Robert F. Rogers

Motoneuron firing patterns underlying fast oscillations in phrenic nerve discharge in the rat

Fast oscillations are ubiquitous throughout the mammalian central nervous system and are especially prominent in respiratory motor outputs, including the phrenic nerves (PhNs). Some investigators have argued for an epiphenomenological basis for PhN high-frequency oscillations because phrenic motoneurons (PhMNs) firing at these same frequencies have never been recorded, although their existence has never been tested systematically. Experiments were performed on 18 paralyzed, unanesthetized, decerebrate adult rats in which whole PhN and individual PhMN activity were recorded. A novel method for evaluating unit-nerve time-frequency coherence was applied to PhMN and PhN recordings. PhMNs were classified according to their maximal firing rate as high, medium, and low frequency, corresponding to the analogous bands in PhN spectra. For the first time, we report the existence of PhMNs firing at rates corresponding to high-frequency oscillations during eupneic motor output. The majority of PhMNs fired only during inspiration, but a small subpopulation possessed tonic activity throughout all phases of respiration. Significant time-varying PhMN-PhN coherence was observed for all PhMN classes. High-frequency, early-recruited units had significantly more consistent onset times than low-frequency, early/middle-recruited and medium-frequency, middle/late-recruited PhMNs. High- and medium-frequency PhMNs had significantly more consistent offset times than low-frequency units. This suggests that startup and termination of PhMNs with higher firing rates are more precisely controlled, which may contribute to the greater PhMN-PhN coherence at the beginning and end of inspiration. Our findings provide evidence that near-synchronous discharge of PhMNs firing at high rates may underlie fast oscillations in PhN discharge.

The role of spinal GABAergic circuits in the control of phrenic nerve motor output

While supraspinal mechanisms underlying respiratory pattern formation are well characterized, the contribution of spinal circuitry to the same remains poorly understood. In this study, we tested the hypothesis that intraspinal GABAergic circuits are involved in shaping phrenic motor output. To this end, we performed bilateral phrenic nerve recordings in anesthetized adult rats and observed neurogram changes in response to knocking down expression of both isoforms (65 and 67 kDa) of glutamate decarboxylase (GAD65/67) using microinjections of anti-GAD65/67 short-interference RNA (siRNA) in the phrenic nucleus. The number of GAD65/67-positive cells was drastically reduced on the side of siRNA microinjections, especially in the lateral aspects of Rexeds laminae VII and IX in the ventral horn of cervical segment C4, but not contralateral to microinjections. We hypothesize that intraspinal GABAergic control of phrenic output is primarily phasic, but also plays an important role in tonic regulation of phrenic discharge. Also, we identified respiration-modulated GABAergic interneurons (both inspiratory and expiratory) located slightly dorsal to the phrenic nucleus. Our data provide the first direct evidence for the existence of intraspinal GABAergic circuits contributing to the formation of phrenic output. The physiological role of local intraspinal inhibition, independent of descending direct bulbospinal control, is discussed.

GABAAergic and Glycinergic Inhibition in the Phrenic Nucleus Organizes and Couples Fast Oscillations in Motor Output

One of the characteristics of respiratory motor output is the presence of fast synchronous oscillations, at rates far exceeding the basic breathing rhythm, within a given functional population. However, the mechanisms responsible for organizing phrenic output into two dominant bands in vivo, medium (MFO)- and high (HFO)-frequency oscillations, have yet to be elucidated. We hypothesize that GABA(A)ergic and glycinergic inhibition within the phrenic motor nucleus underlies the specific organization of these oscillations. To test this, the phrenic nuclei (C(4)) of 14 unanesthetized, decerebrate adult male Sprague-Dawley rats were microinjected unilaterally with either 4 mM strychnine (n = 7) or GABAzine (n = 7) to block glycine or GABA(A) receptors, respectively. Application of GABAzine caused an increase in overall phrenic amplitude during all three phases of respiration (inspiration, postinspiration, and expiration), while the increases caused by strychnine were most pronounced during postinspiration. Neither antagonist produced changes in inspiratory duration or respiratory rate. Power spectral analysis of inspiratory phrenic bursts showed that blockade of inhibition caused significant reduction in the relative power of MFO (GABA(A) and glycine receptors) and HFO (GABA(A) receptors only). In addition, analysis of the coherence between the firing of the ipsi- and contralateral phrenic nerves revealed that HFO coupling was significantly reduced by both antagonists and that of MFO was significantly reduced only by strychnine. We conclude that both GABA(A) and glycine receptors play critical roles in the organization of fast oscillations into MFO and HFO bands in the phrenic nerve, as well as in their bilateral coupling.

Slowly adapting pulmonary stretch receptor spike patterns carry lung distension information

Slowly adapting pulmonary stretch receptors (SARs) provide the respiratory and cardiovascular control systems with information regarding the rate and depth of breathing. Previous information theoretical analysis demonstrated that SAR spike count provides a reliable representation of lung distension. This study examines whether SAR spike patterns may also provide information about lung distension. To investigate this, artificial spike trains were generated with the same number of spikes (but randomized intervals) as those recorded from SARs in response to three different lung inflation volumes in urethane-anesthetized rabbits. Three different spike train classification methods were applied to estimate which stimulus evoked them, and the accuracy with which artificial spike trains were classified was compared to that of real SAR spike trains using the same methods. Because real SAR spike trains were classified with higher accuracies than artificial ones containing the same number of spikes, we conclude that SAR spike patterns, in addition to spike counts, contain information concerning the amplitude of lung distension.

Joint probability-based neuronal spike train classification

Neuronal spike trains are used by the nervous system to encode and transmit information. Euclidean distance-based methods (EDBMs) have been applied to quantify the similarity between temporally-discretized spike trains and model responses. In this study, using the same discretization procedure, we developed and applied a joint probability-based method (JPBM) to classify individual spike trains of slowly adapting pulmonary stretch receptors (SARs). The activity of individual SARs was recorded in anaesthetized, paralysed adult male rabbits, which were artificially-ventilated at constant rate and one of three different volumes. Two-thirds of the responses to the 600 stimuli presented at each volume were used to construct three response models (one for each stimulus volume) consisting of a series of time bins, each with spike probabilities. The remaining one-third of the responses where used as test responses to be classified into one of the three model responses. This was done by computing the joint probability of observing the same series of events (spikes or no spikes, dictated by the test response) in a given model and determining which probability of the three was highest. The JPBM generally produced better classification accuracy than the EDBM, and both performed well above chance. Both methods were similarly affected by variations in discretization parameters, response epoch duration, and two different response alignment strategies. Increasing bin widths increased classification accuracy, which also improved with increased observation time, but primarily during periods of increasing lung inflation. Thus, the JPBM is a simple and effective method performing spike train classification.