Improving Cognition through Dance in Older Filipinos with Mild Cognitive Impairment.

BACKGROUND: People with mild cognitive impairment (MCI) are considered a high-risk population for developing dementia and therefore potential targets for preventive interventions. So far, no pharmacological interventions have proven to be effective. Latest evidence has laid the groundwork for the hypothesis that dancing can have beneficial effect on cognition by improving neuroplasticity. OBJECTIVE: This study aimed to examine whether a structured modular ballroom dance intervention (INDAK) could improve cognition among Filipino older persons with MCI. METHODS: A two-armed, single-blinded, quasi-experimental study was conducted in a community-based population at Marikina City, Philippines. Two hundred and seven participants older than 60 years old with MCI participated through self-assigned allocation to dance (N=101) and control (N=106) groups. The intervention group received INDAK consisting eight types of ballroom dances with increasing complexity lasting one hour, twice a week for 48 weeks. Neurologists and psychologists blinded to the group allocation administered baseline and post intervention assessments using Alzheimer's Disease Assessment Scale - Cognitive (ADAS-Cog), Filipino version of the Montreal Cognitive Assessment (MoCA-P), Boston Naming Test (BNT), Geriatric Depression Scale (GDS), Instrumental Activities of Daily Living (IADL) and Disability Assessment for Dementia (DAD). RESULTS: Baseline sociodemographic and clinical characteristics did not differ between groups. The mean differences between baseline and 48-week assessments were compared between dancers and controls, showing that the intervention group improved in ADAS-Cog, MoCA-P, BNT and GDS. CONCLUSION: INDAK is potentially a novel, ecological and inexpensive non-pharmacological intervention that can improve cognition among older Filipinos with MCI.

Age protects from harmful effects produced by chronic intermittent hypoxia.

Obstructive sleep apnoea (OSA) affects an estimated 3­7% of the adult population, the frequency doubling at ages >60­65 years. As it evolves, OSA becomes frequently associated with cardiovascular, metabolic and neuropsychiatric pathologies defining OSA syndrome (OSAS). Exposing experimental animals to chronic intermittent hypoxia (CIH) can be used as a model of the recurrent hypoxic and O2 desaturation patterns observed in OSA patients. CIH is an important OSA event triggering associated pathologies; CIH induces carotid body (CB)-driven exaggerated sympathetic tone and overproduction of reactive oxygen species, related to the pathogenic mechanisms of associated pathologies observed in OSAS. Aiming to discover why OSAS is clinically less conspicuous in aged patients, the present study compares CIH effects in young (3­4 months) and aged (22­24 months) rats. To define potential distinctive patterns of these pathogenic mechanisms, mean arterial blood pressure as the final CIH outcome was measured. In young rats, CIH augmented CB sensory responses to hypoxia, decreased hypoxic ventilation and augmented sympathetic activity (plasma catecholamine levels and renal artery content and synthesis rate). An increased brainstem integration of CB sensory input as a trigger of sympathetic activity is suggested. CIH also caused an oxidative status decreasing aconitase/fumarase ratio and superoxide dismutase activity. In aged animals, CIH minimally affected CB responses, ventilation and sympathetic-related parameters leaving redox status unaltered. In young animals, CIH caused hypertension and in aged animals, whose baseline blood pressure was augmented, CIH did not augment it further. Plausible mechanisms of the differences and potential significance of these findings for the diagnosis and therapy of OSAS are discussed.

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception.

The cascade of transduction of hypoxia and hypercapnia, the natural stimuli to chemoreceptor cells, is incompletely understood. A particular gap in that knowledge is the role played by second messengers, or in a most ample term, of modulators. A recently described modulator of chemoreceptor cell responses is the gaseous transmitter hydrogen sulfide, which has been proposed as a specific activator of the hypoxic responses in the carotid body, both at the level of the chemoreceptor cell response or at the level of the global output of the organ. Since sulfide behaves in this regard as cAMP, we explored the possibility that sulfide effects were mediated by the more classical messenger. Data indicate that exogenous and endogenous sulfide inhibits adenyl cyclase finding additionally that inhibition of adenylyl cyclase does not modify chemoreceptor cell responses elicited by sulfide. We have also observed that transient receptor potential cation channels A1 (TRPA1) are not regulated by sulfide in chemoreceptor cells.

The Carotid Body Does Not Mediate the Acute Ventilatory Effects of Leptin.

Leptin is a hormone produced mostly in adipose tissue and playing a key role in the control of feeding and energy expenditure aiming to maintain a balance between food intake and metabolic activity. In recent years, it has been described that leptin might also contributes to control ventilation as the administration of the hormone reverses the hypoxia and hypercapnia commonly encountered in ob/ob mice which show absence of the functional hormone. In addition, it has been shown that the carotid body (CB) of the rat expresses leptin as well as the functional leptin-B receptor. Therefore, the possibility exists that the ventilatory effects of leptin are mediated by the CB chemoreceptors. In the experiments described below we confirm the stimulatory effect of leptin on ventilation, finding additionally that the CB does not mediate the instant to instant control of ventilation.

Hypoxic intensity: a determinant for the contribution of ATP and adenosine to the genesis of carotid body chemosensory activity.

Excitatory effects of adenosine and ATP on carotid body (CB) chemoreception have been previously described. Our hypothesis is that both ATP and adenosine are the key neurotransmitters responsible for the hypoxic chemotransmission in the CB sensory synapse, their relative contribution depending on the intensity of hypoxic challenge. To test this hypothesis we measured carotid sinus nerve (CSN) activity in response to moderate and intense hypoxic stimuli (7 and 0% O(2)) in the absence and in the presence of adenosine and ATP receptor antagonists. Additionally, we quantified the release of adenosine and ATP in normoxia (21% O(2)) and in response to hypoxias of different intensities (10, 5, and 2% O(2)) to study the release pathways. We found that ZM241385, an A(2) antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 30.8 and 72.5%, respectively. Suramin, a P(2)X antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 64.3 and 17.1%, respectively. Simultaneous application of both antagonists strongly inhibited CSN discharges elicited by both hypoxic intensities. ATP release by CB increased in parallel to hypoxia intensity while adenosine release increased preferably in response to mild hypoxia. We have also found that the lower the O(2) levels are, the higher is the percentage of adenosine produced from extracellular catabolism of ATP. Our results demonstrate that ATP and adenosine are key neurotransmitters involved in hypoxic CB chemotransduction, with a more relevant contribution of adenosine during mild hypoxia, while vesicular ATP release constitutes the preferential origin of extracellular adenosine in high-intensity hypoxia.

Carotid body function and ventilatory responses in intermittent hypoxia. Evidence for anomalous brainstem integration of arterial chemoreceptor input.

Obstructive sleep apnea is a frequent medical condition consisting in repetitive sleep-related episodes of upper airways obstruction and concurrent events of arterial blood hypoxia. There is a frequent association of cardiovascular diseases and other pathologies to this condition conforming the obstructive sleep apnea syndrome (OSAS). Laboratory models of OSAS consist in animals exposed to repetitive episodes of intermittent hypoxia (IH) which also develop cardiovascular pathologies, mostly hypertension. The overall OSAS pathophysiology appears to be linked to the repetitive hypoxia, which would cause a sensitization of carotid body (CB) chemoreflex and chemoreflex-driven hyperreactivity of the sympathetic nervous system. However, this proposal is uncertain because hyperventilation, reflecting the CB sensitization, and increased plasma CA levels, reflecting sympathetic hyperreactivity, are not constant findings in patients with OSAS and IH animals. Aiming to solve these uncertainties we have studied the entire CB chemoreflex arch in a rat model of IH, including activity of chemoreceptor cells and CB generated afferent activity to brainstem. The efferent activity was measured as ventilation in normoxia, hypoxia, and hypercapnia. Norepinephrine turnover in renal artery sympathetic endings was also assessed. Findings indicate a sensitization of the CB function to hypoxia evidenced by exaggerated chemoreceptor cell and CB afferent activity. Yet, IH rats exhibited marked hypoventilation in all studied conditions and increased turnover of norepinephrine in sympathetic endings. We conclude that IH produces a bias in the integration of the input arising from the CB with a diminished drive of ventilation and an exaggerated activation of brainstem sympathetic neurons.

Effects of cigarette smoke and hypoxia on pulmonary circulation in the guinea pig.

Cigarette smoke (CS) and chronic hypoxia (CH) can produce pulmonary hypertension. Similarities and differences between both exposures and their interaction have not been explored. The aim of the present study was to investigate the effects of CS and CH, as single factors or in combination, on the pulmonary circulation in the guinea pig. 51 guinea pigs were exposed to CS for 12 weeks and 32 were sham-exposed. 50% of the animals in each group were additionally exposed to CH for the final 2 weeks. We measured pulmonary artery pressure (P(pa)), and the weight ratio between the right ventricle (RV) and left ventricle plus the septum. Pulmonary artery contractility in response to noradrenaline (NA), endothelium-dependent vasodilatation and distensibility were evaluated in organ bath chambers. The number of small intrapulmonary vessels showing immunoreactivity to smooth muscle (SM) α-actin and double elastic laminas was assessed microscopically. CS and CH induced similar increases of P(pa) and RV hypertrophy (p<0.05 for both), effects that were further enhanced when both factors were combined. CH increased the contractility to NA (p<0.01) and reduced the distensibility (p<0.05) of pulmonary arteries. Animals exposed to CS showed an increased number of small vessels with positive immunoreactivity to SM α-actin (p<0.01) and those exposed to CH a greater proportion of vessels with double elastic laminas (p<0.05). We conclude that CH amplifies the detrimental effects of CS on the pulmonary circulation by altering the mechanical properties of pulmonary arteries and enhancing the remodelling of pulmonary arterioles.

Spermine attenuates carotid body glomus cell oxygen sensing by inhibiting L-type Ca²(+) channels.

An increase in intracellular Ca²(+) is crucial to O2 sensing by the carotid body. Polyamines have been reported to modulate both the extracellular Ca²(+)-sensing receptor (CaR) and voltage-gated Ca²(+) channels in a number of cell types. Using RT-PCR and immunohistochemistry, the predominant voltage-gated Ca²(+) channels expressed in the adult rat carotid body were L (Ca(V)1.2) and N (Ca(V)2.2)-type. CaR mRNA could not be amplified from carotid bodies, but the protein was expressed in the nerve endings. Spermine inhibited the hypoxia-evoked catecholamine release from isolated carotid bodies and attenuated the depolarization- and hypoxia-evoked Ca²(+) influx into isolated glomus cells. In agreement with data from carotid body, recombinant Ca(V)1.2 was also inhibited by spermine. In contrast, the positive allosteric modulator of CaR, R-568, was without effect on hypoxia-induced catecholamine release from carotid bodies and depolarization-evoked Ca²(+) influx into glomus cells. These data show that spermine exerts a negative influence on carotid body O2 sensing by inhibiting L-type Ca²(+) channels.

A revisit to O2 sensing and transduction in the carotid body chemoreceptors in the context of reactive oxygen species biology.

Oxygen-sensing and transduction in purposeful responses in cells and organisms is of great physiological and medical interest. All animals, including humans, encounter in their lifespan many situations in which oxygen availability might be insufficient, whether acutely or chronically, physiologically or pathologically. Therefore to trace at the molecular level the sequence of events or steps connecting the oxygen deficit with the cell responses is of interest in itself as an achievement of science. In addition, it is also of great medical interest as such knowledge might facilitate the therapeutical approach to patients and to design strategies to minimize hypoxic damage. In our article we define the concepts of sensors and transducers, the steps of the hypoxic transduction cascade in the carotid body chemoreceptor cells and also discuss current models of oxygen- sensing (bioenergetic, biosynthetic and conformational) with their supportive and unsupportive data from updated literature. We envision oxygen-sensing in carotid body chemoreceptor cells as a process initiated at the level of plasma membrane and performed by a hemoprotein, which might be NOX4 or a hemoprotein not yet chemically identified. Upon oxygen-desaturation, the sensor would experience conformational changes allosterically transmitted to oxygen regulated K+ channels, the initial effectors in the transduction cascade. A decrease in their opening probability would produce cell depolarization, activation of voltage dependent calcium channels and release of neurotransmitters. Neurotransmitters would activate the nerve endings of the carotid body sensory nerve to convey the information of the hypoxic situation to the central nervous system that would command ventilation to fight hypoxia.

Effects of the polyamine spermine on arterial chemoreception.

Polyamines modulate many biological functions. Here we report a novel inhibitory modulation by spermine of catecholamine release by the rat carotid body and have identified the molecular mechanism underpinning it. We used molecular (RT-PCR and confocal microscopy) and functional (i.e., neurotransmitter release, patch clamp recording and calcium imaging) approaches to test the involvement of: (i) voltage-dependent calcium channels, and; (ii) the extracellular calcium-sensing receptor, CaR, a G protein-coupled receptor which is also activated by polyamines. RT-PCR and immunohistochemistry of isolated carotid bodies revealed that only Ca(v)1.2 and Ca(v)2.2 were expressed in type 1 cells while Ca(v)1.3, Ca(v)1.4, Ca(v)2.1, Ca(v)2.3 and Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, could not be detected. CaR expression was detected exclusively in the nerve endings. In isolated carotid bodies, the hypoxia-dependent (7% O(2) for 10 minutes) and depolarization-evoked catecholamine release were partially suppressed by pre- (and co)-incubation with 500microM spermine. In dissociated type 1 glomus cells intracellular calcium concentration did not change following spermine treatment, but this polyamine did inhibit the depolarisation-evoked calcium influx. Whole-cell patch clamp recordings of HEK293 cells stably transfected with Ca(v)1.2 demonstrated that spermine inhibits this calcium channel. Interestingly, this inhibition was not apparent if the extracellular solution contained a concentration of Ba(2) above 2 mM as the charge carrier. In conclusion, spermine attenuates catecholamine release by the carotid body principally via inhibition of Ca(v)1.2. This mechanism may represent a negative feedback, which limits transmitter release during hypoxia.

Adenosine in peripheral chemoreception: new insights into a historically overlooked molecule--invited article.

In the present article we review in a concise manner the literature on the general biology of adenosine signalling. In the first section we describe briefly the historical aspects of adenosine research. In the second section is presented the biochemical characteristics of this nucleoside, namely its metabolism and regulation, and its physiological actions. In the third section we have succinctly described the role of adenosine and its metabolism in hypoxia. The final section is devoted to the role of adenosine in chemoreception in the carotid body, providing a review of the literature on the presence of adenosine receptors in the carotid body; on the effects of adenosine at presynaptic level in carotid body chemoreceptor cells, as well as, its metabolism and regulation; and at postsynaptic level in carotid sinus nerve activity. Additionally, a review on the effects of adenosine in ventilation was done. This review discusses evidence for a key role of adenosine in the hypoxic response of carotid body and emphasizes new research likely to be important in the future.

The A(2B)-D(2) receptor interaction that controls carotid body catecholamines release locates between the last two steps of hypoxic transduction cascade.

We have recently demonstrated that adenosine controls the release of catecholamines (CA) from carotid body (CB) acting on A(2B) receptors. Here, we have investigated the hypothesis that this control is exerted via an interaction between adenosine A(2B) and dopamine D(2) receptors present in chemoreceptor cells and if it is, the location of this interaction on the CB hypoxic transduction cascade. Experiments were performed in vitro in CB from 3 months rats. The effect of adenosine A(2B) and dopamine D(2) receptor agonists applied alone or conjunctly, was studied on the basal and evoked release (10% O(2) and ionomycin) of CA from CB. We have observed that the inhibitory action of propylnorapomorphine, a D(2) selective agonist, on the normoxic and 10%O(2)-evoked release of CA was abolished by NECA, an A(2) agonist, meaning that an interaction between the D(2) and A(2B) receptors controls the release of CA from CB. Further, propylnorapomorphine inhibits the release of CA evoked by ionomycin, being this effect totally reversed by NECA. The present results provide direct pharmacological evidence that A(2B) and D(2) receptors interact to modulate the release of CA from rat CB between the steps of Ca(2+) entry and increase in intracellular free Ca(2+), and the activation of exocytosis and neurotransmitter release, of the stimulus-secretion coupling process.

Does ageing modify ventilatory responses to dopamine in anaesthetised rats breathing spontaneously?

Dopamine has been widely used in humans in the management of cardiocirculatory shock, and its inhibitory effect on ventilation has received particular attention in clinical situations more prevalent in the elderly. Dopamine has been extensively studied at the carotid body in adult animals but little is known in aged animals. We investigated the ventilatory responses caused by dopamine in 3 and 24 months old rats.Cumulative intracarotid bolus injections of dopamine were performed in anaesthetised and vagotomised rats, in the absence and in the presence of i.v. infusions of domperidone (23.5-1175 nmol Kg(-1) min(-1)). Airflow (V), tidal volume (V(T)), respiratory rate (f), arterial blood pressure and heart rate were monitored and respiratory minute volume (V(E)) calculated. Basal values of V(E) were lower in 24 months rats (322.9+/-18.8 mL Kg(-1) min(-1)) than in 3 months old rats (442.5+/-24.2 mL Kg(-1) min(-1)), mainly due to reductions in V(T). The dose-dependent decreases caused by dopamine (3-100 nmol) in V(T), f and V(E), were totally prevented by section of the carotid sinus nerve and were not modified by ageing. The maximal % antagonism of the inhibitory effect of dopamine on V(E) caused by domperidone was similar in both 3 (74.6+/-2.7) and 24 (70.7+/-0.8) months old rats. Domperidone alone, increased basal V(E) by 59.6+/-16.6 mL min(-1) Kg(-1), and by 11.8+/-1.2 mL min(-1) Kg(-1), respectively in 3 and 24 months old rats (p<0.01).The inhibitory basal tonus caused by dopamine in ventilation was reduced in aged rats, although the decrease in V(E) caused by its exogenous administration remained unchanged.

Effects of mitochondrial poisons on glutathione redox potential and carotid body chemoreceptor activity.

Low oxygen sensing in chemoreceptor cells involves the inhibition of specific plasma membrane K(+) channels, suggesting that mitochondria-derived reactive oxygen species (ROS) link hypoxia to K(+) channel inhibition, subsequent cell depolarization and activation of neurotransmitter release. We have used several mitochondrial poisons, alone and in combination with the antioxidant N-acetylcysteine (NAC), and quantify their capacity to alter GSH/GSSG levels and glutathione redox potential (E(GSH)) in rat diaphragm. Selected concentrations of mitochondrial poisons with or without NAC were tested for their capacity to activate neurotransmitter release in chemoreceptor cells and to alter ATP levels in intact rat carotid body (CB). We found that rotenone (1 microM), antimycin A (0.2 microg/ml) and sodium azide (5mM) decreased E(GSH); NAC restored E(GSH) to control values. At those concentrations mitochondrial poisons activated neurotransmitter release from CB chemoreceptor cells and decreased CB ATP levels, NAC being ineffective to modify these responses. Additional experiments with 3-nitroprionate (5mM), lower concentrations of rotenone and dinitrophenol revealed variable relationships between E(GSH) and chemoreceptor cell neurotransmitter release responses and ATP levels. These findings indicate a lack of correlation between mitochondrial-generated modifications of E(GSH) and chemoreceptor cells activity. This lack of correlation renders unlikely that alteration of mitochondrial production of ROS is the physiological pathway chemoreceptor cells use to signal hypoxia.

Low glucose effects on rat carotid body chemoreceptor cells' secretory responses and action potential frequency in the carotid sinus nerve.

Glucose deprivation (hypoglycaemia) is counterbalanced by a neuroendocrine response in order to induce fast delivery of glucose to blood. Some central neurons can sense glucose, but nevertheless the most important glucose sensors/glycaemia regulators are located outside the brain. Some recent experimental evidence obtained in carotid body (CB) slices and isolated chemoreceptor cells in culture supports a role for the CB in glucose sensing and presumably glucose homeostasis, but this role has been questioned on the basis of a lack of effect of low glucose on the carotid sinus nerve activity. This work was performed in an attempt to clarify if low glucose is or is not a stimulus for the rat CB chemoreceptors. Using freshly isolated intact CB preparations we have monitored the release of catecholamines (CAs) and ATP from chemoreceptor cells in response to several concentrations of glucose, as indices of chemoreceptor cell sensitivity to glycaemia, and the electrical activity in the carotid sinus nerve (CSN), as an index of reflex-triggering output of the CB. We have observed that basal (20% O(2)) and hypoxia (7 and 10% O(2))-evoked release of CAs was identical in the presence of normal (5.55 mm) and low (3, 1 and 0 mm) glucose concentrations. 0 mm glucose did not activate the release of ATP from the CB, while hypoxia (5% O(2)) did. Basal and hypoxia (5% O(2))-induced CSN action potential frequency was identical with 5.55 and 1 mm glucose. Our results indicate that low glucose is not a direct stimulus for the rat carotid body chemoreceptors.

Chemoreception in the context of the general biology of ROS.

Superoxide anion is the most important reactive oxygen species (ROS) primarily generated in cells. The main cellular constituents with capabilities to generate superoxide anion are NADPH oxidases and mitochondrial respiratory chain. The emphasis of our article is centered in critically examining hypotheses proposing that ROS generated by NADPH oxidase and mitochondria are key elements in O(2)-sensing and hypoxic responses generation in carotid body chemoreceptor cells. Available data indicate that chemoreceptor cells express a specific isoform of NADPH oxidase that is activated by hypoxia; generated ROS acting as negative modulators of the carotid body (CB) hypoxic responses. Literature is also consistent in supporting that poisoned respiratory chain can produce high amounts of ROS, making mitochondrial ROS potential triggers-modulators of the CB activation elicited by mitochondrial venoms. However, most data favour the notion that levels of hypoxia, capable of strongly activating chemoreceptor cells, would not increase the rate of ROS production in mitochondria, making mitochondrial ROS unlikely triggers of hypoxic responses in the CB. Finally, we review recent literature on heme oxygenases from two perspectives, as potential O(2)-sensors in chemoreceptor cells and as generators of bilirubin which is considered to be a ROS scavenger of major quantitative importance in mammalian cells.

The role of NADPH oxidase in carotid body arterial chemoreceptors.

O(2)-sensing in the carotid body occurs in neuroectoderm-derived type I glomus cells where hypoxia elicits a complex chemotransduction cascade involving membrane depolarization, Ca(2+) entry and the release of excitatory neurotransmitters. Efforts to understand the exquisite O(2)-sensitivity of these cells currently focus on the coupling between local P(O2) and the open-closed state of K(+)-channels. Amongst multiple competing hypotheses is the notion that K(+)-channel activity is mediated by a phagocytic-like multisubunit enzyme, NADPH oxidase, which produces reactive oxygen species (ROS) in proportion to the prevailing P(O2). In O(2)-sensitive cells of lung neuroepithelial bodies (NEB), multiple studies confirm that ROS levels decrease in hypoxia, and that E(M) and K(+)-channel activity are indeed controlled by ROS produced by NADPH oxidase. However, recent studies in our laboratories suggest that ROS generated by a non-phagocyte isoform of the oxidase are important contributors to chemotransduction, but that their role in type I cells differs fundamentally from the mechanism utilized by NEB chemoreceptors. Data indicate that in response to hypoxia, NADPH oxidase activity is increased in type I cells, and further, that increased ROS levels generated in response to low-O(2) facilitate cell repolarization via specific subsets of K(+)-channels.

Caffeine inhibition of rat carotid body chemoreceptors is mediated by A2A and A2B adenosine receptors.

Caffeine, an unspecific antagonist of adenosine receptors, is commonly used to treat the apnea of prematurity. We have defined the effects of caffeine on the carotid body (CB) chemoreceptors, the main peripheral controllers of breathing, and identified the adenosine receptors involved. Caffeine inhibited basal (IC50, 210 microm) and low intensity (PO2 approximately 66 mm Hg/30 mm K+) stimulation-induced release of catecholamines from chemoreceptor cells in intact preparations of rat CB in vitro. Opposite to caffeine, 5'-(N-ethylcarboxamido)adenosine (NECA; an A2 agonist) augmented basal and low-intensity hypoxia-induced release. 2-p-(2-Carboxyethyl)phenethyl-amino-5'-N-ethylcaboxamido-adenosine hydrochloride (CGS21680), 2-hexynyl-NECA (HE-NECA) and SCH58621 (A2A receptors agents) neither affected catecholamine release nor altered the caffeine effects. The 8-cycle-1,3-dipropylxanthine (DPCPX; an A1/A2B antagonist) and 8-(4-{[(4-cyanophenyl)carbamoylmethyl]-oxy}phenyl)-1,3-di(n-propyl)xanthine (MRS1754; an A2B antagonist) mimicking of caffeine indicated that caffeine effects are mediated by A2B receptors. Immunocytochemical A2B receptors were located in tyrosine hydroxylase positive chemoreceptor cells. Caffeine reduced by 52% the chemosensory discharges elicited by hypoxia in the carotid sinus nerve. Inhibition had two components with pharmacological analysis indicating that A2A and A2B receptors mediate, respectively, the low (17 x 10(-9) m) and high (160 x 10(-6) m) IC50 effects. It is concluded that endogenous adenosine, via presynaptic A2B and postsynaptic A2A receptors, can exert excitatory effects on the overall output of the rat CB chemoreceptors.