Direct biochemical and neuropharmacological identification of dopamine D2-receptors in the rabbit carotid body

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Revisiting cAMP signaling in the carotid body

Chronic carotid body (CB) activation is now recognized as being essential in the development of hypertension and promoting insulin resistance; thus, it is imperative to characterize the chemotransduction mechanisms of this organ in order to modulate its activity and improve patient outcomes. For several years, and although controversial, cyclic adenosine monophosphate (cAMP) was considered an important player in initiating the activation of the CB. However, its relevance was partially displaced in the 90s by the emerging role of the mitochondria and molecules such as AMP-activated protein kinase and O₂-sensitive K⁺ channels. Neurotransmitters/neuromodulators binding to metabotropic receptors are essential to chemotransmission in the CB, and cAMP is central to this process. cAMP also contributes to raise intracellular Ca²⁺ levels, and is intimately related to the cellular energetic status (AMP/ATP ratio). Furthermore, cAMP signaling is a target of multiple current pharmacological agents used in clinical practice. This review (1) provides an outline on the classical view of the cAMP-signaling pathway in the CB that originally supported its role in the O₂/CO₂ sensing mechanism, (2) presents recent evidence on CB cAMP neuromodulation and (3) discusses how CB activity is affected by current clinical therapies that modify cAMP-signaling, namely dopaminergic drugs, caffeine (modulation of A_2A/A_2B receptors) and roflumilast (PDE4 inhibitors). cAMP is key to any process that involves metabotropic receptors and the intracellular pathways involved in CB disease states are likely to involve this classical second messenger. Research examining the potential modification of cAMP levels and/or interactions with molecules associated with CB hyperactivity is currently in its beginning and this review will open doors for future explorations.

Opposing effects on the phrenic motor pathway attributed to dopamine-D1 and -D3/D2 receptor activation

Previous in vivo studies revealed that dopamine-D1-agonists elevate excitability of ventral respiratory column (VRC) neurons and increase discharge activity in the phrenic motor output through actions in the brainstem. In this in vivo study performed on pentobarbital-anesthetized cats, we show that D1-agonists (SKF-38393, dihydrexidine) given intravenously enhanced discharge activity in VRC inspiratory neurons and the phrenic nerve in two stages; discharge intensity first increased to a peak and then discharge duration increased. Cross-correlation analysis of VRC inspiratory neuron and phrenic nerve discharges showed that both stages increased strength of coupling between medullary inspiratory neurons and the phrenic motoneuron output. Intracellular recording and microiontophoresis experiments indicated that D1-agonists produced their stimulatory effects indirectly through actions on synaptic inputs to VRC inspiratory neurons. Because other laboratories have provided evidence that dopamine acting on other types of receptors depresses respiratory neuron excitability we tested the effects of piribedil, an agonist that activates receptors of the generally depressant D3/D2-dopamine receptor family, on phrenic nerve activity. Piribedil depressed phrenic nerve inspiratory discharge intensity, prolonged discharge duration, slowed burst frequency and slowed rate of action potential augmentation. The effects of piribedil were partially counteracted by intravenous injection of dihydrexidine. We propose that under normal, steady state conditions, D1-receptor-mediated excitatory modulation of phrenic motor output overrides D3/D2-receptor mediated inhibition.

Acute hypoxia modifies cAMP levels induced by inhibitors of phosphodiesterase-4 in rat carotid bodies, carotid arteries and superior cervical ganglia

BACKGROUND AND PURPOSE

Phosphodiesterase (PDE) inhibitors are useful to treat hypoxia-related diseases and are used in experiments studying the effects of oxygen on 3'-5'-cyclic adenosine monophosphate (cAMP) production. We studied the effects of acute hypoxia on cAMP accumulation induced by PDE inhibitors in oxygen-specific chemosensors, the carotid bodies (CBs) and in non-chemosensitive CB-related structures: carotid arteries (CAs) and superior cervical ganglia (SCG).

EXPERIMENTAL APPROACH

Concentration-response curves for the effects of a non-specific PDE inhibitor [isobutylmethylxanthine (IBMX) ], PDE4 selective inhibitors (rolipram, Ro 20-1724) and a PDE2 selective inhibitor (erythro-9-(2-hydroxy-3-nonyl)adenine) on cAMP levels were obtained in normoxic (20% O(2)/5% CO(2)) or hypoxic (5% O(2)/5% CO(2)) conditions.

KEY RESULTS

Responses to the PDE inhibitors were compatible with the presence of PDE4 in rat CBs, CAs and SCG but in the absence of PDE2 in CAs and CBs. Acute hypoxia enhanced the effects of IBMX and PDE4 inhibitors on cAMP accumulation in CAs and CBs. In SCG, acute hypoxia reduced cAMP accumulation induced by all the four PDE inhibitors tested. Differences between the effects of Ro 20-1724 and rolipram on cAMP were found in CAs and CBs during hypoxia.

CONCLUSIONS AND IMPLICATIONS

The effects of PDE4 inhibitors could be potentiated or inhibited by acute hypoxia depending on the PDE isoforms of the tissue. The similarities between the characterization of PDE4 inhibitors at the CBs and CAs, under normoxia and hypoxia, did not support a specific role for cAMP in the oxygen-sensing machinery at the CB and suggested that no direct CB-mediated, hyperventilatory, adverse effects would be expected with administration of PDE4 inhibitors.

Functional characterization of phosphodiesterases 4 in the rat carotid body: effect of oxygen concentrations

The non-specific cAMP phosphodiesterase (PDE) inhibitor isobutyl- methylxanthine (IBMX) has been used to manipulate cAMP levels in carotid body (CB) preparations but the characterization of different PDE isoforms in CB has never been performed. PDE4 is one of the PDE families that uses cAMP as a specific substrate and changes its activity and affinity for drug inhibitors according to the degree of its phosphorylation. We investigated the effects of hypoxia on cAMP accumulation induced by different PDE4 inhibitors in the CB based on the hypothesis that acute changes in O(2) could interfere with their affinity.Concentration-response curves for the effects of the PDE4 selective inhibitors, rolipram and Ro 20-1724 and IBMX on cAMP were obtained in CBs, removed from rats and incubated in normoxia (20%O(2)) or hypoxia (5%O(2)).No differences were found between cAMP concentrations in normoxic and hypoxic conditions in the absence of PDE inhibitors. In both conditions, the E(max) calculated for IBMX was similar to that of the specific PDE4 inhibitors. Hypoxia shifted the concentration response curves to the left with the following rank order of potency IBMX> RO 20-1724=rolipram and increased E(max) by about 25%.This pharmacological approach supports the hypothesis that there is PDE4 activity in CBs that is enhanced by acute hypoxia although the low potency of the PDE4 inhibitors to increase cAMP do not support an important role for PDE4 activation in the O(2)-sensing machinery at the CB.

The effects of dopamine on the respiratory system: Friend or foe?

Dopamine (DA) is an immediate precursor of noradrenaline that has stimulatory or inhibitory effects on a variety of adrenergic receptors. DA is primarily used in the management of circulatory shock for its combined vasopressor and inotropic effects, but it may also exert significant effects on the respiratory system Although the respiratory effects of intravenous DA attract less attention than its hemodynamic effects, there is evidence that DA affects ventilation, pulmonary circulation, bronchial diameter, neuromodulation of sensory pulmonary nerves and lung water clearance. Through these complex mechanisms, DA may exert beneficial as well as detrimental effects on respiration. DA may have beneficial effects on the respiratory system by decreasing oedema formation and improving respiratory muscle function, but can also have deleterious effects, by inhibiting ventilation. Hence, DA may be beneficial in lung oedema, but harmful in cases of difficult weaning from mechanical ventilation. DA should be used with caution in patients with heart failure during weaning from mechanical respiration; however, critically ill patients with chronic obstructive pulmonary disease (COPD) do not show this negative effect of DA on ventilatory drive.

Hypoxia transduction by carotid body chemoreceptors in mice lacking dopamine D(2) receptors

Hypoxia-induced dopamine (DA) release from carotid body (CB) glomus cells and activation of postsynaptic D(2) receptors have been proposed to play an important role in the neurotransmission process between the glomus cells and afferent nerve endings. To better resolve the role of D(2) receptors, we examined afferent nerve activity, catecholamine content and release, and ventilation of genetically engineered mice lacking D(2) receptors (D(2)(-/-) mice). Single-unit afferent nerve activities of D(2)(-/-) mice in vitro were significantly reduced by 45% and 25% compared with wild-type (WT) mice during superfusion with saline equilibrated with mild hypoxia (Po(2) approximately 50 Torr) or severe hypoxia (Po(2) approximately 20 Torr), respectively. Catecholamine release in D(2)(-/-) mice was enhanced by 125% in mild hypoxia and 75% in severe hypoxia compared with WT mice, and the rate of rise was increased in D(2)(-/-) mice. We conclude that CB transduction of hypoxia is still present in D(2)(-/-) mice, but the response magnitude is reduced. However, the ventilatory response to acute hypoxia is maintained, perhaps because of an enhanced processing of chemoreceptor input by brain stem respiratory nuclei.

Neurotransmitters in carotid body development

This review examines the possible role of neurotransmitters present in the carotid body on the functional expression of chemosensory activity during postnatal development. In particular, dopamine, acetylcholine, adenosine and neuropeptides are reviewed. Evidence to date shows involvement of these transmitters in signal transmission from the chemoreceptor cells to chemosensory afferent fibers of the sinus nerve, with clear age- or maturation-dependence of some aspects. However, it remains unresolved whether these neurotransmitters, some of which are expressed in the carotid body before birth, are directly involved in the maturation of the functional properties of the carotid chemoreceptors in sensing oxygen or other stimuli during postnatal development.

Dopamine D2 receptor modulation of carotid body type 1 cell intracellular calcium in developing rats

Carotid chemoreceptor type 1 cells release dopamine, which inhibits carotid chemoreceptor activity via dopamine D2 autoreceptors on type 1 cells. Postnatal changes in dopaminergic modulation may be involved in postnatal chemoreceptor development. The present study explores dopaminergic modulation of the intracellular calcium ([Ca(2+)](i)) response to hypoxia in type 1 cells from 1, 3, and 11- to 16-day-old rats. Using fura-2, we studied the effects of quinpirole, a D2 receptor agonist, on type 1 cell [Ca(2+)](i) response to 90-s hypoxia challenges (Po(2) approximately 1-2 mmHg). Cells were sequentially exposed to the following challenges: 1) hypoxia control, 2) hypoxia plus quinpirole, and 3) hypoxia plus quinpirole plus sulpiride (D2 receptor antagonist). In the 11- to 16-day-old group, type 1 cell [Ca(2+)](i) increased approximately 3 to 4-fold over resting [Ca(2+)](i) in response to hypoxia. Quinpirole (10 microM) significantly blunted the peak [Ca(2+)](i) response to hypoxia. Repeat challenge with hypoxia plus 10 microM quinpirole in the presence of 10 microM sulpiride partially restored the hypoxia [Ca(2+)](i) response. In sharp contrast to the older aged group, 10 microM quinpirole had minimal effect on hypoxia response of type 1 cells from 1-day-olds and a small but significant effect at 3 days of age. We conclude that stimulation of dopamine D2 receptors inhibits type 1 cell [Ca(2+)](i) response to hypoxia, consistent with an inhibitory autoreceptor role. These findings suggest dopamine-mediated inhibition and oxygen sensitivity increase with age on a similar time course and do not support a role for dopamine as a major mediator of carotid chemoreceptor resetting.

Dopamine1 receptor agonists reverse opioid respiratory network depression, increase CO2 reactivity

In adult pentobarbital-anesthetized and unanesthetized decerebrate cats, the D(1)R agonists (6-chloro-APB, SKF-38393, dihydrexidine) given intravenously restored phrenic nerve and vagus nerve respiratory discharges and firing of bulbar post-inspiratory neurons after the discharges were abolished by the micro-opioid receptor agonist fentanyl given intravenously. Reversal of opioid-mediated discharge depression was prevented by the D(1)R antagonist SCH23390. Iontophoresis of the micro-opioid receptor agonist DAMGO depressed firing of medullary bulbospinal inspiratory neurons. Co-iontophoresis of SKF-38393 did not restore firing and had no effect on bulbospinal inspiratory neuron discharges when applied alone. The D(1)R agonists given intravenously prolonged and intensified phrenic nerve and bulbospinal inspiratory neuron discharges. They also increased reactivity to CO(2) by lowering the phrenic nerve apnea threshold and shifting the phrenic nerve-CO(2) response curve to lower et(CO(2)) levels. Intravenous fentanyl on the other hand decreased CO(2) reactivity by shifting the phrenic nerve apnea threshold and the response curve to higher et(CO(2)) levels. Fentanyl effects on reactivity were partially reversed by D(1)R agonists.

Hormones and breathing

A number of hormones, including hypothalamic neuropeptides acting as neurotransmitters and neuromodulators in the CNS, are involved in the physiologic regulation of breathing and participate in adjustment of breathing in disease. In addition to central effects, some hormones also control breathing at peripheral chemoreceptors or have local effects on the lungs and airways. Estrogen and progesterone seem to protect from sleep-disordered breathing, whereas testosterone may predispose to it. Progesterone and thyroxine have long been known to stimulate respiration. More recently, several hormones such as corticotropin-releasing hormone and leptin have been suggested to act as respiratory stimulants. Somatostatin, dopamine, and neuropeptide Y have a depressing effect on breathing. Animal models and experimental human studies suggest that also many other hormones may be involved in respiratory control.

Ventilatory responses to acute and chronic hypoxia in mice: effects of dopamine D(2) receptors

We used genetically engineered D(2) receptor-deficient [D(2)-(-/-)] and wild-type [D(2)-(+/+)] mice to test the hypothesis that dopamine D(2) receptors modulate the ventilatory response to acute hypoxia [hypoxic ventilatory response (HVR)] and hypercapnia [hypercapnic ventilatory response (HCVR)] and time-dependent changes in ventilation during chronic hypoxia. HVR was independent of gender in D(2)-(+/+) mice and significantly greater in D(2)-(-/-) than in D(2)-(+/+) female mice. HCVR was significantly greater in female D(2)-(+/+) mice than in male D(2)-(+/+) and was greater in D(2)-(-/-) male mice than in D(2)-(+/+) male mice. Exposure to hypoxia for 2-8 days was studied in male mice only. D(2)-(+/+) mice showed time-dependent increases in "baseline" ventilation (inspired PO(2) = 214 Torr) and hypoxic stimulated ventilation (inspired PO(2) = 70 Torr) after 8 days of acclimatization to hypoxia, but D(2)-(-/-) mice did not. Hence, dopamine D(2) receptors modulate the acute HVR and HCVR in mice in a gender-specific manner and contribute to time-dependent changes in ventilation and the acute HVR during acclimatization to hypoxia.

Dopaminergic modulation of respiratory motor output in peripherally chemodenervated goats

We examined the ventilatory effects of exogenous dopamine (DA) and norepinephrine (NE) administration in chloralose-anesthetized, paralyzed, artificially ventilated adult goats before and after carotid body denervation (CBD). Intravenous (iv) DA bolus injections and slow iv infusions caused dose-dependent inhibition of phrenic nerve activity (PNA) in carotid body (CB)-intact animals during normoxia and hyperoxia but not during hypercapnia. NE administration in CB-intact goats caused dose-dependent inhibition of PNA of similar magnitude to DA trials. The DA D2-receptor agonists quinelorane and quinpirole inhibited PNA, whereas the DA D1-receptor agonist SKF-81297 had no effect. After CBD, the ventilatory depressant effects of DA persisted, but responses were significantly attenuated compared with CB-intact trials. CBD abolished the inhibitory effect of low-dose NE administration but did not alter ventilatory responses to high-dose NE injection. The peripheral DA D2-receptor antagonist domperidone substantially attenuated the inhibitory effects of DA bolus injections and infusions and reversed the inhibitory ventilatory effect of high-dose DA administration to excitation in some animals. The alpha-adrenoceptor antagonist phentolamine had no effect on DA-induced ventilatory depression. Beta-Adrenoceptor stimulation with isoproterenol produced similar hemodynamic effects to DA administration but had no effect on PNA. We conclude that DA and NE exert both CB-mediated and non-CB-mediated inhibitory effects on respiratory motor output in anesthetized goats. The ventilatory depressant effects that persist in peripherally chemodenervated animals are DA D2-receptor mediated, but their exact location remains speculative.

Stimulus-specific mobilization of dopamine and norepinephrine stores in cat carotid body

The catecholamines (CAs), dopamine (DA) and norepinephrine (NE), are synthesized and stored in carotid body chemosensory type I cells. Previous studies in our laboratory demonstrated that low concentrations of nicotine preferentially evoke the release of NE from rabbit type I cells, whereas hypoxia mobilizes DA and NE in proportion to their stores in the tissue. The primary objective of the present study was to examine whether hypoxia, nicotine and elevated concentrations (30 mM) of K+ evoke the preferential release of DA vs. NE from cat carotid bodies superfused in vitro. In this species, where tissue stores of DA and NE are nearly equal, hypoxia evoked the preferential release of DA from normal carotid bodies. This pattern of release evoked by low O2 was also present following chronic removal of the superior cervical ganglion, which eliminated NE contained in the sympathetic innervation to the carotid body. In contrast, nicotine and high-K+ preferentially mobilized NE in these sympathectomized animals. Sympathectomy also reduced the percent of DA (but not NE) content released from type I cells in response to any of the three stimuli. Our findings suggest that chemosensory type I cells possess stimulus-specific mechanisms for CA mobilization and that the sympathetic innervation modulates the metabolism and release of CAs in the cat carotid body.

Carotid body dopamine content and release by short-term hypoxia: effect of haloperidol and alpha methyl paratyrosine

Dopamine (DA) is thought to modulate the transduction of the hypoxic stimulus by the glomus cell in the carotid body (CB). The hypothesis tested here is that presynaptic DA D2 receptors (D2's) located on the type 1 cell function as autoreceptors to control DA release and/or synthesis. The aim of the study was to compare the effects of blocking D2's with haloperidol and DA synthesis with alpha methyl paratyrosine (AMPT) on the in vitro carotid body DA response to hypoxia. 54 CB's sampled from adult rabbits were incubated for one hour in a surviving medium bubbled with either 100% O2 or 8% O2 Sixteen CB's served as control (100% O2: n = 8, 8% O2: n = 8), 18 (100% O2: n = 8, 8% O2: n = 10) were sampled from rabbits pretreated with AMPT and 20 (100% O2: n = 12, 8% O2: n = 8) were incubated with micromolar concentrations of haloperidol. At the end of exposure. DA contained in the carotid body (DACB) and released in the surviving medium (DAr) were measured by HPLC. In 100% O2 DACB was not different between either AMPT or haloperidol and control, but DAr was significantly higher in the haloperidol group compared with control (mean +/- SE: 26.6 +/- 7.4 versus 7.6 +/- 2.0 pmol/h, P < 0.02). In 8% O2, control DACB (576 +/- 133 pmol/CB) was significantly higher than AMPT or haloperidol (respectively 228 +/- 29.6 and 246 +/- 49.9 pmol/CB, P < 0.01) and control DAr (234 +/- 72.3 pmol/h) was also significantly higher than AMPT or haloperidol (respectively 28.8 +/- 5.2 and 40.6 +/- 11.4 pmol/h, P < 0.01). Finally, DAr was significantly larger in 8% O2 than in 100% O2 in control and AMPT groups (P < 0.01), but not in the haloperidol group. The increase in DAr by haloperidol in the resting CB is consistent with the blockade of D2's regulating DA release. The decreased DAr in 8% O2 after AMPT suggests that increased DA synthesis contributes to maintain DA secretion by the type I cell exposed to short term hypoxia. The lack of difference in DAr between 8% O2 and 100% O2 after haloperidol probably reflects non specific--i.e., D2 independent--effect of micromolar concentration of haloperidol on DA synthesis and/or sodium-calcium exchangers during hypoxia.

Developmental expression of tyrosine hydroxylase, D2-dopamine receptor and substance P genes in the carotid body of the rat

Alterations in the level of putative neurotransmitters/neuromodulators and corresponding receptors may be a possible mechanism involved in changes in chemosensitivity of peripheral chemoreceptors in the carotid body during development. Using quantitative in situ hybridization histochemistry, levels of messenger RNAs encoding tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, the D2-dopamine receptor and substance P of newborn rats at postnatal days 0, 2, 14 and 21 were determined. For comparison, during the same time points during development, we also determined the level of expression of these messenger RNAs in the cells of the superior cervical ganglion which are not chemosensitive. Tyrosine hydroxylase and D2-dopamine receptor messenger RNAs were co-localized in many of the cells in both the carotid body and the superior cervical ganglion. In the carotid body, the level of tyrosine hydroxylase messenger RNA expression was greatest at birth, significantly decreased by 48 h postnatal age and remained decreased at 14 and 21 postnatal days. In contrast, D2-dopamine receptor messenger RNA levels significantly increased with postnatal age in the carotid body. This profile of an D2-dopamine receptor was not observed in the superior cervical ganglion where tyrosine hydroxylase and D2-dopamine receptor messenger RNAs levels did not significantly change from postnatal days 0 to 21. Lastly, in the rat carotid body, substance P messenger RNA was not detected. However, substance P messenger RNA was abundant in the nodose and petrosal ganglion. The increasing contribution of carotid body on ventilation with increasing postnatal age is associated with changes in levels of gene expression for tyrosine hydroxylase and D2-dopamine receptor in the carotid body.

Functional and developmental studies of the peripheral arterial chemoreceptors in rat: effects of nicotine and possible relation to sudden infant death syndrome

The drive on respiration mediated by the peripheral arterial chemoreceptors was assessed by the hyperoxic test in 3-day-old rat pups. They accounted for 22.5 +/- 8.8% during control conditions, but only for 6.9 +/- 10.0% after nicotine exposure, an effect counteracted by blockade of peripheral dopamine type 2 receptors (DA2Rs). Furthermore, nicotine reduced dopamine (DA) content and increased the expression of tyrosine hydroxylase (TH) in the carotid bodies, further suggesting that DA mediates the acute effect of nicotine on arterial chemoreceptor function. During postnatal development TH and DA2R mRNA levels in the carotid bodies decreased. Thus, nicotine from smoking may also interfere with the postnatal resetting of the oxygen sensitivity of the peripheral arterial chemoreceptors by increasing carotid body TH mRNA, as well as DA release in this period. Collectively these effects of nicotine on the peripheral arterial chemoreceptors may increase the vulnerability to hypoxic episodes and attenuate the protective chemoreflex response. These mechanisms may underlie the well-known relation between maternal smoking and sudden infant death syndrome.

Localization of dopamine D2 receptor mRNA in glomus cells of the rabbit carotid body by in situ hybridization

The localization of mRNA coding for the dopamine D2 receptor was studied in the rabbit carotid body using in situ hybridization with synthetic 35S-labelled oligodeoxynucleotides. Using autoradiography on cryostat or semi-thin sections, labelling was observed over the cytoplasm of glomus cells, but not over sustentacular cells. A quantitative study showed that labelling intensity (silver grain density) was increased by haloperidol treatment. These results suggest that glomus cells express the dopamine D2 receptor gene and that this expression is regulated.

Effects of dopaminergic blockade upon carotid chemosensory activity and its hypoxia-induced excitation

The effects of domperidone, antagonist of D2 receptors, on arterial chemoreceptor activity were studied in spontaneously breathing and pentobarbitone anesthetized cats, in which recordings of chemosensory impulse activity were obtained simultaneously from both cut carotid (sinus) nerves. Intravenous injections of domperidone 50 micrograms/kg produced a maintained increase in the basal frequency of chemosensory discharges, after which hyperoxic tests (breathing 100% O2 for 30 s) evoked larger falls in the rate of chemosensory impulses. Chemosensory responses evoked by hypoxic hypoxia (100% N2 tests) and by cytotoxic hypoxia (i.v. injections of NaCN) reached higher impulse rates after domperidone treatment. The effects of domperidone reveal that a resting release of dopamine from glomus cells maintains a low level of basal chemosensory activity under normoxic conditions. Domperidone turns off such restraining dopaminergic control and enhances the transient chemosensory responses to hypoxic stimuli. Present data support a modulatory role for dopamine within the chemoreceptor process, but not its participation as excitatory transmitter between glomus cells and sensory nerve endings.

Effects of dopamine on the carotid chemosensory response to hypoxia in newborn kittens

In a previous study, it has been shown that bolus injections of dopamine could either stimulate or inhibit the carotid chemosensory discharge in the kitten (Marchal et al., 1992a). To further characterize dopaminergic mechanisms in the carotid body during development, the effects of a continuous infusion of dopamine on carotid chemosensory activity in air, hypoxia (8% O2 in N2) and hyperoxia (100% O2) were studied in ten anesthetized, paralyzed and artificially ventilated kittens, aged 1 to 21 days and in three adult cats. One carotid sinus nerve was prepared for recording the activity of a single or a few chemosensory afferents. In the kittens, the immediate effect of dopamine at the onset of infusion (10 micrograms/kg/min) was an inhibition of the discharge in five kittens, a progressive excitation in four and no change in one. Four minutes after the onset of dopamine infusion, there was a significant increase in chemosensory activity both in room air (from 4.5 +/- 0.8 impulse/sec to 8.8 +/- 1.4 impulse/sec, mean +/- SEM, P < 0.05) and in hypoxia (from 24.6 +/- 3.7 impulse/sec to 33.4 +/- 5.3 impulse/sec, P < 0.05) but not in hyperoxia (0.5 +/- 0.2 impulse/sec vs 0.7 +/- 0.3 impulse/sec). The adult cats received four successive dopamine infusions at the rate of 2.5, 5, 7.5 and 10 micrograms/kg/min, in an attempt to establish a dose-response relationship. The effects of dopamine infusions were consistent within, but variable between, cats. The onset of dopamine infusion was associated with an inhibition of the discharge in two cats, at all infusion rates. In one of them, chemosensory activity returned quickly to control and the response to hypoxia was enhanced. In the other cat, the inhibition of the discharge persisted for the duration of the infusion, and the response to hypoxia was inhibited. In the third cat, dopamine had no effect on the chemosensory discharge. The patterns of chemosensory responses evoked by dopamine are qualitatively similar in kittens and cats, but the excitatory type of response appear to be more readily elicited in the kitten.

Effects of chronic hypoxia on opioid peptide and catecholamine levels and on the release of dopamine in the rabbit carotid body

Carotid body catecholamine and opioid levels were measured in rabbits exposed for 8 days to an atmosphere of 11% O2 in N2 (PO2 of approximately 80 mm Hg) and during an identical period of recovery, i.e., after 8 days of returning to the control normoxic atmosphere. Carotid bodies show a decrease in dopamine content at day 2. Thereafter, the levels of this biogenic amine increase progressively to peak at day 10, that is, 2 days after returning to a normoxic atmosphere. Finally, dopamine levels start to decrease and reach prehypoxic control levels at day 16, that is, after 8 days of recovery. In contrast, levels of native opioid peptides remain unchanged during the whole duration of the experiment, except for a decrease at day 2 of the hypoxic exposure. Levels of total opioid peptides are also below control values at day 2 of hypoxia, increase above control values on returning to a normoxic atmosphere (maximal levels at days 10-12), and later decrease to reach prehypoxic levels at day 16. As a result of these changes the ratios of dopamine to opioid levels show a progressive increase from day 0 to day 10 of the experiment and then return to control prehypoxic values. Carotid bodies isolated from animals that have been exposed to hypoxia for 8 days synthesize [3H]dopamine from its natural precursor [3H]tyrosine at a rate of 175 pmol/mg of protein/h, which is about double the rate of synthesis found in the carotid bodies of control animals and those allowed to recover for 8 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of cholecystokinin-like and calcitonin-like peptides in infant carotid bodies: a light- and electron-microscopic immunohistochemical study

Previous immunohistochemical studies have identified several regulatory peptides in the carotid body chief cells in both humans and animals. These peptides, together with amines, may be important in the modulation of the chemoreflex by the carotid body. We report the localization and distribution of calcitonin and cholecystokinin-like (CCK) immunoreactivity in chief cells of human infant carotid body by light- and electron-microscopic immunohistochemical techniques. Consecutive sections immunostained with calcitonin and/or CCK antibodies revealed positively stained chief cells, both alone and in clusters, scattered throughout the carotid body lobule. Generally more chief cells were positive for calcitonin than for CCK. This was confirmed by quantitative analysis showing that the ratio of calcitonin to CCK immunoreactive cells was consistently > 2:1 in all cases studied. There was no apparent correlation between the immunoreactivity for the two peptides and the age, sex, or postmortem interval. Calcitonin-like and CCK-like immunoreactivities were localized electron-microscopically over the dense core granules of the chief cells. Calcitonin and CCK-like peptides in carotid body chief cells may act as neurotransmitters or neuromodulators involved in chemoreception.

Expression of D2 dopamine receptor mRNA in the arterial chemoreceptor afferent pathway

Dopamine is a major neurotransmitter in the arterial chemoreceptor pathway. In the present study we wished to determine if messenger RNAs for dopamine D1 and D2 receptor are expressed in carotid body (type I cells), in sensory neurons of the petrosal ganglion which innervate the carotid body and in sympathetic neurons of the superior cervical ganglion. We failed to detect D1 receptor mRNA in any of these tissues. However, we found that D2 receptor mRNA was expressed by dopaminergic carotid body type I cells. D2 receptor mRNA was also found in petrosal ganglion neurons that innervated the carotid sinus and carotid body. In addition, a large number of sympathetic postganglionic neurons in the superior cervical ganglion expressed D2 receptor mRNA.

Effects of dopamine and domperidone on ventilation during isocapnic hypoxia in humans

In order to investigate the role of dopamine in the ventilatory response to sustained, isocapnic hypoxia six subjects were studied three times in each of three pharmacological conditions: (1) in the absence of any drug administration, (2) during i.v. infusion of dopamine (3 micrograms.kg-1.min-1), and (3) after pretreatment with domperidone. Otherwise the experimental protocol was identical on each day and consisted of holding the subjects' end-tidal PO2 at 100 Torr for 10 min, then 50 Torr for 20 min and finally at 100 Torr again for 5 min. End-tidal PCO2 was held constant 2-3 Torr above normal throughout the experiment. Domperidone increased, and dopamine decreased the magnitudes of both the fast on- and off-responses, but neither drug affected the magnitude of the hypoxic ventilatory decline (HVD). The results of this study suggests: (1) that a peripheral dopaminergic mechanism is not involved in the genesis of HVD, and (2) the peripheral chemoreflex may be modulated peripherally to produce HVD.

Chronic treatment with SCH-23390, a selective dopamine D1 receptor blocker decreases preprotachykinin-A mRNA levels in nucleus tractus solitarii of the rabbit: role in respiratory control

Acute intravenous administration of the selective D1 receptor blocker SCH-23390 resulted in an enhanced respiratory motor output as evidenced by the phrenic nerve activity, whereas local perfusion into the region of nucleus tractus solitarii had no effect. The increase in phrenic nerve activity was accompanied by a concomitant increase in the release of substance P in the region of nucleus tractus solitarii as measured by in vivo microdialysis technique. Chronic administration of SCH-23390 via subcutaneously implanted Alzet mini osmotic pumps, significantly decreased the level of preprotachykinin-A mRNA in the region of respiratory relay neurons in nucleus tractus solitarii but was without effect in the ventral medullary surface structure, wherein the central chemoreceptors are thought to be located. A smaller, but significant decrease was also seen in the striatum. The results suggest that chronic treatment with SCH-23390 leads to a disinhibition of an inhibitory dopaminergic input to the neurons in nucleus tractus solitarii from a suprapontine level, which may account for a subsequent inhibition of tachykinin-containing neurons in the nucleus tractus solitarii, the relay station for respiratory reflexes.

Effects of domperidone and medroxyprogesterone acetate on ventilation in man

If endogenous dopamine acts as an inhibitory neurotransmitter in the carotid bodies in man, domperidone (DP), a selective dopamine D-2 receptor antagonist should stimulate carotid bodies and augment ventilation. Furthermore, the combination of a central ventilatory stimulant, medroxyprogesterone acetate (MPA), with a peripheral ventilatory stimulant, DP, may produce an additive/synergistic ventilatory effect. We conducted a double-blind, placebo-controlled (P), cross-over trial comparing MPA 20 mg three times daily (TID) and DP (20 mg TID) alone and together in 8 healthy male human subjects. Drug effects were measured after 7 days, and a two-week drug washout period was allowed. MPA significantly increased alveolar ventilation (VA), and slopes of hypercapnic and hypoxic ventilatory responses. Domperidone alone significantly increased the slope of the hypoxic response; however, VA and PaCO2 did not change significantly. The combination of MPA and DP resulted in ventilatory changes similar to MPA alone. We conclude that in man endogenous dopamine acts as a modulator of chemoreception during hypoxemia, but plays no major role tonically in control of ventilation during normoxemia and normocapnia. Lack of additive effect with combined DP and MPA suggests that these drugs may share the same final common pathway in the process of chemoreception.

Differential stimulus coupling to dopamine and norepinephrine stores in rabbit carotid body type I cells

Recent studies suggest that preneural type I (glomus) cells in the arterial chemoreceptor tissue of the carotid body act as primary transducer elements which respond to natural stimuli (low O2, pH or increased CO2) by releasing chemical transmitter agents capable of exciting the closely apposed afferent nerve terminals. These type I cells contain multiple putative transmitters, but the identity of the natural excitatory agents remains an unresolved problem in carotid body physiology. Characterization of putative transmitter involvement in the response to natural and pharmacological stimuli has therefore become fundamental to further understanding of chemotransmission in this organ. The present study demonstrates that a natural stimulus (hypoxia) evokes the release of dopamine (DA) and norepinephrine (NE) in approximate proportion to their unequal stores in rabbit carotid body (DA release/NE release = 8.2). In contrast, nicotine (100 microM), a cholinomimetic agent thought to act on the nicotinic receptors present on the type I cells, evokes the preferential release of NE (DA release/NE release = 0.17). These findings suggest that distinct mechanisms are involved in a differential mobilization of these two catecholamines from the rabbit carotid body.

Arterial chemoreceptor involvement in salicylate-induced hyperventilation in rats

1. The extent to which peripheral arterial chemoreceptors are involved in the respiratory stimulant action of salicylates has been investigated in rats. 2. Injection of sodium salicylate (200 mg kg-1, single dose i.v.) caused a rapid transient hyperventilation that was not obtained when the carotid chemoreceptors were denervated by section of the carotid sinus nerves. A delayed (10 min) increase in respiration occurred regardless of whether or not the carotid nerves were sectioned. 3. Intravenous infusions of sodium salicylate (0.5 or 4 mg kg-1 min-1) caused hyperventilation in barbiturate-anaesthetized rats. The threshold dose for respiratory stimulation was significantly lower when the carotid sinus nerves were intact than when they were bilaterally sectioned, and the same pattern was observed following intravenous injections of sodium salicylate (cumulative doses) in anaesthetized and conscious rats. 4. Bilateral sectioning of the vagosympathetic nerve trunks did not significantly affect hyperventilation evoked by salicylate, suggesting that this response does not involve actions of salicylate on sensory receptors innervated by these nerves. 5. Administration of salicylate close-arterial to a carotid body, by local perfusion or cross-perfusion of a carotid sinus, led to an increase in respiration when the ipsilateral carotid nerve was intact, but not when it was sectioned. 6. Neuropharmacological studies on anaesthetized rats showed that chemosensory discharge, recorded from a sectioned carotid nerve, increased in response to salicylate injections with a similar dose-response pattern to the hyperventilation. Salicylate had no effect on baroreceptor discharge. 7. We conclude from our experiments that arterial chemoreceptors do contribute to salicylate-induced hyperventilation, and are almost exclusively responsible for the initial phase of the response in rats. Later increases in breathing are independent of reflexes from arterial chemoreceptors and result from actions at other sites, including the CNS. The therapeutic implications of our results are discussed.

Dopamine blockade alters ventilatory acclimatization to hypoxia in goats

Dopamine (DA) is generally accepted to be an inhibitory neurotransmitter in the carotid body (CB). It is released and depleted from the CB by acute hypoxia. From this background we made the hypothesis that hypoxic depletion of CB DA could be responsible for a time-dependent increase in CB afferent output and the early phase of ventilatory acclimatization to hypoxia (VAH) in goats. We reasoned, then, that blockade of DA receptors in the CB would accelerate the time course of VAH in the goat, i.e. produce a greater acute response to hypoxia (first 15 min) followed by a reduced rate of change of the subsequent time-dependent hyperventilation. We tested this hypothesis by exposing 7 adult female goats to up to 28 h of hypobaric hypoxia (PB = 380 Torr) on 3 different occasions separated by at least 2 months. The first was as control. During the second and third exposures different doses of the DA antagonist, domperidone, were administered prior to and during the hypoxic exposure (0.5 mg/kg followed by 0.25 mg/kg every 3 h and in the second study 1.0 mg/kg followed by 0.5 mg/kg every 2 h). The time course of acclimatization was assessed by measurement of arterial blood gases and pH in the awake goats. The data obtained in the first 4-5 h of hypoxia in domperidone treated animals appeared to support the hypothesis. Domperidone treated animals had a significantly greater acute ventilatory response to hypoxia followed by a lower rate of progressive hyperventilation in this period. However, variation in control values, greater respiratory alkalosis and a secondary significant hyperventilation after 6-7 h of hypoxia in the domperidone treated animals prevents a clear conclusion as to the precise role of CB dopaminergic mechanisms in acclimatization to hypoxia. Nevertheless, peripheral DA receptor blockade with domperidone does alter the time course and magnitude of hyperventilation during the first 7 h of hypobaric hypoxia in goats.

Involvement of central serotonergic mechanisms in the cough reflex

To determine the role of central serotonergic systems in modulating the cough reflex, the effects of serotonergic agonists on the respiration and the cough reflex were comparatively studied. Male and female cats were anesthetized with sodium pentobarbital. Respiration and cough reflex were measured using a pneumotachograph via a cannula inserted into the trachea. The cough reflex was elicited by electrical stimuli to the superior laryngeal nerve. Tranylcypromine, a MAO inhibitor, in a dose of 5 mg/kg, i.v., increased the respiration, but depressed the cough reflex. The serotonin precursor 5-hydroxytryptophan (5 mg/kg, i.v.) depressed the respiration and the cough reflex. Haloperidol (2 mg/kg, i.v.) abolished the tranylcypromine-stimulated respiratory responses, and it intensified the tranylcypromine induced cough depression. It is concluded that the increase in serotonin levels in the brain has a depressant influence on the central generating mechanisms of the cough reflex. Furthermore, central dopaminergic mechanisms seem to play a modulating role on the cough reflex.

Domperidone-induced potentiation of ventilatory responses in awake goats

Dopamine has been implicated in maintaining tonic inhibition of carotid body activity. We tested this hypothesis by assessing the ventilatory effects of a peripheral dopamine antagonist, domperidone. The effects of this agent on the ventilatory responses to hypoxia and hypercapnia were also examined. The study was performed in awake carotid body intact and carotid body denervated goats. Resting minute ventilation increased while PaCO2 decreased (4 Torr) following domperidone administration (0.5 mg/kg, I.V.) in carotid body intact goats. This response did not occur in carotid body denervated goats supporting the hypothesis that endogenous dopamine provides tonic inhibition in the carotid body. Hypoxic and hypercapnic ventilatory responses were significantly augmented following domperidone administration in the carotid body intact goats. This supports the concept of dopaminergic modulation of the response of the carotid body to stimuli. Domperidone allows study of carotid chemoreceptor dopaminergic activity in awake animals because of its high affinity for carotid body D2 dopamine receptors and its lack of CNS effects.

Dopaminergic control of respiration as shown by effects of 4-aminopyridine

The dopaminergic control of respiration in conscious and urethane-anaesthetized rabbits, was studied by comparing the respiratory effects of 4-aminopyridine alone (4-AP; 1 mg/kg i.v.) and those after the administration of dopamine antagonists (domperidone and haloperidol; 1 mg/kg each). The respiratory rate in conscious rabbits was increased by 4-AP. After domperidone this increase was reduced and preceded by a transient decrease. In spontaneously breathing, anesthetized rabbits there was a transient reduction after which the respiratory rate was increased by 4-AP; tidal volume was affected in an inverse manner. After domperidone, the excitatory effect of 4-AP on respiratory rate and the inhibitory effect on tidal volume were blocked. The effects of 4-AP on respiratory rate were prevented by vagotomy. In anesthetized, vagotomized, paralyzed and artificially ventilated rabbits (VPV animals) the peak amplitude of the integrated phrenic nerve activity ("phrenic activity') was increased by 4-AP. After pretreatment with haloperidol this effect of 4-AP on phrenic activity was reduced while the respiratory rate was now increased. In VPV animals with denervated carotid bodies the excitatory effect of 4-AP on phrenic activity was strongly enhanced and respiratory rate was increased. These effects were slightly reduced but not blocked by haloperidol. It is concluded that endogenous dopamine is involved in the control of respiration through effects on peripheral mechanisms (inhibition of inspiratory activity and enhancement of respiratory rate) as well as on central mechanisms (stimulation of inspiratory activity and reduction of respiratory rate).

Effects of dopamine superfusion on the activity of rabbit carotid chemoreceptors in vitro

The effect of different concentrations (0.001, 0.01, 0.1 and 1 mM) of dopamine on chemo-afferent activity was studied in the rabbit carotid body superfused in vitro. Excitation was the sole effect observed: it was always present for dopamine tests at 0.1 and 1 mM but was found in only 4 out of 9 tests at 0.01 mM and in 1 out of 5 tests at 0.001 mM. By comparison with a natural stimulus like hypoxia, dopamine excitation was delayed and had a much slower time course. Dopamine antagonists, (+)-butaclamol and haloperidol did not affect the responses to dopamine and to hypoxia. The results were not significantly altered when CO2 was added to the superfusing medium. It is concluded that dopamine is not a likely excitatory transmitter for chemoreception in the rabbit carotid body.

Ventilatory stimulation by dopamine-receptor antagonists in the mouse

Ventilation was measured by a plethysmographic method in awake mice before and after intraperitoneal injection of neuroleptic drugs to test the hypothesis that dopaminergic mechanisms modulate control of breathing in this species. Dose-dependent augmentation of ventilation at rest and during hypoxia, and reduced ventilation during hypercapnia was demonstrated for haloperidol, droperidol, prochlorperazine and chlorpromazine (P less than 0.05 or less for each drug). Doses of drugs causing maximal increase of the ventilatory response to hypoxia were linearly related (r = 0.98, P less than 0.001) to in vitro affinity of the drugs for dopamine receptors. Despite presumed equal dopamine-receptor blockade, the drugs had unequal effects on the ventilatory response to hypoxia. Droperidol augmented hypoxic ventilation to 290% of the control value, chlorpromazine to 250% control, prochlorperazine to 190% control and haloperidol to 120% control. These differences in efficacy were in the same order as the affinities of the drugs for alpha-adrenoceptors. The effect of combined haloperidol (90 nmol kg-1) and varying doses of phentolamine (175-900 nmol kg-1) was assessed to test the hypothesis that alpha-antagonism was a factor in determining the increase in ventilation following dopamine blockade. Phentolamine caused dose-dependent augmentation of the ventilatory effects of haloperidol (P less than 0.01) but had no ventilatory effect when given alone. Carotid body resection in anaesthetized mice abolished the stimulation of hypoxic ventilation caused by droperidol. It is concluded that dopaminergic mechanisms in the carotid body modulate ventilatory control in the awake mouse. The drugs most effective in augmenting hypoxic ventilation are those that block both dopamine and alpha-adrenoceptors.

Cholecystokinin-like immunoreactivity in cat extra-adrenal paraganglia

In the cat peripheral dopaminergic organs such as the carotid body, subclavian bodies, aortico-pulmonary glomera and small intensively fluorescent cell (SIF cell) clusters of the superior cervical ganglion and the nodose ganglion were found to contain cholecystokinin (CCK)-immunoreactive paraganglionic cells. Thus, the extra-adrenal paraganglionic system may serve as a model for studying peripheral interactions of CCKergic and dopaminergic mechanisms.

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.

Changes in carotid body amine levels and effects of dopamine on respiration in rats treated neonatally with capsaicin

Dopamine levels in rat carotid bodies and the effects of intravenous dopamine injections on respiration in adult rats anaesthetized with pentobarbitone have been studied in animals which were treated with capsaicin neonatally. Levels of dopamine were five fold higher in the carotid bodies of capsaicin-treated rats as compared with vehicle-treated controls, but there was no significant difference between capsaicin-treated and vehicle-treated rats in their ID50 values for dopamine-induced respiratory depression. Domperidone, a dopamine D2-receptor antagonist, substantially reduced the respiratory depression caused by dopamine, both in capsaicin-treated and in control animals, suggesting that a D2-receptor was involved in the response. Cutting the carotid sinus nerves greatly reduced the ventilatory-depressant effect of dopamine, showing that sensory receptors, most probably arterial chemoreceptors, were responsible for most of the response. Substantially less reflex hyperventilation was evoked in capsaicin-treated rats by the peripheral chemoreceptor stimulants hypoxia and sodium cyanide, in comparison with the controls, and domperidone did not increase the responsiveness. About 80% of the reflex ventilatory change originated from carotid body chemoreceptors. The hypoventilation caused by breathing 100% O2 was not significantly different in capsaicin-treated rats when compared with controls. Domperidone substantially reduced this response in capsaicin-treated rats, but not in vehicle-treated animals. Dopamine-induced respiratory depression in capsaicin-treated rats was slightly enhanced, rather than reduced, by oxygen breathing; domperidone remained an effective antagonist of dopamine-induced ventilatory depression. Most of the reduction in respiration caused by dopamine in rats anaesthetized with pentobarbitone can be attributed to actions on a dopamine D2-receptor located in the carotid body. However, despite the increased levels of dopamine found in the carotid bodies, the reduced peripheral chemosensitivity observed in anaesthetized capsaicin-treated rats does not appear to result from a change in sensitivity to dopamine.

Blockade of dopamine-induced chemosensory inhibition by domperidone

The sensory discharges from the carotid body chemoreceptors of the cat are transiently inhibited by dopamine (DA) injections. This chemosensory inhibition was effectively blocked by domperidone, a selective antagonist of D2 dopaminoceptors. The basal frequency of spontaneous chemosensory impulses was immediately and sustainly increased after domperidone, suggesting the withdrawal of a tonic inhibition of chemosensory discharges by endogenous DA released from glomus cells. The peripheral dopaminergic modulation of chemoreflexes may be separately blocked by domperidone, a drug unable to cross the blood-brain barrier.

Increased sensitivity of rabbit carotid body chemoreceptors to dopamine after chronic treatment with domperidone

An increase in specific dopamine D2 receptor binding sites was observed in membranes prepared from the carotid bodies of rabbits treated for 8 weeks and then withdrawn for 4-9 days from the D2 antagonist domperidone (2-5 mg/kg per day). Recordings of chemoreceptor afferent discharge from the carotid body also revealed that this change in receptor density was accompanied by an increased sensitivity to the chemodepressant effects of exogenous dopamine. The chemoreceptor responsiveness of the carotid body to hypoxia is blunted in rabbits treated chronically with domperidone, but this can be restored to normal by an acute dose of the D2 antagonist. These experiments provide evidence that is compatible with a chemo-inhibitory role for endogenous dopamine in the rabbit's carotid body. Furthermore, these results suggest that the carotid body provides a useful model for the functional studies of dopamine D2 receptors.