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published_or_final_version / Zoology / Master / Master of Philosophy

published_or_final_version / Surgery / Master / Master of Philosophy

How behaviors are generated by neural circuits is one of the central questions in neurobiology. Under standard culture conditions, Caenorhabditis elegans travel by propagating sinusoidal waves, moving primarily forward, punctuated by brief runs of backing. How these behaviors are generated and altered is not well understood. Using a combination of behavioral analyses and neuronal imaging, I reveal that an activity imbalance between cholinergic A- and B-motoneurons is the key determinant of directional locomotion. Furthermore, heterotypic gap junctions that couple command interneurons and motoneurons of the backward motor circuit, mediated by innexins UNC-7 in AVA and UNC-9 in A-motoneurons, respectively, establish the B>A activity pattern required for forward movement. Loss of this coupling results in both the hyperactivation of AVA backward interneurons revealing the unregulated, endogenous activity of A-motoneurons. With equal A-motoneuron activity levels as B-motoneurons, innexin mutant animals exhibit irregular body bending (kinking) instead of executing forward motion, as well as increased backing. Through a genetic screen, I identified two stomatin-like proteins as regulators of innexin UNC-9 activity that affect C. elegans’ directional movement. The loss of function of stomatin-like unc-1 leads to the same kinker phenotype as unc-7 or unc-9 mutants. Like UNC-9, UNC-1 functions primarily in the A-motoneurons to allow forward motion, suggesting that UNC-1 is required for effective UNC-7-UNC-9 coupling between AVA and A-motoneurons. Dominant mutations in UNC-1, and another stomatin-like protein STO-6, exhibit genetic interactions with these innexin mutants. These mutations partially restore the forward movement of unc-7 mutants, in an UNC-9-dependent manner, indicating that they regulate UNC-9 channel activity in motoneurons to re-establish the B>A-motoneuron activity pattern in the absence of heterotypic gap junctions between interneurons and motoneurons. These studies describe a role of gap junctions as regulators of circuit dynamics by establishing an imbalanced motoneuron activity pattern that favors forward motion, which can be modulated by upper layer inputs. This study also identifies stomatin-like regulators of innexin hemichannel and gap junction function. Future work will focus on understanding mechanisms through which these stomatins regulate the activity of specific innexin channels in C. elegans motoneurons, as well as their contribution to the dynamic output of the C. elegans motor circuit.

Vasomotion is the spontaneous oscillation of vascular tone, occurring due to synchronization of internal calcium fluctuations between multiple vascular smooth muscle cells by gap junction and electrical communication. Although altered vasomotion has been observed in a variety of pathological situations, characterization of these alterations has been lacking. Using a novel method of spectral quantification, and two experimental models known to have altered vascular structure, the present thesis was designed to evaluate whether vasomotion characteristics could be correlated with altered vascular structure. Rats with perinatal iron deficiency (PID) have previously been shown to possess altered vascular structure. When phenylephrine-mediated contractile and acetylcholine-mediated dilatory responses were investigated in PID animals, they both displayed blunted relaxation as compared to control vessels. When vasomotion characteristics were quantified, vessels taken from PID animals exhibited a decreased power in the very low frequency window (VLF <0.2 Hz). Changing vessel oxygenation to 10% O2 from 95% O2 did not result in significant alterations of vasomotion characteristics. The primary frequency of oscillation was investigated with a peak finder, and found to be significantly different compared to control in both the aorta and renal arteries obtained from PID animals. To investigate the effect of antihypertensive treatment (enalapril and hydrochlorothiazide) on gap junction communication, spontaneously hypertensive rats (SHR) were subject to a 2-week intensive angiotensin converting enzyme inhibitor treatment. This treatment resulted in significant vascular structural regression. All vessels (aorta, renal, mesenteric) from treated animals had greater proportions of power in the VLF window, with both the mesenteric and renal vessels exhibiting a primary peak of oscillation around 0.2 Hz; whereas the aorta had a primary peak at 0.12 Hz. Investigating altered gap junction communication with the gap junction blocker 18-α glycyrrhetinic acid, revealed that vascular bed location was the determining factor of vasomotion response. Immunoblotting did not indicate differences in connexin 43, a major gap junction protein in the vascular smooth muscle. These studies suggest that vasomotion characteristics can be used as a method of vascular phenotype investigation; vasomotion characteristics were significantly different in vessels taken from PID and hypertensive animals as compared to control and antihypertensive-treated animals, respectively. / Thesis (Master, Pharmacology & Toxicology) -- Queen's University, 2008-09-26 11:39:44.043

No description available.

No description available.

How behaviors are generated by neural circuits is one of the central questions in neurobiology. Under standard culture conditions, Caenorhabditis elegans travel by propagating sinusoidal waves, moving primarily forward, punctuated by brief runs of backing. How these behaviors are generated and altered is not well understood. Using a combination of behavioral analyses and neuronal imaging, I reveal that an activity imbalance between cholinergic A- and B-motoneurons is the key determinant of directional locomotion. Furthermore, heterotypic gap junctions that couple command interneurons and motoneurons of the backward motor circuit, mediated by innexins UNC-7 in AVA and UNC-9 in A-motoneurons, respectively, establish the B>A activity pattern required for forward movement. Loss of this coupling results in both the hyperactivation of AVA backward interneurons revealing the unregulated, endogenous activity of A-motoneurons. With equal A-motoneuron activity levels as B-motoneurons, innexin mutant animals exhibit irregular body bending (kinking) instead of executing forward motion, as well as increased backing. Through a genetic screen, I identified two stomatin-like proteins as regulators of innexin UNC-9 activity that affect C. elegans’ directional movement. The loss of function of stomatin-like unc-1 leads to the same kinker phenotype as unc-7 or unc-9 mutants. Like UNC-9, UNC-1 functions primarily in the A-motoneurons to allow forward motion, suggesting that UNC-1 is required for effective UNC-7-UNC-9 coupling between AVA and A-motoneurons. Dominant mutations in UNC-1, and another stomatin-like protein STO-6, exhibit genetic interactions with these innexin mutants. These mutations partially restore the forward movement of unc-7 mutants, in an UNC-9-dependent manner, indicating that they regulate UNC-9 channel activity in motoneurons to re-establish the B>A-motoneuron activity pattern in the absence of heterotypic gap junctions between interneurons and motoneurons. These studies describe a role of gap junctions as regulators of circuit dynamics by establishing an imbalanced motoneuron activity pattern that favors forward motion, which can be modulated by upper layer inputs. This study also identifies stomatin-like regulators of innexin hemichannel and gap junction function. Future work will focus on understanding mechanisms through which these stomatins regulate the activity of specific innexin channels in C. elegans motoneurons, as well as their contribution to the dynamic output of the C. elegans motor circuit.

Density functional theory is used to study the physico-chemical effects of two metallic thin films separated by distances in a range of 4-10 amperes. In this condition, the electrons from the metallic thin film surfaces tunnel through the energy barrier existing between the separated thin films, creating an electronic distribution in the gap between films. The characteristics and features of this electronic distribution, such as energy, momentum, and number of electrons, can be traced by quantum mechanical analyses. These same features can be tuned by varying metallic thin film properties like thickness, separation between films, and film chemical nature. The possibility to tune the physical properties of the electrons located in the gap between thin films makes the studied systems promising for applications that range from catalysis to nano-electronics. Molecular oxygen, water, and ethylene were located in the gap between thin films in order to study the physical and chemical effects of having those molecules in the gap between thin films. It was observed that the electron structure in the gap modifies the geometric and electronic structure of those molecules placed in the gap. In the case of molecular oxygen, it was found that the dissociation energy can be tuned by changing the separation between thin films and changing the chemical nature of the surface and overlayer of the thin film. For water, it was found that by tuning the chemical nature of the surface and sub-surface of both metallic thin films, molecular water dissociation can occur. When ethylene was located in the gap between Ti/Pt thin films, the molecule converts in an anion radical adopting the geometry and structure of the activated monomer necessary to initiate chain polymerization. Regarding magnetism, it was found that by the surface interaction between Ti/Pt and Pt thin films, the magnetic moment of the system decreases as the separation between thin films decreases. The phenomenon was explained by changes observed in the number of electronic states at the Fermi level and in the exchange splitting as a function of separation between films. Finally, a system that resembles a p-n junction was proposed and analyzed. The system is a junction of two metallic thin films with different electronic density in the gap between surfaces. These junctions can be the building blocks for many electronic devices.

In this thesis, investigations into the suitability of three 'weak-link' models, designed for the simulation of superconducting cluster systems, are reported. The focus of the investigation is on both the accuracy of the transport properties produced by these models, and the time taken to produce their results. The thesis develops the theory behind a previous approach which was exclusively used to model percolation systems for coverages below the critical coverage. The modifications made allow the simulations to extend to system coverages above the critical coverage. An additional 'current-ramping' algorithm, to simulate the systematic increase or decrease of current forced through the system, is described and explored. The results for the three models are compared, and their suitability for future investigations is discussed.