Phyllis M Gootman

Preliminary evidence for the evolution in complexity of heart rate dynamics during autonomic maturation in neonatal swine

Previous studies suggest that the autonomic nervous system plays an important role in the generation of complex heart rate dynamics. Therefore, we hypothesized that the complexity (irregularity) of cardiac interbeat intervals would evolve with the maturation of autonomic innervation to the heart. Twelve healthy newborn piglets were implanted with ECG transmitters and studied at one or more different ages up to 33 days of age, the period during which pigs develop functional sympathetic innervation of the heart from the stellate ganglia. Three animals underwent right stellate ganglionectomy, two a left stellate ganglionectomy, two a right cardiac vagotomy and five a sham procedure. The statistic, approximate entropy (ApEn), was used to quantify the regularity of interbeat interval fluctuations. Sham-operated animals showed an increase in the standard deviation (SD) and irregularity (ApEn) of cardiac interval fluctuations with increasing age. Right stellate ganglionectomized piglets had lower interbeat interval ApEn values, but similar SDs by 26-27 days of age compared to sham-operated animals. Left stellate ganglionectomy, which affects cardiac inotropy rather than chronotropy, had no effect on cardiac interval irregularity, while vagotomy had an indeterminant effect. The increasing irregularity of interbeat interval dynamics during autonomic maturation and the apparent attenuation of heartbeat irregularity when right stellate ganglion innervation is interrupted, provides empirical support for the notion that complex heartbeat dynamics in the mature animal are the result of a network of autonomic neural pathways that enables an organism to adapt to stress.

Cardiac effects of esophageal stimulation: possible relationship between gastroesophageal reflux (GER) and sudden infant death syndrome (SIDS)

Twenty-six artificially ventilated newborn pigs were subjected to simulated gastroesophageal reflux; saline (10 cc) of varying pH was flushed through the esophagus from below. At a given pH threshold, reflex bradycardia, which could be blocked by atropine, was elicited. Transecting of the superior laryngeal nerves, the recurrent laryngeal nerves, and the pharyngeal plexus nerves did not block the reflex bradycardia. However, bypassing the regions superior to the esophagus with a shunt prevented the bradycardia. These results indicate that bradycardia caused by gastroesophageal reflux is independent of changes in ventilation and may be an important cause of sudden infant death.

The area postrema of newborn swine is activated by hypercapnia : relevance to sudden infant death syndrome?

This study was performed to investigate a role of the neonatal area postrema (AP) in the chemoreceptor response to hypercapnia which is defective in sudden infant death syndrome (SIDS). AP responses to CO2 inhalation were monitored in 1 to 5 week old piglets by mapping neurons that were induced to express the c-fos gene product, Fos--a marker of functional activation. Interpretive confounds were minimized by controlling for hypoxia, the effects of surgical procedures and ambient environmental stressors on neuronal activity (c-fos expression). The AP demonstrated a powerful and reproducible response in neonatal swine breathing 10% CO2 for 1 h. Intensely immunolabeled nuclei were detected throughout the longitudinal extent of the circumventricular organ, and were especially heavily concentrated at rostral levels proximal to obex. Quantitative analysis verified statistically significant increases in numbers of cells that were induced to express Fos-like immunoreactivity (FLI) in the AP of CO2- stimulated piglets as compared to control groups. No detectable age-related differences were observed in AP response patterns. Conclusions. The AP responds to hypercapnic stress in the newborn piglet. A mature circumventricular organ response in the neonate may be crucial in defending against common environmental stressors, such as nicotine exposure--an emetic agent acting via the AP and a major risk factor in SIDS. Hence, a defect of the AP or its network may underlie a loss of state-dependent controls over cardiopulmonary reflex function in SIDS.

Hypotension-induced expression of the c-fos gene in the medulla oblongata of piglets.

Neural networks that mediate the reflex response to baroreceptor withdrawal were explored in Sus scrofa. Induction of c-fos was used as a monitor of synaptic activity in response to hypotension sustained by systemic administration of a peripheral vasodilator, sodium nitroprusside. Patterns of c-fos gene expression were compared between Saffan-anesthetized experimental animals and age-matched normotensive controls administered vehicle. Effects of other variables were controlled including 1 h preoperative accommodation to the novel environment, anesthesia, blood gases and pH. Identical post-stimulus survival periods were allowed for accumulation of transcript. The c-fos protein, Fos, was identified immunocytochemically with two rabbit antisera raised against amino acids 1-131 of Fos or residues 4-17 of synthetic human transcript. Fos was identified in catecholaminergic neurons labeled with an antiserum to tyrosine hydroxylase (TH). Fos was induced in the nucleus tractus solitarii (NTS) of hypotensive piglets. Neurons encoding Fos matched projection patterns of first order visceral afferents. Induction was prominent in the dorsolateral nucleus coinciding with the baroreceptor field. Indices of increased neuronal activity were evident in other baroreceptor terminal sites, e.g., medial subnucleus, the medial commissural field, the intermediate subnucleus and a ventral A2 noradrenergic area. In reticular formation c-fos protein was induced in circumscribed columns in the lateral tegmental field (LTF) extending from facial nucleus to calamus scriptorius. Catecholaminergic (TH-positive) neurons expressed Fos in the porcine C1 and A1 areas of ventrolateral medulla. Fos was also induced in a dorsal intermediate reticular zone of LTF. Minor or inconsistent differences between experimental and control were observed in nucleus raphe pallidus, rostral paramedian reticular formation, upper thoracic intermediolateral cell column, and stellate ganglia. In conclusion, baroreceptor withdrawal in young animals induced patterns of neuronal response along established cardiovascular reflex pathways.

Regional Blood Flow Changes in Neonatal Pigs in Response to Hypercapnia, Hemorrhage and Sciatic Nerve Stimulation

Regional blood flow changes in response to hypercapnia, hemorrhage, and sciatic nerve stimulation were studied in 22 newborn piglets. Femoral, renal and/or carotid arterial flows were recorded simulta

Developmental changes in heart rate variability during exposure to prolonged hypercapnia in piglets

The hypothesis that hypercapnia-induced differences in heart rate variability (HRV) would emerge during early maturation was tested using a developing porcine model. Piglets were randomly assigned to either exposed (10% CO2 for 1 h) or control (100% O2) conditions, and then to one of three study groups: (a) 5-8 days old, (b) 13-15 days old, (c) 26-34 days old. Experiments were performed on pairs of age-and litter-matched animals that were anesthetized, paralyzed, and artificially ventilated. HRV was evaluated using power spectral analysis, SD of differences between successive RR intervals, and cardiac interval analysis. Statistical comparisons of simultaneously studied animals were made at baseline, 15 and 55 min after onset of hypercapnia, and 2 h after offset of hypercapnia. Our analyses revealed that only HRV of 26-34-day-old animals differed significantly from values of control animals. Cardiac intervals of those animals were distributed in such a manner that hypercapnia likely elicited coactivation of sympathetic and parasympathetic systems. Comparison of the distribution of cardiac intervals for other animals showed that 5-8-day-old animals had high frequency of balanced intervals at baseline that remained so during hypercapnia. Given that such coactivation may be neuroprotective, the paucity of balanced intervals in 13-15-day-old animals could mean that the end of the second postnatal week is associated with increased vulnerability.

Postganglionic Sympathetic Discharge in Neonatal Swine

We hypothesized that cardiac and respiratory modulation of postganglionic peroneal activity appeared in an age-related manner. In anesthetized, paralyzed and artificially ventilated piglets, simultaneous recordings of efferent phrenic and peroneal discharges were obtained during hyperoxia(fraction of inspired oxygen, Fio2 = 1.0) and hypoxia (Fio2 = 0.1). Spectral analyses of peroneal and aortic blood pressure signals revealed peaks at the cardiac frequency (3.25-5.0 Hz). Coherence analysis showed that these two signals were highly correlated at those frequencies, providing evidence for baroreceptor entrainment. Statistically significant (p< 0.05) increases of coherence values were observed during hypoxic stimulation. Such results were observed in most animals despite age, and provided evidence of a potent mechanism for insuring vasomotor tone even in newborn animals. In contrast, spontaneous respiration-related peroneal discharges were observed only in animals ≥20 d old. In animals <20 d old, hypoxic stimulation elicited respiration-related discharges in peroneal activity. In many cases, peroneal hypoxic discharges exhibited an immature biphasic response pattern despite the presence of a mature response pattern of phrenic activity. Such findings suggest a developmental lag in the linkages of respiratory and sympathetic controlling networks.

Integrated Cardiovascular Responses to Combined Somatic Afferent Stimulation in Newborn Piglets

Heart rate, mean aortic pressure and arterial flow responses to interactions of afferent stimulation of the sciatic (SNS) and median nerve of the brachial plexus (BNS) were studied in 30 newborn piglets under halothane-N2O anesthesia. High frequency or intensity SNS or BNS stimulation resulted in pressor responses accompanied by significant increases in the femoral flow and carotid and renal resistance. Combined stimulation produced responses smaller than the estimated sum of the responses. Low frequency or intensity SNS or BNS resulted in depressor responses accompanied by significant decreases in mean femoral flow without change in heart rate; combined stimulation produced responses smaller than the estimated sum. When pressor and depressor patterns of stimulation were combined, pressor responses always dominated. The absence of facilitation to any combination of interactions implies that the cardiovascular regulatory system is not fully developed at birth and undergoes, therefore, postnatal maturation.

Autonomic Nervous System Regulation of Heart Rate in the Perinatal Period

In all vertebrates more advanced than elasmobranch, cardiac muscle is innervated by the autonomic nervous system [15]. There are two major divisions of the autonomic nervous system: the parasympathetic originating in the third, seventh, ninth, and tenth cranial nerves, and the sacral spinal cord region; and the sympathetic system, originating in the thoracolumbar regions of the spinal cord. For both subdivisions of the autonomic nervous system, the cells of origin within the central nervous system project to ganglia located peripherally. The cells of these ganglia send their axons to the various effector organs throughout the body. The transmitter for all preganglionic neurons is acetylcoholine (ACh), while the post ganglionic neurons of the parasympathetic nervous system are also cholinergic. With few exceptions, post-ganglionic neurons of the sympathetic nervous system are adrenergic, utilizing norepinephrine (NE) as their neurotransmitter. In all vertebrates the heart is innervated by a parasympathetic inhibitory system, and a sympathetic excitatory system.

Induction of c-fos gene expression by spinal cord transection in Sus scrofa.

Functional responses of primary sensory afferents and spinal cord were monitored in swine subjected to a high cervical (C1) spinal transection. Two and a half hours after transection, dorsal root ganglia and cervical and thoracolumbar spinal segments were processed immunocytochemically for the c-fos gene product, Fos and related antigens. In spinal-transected animals, Fos-like immunoreactivity (FLI) was induced in spinal laminae I, V, VII and X and the intermediolateral cell column but not in sensory ganglia as compared to controls: spinal-intact age-matched littermates. Spinal laminae expressing FLI harbor sympathetic and somatic interneurons and may aid in maintaining sympathetic outflow.