Mechanisms of bi-directional modulation of thalamocortical transmission in barrel cortex by presynaptic kainate receptors

Presynaptic kainate receptors play an important role in synaptic transmission and short-term plasticity to profoundly regulate network activity in many parts of the mammalian brain. In primary sensory neocortex, where short-term synaptic plasticity is important for receptive field structure and information processing, kainate receptors are highly expressed and regulate thalamocortical inputs, particularly during development. However, the mechanisms of the kainate receptor-dependent presynaptic regulation of thalamocortical transmission are unclear. We therefore investigated this issue using electrophysiology in neonatal thalamocortical slices of barrel cortex combined with pharmacology and biochemical analyses. We show that presynaptic kainate receptors can both facilitate or depress synaptic transmission depending on the extent of their activation. This bi-directional regulation is mediated in part by kainate receptors that directly influence thalamocortical axonal excitability, but also likely involves receptors acting at thalamocortical terminals to regulate transmitter release. The efficacy of kainate in regulating thalamocortical transmission is low compared to that reported for other inputs. Consistent with this low efficacy, our biochemical analyses indicate that the presynaptic kainate receptors regulating neonatal thalamocortical inputs likely lack the high kainate affinity GluK4 and 5 subunits. Thus thalamocortical transmission can be bi-directionally regulated by low affinity kainate receptors through two mechanisms. Such presynaptic regulation provides a potentially powerful mechanism to influence sensory processing during development of barrel cortex.