Vignes et al, 1998). The depressant action of kainate receptors at CA1 synapses has been shown to be developmentally regulated, reducing by nearly half in 10-16 week old rats (Clarke & Collingridge, 2002). The mechanism of action of these receptors in the hippocampus is still unclear. It is associated with a change in paired-pulse facilitation while indirect actions via GABA receptors, muscarinic receptors, adenosine A1 receptors or presynaptic depolarisation have been also been ruled out. This suggests that the locus of action is pre-synaptic but not ionotropic in nature. Inhibition of G-protein activity with pertussin toxin has been shown to eliminate a similar action of domoate, a kainate receptor agonist (Frerking et al, 2001). Thus it seems likely that these actions of kainate receptors occur via direct metabotropic effects where these ion channels act via a G-protein mediated signaling pathway.

frequenct facilitation in mossy fibre terminalsPresynaptic kainate receptors in CA1 hippocampal neurons. Mounted on the internet by permission of Lauri et al (2006) Neuron 50, 415-429 © Cell Press

More recently, the physiological role of such receptors was investigated (Lauri et al, 2006). Bath application of the GluK1 antagonist LY382884 to neonatal hippocampal slices resulted in an increase in the kainate receptor-mediated EPSC, suggesting the presence of a tonically active receptor. The increase in EPSC results from a decrease in failures (instances when transmitter is not released following stimulation) indicating a pre-synaptic locus for this receptor. In addition, only synapses that expressed such a kainate receptor underwent frequency facilitation, an effect similar to paired-pulse facilitation, but carried out over a train of 5 stimuli. Thus the role of these receptors appears to be to reduce the probability of transmitter release to enable facilitation to take place.

The receptors involved in this action are unlikely to have the same subunit composition as those involved in the more mature animals. In neonates, the pre-synaptic kainate receptor displays a very high affinity and is activated by the low levels of L-glutamate found in the synapse between bouts of transmitter release, although it's mode of action is still metabotropic in nature. This high affinity kainate receptor is lost during development alongside the facilitation of synaptic response in CA1 synapses.  It is tempting to envisage that the subunit composition of the high affnity kainate receptor is changed during development to become the lower affinity receptor found in more mature synapses that then undergoes further developmental losses.

Thalamocortical synapses

frequency depression in somatosensory cortexKainate autoreceptors. Kainate autoreceptors may be facilitatory or inhibitory depending on where they are expressed in the brain. A train of identical stimuli under control conditions (black traces) may result in either increased or decreased response amplitude. Both effects are blocked by LY382884 (blue traces), demonstrating that they are mediated by GluK1-containing pre-synaptic kainate receptors.

A pre-synaptic, inhibitory kainate autoreceptor has been described on neurons in the barrel cortex in the somatosensory system (Kidd et al, 2002), that can be activated by synaptically released glutamate. During short bursts of high frequency trains of activity, synaptic transmission is depressed by up to 60% or more. This effect is inhibited by LY382884, demonstrating an involvement of GLUK1 containing receptors. In common with the hippocampal kainate autoreceptors described above, functional expression is developmentally regulated, albeit on a shorter timescale. The depression of high frequency activity disappears during the first post-natal week in rats. This receptor may be important as the frequency range at which it depresses synaptic transmission correlates with frequencies that are associated with whisker activity in mature networks.

The developmental regulation of these pre-synaptic receptors also correlates closely with that of post-synaptic kainate receptor found at these same synapses (Kidd & Isaac, 1999). In addition, loss of these receptors is llikely to be due to the loss of kainate-receptor containing synapses rather than gradual replacement of receptors in existing synapses as the response amplitude to quantal glutamate release is unchanged during this period, although the kainate component of the post-synaptic response reduces (Bannister et al 2005).

Post-synaptic receptors

Kainate receptors can be synaptically activatedSynaptically activated kainate receptor-mediated currents at the mossy fibre-CA3 synapse. Mounted on the internet by permission of Vignes & Collingridge (1997) Nature 388, 179-182 © Macmillan Publishers Ltd

Post-synaptic kainate receptors have been found in a number of brain areas, including the hippocampus (Castillo et al, 1997; Vignes & Collingridge, 1997), spinal cord (Li et al, 1999), somatosensory cortex (Kidd & Isaac, 1999), cerebellum (Bureau et al, 2000) and medial entorhinal cortex (West et al, 2007). The mossy fibre-CA3 synapse in the hippocampus is perhaps the most extensively studied for the functions of kainate receptors. While most of the work at this synapse has focussed on pre-synaptic kainate receptors, they are also expressed post-synaptically. A short tetanus (up to 10 shocks @ 100 Hz) evokes an EPSC that is sensitive to kainate receptor antagonists when both AMPA and NMDA receptors are blocked. This current is not seen when other pathways that connect to the CA3 region of the hippocampus are stimulated, such as the associational-commisural fibres. Activation of post-synaptic kainate receptors results in a slow EPSC, in contrast to the fast kinetics of the AMPA receptor. Curiously, exogenously expressed kainate receptors show much faster kinetics than native receptors expressed in brain tissue, although GluK2/GluK5 heteromeric receptors have been demonstrated to have similarly slow decay kinetics (Barberis et al, 2008).

The roles played by these receptors excitatory synaptic transmission and plasticity is still unclear. In the barrels of the somatosensory cortex, kainate receptors can carry up to 30% of the charge, early in development (Kidd & Isaac, 1999). Interestingly,  the AMPA and kainate receptor-mediated currents that are found in this region are never found together in the same synapses. Thus there are synapses that contain functional AMPA receptors and synapses that contain functional kainate receptors, but never both together. This control of receptor distribution/function indicates that each receptor type interacts with the cellular machinery in a very selective fashion. This may mean that they interact with specific proteins to direct their movements.

barrel respresentations in the somatosensory pathwayPost-synaptic kainate receptors in the barrel cortex undergo changes during development. Synapses in the barrel cortex form the last link in the neuronal pathway from the whiskers to the somatosensory cortex. Each barrel is a representation of a single whisker and together they form a 2-D map of the whisker pattern on the animal's snout. The first post-natal week is a critical period for barrel formation and it coincides with a change in the kainate/AMPA receptor ratio. Early after birth, spontaneous currents are predominantly kainate receptor-mediated. However, after 8 days, recorded currents are predominantly AMPA receptor-mediated.This change not only co-incides with the critical period for the development of barrels but is also mirrored following LTP induction. In this context, it is very interesting that in the hippocampus, the intracellular trafficking protein GRIP interacts with AMPA and kainate receptors in such a way as to promote movement in the opposite directions (Hirbec et al, 2003). This protein stabilises kainate receptors at the cell surface but promotes the internalisation of AMPA receptors. Blockade of GRIP function provides an attractive mechanism to achieve such a change in receptor populations.