Increased ventilation and CO2 chemosensitivity in acetylcholinesterase knockout mice

Acetylcholine Modulates the Hormones of the Growth Hormone/Insulinlike Growth Factor-1 Axis During Development in Mice

Pituitary growth hormone (GH) and insulinlike growth factor (IGF)-1 are anabolic hormones whose physiological roles are particularly important during development. The activity of the GH/IGF-1 axis is controlled by complex neuroendocrine systems including two hypothalamic neuropeptides, GH-releasing hormone (GHRH) and somatostatin (SRIF), and a gastrointestinal hormone, ghrelin. The neurotransmitter acetylcholine (ACh) is involved in tuning GH secretion, and its GH-stimulatory action has mainly been shown in adults but is not clearly documented during development. ACh, together with these hormones and their receptors, is expressed before birth, and somatotroph cells are already responsive to GHRH, SRIF, and ghrelin. We thus hypothesized that ACh could contribute to the modulation of the main components of the somatotropic axis during development. In this study, we generated a choline acetyltransferase knockout mouse line and showed that heterozygous mice display a transient deficit in ACh from embryonic day 18.5 to postnatal day 10, and they recover normal ACh levels from the second postnatal week. This developmental ACh deficiency had no major impact on weight gain and cardiorespiratory status of newborn mice. Using this mouse model, we found that endogenous ACh levels determined the concentrations of circulating GH and IGF-1 at embryonic and postnatal stages. In particular, serum GH level was correlated with brain ACh content. ACh also modulated the levels of GHRH and SRIF in the hypothalamus and ghrelin in the stomach, and it affected the levels of these hormones in the circulation. This study identifies ACh as a potential regulator of the somatotropic axis during the developmental period.

Ventilation and the Response to Hypercapnia after Morphine in Opioid-naive and Opioid-tolerant Rats

BACKGROUND

Opioid-related deaths are a leading cause of accidental death, with most occurring in patients receiving chronic pain therapy. Respiratory arrest is the usual cause of death, but mechanisms increasing that risk with increased length of treatment remain unclear. Repeated administration produces tolerance to opioid analgesia, prompting increased dosing, but depression of ventilation may not gain tolerance to the same degree. This study addresses differences in the degree to which chronic morphine (1) produces tolerance to ventilatory depression versus analgesia and (2) alters the magnitude and time course of ventilatory depression.

METHODS

Juvenile rats received subcutaneous morphine for 3 days (n = 116) or vehicle control (n = 119) and were then tested on day 4 following one of a range of morphine doses for (a) analgesia by paw withdraw from heat or (b) respiratory parameters by plethysmography-respirometry.

RESULTS

Rats receiving chronic morphine showed significant tolerance to morphine sedation and analgesia (five times increased ED50). When sedation was achieved for all animals in a dose group (lowest effective doses: opioid-tolerant, 15 mg/kg; opioid-naive, 3 mg/kg), the opioid-tolerant showed similar magnitudes of depressed ventilation (-41.4 ± 7.0%, mean ± SD) and hypercapnic response (-80.9 ± 15.7%) as found for morphine-naive (-35.5 ± 16.9% and -67.7 ± 15.1%, respectively). Ventilation recovered due to tidal volume without recovery of respiratory rate or hypercapnic sensitivity and more slowly in morphine-tolerant.

CONCLUSIONS

In rats, gaining tolerance to morphine analgesia does not reduce ventilatory depression effects when sedated and may inhibit recovery of ventilation.

Effects of cholinesterase inhibitors and serotonin-1A receptor agonists on morphine-induced ventilatory depression and antinociception in rats

Ventilatory depression is a serious side-effect of opioid analgesics. Naloxone, an antagonist of opioid receptors, eliminates not only ventilatory depression but also analgesic effect of opioids. Pharmacological dissociation of adverse reactions from the main action is important clinically and basically. Cholinergic and serotonergic mechanisms are suggested to counteract the opioid-induced ventilatory disturbances, but their influence on analgesia is still controversial. The present study evaluated the effects of cholinesterase inhibitors and serotonin-1A (5-HT1A) receptor agonists on morphine (1.0mg/kg, i.v.)-induced ventilatory depression and analgesia in rats. In anesthetized animals, spontaneous ventilation and hind leg withdrawal reflexes against nociceptive thermal stimuli were measured simultaneously. Physostigmine (0.1 and 0.2mg/kg, i.v.) and donepezil (0.5 and 1.0mg/kg, i.v.) relieved the morphine-induced ventilatory depression and enhanced its antinociception. On the other hand, (±)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.03 and 0.1mg/kg, i.v.) and buspirone (0.1 and 0.3mg/kg, i.v.) did not influence antinociception of morphine while they restored the decreased ventilation. In unanesthetized animals, hypercapnic ventilatory response was measured by using whole-body plethysmography. Physostigmine (0.3mg/kg, i.p.), donepezil (1.0mg/kg, i.p.), 8-OH-DPAT (0.3mg/kg, i.p.) and buspirone (3.0mg/kg, i.p.) all recovered the morphine (10mg/kg, i.p.)-induced depression of hypercapnic ventilatory response. The present study suggests that activation of cholinergic or serotonergic (5-HT1A) mechanisms may be a useful therapeutic approach for morphine-induced ventilatory depression without loss of its analgesic action.

Near-complete adaptation of the PRiMA knockout to the lack of central acetylcholinesterase

Acetylcholinesterase (AChE) rapidly hydrolyzes acetylcholine. At the neuromuscular junction, AChE is mainly anchored in the extracellular matrix by the collagen Q, whereas in the brain, AChE is tethered by the proline-rich membrane anchor (PRiMA). The AChE-deficient mice, in which AChE has been deleted from all tissues, have severe handicaps. Surprisingly, PRiMA KO mice in which AChE is mostly eliminated from the brain show very few deficits. We now report that most of the changes observed in the brain of AChE-deficient mice, and in particular the high levels of ambient extracellular acetylcholine and the massive decrease of muscarinic receptors, are also observed in the brain of PRiMA KO. However, the two groups of mutants differ in their responses to AChE inhibitors. Since PRiMA-KO mice and AChE-deficient mice have similar low AChE concentrations in the brain but differ in the AChE content of the peripheral nervous system, these results suggest that peripheral nervous system AChE is a major target of AChE inhibitors, and that its absence in AChE- deficient mice is the main cause of the slow development and vulnerability of these mice. At the level of the brain, the adaptation to the absence of AChE is nearly complete.

Developmental basis of the rostro-caudal organization of the brainstem respiratory rhythm generator

The Hox genetic network plays a key role in the anteroposterior patterning of the rhombencephalon at pre- and early-segmental stages of development of the neural tube. In the mouse, it controls development of the entire brainstem respiratory neuronal network, including the pons, the parafacial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC). Inactivation of Krox20/Egr2 eliminates the pFRG activity, thereby causing life-threatening neonatal apnoeas alternating with respiration at low frequency. Another respiratory abnormality, the complete absence of breathing, is induced when neuronal synchronization fails to develop in the preBötC. The present paper summarizes data on a third type of respiratory deficits induced by altering Hox function at pontine levels. Inactivation of Hoxa2, the most rostrally expressed Hox gene in the hindbrain, disturbs embryonic development of the pons and alters neonatal inspiratory shaping without affecting respiratory frequency and apnoeas. The same result is obtained by the Phox2a(+/-) mutation modifying the number of petrosal chemoafferent neurons, by eliminating acetylcholinesterase and by altering Hox-dependent development of the pons with retinoic acid administration at embryonic day 7.5. In addition, embryos treated with retinoic acid provide a mouse model for hyperpnoeic episodic breathing, widely reported in pre-term neonates, young girls with Rett's syndrome, patients with Joubert syndrome and adults with Cheyne-Stokes respiration. We conclude that specific respiratory deficits in vivo are assignable to anteroposterior segments of the brainstem, suggesting that the adult respiratory neuronal network is functionally organized according to the rhombomeric, Hox-dependent segmentation of the brainstem in embryos.

Central cholinergic regulation of respiration: nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in brainstem and spinal cord regions involved in the control of breathing. These receptors mediate central cholinergic regulation of respiration and effects of the exogenous ligand nicotine on respiratory pattern. Activation of alpha4* nAChRs in the preBötzinger Complex (preBötC), an essential site for normal respiratory rhythm generation in mammals, modulates excitatory glutamatergic neurotransmission and depolarizes preBötC inspiratory neurons, leading to increases in respiratory frequency. nAChRs are also present in motor nuclei innervating respiratory muscles. Activation of post- and/or extra-synaptic alpha4* nAChRs on hypoglossal (XII) motoneurons depolarizes these neurons, potentiating tonic and respiratory-related rhythmic activity. As perinatal nicotine exposure may contribute to the pathogenesis of sudden infant death syndrome (SIDS), we discuss the effects of perinatal nicotine exposure on development of the cholinergic and other neurotransmitter systems involved in control of breathing. Advances in understanding of the mechanisms underlying central cholinergic/nicotinic modulation of respiration provide a pharmacological basis for exploiting nAChRs as therapeutic targets for neurological disorders related to neural control of breathing such as sleep apnea and SIDS.

Influence of differential expression of acetylcholinesterase in brain and muscle on respiration

A mouse strain with a deleted acetylcholinesterase (AChE) gene (AChE knockout) shows a decreased inspiration time and increased tidal volume and ventilation .To investigate the respective roles of AChE in brain and muscle, we recorded respiration by means of whole-body plethysmography in knockout mice with tissue selective deletions in AChE expression. A mouse strain with the anchoring domains of AChE deleted (del E5+6 knockout mice) has very low activity in the brain and neuromuscular junction, but increased monomeric AChE in serum. A mouse strain with deletion of the muscle specific region of AChE (del i1RR knockout mice) exhibits no expression in muscle, but unaltered expression in the central nervous system. Neither strain exhibits the pronounced phenotypic traits observed in the complete AChE knockout strain. A third strain lacking the anchor molecule PRiMA, has no functional AChE and butyrylcholinesterase (BChE) in brain and an unaltered respiratory function. BChE inhibition by bambuterol decreases tidal volume and body temperature in del E5+6 and i1RR knockout strains, but not in PRiMA deletion or wild-type controls. We find that: (1) deletion of the full AChE gene is required for a pronounced alteration in respiratory phenotype, (2) BChE is involved in respiratory muscles contraction and temperature control in del E5+6 and i1RR knockout mice, and (3) AChE expression requiring a gene product splice to either exons 5 and 6 or regulated by intron1 influences temperature control.

Neural control of breathing: insights from genetic mouse models

Recent studies described the in vivo ventilatory phenotype of mutant newborn mice with targeted deletions of genes involved in the organization and development of the respiratory-neuron network. Whole body flow barometric plethysmography is the noninvasive method of choice for studying unrestrained newborn mice. Breathing-pattern abnormalities with apneas occur in mutant newborn mice that lack genes involved in the development and modulation of rhythmogenesis. Studies of deficits in ventilatory responses to hypercapnia and/or hypoxia helped to identify genes involved in chemosensitivity to oxygen and carbon dioxide. Combined studies in mutant newborn mice and in humans have shed light on the pathogenesis of genetically determined respiratory-control abnormalities such as congenital central hypoventilation syndrome, Rett syndrome, and Prader-Willi syndrome. The development of mouse models has opened up the field of research into new treatments for respiratory-control disorders in humans.

M(1)/M(3) and M(2)/M(4) muscarinic receptor double-knockout mice present distinct respiratory phenotypes

We investigated the role of muscarinic acetylcholine receptors in the control of breathing. Baseline breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O(2)) and hypercapnia (3% and 5% CO(2)), measured by whole-body plethysmography in partially restrained animals, were compared in mice lacking either M(1) and M(3) or M(2) and M(4) muscarinic receptors, and in wild-type matched controls. M(1/3)R double-knockout mice showed at rest an elevated ventilation (V (E)) due to a large (57%) increase in tidal volume (V(T)). Chemosensory ventilatory responses were unaltered. M(2/4)R double-knockout mice were agitated and showed elevated V (E) and breathing frequency (f(R)) at rest when partially restrained, but unaltered V (E) and low f(R) when recorded unrestrained. Chemosensory ventilatory responses were unaltered. The results suggest that M(1) and M(3) receptors are involved in the control of tidal volume, while M(2) and M(4) receptors may be involved in the control of breathing frequency at rest and response to stress.

Adaptation to excess acetylcholine by downregulation of adrenoceptors and muscarinic receptors in lungs of acetylcholinesterase knockout mice

The acetylcholinesterase knockout mouse has elevated acetylcholine levels due to the complete absence of acetylcholinesterase. Our goal was to determine the adaptive changes in lung receptors that allow these animals to tolerate excess neurotransmitter. The hypothesis was tested that not only muscarinic receptors but also alpha(1)-adrenoceptors and beta-adrenoceptors are downregulated, thus maintaining a proper balance of receptors and accounting for lung function in these animals. The quantity of alpha(1A), alpha(1B), alpha(1D), beta(1), and beta(2)-adrenoceptors and muscarinic receptors was determined by binding of radioligands. G-protein coupling was assessed using pseudo-competition with agonists. Phospholipase C activity was measured by an enzymatic assay. Cyclic AMP (cAMP) content was measured by immunoassay. Muscarinic receptors were decreased to 50%, alpha(1)-adrenoceptors to 23%, and beta-adrenoceptors to about 50% of control. Changes were subtype specific, as alpha(1A), alpha(1B), and beta(2)-adrenoceptors, but not alpha(1D)-adrenoceptor, were decreased. In contrast, receptor signaling into the cell as measured by coupling to G proteins, cAMP content, and PI-phospholipase C activity was the same as in control. This shows that the nearly normal lung function of these animals was explained by maintenance of a correct balance of adrenoceptors and muscarinic receptors. In conclusion, knockout mice have adapted to high concentrations of acetylcholine by downregulating receptors that bind acetylcholine, as well as by downregulating receptors that oppose the action of muscarinic receptors. Tolerance to excess acetylcholine is achieved by reducing the levels of muscarinic receptors and adrenoceptors.

Desferrioxamine enhances hypoxic ventilatory response and induces tyrosine hydroxylase gene expression in the rat brainstem in vivo

The iron chelator desferrioxamine (DFO) induces accumulation of the hypoxia-inducible factor (HIF-1), a transcription factor that up-regulates genes involved in adaptative responses to hypoxia. This property makes DFO a potential neuroprotector against hypoxic stress. We investigated in rats the effects of DFO on the ventilatory response to mild hypoxic tests and the expression of tyrosine hydroxylase (TH), a target gene of HIF-1. Two protocols were used, the first with repeated injections of 50 mg/kg DFO every 2 days during a 2-week period. This was aimed at define the time course of the ventilatory responses to a hypoxic test. In the second protocol, rats were given a single injection of 300 mg/kg DFO. Every day over 4 days, the hypoxic ventilatory response was recorded before the animal was sacrificed, and Western blot analysis of TH in the dorsal brainstem cardiorespiratory area was performed. DFO produced a delayed increase in the hypoxic ventilatory response, which appeared in the same time window as TH up-regulation (2-3 days after the bolus injection of DFO). This delay suggests a genic effect of the drug that improves the ventilatory response to hypoxia.

Ablation of vagal preganglionic neurons innervating the extra-thoracic trachea affects ventilatory responses to hypercapnia and hypoxia

This study tested the hypothesis that during hypercapnia or hypoxia, airway-related vagal preganglionic neurons (AVPNs) of the nucleus ambiguus (NA) release acetylcholine (ACh), which in a paracrine fashion, activates ACh receptors expressed by inspiratory rhythm generating cells. AVPNs in the NA were ablated by injecting a saporin- (SA) cholera toxin b subunit (CTb-SA) conjugate into the extra-thoracic trachea (n=6). Control animals were injected with free CTb (n=6). In CTb treated rats, baseline ventilation and ventilatory responses to hypercapnia (5 and 12% CO(2) in O(2)) or hypoxia (8% O(2) in N(2)) were similar (p>0.05) prior to and 5 days after injection. CTb-SA injected rats maintained rhythmic breathing patterns 5 days post injection, however, tachypneic responses to hypercapnia or hypoxia were significantly reduced. The number of choline acetyltransferase (ChAT) immunoreactive cells in the NA was much lower (p<0.05) in CTb-SA rats as compared to animals receiving CTb only. These results suggest that AVPNs participate in the respiratory frequency response to hypercapnia or hypoxia.

Are there non-catalytic functions of acetylcholinesterases? Lessons from mutant animal models

Acetylcholinesterase (AChE) hydrolyses acetylcholine (ACh) ensuring the fast clearance of released neurotransmitter at cholinergic synapses. Many studies led to the hypothesis that AChE and the closely related enzyme butyrylcholinesterase (BChE) may play other, non-hydrolytic roles during development. In this review, we compare data from in vivo studies performed on invertebrate and vertebrate genetic models. The loss of function of ache in these systems is responsible for the appearance of several phenotypes. In all aspects so far studied, the phenotypes can be explained by an excess of the undegraded substrate, ACh, leading to misfunction and pathological alterations. Thus, the lack of AChE catalytic activity in the mutants appears to be solely responsible for the observed phenotypes. None of them appears to require the postulated adhesive or other non-hydrolytic functions of AChE.