Michael B. Harris

Do descending influences alternate to produce episodic breathing

This study examines the episodic breathing patterns of three disparate groups of vertebrates. In an in vitro bullfrog brainstem-spinal cord preparation, episodic breathing was replaced by uniformly spaced breaths following transection caudal to the optic chiasma. The same effect was produced in hibernating squirrels by inhalation of mild anesthesia. Preliminary data suggest that a similar conversion is also produced in hibernating squirrels by vagotomy, in conjunction with blockade of central NMDA-type glutamate receptors. In all cases, even though overall breathing frequency increased, due to elimination of periods of apnea, instantaneous breathing frequency slowed. Seals breathe episodically in sleep and when these animals awaken after the start of a breathing episode, breathing also immediately slows. The data presented here are consistent with the suggestion that in all vertebrates, higher centres can modulate the central rhythm generator for breathing, in both a positive and a negative fashion. During episodic breathing, in the species studied here, these modulating influences alternate in a fashion that produces periods of apnea alternating with periods of relatively high frequency ventilation.

The influence of NMDA receptor-mediated processes on breathing pattern in ground squirrels

The effects of blockade of N-methyl-D-aspartate (NMDA) type glutamate receptors by a non-competitive antagonist (MK-801) on cortical arousal, breathing pattern and ventilatory responses to hypoxia (10% O2 in N2) and hypercapnia (5% CO2 in air) were assessed in anesthetized (urethane) and unanesthetized golden-mantled ground squirrels (Spermophilus lateralis). Intra-cerebroventricular administration of MK-801 did not alter ventilation during wakefulness, although it did alter the pattern (breathing frequency and tidal volume components) of the hypercapnic ventilatory response, and suppressed the ventilatory response to hypoxia. Animals did not sleep following treatment with MK-801, and intravenous administration of MK-801 prevented expression of the sleep-like state normally observed in anesthetized animals. In anesthetized animals MK-801 elevated breathing frequency to levels observed without anesthesia, and suppressed the hypoxic ventilatory response. These data suggest that NMDA-type glutamatergic receptor-mediated processes influence cortical arousal and facilitate depression of breathing frequency during anesthesia and the hypoxic ventilatory response. Such processes are not essential for the hypercapnic ventilatory response.

Apneusis follows disruption of NMDA-type glutamate receptors in vagotomized ground squirrels

The influences of N-methyl-D-aspartate (NMDA) type glutamate receptor antagonism, by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate (MK-801), on breathing pattern and ventilatory chemoresponses, were assessed in anaesthetized vagotomized spontaneously breathing golden-mantled ground squirrels, Spermophilus lateralis. MK-801 was administered by either bilateral pressure micro-injection into a region of the rostral dorsolateral pons, containing the medial and lateral Parabrachial and Kölliker-Fuse nuclei (the Parabrachial complex, PbC), or by systemic injection. Both treatments induced apneusis. These data indicate that functional NMDA receptor-mediated processes located within the PbC terminate inspiration and actively prevent apneusis in vagotomized ground squirrels. Although both hypercapnia and hypoxia stimulated breathing during the apneusis, the responses were generally slight. The breathing frequency component of the hypercapnic ventilatory response was completely eliminated supporting the hypothesis that the PbC is an integral component of the control network for CO(2) chemoreflex responses. Differences in the results of systemic versus PbC MK-801 illustrate that NMDA receptor-mediated processes outside the PbC do influence ventilation. Our data also show that such processes outside the PbC lengthen both inspiration and expiration in this species, slowing ventilation, and are necessary for the expression of the hypoxic ventilatory response.

Vagal feedback is essential for breathing in unanesthetized ground squirrels.

The roles of vagal afferent feedback in terminating inspiration and modulating breathing pattern and ventilatory responses to hypoxia and hypercapnia were assessed in the golden-mantled ground squirrel, Spermophilus lateralis, during wakefulness and urethane anesthesia. Hypoxia increased ventilation primarily through increases in breathing frequency (f(R)) while hypercapnia increased ventilation primarily through increases in tidal volume (V(T)) in both anesthetized and unanesthetized animals. Vagotomy resulted in an increase in tidal volume, a decrease in breathing frequency and ventilation, and depressed ventilatory responses to both hypoxia and hypercapnia in anesthetized animals. In unanesthetized animals vagotomy produced a transient gasp-like breathing pattern that rapidly progressed to a non-obstructive central apnea. These data indicate that vagal feedback shapes ventilation on a breath-by-breath basis during anesthesia and is essential for ventilation in unanesthetized animals. The mechanisms that transform the influences of vagal input on breathing between anesthetized and unanesthetized states remain unclear. Changes in breathing pattern induced by the removal of vagal feedback compromise chemoreflexes.

Is Hibernation Facilitated by an Inhibition of Arousal

Seasonal hibernation is a poorly understood phenomenon and the details of how hibernation induction. maintenance and arousal are controlled are, for the most part, enigmatic. Hibernating golden-mantled ground squirrels, Spermophilus lateralis, were treated with MK- 80 I, a non-competitive antagonist of the NMDA-type glutamate receptor, as part of a larger investigation of the role of these receptors in the control of breathing in this species. Drug treatment prompted arousal from hibernation at a dose of 5.0 mg/kg, but not at a dose of 0.5 mg/kg. We have previously demonstrated that similar treatment prevents sleep and sleep-like states of cortical arousal in unanesthetized and anesthetized squirrels. As removal of NMDAxad type glutamatergic activation resulted in arousal, the data suggest that arousal is normally actively inhibited; glutamatergic inhibition facilitates stable hibernation and NMDA-type glutamatergic blockade results in a disinhibition of arousal mechanisms, resulting in arousal from hibernation. The similarity between the present results and those pertaining to sleep in non-hibernating squirrels supports the theoretical homology between hibernation and sleep. This research was supported by the Natural Sciences and Engineering Research Council of Canada, and results have been submitted for peer-reviewed publication in primary literature.

The ventilatory response to hypercapnia in hibernating golden-mantled ground squirrels Spermophilus lateralis

The hypercapnic ventilatory response of golden-mantled ground squirrels was characterized on consecutive days throughout single bouts of hibernation and on the third day of hibernation bouts during the early, middle, and late portions of the hibernation season, to determine whether ventilatory responses during hibernation are a function of body temperature or hibernation depth. Our measurements confirm that the hypercapnic ventilatory response of this species during hibernation consists almost exclusively of increases in respiratory frequency (fr); tidal volume (VT) changed little. No change in fr or VT occurred as inspired CO₂ was increased from 0% to 2%. Between 2% and 8% inspired CO₂, fr increased linearly. Respiratory frequency remained constant or decreased slightly as CO₂ was elevated beyond 8%. At levels of inspired CO₂ between 10% and 13%, animals would initiate arousal from hibernation and ventilation would increase again through increases in VT and further increases in fr. No significant interbout or intrabout variations were observed in any aspect of the hypercapnic ventilatory response. These data indicate that hypercapnic ventilatory sensitivity, if not exclusively a function of body temperature, is consistent between periods of equal body temperature regardless of possible temperature-independent changes in hibernation depth.