1. An attempt has been made to test the hypothesis that, in the caudal part of nucleus tractus solitarii (NTS) where carotid sinus nerve (CSN) The carotid body is the major peripheral chemoreceptor for hypoxia in humans and other animals, and it plays an important role in augmenting ventilation during hypoxia (Martin-Body, Robson & Sinclair, 1986). In the rat, chemoreceptor afferents from the carotid body travel along the carotid sinus nerve (CSN) and terminate mainly in the caudal nucleus tractus solitarii (NTS; Lipski, McAllen & Spyer, 1977;Donoghue, Felder, Jordan & Spyer, 1984;Housley & Sinclair, 1988). Several neurotransmitters have been found in NTS (Cherniack, Prabhakar, Haxhiu & Runold, 1991). Among them, substance P (Prabhakar, Runold, Yamamoto, Lagercrantz & Euler, 1984;Henry & Sessle, 1985;Lindefors, Yamamoto, Pantaleo, Lagercrantz, Brodin & Ungerstedt, 1986), dopamine (Goiny, Lagercrantz, Srinivasan, Ungerstedt & Yamamoto, 1991) and L-glutamate (Glut; Kazemi, Chiang & Hoop, 1989) are considered to mediate chemoreceptor inputs to NTS. However, which neurotransmitter is released from the CSN in response to peripheral chemoreceptor activation and to what extent it is involved in regulation of respiration has not been determined.In the present study we used unanaesthetized, freely moving rats in which ventilation and in vivo release of Glut in the caudal NTS by microdialysis were measured during peripheral chemoreceptor stimulation by hypoxic hypoxia or doxapram (Dox) infusion in sham-operated and peripherally chemodenervated (carotid body denervated; CBD) rats. Moreover, the effects on ventilation of a local injection of Glut and during hypoxia, of pretreatment with the NMDA-type receptor antagonist MK-801 or the ionotropic receptor antagonist kynurenate into the caudal NTS (Housley & Sinclair, 1988) were investigated. MS 2462, pp. 55-65 55

1. We examined the role of nitric oxide (NO) in respiratory regulation in the nucleus tractus solitarii (NTS), where L-glutamate release associated with peripheral chemoreceptor activation modulates the hypoxic ventilatory response. 2. Experiments were performed in unanaesthetized freely moving rats. First, the effects on the hypoxic ventilatory response of sodium nitroprusside (SNP, a NO donor) or N -monomethyl-L-arginine (L-NMMA, a NO synthase inhibitor), microinjected into the NTS, were investigated. Second, using in vivo microdialysis, changes in extracellular L-glutamate during hypoxia were examined in the presence of L-NMMA. Third, the effect of L-NMMA on ventilatory augmentation by exogenous L-glutamate was examined. Furthermore, we measured extracellular L-citrulline concentration changes during hypoxia in the NTS to assess NO formation indirectly and also examined the effect of MK-801 (an NMDA receptor antagonist) on L-citrulline levels during hypoxia. 3. SNP increased ventilation during both normoxia and hypoxia. L-NMMA did not alter ventilation or L-glutamate levels during normoxia but significantly attenuated the hypoxic ventilatory response and the increase in L-glutamate during hypoxia. The inhibition by L-NMMA was blocked by L-arginine. The ventilatory augmentation by exogenous L-glutamate was attenuated by L-NMMA. L-Citrulline increased during hypoxia, and this increase was inhibited by MK-801. 4. We provide the first in vivo evidence that, in the NTS, NO works as a retrograde messenger in an L-glutamate-releasing positive feedback system contributing to the augmentation of ventilation during hypoxia.

1. The parabrachial nucleus (PBN) is thought to play an important role in cardiorespiratory control. However, the circumstances under which it affects ventilation are still not known. The purpose of the present study was to investigate how the PBN modulates the ventilatory responses to hypercapnia, hypoxia or a resistive load in awake rats with chemical lesions of the PBN. 2. In three groups of rats (with lateral PBN lesion, with Kolliker-Fuse nucleus lesion and control), ventilation was measured under various conditions. 3. There was no difference in the breathing of normal room air in any of the groups. However, the lesioned groups showed a reduced ventilatory response to hyperoxic hypercapnia (inspired CO2 fractions (FIc02) of 3, 5, 8 and 10%) and to graded hypoxia (inspired 02 fractions (FI 02) of 16, 12, 10 and 8 %) compared with the control group. The control group showed a biphasic response to sustained hypoxia (FN,A at 10% for 30 min), known as 'hypoxic depression', while the lesioned groups showed moderate ventilatory exaggeration throughout hypoxia. In response to a resistive load, the lateral PBN lesion group showed no change in ventilatory compensation. 4. The PBN appeared to have a considerable influence on ventilation stimulated in various ways during wakefulness.It has been suggested that the parabrachial nucleus (PBN) is one of the main relays for the transfer of a wide array of autonomic-related information from the more caudal levels of the neuraxis to supracollicular structures (Shannon & Lindsey, 1983;Milner, Joh, Miller & Pickel, 1984;Segers, Shannon & Lindsey, 1985;Berkley & Scofield, 1990). The PBN, in turn, is known to project to several autonomic nuclei interconnecting with the higher brain, and participates in cardiovascular and/or respiratory control (Fulwiler & Saper, 1984;Ward, 1988 (Ling, Karius & Speck, 1993. These disparate findings may be partly due to the structural complexity of the PBN. In addition, considering its location and neural connections, the PBN may prove to be a respiratory modulator, particularly during wakefulness. For example, decerebration around the level of the PBN changes the hypoxic ventilatory response in rats (MartinBody, 1988 (20 000 i.u., I.M.) was administered on the first three post-operative days. During the first 2-3 days the animals were fed with wet mash, but after that they resumed normal feeding and drinking. Subsequently they were kept on a 12 h light-12 h dark schedule with food and water available ad libitum for 1 week until the final experiments, by which time no unfavourable effects of surgery, such as hypophagia or loss of body weight, were present in any group of rats. Experimental preparations Ventilation was measured using the barometric method of plethysmography as described elsewhere (Mizusawa et al. 1994). Briefly, rats were placed in a 7 1 Plexiglass chamber which was connected to a reference chamber through a pressure transducer (model MP-45, + 5 cmH2O; Validyne, Northridge, CA, USA).Respiratory frequency (f) was calculated from th...

Background -Cyclical changes in systemic blood pressure occur during apnoeic episodes in patients with obstructive sleep apnoea (OSA). Although several factors including arterial hypoxaemia, intrathoracic pressure changes, and disruption of sleep architecture have been reported to be responsible for these changes in blood pressure, the relative importance of each factor remains unclear. This study assessed the role of hypoxaemia on the increase in blood pressure during apnoeic episodes. Methods -The blood pressure in apnoeic episodes during sleep and the blood pressure response to isocapnic intermittent hypoxia whilst awake were measured in 10 men with OSA. While asleep the blood pressure was measured non-invasively using a Finapres blood pressure monitor with polysomnography. The response of the blood pressure to hypoxia whilst awake was also measured while the subjects intermittently breathed a hypoxic (5% or 7% oxygen) gas mixture. Each hypoxic gas exposure was continued until a nadir arterial oxygen saturation (nSao2) of less than 75% was reached, or for a period of 100 seconds. The exposure was repeated five times in succession with five interposed breaths of room air in each run. Results -The mean (SD) increase in blood pressure (AMBP) during apnoeic episodes was 42 1 (17.3) mm Hg during rapid eye movement (REM) sleep and 31*9 (12.5) mm Hg during non-REM sleep. The AMBP during apnoeic episodes showed a correlation with the decrease of nSaO2 (ASao2) (r2 = 0.30). The change in blood pressure in response to intermittent hypoxia whilst awake was cyclical and qualitatively similar to that during apnoeic episodes. Averaged AMBP at an Sao2 of 7% and 5% oxygen was 12-6 (5.7) and 13-4 (3.6) mm Hg, respectively, whereas the averaged AMBP at the same ASao2 during apnoeic episodes was 38-4 (15.5) and 45 2 (20.5) mm Hg, respectively.Conclusions -The blood pressure response to desaturation whilst awake was about one third of that during apnoeic episodes.These results suggest that factors other than hypoxia may play an important part in raising the blood pressure during obstructive sleep apnoea. (Thorax 1995;50:28-34)

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