Browsing by Subject "cholinergic"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item A Cholinergic Sensory-Motor Circuit Controls the Male Copulation Behavior in C. elegans(2012-07-16) Liu, YishiThe nervous system coordinates a sequence of muscle movements to give rise to animal behaviors. In complex invertebrates or lab-studied vertebrates, due to the large number of cells in their nervous systems and the complexities of their behaviors, it is difficult to address how circuits process information to direct each motor output of the behavior. In this dissertation, I used the Caenorhabditis elegans male copulation behavior as a model to address how a compact circuit coordinates different behavioral programs. Insertion of a male copulatory organ into a suitable mate is a conserved and necessary behavioral step for most terrestrial mating. However, the detailed molecular and cellular mechanisms for this distinct social interaction have not been elucidated in any animal. During mating, the C. elegans male cloaca is positioned over the hermaphrodite?s vulva as he attempts to insert his copulatory spicules repetitively. Rhythmic spicule thrusts cease when insertion is sensed. Circuit components consisting of sensory/motor neurons and sex muscles for these steps have been previously identified, but it was unclear how their outputs are integrated to generate a coordinated behavior pattern. Here, I show that contraction of the male oblique muscles is required to sustain genital contact between the sexes. These muscles are innervated by the postcloacal sensilla (p.c.s.) sensory/motor neurons, which secret ACh to activate the levamisole-sensitive AChR and the ACR-16-containing ionotropic AChR on the oblique muscles. For spicules to rhythmically thrust during genital contact, activity of the oblique muscles and the gubernacular muscles is transmitted to the spicule protractor muscles instantaneously via gap junctions between these muscles and causes shallow protractor contractions. The rhythmic protractor contractions eventually switch to sustained contraction, as the SPC sensory-motor neurons integrate information of spicule position at the vulva with inputs from the hook and cloacal sensilla. The ERG-like K+ channel, UNC-103, which decreases the spicule circuit excitability, is likely to set a threshold requirement for integration of these inputs, so that sustained spicule muscle contraction is not stimulated by fewer inputs. In addition, I demonstrate that a cholinergic signaling pathway mediated by a muscarinic acetylcholine receptor, GAR-3, is used to enhance the ionotropic AChRs-mediated fast synaptic transmission in the copulation circuit. GAR-3 is expressed in multiple cells of the copulation circuit, but mainly in the cholinergic p.c.s. neurons and SPC neurons. Activation of GAR-3 is coupled to G?q to trigger downstream signal transduction events that modulate neurotransmitter release from these neurons. Males with a loss-of-function allele of the gar-3 gene are defective in inserting their spicules into the hermaphrodite?s vulva efficiently. Since the p.c.s. neurons regulate the male?s contact with the hermaphrodite?s vulva, and the SPC neurons are required for spicule insertion during mating, GAR-3 probably facilitates male mating behavior via enhancing synaptic transmission from these neurons to their postsynaptic partners.Item Artificial Stimulation of Cephalic Cholinergic Sensory Neurons Induces Mating-Like Motor Responses in Male Caenorhabditis elegans(2012-12-06) Midkiff, JamesAll complex organisms possess a nervous system which they use to monitor environmental and internal stimuli. In higher vertebrates, the nervous system is comprised of billions of cells which form highly plastic neural networks from their synapses. These large neural circuits modulate complex behaviors. The nematode roundworm Caenorhabditis elegans uses a small but highly-interconnected nervous system to carry out complex behaviors. The nervous system of C. elegans is a tractable model to determine the effects of changes on a nervous system at the systemic, cellular, genetic, and molecular levels. The C. elegans male?s nervous system detects environmental conditions, mating cues, attractants, repellents, and the location and composition of possible food sources and integrates these inputs to compute the decision of whether or not to mate. Mating behavior in the C. elegans male is regulated at a number of steps by cholinergic signaling from various sensory and sensory-motor neurons, but a comprehensive model of how cholinergic signaling controls this circuit has not yet been elucidated. Previous studies have thoroughly dissected the cellular structure, neural connectivity, and signaling pathways of the male?s peripheral circuits located in the genital regions of the animal?s tail. However, no studies have been conducted to determine what role the cephalic cholinergic neurons have in regulating mating behavior. I hypothesized that cephalic cholinergic neurons exert regulatory control over the male-specific mating circuit. I inserted the transmembrane light-activated ion pore Channelrhodopsin-2 fused to YFP and expressed from the Punc-17small promoter into these neurons and selectively stimulated them using high-intensity blue light. Stimulation induced mating-like behaviors in the male tail consistent with behaviors seen during copulation with a hermaphrodite. Using behavioral assays, I demonstrated that these behaviors were male-specific and only occurred after direct stimulation in the absence of a hermaphrodite. Incidence of mating-like behaviors increased significantly as the worm aged, and the mating circuit retained a memory of the stimulus, indicated by the latency between stimulation and onset of mating-like behaviors. Brief food deprivation, which normally downregulates excitability of the mating circuit via UNC-103 ERG-like K+ channels, caused an unexpected increase in the number of blue light-stimulated behaviors displayed. Pharmacological assays using acetylcholine (ACh) agonists showed that stimulation of the cephalic cholinergic neurons increased propensity for spicule protraction in the presence of an ACh agonist, and partially restored the decline in spicule protraction associated with temporary food deprivation. I sought to identify the cephalic cholinergic neuron or neurons responsible for regulating mating-like behavior in the tail circuits. I looked for a reduction in mating-like behaviors after stimulation after removal of a cephalic cholinergic neuron pair via laser micro-ablation. Two cholinergic and chemosensory neuron pairs in the inner labial sensilla (IL2L/R and IL2VL/R) appear to generate and/or relay the signal that induces mating-like behaviors in the tail. I hypothesize that these neurons sense environmental cues before the male contacts a mate, and modulate lasting motivational changes within the male mating circuit.Item GABAergic systems in a model of age-related cognitive impairment(2012-07-16) LaSarge, Candi LynnWith medical advancements extending the life span, age-related cognitive decline is a growing problem for the United States. A rat model of cognitive aging was used to investigate the GABAergic neurotransmitter system in relation to changes in learning and memory functions. Confocal stereology was used to determine the number of GABAergic and cholinergic projection neurons in the rostral basal forebrain of spatially characterized young and aged male F344 rats. The GABAergic system was then assessed as a potential target for improving age-related cognitive decline using an odor discrimination task sensitive to decline in aging. Performance of aged rats was impaired compared to young rats on the spatial version of the Morris water maze. Notably, a high degree of variability in individual abilities was observed among aged rats such that some aged rats performed on par with young (aged-unimpaired) and others performed outside the range of young, demonstrating impairment (aged-impaired). The number of basal forebrain neurons expressing multiple immunomarkers for GABAergic septohippocampal projection cells was selectively increased in aged-impaired rats in comparison to both young and aged-unimpaired rats. Indeed, among aged rats, worse performance in the water maze was reliably associated with higher GABAergic cell number. The number of cholinergic neurons, quantified in adjacent sections did not differ as a function of chronological age or cognitive status. These data suggest that aging can dysregulate GABAergic systems in circuitry important for learning and memory and such alterations may contribute to age-related cognitive decline. To test whether the GABAergic system may be a viable target for treating age-related cognitive decline, a second cohort of young and aged rats was characterized in an odor discrimination task. Similar to aged rat water maze performance, some aged rats performed odor learning discrimination problems on par with the young cohort (i.e. aged-unimpaired) and some aged rats were impaired compared to young (i.e. aged-impaired). Using a within-subjects design, the GABA(B) antagonist, CGP 55845 completely ameliorated odor discrimination learning deficits in aged-impaired rats in a dose-dependent manner. These data support the hypothesis that the GABAergic system should be a novel target for therapies aimed at treating age-related cognitive decline.