GLUTAMATE LOCALLY ACTIVATES DENDRITIC OUTPUTS OF THALAMIC INTERNEURONS

The relay of information through thalamus to cortex is dynamically gat ed, as illustrated by the retinogeniculocortical pathway(1). Important to this is the inhibitory interneuron in the lateral geniculate nucle us (LGN). For the typical neuron, synaptic information arrives through postsynaptic dendrites and is transmitted by axon terminals. However, the typical thalamic interneuron, in addition to conventional axonal outputs, has distal dendrites that serve both pre- and postsynaptic ro les(2-6). These dendritic terminals participate in curious and enigmat ic triadic arrangements, in which each contacts a relay cell dendrite and is contacted by a glutamatergic retinal terminal that innervates t he same relay cell dendrite. Here we show that agonists of the metabot ropic glutamate receptor (mGluR) activate dendritic terminals of inter neurons in the absence of action potentials, thereby inhibiting the po stsynaptic relay neuron. Somatic recordings from LGN interneurons reve al that there is no response to mGluR agonists, suggesting that their dendritic terminals are electrically isolated from their somata and ax ons, consistent with anatomical modelling of these cells', Our results offer insight into the functioning of triadic circuitry and indicate that thalamic interneurons can perform independent computations expres sed through axonal as opposed to dendritic outputs.