Makoto Funahashi

The role of area postrema neurons expressing H-channels in the induction mechanism of nausea and vomiting

The area postrema is one of the circumventricular organs, lacks a blood-brain barrier, and is well known as the chemoreceptor trigger zone for emesis. Area postrema neurons are sensitive to emetic chemical substances carried in the blood plasma. Our previous study demonstrated the presence of 3 types of neurons characterized by different ion channels expressed in each cell type, but the type or types of area postrema neurons involved in the induction of nausea and/or emesis have remained unclear. To clarify the role of the most populous cells, which express the hyperpolarization-activated cation channel (H-channel), in induction of nausea and/or emesis, we investigated the effects of ZD7288 (an H-channel inhibitor) on apomorphine-induced conditioned taste aversion (CTA) to saccharin and c-Fos expression in the area postrema. We found that ZD7288 inhibited the acquisition of CTA and reduced apomorphine-induced c-Fos expression in the area postrema, indicating the involvement of the cells expressing H-channels in the induction of nausea and/or emesis. Finally, we discuss the role of cells expressing H-channels in the mechanism of nausea and/or vomiting.

Peripheral N-Methyl-D-Aspartate Receptors Modulate Nonadrenergic Noncholinergic Lower Esophageal Sphincter Relaxation in Rabbits

We investigated the role of peripheral N-methyl-d-aspartate (NMDA) receptors in the myenteric plexus in mediating nonadrenergic noncholinergic (NANC) nitrergic relaxation of the lower esophageal sphincter (LES). Isometric contraction of LES strips from Japanese White rabbits was measured. NANC relaxation was induced by KCl (30 mM) in the presence of atropine and guanethidine. The concentration of 3′,5′-cyclic guanosine monophosphate (cGMP) was measured using a radioimmunoassay. The muscle strips were exposed to diethyldithiocarbamic acid (DETCA; 3 mM) to inactivate Cu/Zn superoxide dismutase. MK801 (5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imine) inhibited NANC relaxation in a concentration-dependent manner (EC50 = 1.5 × 10−5 M), accompanied by a decrease in cGMP production. NMDA induced a concentration-dependent relaxation, which was antagonized by MK801. NMDA stimulated cGMP production, which was inhibited by NG-nitro-l-arginine. Superoxide dismutase (100 U/mL) shifted the concentration-response relationship of MK801-mediated inhibition of NANC relaxation to the right (EC50 = 3.4 × 10−5 M), whereas catalase did not. Treatment with DETCA shifted the concentration-response relationships of pyrogallol-, ketamine- and MK801-mediated inhibition of NANC relaxation to the left. These findings suggest that the peripheral NMDA receptors mediate NANC smooth muscle relaxation, and modulate it, in part, through extracellular production of superoxide anions, thus eliminating the relaxant effect of endogenous nitric oxide.

Intravenous anesthetics inhibit nonadrenergic noncholinergic lower esophageal sphincter relaxation via nitric oxide-cyclic guanosine monophosphate pathway modulation in rabbits

BackgroundNonadrenergic noncholinergic (NANC) nerves have important roles in the regulation of the lower esophageal sphincter (LES) motility and function. The effects of thiopental, ketamine, and midazolam on NANC LES relaxation were investigated. MethodsThe isometric tension of circular muscle strips from Japanese White rabbits was examined. The NANC relaxation was induced by KCl (30 mm) in the presence of atropine (3 × 10−6 m) and guanethidine (3 × 10−6 m). The modifications of the NANC and sodium nitroprusside (SNP; 10−5 m)-induced relaxation by the anesthetics were examined. The content of 3′,5′-cyclic guanosine monophosphate (cGMP) was measured by radioimmunoassay. ResultsThe KCl-induced relaxation was abolished by pretreating with tetrodotoxin (10−6 m). The NANC relaxation was inhibited in the presence of NG-nitro-l-arginine (L-NNA; 3 × 10−5 m), methylene blue (10−6 m), apamin (10−7 m), and glibenclamide (10−5 m). The SNP-induced relaxation was inhibited by methylene blue but was not affected by tetrodotoxin, L-NNA, apamin, or glibenclamide. Ketamine (EC50 = 8.8 × 10−5 m) and midazolam (EC50 = 4.8 × 10−6 m) suppressed the NANC response in a concentration-dependent manner, leaving SNP-induced response unchanged. Thiopental altered neither of the relaxations. cGMP content was decreased in the presence of ketamine and midazolam. ConclusionThe NANC relaxation was mediated by nitric oxide and by low-conductance calcium- and adenosine triphosphate–sensitive potassium channels of smooth muscle. The modulation of the nitric oxide–cGMP pathway was related, at least in part, to the inhibitory actions of ketamine and midazolam on the NANC LES relaxation.

Electrophysiologically identified presynaptic mechanisms underlying amylinergic modulation of area postrema neuronal excitability in rat brain slices.

Amylin, which is co-secreted together with insulin by pancreatic beta cells, is considered to be an important peptide hormone involved in the control of feeding behavior and energy homeostasis. Although the area postrema has been implicated to be a primary target of amylin, there are no studies of the mechanisms by which amylin may alter the excitability of area postrema neurons. To investigate the mechanism for amylinergic modulation of neuronal excitability, we performed perforated patch-clamp recordings from area postrema neurons in rat brainstem slices. Amylin-induced changes in excitatory responses, such as increases in the frequency of mEPSCs (miniature excitatory postsynaptic currents) and changes in the amplitude distribution of mEPSCs, were found in cells not displaying the hyperpolarization-activated cation current (I(h)). Area postrema cells displaying I(h) did not respond to amylin application. Inhibitory responses to amylin were never encountered. Bath application of CNQX (AMPA type glutamate receptor antagonist) abolished the effects of amylin. Depolarization of cells during amylin application was sufficient at 1 μM to cause action potential discharge by responding cells. We conclude that amylin receptors are located mostly on presynaptic glutamatergic terminals connecting to the area postrema neurons not displaying I(h) and amylin concentrations can increase glutamate release enough to cause cell firing. Modulation of amylinergic activity may offer a novel target to influence food intake and obesity.

Contralateral dominance of corticomuscular coherence for both sides of the tongue during human tongue protrusion: An MEG study.

Sophisticated tongue movements contribute to speech and mastication. These movements are regulated by communication between the bilateral cortex and each tongue side. The functional connection between the cortex and tongue was investigated using oscillatory interactions between whole-head magnetoencephalographic (MEG) signals and electromyographic (EMG) signals from both tongue sides during human tongue protrusion compared to thumb data. MEG-EMG coherence was observed at 14-36 Hz and 2-10 Hz over both hemispheres for each tongue side. EMG-EMG coherence between tongue sides was also detected at the same frequency bands. Thumb coherence was detected at 15-33 Hz over the contralateral hemisphere. Tongue coherence at 14-36 Hz was larger over the contralateral vs. ipsilateral hemisphere for both tongue sides. Tongue cortical sources were located in the lower part of the central sulcus and were anterior and inferior to the thumb areas, agreeing with the classical homunculus. Cross-correlogram analysis showed the MEG signal preceded the EMG signal. The cortex-tongue time lag was shorter than the cortex-thumb time lag. The cortex-muscle time lag decreased systematically with distance. These results suggest that during tongue protrusions, descending motor commands are modulated by bilateral cortical oscillations, and each tongue side is dominated by the contralateral hemisphere.

Somatosensory evoked magnetic fields following tongue and hard palate stimulation on the preferred chewing side

Although oral sensory feedback is essential for mastication, whether the cortical activity elicited by oral stimulation is associated with the preferred chewing side (PCS) is unclear. Somatosensory evoked fields were measured in 12 healthy volunteers (6 with the right side as the PCS and 6 with the left side as the PCS) following tongue and hard palate stimulation. Three components were identified over the contralateral (P40m, P60m, and P80m) and ipsilateral [P40m(I), P60m(I), and P80m(I)] hemispheres. Since no component was consistently detected across subjects, we evaluated the cortical activity over each hemisphere using the activated root-mean-square (aRMS), which was the mean amplitude of the 18-channel RMS between 10 and 150ms. For tongue stimulation, the aRMS for each hemisphere was 8.23 ± 1.55 (contralateral, mean ± SEM) and 4.67 ± 0.88 (ipsilateral)fT/cm for the PCS, and 5.11 ± 1.10 (contralateral) and 4.03 ± 0.82 (ipsilateral)fT/cm for the non-PCS. For palate stimulation, the aRMS was 5.35 ± 0.58 (contralateral) and 4.62 ± 0.67 (ipsilateral)fT/cm for the PCS, and 4.63 ± 0.56 (contralateral) and 4.14 ± 0.60 (ipsilateral)fT/cm for the non-PCS. For hard palate stimulation, the aRMS did not differ between the PCS and non-PCS, whereas for tongue stimulation, the contralateral hemisphere aRMS was significantly greater for the PCS than for the non-PCS. Thus, our results show that lateralized cortical activation was associated with the PCS for tongue, but not hard palate, stimulation; a potential reason for this may be the different sensory-inputs between these two areas, specifically the presence or absence of fine motor function.

Effects of treadmill exercise on the LiCl-induced conditioned taste aversion in rats

Studies have shown that exercise can enhance learning and memory. Conditioned taste aversion (CTA) is an avoidance behavior induced by associative memory of the taste sensation for something pleasant or neutral with a negative visceral reaction caused by the coincident action of a toxic substance that is tasteless or administered systemically. We sought to measure the effects of treadmill exercise on CTA in rats by investigating the effects of exercise on acquisition, extinction and spontaneous recovery of CTA. We made two groups of rats: an exercise group that ran on a treadmill, and a control group that did not have structured exercise periods. To condition rats to disfavor a sweet taste, consumption of a 0.1% saccharin solution in place of drinking water was paired with 0.15M LiCl (2% body weight, i.p.) to induce visceral discomfort. We measured changes of saccharin consumption during acquisition and extinction of CTA. The exercise and no-exercise groups both acquired CTA to similar levels and showed maximum extinction of CTA around 6 days after acquisition. This result indicates that exercise affects neither acquisition nor extinction of CTA. However, in testing for preservation of CTA after much longer extinction periods that included exercise or not during the intervening period, exercising animals showed a significantly lower saccharin intake, irrespective of having exercised or not during the conditioning phase of the trial. This result suggests that exercise may help to preserve aversive memory (taste aversion in this example) as evidence by the significant spontaneous recovery of aversion in exercising animals.

Development of inhibitory synaptic transmission to the superior salivatory nucleus in rats

The primary parasympathetic center of the submandibular and sublingual salivary glands is the superior salivatory (SS) nucleus, neurons of which receive excitatory (glutamatergic) and inhibitory (GABAergic and glycinergic) synaptic transmissions in rats. In the present study, to examine postnatal neural development, we focused on inhibitory transmission to the SS neurons in neonatal rats from postnatal day 2 (P2) to P14. Conventional and gramicidin-perforated whole-cell patch-clamp techniques were applied to the neurons in brainstem slices. The decay time constants of GABAergic and glycinergic postsynaptic currents (PSCs) consisted of fast (tau(fast)) and slow (tau(slow)) components. Both tau(fast) and tau(slow) of PSC components tended to become faster with development. The equilibrium potential of Cl(-) (E(Cl-)) was estimated from the reversal potentials of total PSCs (GABAergic plus glycinergic). The E(Cl-) in the P8-P14 group was significantly more negative than E(Cl-) in the P2-P7 group. Exogenous GABA application at the resting potentials produced depolarization in 83% of SS neurons at P2-P7 and accompanied the action potential in some neurons. In contrast, at P8-P14, GABA evoked hyperpolarization in 78% of SS neurons; therefore, SS neurons did not acquire mature inhibitory systems until P14. The development of SS neurons is discussed as compared with the development of peripheral salivary gland tissue and brainstem neurons that participate in oral motor and sensory functions.

Cortico-muscular synchronization by proprioceptive afferents from the tongue muscles during isometric tongue protrusion

Tongue movements contribute to oral functions including swallowing, vocalizing, and breathing. Fine tongue movements are regulated through efferent and afferent connections between the cortex and tongue. It has been demonstrated that cortico-muscular coherence (CMC) is reflected at two frequency bands during isometric tongue protrusions: the beta (β) band at 15-35Hz and the low-frequency band at 2-10Hz. The CMC at the β band (β-CMC) reflects motor commands from the primary motor cortex (M1) to the tongue muscles through hypoglossal motoneuron pools. However, the generator mechanism of the CMC at the low-frequency band (low-CMC) remains unknown. Here, we evaluated the mechanism of low-CMC during isometric tongue protrusion using magnetoencephalography (MEG). Somatosensory evoked fields (SEFs) were also recorded following electrical tongue stimulation. Significant low-CMC and β-CMC were observed over both hemispheres for each side of the tongue. Time-domain analysis showed that the MEG signal followed the electromyography signal for low-CMC, which was contrary to the finding that the MEG signal preceded the electromyography signal for β-CMC. The mean conduction time from the tongue to the cortex was not significantly different between the low-CMC (mean, 80.9ms) and SEFs (mean, 71.1ms). The cortical sources of low-CMC were located significantly posterior (mean, 10.1mm) to the sources of β-CMC in M1, but were in the same area as tongue SEFs in the primary somatosensory cortex (S1). These results reveal that the low-CMC may be driven by proprioceptive afferents from the tongue muscles to S1, and that the oscillatory interaction was derived from each side of the tongue to both hemispheres. Oscillatory proprioceptive feedback from the tongue muscles may aid in the coordination of sophisticated tongue movements in humans.

Modulation of stimulus-induced 20-Hz activity for the tongue and hard palate during tongue movement in humans.

OBJECTIVEnModulation of 20-Hz activity in the primary sensorimotor cortex (SM1) may be important for oral functions. Here, we show that 20-Hz event-related desynchronization/synchronization (20-Hz ERD/ERS) is modulated by sensory input and motor output in the oral region.nnnMETHODSnMagnetic 20-Hz activity was recorded following right-sided tongue stimulation during rest (Rest) and self-paced repetitive tongue movement (Move). To exclude proprioception effects, 20-Hz activity induced by right-sided hard palate stimulation was also recorded. The 20-Hz activity in the two conditions was compared via temporal spectral evolution analyses.nnnRESULTSn20-Hz ERD/ERS was detected over bilateral temporoparietal areas in the Rest condition for both regions. Moreover, 20-Hz ERS was significantly suppressed in the Move condition for both regions.nnnCONCLUSIONSnDetection of 20-Hz ERD/ERS during the Rest condition for both regions suggests that the SM1 functional state may be modulated by oral stimulation, with or without proprioceptive effects. Moreover, the suppression of 20-Hz ERS for the hard palate during the Move condition suggests that the stimulation-induced functional state of SM1 may have been modulated by the movement, even though the movement and stimulation areas were different.nnnSIGNIFICANCEnSensorimotor function of the general oral region may be finely coordinated through 20-Hz cortical oscillation.