Thomas A. Houpt - US grants

Conditioned taste aversion (CTA) is a form of associative learning in which an animal avoids and reacts aversively to the taste of a food that has previously been paired with illness. CTA has been described in many species, from invertebrates to human, and has important clinical implications in drug and radiation therapies. It may also serve as a model for behavioral plasticity and altered responsiveness in ingestive behaviors, such as are seen in eating disorders. We have recently discovered that the expression of a CTA previously acquired by pairing intraoral infusions of 5% sucrose with lithium chloride is correlated with the expression of the immediate-early gene c-Fos in the medial intermediate region of the nucleus of the solitary tract (iNTS) in the rat. One recent experiments have demonstrated that the induction of c-Fos in the iNTS appears to be a specific and quantifiable neuronal correlate of CTA expression. In this proposal, we hypothesize that c-Fos expression reveals functionally important brain sites mediating CTA expression, and that c-Fos expression in at some of these sites may play a physiological role in CTA acquisition and expression. We will characterize the time course and anatomical distribution of c-Fos expression in the rat brain by in situ hybridization and immunohistochemistry induced by intraoral infusions of sucrose before and after CTA acquisition and after LiC1 injection to identify neuronal populations activated by conditioned and unconditioned stimuli. We will then make fiber-sparing, excitotoxic lesions directed at 1) novel brain sites we have identified the express c-Fos after CTA expression and 2) brain sites previously demonstrated in earlier studies to be required for CTA. Alterations in CTA expression induced by lesions will be measured by quantifying intake, taste reactivity, and c-Fos expression. This proposal presents a novel approach to understanding CTA by attempting to correlate the effects of fiber-sparing lesions on intake and taste reactivity (to distinguish passive avoidance from active rejection) with the effects of the lesion on the pattern of c-Fos expression. Our working hypothesis is that changes in the pattern of c-Fos expression reveal changes in the properties of the network involved in CTA. These experiments will contribute to an understanding of the neural pathways and molecular mechanisms underlying acquired changes in food preferences that occur and may contribute to understanding the pathology of ingestive behaviors, such as occur in obesity, anorexia nervosa, and bulimia nervosa.

Advances in magnetic resonance imaging (MRI) are driving the development of MRI machines beyond conventional static magnetic field strengths to fields of 4-9 tesla (T). Little is known about the sensory or physiological effects of high strength static magnetic fields on mammals and humans. We have recently discovered that 30 min exposure to a 9.4 T field has behavioral and neural effects in rats. At the behavioral level, magnetic field exposure induced a conditioned taste aversion (CTA) after pairing with the taste of saccharin. CTA has proven to be a sensitive index of visceral perturbation or malaise induced by a treatment; therefore the magnetic field may be experienced by the rat as an aversive stimulus. At the neural level, the same exposure induced specific and significant c-Fos immunoreactivity in brainstem visceral relays (e.g. the nucleus of the solitary tract and parabrachial nucleus) and in vestibular nucluei (e.g., medial vestibular nucleus). Both the behavioral response and the pattern of c-Fos activation are similar to the effects of vestibular disturbances, such as rotation. We hypothesize that the magnetic field activates the rats' vestibular apparatus, causing vertigo; this would b consistent with reports of vertigo and nausea in humans exposed to 4 T fields. These findings suggest that CTA and c-Fos expression can be used in an animal model of the effects of high-strength, static magnetic fields. We propose to determine the sensitivity of rats using the large-bore, high-strength NMR magnets available at the National High Magnetic Field Laboratory. We will make lesions of sensory sites and nerves to determine the pathways for detection of the magnetic field. We will probe the underlying pharmacology with anti-emetics and other drugs that may attenuate the effects of the field. The acute behavioral effects will be measured by observational scoring; aversive or delayed effects will be measured by CTA expression; and the neural response will be quantified by c-Fos expression throughout the brain. These experiments will help predict the effects of future high-strength MRI on humans, and contribute to understanding the neural pathways underlying the effects.

DESCRIPTION (provided by applicant): Advances in magnetic resonance imaging (MRI) are driving the development of faster and higher resolution MRI machines. While MRI machines with static magnetic fields of 1 to 2 tesla (T) and resolutions of 2 mm3 are standard in clinical use, higher resolution requires stronger fields. Little is known about the sensory or physiological effects of static magnetic fields of high strength on mammals and humans. Using the large- bore, high-strength NMR magnets at the U.S. National High Magnetic Field Laboratory at Florida State University, we have discovered that exposure to 7T or higher magnetic fields has behavioral and neural effects on rats. At the behavioral level, magnetic field exposure induced a conditioned taste aversion after pairing with the taste of saccharin. At the neural level, the same exposure induced specific and significant c- Fos , a marker of neuronal activation, in the rat brainstem. c-Fos was observed in visceral and stress relays and in vestibular nuclei (e.g. the medial vestibular nucleus). Both the behavioral response and the pattern of c-Fos activation are similar to the effects of vestibular disturbance, such as rotation and motion sickness. In the previous funding period, labyrinthectomy was found to block all these effects;the specific inner ear organ responsible for magnetic field transduction is still unknown, however. Two other notable findings were: 1) preexposure to 14T reduces responsiveness upon later exposure, suggesting either sensory adaptation or damage;2) after rats are trained to climb a ladder through the 14T magnet, they not only refuse to traverse 14T on subsequent trials, but they will not cross the 2T field line. Here we propose: to explore the specific vestibular sites that transduce magnetic field reponses using using pharmacology and mutant mouse strains that lack otolith or semicircular organs;to define the parameters of preexposure that may distinguish adaptation from damage;and to determine the thresholds for aversion and detection using a) ladder-climbing through different magnetic fields and b) conditioned suppression with magnetic fields as the cue. This proposal is relevant to public health in two ways: First, patients undergoing MRI examinations are routinely exposed to high strength static magnetic fields of 3T, and higher strength MRI machines are in development. This proposal explores the effects of similar magnetic fields on the vestibular system that might cause acute motion sickness or long-term changes in vestibular function after repeated exposure. Secondly, vestibular dysfunction is a common complaint and cause of injury (e.g. falling in the elderly), and this proposal contains novel approaches to investigating the inner ear and vestibular system