It is well established that pacemaker neurons in the brainstem provide automatic control of breathing for metabolic homeostasis and survival. During waking spontaneous breathing, cognitive and emotional demands can modulate the intrinsic brainstem respiratory rhythm. However the neural circuitry mediating this modulation is unknown. Studies of supra-pontine influences on the control of breathing have implicated limbic/paralimbic-bulbar circuitry, but these studies have been limited to either invasive surgical electrophysiological methods or neuroimaging during substantial respiratory provocation. Here we probed the limbic/paralimbic-bulbar circuitry for respiratory related neural activity during unlabored spontaneous breathing at rest as well as during a challenging cognitive task (sustained random number generation). Functional magnetic resonance imaging (fMRI) with simultaneous physiological monitoring (heart rate, respiratory rate, tidal volume, end-tidal CO2) was acquired in 14 healthy subjects during each condition. The cognitive task produced expected increases in breathing rate, while end-tidal CO2 and heart rate did not significantly differ between conditions. The respiratory cycle served as the input function for breath-by-breath, event-related, voxel-wise, random-effects image analyses in SPM5. Main effects analyses (cognitive task + rest) demonstrated the first evidence of coordinated neural activity associated with spontaneous breathing within the medulla, pons, midbrain, amygdala, anterior cingulate and anterior insular cortices. Between-condition paired t-tests (cognitive task > rest) demonstrated modulation within this network localized to the dorsal anterior cingulate and pontine raphe magnus nucleus. We propose that the identified limbic/paralimbic-bulbar circuitry plays a significant role in cognitive and emotional modulation of spontaneous breathing.

SUMMARYGlutamate is a key regulatory neurotransmitter in the triphasic central pattern generator controlling feeding behavior in the pond snail, Helisoma trivolvis. It excites phase two motor neurons while inhibiting those in phases one and three. However, the receptors that mediate this regulation are only partially characterized. The purpose of these experiments was to further characterize the glutamate receptors on three buccal neurons modulated by glutamate. Intracellular recordings from B5, B19 and B27 neurons were taken during the perfusion of isolated buccal ganglia with agonists that are selective for different vertebrate glutamate receptors. The firing rate of all three neurons was inhibited in a dose-dependent manner by glutamate, including that of B27, a phase 2 motor neuron known to be excited by glutamate in vivo. Quisqualate also reduced the firing rate in all three neurons, and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a relatively non-selective metabotropic glutamate receptor (mGluR) agonist, reduced the firing rate in B5 neurons, but not in B19 or B27 neurons. Agonists selective for vertebrate group I, II and III mGluRs did not affect the firing rate in any of the Helisoma buccal neurons tested, suggesting that mGluR agonist binding sites on these neurons do not closely resemble those on any vertebrate mGluR subtypes. An increase in frequency of action potentials was observed in all three cell types in the presence of 100·mol·l -1 kainate (KA), suggesting the presence of excitatory (S)-␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/KA-like receptors. However, electrotonic coupling between B19 and B27 neurons, and a lack of effect of KA on isolated B19 neurons suggest the excitatory effects of KA on this neuron are indirect. These findings suggest the presence of multiple glutamate receptor subtypes in molluscan neurons that do not always resemble vertebrate receptors pharmacologically.