Irene C. Solomon

Automatic Selection of the Threshold Value

Calculation of approximate entropy (ApEn) requires a priori determination of two unknown parameters, m and r. While the recommended values of r, in the range of 0.1-0.2 times the standard deviation of the signal, have been shown to be applicable for a wide variety of signals, in certain cases, r values within this prescribed range can lead to an incorrect assessment of the complexity of a given signal. To circumvent this limitation, we recently advocated finding the maximum ApEn value by assessing all values of r from 0 to 1, and found that maximum ApEn does not always occur within the prescribed range of r values. Our results indicate that finding the maximum ApEn leads to the correct interpretation of a signals complexity. One major limitation, however, is that the calculation of all choices of r values is often impractical due to the computational burden. Our new method, based on a heuristic stochastic model, overcomes this computational burden, and leads to the automatic selection of the maximum ApEn value for any given signal. Based on Monte Carlo simulations, we derive general equations that can be used to estimate the maximum ApEn with high accuracy for a given value of m. Application to both synthetic and experimental data confirmed the advantages claimed with the proposed approach.

r

Recent studies have suggested that cell-to-cell coupling, which occurs via gap junctions, may play a role in CO(2) chemoreception. Here, we used immunoblot and immunohistochemical analyses to investigate the presence, distribution, and cellular localization of the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) in putative CO(2)-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed that both Cx subtypes were expressed in putative CO(2)-chemosensitive brainstem regions; however, regional differences in expression were observed. Immunohistochemical experiments confirmed Cx expression in each of the putative CO(2)-chemosensitive brainstem regions, and further demonstrated that Cx26 and Cx32 were found in neurons and Cx26 was also found in astrocytes in these regions. Thus, our findings suggest the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction proteins Cx26 and Cx32, at least in part, form the neuroanatomical substrate for this gap junctional communication, which is hypothesized to play a role in central CO(2) chemoreception.

for Approximate Entropy

Severe brain hypoxia results in respiratory excitation and an increase in sympathetic nerve activity. Respiratory excitation takes the form of gasping which is characterized by an abrupt onset, high amplitude, short duration burst of inspiratory activity. Recent evidence suggests that centrally-mediated hypoxic respiratory and sympathetic excitation may result from direct hypoxic stimulation of discrete hypoxia chemosensitive sites in the medulla. Thus, medullary regions involved in the generation and modulation of respiratory and sympathetic vasomotor output may contain neurons which function as central oxygen detectors, acting as medullary analogs to the peripheral (arterial) chemoreceptors. This review focuses on the medullary sites and mechanisms proposed to mediate hypoxic respiratory and sympathetic excitation in anesthetized, chemodeafferented animals, and provides the evidence suggesting a role for central oxygen detectors in the control of breathing and sympathetic vasomotor output.

Localization of connexin26 and connexin32 in putative CO2-chemosensitive brainstem regions in rat

The pre‐Bötzinger complex (pre‐BötC) is hypothesized to be the site for respiratory rhythm generation in mammals. Studies examining the cellular mechanisms mediating rhythm generation have focused on the role of chemically mediated synaptic interactions; however, electrotonic synaptic interactions (i.e., electrotonic coupling), which occur by means of gap junctions, may also play a role. Here, we used immunoblot and immunohistochemical analyses to determine whether the pre‐BötC contains the gap junction proteins necessary for electrotonic communication and whether the presence and distribution of these gap junction proteins show a developmental change in expression. We found that both connexin26 (Cx26) and connexin32 (Cx32) were expressed in pre‐BötC neurons of neonatal and adult rats; however, the relative amounts and their distribution varied by age. Cx26 labeling was seen in a high proportion of pre‐BötC neurons in neonatal rats ≤ 7 days postnatal (P7) but declined with increasing age. In contrast, Cx32 labeling was sparse in pre‐BötC neurons of neonatal rats ≤ P7, but increased with increasing age; the highest proportion was seen in adult rats. These data suggest the potential for gap junctional communication in the pre‐BötC of both neonatal and adult rats, and we propose that the gap junction proteins Cx26 and Cx32 form the neuroanatomic substrate for this gap junctional communication, which may be important in the synchronization of neural activity generating respiratory rhythm.J. Comp. Neurol. 440:12–19, 2001.

Excitation of phrenic and sympathetic output during acute hypoxia: contribution of medullary oxygen detectors.

Recent evidence indicates that gap junctions play a more prominent role in normal functioning of the mammalian central nervous system (CNS) than was once believed. Accumulating evidence from both neonatal and adult rodents indicates that gap junctions participate in multiple aspects of respiratory control, including central CO(2) chemoreception, respiratory rhythmogenesis, and respiratory motoneuron output. This review provides an overview of gap junction neurobiology in the mammalian CNS and presents the anatomical and electrophysiological evidence for gap junctions in CO(2) chemoreception and respiratory control.

Differential expression of connexin26 and connexin32 in the pre-Bötzinger complex of neonatal and adult rat

Recent work from our laboratory has demonstrated that the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) are expressed in neurons in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Whether the recently identified neuron-specific gap junction protein connexin36 (Cx36) is also present in these brainstem regions remains to be determined. Therefore, in the current experiments, immunoblot and immunohistochemical protocols were used to investigate the regional distribution and cellular localization of Cx36 in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed Cx36 expression in putative CO2-chemosensitive brainstem regions in each of the age groups examined, although both regional and developmental differences in the relative expression levels were detected. Immunohistochemical analyses confirmed Cx36 expression in neurons in each of the putative CO2-chemosensitive brainstem regions and revealed both somal and dendritic labeling patterns. These findings provide additional morphological evidence supporting the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction protein Cx36 also contributes to the neuroanatomical substrate for gap junctional communication, which is hypothesized to play a role in central CO2 chemoreception.

Gap junctions in CO2-chemoreception and respiratory control

We have previously demonstrated that chemical stimulation of the pre-Bötzinger complex (pre-BötC) in the anesthetized cat produces either phasic or tonic excitation of phrenic nerve discharge. This region is characterized by a mixture of inspiratory-modulated, expiratory-modulated, and phase-spanning (including pre-inspiratory (pre-I)) neurons; however, its influence on expiratory motor output is unknown. We, therefore, examined the effects of chemical stimulation of the pre-BötC on expiratory motor output recorded from the caudal iliohypogastric (lumbar, L(2)) nerve. We found that unilateral microinjection of DL-homocysteic acid (DLH; 10 mM; 10-20 nl) into 16 sites in the pre-BötC enhanced lumbar nerve discharge, including changes in timing and patterning similar to those previously reported for phrenic motor output. Both increased peak amplitude and frequency of phasic lumbar bursts as well as tonic excitation of lumbar motor activity were observed. In some cases, evoked phasic lumbar nerve activity was synchronized in phase with phrenic nerve discharge. These findings demonstrate that chemical stimulation of the pre-BötC not only excites inspiratory motor activity but also excites expiratory motor output, suggesting a role for the pre-BötC in generation and modulation of inspiratory and expiratory rhythm and pattern.

Connexin36 distribution in putative CO2-chemosensitive brainstem regions in rat

Numerous experimental preparations from neonatal rodents have been developed to study mechanisms responsible for respiratory rhythm generation. Amongst them, the in vivo anesthetized neonatal rat preparation and the in vitro medullary slice preparation from neonatal rat are commonly used. These two preparations not only contain a different extent of the neuroanatomical axis associated with central respiratory control, but they are also studied under markedly different conditions, all of which may affect the complex dynamics underlying the central inspiratory neural network. Here, we evaluated the approximate entropy (ApEn) underlying inspiratory motor bursts as an index of inspiratory neural network complexity from each preparation to address this possibility. Our findings suggest that the central inspiratory neural network of the in vivo anesthetized neonatal rat exhibits lower complexity (i.e., more order) than that observed in the in vitro transverse medullary slice preparation, both of which are substantially lower than that observed in more intact in vitro (e.g., arterially-perfused rat) and mature in vivo (e.g., anesthetized rat, piglet, cat) preparations. We suggest that additional studies be conducted to identify the precise mechanisms responsible for the differences in central inspiratory neural network complexity between these two neonatal rat preparations.

Modulation of expiratory motor output evoked by chemical activation of pre-Bötzinger complex in vivo

The caudal ventrolateral medulla (CVLM) participates in the central control of airway caliber. For example, both electrical and chemical stimulation of the CVLM decrease total lung resistance by withdrawing cholinergic input to airway smooth muscle. Although cell bodies in the CVLM have been shown to play an important role in mediating the central control of airway caliber, the pharmacological mechanism in this brainstem region responsible for causing this airway dilation is unknown. We, therefore, examined the role played by ionotropic excitatory amino acid receptors in the CVLM in the control of airway caliber in chloralose-anesthetized dogs. We found that microinjection of 3.9 pmol of NMDA or AMPA or quisqualate into 12 sites in the CVLM decreased total lung resistance by 1.5 +/- 0.2 cm H2O l(-1) s(-1) (p < 0.05), and that microinjection of 3.9 pmol of kainic acid into 9 in the CVLM decreased total lung resistance by 0.5 +/- 0.1 cm H2O l(-1) s(-1) (p < 0.05). The decrease in total lung resistance evoked by either NMDA or AMPA or quisqualate was not different (p > 0.05) while that evoked by kainic acid was significantly smaller. Additionally, microinjection of NMDA or AMPA or quisqualate caused a small but significant decrease in mean arterial pressure and heart rate (p < 0.05). These experiments demonstrate that the airway dilation evoked by stimulation of excitatory amino acid receptors in the CVLM is mediated by both NMDA and non-NMDA receptors.

Respiratory Network Complexity in Neonatal Rat in vivo and in vitro

Most models of central pattern generators (CPGs) involve two distinct nuclei mutually inhibiting one another via synapses. Here, we present a single-nucleus model of biologically realistic Hodgkin-Huxley neurons with random gap junction coupling. Despite no explicit division of neurons into two groups, we observe a spontaneous division of neurons into two distinct firing groups. In addition, we also demonstrate this phenomenon in a simplified version of the model, highlighting the importance of afterhyperpolarization currents (I AHP) to CPGs utilizing gap junction coupling. The properties of these CPGs also appear sensitive to gap junction conductance, probability of gap junction coupling between cells, topology of gap junction coupling, and, to a lesser extent, input current into our simulated nucleus.