CO2-induced prolongation of expiratory time during early development

Role of fast inhibitory synaptic transmission in neonatal respiratory rhythmogenesis and pattern formation

Several studies have investigated the general role of chloride-based neurotransmission (GABA_A and glycinergic signaling) in respiratory rhythmogenesis and pattern formation. In several brain regions, developmental alterations in these signaling pathways have been shown to be mediated by changes in cation-chloride cotransporter (CC) expression. For instance, CC expression changes during the course of neonatal development in medullary respiratory nuclei and other brain/spinal cord regions in a manner which decreases the cellular import, and increases the export, of chloride ions, shifting reversal potentials for chloride to progressively more negative values with maturation. In slice preparations of the same, this is related to an excitatory-to-inhibitory shift of GABA_A- and glycinergic signaling. In medullary slices, GABA_A-/glycinergic signaling in the early neonatal period is excitatory, becoming inhibitory over time. Additionally, blockade of the Na⁺/K⁺/2Cl^- cotransporter, which imports these ions via secondary active transport, converts excitatory response to inhibitory ones. These effects have not yet been demonstrated at the individual respiratory-related neuron level to occur in intact (in vivo or in situ) animal preparations, which in contrast to slices, possess normal network connectivity and natural sources of tonic drive. Developmental changes in respiratory rhythm generating and pattern forming pontomedullary respiratory circuitry may contribute to critical periods, during which there exist increased risk for perinatal respiratory disturbances of central, obstructive, or hypoxia/hypercapnia-induced origin, including the sudden infant death syndrome. Thus, better characterizing the neurochemical maturation of the central respiratory network will enhance our understanding of these conditions, which will facilitate development of targeted therapies for respiratory disturbances in neonates and infants.

Laryngeal reflex apnea in neonates: effects of CO2 and the complex influence of hypoxia

We have examined influence of hypocapnia, mild hypercapnia and hypoxia on the durations of fictive apnea and respiratory disruption elicited by injection of 0.1ml of water into the laryngeal lumen-the laryngeal chemoreflex (LCR)-in 20 unanesthetized, decerebrate, vagotomized piglets aged 4-10 days that were paralyzed and ventilated with a constant frequency and tidal volume. The LCR was enhanced by hypocapnia and attenuated by hypercapnia as reported by others. The responses to laryngeal stimulation during hypoxia were varied and complex: some animals showed abbreviated responses during the tachypnea of early hypoxia, followed after 10-15min by more prolonged apnea and respiratory disruption accompanying the reduction in ventilatory activity that commonly occurs during sustained hypoxia in neonates. We speculate that this later hypoxic enhancement of the LCR may be due to accumulation of adenosine in the brain stem.

Activation of central adenosine A2A receptors enhances superior laryngeal nerve stimulation-induced apnea in piglets via a GABAergic pathway

Activation of the laryngeal mucosa results in apnea that is mediated through, and can be elicited via electrical stimulation of, the superior laryngeal nerve (SLN). This potent inhibitory reflex has been suggested to play a role in the pathogenesis of apnea of prematurity and sudden infant death syndrome, and it is attenuated by theophylline and blockade of GABAA receptors. However, the interaction between GABA and adenosine in the production of SLN stimulation-induced apnea has not been previously examined. We hypothesized that activation of adenosine A2A receptors will enhance apnea induced by SLN stimulation while subsequent blockade of GABAA receptors will reverse the effect of A2A receptor activation. The phrenic nerve responses to increasing levels of SLN stimulation were measured before and after sequential intracisternal administration of the adenosine A2A receptor agonist CGS ( n = 10) and GABAA receptor blocker bicuculline ( n = 7) in ventilated, vagotomized, decerebrate, and paralyzed newborn piglets. Increasing levels of SLN stimulation caused progressive inhibition of phrenic activity and lead to apnea during higher levels of stimulation. CGS caused inhibition of baseline phrenic activity, hypotension, and enhancement of apnea induced by SLN stimulation. Subsequent bicuculline administration reversed the effects of CGS and prevented the production of apnea compared with control at higher SLN stimulation levels. We conclude that activation of adenosine A2A receptors enhances SLN stimulation-induced apnea probably via a GABAergic pathway. We speculate that SLN stimulation causes endogenous release of adenosine that activates A2A receptors on GABAergic neurons, resulting in the release of GABA at inspiratory neurons and subsequent respiratory inhibition.

Adenosine A2A receptors mediate GABAergic inhibition of respiration in immature rats

Adenosine is a known inhibitor of respiratory output during early life. In this study we investigated the developmental changes in adenosine A2A-receptor activation on respiratory timing, as well as the relationship between adenosine and GABA. The specific adenosine A2A-receptor agonist CGS-21680 (CGS) or vehicle control was injected into the fourth ventricle of 14-day (n = 9), 21-day (n = 9), and adult (n = 5) urethane-anesthetized rats while diaphragm electromyogram was monitored as an index of respiratory neural output. CGS injection resulted in a decrease in frequency and/or apnea in all 14-day-old rats and in 66% of 21-day-old rats. There was no effect of CGS injection on respiratory timing in adult rats. Prior injection of the GABA(A)-receptor blocker bicuculline at 14 and 21 days eliminated the CGS-induced decrease in frequency and apnea. We conclude from these studies that the inhibitory effect of A2A-receptor activation on respiratory drive is age dependent and is mediated via GABAergic inputs to the inspiratory timing neural circuitry. These findings demonstrate an important mechanism by which xanthine therapy alleviates apnea of prematurity.

Adenosine A2A receptors interact with GABAergic pathways to modulate respiration in neonatal piglets

GABA and adenosine contribute to respiratory inhibition in early postnatal life. In this study the adenosine A2A receptor agonist CGS21680 was used to evaluate adenosine receptor specificity and the interrelation of adenosine and GABA in the inhibition of inspiratory drive. In neonatal piglets (n = 10), CGS21680 was injected into the fourth ventricle resulting in apnea and/or decreased burst area and frequency of phrenic discharge. Phrenic burst area decreased to 58.9 +/- 8.6% (S.E.M.) after CGS21680 injection (control = 91.8 +/- 1.0%). Expiratory time increased 261.0 +/- 59.9% after CGS21680 from control (87.7 +/- 2.7%). When bicuculline was injected locally within the rostral ventrolateral medulla (n = 5), or into the fourth ventricle (n = 5), the CGS21680 induced inhibition of phrenic was abolished. To define expression of A2A receptor at the message level (mRNA), we employed in situ hybridization with a digoxigenin-coupled oligonucleotide. Adenosine A2A receptor mRNA was expressed in regions of the medulla oblongata known to contain GABAergic neurons. We conclude that GABAergic inputs affecting respiratory timing and inspiratory drive are modulated by activation of A2A receptors. These findings offer new insight into the mechanism whereby xanthine therapy diminishes apnea of prematurity.

Hypercapnic ventilatory response in mice lacking the 65 kDa isoform of Glutamic Acid Decarboxylase (GAD65)

Background

Recent reports have shown that there are developmental changes in the ventilatory response to hypercapnia in the rat. These are characterized by an initial large response to carbon dioxide immediately after birth followed by a decline with a trough at one week of age, followed by a return in sensitivity. A second abnormality is seen at postnatal day 5 (P5) rats in that they cannot maintain the increase in frequency for 5 min of hypercapnia. In mice lacking GAD65 the release of GABA during sustained synaptic activation is reduced. We hypothesized that this developmental pattern would be present in the mouse which is also less mature at birth and that GABA mediates this relative respiratory depression.

Methods

In awake C57BL/6J and GAD65-/- mice the ventilatory response to 5% carbon dioxide (CO2) was examined at P2, P4, P6, P7, P12.5, P14.5 and P21.5, using body plethysmography.

Results

Minute ventilation (VE) relative to baseline during hypercapnia from P2 through P7 was generally less than from P12.5 onwards, but there was no trough as in the rat. Breaking VE down into its two components showed that tidal volume remained elevated for the 5 min of exposure to 5% CO2. At P6, but not at other ages, respiratory frequency declined with time and at 5 min was less that at 2 and 3 min. GAD65-/- animals at P6 showed a sustained increase in respiratory rate for the five mins exposure to CO2.

Conclusion

These results show, that in contrast to the rat, mice do not show a decline in minute ventilatory response to CO2 at one week of age. Similiar to the rat at P5, mice at P6 are unable to sustain an increase in CO2 induced respiratory frequency and GAD65 contributes to this fall off.

Hypercapnia-induced activation of brainstem GABAergic neurons during early development

During early development, GABAergic mechanisms contribute to the regulation of respiratory timing in response to CO2. In 5-7 day old piglets, a double labeling technique was used to determine whether GABA-containing neurons are activated by normoxic hypercapnia (10% CO2, 21% O2, and 69% N2). The c-Fos gene encoded protein (c-Fos) was employed to localize CO2 activated cells within the piglet medulla oblongata. Parvalbumin was used as a marker for GABAergic neurons. In animals breathing room air, only scant c-Fos-like immunoreactive neurons were observed. A marked increase in c-Fos positive cells was induced after a 60 min exposure to hypercapnia. Colocalization studies revealed that hypercapnia significantly increased c-Fos expression in GABA-containing neurons in the medulla oblongata, especially in the ventral aspect of the medulla, within the Bötzinger region, the gigantocellular reticular nucleus, and the caudal raphe nuclei. Only a few double-labeled cells were observed within the nucleus tractus solitarius. Therefore, brainstem GABAergic neurons are part of the neural networks that respond to CO2 and may contribute to respiratory frequency responses to hypercapnia during early development.

Inhibition of Na(+)/H(+) exchanger type 3 reduces duration of apnea induced by laryngeal stimulation in piglets

Reflexes from the larynx induce cessation of breathing in newborn animals. The magnitude of respiratory inhibition is inversely related to the level of central chemical input. Recent studies indicate that selective inhibition of Na(+)/H(+) exchanger type 3 (NHE3) activates CO(2)/H(+)-sensitive neurons, resembling the responses evoked by hypercapnic stimuli. Hence, the use of NHE3 inhibitors may reduce reflexly mediated respiratory depression and duration of apnea in the neonatal period. This possibility was examined in decerebrate, vagotomized, ventilated, and paralyzed piglets by testing the effects of i.v. administration of NHE3 blocker S8218 on the response of phrenic nerve amplitude, frequency, and duration of apnea induced by graded electrical stimulation of the superior laryngeal nerve. Superior laryngeal nerve stimulation caused a significant decrease in phrenic nerve amplitude, frequency, minute phrenic activity, and inspiratory time (all p < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea both during and after cessation of stimulation. NHE3 blocker S8218 reduced the superior laryngeal nerve stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (all p < 0.05) and reduced superior laryngeal nerve stimulation-induced apnea and duration of poststimulation apnea (p < 0.05). In six other pigs the brain concentrations of S8218 were measured at different intervals after i.v. administration of the drug and were found to be higher in the brain tissue than plasma at all intervals. These findings suggest that the use of NHE3 blockers may decrease the duration of apnea and possibly reduce the pathophysiologic consequences of potentially life-threatening apnea in infants.

Endogenous nitric oxide modulates responses of tissue and airway resistance to vagal stimulation in piglets

The role of endogenous nitric oxide (NO) in modulating the excitatory response of distal airways to vagal stimulation is unknown. In decerebrate, ventilated, open-chest piglets aged 3-10 days, lung resistance (RL) was partitioned into tissue resistance (Rti) and airway resistance (Raw) by using alveolar capsules. Changes in RL, Rti, and Raw were evaluated during vagal stimulation at increasing frequency before and after NO synthase blockade with N(omega)-nitro-L-arginine methyl ester (L-NAME). Vagal stimulation increased RL by elevating both Rti and Raw. NO synthase blockade significantly increased baseline Rti, but not Raw, and significantly augmented the effects of vagal stimulation on both Rti and Raw. Vagal stimulation also resulted in a significant increase in cGMP levels in lung tissue before, but not after, L-NAME infusion. In seven additional piglets after RL was elevated by histamine infusion in the presence of cholinergic blockade with atropine, vagal stimulation failed to elicit any change in RL, Rti, or Raw. Therefore, endogenous NO not only plays a role in modulating baseline Rti, but it opposes the excitatory cholinergic effects on both the tissue and airway components of RL. We speculate that activation of the NO/cGMP pathway during cholinergic stimulation plays an important role in modulating peripheral as well as central contractile elements in the developing lung.

Frequency responses to hypoxia and hypercapnia in carotid body-denervated conscious rats

The ventilatory response to brief, severe hypoxia is biphasic consisting of an initial facilitation followed by a slowing of breathing frequency (fR). After the hypoxic stimulus is removed, fR drops below baseline levels. This phenomenon is called the post-hypoxic frequency decline (phfd). These fR changes are due to reciprocal changes in expiratory time (TE), mediated by the ventrolateral pontine A5 region (J. Physiol. (London) 497 (1996) 79; Am. J. Physiol. 274 (1998) R1546). The purpose of this study was to determine if carotid body input is required for full manifestation of phfd by quantifying ventilation in intact and carotid sinus denervated rats in response to hypoxic, and contrasted with hypercapnic stimuli. Following carotid denervation the initial facilitation of fR was eliminated in response to hypoxia, but the phfd remained. In contrast the pattern in response to increased CO2 remained constant before and after carotid denervation. These results suggest that phfd is not dependent upon carotid body stimulation, but is mediated centrally.

Differential effects of hypercapnia on expiratory phases of respiration in the piglet

Hypercapnia induces prolongation of expiratory time (TE) during early development. In the present study, we determined the response to steady state hypercapnia of three neural phases of the total respiratory cycle, inspiration (TI), stage 1 or passive expiration, TE-1 and stage 2 or active expiration, TE-2. Experiments were performed in decerebrate, vagotomized, spontaneously breathing piglets aged 5-10 days. Neural phases of the respiratory cycle were based on electrical activities of the thyroarytenoid (TA, laryngeal adductor) and triangularis sternii (TS, chest wall expiratory muscle) in relation to diaphragm (D) activity. We observed that hypercapnia induced prolongation of both expiratory phases. The greater prolongation of TE-1 was associated with an increase in TA activity and an increase in laryngeal resistance, which peaked early in TE-1, and then progressively decreased. These findings demonstrate that, in early postnatal life, a hypercapnia induced increase in respiratory drive is associated with centrally mediated prolongation of both phases of expiration, a greater prolongation of TE-1, and an increase in laryngeal resistance during post-inspiration. We speculate that the latter serves to optimize gas exchange by reducing large fluctuations in functional residual capacity.

Central GABAergic mechanisms are involved in apnea induced by SLN stimulation in piglets

Stimulation of the superior laryngeal nerve (SLN) results in apnea in animals of different species, the mechanism of which is not known. We studied the effect of the GABA(A) receptor blocker bicuculline, given intravenously and intracisternally, on apnea induced by SLN stimulation. Eighteen 5- to 10-day-old piglets were studied: bicuculline was administered intravenously to nine animals and intracisternally to nine animals. The animals were anesthetized and then decerebrated, vagotomized, ventilated, and paralyzed. The phrenic nerve responses to four levels of electrical SLN stimulation were measured before and after bicuculline. SLN stimulation caused a significant decrease in phrenic nerve amplitude, phrenic nerve frequency, minute phrenic activity, and inspiratory time (P < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea during stimulation that often persisted beyond cessation of the stimulus. Bicuculline, administered intravenously or intracisternally, decreased the SLN stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (P < 0.05). Bicuculline also reduced SLN-induced apnea and duration of poststimulation apnea (P < 0.05). We conclude that centrally mediated GABAergic pathways are involved in laryngeal stimulation-induced apnea.

Depression of ventilatory responses after daily, cyclic hypercapnic hypoxia in piglets

Ventilatory responses (VRs) were measured via a sealed face mask and pneumotachograph in 30 unsedated, mixed-breed miniature piglets at 12.6 +/- 2.3 days of age (day 1) and then repeated after seven daily 24-min exposures to 10% O(2)-6% CO(2) [hypercapnic hypoxia (HH)]. Arterial blood was sampled at baseline, after 10 min of exposure, and after 10 min of recovery. VRs included hypoxia (10% O(2) in N(2)), hypercapnia (6% CO(2) in air), and HH (10% O(2)-6% CO(2)-balance N(2)). Treatment groups (n = 10 each) were exposed to 24 min of HH from day 2 to 8 as sustained HH (24 min of HH and then 24 min of air) or cyclic HH (4 min of HH alternating with 4 min of air). Day 1 and 9 data were compared in treatment and control groups. After cyclic HH, respiratory responses to CO(2) were reduced during hypercapnia and during HH (P < 0.001 vs. control for minute ventilation in both). In both treatment groups, time to peak minute ventilation was delayed in hypoxia (P = 0.02, ANOVA), and response amplitude was increased (P < 0.001 and P = 0.003, sustained and cyclic HH, respectively, vs. control). Respiratory pattern was also altered during the VRs and among treatment groups. Stimulus presentation characteristics exert effects on VRs that are independent of those elicited by daily HH.

Lesion or muscimol in the rostral ventral medulla reduces ventilatory output and the CO(2) response in decerebrate piglets

Developmental abnormalities have been described in the arcuate nucleus of sudden infant death syndrome (SIDS) victims. The arcuate nucleus has putative homologues in chemosensitive areas of the ventral medulla in animals. We refer to some of these areas collectively as the rostral ventral medulla (RVM). In the RVM of decerebrate piglets 2-15 days of age, we studied the effects of electrolytic lesions (n=7) or microdialysis of muscimol (n=15), a GABAA receptor agonist, on ventilatory output and the response to hypercapnia. Lesions caused a 66.7+/-17.3% reduction in eupneic phrenic minute activity (MA) and abolished the response to hypercapnia. Muscimol dialysis caused a 32.4+/-10.4% reduction in MA with a significant downward displacement of the response to hypercapnia with no significant effect on the slope. We conclude that the piglet RVM contains neurons of vital importance in the maintenance of normal breathing and the response to systemic CO(2). We hypothesize that dysfunction of homologous regions in the human infant could lead to impaired ability to respond to hypercapnia and could potentially be involved in the pathogenesis of SIDS.

Recurrent hypoxic exposure and reflex responses during development in the piglet

The effects of recurrent hypoxia on cardiorespiratory reflexes were characterized in anesthetized piglets at 2-10 d (n=15), 2-3 weeks (n=11) and 8-10 weeks (n=8). Responses of phrenic and hypoglossal electroneurograms (ENG(phr) and ENG (hyp)) to hypoxia (8% 0(2), bal N(2), 5 min), hypercapnia (7% CO(2) bal O(2), 5 min) and intravenous capsaicin were tested before and after recurrent exposure to 11 episodes of hypoxia (8% O(2) bal N(2), 5 min). In piglets 2-10 d, ENG(phr) response to hypoxia declined in proportion to the number of hypoxic exposures; however, ENG (hyp) response to hypoxia was unchanged. In piglets at 2-10 d, intracisternal injection of bicuculline (GABA(A) receptor antagonist) reversed effects of recurrent hypoxia on ENG(phr) hypoxic response, eliminated apnea during hypoxia, as well as the delay in appearance of ENG(phr) after hypoxia. The ENG(phr) response to 7% CO(2) inhalation also decreased after recurrent hypoxia; however, the ENG(phr) response to C-fiber stimulation by capsaicin was unaltered. Piglets at 2-3 and 8-10 weeks were resistant to the depressive effects of recurrent hypoxia on respiratory reflex responses. We conclude that the response of the anesthetized newborn piglet to recurrent hypoxia is dominated by increasing inhibition of phrenic neuroelectrical output during successive hypoxic exposures. Central GABAergic inhibition may contribute significantly to the cumulative effects of repeated hypoxia in the newborn piglet experimental model.

Maturation attenuates the effects of cGMP on contraction, [Ca2+]i and Ca2+ sensitivity in ovine basilar arteries

The present study explores the hypothesis that age-related variations in cerebrovascular responses to vasodilators reflect corresponding age-dependent differences in the mechanisms coupling changes in cytosolic cGMP to vasorelaxation. The experiments focused on cGMP's ability to decrease either [Ca2+]i or myofilament Ca2+ sensitivity, because both effects can contribute to cGMP-induced vasodilation. Use of the cGMP analog 8-pCPT-cGMP minimized problems associated with limited cell permeation or cGMP hydrolysis. In fetal basilars contracted with 10 microM serotonin, the EC30 for 8-pCPT-cGMP-induced relaxation was 6 microM. In fura-2 loaded fetal basilars, pretreatment with 6 microM 8-pCPT-cGMP significantly depressed the sensitivity of [Ca2+]i to 5HT, and also myofilament sensitivity to calcium, but only in fetal arteries. In fetal basilar arteries contracted with 120 mM potassium, the EC30 for 8-pCPT-cGMP-induced relaxation was 25 microM. In fura-2 loaded ovine arteries, pretreatment with 25 microM 8-pCPT-cGMP had no effect on the ability of graded concentrations of potassium to elevate [Ca2+]i but reduced potassium's ability to induce contraction and attenuated myofilament calcium sensitivity; these latter effects were significant only in fetal arteries. In alpha-toxin permeabilized preparations, 25 microM 8-pCPT-cGMP significantly depressed both basal- and agonist-stimulated myofilament calcium sensitivity, only in fetal but not in adult basilars. Together, these results demonstrate that: (1) sensitivity to cGMP is greater in fetal than adult sheep arteries independent of method of contraction; (2) cGMP can reduce [Ca2+]i but only in agonist-contracted and not in potassium-contracted arteries; (3) and cGMP attenuates myofilament calcium sensitivity regardless of method of contraction. Overall, the data demonstrate that variations in the ability of cGMP to produce vasodilatation reflect age-, artery-, and agonist-dependent differences in the combination of mechanisms mediating responses to cGMP.