Chiung-Chun Huang

Presynaptic D2 dopaminergic receptors mediate inhibition of excitatory synaptic transmission in rat neostriatum

The effect of dopamine (DA) on excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular- and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of DA (0.01-10 microM) reversibly decreases EPSP in a concentration-dependent manner and with a estimated IC50 of 0.3 microM. In addition, the inhibitory effect induced by DA at a low concentration (0.1 microM) was antagonized by sulpiride (1-10 nM), a selective D2 dopaminergic receptor antagonist. However, D1 dopaminergic receptor antagonist SKF-83566 (1-5 microM) did not affect the blocking effect by DA 0.1 microM. Based on these findings, we conclude that DA at a low concentration (< or = 0.1 microM) reduced the excitatory response of neostriatal neurons following cortical stimulation via the activation of D2, but not D1 dopaminergic receptors, located on the terminals of corticostriatal neurons.

Characterization of the anoxia-induced long-term synaptic potentiation in area CA1 of the rat hippocampus.

1 The purpose of the present study was to characterize the mechanisms underlying the anoxia‐induced long‐term potentiation (LTP) of glutamatergic synaptic transmission in the CA1 region of rat hippocampus by use of intracellular recordings in vitro. 2 In response to superfusion of an anoxic medium equilibrated with 95% N2–5% CO2, the initial slope (measured within 3u2003ms from the onset of the synaptic response) of the excitatory postsynaptic potential (e.p.s.p.) generated in the hippocampal CA1 neurones by stimulation of Schaffer collateral‐commissural afferent pathway was significantly decreased by 91.3±4.9% (n=10) within 10u2003min of the anoxic episode. The reduction of the initial slope of the e.p.s.p. was accompanied by a transient membrane hyperpolarization followed by a sustained depolarization (10.8±1.7u2003mV, n=10), along with a reduction in membrane input resistance (69.3±4.8% of control, n=10). On return to reoxygenated medium, the e.p.s.p. slope returned to the control value within 8–10u2003min and was subsequently and progessively potentiated to reach a plateau (195.6±14.7% of control, n=10) 15–20u2003min after return to control ACSF. This anoxic episode‐induced persistent potentiation of synaptic transmission lasted for more than 1u2003h and was termed anoxic LTP. 3 The anoxic episode induced a persistent potentiation of the initial slopes of both pharmacologically isolated α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazola‐propionate (AMPA) receptor‐mediated e.p.s.p. (e.p.s.p.AMPA) and N‐methyl‐D‐aspartate (NMDA) receptor‐mediated e.p.s.p. (e.p.s.p.NMDA) with a similar time course and magnitude. The sensitivity of postsynaptic neurones to NMDA (10u2003μM), but not to AMPA (10u2003μM) was also persistently potentiated following the anoxic episode. In addition, the anoxia‐induced LTP of the initial slope of e.p.s.p.AMPA was accompanied by a decrease in the magnitude of paired‐pulse facilitation (PPF; from 106.8±17.6 to 46.6±18.4%, n=6), a phenomenon which was associated with presynaptic transmitter release mechanisms. 4 The induction of the anoxic LTP is dependent on the extracellular Ca2+ concentration. The induction of the anoxic LTP was completely abolished when the external Ca2+ was removed and substituted with equimolar Mg2+. Moreover, the anoxic LTP was completely abolished in neurones intracellularly recorded with Ca2+ chelator bis‐(O‐aminophenoxy)‐N,N,N′,N′‐tetraacetic acid (BAPTA, 500u2003mM). 5 Occlusion experiments were performed to examine whether the sustained enhancement of the initial slope of the e.p.s.p. produced by tetanic stimulation and the anoxic episode share common cellular mechanisms. Three episodes of tetanic stimulation were delivered to saturate the LTP, following which a long period (15u2003min) of anoxia failed to cause a further potentiation of the initial slope of the e.p.s.p. Similarly, prior induction of anoxic LTP also significantly attenuated the subsequent synaptic potentiation induced by a high‐frequency tetanic stimulation (100u2003Hz for 1u2003s duration). These data imply that these two forms of synaptic plasticity may share a common cellular mechanism. 6 These results provide strong evidence that the generation of the anoxia‐induced LTP of glutamatergic synaptic transmission in the CA1 region of rat hippocampus probably involves both of the presynaptic and postsynaptic loci. The mechanisms underlying the persistent potentiation are likely to be attributable to an enhancement of presynaptic glutamate release and a selective upregulation of postsynaptic NMDA receptor‐mediated synaptic response through the Ca2+‐dependent processes.

Valproic acid suppresses the synaptic response mediated by the NMDA receptors in rat amygdalar slices.

The mechanism of action of the anticonvulsant drug valproic acid (VPA) was studied in rat amygdaloid slices using intracellular recording techniques. In the presence of bicuculline (20 microM), stimulation of the endopyriform nucleus evoked an excitatory postsynaptic potential (EPSP) followed by a paroxysmal depolarizing shift (PDS). Superfusion of VPA (2 mM) reversibly suppressed the PDS. Synaptic response mediated by the N-methyl-D-aspartate (NMDA) receptors (EPSPNMDA) was isolated pharmacologically by application of a solution containing nonNMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and gamma-aminobutyric acid receptor antagonist bicuculline (20 microM). VPA (0.2-10 mM) reversibly reduced the amplitude of the EPSPNMDA in a dose-dependent manner. Higher concentration of VPA (10 mM), in addition, suppressed the normal synaptic transmission. These results suggest that VPAs anticonvulsant effect is due, at least in part, to its blocking action on the EPSPNMDA.

Presynaptic inhibition of excitatory neurotransmission by lamotrigine in the rat amygdalar neurons

Lamotrigine (LAG) is a new antiepileptic drug which is licensed as adjunctive therapy for partial and secondary generalized seizures. In the present study, the mechanisms responsible for its antiepileptic effect were studied in rat amygdaloid slices using intracellular recording and whole‐cell patch clamp techniques. Bath application of LAG (50 μM) reversibly suppressed the excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) evoked by stimulating ventral endopyriform nucleus. Synaptic response mediated by the N‐methyl‐D‐aspartate (NMDA) receptor (EPSP NMDA) was isolated pharmacologically by application of a solution containing non‐NMDA receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX,10 μM) and γ‐aminobutyric acidA receptor antagonist bicuculline (20 μM). LAG produced a parallel inhibition of EPSP NMDA. Postsynaptic depolarization induced by α‐amino‐5‐methyl‐4‐isoxazole propionate (AMPA) was not altered by LAG. In addition, LAG increased the ratio of the second pulse response to the first pulse response (P2/P1), which is consistent with a presynaptic mode of action.

Sustained enhancement of NMDA receptor-mediated synaptic potential by isoproterenol in rat amygdalar slices ☆

The effect of isoproterenol (Iso) on synaptic transmission mediated by the N-methyl-D-aspartate (NMDA) receptors (EPSPNMDA) was investigated in slices of rat amygdala using intracellular recording techniques. EPSPNMDA was isolated pharmacologically by application of a solution containing the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and GABAA receptor blocker bicuculline (20 microM). Superfusion of Iso (15 microM) produced a long-lasting enhancement of EPSPNMDA. Pretreatment the slices with propranolol (10 microM) completely prevented the effect of Iso confirming the mediation by beta-adrenergic receptors. These results provide the direct evidence for adrenergic modulation of excitatory amino acid neurotransmission in the vertebrate central nervous system.

Muscarinic depression of excitatory synaptic transmission mediated by the presynaptic M3 receptors in the rat neostriatum

The effect of carbachol on the excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of carbachol (0.01-3 microM) reversibly decreases the EPSP amplitude in a concentration-dependent manner and with an estimated IC50 of 0.3 microM. While, neither the N-methyl-D-aspartate (NMDA, 100 microM)- nor (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 100 microM)-induced response was affected by carbachol (0.1 microM). In addition, the inhibitory effect induced by carbachol at a low concentration of 0.1 microM was attenuated by 4-diphenylacetoxy-N,N-methyl-piperidine (4-DAMP), a selective M3 muscarinic receptor antagonist. However, other muscarinic subtype (M1 or M2) antagonists could also block the inhibitory effect by carbachol 0.1 microM. The rank order of antagonist potency was: 4-DAMP (M3 antagonist) > methoctramine (M2 antagonist) > pirenzepine (M1 antagonist). Based on these findings, we conclude that carbachol at a low concentration (< or = 0.1 microM) reduced the excitatory response of neostriatal neurons following cortical stimulation via presynaptic M3 muscarinic receptors located on the terminals of corticostriatal neurons.

Enhancement of NMDA receptor-mediated synaptic potential by isoproterenol is blocked by Rp-adenosine 3′,5′-cyclic monophosphothioate

The intracellular mechanisms underlying the facilitatory action of isoproterenol (Iso) on the NMDA receptor-mediated synaptic potential (EPSPNMDA) was investigated in an in vitro slice preparation of rat amygdala. Intracellular recordings were made from basolateral amygdala neurons in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and picrotoxin (50 microM) which block non-NMDA and GABAA receptors, respectively. Superfusion of Iso (15 microM) produced a sustained increase in EPSPNMDA. Rp-adenosine-3,5-cyclic monophosphotioate (Rp-cAMPS), a potent inhibitor of protein kinase A (PKA) alone decreased the amplitude of EPSPNMDA below baseline values and prevented the subsequent potentiation by Iso. Forskolin, a direct activator of adenylate cyclase, mimics the effect of Iso, and Rp-cAMPS also reversed forskolin-induced enhancement of EPSNMDA. These results suggest that cAMP-dependent protein kinase mediates the enhancement of EPSPNMDA by Iso in the amygdala.

Paired-pulse depression of the N-methyl-D-aspartate receptor-mediated synaptic potentials in the amygdala.

1 An in vitro slice preparation of rat amygdala was used to study the paired‐pulse depression of the N‐methyl‐D‐aspartate (NMDA) receptor‐mediated synaptic potential e.p.s.p.NMDA. 2 The e.p.s.p.NMDA was isolated pharmacologically by applying a solution containing the non‐NMDA receptor antagonist, 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) and the γ‐aminobutyric acidA (GABAA) blocker picrotoxin and increasing the stimulus intensity. 3 When two stimuli of identical strength were applied in close succession, the second e.p.s.p.NMDA was depressed. This paired‐pulse depression was seen with interstimulus intervals of between 100 ms and 2000 ms; the maximal depression was observed at interval of 200 ms. 4 Superfusion of phaclofen or 2‐hydroxy‐saclofen inhibited the paired‐pulse depression indicating the involvement of GABAB receptors. 5 Bath applications of Ba2+ or intracellular injection of Cs+ to block post‐ but not presynaptic GABAB receptors failed to inhibit the paired‐pulse depression (PPD). 6 Incubation of slices with pertussis toxin prevented the postsynaptic hyperpolarization induced by baclofen. The PPD of e.p.s.p.NMDA, however, was not affected by pertussis toxin treatment. 7 These results suggest that GABA released by the first stimulus acts on GABAB receptors to suppress the second e.p.s.p.NMDA via mechanisms other than activation of a postsynaptic GABAB receptor‐coupled K+ conductance.

Blockade of isoproterenol-induced synaptic potentiation by tetra-9-aminoacridine in the rat amygdala ☆

The effects of tetrahydro-9-aminoacridine (THA) on beta-adrenoceptor activation-induced synaptic potentiation were studied in brain slices of the rat amygdala using intracellular recording techniques. To exclude the involvement of N-methyl-D-aspartate (NMDA) receptors, all the experiments were performed in the presence of NMDA receptor antagonist, D-APV (50 microM). Bath application of isoproterenol (Iso; 15 microM) results in a long-lasting enhancement of the amplitude of excitatory postsynaptic potentials (EPSPs) to 200 +/- 6% of baseline. Forskolin, which directly activates adenyl cyclase, produces a similar effect suggesting that Iso may act through a cyclic AMP-dependent mechanism. Pretreatment of the slices with THA (300 microM) completely abolishes the Iso- and forskolin-induced synaptic potentiation. We hypothesize that the locus of THA/beta-adrenoceptor interaction is presynaptic; the underlying mechanism is likely due to THAs depression of transmitter release via a presynaptic blockade of voltage-dependent Ca2+ channels.

Conantokin-T selectively antagonizes N-methyl-d-aspartate-evoked responses in rat hippocampal slice

This study investigated the mode of action of conantokin-T, a 21 amino acid peptide toxin isolated from the venom of the fish-hunting cone snail Conus tulipa, on excitatory synaptic transmission in rat hippocampal slices using intracellular recording techniques. Superfusion of conantokin-T (1-500 nM) specifically and irreversibly decreased the pharmacologically isolated N-methyl-D-aspartate receptor (NMDA)-mediated excitatory postsynaptic potential (EPSPNMDA) in a concentration-dependent manner but had no effect on normal excitatory synaptic transmission (EPSP). The sensitivity of postsynaptic neurons to NMDA but not to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid was also antagonized by conantokin-T pretreatment. In addition, the conantokin-T-induced depression of EPSPNMDA could be antagonized by prior treatment of hippocampal slices with either DL-2-amino-5-phosphonovaleate (10 microM) or ifenprodil (20 microM). However, 7-chlorokynurenic acid (1 microM) had no effect on the action of conantokin-T. These findings indicated that conantokin-T modulates the NMDA receptor by an interaction with its glutamate binding site and polyamine recognition site.