Besides sensory input from whiskers, higher order posteromedial nucleus (PoM) of the thalamus, receives rich and complex cortical feedback projections of modulatory and driving type (from layer 6 and 5, respectively). The function of these recurrent pathways is under control of the brain stem neuromodulatory systems. In this study we investigated cholinergic influence on facilitatory properties of cortical layer 6 input to PoM cells. For this purpose we prepared “classical” thalamocortical slices from brains of young (21-28 days) rats. Such slices are suitable for studying the physiology of synapses made on PoM cells by axons from cortical layer 6, because fibers descending from layer 5 are mostly cut off. Membrane potential of PoM neurons was recorded by whole-cell patch-clamp method while cholinergic agonist carbachol (5-8 µM) was added to the bath to activate cholinergic receptors. To evoke excitatory postsynaptic potentials (EPSPs), repetitive series of 5 electrical stimuli at 20 Hz frequency (15 s inter-trial interval, baseline membrane voltage at -56 mV) were applied by stimulation electrode placed at the cortico-thalamic fiber tract in the internal capsule. Bicuculline was present in the bath to block GABAA receptors. We found that carbachol led to almost threefold decrease of the first EPSP’s amplitude in the train. At the same time, however, the ratio between the second and the first postsynaptic potential (paired pulse facilitation) became nearly two times bigger. Moreover, with activated cholinergic receptors, the following EPSPs in the train also grew faster in amplitude. Our results indicate that modulatory cholinergic system may increase the frequency potentiation of cortical layer 6 input to PoM, most likely due to decrease of transmitter release through activation of presynaptic cholinergic receptors. This research and NS was supported by the Foundation for Polish Science through International PhD Program in Neurobiology.
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