Optical recording of spontaneous respiratory neuron activity in the rat brain stem

Identification of neurotransmitters and co-localization of transmitters in brainstem respiratory neurons

Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.

Novel axonal projection from the caudal end of the ventrolateral medulla to the intermediolateral cell column

We used an optical imaging technique to investigate whether axons of neurons in the caudal end of the ventrolateral medulla (CeVLM), as well as axons of neurons in the rostral ventrolateral medulla (RVLM), project to neurons in the intermediolateral cell column (IML) of the spinal cord. Brain stem-spinal cord preparations from neonatal normotensive Wistar-Kyoto and spontaneously hypertensive rats were stained with a voltage-sensitive dye, and responses to electrical stimulation of the IML at the Th2level were detected as changes in fluorescence intensity with an optical imaging apparatus (MiCAM-01). The results were as follows: 1) depolarizing responses to IML stimulation during low-Ca high-Mg superfusion were detected on the ventral surface of the medulla at the level of the CeVLM, as well as at the level of the RVLM, 2) depolarizing responses were also detected on cross sections at the level of the CeVLM, and they had a latency of 24.0 ± 5.5 (SD) ms, 3) antidromic action potentials in response to IML stimulation were demonstrated in the CeVLM neurons where optical images were detected, and 4) glutamate application to the CeVLM increased the frequency of excitatory postsynaptic potentials (EPSPs) and induced depolarization of the IML neurons. The optical imaging findings suggested a novel axonal and functional projection from neurons in the CeVLM to the IML. The increase in EPSPs of the IML neurons in response to glutamate application suggests that the CeVLM participates in the regulation of sympathetic nerve activity and blood pressure and may correspond to the caudal pressor area.

Imaging of respiratory-related population activity with single-cell resolution

The pre-Bötzinger complex (PBC) in the rostral ventrolateral medulla contains a kernel involved in respiratory rhythm generation. So far, its respiratory activity has been analyzed predominantly by electrophysiological approaches. Recent advances in fluorescence imaging now allow for the visualization of neuronal population activity in rhythmogenic networks. In the respiratory network, voltage-sensitive dyes have been used mainly, so far, but their low sensitivity prevents an analysis of activity patterns of single neurons during rhythmogenesis. We now have succeeded in using more sensitive Ca(2+) imaging to study respiratory neurons in rhythmically active brain stem slices of neonatal rats. For the visualization of neuronal activity, fluo-3 was suited best in terms of neuronal specificity, minimized background fluorescence, and response magnitude. The tissue penetration of fluo-3 was improved by hyperosmolar treatment (100 mM mannitol) during dye loading. Rhythmic population activity was imaged with single-cell resolution using a sensitive charge-coupled device camera and a x20 objective, and it was correlated with extracellularly recorded mass activity of the contralateral PBC. Correlated optical neuronal activity was obvious online in 29% of slices. Rhythmic neurons located deeper became detectable during offline image processing. Based on their activity patterns, 74% of rhythmic neurons were classified as inspiratory and 26% as expiratory neurons. Our approach is well suited to visualize and correlate the activity of several single cells with respiratory network activity. We demonstrate that neuronal synchronization and possibly even network configurations can be analyzed in a noninvasive approach with single-cell resolution and at frame rates currently not reached by most scanning-based imaging techniques.

Spatiotemporal Activity Patterns During Respiratory Rhythmogenesis in the Rat Ventrolateral Medulla

One of the most important brain rhythms is that which generates involuntary breathing movements. The lower brain stem contains neural circuitry for respiratory rhythm generation in mammals. To date, microsectioning and selective lesioning studies have revealed anatomical regions necessary for respiratory rhythmogenesis. Although respiratory neurons distributed within these regions can be identified by their firing patterns in different phases of the respiratory cycle, conventional electrophysiology techniques have limited the study of spatial organization within this network. Optical imaging techniques offer the potential for monitoring the spatiotemporal activity of large groups of neurons simultaneously. Using high-speed voltage-sensitive dye imaging and spatial correlation analysis in an arterially perfused in situ preparation of the juvenile rat, we determined the spatial distribution of respiratory neuronal activity in a region of the ventrolateral respiratory group containing the pre-Bötzinger complex (pBC) during spontaneous eupneic breathing. While distinctly pre- and postinspiratory-related responses were spatially localizable on length scales less than 100 μm, we found the studied area on whole exhibited a spatial mixture of phase-spanning and postinspiratory-related activity. Additionally, optical recordings revealed significant widespread hyperpolarization, suggesting inhibition in the same region during expiration. This finding is consistent with the hypothesis that inhibitory neurons play a crucial role in the inspiration-expiration phase transition in the pBC. To our knowledge this is the first optical imaging of a near fully intact in situ preparation that exhibits both eupneic respiratory activity and functional reflexes.

A Long-Time, High Spatiotemporal Resolution Optical Recording System for Membrane Potential Activity via Real-Time Writing to the Hard Disk

Using real-time hard disk recording, we have developed an optical system for the long-duration detection of changes in membrane potential from 1,020 sites with a high temporal resolution. The signal-to-noise ratio was sufficient for analyzing the spreading pattern of excitatory waves in frog atria in a single sweep.

Optical imaging of respiratory neuron activity from the dorsal view of the lower brainstem

1. We visualized respiratory-related neuron network activity in the dorsal part of the pons and medulla of an in vitro preparation from newborn rats by optical recordings using a voltage-sensitive dye. We measured optical signals from several seconds before to several seconds after the inspiratory phase using the inspiratory motor nerve discharge as the trigger signal and we averaged the optical signals of 20-50 respiratory cycles to obtain an optical image correlating specifically to inspiratory activity. 2. Four areas that were excited or inhibited corresponding to the respiratory cycles were detected. (i) The most rostral activity was in the rostral and lateral parts of the pons, with activity mainly in the inspiratory phase, corresponding to the pontine-respiratory group. (ii) In the midpontine level, inspiratory activity followed by long-lasting hyperpolarization appeared in the midlateral parts. This part was presumed to reflect activity in the locus coeruleus. The hyperpolarization became almost negligible after treatment with the alpha-adrenergic antagonist, phentolamine. (iii) In the dorsal medulla, the predominantly inspiratory activity was detected at the rostral level of the area postrema. This part was considered to reflect activity mainly of the hypoglossal nucleus. (iv) At a similar level, we also detected weak and disperse inspiratory activity extending more laterally and caudally than that of the hypoglossal nucleus activity. This might reflect activity of the dorsal respiratory group. 3. In conclusion, the present optical recording study revealed that the dorsal part of the lower brainstem in the in vitro preparation is noticeably active as well as the ventral part shown in the previous study. This method is very useful for analysis of pharmacological properties, as well as the spatio-temporal pattern of respiratory-related network activity in the brainstem.

Connections between respiratory neurones in the neonatal rat transverse medullary slice studied with cross-correlation

In the transverse medullary slice prepared from neonatal rats the hypoglossal nerve rootlets exhibit a bursting 'respiratory' rhythm as do neurones in the pre-Bötzinger complex (PBC). We used cross-correlation analysis of the rhythmic multiunit discharges recorded from hypoglossal nerve rootlets, hypoglossal nucleus neurones and PBC neurones to investigate the connections between these groups. All cross-correlograms computed between left and right hypoglossal nerves, and between hypoglossal neurones and contralateral hypoglossal nerves, displayed central peaks with broad half-amplitude widths (mean +/- S.D. of 29.6 +/- 10.4 and 37.3 +/- 6.0 ms, respectively), which we interpreted as evidence for activation from a common source. Five of the 18 cross-correlograms computed between left and right PBC neurones displayed peaks either side of time zero with narrower half-amplitude widths (mean +/- S.D. of 9.3 +/- 1.9 ms) superimposed on broader central peaks, which we interpreted as evidence for mutual excitation and common activation, respectively. Cross-correlograms computed between PBC neurones and contralateral hypoglossal neurones or nerves did not display consistent features, but some of those computed between PBC and ipsilateral hypoglossal neurones (two of eight) or nerves (two of five) displayed peaks with broad half-amplitude widths (mean +/- S.D. of 36.8 +/- 6.9 ms), offset from time zero by 6 ms (except for one at 18 ms), which we interpreted as evidence for excitation of hypoglossal neurones and motoneurones by PBC neurones. We concluded that rhythm is synchronised between left and right sides by mutual excitatory connections between left and right PBC neurones. The rhythm is transmitted to ipsilateral hypoglossal neurones by a paucisynaptic pathway. Both hypoglossal neurones and PBC neurones receive a common activation from as yet unidentified sources.

A novel functional neuron group for respiratory rhythm generation in the ventral medulla

We visualized respiratory neuron activity covering the entire ventral medulla using optical recordings in a newborn rat brainstem-spinal cord preparation stained with voltage-sensitive dye. We measured optical signals from several seconds before to several seconds after the inspiratory phase using the inspiratory motor nerve discharge as the trigger signal; we averaged the optical signals of 50-150 respiratory cycles to obtain an optical image correlating particularly to inspiratory activity. The optical images we obtained from the ventral approach indicated that neuron activity first appeared during the respiratory cycle in the limited region of the rostral ventrolateral medulla (RVLM), preceding the onset of inspiratory activity by approximately 500 msec. During the inspiratory phase, plateau activity appeared in the more caudal ventrolateral medulla at the level of the most rostral roots of the XIIth nerve. Comparison with electrophysiological recordings from respiratory neurons in the RVLM suggested that the optical signals preceding the inspiratory burst reflect preinspiratory neuron activity in this area. This RVLM area was determined to be ventrolateral to the facial nucleus and close to the ventral surface. We referred to this functional neuron group as the para-facial respiratory group (pFRG). Partial, bilateral electrical lesioning of the pFRG significantly reduced the respiratory frequency, together with changes in the spatiotemporal pattern of respiratory neuron activity. Our findings suggest that the pFRG comprises a neuronal population that is involved in the primary respiratory rhythm generation in the rostrocaudally extending respiratory neuron network of the medulla.

Breathing: rhythmicity, plasticity, chemosensitivity

Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.