Distinct effects of orexin A on spontaneous and evoked synaptic currents in the rat nucleus tractus solitarius.

Orexins are produced in hypothalamic areas and orexin-containing neurons are distributed in widespread areas of the central nervous system. Orexins regulate several physiological functions such as arousal, food intake and autonomic control. The presence of orexin-containing neuron terminals and orexin receptors has been confirmed in the nucleus tractus solitarius (NTS), which receives primary afferent fibers from peripheral organs including baroreceptors. However, the neuronal effects of orexin-1 receptor (OX1R) activation were not examined. Here, we aimed to determine the effects of OX1R activation on excitatory synaptic transmission. OX1R activation increased the frequency of spontaneous excitatory synaptic currents (sEPSCs). This effect was blocked by the prior application of L-NAME. In contrast, the amplitude of evoked excitatory postsynaptic currents (eEPSCs) was suppressed by OX1R activation, and this effect was prevented by a cannabinoid receptor 1 blocker, AM251, but not by the pretreatment with L-NAME. Altogether, these results suggest that OX1R activation increases sEPSCs frequency by stimulating NO production, whereas it inhibits eEPSCs by releasing endocannabinoids in the NTS. Thus, OX1R activation had distinct effects on spontaneous and evoked excitatory synaptic transmissions in the NTS.

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 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.

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.

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.

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.

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.

Dextromethorphan inhibits the glutamatergic synaptic transmission in the nucleus tractus solitarius of guinea pigs.

Dextromethorphan (DEX) is a widely used non-opioid antitussive. However, the precise site of action and its mechanism were not fully understood. We examined the effects of DEX on AMPA receptor-mediated glutamatergic transmission in the nucleus tractus solitarius (NTS) of guinea pigs. Excitatory postsynaptic currents (evoked EPSCs: eEPSCs) were evoked in the second-order neurons by electrical stimulation of the tractus solitarius. DEX reversibly decreased the eEPSC amplitude in a concentration-dependent manner. The DEX-induced inhibition of eEPSC was accompanied by an increased paired-pulse ratio. Miniature EPSCs (mEPSCs) were also recorded in the presence of Cd(2+) or tetrodotoxin. DEX decreased the frequency of mEPSCs without affecting their amplitude. Topically applied AMPA provoked an inward current in the neurons, which was unchanged during the perfusion of DEX. BD1047, a σ-1-receptor antagonist, did not block the inhibitory effect of DEX on the eEPSCs, but antagonized the inhibition of eEPSCs induced by SKF-10047, a σ-1 agonist. Haloperidol, a σ-1 and -2 receptor ligand, had no influence on the inhibitory action of DEX. These results suggest that DEX inhibits glutamate release from the presynaptic terminals projecting to the second-order NTS neurons, but this effect of DEX is not mediated by the activation of σ receptors.

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.

Involvement of platelet-derived growth factor-BB and its receptor-beta in hypoxia-induced depression of excitatory synaptic transmission in the nucleus tractus solitarius of mice.

The role of platelet-derived growth factor (PDGF)-BB / PDGF receptor (PDGFR)-beta signal in inhibition of synaptic transmission by hypoxia is unclear. In the nucleus tractus solitarius neurons, hypoxia with N(2) or NaCN decreased the amplitude of excitatory postsynaptic currents (EPSCs) similarly in wild type (WT) and PDGFR-beta gene-knockout (KO) mice. Recovery of EP SCs after a high concentration of NaCN in KO mice was significantly faster than that in WT mice, while recovery after a low concentration of NaCN or N(2) was not different between both mice. These results suggest that the PDGF-BB / PDGFR-beta signal modulates the excitatory synaptic transmission during hypoxia.

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.

N-methyl-D-aspartate mechanisms in depolarization of augmenting expiratory neurons during the expulsive phase of fictive cough in decerebrate cats.

Cough reflex is characterized by a large expulsive phase for expelling the mucus or particles from the airway. The present study investigated the involvement of N-methyl-D-aspartate (NMDA) mechanisms in the expulsive phase of cough reflex using decerebrate and paralyzed cats. A fictive cough was induced by repetitive stimulation of the superior laryngeal nerve, which was characterized by an increased inspiratory discharge in the phrenic nerve (the stage 1 of fictive cough; SC1) and large spindle-shaped discharge in the iliohypogastric nerve (the stage 2 of fictive cough; SC2). Intravenous injection of an antagonist of NMDA receptors, dizocilpine (0.1mg/kg), increased the threshold intensity of stimulation for inducing a fictive cough. The SC2 iliohypogastric response was more vulnerable to dizocilpine than the SC1 phrenic response. Membrane potential of augmenting expiratory (aug-E) neurons was recorded from the caudal ventral respiratory group. Aug-E neurons showed a large depolarization with a high frequency discharge during the SC2 in major cases (n=35) and hyperpolarization in minor cases (n=6). Dizocilpine inhibited the occurrence of these SC2 responses of aug-E neurons without any effect on the basal respiratory fluctuations of membrane potential. This drug had no significant effect on waves of excitatory and inhibitory postsynaptic potentials evoked in aug-E neurons by single pulse stimulation of the SLN. The present results demonstrated that NMDA mechanisms contribute preferentially to the expulsive phase response in aug-E neurons during fictive cough reflex.

Platelet-derived growth factor (PDGF)-BB inhibits AMPA receptor-mediated synaptic transmission via PDGF receptor-beta in murine nucleus tractus solitarius.

Although platelet-derived growth factor (PDGF)-BB activates PDGF receptor-beta (PDGFR-beta) and, in turn, inhibits the glutamate N-methyl-D-aspartate (NMDA) receptor function, whether PDGF-BB modulates the CNS function mediated by another glutamate receptors, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors, remains poorly understood. Here we now report the inhibitory effect of PDGF-BB on the AMPA receptor function in the nucleus tractus solitarius (NTS) by using slice patch-clamp techniques. Excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of the tractus solitarius in mouse NTS second-order neurons. EPSCs were nearly completely eliminated by CNQX but not by MK-801, implying mediation through non-NMDA receptors. PDGF-BB significantly decreased the amplitude of EPSCs without affecting the mean decay time constant. This inhibitory effect was transient and reversible after removing PDGF-BB. Furthermore, PDGF-BB significantly reduced the amplitude of AMPA-induced currents in NTS neurons, which showed that PDGF-BB could suppress the AMPA receptor-mediated excitatory input via the postsynaptic mechanism. The inhibitory effect of PDGF-BB on EPSCs was not observed in mutant mice with conditional deletion of the PDGFR-beta gene in neurons. Together, these studies suggest that the PDGF-B/PDGFR-beta axis inhibits the AMPA receptor-mediated synaptic transmission that comprises the major part of the primary afferent to the NTS second-order neuron. The detected inhibitory action may be involved in the CNS regulation of the respiratory response.

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.

Functional and morphological organization of the nucleus tractus solitarius in the fictive cough reflex of guinea pigs.

Projection of the superior laryngeal nerve (SLN) afferent fibers into the nucleus tractus solitarius (NTS) was investigated using a fluorescent tracer in guinea pigs. High density of fluorescence was detected in the ipsilateral NTS extending from 0.5 mm caudal to 1.2 mm rostral to the obex. At coronal slices, the fluorescent granules, lines and patches were located in the interstitial, medial and dorsal regions of NTS. Fluorescence was also found in the dorsal region of contralateral commissural NTS. Microstimulation of the rostral NTS, which corresponded to the region showing the strong fluorescence, induced an increase in the inspiratory discharge of phrenic nerve that was immediately followed by a large burst discharge of the iliohypogastric nerve in decerebrate, paralyzed and artificially ventilated guinea pigs. This serial response of the two nerves was identical to that induced by electrical stimulation of the SLN. Intravenous injection of codeine suppressed both NTS and SLN-induced responses. The SLN-induced response was inhibited by microinjection of codeine into the ipsilateral NTS and abolished by lesion of the ipsilateral NTS. These results suggest that the NTS has an integrative function in production of cough reflex and is possible sites of action of central antitussive agents.

Effects of progesterone on apneic events during behaviorally defined sleep in male rats.

Drug therapy with progesterone has been applied to the patients with sleep apnea syndrome, but its clinical efficacy is equivocal. In the present study, we examined the effects of progesterone (1 and 30 mg/kg, i.p.) on the apneic events during behaviorally defined sleep in male rats at 4, 14 and 26 weeks of age by using a whole body plethysmographic measurement. The number of events of spontaneous apnea (SA) and post-sigh apnea (PSA) increased with aging. The duration of SA or PSA was also prolonged in old rats. A low dose (1 mg/kg) of progesterone significantly decreased the number of both SA and PSA, and this effect increased in an age-dependent manner. However, progesterone had no effect on the duration of SA and PSA. Neither the basal respiratory rate nor the total sleep time was changed. On the other hand, a higher dose (30 mg/kg) of progesterone had no effect on the number of SA and PSA, while it prolonged the duration of PSA. It also prolonged the total sleep time without affecting the basal respiratory rate. Pretreatment with mifepristone (5 mg /kg, i.p.), an antagonist of progesterone receptors, inhibited the effects of the low dose of progesterone, but did not show any antagonistic effect on the high dose-induced changes. These results suggest that the progesterone-mediated mechanisms are involved, at least partly, in respiratory function during sleep and the progesterone therapy is possibly effective within an appropriate dose range for the sleep apnea syndrome.

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.

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.

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.