Brain stem lesion size determined by DEAD red or conjugation of neurotoxin to fluorescent beads

Glia modulation of the extracellular milieu as a factor in central CO2 chemosensitivity and respiratory control

We discuss the influence of astrocytes on respiratory function, particularly central CO2 chemosensitivity. Fluorocitrate (FC) poisons astrocytes, and studies in intact animals using FC provide strong evidence that disrupting astrocytic function can influence CO2 chemosensitivity and ventilation. Gap junctions interconnect astrocytes and contribute to K+ homeostasis in the extracellular fluid (ECF). Blocking gap junctions alters respiratory control, but proof that this is truly an astrocytic effect is lacking. Intracellular pH regulation of astrocytes has reciprocal effects on extracellular pH. Electrogenic sodium-bicarbonate transport (NBCe) is present in astrocytes. The activity of NBCe alkalinizes intracellular pH and acidifies extracellular pH when activated by depolarization (and a subset of astrocytes are depolarized by hypercapnia). Thus, to the extent that astrocytic intracellular pH regulation during hypercapnia lowers extracellular pH, astrocytes will amplify the hypercapnic stimulus and may influence central chemosensitivity. However, the data so far provide only inferential support for this hypothesis. A lactate shuttle from astrocytes to neurons seems to be active in the retrotrapezoid nucleus (RTN) and important in setting the chemosensory stimulus in the RTN (and possibly other chemosensory nuclei). Thus astrocytic processes, so vital in controlling the constituents of the ECF in the central nervous system, may profoundly influence central CO2 chemosensitivity and respiratory control.

Pressor responses to nasal stimulation are unaltered after disrupting the CPA

Stimulation of either the caudal pressor area (CPA) in the most caudal ventrolateral medulla with glutamate, or the nasal mucosa with ammonia vapors, induces an increase in mean arterial blood pressure (MABP). In the present study, we determined if neurons in the CPA serve as a relay for the increase in MABP seen after nasal stimulation. Ammonia vapors stimulated the nasal mucosa of rats anesthetized with either urethane alone or ketamine/xylazine and urethane to induce an increase in MABP, a bradycardia, and an apnea. Bilateral injections (50 nl) of glycine (1 M) or muscimol (2 mM) were placed in the CPA and the nasal mucosa again stimulated. The increases in MABP, the bradycardia and the duration of apnea to nasal stimulation were unchanged after either injection. However, resting MABP and HR were decreased significantly after glycine injections and resting MABP and resting respiratory rate were decreased after muscimol injections. The increase in MABP seen with nasal stimulation also did not change after multiple bilateral injections (3x40 nl) of ibotenate (5 microg/microl) in the CPA, but the bradycardia was eliminated and the duration of apnea was significantly shorter. These results suggest that the increase in MABP induced by nasal stimulation is via routes that do not include neurons in the CPA.

Raphe magnus nucleus is involved in ventilatory but not hypothermic response to CO2

There is evidence that serotonin [5-hydroxytryptamine (5-HT)] is involved in the physiological responses to hypercapnia. Serotonergic neurons represent the major cell type (comprising 15-20% of the neurons) in raphe magnus nucleus (RMg), which is a medullary raphe nucleus. In the present study, we tested the hypothesis 1) that RMg plays a role in the ventilatory and thermal responses to hypercapnia, and 2) that RMg serotonergic neurons are involved in these responses. To this end, we microinjected 1) ibotenic acid to promote nonspecific lesioning of neurons in the RMg, or 2) anti-SERT-SAP (an immunotoxin that utilizes a monoclonal antibody to the third extracellular domain of the serotonin reuptake transporter) to specifically kill the serotonergic neurons in the RMg. Hypercapnia caused hyperventilation and hypothermia in all groups. RMg nonspecific lesions elicited a significant reduction of the ventilatory response to hypercapnia due to lower tidal volume (Vt) and respiratory frequency. Rats submitted to specific killing of RMg serotonergic neurons showed no consistent difference in ventilation during air breathing but had a decreased ventilatory response to CO(2) due to lower Vt. The hypercapnia-induced hypothermia was not affected by specific or nonspecific lesions of RMg serotonergic neurons. These data suggest that RMg serotonergic neurons do not participate in the tonic maintenance of ventilation during air breathing but contribute to the ventilatory response to CO(2). Ultimately, this nucleus may not be involved in the thermal responses to CO(2).

Transient attenuation of CO2 sensitivity after neurotoxic lesions in the medullary raphe area of awake goats

The major objective of this study was to gain insight into whether under physiological conditions medullary raphe area neurons influence breathing through CO(2)/H(+) chemoreceptors and/or through a postulated, nonchemoreceptor modulatory influence. Microtubules were chronically implanted into the raphe of adult goats (n = 13), and breathing at rest (awake and asleep), breathing during exercise, as well as CO(2) sensitivity were assessed repeatedly before and after sequential injections of the neurotoxins saporin conjugated to substance P [SP-SAP; neurokinin-1 receptor (NK1R) specific] and ibotenic acid (IA; nonspecific glutamate receptor excitotoxin). In all goats, microtubule implantation alone resulted in altered breathing periods, manifested as central or obstructive apneas, and fractionated breathing. The frequency and characteristics of the altered breathing periods were not subsequently affected by injections of the neurotoxins (P > 0.05). Three to seven days after SP-SAP or subsequent IA injection, CO(2) sensitivity was reduced (P < 0.05) by 23.8 and 26.8%, respectively, but CO(2) sensitivity returned to preinjection control values >7 days postinjection. However, there was no hypoventilation at rest (awake, non-rapid eye movement sleep, or rapid eye movement sleep) or during exercise after these injections (P > 0.05). The neurotoxin injections resulted in neuronal death greater than three times that with microtubule implantation alone and reduced (P < 0.05) both tryptophan hydroxylase-expressing (36%) and NK1R-expressing (35%) neurons at the site of injection. We conclude that both NK1R- and glutamate receptor-expressing neurons in the medullary raphe nuclei influence CO(2) sensitivity apparently through CO(2)/H-expressing chemoreception, but the altered breathing periods appear unrelated to CO(2) chemoreception and thus are likely due to non-chemoreceptor-related neuromodulation of ventilatory control mechanisms.

Dorsomedial hypothalamic sites where disinhibition evokes tachycardia correlate with location of raphe-projecting neurons

Disinhibition of neurons in the region of the dorsomedial hypothalamus (DMH) elicits sympathetically mediated tachycardia in rats through activation of the brain stem raphe pallidus (RP), and this same mechanism appears to be largely responsible for the increases in heart rate (HR) seen in air jet stress in this species. Neurons projecting to the RP from the DMH are said to be concentrated in a specific subregion, the dorsal hypothalamic area (DA). Here, we examined the hypothesis that the location of RP-projecting neurons in the DA correspond to the sites at which microinjection of bicuculline methiodide (BMI) evokes the greatest increases in HR. To determine the distribution of RP-projecting neurons in the DA, cholera toxin B was injected in the RP in four rats. A consistent pattern of retrograde labeling was seen in every rat. In the hypothalamus, RP-projecting neurons were most heavily concentrated midway between the mammillothalamic tract and the dorsal tip of the third ventricle dorsal to the dorsomedial hypothalamic nucleus approximately 3.30 mm caudal to bregma. In a second series of experiments, the HR response to microinjections of BMI (2 pmol/5 nl; n = 76) was mapped at sites in the DA and surrounding areas in 22 urethane-anesthetized rats. All injection sites were located from 2.56 to 4.16 mm posterior to bregma, and the microinjections that evoked the largest increase in HR (i.e., >100 beats/min in some instances) were located in a region where RP-projecting neurons were most densely concentrated. Thus RP-projecting neurons in the DA may mediate DMH-induced tachycardia and thus play a role in stress-induced cardiac stimulation.

CO2, brainstem chemoreceptors and breathing

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.