Cameron J. Grover

The preBötzinger complex of the lower brainstem generates two breathing-related rhythms: one for inspiration on a timescale of seconds, and another that produces larger amplitude sighs on the order of minutes. We hypothesize that these two disparate rhythms emerge in tandem wherein recurrent excitation gives rise to the inspiratory rhythm while a calcium oscillator generates sighs; distinct neuronal populations are not required. We present several mathematical models that instantiate our working hypothesis including: (1) an activity (firing rate) model and (2) a minimal spiking network model. Both modeling frameworks corroborate the single-population rhythmogenic hypothesis.

The preBötzinger Complex (preBötC) of the lower brainstem generates two breathing-related rhythms: one for inspiration on a timescale of seconds and another that produces larger amplitude sighs on the order of minutes. Their underlying mechanisms and cellular origins remain incompletely understood. We resolve these problems via a joint experiment and modeling approach. Blocking purinergic gliotransmission does not perturb either rhythm and imaging experiments show that both rhythms emanate from the same glutamatergic neuron population. We hypothesized that these two disparate rhythms emerge in tandem wherein recurrent excitation gives rise to inspiratory rhythm while a calcium oscillator generates sighs; there is no obligatory role for gliotransmission, hyperpolarization activated mixed cationic current ( I _h ) in neurons, or synaptic inhibition-mediated coupling of separate populations. We developed a mathematical model that instantiates our working hypothesis. Tests of model predictions validate the single-population rhythmogenic framework, reproducing disparate breathing-related frequencies and the ability for inspiratory and sigh rhythms to be separately regulated in support of respiration under a wide array of conditions. Here we show how a single neuron population exploits two cellular tool-kits: one involving voltage-dependent membrane properties and synaptic excitation for inspiratory breathing (eupnea) and an intracellular biochemical oscillator for sighs, which ventilate and maintain optimal function in the compliant mammalian lung. SIGNIFICANCE STATEMENT: Breathing consists of two vital rhythms: one for eupnea that serves periodic physiological gas exchange and the other for sighs, which are larger breaths that occur minutes apart and serve to optimize pulmonary function. These rhythms with disparate frequencies emerge via a mechanism that is simpler than previously envisaged: it results from one neuron population (not two as previously thought) without need for gliotransmission or synaptic inhibition-mediated coupling of neuronal populations. We show that a low-frequency intracellular calcium oscillation underlies sighs and functions in parallel with the higher-frequency voltage-dependent network oscillation that drives eupnea. Exploiting two separate cellular tool kits enables quasi-independent breathing rhythms, which are unique features of breathing in mammals with compliant lungs.

The preBötzinger complex (preBötC) gives rise to two types of breathing behavior under normal physiological conditions: eupnea and sighing. Here, we examine the neural mechanisms that couple their underlying rhythms. We measured breathing in awake intact adult mice and recorded inspiratory rhythms from the preBötC in neonatal mouse brainstem slice preparations. We show previously undocumented variability in the temporal relationship between sigh breaths or bursts and their preceding eupneic breaths or inspiratory bursts. Investigating the synaptic mechanisms for this variabilityin vitro, we further show that pharmacological blockade of chloride-mediated synaptic inhibition strengthens inspiratory-to-sigh temporal coupling. These findings contrast with previous literature, which suggested glycinergic inhibition linked sigh bursts to their preceding inspiratory bursts with minimal time intervals. Furthermore, we verify that pharmacological disinhibition did not alter the duration of the prolonged interval that follows a sigh burst before resumption of the inspiratory rhythm. These results demonstrate that synaptic inhibition does not enhance coupling between sighs and preceding inspiratory events or contribute to post-sigh apneas. Instead, we conclude that excitatory synaptic mechanisms coordinate inspiratory (eupnea) and sigh rhythms.

The preBötzinger complex (preBötC) generates the rhythm and rudimentary motor pattern for inspiratory breathing movements. Here, we test “burstlet” theory (Kam et al., 2013a), which posits that low amplitude burstlets, subthreshold from the standpoint of inspiratory bursts, reflect the fundamental oscillator of the preBötC. In turn, a discrete suprathreshold process transforms burstlets into full amplitude inspiratory bursts that drive motor output, measurable via hypoglossal nerve (XII) dischargein vitro. We recap observations by Kam and Feldman in neonatal mouse slice preparations: field recordings from preBötC demonstrate bursts and concurrent XII motor output intermingled with lower amplitude burstlets that do not produce XII motor output. Manipulations of excitability affect the relative prevalence of bursts and burstlets and modulate their frequency. Whole-cell and photonic recordings of preBötC neurons suggest that burstlets involve inconstant subsets of rhythmogenic interneurons. We conclude that discrete rhythm- and pattern-generating mechanisms coexist in the preBötC and that burstlets reflect its fundamental rhythmogenic nature.

The preBötzinger complex (preBötC) generates the rhythm and rudimentary motor pattern for inspiratory breathing movements. Here, we test ‘burstlet’ theory (Kam, Worrell, Janczewski, et al. 2013), which posits that low amplitude burstlets, subthreshold from the standpoint of inspiratory bursts, reflect the fundamental oscillator of the preBötC. In turn, a discrete suprathreshold process transforms burstlets into full amplitude inspiratory bursts that drive motor output, measurable via hypoglossal nerve (XII) discharge in vitro . We recap observations by Kam & Feldman: field recordings from preBötC demonstrate bursts and concurrent XII motor output intermingled with lower amplitude burstlets that do not produce XII motor output. Manipulations of excitability affect the relative prevalence of bursts and burstlets and modulate their frequency. Whole-cell and photonic recordings of preBötC neurons suggest that burstlets involve inconstant subsets of rhythmogenic interneurons. We conclude that discrete rhythm- and pattern-generating mechanisms coexist in the preBötC and that burstlets reflect its fundamental rhythmogenic nature.

First principles electronic structure studies on the ground state geometries, stability, and the electronic structure of Al_nNa_m and Al_nMg_m (m ≤ 3, n + m ≤ 15) clusters have been carried out to examine the nature of bonding between Na or Mg and Al. Identifying whether the bonding is ionic or metallic in bulk materials is typically straightforward; however, in small clusters where quantum confinement is important, the nature of bonding may become unclear. We have performed a critical analysis of the bonding in these bimetallic clusters using charge analysis, electrical dipole moments, hybridization of the atomic orbitals, the Laplacian of the charge density at the bond critical points, and the change in the bonding energy between neutral and anionic forms of the cluster. For Na_nAl_m clusters, we find that the Na binding is primarily ionic, while the bonding in Al_nMg_m is primarily metallic. We find that the Mulliken population of the 3p orbital of Na and Mg can provide a rapid assessment of the nature of bonding. We also find that the Hirshfeld charge and dipole moments are effective indicators, when placed in context. We found that the Laplacian of the charge density at the bond critical points can be misleading in identifying whether the bonding is ionic or metallic in small clusters.

We report a joint experimental and theoretical investigation into the geometry, stability, and reactivity with oxygen of alloy metal clusters Al_nMg_m^– (4 ≤ n+m ≤ 15; 0 ≤ m ≤ 3). Considering that Al and Mg possess three and two valence electrons, respectively, clusters with all possible valence electron counts from 11 to 46 are studied to probe the magic numbers predicted by the spherical jellium model, and to determine whether enhanced stability and reduced reactivity may be found for some Al_nMg_m^– at non-magic numbers. Al₅Mg₂^– and Al₁₁Mg₃^– exhibit enhanced stability corresponding to the expected magic numbers of 20 and 40 electrons, respectively; while Al₇Mg₃^–, Al₁₁Mg^–, and Al₁₁Mg₂^– turn out to be unexpectedly stable at electron counts of 28, 36, and 38, respectively. The enhanced stability at non-magic numbers is explained through a crystal-field-like splitting of degenerate shells by the geometrical distortions of the clusters. Al_nMg_m^– clusters appear to display higher oxidation than pure Al_n^– clusters, suggesting that the addition of Mg atoms enhances the combustion of pure aluminum clusters.