Intrinsic Properties and Neuromodulator Regulation of Neuronal Excitability of Intralaminar Thalamic Neurons
The parafascicular nucleus, one of the intralaminar thalamic nuclei, projects to many nuclei of the basal ganglia and several widespread neocortical areas. The parafascicular nucleus may have a role in arousal and motor activity. Based on morphological and electrophysiological properties, I found two distinct subtypes of neurons: diffuse and bushy. Diffuse neurons are characterized by a small number of long, poorly branched dendrites and these neurons are the most prevalent subtype in the parafascicular nucleus. Bushy neurons have multiple radial branching dendrites with complex arborization and appear similar to thalamocortical neurons of primary sensory thalamic nuclei. Diffuse neurons have a more depolarized resting membrane potential and have higher input resistances than bushy neurons. Burst discharge, which results from activation of the transient, low threshold calcium current, was present in all bushy neurons; however, burst discharge was absent in the majority of diffuse neurons. Diffuse neurons primarily project to the basal ganglia, whereas bushy neurons predominantly project to frontal cortex. My results demonstrate that the parafascicular nucleus consists of two neuronal subtypes that have distinct cytoarchitecture and intrinsic properties; the latter could influence their role in arousal and motor activity. I next examined the actions of cholinergic agonists on rat parafascicular nucleus neurons. The muscarinic receptor agonist, acetyl-beta-methylcholine (MCh), hyperpolarized the majority of neurons by increasing a resting potassium conductance via the activation of M2 muscarinic receptors. In a smaller population, MCh, via M1 muscarinic receptor activation, produced a depolarization mediated by a decreased potassium conductance. Similar cholinergic responses were observed in both bushy and diffuse neuronal subtypes, and these actions could play a significant role in altering motor-related activities during different arousal states. I also tested the actions of neurokinins on parafascicular nucleus neurons excitability. Substance P, neurokinin A, and neurokinin B strongly depolarized the majority of neurons tested via a decreased potassium conductance. The neurokinins can alter the excitability of parafascicular nucleus neurons, and thereby have a significant influence on the basal ganglia circuit and cortical activity.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign