Histochemical correlates of firing characteristics in the oculomotor system of rhesus monkey

Mayadalı, Ümit Suat (2022): Histochemical correlates of firing characteristics in the oculomotor system of rhesus monkey. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)

Abstract

Quick and accurate eye movements are achieved through the precise and well-timed firing of various neurons in the oculomotor circuit, which is determined by their cell characteristics. To date, intrinsic membrane properties of motor and premotor neurons enabling healthy saccade generation have not been studied in the monkey brainstem. This thesis investigates ion channel and transmitter-related profiles of motoneurons in the abducens and trochlear nuclei, and various premotor neuron groups such as burst neurons (BN), omnipause neurons (OPN) and vestibular Y-group neurons in a comparative manner. The overarching aim of this work is therefore to establish the histochemical profiles of oculomotor neurons concerning their fast-firing and bursting capacity. This goal was pursued with the immunolabeling-based examination of all aforementioned neuron populations for voltage-gated potassium channels (Kv), which facilitate the fast-firing capacity of neurons. Next, the expression of low-voltage activated (LVA) cation channels was investigated to assess the bursting characteristics of motoneurons and premotor BNs of the saccadic circuitry. Lastly, the expression of transmitter-related proteins in motoneurons of the abducens and trochlear nuclei was investigated to address anatomical and physiological differences found between their subpopulations. Motoneurons of extraocular muscles are classified by their innervation of slow contracting, fatigue-resistant multiply-innervated (MIF) or fast-contracting, fatigable singly-innervated (SIF) fibers. A previous concept based on their anatomical segregation suggested that MIF motoneurons mediate slow eye movements or gaze holding, whereas SIF motoneurons generate fast eye movements. However, this view has been challenged recently as electrophysiological recordings in cat revealed that both groups are active for all eye movements with a burst/tonic discharge, albeit with different dynamic properties. Therefore, my first aim was to establish ion channel and transmitter profiles of SIF and MIF motoneurons to investigate their differences and to address the incongruencies between anatomical, histochemical and electrophysiological data. Histochemical analyses revealed significant differences between MIF and SIF motoneurons, as well as abducens internuclear neurons, in terms of Kv and LVA channel and transmitter-related protein immunolabeling. These findings provided strong evidence explaining the physiological differences between these neuronal groups without conflicting established histochemical and anatomical signatures. My second aim was the investigation of intrinsic membrane properties of premotor BNs, which relay calculated saccadic signals to motoneurons. According to neuromimetic computational models based on clinical findings, BNs supposedly express LVA channels i.e. T-type voltage-activated calcium (Cav3) channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. These channels produce post-inhibitory rebound bursting in neurons that receive a sustained inhibition, and subsequently are disinhibited. Therefore, the model predicts that the bursting of BNs should be in response to the cessation of tonic inhibition they receive from OPNs. Excitatory BNs of the vertical and inhibitory BNs of the horizontal saccades similarly expressed LVA channels, confirming the prediction of neuromimetic models of eye movements. According to these models, saccadic disorders such as progressive supranuclear palsy can be explained by a dysfunction of LVA channels in BNs. Here, I provide the histochemical substrate for this hypothesis. My third aim was to establish a module of co-expressed histochemical markers that could delineate distinct firing patterns observed in various neurons of the oculomotor system. The investigation of Kv channels in neurons of the oculomotor system revealed that Kv1.1&Kv3.1 were expressed in all fast-firing neurons of the brainstem saccadic circuitry; including both premotor BN populations, tonic-firing OPNs and dorsal Y-group neurons, burst/tonic firing SIF motoneurons and internuclear neurons, but not in MIF motoneurons. These findings suggest Kv channel function as a prerequisite to the precise and well-timed activity of various neurons in the oculomotor system. However, Kv channel immunolabeling is not sufficient to delineate distinct firing characteristics of these neurons. Furthermore, no pattern of co-expression with LVA channels or transmitter-related proteins was observed to suggest any distinctions between tonic- or burst-firing in neurons of the oculomotor circuitry. Overall, this research solidified the immunohistochemical method as a valid tool for indirectly assessing the physiological properties of various neurons in the monkey oculomotor system. This methodology was validated firstly in a bottom-up approach, as used in the examination of motoneurons based on anatomical and electrophysiological findings. Then it was validated in a top-down approach, as used in the examination of premotor BNs based on clinical and mathematical hypotheses. Establishing the histochemical profiles of various neurons related to their firing patterns paves the way for the post-mortem evaluation of human clinical cases with saccadic disorders.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Graduate School of Systemic Neurosciences (GSN)
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 28. September 2022
• 1. Bericht­erstatter:in: Horn-Bochtler, Anja