Biomedical subjects

G G Haddad

Publications and source records attributed to G G Haddad.

At least 19 recordsLinked to original sources

PubMed1992

Ontogeny and distribution of GABAA receptors in rat brainstem and rostral brain regions.

Previous studies from our laboratory and others have shown that there are major age-related differences in brainstem neuronal function. Since GABAA receptors are major targets for GABA-mediated inhibitory modulation and play a key role in regulating cardiorespiratory function, especially during O2 deprivation, we examined differences in GABAA receptor density and distribution during postnatal development. Using quantitative receptor autoradiography, the present study was performed to examine the postnatal expression of GABAA receptors in the rat brainstem and rostral brain areas at five ages, i.e. postnatal day 1 (P1), P5, P10, P21 and P120. Ten-micrometer brain sections at different brain levels were labelled with [3H]muscimol in Tris-citrate buffer. We found that (i) GABAA receptors appeared very early in almost all the brainstem as well as rostral areas; (ii) at P1, the brainstem had a higher GABAA receptor binding density than rostral areas and its density peaked at P5 or P10; and (iii) receptor densities of the cerebellum and rostral brain areas such as cortex, thalamus and dentate gyrus increased with age, especially between P10 and P21, but most other subcortical areas like caudate-putamen and hippocampal CA1 area did not increase remarkably after birth. We conclude that: (i) GABAA receptors exist in most brain areas at birth; (ii) there are several patterns of postnatal development of GABAA receptors in the CNS with dramatic differences between the brainstem and cortex; (iii) brainstem functions rely more on GABAA receptors in early postnatal life than at more mature stages. We speculate that GABAA receptors develop earlier in phylogenetically older structures (such as brainstem) than in newer brain regions (such as cortex).

Aging↗

PubMed1992

Role of ATP-sensitive K+ channels during anoxia: major differences between rat (newborn and adult) and turtle neurons.

1. It is well known that anoxia induces an increase in extracellular K+. The underlying mechanisms for the increase, however, are not well understood. In the present study, we performed electrophysiological, pharmacological and receptor autoradiographic experiments in an attempt to examine K+ ionic homeostasis during anoxia. Ion-selective microelectrodes were employed to measure intracellular and extracellular K+ activity from hypoglossal neurons in brain slices. 2. During 3-4 min anoxia, adult hypoglossal neurons lose a large amount of their intracellular K+ and this contributes in a major way to the 8-fold increase in extracellular K+. 3. Loss of intracellular K+ from hypoglossal neurons is, to a great extent, due to activation of certain specific K+ channels. Glibenclamide, a potential sulphonylurea ligand and a specific blocker of ATP-sensitive K+ (KATP) channels, has no effect on K+ homeostasis during oxygenated states, but almost halves the anoxia-induced increase in extracellular K+ in the adult rat. 4. [3H]glibenclamide autoradiography shows that the hypoglossal nucleus in the adult rat has high sulphonylurea receptor density, a finding that is consistent with our electrophysiological observation. 5. Since we have previously shown that newborn mammals and reptiles are more resistant to O2 deprivation than adult mammals, we performed comparative studies among adult rat, newborn rat and adult turtle. In sharp contrast to the adult rat, extracellular K+ activity in newborn rat and adult turtle brain increases little (10 to 100 times less than the adult rat) and glibenclamide has a small and insignificant effect on K+ efflux in the newborn rat and none in the turtle. Glibenclamide receptor binding sites are much lower in the newborn rat than in the adult rat central nervous system (CNS) and barely detectable in the turtle brain. 6. These results support the hypothesis that in the adult rat, K+ is lost during anoxia from neurons through sulphonylurea receptor or KATP channels in a major way. Generally, however, KATP channels are poorly expressed in the newborn rat and adult turtle CNS and have little role to play during O2 deprivation.

Animals↗

PubMed1992

Cl- and Na+ homeostasis during anoxia in rat hypoglossal neurons: intracellular and extracellular in vitro studies.

1. To understand the mechanisms which lead to acute neuronal swelling during anoxia, we studied the ionic movements of Cl- and Na+ during O2 deprivation in the hypoglossal (XII) neurons of rat brain slices using double-barrelled ion-selective microelectrodes. 2. Baseline extracellular Cl- and Na+ activities ([Cl-]o, [Na+]o) were 128.3 +/- 7.4 and 150.0 +/- 3.4 mM respectively (n = 12) in the adult. Similar baseline values were obtained from neonatal brain slices. 3. During a period of anoxia (4 min), [Na+]o decreased by about 40 mM in adult slices while [Na+]o did not show any significant change in the neonate (n = 12). Although anoxia induced a significant decrease of [Cl-]o in both adult and neonate, [Cl-]o dropped 7 times more in the adult than in the neonate (n = 12). 4. Intracellular Cl- activity ([Cl-]i) was studied in twenty-seven adult hypoglossal cells. All showed an increase in [Cl-]i) was studied in twenty-seven adult hypoglossal cells. All showed an increase in [Cl-]i with O2 deprivation. Detailed analysis carried out on ten hypoglossal neurons showed a baseline [Cl-]i of 11.4 +/- 4.5 mM and an increase in [Cl-]i by 20.6 +/- 7.2 mM during O2 limitation. 5. Baseline [Cl-]i in neonatal XII neurons was similar to that of the adult. Anoxia, however, produced an increase in [Cl-]i by only 4.5 +/- 2.4 mM (n = 7). This increase in [Cl-]i was significantly less than that in the adult (P less than 0.001). Prolonged anoxia (6-12 min) in the neonate led to a more substantial increase in [Cl-]i, an observation consistent with the decrease in [Cl-]o after prolonged O2 deprivation. 7. We conclude that during anoxia: (1) intracellular [Cl-] increases in the adult and this most likely occurs because of entry of extracellular Cl- into the cytosol and (2) there is a major maturational difference in mechanisms regulating Cl- and Na+ homeostasis between newborn and adult brain tissue. We speculate that these mechanisms may account, at least partially, for the relative tolerance to anoxia in the newly born.

Animals↗

PubMed1992

Comparative responses of brain stem and hippocampal neurons to O2 deprivation: in vitro intracellular studies.

Most mammalian neurons are known to be sensitive to oxygen availability, but the nature of the sensitivity is not well understood. Previous results have suggested that brain stem neurons may respond differently than cortical neurons during oxygen deprivation. We pursued this hypothesis by examining the time course of change in membrane potential (Vm) and input resistance (Rn) during periods of reduced oxygen availability in a tissue slice preparation. Since extracellular potassium is an important factor determining resting membrane potential, extracellular K+ activity, (K+o), was also measured. Adult rat neurons from three regions were recorded: hippocampal CA1 region, hypoglossal nucleus (XII), and dorsal vagal motor nucleus (DMNX). At the end of a 5-min hypoxic exposure, all neurons depolarized and this depolarization was greatest in XII (28.8 +/- 3.2 mV) compared with DMNX (17.8 +/- 3.7 mV) and CA1 (6.7 +/- 4.4 mV). K+o increased in all regions and was larger in DMNX (7.1 +/- 2.6 mM) and XII (5.3 +/- 2.1 mM) compared with CA1 (2.2 +/- 1.4 mM). During more severe oxygen deprivation (anoxia), neurons also depolarized at different rates with XII greater than DMNX greater than CA1. K+o increased markedly (28-36 mM) by 5 min into anoxia, and no statistical difference was observed between regions. From these results we conclude that 1) all cells tested were depolarized after 5 min of hypoxia; however, regional variability exists in the sensitivity to hypoxia; brain stem neurons depolarize faster than cortical neurons; 2) during anoxia, all brain stem and cortical neurons show a major depolarization, and 3) these differences in membrane potential cannot be solely attributed to changes in extracellular K+.

Brain Stem↗

PubMed1992

Oxidative and glycolytic pathways in rat (newborn and adult) and turtle brain: role during anoxia.

Using enzyme histochemistry and in vitro electrophysiological recordings in brain slices, we studied 1) the relative activity of cytochrome c oxidase (Cytox) and hexokinase (HK) and 2) cellular function by examining ionic homeostasis across cell membranes in the turtle and newborn (5 days old) and adult rat central nervous system. We found that Cytox was higher in the rostral than in the caudal brain regions of the adult rat and that the activity in the newborn is at least as high as in the adult rat. In contrast, adult turtles had very low Cytox activity throughout the central nervous system. Compared with that in the adult rat, HK activity in the newborn was generally lower in the rostral brain and cerebellum but similar or higher in the brain stem and spinal cord. In the turtle, HK activity was higher in the cerebellum, brain stem, and ventral horn of the spinal cord than in those in the rat. During anoxia, extracellular K+ increased by approximately 10-fold (from 3.2 to approximately 32 mM) in the adult brain stem but only by 2.6 mM in newborn rats. After glycolysis was blocked with iodoacetic acid (10-20 mM), extracellular K+ increased remarkably in both adult and newborn rats to approximately 35 mM. In contrast, the turtle brain tissue showed a slight and insignificant increase in extracellular K+ during complete anoxia or with iodoacetic acid; there was a modest increase in K+ when anoxia and iodoacetate were administered together. We conclude that 1) the newborn rat brain must rely either on higher glycolytic capacity or on a reduction of metabolic rate during O2 deprivation and 2) the turtle brain can subsist on nonglucose fuels or on fuels not requiring the citric acid cycle and the electron transfer chain.

Aging↗

PubMed1992

Autoresuscitation: a survival mechanism in piglets.

Piglets were studied to determine 1) the cardiovascular and neurophysiological effects of prolonged laryngeal-induced respiratory inhibition (n = 7) and 2) whether these effects were modulated by autonomic blockade (n = 6). Respiration, electrocardiogram, electroencephalogram (EEG), and blood pressure were recorded, and blood gases were measured. During continuous laryngeal stimulation in the presence of light anesthesia, apnea was interrupted every 1-2.5 min by clusters of two to six breaths. Compared with control, these breaths had a significantly greater tidal volume (430 +/- 30% of control), shorter inspiratory time (87 +/- 5%), and longer expiratory time (124 +/- 15%) and, thus, were of a gasping nature. With each cluster of gasps, arterial PO2 increased from 15 +/- 2 to 56 +/- 5 Torr, heart rate from 84 +/- 7 to 161 +/- 5 beats/min, and mean blood pressure from 48 +/- 4 to 106 +/- 6 mmHg. The EEG became flat by 1 min after the onset of apnea and remained isoelectric throughout the stimulus period. Cyclical gasps were not affected by sympathetic or parasympathetic blockade. These data show that, despite EEG silence, piglets can autoresuscitate by initiating gasps that are not dependent on autonomic integrity. These gasps markedly improve cardiovascular status and may sustain animals for a prolonged period of time.

Animals↗

PubMed1992

Differential responses of neocortical neurons to glucose and/or O2 deprivation in the human and rat.

1. Intracellular recordings were performed in human and rat neocortical neurons with in vitro brain slice techniques. Baseline cellular properties and the effect of O2 and glucose deprivation on these neurons were studied. 2. Intracellular labelings of electrophysiologically identified neurons showed that most neurons recorded from layers 4 and 5 of the neocortex in both rats and humans were pyramidal cells with a regular-spiking or a burst firing pattern. 3. A period of complete anoxia (4-5 min) induced little or no change in membrane potential (Vm) in rat and human neocortical neurons, contrasting with the major depolarization we have previously observed in rat brainstem neurons during a similar period of anoxia. Evident depolarization occurred only when the slices were exposed to a more prolonged period of anoxia (> 7 min in rats and > 10 min in humans). 4. Membrane input resistance (Rm) of neocortical neurons decreased in both species during anoxia. In human neocortical neurons, Rm decreased by a mean of 22% with a marked increase in rheobase and suppression in spontaneous excitatory postsynaptic potentials (EPSPs). Interestingly, the increase in rheobase in human cells occurred even at an early stage (post 2-3 min in anoxia), when Vm and Rm had not yet changed. 5. Perfusing slices with a glucose-free medium for 1-2 h produced a relatively modest change in Vm (mean congruent to 28 mV). However, combined deprivation of both glucose and O2 resulted in a major depolarization (mean congruent to 50 mV) within 5-10 min in both human and rat neocortical neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1992

Adult criteria for obstructive sleep apnea do not identify children with serious obstruction.

Although obstructive sleep apnea (OSA) occurs in the pediatric population, diagnostic criteria have not been established. Since criteria for adult OSA are well established, we asked whether commonly used adult criteria, such as the apnea index (based on the number of obstructive apnea [OA] events per hour), would identify children with serious sleep-related upper airway obstruction. Polysomnographic data were analyzed from 20 children (ages 8 months to 16 yr) with clinical evidence of upper airway obstruction during sleep (loud snoring and labored breathing) and who had cyclic oscillations of oxyhemoglobin saturation (SaO2) during sleep. The overnight studies included sleep state (EEG, EOG, and EMG), SaO2, ECG, nasal (end-tidal CO2) and oral (thermistor) airflow, chest and abdominal movement (inductance plethysmography), and video camera and behavioral observations. Measurements included the number of obstructive events > or = 10 s, the number of desaturations (> or = 5% decrease lasting > or = 5 s), the number of desaturation episodes to < 90%, < 85%, and < 80% lasting > 5 s, and the percentage of sleep time with SaO2 values < 90%. Gas exchange was impaired as evidenced by cyclic decreases in SaO2 and elevated PETCO2 values (maximum value 58 +/- 6 mm Hg). The children experienced 175 +/- 168 (range 6 to 609) episodes of decreased SaO2 > 5%, with an average minimum SaO2 of 66 +/- 13% (range 30 to 85%). The average number of apnea events was only 1.9 +/- 3.2 events/h (range 0 to 10.4).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed1991

O2 tension in adult and neonatal brain slices under several experimental conditions.

Brain tissue O2 tension (pO2) was measured in brainstem slices of adult and neonatal rats using carbon fiber polarographic microelectrodes. These studies were performed in order to examine the relation between pO2 and a variety of experimental conditions including temperature, distance from slice surface, brain region, animal age, tissue thickness and ambient O2 levels. Baseline brain tissue pO2 was inversely proportional to temperature and depth from slice surface. White matter had a much higher pO2 than gray matter. Tissue thickness and animal age had major effects on tissue pO2. In slices of 800 microns thick at 37 degrees C, for example, brain tissue pO2 in the adult dropped to 0 mm Hg at a depth of 200-300 microns, but remained above 45 mm Hg throughout neonatal (3-10 days) slices, when O2 tension in the perfusate was about 600 mm Hg. In thicker neonatal slices (1500 microns), pO2 decreased also to 0 mm Hg in deep areas. An N2 environment produced a rapid reduction in pO2 to 0 mm Hg within 15 s, and O2 levels of 21, 10 and 5% induced graded pO2 minima and graded latencies to reach each pO2 nadir. We conclude that: (1) tissue thickness has a major effect on tissue pO2 level: pO2 can reach zero if the slice is thicker than 600 microns in the adult and 1500 microns thick in the neonate; (2) pO2 level is higher in neonatal brain tissue at all ambient O2 concentrations than in the adult; and (3) graded hypoxia produces patterned and graded reductions in tissue pO2.

Aging↗

PubMed1991

Major differences in CNS sulfonylurea receptor distribution between the rat (newborn, adult) and turtle.

Our previous results have shown that KATP channels play an important role in K+ efflux and extracellular K+ accumulation in the rat brain, and this role was quantitatively more important in the adult than in the newborn brain. The purpose of this study was to localize by autoradiographic techniques the binding sites of glibenclamide, a potent sulfonylurea ligand that targets KATP channels, in the adult and newborn rat central nervous system (CNS). Since the adult turtle is resistant to anoxia, we also compared the rat to the turtle brain sulfonylurea receptor distribution. In all three animal groups (newborn rat, adult rat, adult turtle), specific glibenclamide binding was saturable. Scatchard plots were curvilinear in the rat, thus suggesting that glibenclamide binds to two types of sites, i.e., high and low affinity sites. Scatchard analysis on turtle brain tissue showed evidence of one binding site only. We also found that the distribution of glibenclamide binding sites was heterogeneous in the adult rat CNS with a higher density in rostral than in caudal regions. The highest binding densities were seen in the cortex, hippocampus, cerebellum, substantia nigra, and a few thalamic nuclei; intermediate densities were observed in the basal ganglia, septum, thalamus, and the hypoglossal nucleus. There was a low density in most areas of the hypothalamus, midbrain, brainstem, and spinal cord. Compared with the adult rat, the newborn had a very homogeneous distribution of binding sites and densities were very low throughout the CNS; the level of binding density was even lower in some regions undetectable in the adult turtle. Our results indicate that (1) there are high and low affinity sulfonylurea receptors in the rat CNS, (2) there is a striking heterogeneity in the distribution and density of sulfonylurea receptors in the adult rat CNS and this is in sharp contrast to the homogeneous distribution and low density in both newborn rat and adult turtle; (3) sulfonylurea receptors increase in number postnatally in the rat since binding density increases and the Kd in the newborn rat is similar to that in the adult rat. We speculate that KATP channels and sulfonylurea receptors are poorly developed in the turtle and develop mostly after birth in the rat, reaching highest density in adulthood.

ATP-Binding Cassette Transporters↗

PubMed1991

Ontogeny and distribution of opioid receptors in the rat brainstem.

The distribution and postnatal ontogeny of opioid receptors have been investigated using in vitro quantitative receptor autoradiography. Rats were studied at postnatal day 1 (P1), P5, P10, P21 and P120 (adult). Opioid receptor sites for (D-Ala2,N-MePhe4,Gly-ol5)-enkephalin (DAMGO) binding were labelled with 4 nM of 3H-DAMGO; (D-Ala2,D-Leu5)-enkephalin (DADLE) binding sites were labelled with 4 nM of 3H-DADLE in the presence of 1 microM unlabelled mu-agonist (N-MePhe3,D-Pro4)-morphiceptin (PL107). We found that both binding sites have strikingly different distributional patterns. [3H]DADLE binding sites were rather homogeneous, whereas the distribution of [3H]DAMGO binding was very heterogeneous with the highest density in the nucleus of the solitary tract (NTS), ambiguus nucleus, dorsal motor nucleus of the vagus and the parabrachial areas. [3H]DAMGO binding density was 2- to 40-fold higher than [3H]DADLE binding sites in most brainstem nuclei. [3H]DAMGO binding sites appeared in most brainstem nuclei at birth, with a high density in cardiorespiratory-related nuclei, whereas [3H]DADLE binding sites were too scarce to be quantitated at P1. Both binding sites increased with age, but the developing patterns depended on the nucleus and the type of binding site. In most areas, the densities of both binding sites reached a maximum between P10 and P21 and then decreased to an adult level, but in some nuclei (e.g. the caudal part of the NTS and dorsal raphe nucleus), [3H]DAMGO binding sites kept increasing until adulthood. In contrast with the brainstem, cortical areas had a lower binding density in the newborn and reached peak levels later than brainstem regions (post P21). We conclude that (1) since [3H]DAMGO binding sites mainly reflect mu-receptors and [3H]DADLE binding sites delta-receptors (in the presence of PL017), the brainstem is essentially a mu-receptor region through delta-receptors are present; (2) both opioid receptors are present at birth but delta-receptors are very scarce in the newborn; (3) both receptors increase with age, but the time course depended on various nuclei and receptor types; (4) cardiorespiratory-related nuclei have high density of mu-receptors at all ages; and (5) opioid receptors develop earlier in the brainstem than in the cortex.

Animals↗

PubMed1991

Developmental changes in neuromuscular transmission in the rat diaphragm.

Neuromuscular transmission was studied in diaphragms from rats of three ages, 4-7 days old, 11-12 days old, and adults with the use of an in vitro phrenic nerve-hemidiaphragm preparation. Each hemidiaphragm was stimulated via either muscle or nerve with 1-s stimulus trains at frequencies from 10 to 100 Hz. The patterns of force development obtained in response to the two routes of stimulation were compared for each group. Diaphragms from adults developed maximum force in response to stimulation of approximately 40 Hz with no significant decrease in force at higher frequencies. Within each stimulus train, once peak force was achieved, it was maintained for the remainder of the stimulus and responses to nerve and muscle stimulation were almost identical. In contrast, diaphragms from 4- to 7-day-old rats developed maximum force at approximately 20 Hz; stimulation at greater than or equal to 60 Hz induced significantly less peak force. This decrease in peak force at higher frequencies was significantly larger for nerve than for muscle stimulation. In addition, during each nerve stimulus train diaphragms from 4- to 7-day-old rats were unable to maintain peak force, which decreased at frequencies greater than 20 Hz. The decrease in force reached approximately 50% of peak at stimulation frequencies greater than or equal to 60 Hz. Diaphragms from 11- to 12-day-old rats showed intermediate responses. Based on the responses to phrenic nerve stimulation, we conclude that the neonatal rat diaphragm shows marked neuromuscular transmission failure that is not seen in the adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1991

Effect of anoxia on intracellular and extracellular potassium activity in hypoglossal neurons in vitro.

1. A brain slice preparation was used to study the hypoglossal (XII) neuronal response to anoxia. Both intra- and extracellular potassium activities (K+i,K+o) were measured by the use of ion-selective microelectrodes, and K+ flux was assessed by the use of pharmacologic blockers. 2. Extracellular recordings showed that a short period of anoxia (4 min) induced an increase in K+o of 26.4 +/- 7.5 mM (mean +/- SD, n = 20) in the XII nucleus of adult rats. 3. Intracellular recordings (n = 31) in XII neurons showed a substantial decrease in K+i during anoxia. Fourteen neurons were analyzed in detail and these showed that XII neurons depolarized to -25.3 +/- 7.7 mV, whereas K+i dropped from 93.6 +/- 14.9 to 32 +/- 9.0 mM. These results strongly suggested that K+ is lost from XII neurons during anoxia. 4. Although the extracellular space (ECS) shrank by approximately 50% during anoxia, the possibility that the increase in K+o and decrease in K+i were mainly caused by shrinkage of the ECS and swelling of intraneuronal space was excluded to a great degree because the changes in K+i and K+o during anoxia were relatively very large. 5. To study the mechanisms by which K+ is lost from XII neurons, we used several pharmacologic blockers. High concentration of ouabain (10 mM) and strophanthidin (80 microM) increased K+o from baseline (3-4 mM) to 40.9 +/- 2.5 mM (n = 6) but did not abolish an additional anoxia-induced increase in K+o, suggesting that mechanisms other than Na(+)-K(+)-adenosine triphosphatase inhibition were also responsible for the anoxia-induced K+ leakage.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Effect of metabolic inhibition on the excitability of isolated hippocampal CA1 neurons: developmental aspects.

1. The effects of brief exposures to hypoxia on the membrane currents of isolated hippocampal CA1 neurons were studied with the use of the whole-cell variation of the patch-clamp technique. Neurons were acutely dissociated from immature (day 2-7) and mature (day 21-43) rats. 2. In the current-clamp mode, Na-cyanide (CN) hyperpolarized both mature and immature neurons. In the voltage-clamp mode, CN decreased the magnitude of the hyperpolarizing holding current in both age groups. 3. CN did not have a consistent effect on the voltage-dependent calcium and potassium currents of immature and mature CA1 neurons but decreased the voltage-dependent inward current of neurons at both ages. This effect was age dependent: the inward current of immature neurons decreased by only 10%, but that of mature neurons decreased by approximately 40%. 4. The decrease in the magnitude of the hyperpolarizing holding current and the depression of the voltage-dependent inward current of mature neurons were observed during brief exposure to N2 (PO2 = 0), indicating that the electroresponses observed with CN were the result of blocking oxidative respiration. 5. The hypoxia-sensitive inward current was blocked by tetrodotoxin (TTX) but was not blocked by cadmium or cesium + tetraethylammonium (TEA). Therefore this current was identified as the voltage-dependent, fast-inactivating sodium current (INa). 6. The isolated sodium current was studied with the use of cadmium to block calcium and TEA + cesium to block potassium currents. In mature neurons, CN left-shifted the steady-state inactivation curve for INa and slowed the deactivation kinetics of INa. CN caused little or no change in INa activation, fast inactivation, recovery from inactivation, or current-voltage (I-V) relationship. 7. We conclude that brief exposures to CN and hypoxia alter the intrinsic excitability of CA1 neurons by at least two mechanisms: 1) alterations in leakage currents and 2) alterations in the fast Na+ conductance that are maturationally dependent. We propose that the alterations in the Na+ conductance may play an adaptive role by reducing O2 demands and thus possibly delaying neuronal injury.

Aging↗

PubMed1990

O2 deprivation induces a major depolarization in brain stem neurons in the adult but not in the neonatal rat.

1. In order to study the neuronal response to hypoxia with maturation, hypoglossal neurons from adult and neonatal rat (3-7, 14-16, 21 and 28 days) brain stem slices were subjected to O2 deprivation. All neurons depolarized and showed no evidence of hyperpolarization at any time during the hypoxic period. 2. The magnitude of depolarization was about three-fold larger in adult hypoglossal neurons (mean = 32.0 mV) than in young neonatal neurons (mean = 10.4-11.2 mV) during hypoxic exposure (15-20 Torr) of 5 min. During longer periods of hypoxia of 15-30 min, neonatal cells showed an increase in the magnitude of depolarization reaching a level close to 80% of that in the adult. 3. In the early phase of hypoxia, adult neurons increased peak and steady-state spike frequency to induced current injections. Later, both spike frequencies decreased and, in 1/2 of adult neurons, there was a depolarization block. Input resistance (RN) of most adult neurons increased during hypoxia (RN = 180% of control after 5 min). Though neonatal neurons increased firing frequency, none had depolarization block and there was no increase in RN. 4. Tetrodotoxin (TTX), tetraethylammonium (TEA), apamin, high Mg2+/low Ca2+ solutions and intracellular ethyleneglycol-bis-(beta-aminoethylether)N,N,N',N'-tetrac etic acid (EGTA) did not reduce the magnitude of depolarization in hypoglossal neurons of 4-week-old and adult rats. Strophanthidin application depolarized hypoglossal neurons but decreased RN. 5. Ion-selective electrodes used to measure K+ concentrations in the extracellular fluid (Ko+) revealed a major increase in Ko+ (mean = 3.2 mM) in the adult hypoglossal area but not in the newborn tissue (mean = 0.65 mM). This probably reflects a difference in the amount of K+ efflux between neonatal and adult hypoglossal neurons. Shrinkage in the extracellular compartment in the adult may also account for some of the difference. 6. These results suggest that neonatal neuronal tissue is more tolerant to hypoxia than the adult, with the inherent cellular properties being maintained in newborn but not in adult neurons. The difference in membrane depolarization during hypoxia between the neonate and the adult can be attributed, in part, to differences in Ko+. The reason(s) for the difference in Ko+ is not known but could be due, to some extent, to different rates of intracellular ATP depletion and failure of the Na(+)-K+ pump.

Action Potentials↗

PubMed1990

Prolonged apnea and impaired survival in piglets after sinus and aortic nerve section.

We examined the effects of carotid body denervation (CX, n = 9), CX + aortic nerve section (CAX, n = 9), and sham surgery (SHAM, n = 7) on cardiorespiratory and metabolic function in young piglets (less than 9 days). For comparison, 1-mo-old pigs were also studied. Studies were performed 1 day after surgery, during which time ventilation (barometric plethysmography), heart rate, blood pressure, arterial blood gases, and electroencephalogram were recorded under normoxia. CX and CAX piglets hypoventilated (arterial PCO2 = 47.1 +/- 2.6 and 45.4 +/- 3.1 Torr, respectively) compared with SHAM piglets (arterial PCO2 = 36.4 +/- 1.5 Torr). CX piglets had an average of 8.0 +/- 3.0 apneas/h, lasting, on average, 26 +/- 3 s. CAX piglets averaged 17.2 +/- 7.9 apneas/h, lasting 30 +/- 5 s. Such long apneas were never observed in SHAM animals. Mean heart rate and blood pressure in denervated piglets were not significantly different from those in SHAM piglets. In animals followed up poststudy, significantly high mortality was observed in CX (5 of 9) and CAX (6 of 9) piglets by 7 days after surgery but not in SHAM animals (0 of 7) despite identical environmental and feed conditions (P less than 0.05; chi 2). One-month-old denervated animals showed periodic breathing and hypoventilation, but none died. These results suggest that in the newborn piglet 1) peripheral chemoreceptors have an active role in maintaining normal ventilation and avoidance of prolonged apnea and 2) survivability in early life is critically dependent on peripheral chemoreceptors.

Afferent Pathways↗

PubMed1990

Membrane and cellular properties in oscillating networks: implications for respiration.

Because of a number of major advances in the past one to two decades, there is little doubt that the inherent cellular and membrane properties of neurons in an oscillating network play an important role in shaping the output of that network. There are a number of such examples in vertebrate and invertebrate systems. In this review, we present some of the newer methods that have been used in the identification of membrane properties and detail some cellular studies performed in both vertebrate (locomotion and sleep/waking rhythms) and invertebrate network systems (escape swimming in Tritonia diomedia and pyloric rhythm in Panulirus interruptus). Studies examining the cellular or membrane properties of respiratory neurons have been scarce until recently. The importance of these properties in dictating respiratory rhythm generation and output in the mature and immature animal is not yet known; however, we put this issue in perspective by building a parallel between mammalian respiration and other vertebrate networks that have been better investigated and characterized.

Animals↗

PubMed1990

Biophysical properties of hypoglossal neurons in vitro: intracellular studies in adult and neonatal rats.

A brain stem slice preparation from adult and neonatal (less than or equal to 12 days old) rats and intracellular recordings were used to examine the cellular properties of neurons within the hypoglossal (HYP) nucleus. Resting membrane potential (Vm) for adult hypoglossal neurons was -80 +/- 2 (SE) mV. Rheobase was 2.1 +/- 0.4 nA, and input resistance (RN) was 20.8 +/- 1.5 M omega and decreased during the hyperpolarizing period ("sag"). Compared with adult HYP cells, newborn HYP neurons had significantly lower resting potentials (Vm = -73 +/- 2 mV), lower rheobase (0.7 +/- 0.2 nA), and higher RN (27.6 +/- 3.9 M omega). Single action potentials, elicited by short depolarizing-current pulses, were followed by a slow afterhyperpolarization in adult [6.4 +/- 0.3 mV, time constant (tc) 31.0 +/- 1.2 ms] and newborn cells (7.4 +/- 0.2 mV, tc 37.2 +/- 8.2 ms). Prolonged outward current (2 s) produced little spike frequency adaptation in either adult or newborn neurons. Onset of spike activity was not delayed by hyperpolarizing pulses preceding depolarizations. In addition, pharmacological experiments showed that HYP neurons have a tetrodotoxin-sensitive Na+ current and a delayed and an inward rectifier current but no major Ca2+ current. We conclude the following. 1) Electrophysiological membrane properties mature postnatally in HYP neurons; some of these developmental changes can be ascribed to an increase in soma size and dendritic outgrowth but others cannot. 2) Adult HYP neurons, compared with other brain stem neurons (i.e., vagal cells or cells in the nucleus tractus solitarius), are not endowed with major Ca2+ currents or K+ currents such as the A current and the Ca2(+)-activated K+ current.

Aging↗