1. In midcollicular decerebrate, unanesthetized, paralyzed cats ventilated with a cycle-triggered pump system, the properties of high-frequency oscillations (HFOs, 50-100 Hz) in membrane potentials (MPs) of medullary inspiratory (I) and expiratory (E) cells were studied. Simultaneous recordings were taken from bilateral phrenic and recurrent laryngeal (RL) nerves and from cells in the intermediate ventral respiratory group (intVRG, 0-1 mm rostral to the obex) or the caudal ventral respiratory group (cVRG, 2-4 mm caudal to the obex). 2. Spectral coherence analyses were used to detect the presence of HFOs during I in I and E cell MPs. Cross-correlation histograms (CCHs) between the cell and phrenic signals were used to ascertain cell-nerve HFO phase relations and to identify cells as RL motoneurons. Of the 103 cells that had significant HFOs (cell-phrenic coherences > or = 0.1), measurable HFO peak lags in the CCH were seen in 53 cells: 1) RL cells (9 I cells and 7 E cells); and 2) other types of cell (8 intVRG I cells, 18 intVRG E cells, and 11 cVRG E cells). These cells had high HFO correlations; the cell-phrenic coherence range was 0.35-0.94, with a mean HFO frequency of 58 Hz. 3. The cell-phrenic HFO lag (in ms) was measured in the CCH as the lag of the primary peak (peak located nearest to 0 lag). The phase lag was defined as (lag of primary peak in ms)/(HFO period in ms). The phase lags differed markedly between two subsets of cells: 1) RL I cells had HFO depolarization peaks that lagged the phrenic HFO peaks (average cell-phrenic phase lag = -0.18); and 2) the non-RL cells, regardless of location (intVRG or cVRG) and type (I or E), had HFO depolarization peaks leading (preceding) the phrenic HFO peaks (average cell-phrenic phase lag = 0.28). In addition, the cVRG E cells had significantly shorter cell-phrenic phase lags than the intVRG E cells (0.23 vs. 0.31, respectively). 4. These lags can be compared with the (I unit)-phrenic phase lags (average approximately 0.3) found in earlier extracellular studies. 1) There is a transmission delay of about one half HFO cycle from excitatory I cells to RL I cells. 2) Because a depolarization peak in the MP of an E cell corresponds to the start of a hyperpolarizing wave, the excitatory bulbospinal pathways from I cells have transmission times comparable with those of the inhibitory intramedullary pathways from I cells to E cells. 5. These results indicate that study of HFO phase relations can furnish useful information on functional connectivity of medullary respiratory neurons during the I phase.

In seven decerebrate cats, recordings were taken from the preganglionic cervical sympathetic (CSy) nerves and from 74 individual CSy fibres. Correlation and spectral analyses showed that nerve and fibre discharges had several types of rhythm that were coherent (correlated) between population and unit activity: respiratory, ‘3 Hz’ (2–6 Hz, usually cardiac related), and ‘10 Hz’ (7–13 Hz). Almost all units (73/74) had respiratory modulation of their discharge, either phasic (firing during only one phase) or tonic (firing during both the inspiratory (I) and expiratory (E) phases). The most common pattern consisted of tonic I‐modulated firing. When the vagi were intact, lung afferent input during I greatly reduced CSy unit and nerve discharge, as evaluated by the no‐inflation test. The incidence of unit‐nerve coherent fast rhythms (3 Hz or 10 Hz ranges) depended on unit discharge pattern: they were present in an appreciable fraction (30/58 or 52 %) of tonic units, but in only a small fraction (2/15 or 13 %) of phasic units. When baroreceptor innervation (aortic depressor amd carotid sinus nerves) was intact, rhythms correlated to the cardiac cycle frequency were found in 20/34 (59 %) of units. The cardiac origin of these rhythms was confirmed by residual autospectral and partial coherence analysis and by their absence after baroreceptor denervation. The 10 Hz coherent rhythm was found in 7/34 units when baroreceptor innervation was intact, where it co‐existed with the cardiac‐locked rhythm; after barodenervation it was found in 9/50 neurones. Where both rhythms were present, the 10 Hz component was sometimes synchronized in a 3:1 ratio to the 3 Hz (cardiac‐related) frequency component. The tonic and phasic CSy units seem to form distinct populations, as indicated by the differential responses to cardiac‐related afferent inputs when baroreceptor innervation is intact. The high incidence of cardiac‐related correlation found among tonic units suggests that they are involved in vasomotor regulation. The high incidence of respiratory modulation of discharge suggests that the CSy units may be involved in regulation of the nasal vasculature and consequent ventilation‐related control of nasal airway resistance.

1. In vagotomized, paralyzed, decerebrate cats, simultaneous recordings were taken from one or more sympathetic nerves [cervical sympathetic (CS), inferior cardiac (IC), splanchnic (SP)] and from medullary neurons in vasomotor-related regions. Coherence analyses were used to ascertain the presence of sympathetic rhythms (2-6 Hz or "3-Hz rhythm," 7-13 Hz or "10-Hz rhythm") that were correlated between different signals. The occurrence of a significant peak at such a frequency in a unit-nerve coherence spectrum allowed the identification of a medullary neuron as sympathetic related. 2. A serendipitous example is given of a rostral ventrolateral medullary neuron that had significant unit-nerve 10-Hz coherence peaks for three sympathetic nerves (CS, IC, SP); but, as revealed by partial coherence analysis, the unit activity's correlation with one nerve's activity could be partially or completely dependent on its correlation with other nerve activities. Thus in this case the unit-CS and unit-IC coherences at 10 Hz were completely dependent on the SP rhythm, whereas the unit-SP coherence was not significantly affected by the CS and IC rhythms. This asymmetry suggests that the neuron was preferentially connected to SP-generating medullary circuits. 3. This example indicates the strength of partial coherence analysis as a means of studying differential connectivity between medullary sympathetic-related neurons and sympathetic output neuron populations.

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Breathing in mammals starts in the foetus and acquires a vital importance at birth. The ability to produce rhythmic motor behaviours linked to respiratory function is a property of the brainstem reticular formation, which has been remarkably conserved during the evolution of vertebrates. Therefore, to understand the biological basis of the breathing behavior, we are investigating conservative developmental mechanisms orchestrating the organogenesis of the brainstem. In vertebrates, the hindbrain is one of the vesicles that appears at the anterior end of the neural tube of the embryo. Further morphogenetic subdivision ensues whereby the hindbrain neuroepithelium becomes partitioned into an iterated series of compartments called rhombomeres. The segmentation process is believed to determine neuronal fates by encoding positional information along the rostro-caudal axis. Before and at the onset of segmentation, genes encoding transcription factors such as Hox, kreisler, are expressed in domains corresponding to the limits of future rhombomeres. Inactivation of these genes specifically disturbs the rhombomeric pattern of the hindbrain. The presentation will address the problem of whether this primordial rhombomeric organisation influences later function of respiratory control networks in chicks and mice. Experiments were performed in embryos and after birth in transgenic mice. They show that, although expression of developmental genes and hindbrain segmentation are transient events of early embryonic development, they are important for the process of respiratory rhythm generation by brainstem neuronal networks. We have found in chick that at the end of the period of segmentation, the hindbrain contains neuronal rhythm generators that conform to the rhombomeric anatomical pattern. We have also identified a minimal rhombomeric motif allowing the post-segmental maturation of a specific (GABAergic) rhythm-promoting circuit. Furthermore, in vivo and in vitro analysis of neurons in transgenic mice revealed postnatal respiratory phenotypes associated with defects of central pontine and/or afferent respiratory control in Krox-20, Hoxa1 and kreisler mutants. Neonatal respiratory phenotypes are also induced in mice by treatment with low doses of retinoic acid that slightly change the early embryonic development of the Pons. Altogether, these experiments indicate that segmentation-related specifications of the hindbrain rhythmic neuronal network influences the respiratory patterns after birth. Therefore, early developmental processes have to be taken into account to understand normal and pathological diversity of the breathing behaviour in vertebrates. Acknowledgement:Supported by HFSP RG101/97, ACI (BDPI) 2000, CEE BIO4CT, ICCTI PRAXIS XXI (BD/11299/97). Development of gill and lung breathing in amphibia MJ Gdovin, VV Jackson, JC Leiter Division of Life Sciences, University of Texas at San Antonio, TX, USAIn the 25 morphological stages of larval bullfrog development there exists a gradual transition from gill to lung ventilat...