Confocal calcium imaging analysis of respiratory-related burst activity in the parafacial region

Revisiting the two rhythm generators for respiration in lampreys

In lampreys, respiration consists of a fast and a slow rhythm. This study was aimed at characterizing both anatomically and physiologically the brainstem regions involved in generating the two rhythms. The fast rhythm generator has been located by us and others in the rostral hindbrain, rostro-lateral to the trigeminal motor nucleus. More recently, this was challenged by researchers reporting that the fast rhythm generator was located more rostrally and dorsomedially, in a region corresponding to the mesencephalic locomotor region. These contradictory observations made us re-examine the location of the fast rhythm generator using anatomical lesions and physiological recordings. We now confirm that the fast respiratory rhythm generator is in the rostro-lateral hindbrain as originally described. The slow rhythm generator has received less attention. Previous studies suggested that it was composed of bilateral, interconnected rhythm generating regions located in the caudal hindbrain, with ascending projections to the fast rhythm generator. We used anatomical and physiological approaches to locate neurons that could be part of this slow rhythm generator. Combinations of unilateral injections of anatomical tracers, one in the fast rhythm generator area and another in the lateral tegmentum of the caudal hindbrain, were performed to label candidate neurons on the non-injected side of the lateral tegmentum. We found a population of neurons extending from the facial to the caudal vagal motor nuclei, with no clear clustering in the cell distribution. We examined the effects of stimulating different portions of the labeled population on the respiratory activity. The rostro-caudal extent of the population was arbitrarily divided in three portions that were each stimulated electrically or chemically. Stimulation of either of the three sites triggered bursts of discharge characteristic of the slow rhythm, whereas inactivating any of them stopped the slow rhythm. Substance P injected locally in the lateral tegmentum accelerated the slow respiratory rhythm in a caudal hindbrain preparation. Our results show that the fast respiratory rhythm generator consists mostly of a population of neurons rostro-lateral to the trigeminal motor nucleus, whereas the slow rhythm generator is distributed in the lateral tegmentum of the caudal hindbrain.

Glycine and GABAA receptors suppressively regulate the inspiratory-related calcium rise in the thoracic inspiratory cells of the neonatal rat

We previously demonstrated that in an isolated brainstem-spinal cord preparation from neonatal rats, a local bath application of strychnine (a broad antagonist of glycine and GABA_A receptors) to the spinal cord enhances thoracic inspiratory motor activity. Herein, to investigate the involvement of the inspiratory spinal interneurons that provide excitatory input to the motoneuron, we conducted calcium imaging using this preparation. Oregon Green 488 BAPTA-1 AM, a fluorescent calcium indicator, was injected into the ventromedial surface of the thoracic cord. In all cells that showed inspiratory-related fluorescence changes > 2% of the baseline fluorescence intensity, the inspiratory-related fluorescence change decreased when the focal depth was deepened. The application of strychnine to the spinal cord increased the inspiratory-related intracellular calcium rise in these cells. These results suggest that the enhancement of inspiratory interneuron activity could be involved in this enhancement of inspiratory motor activity.

Intrinsic properties and synaptic connectivity of Phox2b-expressing neurons in rat rostral parvocellular reticular formation

The paired-like homeobox 2b gene (Phox2b) is critical for the development of the autonomic nervous system. We have previously demonstrated the distinct characteristics of Phox2b-expressing (Phox2b⁺) neurons in the reticular formation dorsal to the trigeminal motor nucleus (RdV), which are likely related to jaw movement regulation. In this study, we focused on Phox2b⁺ neurons in the rostral parvocellular reticular formation (rPCRt), a critical region for controlling orofacial functions, using 2-11-day-old Phox2b-EYFP rats. Most Phox2b⁺ rPCRt neurons were glutamatergic, but not GABAergic or glycinergic. Approximately 65 % of Phox2b⁺ rPCRt neurons fired at a low frequency, and approximately 24 % of Phox2b⁺ rPCRt neurons fired spontaneously, as opposed to Phox2b⁺ RdV neurons. Stimulation of the RdV evoked inward postsynaptic currents in more than 50 % of Phox2b⁺ rPCRt neurons, while only one Phox2b⁺ rPCRt neuron responded to stimulation of the nucleus of the solitary tract. Five of the 10 Phox2b⁺ neurons sent their axons that ramified within the trigeminal motor nucleus (MoV). Of these, the axons of the two neurons terminated within both the MoV and rPCRt. Our findings suggest that Phox2b⁺ rPCRt neurons have distinct electrophysiological and synaptic properties that may be involved in the motor control of feeding behavior.

Calcium Imaging Analysis of Cellular Responses to Hypercapnia and Hypoxia in the NTS of Newborn Rat Brainstem Preparation

It is supposed that the nucleus of the solitary tract (NTS) in the dorsal medulla includes gas sensor cells responsive to hypercapnia or hypoxia in the central nervous system. In the present study, we analyzed cellular responses to hypercapnia and hypoxia in the NTS region of newborn rat in vitro preparation. The brainstem and spinal cord were isolated from newborn rat (P0-P4) and were transversely cut at the level of the rostral area postrema. To detect cellular responses, calcium indicator Oregon Green was pressure-injected into the NTS just beneath the cut surface of either the caudal or rostral block of the medulla, and the preparation was superfused with artificial cerebrospinal fluid (25–26°C). We examined cellular responses initially to hypercapnic stimulation (to 8% CO₂ from 2% CO₂) and then to hypoxic stimulation (to 0% O₂ from 95% O₂ at 5% CO₂). We tested these responses in standard solution and in two different synapse blockade solutions: (1) cocktail blockers solution including bicuculline, strychnine, NBQX and MK-801 or (2) TTX solution. At the end of the experiments, the superfusate potassium concentration was lowered to 0.2 from 3 mM to classify recorded cells into neurons and astrocytes. Excitation of cells was detected as changes of fluorescence intensity with a confocal calcium imaging system. In the synaptic blockade solutions (cocktail or TTX solution), 7.6 and 8% of the NTS cells responded to hypercapnic and hypoxic stimulation, respectively, and approximately 2% of them responded to both stimulations. Some of these cells responded to low K⁺, and they were classified into astrocytes comprising 43% hypercapnia-sensitive cells, 56% hypoxia-sensitive cells and 54% of both stimulation-sensitive cells. Of note, 49% of the putative astrocytes identified by low K⁺ stimulation were sensitive to hypercapnia, hypoxia or both. In the presence of a glia preferential blocker, 5 mM fluoroacetate (plus 0.5 μM TTX), the percentage of hypoxia-sensitive cells was significantly reduced compared to those of all other conditions. This is the first study to reveal that the NTS includes hypercapnia and hypoxia dual-sensitive cells. These results suggest that astrocytes in the NTS region could act as a central gas sensor.

Analysis of the neuronal network of the medullary respiratory center in transgenic rats expressing archaerhodopsin-3 in Phox2b-expressing cells

Preinspiratory (Pre-I) neurons in the parafacial respiratory group (pFRG) comprise one of the respiratory rhythm generators in the medulla of the neonatal rat. A subgroup of pFRG/Pre-I neurons expresses the transcription factor Phox2b. To further analyze detailed neuronal mechanisms of respiratory rhythm generation in the neonatal rat, we developed a transgenic (Tg) rat line in which Phox2b-positive cells expressed archaerhodopsin-3 (Arch). Brainstem-spinal cord preparations were isolated from 0-2-day-old Tg newborn rats and were superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4, at 25-26 °C. Inspiratory fourth cervical ventral root (C4) activity was monitored, and membrane potentials of neurons in the pFRG including Pre-I and inspiratory neurons were recorded. Phox2b-positive cells in the Tg rats were essentially positive for enhanced green fluorescent protein (EGFP) signals (reporter for Arch) in the pFRG. Continuous photo-stimulation of the rostral ventral medulla for up to 90 s by covering the pFRG with green laser light (532 nm) induced a decrease of respiratory rate measured at C4 accompanied by membrane hyperpolarization of Phox2b-positive pFRG/Pre-I neurons. In contrast, Phox2b-negative inspiratory neurons were not hyperpolarized during the photo-stimulation. Our findings showed that Phox2b-expressing pFRG/Pre-I neurons are involved in the maintenance of the basic respiratory rhythm in neonatal rat.