Reduction of glutamatergic activity through cholinergic dysfunction in the hippocampus of hippocampal cholinergic neurostimulating peptide precursor protein knockout mice.

Cholinergic activation can enhance glutamatergic activity in the hippocampus under pathologic conditions, such as Alzheimer's disease. The aim of the present study was to elucidate the relationship between glutamatergic neural functional decline and cholinergic neural dysfunction in the hippocampus. We report the importance of hippocampal cholinergic neurostimulating peptide (HCNP) in inducing acetylcholine synthesis in the medial septal nucleus. Here, we demonstrate that HCNP-precursor protein (pp) knockout (KO) mice electrophysiologically presented with glutamatergic dysfunction in the hippocampus with age. The impairment of cholinergic function via a decrease in vesicular acetylcholine transporter in the pre-synapse with reactive upregulation of the muscarinic M1 receptor may be partly involved in glutamatergic dysfunction in the hippocampus of HCNP-pp KO mice. The results, in combination with our previous reports that show the reduction of hippocampal theta power through a decrease of a region-specific choline acetyltransferase in the stratum oriens of CA1 and the decrease of acetylcholine concentration in the hippocampus, may indicate the defined cholinergic dysfunction in HCNP-pp KO mice. This may also support that HCNP-pp KO mice are appropriate genetic models for cholinergic functional impairment in septo-hippocampal interactions. Therefore, according to the cholinergic hypothesis, the model mice might are potential partial pathological animal models for Alzheimer's disease.

Modulation of excitatory synaptic transmissions by TRPV1 in the spinal trigeminal subnucleus caudalis neurons of neuropathic pain rats.

The present study examined contribution of the transient receptor potential vanilloid 1 channel (TRPV1) to the chronic orofacial pain. Bilateral partial nerve ligation (PNL) of the mental nerve, a branch of trigeminal nerve, was performed to induce neuropathic pain. The withdrawal threshold in response to mechanical stimulation of the lower lip skin was substantially reduced after the surgery in the PNL rats while it remained unchanged in the sham rats. This reduction in the PNL rats was alleviated by pregabalin injected intraperitoneally (10 mg/kg) and intracisternally (10, 30, 100 µg). Furthermore, an intracisternal injection of AMG9810, an antagonist of TRPV1, (1.5, 5.0 µg) attenuated the reduction of withdrawal threshold. Spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) were recorded from the spinal trigeminal subnucleus caudalis (Vc) neurons in the brainstem slice, which receive the orofacial nociceptive signals. In the PNL rats, superfusion of capsaicin (0.03, 0.1 µM) enhanced their frequency without effect on the amplitude and the highest concentration (0.3 µM) increased both the frequency and amplitude. In the sham rats, only 0.3 µM capsaicin increased their frequency. Thus, capsaicin-induced facilitation of sEPSCs and mEPSCs in the PNL rats was significantly stronger than that in the sham rats. AMG9810 (0.1 µM) attenuated the capsaicin's effect. Capsaicin was ineffective on the trigeminal tract-evoked EPSCs in the PNL and sham rats. These results suggest that the chronic orofacial pain in the PNL model results from facilitation of the spontaneous excitatory synaptic transmission in the Vc region through TRPV1 at least partly.

Effects of progesterone on hypoxia-induced inhibition of excitatory synaptic transmission in the rat nucleus tractus solitarius.

The present study evaluated the ability of progesterone to alleviate the synaptic transmission disturbed by hypoxia in the nucleus tractus solitarius (NTS). Hypoxia with N2 inhibited spontaneous and tractus solitarius-evoked excitatory postsynaptic currents (sEPSCs and eEPSCs) in NTS neurons of the rat brainstem slice. An additional application of progesterone counteracted the hypoxia-induced inhibition of sEPSCs and eEPSCs without affecting the baseline currents. This effect of progesterone occurred rapidly and reversibly. Progesterone had neither effect on sEPSCs nor eEPSCs in normoxia. These results suggest that progesterone restores hypoxia-induced disturbance of the NTS glutamatergic transmission, presumably by a presynaptic, non-genomic mechanism.

Involvement of presynaptic TRPV1 channels in prostaglandin E2-induced facilitation of spontaneous synaptic transmission in the rat spinal trigeminal subnucleus caudalis.

Prostaglandin E2 (PGE2) synthesized in the central nervous system influences various physiological functions including nociception. Recently, we have demonstrated that PGE2 facilitates spontaneous synaptic transmission through presynaptic EP1 receptors in the spinal trigeminal subnucleus caudalis (Vc) neurons that receive nociceptive signals from the orofacial area. Increasing evidence suggests that the action of PGE2 is related to activation of transient receptor potential vanilloid 1 (TRPV1) channels. The present study investigated whether TRPV1 channels contribute to the facilitatory effect of PGE2 on synaptic transmission in the Vc neurons. Spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded from Vc neurons in the rat brainstem slice by whole-cell patch-clamp mode. Superfusion of capsaicin (0.3, 1.0 µM) concentration-dependently increased the frequency of both sEPSCs and sIPSCs without any significant effect on their amplitude. The effect of capsaicin was completely abolished by a TRPV1 channel blocker AMG9810 (0.1 µM). PGE2 (5.0 µM) increased the frequency of sEPSCs and sIPSCs. This facilitatory effect of PGE2 was attenuated by AMG9810 and in neurons desensitized by repeated application of capsaicin. While a low concentration of either PGE2 (1.0 µM) or capsaicin (0.1 µM) had an insignificant effect on the sEPSCs and sIPSCs, co-application of these drugs increased their frequency. The present study demonstrated involvement of the presynaptic TRPV1 channels in PGE2-induced facilitation of spontaneous synaptic transmissions and suggests interaction of PGE2 with TRPV1 channels in modification of nociceptive signals from the orofacial area to the Vc neurons.

Involvement of the cAMP-Dependent Pathway in Dextromethorphan-Induced Inhibition of Spontaneous Glutamate Transmission in the Nucleus Tractus Solitarius Neurons of Guinea Pigs.

Dextromethorphan (DEX) presynaptically decreases glutamatergic transmission in second-order neurons of the nucleus tractus solitarius (TS). To clarify the inhibitory mechanism of DEX, the present study examined the interaction of DEX with cAMP. The effects of DEX on miniature and TS-evoked excitatory postsynaptic currents (mEPSCs and eEPSCs) were recorded under activation of the cAMP-dependent pathway using the brainstem slices. An increase in cAMP by forskolin counteracted the inhibitory effect of DEX on mEPSCs. Eight-Bromo-cAMP and N-ethylmaleimide also attenuated the DEX effect. However, forskolin had negligible effects on the DEX-induced inhibition of eEPSCs. This suggests that DEX decreases spontaneous glutamate release by inhibiting the cAMP-dependent pathway and synchronous release by another unknown mechanism.

Hippocampal Cholinergic Neurostimulating Peptide as a Possible Modulating Factor against Glutamatergic Neuronal Disability by Amyloid Oligomers.

Despite having pathological changes in the brain associated with Alzheimer's disease (AD), some patients have preserved cognitive function. A recent epidemiological study has shown that diet, exercise, cognitive training, and vascular risk monitoring interventions may reduce cognitive decline in at-risk elderly people in the general population. However, the details of molecular mechanisms underlying this cognitive function preservation are still unknown. Previous reports have demonstrated that enriched environments prevent the impairment of hippocampal long-term potentiation (LTP) through ß2-adrenergic signals, when LTP is incompletely suppressed by synthetic amyloid-ß (Aß) oligomers. The cholinergic network from the medial septal nucleus (MSN) is also a main modulating system for hippocampal glutamatergic neural activation through nicotinergic and/or muscarinergic acetylcholine receptors. Previously, we reported the importance of a cholinergic regulator gene in the MSN, hippocampal cholinergic neurostimulating peptide (HCNP). By using hippocampal sections from mice, we here demonstrated that the cholinergic neural activation from the MSN enhanced the glutamatergic neuronal activity during unsaturated LTP but not during saturated LTP. Synthetic Aß oligomers suppressed the hippocampal glutamatergic activity in a concentration-dependent manner. Furthermore, HCNP, as well as a cholinergic agonist acting through the muscarinic M1 receptor, prevented the suppression of hippocampal glutamatergic neuronal activity induced by synthetic Aß oligomers. This result suggests that the persisting cholinergic activation might be a potential explanation for the individual differences in cognitive effects of AD pathological changes.

Reversal of morphine-induced respiratory depression by doxapram in anesthetized rats.

The present study was undertaken to investigate whether doxapram, a blocker of tandem pore K(+) (TASK-1/-3) channels, is a useful tool for recovery from morphine-induced ventilatory disturbances. Spontaneous ventilation and the hind leg withdrawal response against noxious thermal stimulation were recorded simultaneously in anesthetized rats. Morphine (1.0mg/kg, i.v.) decreased the minute volume resulting from depression of the ventilatory rate and tracheal airflow. Concomitantly, it prolonged the latency of withdrawal response against the thermal stimulation. Subsequent intravenous injection of doxapram recovered the morphine-induced ventilatory depression. This effect of doxapram declined rapidly after a single injection (1.0-3.0mg/kg, i.v.) but persisted with a continuous infusion (0.33mg/kg/min). Neither single injection nor continuous infusion of doxapram had any detectable effect on the analgesic potency of morphine. The central respiratory activity was recorded from the phrenic nerve in anesthetized, vagotomized, paralyzed and artificially ventilated rats. Morphine (3.0mg/kg, i.v.) induced respiratory depression, characterized by a prolonged plateau-like inspiratory discharge (apneustic discharge) in the phrenic nerve. Doxapram (10mg/kg, i.v.) restored the morphine-induced apneustic discharge to an augmenting inspiratory discharge. This study demonstrated that doxapram counteracted morphine-induced respiratory depression by stimulating the central respiratory network without compromising morphine antinociception. These results support the clinical use of doxapram for amelioration of ventilatory disturbances in patients treated with opioids.

Mechanisms of pentazocine-induced ventilatory depression and antinociception in anesthetized rats.

This study was performed to clarify mechanisms underlying pentazocine-induced ventilatory depression and antinociception. Spontaneous ventilation and hind leg withdrawal response against nociceptive thermal stimulation were simultaneously recorded in anesthetized rats. Pentazocine decreased minute volume resulting from depression of the ventilatory rate and tracheal airflow, and prolonged the latency of withdrawal response. Pre-treatment of ß-funaltorexamine, but not nor-binaltorphimine, significantly attenuated pentazocine-induced ventilatory depression, while either antagonist weakened its analgesic potency. Comparing with effects of fentanyl and U50488, the present results suggest that ventilatory depression induced by pentazocine is mediated by mainly µ receptors and analgesia by both µ and κ receptors.

Effects of prostaglandin E2 on synaptic transmission in the rat spinal trigeminal subnucleus caudalis.

The spinal trigeminal subnucleus caudalis (Vc) receives preferentially nociceptive afferent signals from the orofacial area. Nociceptive stimuli to the orofacial area induce cyclooxygenase both peripherally and centrally, which can synthesize a major prostanoid prostaglandin E2 (PGE2) that implicates in diverse physiological functions. To clarify the roles of centrally-synthesized PGE2 in nociception, effects of exogenous PGE2 on synaptic transmission in the Vc neurons were investigated in the rat brainstem slice. Spontaneously occurring excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded, respectively, under pharmacological blockade of inhibitory and excitatory transmission by whole-cell patch-clamp mode. Perfusion of PGE2 (1-5 µM) increased the frequency of sIPSCs in a concentration-dependent manner but had no significant effect on the amplitude. Similarly to the effects on sIPSCs, PGE2 increased the sEPSC frequency without any effect on the amplitude. These facilitatory effects of PGE2 on spontaneous synaptic transmissions were blocked by an EP1 antagonist SC19220 but not by an EP4 antagonist AH23848. Electrical stimulation of the trigeminal tract evoked short latency EPSCs (eEPSCs) in the Vc neurons. PGE2 (5 µM) was ineffective on the eEPSCs. The present study demonstrated that PGE2 facilitated spontaneous synaptic transmissions in the Vc neurons through activating the presynaptic EP1 receptors but had no effect on the trigeminal tract-mediated excitatory transmission. These results suggest that centrally-synthesized PGE2 modifies the synaptic transmission in the Vc region, thereby contributing to the processing of nociceptive signals originated from the orofacial area.

Enhancement of long-term potentiation via muscarinic modulation in the hippocampus of HCNP precursor transgenic mice.

Hippocampal cholinergic neurostimulating peptide (HCNP) regulates acetylcholine synthesis in the septal hippocampus through the quantitative increase of choline acetyltransferase levels in the septal nucleus both in vitro and in vivo. Additionally, HCNP-precursor protein transgenic (HCNP-pp Tg) mice display depressive behavior. To examine the physiological function of HCNP and/or HCNP-pp on hippocampal neural activity, we investigated whether overexpression of HCNP-pp strengthened the efficiency of neural activity in the hippocampus. Long-term potentiation (LTP) of excitatory synaptic transmission was induced by a tetanic stimulation of the Schaffer collateral-commissural fibers (SCs) in mouse hippocampal slices. LTP in HCNP-pp Tg mice was significantly enhanced when compared with wild-type littermate (WT) mice. This facilitation of LTP in HCNP-pp Tg mice was blocked by atropine or pirenzepine, but not by mecamylamine. In contrast, LTP in WT mice was not affected by atropine, but enhanced by carbachol. However, neither difference in the input-output relationship of field excitatory postsynaptic potentials nor in the facilitation ratio in paired-pulse stimulation of the SCs was observed between HCNP-pp Tg and WT mice, indicating that presynaptic glutamate release in HCNP-pp Tg mice is similar to that of WT mice. These results suggest that muscarinic (M1) modulation of glutamatergic postsynaptic function may be involved in strengthening LTP in HCNP-pp Tg mice.

Blockade of phosphodiesterase 4 reverses morphine-induced ventilatory disturbance without loss of analgesia.

AIMS: Ventilatory disturbance is a fatal side-effect of opioid analgesics. Separation of analgesia from ventilatory depression is important for therapeutic use of opioids. It has been suggested that opioid-induced ventilatory depression results from a decrease in adenosine 3',5'-cyclic monophosphate content in the respiratory-related neurons. Therefore, we examined the effects of caffeine, a methylxanthine non-selective phosphodiesterase (PDE) inhibitor with adenosine antagonistic activity, and rolipram, a racetam selective PDE4 inhibitor, on ventilatory depression induced by morphine. MAIN METHODS: Spontaneous ventilation and paw withdrawal responses to nociceptive thermal stimulation were measured in anesthetized rats simultaneously. The efferent discharge of the phrenic nerve was recorded in anesthetized, vagotomized, paralyzed and artificially ventilated rats. KEY FINDINGS: Rolipram (0.1 and 0.3 mg/kg, i.v.) and caffeine (3.0 and 10.0 mg/kg, i.v.) relieved morphine (1.0 mg/kg, i.v.)-induced ventilatory depression but had no discernible effect on its analgesic action. Rolipram (0.3 and 1.0 mg/kg, i.v.) and caffeine (10.0 and 20.0 mg/kg, i.v.) recovered morphine (3.0 mg/kg, i.v.)-induced prolongation and flattening of inspiratory discharge in the phrenic nerve. SIGNIFICANCE: Inhibition of PDE4 may be a possible approach for overcoming morphine-induced ventilatory depression without loss of analgesia.

Modulation of glutamatergic transmission by presynaptic N-methyl-D-aspartate mechanisms in second-order neurons of the rat nucleus tractus solitarius.

The present study investigated the physiological function of presynaptic N-methyl-d aspartate (NMDA) mechanisms in glutamatergic transmission in the rat nucleus tractus solitarius (NTS). Membrane currents were recorded from the NTS second-order neurons by using whole-cell patch pipettes including MK-801 to block postsynaptic NMDA receptors. All experiments were performed under blockade of inhibitory synaptic transmission. Co-application of NMDA and d-serine decreased the tractus solitarius (TS)-evoked excitatory postsynaptic currents (eEPSCs) in 7/12 (58%) of neurons, and increased the paired pulse ratio. The remaining neurons were insensitive to NMDA and d-serine. Application of an NMDA antagonist D-AP5 had no effect on eEPSCs in all 8 neurons tested. Action potential-independent EPSCs (miniature EPSCs; mEPSCs) were recorded in the presence of tetrodotoxin. Co-application of NMDA and d-serine increased the mEPSC frequency but had no significant effect on the amplitude in 5/28 (18%) of neurons. D-AP5 decreased the mEPSC frequency without effect on the amplitude in 6/18 (33%) of neurons. This study demonstrated that (1) NMDA receptors were presynaptically distributed in a subset of NTS second-order neurons and that (2) the presynaptic NMDA receptors played an inhibitory role in TS-mediated release of glutamate and a facilitatory role in spontaneous release of glutamate. The present results suggest that the activation of presynaptic NMDA receptors modulates glutamatergic transmissions in the rat NTS second-order neurons.

The NADPH oxidase inhibitor diphenyleneiodonium activates the human TRPA1 nociceptor.

Transient receptor potential ankyrin 1 (TRPA1) is a Ca(2+)-permeable nonselective cation channel expressed in neuronal and nonneuronal cells and plays an important role in acute and inflammatory pain. Here, we show that an NADPH oxidase (NOX) inhibitor, diphenyleneiodonium (DPI), functions as a TRPA1 activator in human embryonic kidney cells expressing human TRPA1 (HEK-TRPA1) and in human fibroblast-like synoviocytes. Application of DPI at 0.03-10 µM induced a Ca(2+) response in HEK-TRPA1 cells in a concentration-dependent manner. The Ca(2+) response was effectively blocked by a selective TRPA1 antagonist, HC-030031 (HC). In contrast, DPI had no effect on HEK cells expressing TRPV1-V4 or TRPM8. Four other NOX inhibitors, apocynin (APO), VAS2870 (VAS), plumbagin, and 2-acetylphenothiazine, also induced a Ca(2+) response in HEK-TRPA1 cells, which was inhibited by pretreatment with HC. In the presence of 5 mM glutathione, the Ca(2+) response to DPI was effectively reduced. Moreover, mutation of cysteine 621 in TRPA1 substantially inhibited the DPI-induced Ca(2+) response, while it did not inhibit the APO- and VAS-induced responses. The channel activity was induced by DPI in excised membrane patches with both outside-out and inside-out configurations. Internal application of neomycin significantly inhibited the DPI-induced inward currents. In inflammatory synoviocytes with TRPA1, DPI evoked a Ca(2+) response that was sensitive to HC. In mice, intraplantar injection of DPI caused a pain-related response which was inhibited by preadministration with HC. Taken together, our findings demonstrate that DPI and other NOX inhibitors activate human TRPA1 without mediating NOX.

Modulation of glutamatergic transmission by metabotropic glutamate receptor activation in second-order neurons of the guinea pig nucleus tractus solitarius.

Activity of second-order relay neurons in the nucleus tractus solitarius (NTS) is regulated by peripheral and intrinsic synaptic inputs, and modulation of those inputs by metabotropic glutamate receptors (mGluRs) has been proposed. This study investigated effects of mGluR activation on glutamatergic transmission in the NTS second-order neurons of guinea pigs. Whole-cell patch-clamp recordings from the brainstem slices revealed that activation of mGluRs exerted its effects on the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) but not on the amplitude. The sEPSC frequency was increased by an agonist of group I mGluRs, and it was decreased by an mGluR1 antagonist but not by an mGluR5 antagonist. The agonists of group II and III mGluRs decreased the sEPSC frequency, while their antagonists alone had no effect. Perfusion of cystine or TBOA, either of which elevates extracellular glutamate concentration, resulted in an increase in the sEPSC frequency, leaving the amplitude unchanged. The increased frequency of sEPSCs was returned to control by an mGluR1 antagonist. The tractus solitarius-evoked EPSCs were not altered by an agonist of group I mGluRs, whereas they were decreased along with an increase in paired-pulse ratio by agonists of group II and III mGluRs. These results suggest that mGluRs are present at the presynaptic sites in the NTS second-order neurons in guinea pigs. The mGluR1s function to facilitate the release of glutamate from axon terminals of intrinsic interneurons and the group II and III mGluRs play an inhibitory role in glutamatergic transmission.

Pharmacological strategy for overcoming opioid-induced ventilatory disturbances.

Opioids are among the most frequently used analgesics for treatment of severe pain. However, certain of their side-effects, particularly ventilatory disturbances, often restrict their use. Separation of analgesia from respiratory depression has long been a goal in the basic research and therapeutic use of opioids. This report briefly describes opioid-induced respiratory depression and possible pharmacological strategies to counteract this without affecting analgesia.

A model of the central regulatory system for cough reflex.

Cough is an important defensive reflex that eliminates particles and secretions from the airways and protects the lower airways from the aspiration of foreign materials. Although the classical cough center is thought to be situated in or around the nucleus tractus solitarius (NTS) of the brainstem, our understanding of its profile is still incomplete. Accumulating evidence suggests a new concept of the central regulatory system for cough reflex. The cough pattern generator in the brainstem appears to be identical to the respiratory pattern generator and to function by reshaping of the discharge pattern of respiratory neurons. The generated cough motor task is transmitted to spinal motoneurons through the descending respiratory pathways. The cough-gating mechanism receives the peripheral tussigenic information through the relay neurons in the NTS and activates such a functionally flexible pattern generator by producing triggering signals. This review focuses on the cough-gating neurons that constitute the gating mechanism and play a crucial role in the generation of cough reflex.

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.

High doses of oseltamivir phosphate induce acute respiratory arrest in anaesthetized rats.

It has been reported that one of the serious adverse events after the treatment of oseltamivir phosphate (OP) for influenza patients is sudden death resulting from cardiorespiratory arrest. To investigate the aetiology of such an adverse consequence, we examined effects of OP (expressed as free base) on blood pressure and ventilation in anaesthetized rats with vagotomy. Intravenous OP (30-200 mg/kg) caused dose-dependent hypotension and bradycardia in spontaneously breathing animals. Concomitantly with changes in blood pressure, the tracheal airflow increased. The ventilatory rate hastened during the injection and then transiently slowed around 1 min. after the administration (transient hypopnea). Thereafter, it gradually returned to control. The hypopnea increased with increasing dose and ventilatory arrest occurred at 200 mg/kg. Intraduodenal OP (500-1000 mg/kg) provoked cardioventilatory arrest 72-218 min. after the injection. Oseltamivir carboxylate (100-200 mg/kg, i.v.), an active metabolite of OP, had no significant effect on ventilation and blood pressure. In artificially ventilated animals, intravenous OP caused slowing of the respiratory rate around 1 min. after the injection in a dose-dependent manner. This effect of OP waned in 5 min. after the administration. The amplitude of phrenic nerve discharge was not changed at lower doses (30-100 mg/kg). The phrenic nerve stopped to discharge immediately after higher doses (150-200 mg/kg). We demonstrated that OP causes central suppression of the respiratory function in rats and suggest a relationship between the OP-induced cardiorespiratory arrest and sudden death observed in influenza patients after taking OP.

Distinct modulatory effects of 5-HT on excitatory synaptic transmissions in the nucleus tractus solitarius of the rat.

The second-order relay neurons in the nucleus tractus solitarius (NTS) receive numerous peripheral afferent inputs mainly from the vagus nerve. Their activity is modified by several neuromodulators and hence autonomic responses are properly regulated. Serotonin (5-HT) is an important candidate for such neuromodulators, since serotonergic inputs and distribution of 5-HT receptors are provided in the NTS. However, its mechanism of action remains unclear. To evaluate the serotonergic modulation of synaptic transmission, we examined the effects of 5-HT (1.0-10.0 µM) on the solitary tract-evoked excitatory postsynaptic currents (eEPSCs) and spontaneously occurring EPSCs (sEPSCs) in the preselected second-order neurons of the rat NTS. 5-HT concentration-dependently decreased the amplitude of eEPSCs, which was accompanied by an increase in paired-pulse ratio. The inhibitory effect of 5-HT was mimicked by α-methylserotonin and blocked by ketanserin. 5-HT had no effect on the inward current induced in the NTS neurons by topically applied α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). On the other hand, 5-HT increased the frequency of sEPCSs without effect on their amplitude. This excitatory effect of 5-HT was mimicked by 2-methylserotonin and antagonized by ondansetron. The results suggest a dual modulation of the excitatory synaptic transmission by 5-HT in the NTS; presynaptic inhibition of the peripheral inputs synapsing to the relay neurons via 5-HT(2) receptors and presynaptic excitation of inputs from the intrinsic local network via 5-HT(3) receptors. These effects of 5-HT may provide important means of optimizing the autonomic responses mediated by the NTS network.

M3-receptor activation counteracts opioid-mediated apneusis, but the apneusis per se is not necessarily related to an impaired M3 mechanism in rats.

AIMS: Morphine slows the respiratory cycle due to a predominant prolongation of inspiration (apneusis) by postponing the spontaneous termination of inspiration (inspiratory off-switching). The present study investigates whether the morphine-induced apneusis results from impairment of cholinergic mechanisms in the central respiratory network. MAIN METHODS: The efferent discharge was recorded from the phrenic nerve in artificially ventilated and anesthetized rats with vagotomy. All drugs were injected intravenously. KEY FINDINGS: The phrenic nerve displayed an augmenting discharge during inspiration and arrest of discharge during expiration in normal condition. Administration of morphine (0.3-10.0mg/kg) dose-dependently provoked apneusis characterized by a long-lasting, plateau inspiratory discharge of the phrenic nerve. It shortened the expiratory duration. Subsequent administration of physostigmine (0.1mg/kg) restored the morphine-induced apneusis to eupnea with a partial recovery of the augmenting inspiratory discharge. This modification of physostigmine was blocked by a non-specific muscarinic antagonist scopolamine (3.0mg/kg), leading to re-prolongation of inspiration. A similar antagonism was affected by an antagonist of M3 cholinergic receptors, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 1.0 and 10.0mg/kg) but not by an antagonist of M1 cholinergic receptors, pirenzepine (1.0 and 10.0mg/kg). SIGNIFICANCE: These results demonstrate that the activation of endogenous M3 cholinergic mechanisms counteracts the morphine-induced apneusis.