Ann C. Bonham - US grants

It is well known that area postrema neurons augment baroreflex and cardiopulmonary reflex regulation of the sympathetic nervous system. Very little is known regarding the CNS sites, mechanisms, and pharmacology involved in the augmentation. Our goal is to identify the central pathways, mechanisms, and neurotransmitter/neuromodulators whereby area postrema neurons augment baroreflex and cardiopulmonary reflex control of sympathetic nerve activity. We propose that augmentation occurs early in the afferent pathways at the level of nucleus tractus solitarius (NTS) where area postrema neurons increase the sensitivity of NTS neurons to inputs from baroreceptor or cardiopulmonary afferents. We showed electrophysiologically that area postrema and aortic baroreceptor afferents converge onto the same neurons in the NTS and interact in a facilitatory manner. Same was true for area postrema and vagal afferents, suggesting a basis for area postrema augmentation of cardiopulmonary receptor-mediated sympathoinhibition. We propose to extend this work to test the following specific hypotheses: 1. Activation of area postrema neurons augments cardiopulmonary receptor-evoked sympathoinhibition increasing the responsiveness of NTS cells to input from cardiopulmonary receptors. 2. Augmentation by area postrema neurons of the central processing of both baroreceptor and cardiopulmonary receptor afferent input by NTS neurons is mediated by a direct route from area postrema to the NTS and by an indirect route from area postrema to the parabrachial nucleus (PBN) and back to the NTS. 3. Augmentation involves both glutamatergic and noradrenergic synapses in NTS. Hypotheses will be tested in pentobarbital-anesthetized rabbits. These aims are: 1. determine whether excitation of area postrema neurons augments the responsiveness of left atrial receptor-sensitive neurons in the NTS to activate the left atrial receptors; 2. determine whether interruption of synaptic activity in PBN alters the ability of area postrema neurons to evoke spikes from NTS neurons and/or to augment the processing by those NTS neurons of either left atrial receptor or baroreceptor input; 3. determine whether PBN neurons that receive excitatory inputs from area postrema send efferent projections to the NTS; 4. determine the neurotransmitter/neuromodulator(s) by which stimulation of area postrema neurons augment responsiveness of myelinated left atrial receptor- sensitive NTS neurons to left atrial receptor activation and of baroreceptor-sensitive cells to baroreceptor activation; 5. determine whether area postrema neurons which project to the NTS (where left atrial receptor or baroreceptor afferents terminate) or to the PBN are excited by local injections of vasopressin, which is implicated at acting within the area postrema to augment baroreflex and cardiopulmonary reflex function.

DESCRIPTION (Adapted from the applicant's abstract): Within the central network of the baroreflex pathway, baroreceptor signals are integrated to rapidly regulate arterial blood pressure. Much of the integration is in the nucleus of the solitary tract (NTS), where the baroreceptor signals are first processed. In the NTS, baroreceptor signals are transmitted by the fast ionotropic glutamate receptors (iGluRs). At high, yet physiologically relevant frequencies of baroreceptor input, the signal transmission is depressed. This frequency-dependent depression of transmission early in the pathway likely serves to accommodate information-transfer at more distal synapses to optimize reflex function. Still in question is the mechanism underlying the frequency-dependent depression. In other neural networks, slow-acting G protein-linked metabotropic glutamate receptors (mGluRs) have been shown to provide both acute and long-term modulation of fast glutamatergic transmission mediated by the iGluRs. The focal point of this research proposal is that mGluRs operate similarly at baroreceptor terminal-NTS synapses. The Specific Hypothesis to be tested is that at low frequencies of baroreceptor input, glutamate is released from baroreceptor terminals and is sufficient to activate postsynaptic iGluRs to mediate synaptic transmission and to activate postsynaptic mGluRs to evoke a small slow excitation; however, the amount of glutamate released with low frequency stimulation is insufficient to activate presynaptic mGluRs. As a result presynaptic mGluRs do not affect synaptic transmission. As baroreceptor input frequency is increased, glutamate is released in sufficient amounts to diffuse further into the synaptic cleft to activate presynaptic mGluRs which decreases further glutamate release and ultimately reduces synaptic transmission. Four Specific Aims will address this hypothesis. Aim 1 takes advantage of an intact baroreflex circuitry in vivo to determine whether activation of presynaptic mGluRs at NTS synapses depresses baroreceptor signal transmission in a frequency-dependent manner. Aims 2-4 use voltage clamp analysis to isolate systematically the roles of pre- and postsynaptic mGluRs on synaptic transmission at increasing frequencies of visceral input and at different membrane potentials of the postsynaptic cells. It is anticipated that knowing how mGluRs modulate baroreceptor signal transmission will advance our understanding of acute and perhaps long-term changes in baroreflex function.