Electrophysiological analysis of exogenous and endogenous adenosine actions in the rat and human hippocampus in vitro

Adenosine acts in at least three different ways at A₁ receptors in order to reduce excitability and signal transfer. In the hippocampus, it causes in a steady state outward (potassium) current which is blocked by barium ions and not sensitive to voltage; (ii) an increase in the calcium and cyclic AMP-dependent current 1AHP which is responsible for a long lasting afterhyperpolarization and the accommodation of action potential firing; (iii) a presynaptic reduction of excitatory but not inhibitory transmitter release. The mechanism of this effect is unclear. It is, in contrast to the former, not sensitive to pertussis toxin. When the adenosine elicited potassium currents at the postsynaptic site are blocked, slow inward currents, normally carried by calcium, are unaffected by adenosine. A2 receptor mediated actions have not been found in the hippocampus. While the precise analysis of these effects has been done largely on rats they were all found in human hippocampal slices as well. © 1993 Wiley-Liss, Inc.     

Electrophysiological actions of felbamate on rat striatal neurones.

1. We have investigated the effects of the anticonvulsant drug, felbamate (FBM), on striatal neurones, recorded in vitro by using both intracellular and extracellular conventional recordings in slices and whole-cell recordings in acutely isolated neurones. 2. FBM, at therapeutically relevant concentrations (30-300 microM) showed multiple mechanisms of action. Like other antiepileptic drugs, FBM (30-300 microM) showed a direct inhibitory action on current-evoked firing discharge of striatal neurones. A patch-clamp analysis of this effect revealed a dose-related reduction of voltage-dependent sodium (Na+) currents (10-100 microM), with a half inhibiton dose (IC50) value of 28 microM. 3. We also tested whether FBM affected corticostriatal glutamate transmission. In control medium (1.2 mM external magnesium), both extracellularly recorded field potentials and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) evoked by cortical stimulation were no affected by bath application of 30-300 microM FBM. 4. When magnesium was removed from the perfusing solution, a procedure which reveals a N-methyl-D-aspartate (NMDA)-mediated component in the corticostriatal synaptic potential, FBM (30-300 microM) produced a dose-dependent reduction of the amplitude of both the field potential and the e.p.s.p. 5. FBM reduced the inward currents produced either by bath or by focal applications of 30 microM NMDA, finding consistent with the hypothesis that the observed reduction of the NMDA-mediated component of the synaptic potentials may be caused at postsynaptic level. 6. The reduction of the NMDA-mediated component of the synaptic transmission by FBM and its depressant effect on the voltage-dependent Na+ channels, may account for the antiepileptic action of this drug. Moreover, the pharmacological properties of FBM might render this drug interesting as a neuroprotectant agent.     

Electrophysiological actions of GABAB agonists and antagonists in rat dorso-lateral septal neurones in vitro.

1. The actions of GABAB-receptor agonists and antagonists on rat dorso-lateral septal neurones in vitro were recorded with intracellular microelectrodes. 2. In the presence of 1 microM tetrodotoxin to prevent indirect neuronal effects caused by action potential-dependent neurotransmitter release, bath application of baclofen (0.1-30 microM) or SK&F 97541 (0.01-3 microM) evoked concentration-dependent hyperpolarizations which reversed close to the potassium equilibrium potential; the EC50S were 0.55 and 0.05 microM, respectively. No significant desensitization was observed during prolonged agonist exposure (< or = 10 min). 3. Hyperpolarizations induced by baclofen were antagonized in a competitive manner by the following GABAB-receptors antagonists (calculated pA2 values in parentheses): CGP 36742 (4.0), 2-OH saclofen (4.2), CGP 35348 (4.5), CGP 52432 (6.7) and CGP 55845A (8.3). Responses to SK&F 97541 were also antagonized by CGP 55845A (pA2 = 8.4). 4. The amplitude of the late, GABAB receptor-mediated inhibitory postsynaptic potential (i.p.s.p.) was reduced by the GABAB antagonists as follows (means +/- s.e.mean): CGP 55845A (1 microM) 91 +/- 5%, CGP 52432 (1 microM) 64 +/- 5%, CGP 35348 (100 microM) 82 +/- 5%, CGP 36742 (100 microM) 76 +/- 8%, and 2-OH saclofen (100 microM) 68 +/- 3%. 5. It is concluded that neurones in the rat dorso-lateral septal nucleus express conventional GABAB receptors, which are involved in the generation of slow inhibitory postsynaptic potentials. CGP 55845A is the most potent GABAB receptor antagonist described in this brain area.     

Toluene exposure during brain growth spurt and adolescence produces differential effects on N-methyl-D-aspartate receptor-mediated currents in rat hippocampus

Toluene, an industrial organic solvent, is voluntarily inhaled as drug of abuse. Because inhibition of N-methyl-d-aspartate (NMDA) receptors is one of the possible mechanisms underlying developmental neurotoxicity of toluene, the purpose of the present study was to examine the effects of toluene exposure during two major neurodevelopmental stages, brain growth spurt and adolescence, on NMDA receptor-mediated current. Rats were administered with toluene (500 mg/kg, i.p.) or corn oil daily over postnatal days (PN) 4-9 (brain growth spurt) or PN 21-26 (early adolescence). Intracellular electrophysiological recordings employing in CA1 pyramidal neurons in the hippocampal slices were performed during PN 30-38. Toluene exposure during brain growth spurt enhanced NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) by electrical stimulation, but impaired the paired-pulse facilitation and NMDA response by exogenous application of NMDA. Toluene exposure during adolescence resulted in an increase in NMDA receptor-mediated EPSCs and a decrease in exogenous NMDA-induced currents, while lack of any effect on paired-pulse facilitation. These findings suggest that toluene exposure during brain growth spurt and adolescence might result in an increase in synaptic NMDA receptor responsiveness and a decrease in extrasynaptic NMDA receptor responsiveness, while only toluene exposure during brain growth spurt can produce presynaptic modulation in CA1 pyramidal neurons. The functional changes in NMDA receptor-mediated transmission underlying developmental toluene exposure may lead to the neurobehavioral disturbances.     

Local anaesthetic actions of cocaine: effects on excitatory and inhibitory synaptic responses in the hippocampus in vitro.

1. The basis for the proconvulsant action of cocaine was investigated in the CA1 region of the rat hippocampal slice in vitro. 2. Superfusion with 100 microM cocaine depressed inhibitory and excitatory postsynaptic potentials recorded intracellularly from CA1 pyramidal neurones; both types of potentials were inhibited to an equal extent. When inhibition was assessed using extracellular recording of population spike responses before and after conditioning impulses, there did not appear to be any selective effect upon either recurrent or feed-forward gamma-aminobutyric acid (GABA)ergic inhibition. 3. Not all responses showed equivalent sensitivity to the local anaesthetic actions of cocaine. In particular, the antidromic population spike evoked by stimulation of the alveus was significantly more sensitive than the presynaptic fibre spike elicited by stimulation of stratum radiatum. 4. The rate of interictal spiking in hippocampus, induced by penicillin and increased potassium in the perfusion medium, was depressed by superfusion with cocaine in the range 5-100 microM. 5. These results suggest that cocaine does not have a selective depressant effect upon inhibitory pathways in the CA1 region of the hippocampus. Although the hippocampus shows epileptiform activity following systemic administration of local anaesthetics such as cocaine in the intact rat, this effect may not reflect a direct hippocampal site of drug action.     

Desensitization of N-methyl-D-aspartate receptors in neurons dissociated from adult rat hippocampus.

Desensitization of the N-methyl-D-aspartate (NMDA) receptor-channel complex was studied in isolated rat hippocampal neurons using a fast drug application system. 1) Desensitization rate was slower at more negative membrane potentials and when external [Ca2+] was lowered. 2) In the presence of 10 microM glycine, 2-amino-5-phosphonovalerate neither induced desensitization nor prevented recovery from it. 3) Preincubation in 500 microM aspartate or 10 microM glycine alone elicited desensitization only weakly or not at all. 4) Aspartate appeared to bind at its receptor site in the absence of glycine, and vice versa. It is proposed that, for the NMDA receptor, channel opening is necessary for the occurrence of desensitization and, thus, that desensitization involves structural changes in the channel-lining section of the protein rather than the glycine or NMDA binding sites.     

Estrogen receptor-mediated actions of polyphenolic catechins in vivo and in vitro.

Recent investigations have demonstrated that polyphenolic catechins inhibit breast cancer cell proliferation and tumor growth. However, the ER-mediated effects of the three predominant catechins (EGCG, ECG, and EGC) have not been extensively examined in vitro or in vivo. Therefore, EGCG, ECG, and EGC were examined for their ability to compete with [(3)H]-17beta-estradiol ([(3)H]-E(2)) for binding to ERalpha and ERbeta and to elicit reporter gene activity in MCF-7 human breast cancer cells transiently transfected with either chimeric ERalpha or ERbeta. EGCG and ECG displaced [(3)H]-E(2) from GST-hERalphadef (D, E, and F domains of human ERalpha fused to GST) or from full-length human ERbeta. Additionally, only EGCG elicited Gal4-hERalphadef and Gal4-mERbetadef-mediated reporter gene expression (EC(50) values: 28 and 19 micro M, respectively) in MCF-7 cells cotransfected with a Gal4-regulated luciferase reporter gene. In cotreatment experiments, EGCG (1-50 micro M) and ECG (1 micro M) decreased E(2)-induced (1 nM) ERbeta-mediated gene expression 35-50%. In vivo, no catechin induced estrogenic responses (uterine weight or uterine peroxidase activity) in immature C57BL/6 mice. However, when mice were cotreated with E(2) (10 micro g/kg/day, 3 days) and either EGCG (30 and 50 mg/kg/day, 3 days) or ECG (50 mg/kg/day, 3 days), uterine peroxidase activity was increased 2.3-fold above that elicited by E(2) alone. In conclusion, EGCG and ECG bind to ERalpha and ERbeta, but only EGCG elicited ER-mediated gene expression in vitro. However, both of these compounds moderately increased E(2)-inducible responses in vivo.     

The novel antipsychotic drug lurasidone enhances N-methyl-D-aspartate receptor-mediated synaptic responses

N-Methyl-D-aspartate (NMDA) receptor (NMDAR) hypofunction has been postulated to contribute to the cognitive deficit of schizophrenia. In this study, we examined the effect of lurasidone (Latuda; Dainippon Sumitomo Pharma Co. Ltd., Tokyo, Japan), a newly approved atypical antipsychotic drug (APD), on NMDAR synaptic function in rat frontal cortical pyramidal neurons. In vivo administration of lurasidone produced a significant and selective enhancement of NMDAR-mediated synaptic responses and surface expression of NR2A and NR2B subunits. Lurasidone has high affinity for serotonin 5-HT(1A), 5-HT(2A), and 5-HT(7) receptors and dopamine D(2) receptors. In vivo administration of the 5-HT(7) receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2 -(2-(4-methyl-1-piperidinyl)ethyl)pyrrolidine (SB-269970) mimicked the enhancing effect of lurasidone on NMDAR responses, whereas the D(2) receptor antagonist haloperidol failed to do so. Previous studies have found that short-term administration of lurasidone reverses the cognitive impairment induced by subchronic administration of phencyclidine (PCP), an NMDAR noncompetitive antagonist. In this study, we found that lurasidone, as well as the prototypical atypical APD clozapine, restored NMDAR-mediated synaptic responses to normal levels in the PCP model of schizophrenia. These results suggest that NMDAR is the potential key molecular target of lurasidone, possibility via antagonizing 5-HT(7) receptors, which is consistent with evidence that 5-HT(7) receptor antagonism contributes to cognitive enhancement by atypical APDs in patients with schizophrenia.     

Nebracetam (WEB 1881FU) prevents N-methyl-D-aspartate receptor-mediated neurotoxicity in rat striatal slices.

The effects of nebracetam were investigated on N-methyl-D-aspartate (NMDA) receptor- and voltage-operated Ca2+ channels (VOCC)-mediated neural dysfunction by directly monitoring the real-time dynamics of dopamine released from rat striatal slices. Nebracetam (10(-5) and 10(-4) M) completely protected against striatal dopaminergic impairment induced by L-glutamate and NMDA, respectively. BAY K-8644-evoked striatal dysfunction was not blocked by nebracetam (10(-4) M). Therefore, nebracetam seems to produce a neuroprotective action by interacting, at least in part, with NMDA receptor-operated Ca2+ channels.     

Electrophysiological actions of phencyclidine in hippocampal slices from the rat

The electrophysiological effects of phencyclidine were examined in the CA region of slice preparations of the hippocampus of the rat. The only direct effects of phencyclidine on monosynaptic transmission in the Schaffer collateral/commissural input to the CA region were local anesthetic-like actions, that were observed at concentration greater than or equal to 50 microM, and were characterized by decreases in the amplitude of both orthodromically- and antidromically-evoked potentials. However, phencyclidine, at concentrations greater than 0.5 microM, increased the apparent sensitivity of slices to perfusion with norepinephrine (NE), such that a previously subthreshold concentration of NE significantly increased the magnitude of the population spike response through an action at beta-adrenergic receptors. This action of phencyclidine appears to reflect its ability to block the high affinity uptake of NE into nerve terminals in the slice preparation. Because these responses were observed at concentrations of phencyclidine which are found in the brains of abusers of phencyclidine, they may play a significant role in the overall neuropharmacological response to phencyclidine.     

Opioid receptor regulation of muscarinic acetylcholine receptor-mediated synaptic responses in the hippocampus

A common feature of many synapses is their regulation by neurotransmitters other than those released from the presynaptic terminal. This aspect of synaptic transmission is often mediated by activation of G protein coupled receptors (GPCRs) and has been most extensively studied at amino acid-mediated synapses where ligand gated receptors mediate the postsynaptic signal. Here we have investigated how opioid receptors modulate synaptic transmission mediated by muscarinic acetylcholine receptors (mAChRs) in hippocampal CA1 pyramidal neurones. Using a cocktail of glutamate and gamma-amino-butyric acid (GABA) receptor antagonists a slow pirenzepine-sensitive excitatory postsynaptic potential (EPSP(M)) that was associated with a small increase in cell input resistance could be evoked in isolation. This response was enhanced by the acetylcholine (ACh) esterase inhibitor physostigmine (1 microM) and depressed by the vesicular ACh transport inhibitor vesamicol (50 microM). The mu-opioid receptor agonists DAMGO (1-5 microM) and etonitazene (100 nM), but not the delta- and kappa-opioid receptor selective agonists DTLET (1 microM) and U-50488 (1 microM), potentiated this EPSP(M) (up to 327%) without affecting cell membrane potential or input resistance; an effect that was totally reversed by naloxone (5 microM). In contrast, postsynaptic depolarizations and increases in cell input resistance evoked by carbachol (3 microM) were unaffected by DAMGO (1-5 microM) but were abolished by atropine (1 microM). Taken together these data provide good evidence for a mu-opioid receptor-mediated presynaptic enhancement of mAChR-mediated EPSPs in hippocampal CA1 pyramidal neurones.     

Electrophysiological actions of alfentanil: intracellular studies in the rat locus coeruleus neurones.

1. The electrophysiological effects of alfentanil on 156 neurones of the rat locus coeruleus were investigated by use of intracellular recordings from the in vitro brain slice preparation. 2. Bath application of alfentanil (5-100 nM) reversibly decreased the firing rate of all neurones tested in a dose-dependent manner, with an IC50 4.1.nM. 3. Based on inhibition of the spontaneous firing rate, alfentanil was 22 times more potent than morphine. 4. At 100 nM, alfentanil produced a complete inhibition of firing of all neurones tested (n = 62); the inhibition was accompanied by a membrane hyperpolarization 17.0 +/- 0.8 mV (range 6.1-30.3 mV, n = 62) and a reduction in input resistance 26.4 +/- 1.7% (range 6.5-53%, n = 51). 5. The effects of alfentanil were antagonized by naloxone, with a dissociation equilibrium constant of 2.7 +/- 0.4 nM (n = 6). 6. The reversal potential for the alfentanil-induced hyperpolarization was -110 +/- 2 mV (n = 9), which is approximately the potassium equilibrium potential. 7. The alfentanil-induced hyperpolarization was blocked by caesium chloride and barium chloride. 8. These results indicate that alfentanil binds to mu-opioid receptors on the cell membrane of neurones of the locus coeruleus. This leads to opening of the inward-going rectification potassium channels, resulting in the observed hyperpolarization of the membrane.     

Electrophysiological evidence for the implication of cholecystokinin in the modulation of the N-methyl-D-aspartate response by sigma ligands in the rat CA3 dorsal hippocampus

Previous studies from our laboratory have demonstrated that low doses of selective sigma () ligands potentiate the response of pyramidal neurones to N-methyl-D-aspartate (NMDA) in the CA₃ region of the rat dorsal hippocampus. It has also been found that the neuropeptide cholecystokinin (CCK) is involved in the effects induced by ligands on colonic motility. The present experiments were undertaken to determine if this interaction is also present in the rat dorsal hippocampus. Using microiontophoresis and in vivo extracellular recordings of rat CA₃ dorsal hippocampus pyramidal neurones, we assessed the effects of CCK_A and CCK_B receptor antagonists on the potentiation of the NMDA response, induced by the intravenous administration of low doses of the ligands 1,3-di(2-tolyl)guanidine (DTG), (+)-pentazocine and JO-1784. The potentiation of the NMDA response induced by these ligands was abolished by the selective CCK_A receptor antagonist SR 27897, but not by the CCK_B antagonist Cl-988. CCK-8S, applied with a low current, insufficient to induce by itself an increase of the firing activity, markedly potentiated the response of NMDA without affecting significantly that of quisqualate. SR 27897, but not Cl-988, significantly reduced the potentiation of the NMDA response by CCK-8S. These results suggest the existence of a functional interaction between CCK and receptor-mediated effects in the dorsal hippocampus.     

Comparison of peptide and nonpeptide receptor-mediated responses in rat tail artery.

Chronic uremia and metabolic acidosis impair vascular responses to norepinephrine (NE) and also cause multiple metabolic defects in skeletal muscle. These studies were conducted to determine whether decreased vascular responsiveness resulted from putative second messenger metabolism. Tail arteries were used from rats with metabolic acidosis or nonacidotic uremia and from normal controls. In normal arteries, the maximal responses were the same for arginine vasopressin (AVP), NE, and mixtures of the two agonists, suggesting that the two receptor types use the same transduction mechanism. Nonetheless, qualitative differences exist between AVP- and NE-induced responses: (a) Concentration-response curves are steeper for AVP than for NE, (b) EC50 values are similar for AVP between inositol phosphates (IP assays) and contraction, but not for NE, and (c) arteries from rats with metabolic acidosis or uremia show selective blunting of biochemical and contractile responses to NE but not to AVP. We conclude that these metabolic derangements selectively affect alpha-adrenergic receptors, but not AVP receptors or the transduction mechanism leading to contraction.     

The actions of 3-aminopropanephosphinic acid at GABAB receptors in rat hippocampus.

The actions of 3-aminopropanephosphinic acid (APPA) were examined using whole-cell patch-clamp recording in rat hippocampal slice. In recordings from neurons in subfield CA1 of slices from young (2-4 weeks) and adult (greater than 2 month) rats, APPA (0.5-50 microM) produced membrane hyperpolarization and outward current under voltage-clamp. APPA also inhibited excitatory postsynaptic potentials with an IC50 of 2.3 microM, and reduced inhibitory postsynaptic potentials at concentrations from 0.1 to 1 microM. The hyperpolarizing and synaptic depressant effects of APPA were reduced by 2-OH-saclofen an antagonist at the B-type receptor for the neurotransmitter gamma-aminobutyric acid (GABA). In this preparation APPA exhibited potencies similar to those previously reported for the GABAB receptor agonist baclofen. APPA was much less effective in inhibiting synaptic transmission measured using field potential recordings. The observations made with whole-cell patch-clamp recording indicate that in hippocampus APPA acts as a potent agonist at presynaptic GABAB receptors associated with both excitatory and inhibitory synapses, and also activates postsynaptic GABAB receptors.     

N-methyl-D-aspartate increases cytoplasmic free calcium in mouse hippocampus.

The effect of N-methyl-D-aspartate (NMDA) and L-glutamate on the concentration of intracellular free calcium (Cai) and on uptake of the calcium was determined in microsacs and synaptosomes isolated from mouse brain. L-Glutamate and NMDA increased Cai in hippocampal microsacs but had little or no effect on Cai in microsacs, isolated from cortex or cerebellum or in synaptosomes. N-Methyl-D-aspartate also increased uptake of calcium, measured using 45Ca into hippocampal microsacs. The EC50 values for NMDA-stimulated increases in Cai and uptake of calcium in microsacs were about 30 microM. Maximum responses were observed with 100 microM NMDA. Increases in Cai stimulated by NMDA were dependent on extracellular calcium, indicating that NMDA increased Cai in microsacs by increasing conductance through an NMDA receptor-operated cation channel, rather than by releasing calcium from intracellular stores. The NMDA antagonists, 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), 2-amino-5-phosphonopentanoic acid (AP-5), magnesium and zinc blocked responses to NMDA. This demonstrates NMDA-mediated effects on ion flux in a cell-free preparation from brain. This preparation may be useful for study of the in vitro effects of drugs or toxins on NMDA receptors in brain.     

The actions of diazepam and phenytoin on a low dose penicillin epileptiform focus in the anaesthetised rat.

1 A technique is described for the induction of an acute short-lived epileptiform focus by the local injection of benzyl penicillin 25 units into the cerebral cortex of rats anaesthetized with urethane. The effects of anticonvulsants on this microfocus have been studied. 2 Phenytoin 20 mg/kg and diazepam 1 mg/kg intravenously significantly reduced the frequency of the normally occurring surface positive electrocorticogram (ECoG) plateaux and caused a greater decrease in the frequency of the epileptiform spikes normally associated with these plateaux. 3 A novel effect was seen in that both anticonvulsant drugs significantly reduced the amplitude of the epileptiform spike within the focus, as well as causing an even larger decrease in the amplitude of the propagated spike. The contribution of the anticonvulsant drug effect on the focal spike to its effect on propagated activity is discussed.     

5-HT4 receptor-mediated modulation of 5-HT release in the rat hippocampus in vivo.

1. In the present study, the ability of the 5-hydroxytryptamine, receptor (5-HT4 receptor) to modulate the release of 5-HT in the hippocampus of freely-moving rats was investigated by the in vivo microdialysis technique. 2. The 5-HT4 receptor agonist, renzapride (1.0-100 microM, administered via the microdialysis probe) increased extracellular hippocampal levels of 5-HT in concentration-dependent manner (approximately 200% maximal increase). The ability of renzapride (100 microM, administered via the microdialysis probe) to elevate extracellular levels of 5-HT remained in the presence of the selective 5-HT reuptake blocker, paroxetine (1.0 microM, administered via the microdialysis probe). Furthermore, another 5-HT4 receptor agonist 5-methoxytryptamine (5-MeOT; 10 microM, administered via the microdialysis probe, in the presence of the non-5-HT4 5-HT receptor antagonists pindolol (10 microM) and methysergide (10 microM)) maximally elevated extracellular levels of 5-HT by approximately 450% in the rat hippocampus. The elevation of extracellular 5-HT levels induced by either renzapride (100 microM) or 5-MeOT (10 microM) was completely prevented by combined administration of the selective 5-HT4 receptor antagonist, GR113808 (100 nM, administered via the microdialysis probe). GR113808 (100 nM, administered via the microdialysis probe) administered alone, however, reduced extracellular hippocampal 5-HT levels by some 60%. 3. Systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.1 mg kg-1, s.c.) reduced extracellular levels of 5-HT in the rat hippocampus by approximately 40%. Prior administration of 8-OH-DPAT (0.1 mg kg-1, s.c.), with an associated reduction of extracellular hippocampal 5-HT levels by approximately 40-50%, however, failed to prevent a subsequent elevation of extracellular levels of 5-HT induced by renzapride (100 microM, administered via the microdialysis probe). 4. Systemic administration of the 5-HT4 receptor agonist, renzapride (0.25 and 1.0 mg kg-1, i.p.) increased extracellular levels of 5-HT in the hippocampus in a dose-dependent manner. The higher dose of renzapride increasing extracellular 5-HT levels by some 200%. The selective 5-HT4 receptor antagonist, GR125487D (1.0-100 micrograms kg-1, i.p.) caused a dose-dependent reduction in extracellular levels of 5-HT in the hippocampus (maximally approximately 80% reduction). Prior administration of GR125487D (10 micrograms kg-1, i.p.) prevented the elevation of extracellular levels of 5-HT induced by renzapride (1.0 mg kg-1, i.p.). 5. In conclusion, the present study provides evidence that activation of the 5-HT4 receptor facilitates 5-HT release in the rat hippocampus in vivo.     

Electrophysiological actions of the plant alkaloid 6-benzoylheteratisine in rat hippocampal slices

The effects of the Aconitum alkaloid 6-benzoylheteratisine on neuronal activity was investigated in the in vitro slice preparation of rat hippocampus by extracellular recording of the stimulus-evoked population spike. 6-Benzoylheteratisine (0.01-10 μΜ) depressed the orthodromic and antidromic population spike in a concentration-dependent manner.The action of the drug was activity-dependent. The latency of onset of the inhibition was accelerated when the frequency of electrical stimulation had been increased. Furthermore, the effect of 6-benzoylheteratisine was evaluated in two different models of epileptiform activity induced either by blockade of GABA receptors by bicuculline (10 μΜ) or by a nominal Mg²⁺-free bathing medium. Due to the activity-dependent mode of action, this drug effectively reduced the number and the size of the synaptically evoked population spikes in the presence of bicuculline or nominal Mg²⁺-free bathing medium, respectively. Received: 23 September 1996 / Accepted: 2 January 1997