Pamela J. Hornby - US grants

The overall goals of my research are to determine the mechanisms of action and neurocircuitry in the brain that regulate gastrointestinal (GI) function and which may contribute to alterations in GI function resulting from stress and disease. This information may be used as a basis for developing new drug therapies that are targeted to work at key central nervous system (CNS) sites to control or prevent GI dysfunction. The dorsal motor nucleus of the vagus (DMV) is the 'motor output center' in the medulla where neurons innervating the GI tract are located. Two other nuclei, the nucleus raphe obscurus of the medulla (NRO) and the paraventricular nucleus of the hypothalamus (PVN), control GI function through connections to the DMV and contain neurotransmitters known to be important in GI function. However, there is little information on the organization of PVN and NRO projections and the neurotransmitters in the DMV that control different functions of the GI tract. To date there is almost no information on how sympathetic outflow to the GI system is controlled by the brain. Neurons in the rostroventrolateral medulla (RVL) provide tonic activation of sympathetic preganglionic neurons in the intermediolateral cell column in the spinal cord, and activation of RVL neurons alters GI function. This proposal will determine how neurotransmitter inputs from the PVN (corticotropin releasing hormone [CRF] oxytocin [OXY] and vasopressin [AVP]) and NRO (serotonin [5HT], thyrotropin releasing hormone [TRH], and substance P [SP]) as well as GABA and norepinephrine, control different aspects of GI function through pathways involving the DMV and RVL. The experiments in this proposal will test the hypotheses that: (1) neurotransmitters are organized viscerotopically within the DMV and control of different aspects of GI function, (2) neurotransmitters from the NRO and PVN are selective in terms of their innervation of regions of the DMV, and their influence on GI function, (3) neurotransmitter-identified pathways control sympathetic outflow to the GI tract through the RVL and (4) mild stress causes gastric mucosal damage and alteration of GI transit through PVN and NRO autonomic pathways, and that these effects can be prevented by specific receptor drugs targeted at these nuclei. To quantify the relative differences in transmitter staining in subregions of these nuclei, image analysis of the optical density will be performed on immunocytochemically stained sections, then the potential relationship of identified neurotransmitters to cell bodies projecting to target organs will be assessed by combined immunocytochemical-retrograde tracing techniques. These observed relationships will be confined at the electron microscopic level. To assess the function of this neural circuitry, microinjection of drugs will be performed into brain nuclei of anesthetized animals while recording the indices of GI function. Finally, drugs will be microinjected to attempt to abolish the effects of acute stress or ICV administration of CRF on gastric mucosal damage and GI transit in awake rats.

The overall goals of my research are to determine the mechanisms of action and neurocircuitry in the brain that regulate gastrointestinal (GI) function and which may contribute to alterations in GI function resulting from stress and disease. This information may be used as a basis for developing new drug therapies that are targeted to work at key central nervous system (CNS) sites to control or prevent GI dysfunction. The dorsal motor nucleus of the vagus (DMV) is the 'motor output center' in the medulla where neurons innervating the GI tract are located. Two other nuclei, the nucleus raphe obscurus of the medulla (NRO) and the paraventricular nucleus of the hypothalamus (PVN), control GI function through connections to the DMV and contain neurotransmitters known to be important in GI function. However, there is little information on the organization of PVN and NRO projections and the neurotransmitters in the DMV that control different functions of the GI tract. To date there is almost no information on how sympathetic outflow to the GI system is controlled by the brain. Neurons in the rostroventrolateral medulla (RVL) provide tonic activation of sympathetic preganglionic neurons in the intermediolateral cell column in the spinal cord, and activation of RVL neurons alters GI function. This proposal will determine how neurotransmitter inputs from the PVN (corticotropin releasing hormone [CRF] oxytocin [OXY] and vasopressin [AVP]) and NRO (serotonin [5HT], thyrotropin releasing hormone [TRH], and substance P [SP]) as well as GABA and norepinephrine, control different aspects of GI function through pathways involving the DMV and RVL. The experiments in this proposal will test the hypotheses that: (1) neurotransmitters are organized viscerotopically within the DMV and control of different aspects of GI function, (2) neurotransmitters from the NRO and PVN are selective in terms of their innervation of regions of the DMV, and their influence on GI function, (3) neurotransmitter-identified pathways control sympathetic outflow to the GI tract through the RVL and (4) mild stress causes gastric mucosal damage and alteration of GI transit through PVN and NRO autonomic pathways, and that these effects can be prevented by specific receptor drugs targeted at these nuclei. To quantify the relative differences in transmitter staining in subregions of these nuclei, image analysis of the optical density will be performed on immunocytochemically stained sections, then the potential relationship of identified neurotransmitters to cell bodies projecting to target organs will be assessed by combined immunocytochemical-retrograde tracing techniques. These observed relationships will be confined at the electron microscopic level. To assess the function of this neural circuitry, microinjection of drugs will be performed into brain nuclei of anesthetized animals while recording the indices of GI function. Finally, drugs will be microinjected to attempt to abolish the effects of acute stress or ICV administration of CRF on gastric mucosal damage and GI transit in awake rats.

DESCRIPTION: (Adapted from the Applicant's Abstract): Impairment of gastric adaptive relaxation reflexes is an underlying cause of gastroparesis. It may also contribute to gastroesphageal reflux disease and metaplasia (Barretts). In the hindbrain, vagal motor output to the gastric fundus is from preganglionic cholinergic neurons that are both excitatory and inhibitory. The inhibitory pathway is termed non-adrenergic non-cholinergic (NANC) and evokes relaxation of the stomach via enteric nitric oxide (NO) and vasoactive polypeptide inhibitory motor neurons. To manipulate this pathway we currently use pharmacological and molecular antisense techniques in rats. However, it is clear that the most powerful molecular tools are in mice with targeted genetic mutations. Therefore, in this exploratory proposal we will modify our physiological recordings and stereotaxic brain microinjections for use in mice. This would be a unique approach in the field of brain-gut interactions. In rats, we (and others) have successfully measured gastric relaxation by intragastric balloon pressure. However, the extent of gastric relaxation is limited by the inherent compliance of the intragastric balloon and the initial imparting pressure. This can be circumvented by use of a barostat, which measures changes in intragastric volume using an open system in large animals and human. Barostats have recently been miniaturized for rodents. The first hypothesis is that gastric relaxation is evoked by GABAb and NK1 receptors in neurons of the dorsal motor nucleus of the vagus (DMN) of mice. Mice will receive mid-collicular decerebration to circumvent potentially confounding effects of anesthesia and low baseline gastric tone. NO mediates gastric relaxation at many sites in the brain-gut 'highway'. Specifically, gastric relaxation is regulated by hindbrain NO input into the dorsal vagal complex, by preganglionic NO containing vagal neurons and by inhibitory motor neurons at the neuromuscular junction. The second hypothesis is that cholinergic tone withdrawal is retained in centrally evoked gastric relaxation in NO synthase knockout mice. We predict that the extent of gastric relaxation is impaired compared to control mice. This proposal will enable us to surmount the difficulties associated with physiological miniaturization. Thus we will be able to exploit genetically engineered mice using state-of-the-art equipment for measuring intragastric volume. This will be critical for understanding how the underlying genetic components translate into alterations in neural control of gastric motor function.