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UBC Theses and Dissertations

Medial lemniscal evoked responses in thalamic ethmoid neurons Lee, Stephanie G.

Abstract

This thesis describes electrophysiological and pharmacological properties of neurons in the ethmoid nucleus of the rat thalamus. According to the atlas by Paxinos and Watson, the ethmoid nucleus is located dorsal to the medial lemniscus, the major somatosensory input to the thalamus. The ethmoid also lies ventral to the parafascicular nucleus, caudal to the ventrobasal thalamus and rostral to the scaphoid nucleus. The ethmoid is considered a higher order nucleus, which implies that it serves as a link in corticothalamo- cortical pathways that process sensory information. The literature on this nucleus is scarce and this thesis represents the first known attempt to study these neurons. Hence, a major objective of this thesis was to determine the passive and active properties of ethmoid neurons. Recent evidence from this laboratory has shown that stimulation of the medial lemniscus produced glycinergic and GABAergic inhibition in ventrobasal neurons. This inhibition was not sensitive to ionotropic glutamate receptor antagonism by kynurenate. A prediction from these studies was that the ethmoid was an intermediate nucleus in a circuit between the medial lemniscus and neurons of the ventrobasal thalamus. Thus, a key aim of this thesis was to examine the possible involvement of ethmoid neurons in this novel circuit. This thesis provides evidence that ethmoid neurons have passive and active properties similar to other neurons of the dorsal thalamus. Ethmoid neurons have a mean resting membrane potential of ~ -53 mV, a mean input resistance of ~670 MΩ and a mean membrane time constant of ~64 ms. Ethmoid neurons also have the ability to generate spikes in the tonic and burst firing modes. The active properties were sensitive to blockade by internal application of QX-314, a quaternary blocker of Na⁺ channels and by extracellular application of Ni2 ⁺ , a Ca2 ⁺ channel antagonist. These observations are consistent with Na⁺ dependent action potentials when a neuron is in the tonic firing mode and low threshold Ca2 ⁺ spikes when in the burst firing mode. We showed that stimulation of the medial lemniscus evoked depolarizations in all recorded ethmoid neurons. We categorized the depolarization responses into two groups. Group I were monophasic depolarizations and group II were biphasic depolarizations. The medial lemniscal evoked depolarization of ethmoid neurons persisted in the presence of kynurenate. These observations are consistent with the participation of ethmoid neurons in a circuit that results in glycinergic inhibition in ventrobasal neurons. This thesis marks the first known attempt to study neurons of the ethmoid nucleus. The observations provide evidence for functional similarities of ethmoid neurons to other thalamic neurons, as well as evidence for novel inputs activated by stimulation of the medial lemniscus.

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