Altering cannabinoid signaling during development disrupts neuronal activity

In adult cortical tissue, recruitment of GABAergic inhibition prevents the progression of synchronous population discharges to epileptic activity. However, at early developmental stages, GABA is excitatory and thus unable to fulfill this role. Here, we report that retrograde signaling involving endocannabinoids is responsible for the homeostatic control of synaptic transmission and the resulting network patterns in the immature hippocampus. Blockade of cannabinoid type 1 (CB1) receptor led to epileptic discharges, whereas overactivation of CB1 reduced network activity in vivo. Endocannabinoid signaling thus is able to keep population discharge patterns within a narrow physiological time window, balancing between epilepsy on one side and sparse activity on the other, which may result in impaired developmental plasticity. Disturbing this delicate balance during pregnancy in either direction, e.g., with marijuana as a CB1 agonist or with an antagonist marketed as an antiobesity drug, can have profound consequences for brain maturation even in human embryos.

Persistent epileptiform activity induced by low Mg2+ in intact immature brain structures

We have determined the properties of seizures induced in vitro during the first postnatal days using intact rat cortico-hippocampal formations (CHFs) and extracellular recordings. Two main patterns of activity were generated by nominally Mg2+-free ACSF in hippocampal and cortical regions: ictal-like events (ILEs) and late recurrent interictal discharges (LRDs). They were elicited at distinct developmental periods and displayed different pharmacological properties. ILEs were first observed in P1 CHFs 52 +/- 7 min after application of low-Mg2+ ACSF (frequency 1.5 +/- 0.3 h-1, duration 86 +/- 3 s). There is a progressive age-dependent maturation of ILEs characterized by a decrease in their onset and an increase in their frequency and duration. ILEs were abolished by d-APV and Mg2+ ions. From P7, ILEs were followed by LRDs that appeared 89 +/- 8 min after application of low-Mg2+ ACSF (frequency approximately 1 Hz, duration 0.66 s, amplitude 0.31 +/- 0.03 mV). LRDs were no longer sensitive to d-APV or Mg2+ ions and persisted for at least 24 h in low-Mg2+ or in normal ACSF. ILEs and LRDs were synchronized in limbic and cortical regions with 10-40 ms latency between the onsets of seizures. Using a double chamber that enables independent superfusion of two interconnected CHFs, we report that ILEs and LRDs generated in one CHF propagated readily to the other one that was being kept in ACSF. Therefore, at a critical period of brain development, recurrent seizures induce a permanent form of hyperactivity in intact brain structures and this preparation provides a unique opportunity to study the consequences of seizures at early developmental stages.

Morphology of CA3 non-pyramidal cells in the developing rat hippocampus

Although several investigations have shown that the local GABAergic circuit in the rat hippocampus is functional very early in development, this result has not been yet completed by the investigation of the full dendritic and axonal arborization of the neonatal interneurones. In the present study, intracellular injection of biocytin was used to assess the branching pattern of interneurones in the hippocampal CA3 region of rat between 2 and 6 days of age. Based on their dendritic morphology, the biocytin-filled interneurones were divided into four classes: bipolar, stellate, pyramidal-like and fusiform interneurones. About half of the biocytin-filled neonatal interneurones exhibited dendritic or somatic filopodial processes. The axonal arbors of the filled-interneurones were widely spread into the CA3 region, and in four out of nine cases extended beyond the CA3 region to branch into the CA1 region. These results show that, despite immature features, the filopodial processes, the hippocampal interneurones are well developed early in development at a time when their target cells, the pyramidal neurones, are still developing. These observations are consistent with a trophic role that GABA may play early in development.

Characterization in cultured cerebellar granule cells and in the developing rat brain of mRNA variants for the NMDA receptor 2C subunit

N-Methyl-D-aspartate (NMDA) receptors are heteromeric structures resulting from the association of at least two distantly related subunit types, NR1 and one of the four NR2 subunits (NR2A-NR2D). When associated with NR1, the NR2 subunits impose specific properties to the reconstituted NMDA receptors. Although the NR1 mRNAs are expressed in the majority of central neurons, the NR2 subunits display distinct patterns of expression in the developing and adult rat brain. The NR2C subunit is barely expressed in the rat forebrain, whereas its expression increases substantially in the granule cells in the course of cerebellar development. We have identified novel NR2C splice variants in cultured cerebellar granule cells as well as in the developing cerebellum. When compared with the prototypic NR2C mRNA, these variants carry one (NR2Cb) or two (NR2Cd) insertions or a deletion (NR2Cc) and encode putative NR2C polypeptides that terminate between the third and fourth membrane segments or between the first and second membrane segments. RT-PCR analysis and in situ hybridization show that expression of the splice variants is developmentally regulated, both in the cerebellum and in the hippocampus. Electrophysiological recordings and microfluorimetry emissions in transfected human embryonic kidney 293 cells indicate that the NR2Cb variant, when expressed in combination with NR1, does not contribute to the formation of functional receptor channels. The significance of theses findings is discussed.

The establishment of GABAergic and glutamatergic synapses on CA1 pyramidal neurons is sequential and correlates with the development of the apical dendrite

We have performed a morphofunctional analysis of CA1 pyramidal neurons at birth to examine the sequence of formation of GABAergic and glutamatergic postsynaptic currents (PSCs) and to determine their relation to the dendritic arborization of pyramidal neurons. We report that at birth pyramidal neurons are heterogeneous. Three stages of development can be identified: (1) the majority of the neurons (80%) have small somata, an anlage of apical dendrite, and neither spontaneous nor evoked PSCs; (2) 10% of the neurons have a small apical dendrite restricted to the stratum radiatum and PSCs mediated only by GABA(A) receptors; and (3) 10% of the neurons have an apical dendrite that reaches the stratum lacunosum moleculare and PSCs mediated both by GABA(A) and glutamate receptors. These three groups of pyramidal neurons can be differentiated by their capacitance (C(m) = 17.9 +/- 0.8; 30.2 +/- 1.6; 43.2 +/- 3.0 pF, respectively). At birth, the synaptic markers synapsin-1 and synaptophysin labeling are present in dendritic layers but not in the stratum pyramidale, suggesting that GABAergic peridendritic synapses are established before perisomatic ones. The present observations demonstrate that GABAergic and glutamatergic synapses are established sequentially with GABAergic synapses being established first most likely on the apical dendrites of the principal neurons. We propose that different sets of conditions are required for the establishment of functional GABA and glutamate synapses, the latter necessitating more developed neurons that have apical dendrites that reach the lacunosum moleculare region.

Late embryonic expression of AMPA receptor function in the CA1 region of the intact hippocampus in vitro

Studies in slices suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic currents are not present in CA1 (Cornu ammonis) pyramidal neurons at birth (P0). We have re-examined this issue in the rat intact hippocampal formation (IHF) in vitro. Injections of biocytin or carbocyanine show that the temporo-ammonic, commissural and Schaffer collateral pathways are present at birth in the marginal zone of CA1. Electrical stimulation of these pathways evoked field excitatory postsynaptic potentials (fEPSPs) in the marginal zone of CA1 from embryonic day 19 (E19) to postnatal day 9 (P9). These fEPSPs are mediated by synaptic AMPA receptors as they are reduced or completely blocked by: (i) tetrodotoxin; (ii) high divalent cation concentrations; (iii) the adenosine A1 receptor agonist CPA; (iv) anoxic episodes; (v) the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI-53655) or the mixed AMPA-kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX). The amplitude of the fEPSPs is also reduced by D(-)-2-amino-5-phosphonopentanoic acid (D-APV) and its duration is increased by bicuculline suggesting the participation of N-methyl-D-aspartate (NMDA) and GABAA (gamma-aminobutyric acid) receptors. Finally, AMPA receptor-mediated fEPSPs are also recorded in P0 slices, but they are smaller and more labile than in the IHF. Our results suggest that in embryonic CA1 neurons, glutamate acting on AMPA receptors already provides a substantial part of the excitatory drive and may play an important role in the activity-dependent development of the hippocampus. Furthermore, the IHF may be a convenient preparation to investigate the properties of the developing hippocampus.

Contributions of AMPA and GABA(A) receptors to the induction of NMDAR-dependent LTP in CA1

The contributions of (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and gamma-aminobutyric acid (GABA[A]) receptors in the induction of long-term potentiation (LTP) have been studied in the CA1 region of the rat hippocampus. The results suggest that: (1) in physiological conditions, AMPARs are necessary for the induction of N-methyl-D-aspartate receptor (NMDAR)-dependent LTP since LTP cannot be elicited in the presence of the AMPAR antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Although a NMDAR-dependent LTP occurs in the presence of a GABA(A) antagonist and high concentrations of divalents cations, blockade of AMPARs leads to a voltage-dependent calcium channels (VDCC)-dependent LTP since its induction is blocked by nifedipine and not by APV. (2) The bicarbonate-induced GABA(A) receptor-mediated depolarizing response is not necessary in the induction of NMDAR-dependent or VDCC-dependent LTP since induction of these two types of LTP were not blocked by acetazolamide or in a nominally bicarbonate-free solution.

Effects of chronic diazepam treatment on pre- and postsynaptic 5-HT1A receptors in the rat brain

Biochemical and electrophysiological approaches were used to assess possible changes in 5-HT1A receptors in the rat brain after long-term treatment with an anxiolytic benzodiazepine. Rats were treated with diazepam (2 mg/kg i.p. daily) during 14 days and then untreated for 1 day (protocol A) or 5 days (protocol C) until they were killed for in vitro investigations on 5-HT1A receptors. In addition, other rats (protocol B) received the same 14-day treatment with diazepam, followed by 1 mg/kg of the drug on days 15 and 16, and 0.5 mg/kg on days 17 and 18, and were killed 24 h after the last injection. In vitro binding and quantitative autoradiographic experiments with [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) showed that the characteristics of 5-HT1A receptor binding sites in the hippocampus and the dorsal raphe nucleus were not significantly altered by the administration of diazepam under the treatment protocols A, B and C. Furthermore, in vitro electrophysiological recordings of serotoninergic neurons in the dorsal raphe nucleus of brain stem slices revealed no modification in the sensitivity of somatodendritic 5-HT1A autoreceptors in rats treated with diazepam according to the protocols A and B. However, under the conditions of protocol C, the potency of 8-OH-DPAT to depress the firing rate of serotoninergic neurons was significantly enhanced, as expected of a hypersensitivity of somatodendritic 5-HT1A autoreceptors. These data support the hypothesis that some functional changes in these receptors could occur during benzodiazepine withdrawal. However, they do not support the idea of a reduced anxiolytic efficacy of 5-HT1A receptor agonists as a result of prior treatment with a benzodiazepine.

The calcium-dependent transient inactivation of recombinant NMDA receptor-channel does not involve the high affinity calmodulin binding site of the NR1 subunit

N-methyl-D-aspartate (NMDA) receptor function can be regulated by direct binding of calmodulin to a low and high affinity (C1 exon cassette) site in the C-terminal region of the NR1 subunit. To evaluate the involvement of the high affinity binding site in the transient inactivation of the NMDA receptor-channels by intracellular calcium, several splice variants of the NR1 subunit have been individually co-transfected with the NR2A subunit in HEK 293 cells. The transient Ca2+ induced inactivation (40-50%) of the heteromeric receptors was similar whether the NR1 variants contained (NR1-1a, 1b) or lacked (NR1-2a, 2b, 4a, 4b) the C1 exon cassette bearing the high affinity binding site for calmodulin. This demonstrates that this site is not involved in the Ca2+ dependent transient inactivation of NMDA receptors.

Redox modulation of synaptic responses and plasticity in rat CA1 hippocampal neurons

Effects of redox reagents on excitatory and inhibitory synaptic responses as well as on the bidrectional plasticity of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses were studied in CA1 pyramidal neurons in rat hippocampal slices. The oxidizing agent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM) did not affect AMPA, GABAA or GABAB receptor-mediated synaptic responses or the activation of presynaptic metabotropic receptors. However, DTNB irreversibly decreased (by approximately 50%) currents evoked by focal application of NMDA. DTNB also decreased the NMDA component of the EPSC. The reversal potential of NMDA currents and the Mg2+ block were not modified. In the presence of physiological concentrations of Mg2+ (1.3 mM), DTNB did not affect the NMDA receptor-dependent induction of long-term potentiation (LTP) or long-term depression (LTD) expressed by AMPA receptors. In contrast, DTNB fully prevented LTP and LTD induced and expressed by NMDA receptors. Plasticity of NMDA receptor-mediated synaptic responses could be reinstated by the reducing agent tris-(2-carboxyethyl) phosphine (TCEP, 200 microM). These results suggest that persistent, bidirectional changes in synaptic currents mediated by NMDA receptors cannot be evoked when these receptors are in an oxidized state, whereas NMDA-dependent LTP and LTD are still expressed by AMPA receptors. Our observations raise the possibility of developing therapeutic agents that would prevent persistent excitotoxic enhancement of NMDA receptor-mediated events without blocking longterm modifications of AMPA receptor-mediated synaptic responses, thought to underlie memory processes.

Epileptiform activity but not synaptic plasticity is blocked by oxidation of NMDA receptors in a chronic model of temporal lobe epilepsy

Simultaneous extracellular recordings were performed in stratum radiatum and stratum pyramidale of hippocampal slices 7 days following unilateral intracerebroventricular injections of kainic acid. In this ex vivo experimental model of human temporal lobe epilepsy, stimulation of the surviving commissural fibres in stratum radiatum produced graded epileptiform activity in the CA1 area. The oxidizing reagent 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) acting at NMDA receptors redox sites decreases NMDA receptor-mediated responses by half and suppresses evoked epileptiform discharges. We have examined the effect of DTNB on NMDA-dependent bidirectional synaptic plasticity and EPSP/spike coupling. DTNB treatment did not prevent either long-term potentiation induced by tetanic stimulation or long-term depression induced by low frequency stimulation of field EPSPs. Application of DTNB alone did not induce EPSP/spike dissociation. However, both high and low frequency stimulations induced EPSP/spike potentiation indicating that neurons had a high probability to discharge in synchrony. These results suggest that oxidizing reagents may provide novel antiepileptic treatments since they decrease NMDA-dependent evoked epileptiform activity but do not interfere with either NMDA-dependent synaptic plasticity or the probability of synchronous discharge.

Enhanced NMDAR-dependent epileptiform activity is controlled by oxidizing agents in a chronic model of temporal lobe epilepsy

1. Graded N-methyl-D-aspartate receptor (NMDAR)-dependent epileptiform discharges were recorded from ex vivo hippocampal slices obtained from rats injected a week earlier with an intracerebroventricular dose of kainic acid. Intracellular recordings from pyramidal cells of the CA1 area showed that glutamate NMDAR actively participated in synaptic transmission, even at resting membrane potential. When NMDAR were pharmacologically isolated, graded burst discharges could still be evoked. 2. The oxidizing reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM, 15 min) suppressed the late part of the epileptiform burst that did not recover after wash but could be reinstated by the reducing agent tris (2-carboxyethyl) phosphine (TCEP, 200 microM, 15 min) and again abolished with the NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-APV). 3. Pharmacologically isolated NMDAR-mediated responses were decreased by DTNB (56 +/- 10%, mean +/- SD, n = 6), an effect reversed by TCEP. 4. When only the fast glutamateric synaptic component was blocked, NMDA-dependent excitatory postsynaptic potentials (EPSPs) could be evoked despite the presence of underlying fast and slow inhibitory postsynaptic potentials (IPSPs). DTNB decreased EPSPs to 48 +/- 12% (n = 5) of control. 5. Since a decrease of the NMDAR-mediated response by +/- 50% is sufficient to suppress the late part of the burst, we suggest that epileptiform activity can be controlled by manipulation of the redox sites of NMDAR. Our observations raise the possibility of developing new anticonvulsant drugs that would spare alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-R (AMPAR)-mediated synaptic responses and decrease NMDAR-mediated synaptic transmission without blocking it completely.

Electrophysiological, biochemical, neurohormonal and behavioural studies with WAY-100635, a potent, selective and silent 5-HT1A receptor antagonist

Although considerable progress has been made in characterising the 5-HT1A receptor using agonists, partial agonists or non-selective antagonists, further studies of 5-HT1A receptor function have been hindered by the lack of highly selective antagonists. The term 'silent' antagonist has been used for such compounds in order to distinguish them unequivocally from several 5-HT1A receptor partial agonists which were initially designated 'antagonists'. In this report we provide a comprehensive review of the biochemical, pharmacological and behavioural properties of the first potent, selective and silent 5-HT1A receptor antagonist, WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2- pyridinyl)cyclohexanecarboxamide trihydrochloride). WAY-100635 had an IC50 (displacement of specific [3H]8-OH-DPAT binding to 5-HT1A receptors in the rat hippocampus) of 1.35 nM and was > 100-fold selective for the 5-HT1A site relative to a range of other CNS receptors. [3H]WAY-100635 was also characterised as the first 5-HT1A antagonist radioligand, displaying the same regional distribution of binding sites as [3H]8-OH-DPAT in rat brain. As would be expected for the binding of an antagonist to a G-protein-coupled receptor, the Bmax of [3H]WAY-100635 specific binding was consistently 50-60% greater than that of the agonist radioligand, [3H]8-OH-DPAT. Mn2+, but not guanine nucleotides, inhibited [3H]WAY-100635-specific binding. [3H]WAY-100635 was also shown to bind selectively to brain 5-HT1A receptors in vivo, following intravenous administration to mice. In vitro electrophysiological studies demonstrated that WAY-100635 had no 5-HT1A receptor agonist actions, but dose-dependently blocked the effects of agonists at both the postsynaptic 5-HT1A receptor in the CA1 region of the hippocampus, and the somatodendritic 5-HT1A receptor located on dorsal raphe 5-HT neurones. In vivo, WAY-100635 also dose-dependently blocked the ability of 8-OH-DPAT to inhibit the firing of dorsal raphe 5-HT neurones, and to induce the '5-HT syndrome', hypothermia, hyperphagia and to elevate plasma ACTH levels. In the mouse light/dark box anxiety model, WAY-100635 induced anxiolytic-like effects. WAY-100635 had no intrinsic effect on cognition in the delayed-matching-to-position model of short-term memory in the rat, but reversed the disruptive effects of 8-OH-DPAT on motor motivational performance. These data clearly demonstrate that WAY-100635 is the first potent, selective and silent 5-HT1A receptor antagonist. Furthermore, [3H]WAY-100635 is the first antagonist radioligand to become available for 5-HT1A receptor binding studies both in vitro and in vivo. The positive effects of WAY-100635 in an anxiety model also indicate that a postsynaptic 5-HT1A receptor antagonist action may contribute to the anxiolytic properties of 5-HT1A receptor partial agonists.

NMDA receptor redox sites: are they targets for selective neuronal protection?

NMDA receptors play a central role in neuronal plasticity and in several pathological situations. Transient activation of this receptor triggers long-term potentiation, whereas sustained activation leads to cell death. Evidence for control of this activity by a redox site in cell cultures, brain tissues and in recombinant NMDA receptors are discussed by Henri Gozlan and Yehezkel Ben-Ari. The characteristics of this modulation and the consequences of redox state modifications on NMDA-mediated events are examined in vitro under physiological and pathological conditions. Since metabolic disorders enhance NMDA receptor function, the redox site could constitute a new target for selectively preventing in vivo the deleterious consequences of overactivation without blocking neuronal plasticity mediated by NMDA receptors.

In CA1 hippocampal neurons, the redox state of NMDA receptors determines LTP expressed by NMDA but not by AMPA receptors

1. Using extracellular recording techniques in the CA1 region of the rat hippocampus, we have evaluated the effects of the redox reagents 5,5O-dithiobis-2-nitrobenzoic acid (DTNB) and tris (carboxyethyl) phosphine (TCEP) on long-term potentiation (LTP) expressed by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors. In physiological conditions a high-frequency stimulation (HFS) of Schaffer collateral-commissural fibers induced a LTP expressed by a persistent increase (73 +/- 13%, mean +/- SE, n = 8/10) of AMPA field potentials (LTPA). In the presence of 10 microM of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and reduced concentration of Mg2+ (0.1 mM) to boost NMDA receptors, the HFS induced LTP of NMDA field potentials (LTPN; 62 +/- 11%, n = 8/10). 2. The thiol-oxidizing reagent DTNB (200 microM) reduced, by 46 +/- 5% (n = 24), NMDA-receptor field potentials (NMDA-FP), and this effect could not be reversed by extensive washing. The disulfide-reducing agent TCEP (200 microM) slightly increased AMPA-FP and reversed the DTNB-induced inhibition of NMDA-FP. 3. DTNB (200 microM, 10 min), and TCEP (200 microM, 20 min), had no effect on AMPA-FP (98 +/- 3% and 101 +/- 5%, respectively, n = 12). 4. DTNB (200 microM, 15 min) did not prevent the induction or expression of LTPA (-12 and -5%, respectively, n = 8/8). Similar results were observed with TCEP (200 microM, 20 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple forms of long-term potentiation and multiple regulatory sites of N-methyl-D-aspartate receptors: role of the redox site

Long-term potentiation (LTP) is a form of synaptic plasticity thought to be involved in learning and memory. Although extensively studied, mainly in the CA1 region of the hippocampus, the mechanisms underlying the induction and expression of LTP are poorly elucidated. This is probably due to the fact that LTP is not a unique process and indeed recent studies have shown that several forms of LTP could be generated depending on the experimental conditions. Furthermore, LTP is generally associated with a long-lasting increase of the synaptic efficacy of AMPA receptors but an increasing number of data also suggested that NMDA receptors could be potentiated as well. NMDA receptor responses are modulated by a large number of extracellular and intracellular events, providing additional possibilities for the generation of LTP. The role of these different modulatory sites of the NMDA receptor and their relation with LTP are reviewed with a particular attention to the redox site which seems to be a selective target to distinguish between AMPA and NMDA-LTP.

The selective 5-HT1A antagonist radioligand [3H]WAY 100635 labels both G-protein-coupled and free 5-HT1A receptors in rat brain membranes

The tritiated derivative of the novel silent 5-HT1A receptor antagonist WAY 100635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide] was tested as a potential radioligand of 5-HT1A receptors in the rat brain. Binding assays with membranes from various brain regions showed that [3H]WAY 100635 specifically bound to a homogeneous population of sites, with a Kd of 0.10 nM. The regional distribution of [3H]WAY 100635 specific binding sites, as assessed in membrane binding assays and by autoradiography of labelled brain sections, superimposed exactly over that of 5-HT1A receptors specifically labelled by [3H]8-hydroxy-2-(di-n-propylamino) tetralin ([3H]8-OH-DPAT). Furthermore, the positive correlation (r = 0.96) between the respective pKi values of a large series of ligands as inhibitors of the specific binding of [3H]WAY 100635 and [3H]8-OH-DPAT in hippocampal membranes indicated that their pharmacological properties were similar. Nevertheless, marked differences also existed between [3H]8-OH-DPAT and [3H]WAY 100635 specific binding, as the former was inhibited by 1-100 microM GTP and GppNHp, whereas the latter was enhanced by these guanine nucleotides. In contrast, Mn2+ (1-10 mM) increased the specific binding of [3H]8-OH-DPAT, but inhibited that of [3H]WAY 100635. Treatment of membranes with N-ethylmaleimide (1-5 mM) markedly reduced their capacity to specifically bind [3H]8-OH-DPAT, but slightly increased (at 1 mM) or did not affect (at 5 mM) their [3H]WAY 100635 specific binding capacity. Finally, the Bmax of [3H]WAY 100635 specific binding sites was regularly 50-60% higher than that of [3H]8-OH-DPAT in the same membrane preparations from various brain regions (hippocampus, septum, cerebral cortex). These data are compatible with the idea that whereas [3H]8-OH-DPAT only binds to G-protein-coupled 5-HT1A receptors, [3H]WAY 100635 is a high affinity ligand of both G-protein-coupled and free 5-HT1A receptor binding subunits in brain membranes.

5-HT3 receptor antagonism by anpirtoline, a mixed 5-HT1 receptor agonist/5-HT3 receptor antagonist

1. The aim of this study was to provide evidence that anpirtoline, which is an agonist at 5-HT1B and 5-HT1D receptors and also displays submicromolar affinity for 5-HT1A recognition sites, in addition, acts as an antagonist at 5-HT3 receptors. 2. In radioligand binding studies on rat brain cortical membranes, anpirtoline inhibited specific binding of [3H]-(S)-zacopride to 5-HT3 receptor recognition sites (pKi: 7.53). 3. In N1E-115 neuroblastoma cells in which [14C]-guanidinium was used as a tool to measure cation influx through the 5-HT3 receptor channel, the 5-HT-induced influx was concentration-dependently inhibited by anpirtoline. In this respect, anpirtoline mimicked other 5-HT3 receptor antagonists; the rank order of potency was ondansetron > anpirtoline > metoclopramide. 4. The concentration-response curve for 5-HT as a stimulator of [14C]-guanidinium influx was shifted to the right by anpirtoline (apparent pA2: 7.78). 5. In urethane-anaesthetized rats, anpirtoline inhibited (at lower potency than zacopride and tropisetron) the 5-HT- or phenylbiguanide-induced bradycardia (Bezold-Jarisch reflex), but did not induce this reflex by itself. 6. Intravenous infusion of cisplatin in the domestic pig caused a consistent emetic response which was antagonized by anpirtoline. 7. It is concluded that anpirtoline, which was previously characterized as a 5-HT1 receptor agonist also proved to be a 5-HT3 receptor antagonist in several experimental models and, hence, exhibits a unique pattern of properties at different 5-HT receptors.

Selective in vivo labelling of brain 5-HT1A receptors by [3H]WAY 100635 in the mouse

The novel selective 5-HT1A receptor antagonist radioligand [3H]WAY 100635 ([O-methyl-3H]N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridyl)cyclohexane-carboxamide) was injected i.v. to mice in an attempt to label in vivo central 5-HT1A receptors. Although 5 min after the i.v. injection of [3H]WAY 100635 (4-7.6 muCi per mouse) the amount of tritium found in the whole brain only accounted for 1.5-1.8% of the injected radioactivity, regional differences in 3H accumulation already corresponded to those of 5-HT1A receptor density. Optimal data were obtained 1 h after [3H]WAY 100635 injection as the distribution of 3H in brain was exactly that of 5-HT1A receptor binding sites in mouse brain sections labelled in vitro with [3H]WAY 100635. In particular, high level of labelling was found in the lateral septum, gyrus dentatus and CA1 area of Ammon's horn in the hippocampus, dorsal raphe nucleus and entorhinal cortex. No labelling was found in he substantia nigra, and 3H accumulated in the cerebellum represented only 12-14% of that found in the hippocampus. Pretreatment with various drugs indicated that only 5-HT1A receptor ligands were able to decrease the accumulation of 3H in all the brain areas examined except in the cerebellum. Assuming that only non-specific binding took place in the latter structure, it was possible to calculate the ID50 values of 5-HT1A receptor agonists (8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), S 14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphthyl+ ++)piperazine) and S 20499 ((+)-4-[N-(5-methoxy-chroman-3-yl)-N-propylamino]butyl-8- azaspiro-(4,5)-decane-7,9-dione)) and antagonists (spiperone, (-)-tertatolol, (+)-WAY 100135 (N-tert-butyl-3,4-(2-methoxyphenyl)piperazin-1-yl-2-phenyl- propanamide)) as inhibitors of 3H accumulation in the hippocampus of [3H]WAY 100635-injected mice. Comparison of these values with the in vitro affinity of the same ligands for hippocampal 5-HT1A receptors revealed marked variations in the capacity of 5-HT1A receptor agonists and antagonists to reach the brain when injected via the subcutaneous route in mice.

Anoxic LTP is mediated by the redox modulatory site of the NMDA receptor

1. The effects of redox reagents, 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) and tris(carboxyethyl)phosphine (TCEP), on anoxia-induced long-term potentiation (LTP) were investigated in CA1 hippocampal neurons using extracellular recording techniques. Experiments were performed in the presence of 0.1 mM MgCl2 and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to pharmacologically isolate N-methyl-D-aspartate (NMDA) receptor-mediated responses. 2. DTNB (200 microM), a thiol oxidizing reagent, reduces by 52 +/- 9% (mean +/- SE) (n = 9/9) NMDA-receptor field potentials evoked by electrical stimulation of Schaffer collaterals and this effect could not be reversed by extensive washing. Nearly the same reduction of the initial response was obtained with different concentrations of DTNB (100 and 500 microM), but the time required to reach the maximal inhibition was concentration-dependent. 3. In keeping with an earlier study oxygen and glucose deprivation for 2-3 min induced a long-term potentiation (LTP) of the NMDA receptor response (+65 +/- 16%, n = 4/6). This potentiation was reversed by DTNB (100-500 microM) (-47 +/- 18%; n = 4/4) and the initial LTP could not be restored upon extensive washing of the drug. 4. TCEP (200 microM), a reagent which reduces S-S bond, amplified the electrically evoked NMDA-receptor EPSP (+27 +/- 12%; n = 3). In addition, TCEP (200 microM), nearly completely reversed the effect of DTNB (200 microM) on anoxia-induced LTP (+56 +/- 19%; n = 3/3). Preliminary results also indicate that TCEP occlude anoxic-LTP (n = 3/4). 5. Following DTNB (200 microM) treatment, oxygen and glucose deprivation did not generate anoxic LTP and extensive washing did not restore a potentiated NMDA field potential. 6. These observations strongly suggest that the redox site of the NMDA receptor is involved in the induction and the maintenance of the anoxic LTP of the NMDA receptor-mediated response in CA1.

Anoxic LTP sheds light on the multiple facets of NMDA receptors

Hippocampal neurones in the CA1 region have become a model system to study the mechanisms of long-term potentiation (LTP) and memory processes. The CA1 region is also highly vulnerable to ischaemic or anoxic episodes which induce a selective and delayed degeneration of pyramidal neurones. In CA1 neurones, anoxic episodes generate a novel form of LTP to which we refer as anoxic LTP. In common with tetanic LTP, the induction of anoxic LTP is voltage- and NMDA receptor-dependent. However, in contrast with tetanic LTP, the expression of anoxic LTP is mediated exclusively by NMDA receptors. These observations suggest that anoxic-ischaemic episodes trigger a switch in favour of NMDA receptor-operated synaptic transmission. We suggest that the multiple forms of NMDA receptor-dependent LTPs are determined by extracellular and intracellular modulatory sites of this receptor.

Production and characterization of polyclonal antibodies recognizing the intracytoplasmic third loop of the 5-hydroxytryptamine1A receptor

The portion of the complementary DNA encoding the third intracellular loop of the rat 5-hydroxytryptamine1A (serotonin) receptor was subcloned into the vector pGEX-KG and expressed in Escherichia coli as a fusion protein coupled with the glutathione S-transferase of Schistosoma japonicum. The fusion protein was purified on a glutathione-agarose affinity column and used to immunize rabbits for the production of polyclonal anti-5-hydroxytryptamine1A receptor antibodies. Enzyme-linked immunosorbent assay revealed that antibodies were produced as early as one month after the first injection of the fusion protein, and immune response plateaued at a maximum after the third (monthly) booster injection. These antibodies only marginally affected the specific binding of [3H]8-hydroxy-2-(di-n-propyl-amino) tetralin to solubilized and membrane bound 5-hydroxytryptamine1A receptors, and did not interfere with serotonin-induced inhibition of forskolin-stimulated adenylate cyclase negatively coupled to 5-hydroxytryptamine1A receptors in rat hippocampal membranes. However, antibodies were able to immunoprecipitate 5-hydroxytryptamine1A receptor binding sites solubilized from rat hippocampal membranes. The distribution of immunoautoradiographic labelling and immunohistochemical staining of rat brain sections exposed to the antibodies raised against the fusion protein superimposed to that of 5-hydroxytryptamine1A receptor binding sites labelled by specific radioligands, with marked enrichment in the limbic areas (dentate gyrus and CA1 area in the hippocampus, lateral septum, entorhinal cortex) and the anterior raphe nuclei. The differential cellular location of immunoreactivity within the hippocampus (where dendritic fields but not pyramidal cell somas were immunostained) and the median raphe nucleus (where the plasmic membrane of somas was strongly immunoreactive) suggests that the addressing of 5-hydroxytryptamine1A receptors might differ from one neuronal cell type to another.

[NMDA receptor and long-term potentiation]

In order to evaluate the role of the NMDA receptor redox site in long-term potentiation (LTP), we have investigated the effects of two redox reagents, 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) and tris(carboxyethyl)phosphine (TCEP) on the induction and expression of various forms of LTP. DTNB a thiol-oxidizing agent, irreversibly reduces by 50% NMDA receptor EPSP. In the presence of DTNB, the induction of tetanic and anoxic LTP are prevented. When tetanic or anoxic LTP were generated first, DTNB completely reverses the potentiation and TCEP a disulfide-reducing agent restores LTP to its initial level. These redox agents have no effect on AMPA synaptic transmission and did not significantly modify the induction and the expression of tetanic AMPA-LTP. These results suggest that thiol-oxidizing compounds might be useful for the treatment of cerebral ischemia.

NMDA redox site modulates long-term potentiation of NMDA but not of AMPA receptors

We have compared the effects of redox drugs on long-term potentiation mediated by AMPA or NMDA receptors. A reducing and an oxidizing agent had no effect on long-term potentiation mediated by AMPA receptors. In contrast, the induction of long-term potentiation mediated by NMDA receptors was prevented by a thiol oxidizing drug and restored by a disulfide reducing agent.

Interactions of lesopitron (E-4424) with central 5-HT1A receptors: in vitro and in vivo studies in the rat

Previous studies have shown that the 5-HT1A receptor ligand, lesopitron (E-4424, 2-[4-[4-(4-chloro-1-pyrazolyl)butyl]-1-piperazinyl]pyrimidine), exerts potent anxiolytic-like effects in rodents and monkeys (Costall et al., 1992, J. Pharmacol. Exp. Ther. 262, 90). In an attempt to determine whether these effects are really mediated through the interaction of lesopitron with central 5-HT1A receptors, we investigated the agonistic and/or antagonistic nature of this interaction under in vitro and in vivo conditions in the rat. In vitro binding and autoradiographic studies with [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and [3H]lesopitron as radioligands confirmed that lesopitron binds to 5-HT1A receptors in the rat brain with a relatively high affinity (pKi = 7.35). As expected of a full agonist at postsynaptic 5-HT1A receptors, lesopitron (IC50 = 125 nM) inhibited forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes to the same extent as 5-HT, and this effect was preventable by potent 5-HT1A receptor antagonists such as (-)-tertatolol, (-)-propranolol and N-tert-butyl-3,4-(2-methoxyphenyl)piperazin-1-yl-2-phenyl- propanamide ((+)-WAY 100135). As previously shown for agonists acting at the somato-dendritic 5-HT1A autoreceptors in the dorsal raphe nucleus, lesopitron inhibited the firing of serotoninergic neurons both in vitro (in brainstem slices, IC50 = 120 nM) and in vivo (in chloral hydrate-anaesthetized rats, ID50 = 35 micrograms/kg i.v.), and this effect was preventable by (-)-tertatolol. Interestingly, the inhibition of the discharge due to lesopitron lasted for only a few minutes both in vitro and in vivo whereas the anxiolytic-like properties of this drug lasted for hours after a single injection in mice (Costall et al., 1992). In addition, the doses required for the stimulation of pre- and postsynaptic 5-HT1A receptors were markedly higher than those producing significant anxiolytic-like effects in rodents (Costall et al., 1992). It is therefore unlikely that the anxiolytic-like properties of lesopitron involve its stimulatory action at central 5-HT1A receptors.

Differential effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on various 5-HT receptor binding sites in the rat brain

The effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), an alkylating agent producing irreversible blockade of various membrane bound receptors in brain, were investigated on four different types of serotonin receptors, 5-HT1A, 5-HT1B, 5-HT2A and 5-HT3, in various brain regions in the rat. In addition, the fate of central benzodiazepine- and "R"-zacopride-specific binding sites was also examined in rats treated with EEDQ. Membrane binding assays and/or quantitative autoradiography with appropriate radioligands indicated that EEDQ inactivated 5-HT1A, 5-HT1B and 5-HT2A sites, but was poorly active on 5-HT3, benzodiazepine and "R" sites. Among the receptors affected by EEDQ, hippocampal 5-HT1A sites were the most sensitive to the alkylating agent (ID50 approximately 1 mg/kg i.p.), followed by the cortical 5-HT2A (ID50 approximately 3 mg/kg i.p.) and the striatal 5-HT1B (ID50 approximately 6 mg/kg i.p.) sites. Pretreatment by selective ligands partially protected hippocampal 5-HT1A sites from irreversible inactivation by EEDQ (10 mg/kg i.p.) with the following order of efficacy: WAY 100635 > spiperone > BMY 7378 > ipsapirone. Similarly, pretreatment by spiperone (5 mg/kg i.p.) also reduced the ability of EEDQ to inactivated cortical 5-HT2A receptors. Analyses of the time-course recovery of respective binding sites after EEDQ administration showed that the turnover rate of 5-HT1A sites did not significantly differ in the dorsal raphe nucleus and in various forebrain areas (hippocampus, septum, cerebral cortex; half-life: approximately 4 days), but was lower than that of cortical 5-HT2A sites (half-life: 2.9 days).

Central pre- and postsynaptic 5-HT1A receptors in rats treated chronically with a novel antidepressant, cericlamine

Biochemical and electrophysiological approaches were used to assess the possible changes in 5-hydroxytryptamine (serotonin) 5-HT1A receptors in the rat brain after a long-term treatment with cericlamine [2-(3,4-dichlorobenzyl)-2-dimethylamino-1-propanol], a novel serotonin reuptake inhibitor with antidepressant properties. Possible changes in other serotonin receptor binding sites (5-HT2A, 5-HT2C and 5-HT3) were also investigated after this treatment. Cericlamine was injected for 2 weeks at a dose (16 mg/kg i.p., twice daily) that ensured complete prevention of 4-methyl-alpha-ethyl-meta-tyramine-induced depletion of brain serotonin. In vitro binding and quantitative autoradiographic studies showed that neither 5-HT1A, 5-HT2A, 5-HT2C nor 5-HT3 receptor binding sites in various brain areas were affected by the 14-day treatment with cericlamine. Although forskolin-stimulated adenylate cyclase activity was significantly increased in hippocampal homogenates from cericlamine-treated rats, the reduction in this enzymatic activity due to 5-HT1A receptor stimulation by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was unchanged in these animals as compared with controls. In contrast, in vitro and in vivo electrophysiological recordings of serotoninergic neurons in the dorsal raphe nucleus revealed a clearcut functional desensitization of somatodendritic 5-HT1A autoreceptors. Thus the potency of 8-OH-DPAT and ipsapirone to depress the firing rate of these neurons in brain stem slices was significantly reduced after the 2-week treatment with cericlamine. In vivo, the potency of an injection of cericlamine to inhibit the discharge of serotoninergic neurons was also markedly less in rats that had been pretreated for 2 weeks with this drug as compared with controls. However, the inhibitory effects of systemically injected 8-OH-DPAT and ipsapirone on the electrical activity of serotoninergic neurons were as pronounced in cericlamine-treated rats as in controls. In addition, the reduction in serotonin synthesis due to an acute treatment with 8-OH-DPAT (0.1 or 0.3 mg/kg s.c.) was not significantly different in both groups of rats. These data support the idea that postsynaptic (in the hippocampus) and somatodendritic (in the dorsal raphe nucleus) 5-HT1A receptors are differently regulated in the rat brain, because only the latter receptors desensitized after a long-term blockade of serotonin reuptake by cericlamine. They also suggest that the inhibitory influence of systemically administered direct 5-HT1A agonists such as 8-OH-DPAT and ipsapirone on the electrical and metabolic activity of serotoninergic neurons does not result solely from the stimulation of somatodendritic 5-HT1A autoreceptors.

Characterisation of the non-5-HT3 high-affinity 'R' binding site for (R)-zacopride in brain and other tissues

Previous studies showed that whereas the potent 5-HT3 receptor antagonist (S)-[3H]zacopride only labels 5-HT3 receptor binding sites, the (R)-enantiomer, (R)-[3H]zacopride, labels these receptors and another class of high-affinity binding sites, named the R sites, in membranes from the rat cerebral cortex and NG 108-15 clonal cells (Kidd et al., Eur. J. Pharmacol. 211, 133, 1992). Further studies of R sites revealed that they existed not only in the cerebral cortex but also in various other areas of the rat brain and spinal cord. In addition, R sites were also found in post-mortem human brain tissues. Both in the rat and in man, the regional distribution of central R sites was markedly different from that of 5-HT3 receptors specifically labelled with (S)-[3H]zacopride. Under appropriate conditions for the specific labelling of R sites (with (R)-[3H]zacopride in the presence of 1.0 microM ondansetron to saturate 5-HT3 receptor binding sites--and 0.1 mM mianserin for the determination of non-specific binding), these R sites were also found in rat peripheral tissues (intestine > spleen > kidney > testicles = liver > adrenals > lung > heart). At least in the kidney and the liver, the pharmacological profile of R sites corresponded exactly to that found in NG 108-15 cells. R sites were also detected in membranes from C6 glioma cells and glial cells cultured from the whole cortex of new born rats. In contrast, no specific binding of (R)-[3H]zacopride to R sites could be found in membranes from N1E-115 neuroblastoma cells. Conversely, 5-HT3 receptors could be labelled by (S)-[3H]zacopride in the latter cells but not in C6 glioma and cultured glial cells. As expected from their glial location, the density of R sites increased in the rat hippocampus lesioned with kainic or ibotenic acid to induce local gliosis. In contrast, the density of hippocampal 5-HT3 receptors was unchanged in lesioned rats. Finally, the determination of the apparent molecular size of R sites by radiation inactivation gave a value (approximately 30 kDa) which was significantly lower than that of 5-HT3 receptor binding sites in the rat entorhinal cortex (40 kDa) and NG 108-15 cells (57 kDa). All these data clearly showed that R sites and 5-HT3 receptors are different molecular species. Whether R sites mediate the 5-HT3 receptor-unrelated actions of (R)-zacopride deserves further investigations.

SR 57227A: a potent and selective agonist at central and peripheral 5-HT3 receptors in vitro and in vivo

SR 57227A (4-amino-(6-chloro-2-pyridyl)-1 piperidine hydrochloride) is a novel compound with high affinity and selectivity for the 5-HT3 receptor. The compound had affinities (IC50) varying between 2.8 and 250 nM for 5-HT3 receptor binding sites in rat cortical membranes and on whole NG 108-15 cells or their membranes in vitro, assayed under various conditions with [3H]S-zacopride or [3H]granisetron as radioligand. Like reference 5-HT3 receptor agonists, SR 57227A stimulated the uptake of [14C]guanidinium into NG 108-15 cells in the presence of substance P (EC50 = 208 +/- 16 nM) and contracted the isolated guinea-pig ileum (EC50 = 11.2 +/- 1.1 microM), effects that were antagonised by the 5-HT3 receptor antagonist tropisetron. The agonist effect of SR 57227A was also observed in vivo, as the compound elicited the Bezold-Jarisch reflex in anesthetised rats (ED50 = 8.3 micrograms/kg i.v.), an effect that was blocked by tropisetron and R,S-zacopride, but not by methysergide. When injected unilaterally into the mouse striatum, SR 57227A, like 2-methyl-5-HT, elicited contralateral turning behaviour which was antagonised by ondansetron. Furthermore, microiontophoretic application of SR 57227A markedly inhibited the firing rate of rat cortical neurones, an effect antagonised by tropisetron. Finally, in contrast to reference 5-HT3 agonists, SR 57227A bound to 5-HT3 receptors on mouse cortical membranes after systemic administration (ED50 = 0.39 mg/kg i.p. and 0.85 mg/kg p.o.). These results suggest that SR 57227A is a potent agonist at peripheral and central 5-HT3 receptors, both in vitro and in vivo. In view of the dearth of 5-HT3 receptor agonists which are capable of crossing the blood-brain barrier, SR 57227A may be useful in the characterisation of the neuropharmacological effects produced by the stimulation of these receptors.

5-HT3 receptors in the rat central nervous system are mainly located on nerve fibres and terminals

Autoradiographic and membrane binding studies with [3H](R,S)- or [3H](S)-zacopride were performed in combination with lesions using various neurotoxins in an attempt to identify which neuronal cell types are endowed with 5-HT3 receptors in the rat central nervous system. Lesions of noradrenergic (by DSP-4), dopaminergic (by 6-hydroxydopamine) and serotonergic (by 5,7-dihydroxytryptamine) systems had little effect generally on the density of 5-HT3 receptors labelled with [3H](R,S)- or [3H](S)-zacopride in various regions of the brain and the spinal cord. The only exception was the amygdala where a significant loss (approximately -20%) of 5-HT3 receptors labelled by [3H](R,S)-zacopride was associated with the selective lesion of serotonergic fibres by 5,7-dihydroxytryptamine. Microinjection of kainic or ibotenic acid into the dorsal and ventral hippocampus reduced the density of 5-HT1A receptors labelled with [3H]8-OH-DPAT (approximately -45%) as expected from their known location on intrinsic neuronal cell bodies and/or dendrites. In contrast, the same lesion did not affect 5-HT3 receptors, suggesting their location on fibres 'en passage'. At the spinal level, 5-HT3 receptors were found to exist on primary afferent fibres terminating within the superficial layers of the dorsal horn, as shown by the marked reduction in the local autoradiographic labelling by [3H](S)-zacopride after either dorsal rhizotomy (-81%) or neonatal capsaicin treatment (-72%). These data suggest that 5-HT3 receptors in the central nervous system are generally located presynaptically on nerve terminals or fibres of non-monoaminergic neurones.

(-)Tertatolol is a potent antagonist at pre- and postsynaptic serotonin 5-HT1A receptors in the rat brain

The potential 5-HT1A antagonist properties of the beta-antagonist tertatolol were assessed using biochemical and electrophysiological assays in the rat. (+/-) Tertatolol bound with high affinity (Ki = 38 nM) to 5-HT1A sites labelled by [3H]8-OH-DPAT in hippocampal membranes. The (-)stereoisomer (Ki = 18 nM) was about 50-fold more potent than the (+)stereoisomer (Ki = 864 nM) to inhibit the specific binding of [3H]-8-OH-DPAT. As expected of a 5-HT1A antagonist, (-)tertatolol prevented in a concentration-dependent manner (Ki = 24 nM) the inhibitory effect of 8-OH-DPAT on forskolin-stimulated adenylate cyclase activity in rat hippocampal homogenates. Furthermore in vivo pretreatment with (-)tertatolol (5 mg/kg s.c.) significantly reduced the inhibitory influence of 8-OH-DPAT (0.3 mg/kg s.c.) on the accumulation of 5-hydroxytryptophan in various brain areas after the blockade of aromatic L-amino acid decarboxylase by NSD-1015 (100 mg/kg i.p.). In vitro (in brainstem slices; Ki approximately 50 nM) and in vivo (in chloral hydrate anaesthetized rats; ID50 approximately 0.40 mg/kg i.v.), (-)tertatolol prevented the inhibitory effects of the 5-HT1A receptor agonists 8-OH-DPAT, ipsapirone and lesopitron on the firing rate of serotoninergic neurones within the dorsal raphe nucleus. In about 25% of these neurones, the basal firing rate was significantly increased by (-)tertatolol (up to +47% in vitro, and +30% in vivo). These data indicate that (-)tertatolol is a potent competitive antagonist at both pre (in the dorsal raphe nucleus)-and post (in the hippocampus)-synaptic 5-HT1A receptors in the rat brain.

New methoxy-chroman derivatives, 4[N-(5-methoxy-chroman-3-yl)N- propylamino]butyl-8-azaspiro-(4,5)-decane-7,9-dione [(+/-)-S 20244] and its enantiomers, (+)-S 20499 and (-)-S 20500, with potent agonist properties at central 5-hydroxytryptamine1A receptors

The potential interaction of the new methoxy-chroman derivatives: (+/-)-S 20244 (4-[N-(5-methoxy-chroman-3-yl)N-propylamino]butyl-8-azaspiro- (4,5)-decane-7,9-dione) and its enantiomers (+)-S 20499 and (-)-S 20500 with central 5-hydroxytryptamine1A (5-HT1A) receptors was assessed using biochemical and electrophysiological tests in the rat. In vitro binding assays revealed that these drugs bound with high affinity to 5-HT1A sites in hippocampal membranes (Ki: 0.19 nM for (+)-S 20499, 0.95 nM for (-)-S 20500 and 0.35 nM for the racemate (+/-) S 20244). As seen with the prototypical 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin, (+/-)-S 20244, (+)-S 20499 and (-)-S 20500 inhibited forskolin-activated adenylate cyclase in hippocampal homogenates with potencies corresponding to their respective affinities for 5-HT1A sites. The maximal inhibitory effect of the chroman derivatives was not additive with that of 8-hydroxy-2-(di-n- propylamino)tetralin and could be competitively reduced by 5-HT1A antagonists such as (-)-propranolol and (+/-)-tertatolol. Electrophysiological recordings within the dorsal raphe nucleus both in vitro (in brain-stem slices) and in vivo (in chloral hydrate anesthetized rats) showed that (+)-S 20499, (+/-)-S 20244 and (-)-S 20500 induced, in that order of (decreasing) potency, a dose-dependent reduction in the spontaneous firing of serotoninergic neurons. In vitro, as well as in vivo, the inhibitory influence of the chroman derivatives on the discharge frequency of serotoninergic neurons could be competitively antagonized by (+/-)-tertatolol. Finally, oral administration of increasing doses of the most potent enantiomer, (+)-S 20499, induced a marked reduction in the rate of 5-HT turnover, without affecting that of dopamine, in various brain areas. All these biochemical and electrophysiological data indicate that (+)-S 20499 is a highly potent agonist at both presynaptic (i.e., somatodendritic) and postsynaptic 5-HT1A receptors in the rat brain.

Quantification of 5-hydroxytryptamine1A receptors in the cerebellum of normal and X-irradiated rats during postnatal development

5-Hydroxytryptamine1A receptors were studied in rats during the first postnatal month in the normal cerebellum and in the granule cell-deprived cerebellum produced by X-irradiation at postnatal day 5. Quantitative autoradiographic studies on sagittal sections of cerebellar vermis, using [1251]BH-8-MeO-N-PAT as radioligand or specific anti-receptor antibodies, revealed that 5-hydroxytryptamine1A receptors existed in the molecular/Purkinje cell layer but at variable density from one lobule to another. Thus, in both normal and X-irradiated rats, the posterior lobules were more heavily labelled than the anterior ones, and the density of 5-hydroxytryptamine1A sites decreased progressively in all the cerebellar folia down to hardly detectable levels at postnatal day 21. However, the intensity of labelling remained higher at postnatal day 8 and postnatal day 12 in X-irradiated rats than in age-paired controls. Measurements of [3H]8-OH-DPAT specific binding to membranes from whole cerebellum confirmed that the density of 5-hydroxytryptamine1A sites per mg membrane protein (Bmax) was higher in X-irradiated animals than in age-paired controls. However, on a "per cerebellum" basis, no significant difference could be detected between the total number of 5-hydroxytryptamine1A sites, which progressively increased in both control and X-irradiated animals during the first postnatal month. These results therefore show that 5-hydroxytryptamine1A receptors are not located on developing granule cells. The progressive decrease in 5-hydroxytryptamine1A receptor density during the first postnatal month did not reflect a transient expression of 5-hydroxytryptamine1A receptors in the cerebellum of newborn rats, but resulted from the progressive "dilution" of these sites in this growing structure. The higher density of 5-hydroxytryptamine1A sites in X-irradiated rats simply reflected a lower "dilution" due to the delayed growth of the cerebellum in these animals.

Baroreceptor reflex inhibition induced by the stimulation of serotonin3 receptors in the nucleus tractus solitarius of the rat

Previous studies suggested that in the nucleus tractus solitarius, cardiovascular responses to serotonin may involve the simultaneous activation of more than one receptor subtype. In the present study, the cardiovascular effects of the local application of serotonin and different serotonin3 agonists and antagonists into the nucleus tractus solitarius were analysed in intact and unilaterally ganglionectomized rats. Unilateral injections of serotonin (5-15 nmol) produced a dose-dependent increase in blood pressure and partially antagonized the arterial baroreflex responses evoked by an i.v. injection of phenylephrine. Similar blood pressures response were obtained after unilateral microinjections of phenylbiguanide (5 nmol) and 2-methyl-serotonin (5 nmol), two serotonin3 receptor agonists. Bilateral microinjections of serotonin or phenylbiguanide produced more pronounced blood pressure effects and antagonized completely the baroreflex responses. Both blood pressure and baroreflex effects were antagonized by prior injections of specific serotonin3 antagonists such as zacopride (100 pmol) and ondansetron (100 pmol). Concomitant autoradiographic studies performed in intact and ganglionectomized rats, using [125I]iodozacopride, confirmed that serotonin3 receptors in the nucleus tractus solitarius are mainly located on vagal afferent fibers. In addition, serotonin microinjections made in the nucleus tractus solitarius ipsilateral to the ganglionectomy revealed a significant reduction in cardiovascular responses compared to intact animals. These results suggest that in the nucleus tractus solitarius of the rat, serotonin is involved in the reflex regulation of blood pressure through the stimulation of serotonin3 receptors presumably located on vagal afferent fibers. Since bicuculline antagonized the serotonin-mediated pressor responses, a serotonin3-dependent activation of an inhibitory GABAergic system within the nucleus tractus solitarius might be involved in blood pressure regulatory mechanisms.

Anxiolytic-like actions of anpirtoline in a mouse light-dark aversion paradigm

The anxiolytic-like potential of anpirtoline was assessed in a mouse light/dark aversion test. Anpirtoline (1.0 ng kg(-1)-1.0 micrograms kg-1 i.p.) reduced the aversive responding of mice. This was detected as an increase in the latency to locate the non-aversive compartment and by decreases in the percentage of the time spent in the dark compartment, and the numbers of rears and line crossings in the dark compartment. In radioligand binding studies anpirtoline displayed submicromolar affinity for 5-HT1A, 5-HT1B and 5-HT3 receptor recognition sites (Ki = 151, 28 and 30 nM, respectively) and more modest affinity for 5-HT2 receptor recognition sites (Ki = 1.48 microM). It is concluded that anpirtoline has a unique spectrum of affinity for 5-HT receptor subtypes, its interaction with which may account for its anxiolytic-like activity.

Quantitative autoradiographic mapping of 5-HT3 receptors in the rat CNS using [125I]iodo-zacopride and [3H]zacopride as radioligands

Substitution of the chlorine atom by a radio-iodine in position 5 in the zacopride molecule yielded [125I]iodo-zacopride that bound with high affinity (Kd = 4.3 nM) to 5-HT3 receptors in the rat central nervous system. Assays with membranes from the posterior (mainly entorhinal) cortex confirmed that the pharmacological properties and regional distribution of [125I]iodo-zacopride-specific binding sites were identical with those of 5-HT3 sites labelled by the reference radioligand [3H]zacopride. Autoradiographic investigations for the visualization and quantification of 5-HT3 receptors yielded similar results with both radioligands, but autoradiograms could be obtained after only 1-3 days of exposure of sections labelled with [125I]iodo-zacopride, instead of 4-6 months using [3H]zacopride. The highest density of 5-HT3 sites was found in the nucleus tractus solitarius followed by, in decreasing order, the dorsal motor nucleus of the vagus nerve, the superficial layers of the dorsal horn in the spinal cord, the nucleus of the spinal tract of the trigeminal nerve, and the area postrema. Significant labelling of 5-HT3 receptors was also observed in limbic areas (amygdala, hippocampus, frontal and entorhinal cortex), and to a much lower extent in the dorsal raphe nucleus, striatum, and substantia nigra. These multiple locations further support the idea that 5-HT3 receptors are probably involved in several 5-HT-mediated functions in the central nervous system.

Anpirtoline, a novel, highly potent 5-HT1B receptor agonist with antinociceptive/antidepressant-like actions in rodents

1. The purpose of the present study was to relate the effects of the novel drug, anpirtoline, on 5-hydroxytryptamine (5-HT) receptor subtypes to its antinociceptive and antidepressant-like actions in rodents. 2. Binding assays with rat brain membranes have shown that anpirtoline bound with a much higher affinity to 5-HT1B receptor (Ki = 28 nM) than to 5-HT1A (Ki = 150 nM) and 5-HT2 (Ki = 1.49 microM) receptors. 3. Like 5-HT, anpirtoline concentration-dependently inhibited forskolin-stimulated adenylate cyclase activity in homogenates from the rat substantia nigra. Both effects were not additive, and could be prevented by 5-HT1B receptor antagonists such as propranolol and penbutolol. 4. In superfused rat and pig brain cortex slices preincubated with [3H]-5-HT, the electrically evoked tritium overflow was inhibited by anpirtoline and 5-HT. Whereas 5-HT was equipotent in both tissues (EC50 = 69 nM), anpirtoline was markedly less potent in pig brain cortex slices (EC50 = 1190 nM) than in rat brain cortex slices (EC50 = 55 nM). The concentration-response curve for anpirtoline was shifted to the right by metitepine in both preparations. 5. In the social behaviour deficit test, anpirtoline and trifluoromethylphenyl-piperazine were effective in reversing the isolation-induced impairments in mice, an effect shown only by compounds with agonist properties at the 5-HT1B receptor. 6. In the electrostimulated pain test using mice, anpirtoline dose-dependently increased the pain threshold with an ED50 of 0.52 mg kg-1, i.p. The antinociceptive activity of anpirtoline was abolished by pretreatment with cyproheptadine or propranolol.7. In the forced swimming test in rats, anpirtoline induced a dose-related increase in swimming activity. With an ED50 value of 4.6mgkg-1, i.p., anpirtoline was 4 times more potent than the two standard compounds imipramine and desipramine. The decrease of immobility time or the increase of active periods in this model of behavioural despair is suggested to be characteristic of antidepressant drugs.8. Anpirtoline exhibits both antinociceptive and antidepressant-like activities in animals. It is probable that anpirtoline elicits these pharmacological effects via its agonist effect on 5-HT1B and 5-HT1A receptors.

Characterization of a novel 5-HT4 receptor antagonist of the azabicycloalkyl benzimidazolone class: DAU 6285

Three chemical classes of serotonin 5-HT4 receptor agonists have been identified so far: 5-substituted indoles (e.g. 5-HT), benzamides (e.g. renzapride) and benzimidazolones (e.g. BIMU 8). In a search for 5-HT4 receptor antagonists, we have discovered that the benzimidazolone derivative DAU 6285 (for structure see text), is 3-5 times more potent than tropisetron in blocking 5-HT, renzapride and BIMU 8 induced stimulation of adenylate cyclase activity in mouse embryo colliculi neurons. Schild plot analysis yielded Ki values of 220, 181 and 255 nmol/l, respectively. In addition, DAU 6285 showed poor activity as a 5-HT3 receptor ligand with respect to tropisetron, as demonstrated by in vitro binding studies (Ki, 322 vs 2.8 nmol/l) and by its antagonistic activity in the Bezold-Jarisch reflex test (ID50, 231 vs 0.5 micrograms/kg, i.v.). No significant binding (Ki greater than 10 mumol/l) of DAU 6285 to serotonergic 5-HT1A, 5-HT1B, 5-HT1C, 5-HT1D, and 5-HT2 receptors as well as to adrenergic alpha 1, alpha 2, dopaminergic D1, D2 or muscarinic M1-M3 receptor subtypes was found. The data indicate that DAU 6285 has a somewhat higher affinity than tropisetron for 5-HT4 receptors, a property confirmed in functional tests, and much lower affinity than tropisetron for 5-HT3 receptors. The compound represents a new interesting tool for investigating the pharmacological and physiological properties of 5-HT4 receptors.

5-HT3 receptor antagonists reverse helpless behaviour in rats

The effects of the 5-HT3 receptor antagonists, zacopride, ondansetron and ICS 205-930, were investigated in an animal model of depression, the learned helplessness test. Rats previously subjected to a session of 60 inescapable foot-shocks exhibited a deficit of escape performance in three subsequent shuttle-box sessions. The 5-HT3 receptor antagonists administered i.p. twice daily on a chronic schedule (zacopride 0.03-2 mg/kg per day; ondansetron and ICS 205-930: 0.125-2 mg/kg per day) reduced the number of escape failures at low to moderate daily doses. This effect was not observed with the highest dose(s) of zacopride, ondansetron and ICS 205-930 tested. These results indicate that 5-HT3 antagonists may have effects like those of conventional antidepressants in rats.

The potent 5-HT3 receptor antagonist (R)-zacopride labels an additional high affinity site in the central nervous system

The binding characteristics of [3H](R)- and [3H](S)-zacopride were investigated in membranes from the rat entorhinal cortex and NG 108-15 clonal cells. In contrast to [3H](S)-zacopride which bound solely to 5-HT3 receptors, [3H](R)-zacopride recognized another class of binding sites, called the (R)-sites, in both membrane preparations. In addition to (R)-zacopride (Ki = 3-11 nM), only (R)-iodo-zacopride, (R)-dechloro-zacopride, prazosin and mianserin exhibited high to moderate affinity for the (R)-sites, whose possible functions remain to be established.

[Preclinical pharmacology of amoxapine and amitriptyline. Implications of serotoninergic and opiodergic systems in their central effect in rats]

The effects of two antidepressant drugs, amoxapine and amitriptyline, that belong to distinct chemical classes, have been examined on various biochemical parameters related to serotoninergic and opioidergic neurotransmission in the rat brain and spinal cord. In vitro binding studies showed that both amoxapine and amitriptyline interact in the nanomolar range with 5-HT2 receptors labelled by [3H]ketanserin in cortical membranes. By contrast, neither amoxapine nor amitriptyline can be considered as possible ligands of 5-HT1A and 5-HT1B receptors because their affinities for these sites are in the micromolar range (or even worse). Interestingly, amoxapine binds with a good affinity (IC50 = 0.30 microM) to 5-HT3 receptors labelled by [3H]zacopride in cortical membranes. Complementary experiments using the 5-HT3-dependent Bezold-Jarisch reflex confirmed that amoxapine really acts in vivo as a 5-HT3 antagonist (IC50 = 50 micrograms/kg i.v.), whereas amitriptyline is essentially inactive on 5-HT3 receptors. The second part of this study consisted of looking for possible changes in central 5-HT receptors 24 h after either a single or a repeated (for 14 days) treatment with amoxapine (10 mg/kg i.p. each day) or amitriptyline (10 mg/kg i.p.). A marked decrease in the density of 5-HT2 receptors was found in the cerebral cortex in both treatment groups. By contrast, neither 5-HT1A nor 5-HT1B receptors were significantly affected in any brain region studied. Finally we explored whether acute and/or chronic administration of amoxapine or amitriptyline affected the levels of opioid peptides and the mu and delta classes of opioid receptors in various regions of the brain and the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of tryptophan residue(s) in the specific binding of agonists/antagonists to 5-HT3 receptors in NG108-15 clonal cells

Chemical modification of the 5-HT3 receptors in membranes from NG108-15 hybridoma cells was achieved using protein modifying reagents specific for various amino acid residues: N-bromosuccinimide for tryptophan, dithiothreitol for cystine, sodium tetrathionate for cysteine, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline for aspartic and glutamic acids, diethylpyrocarbonate for histidine, tetranitromethane for tyrosine and 2,3-butanedione for arginine. Among all the reagents tested, N-bromosuccinimide produced the largest alteration in the specific binding of [3H]zacopride onto 5-HT3 receptors. A significant reduction in Bmax (approximately 50%) with no change in Kd were noted on [3H]zacopride specific binding to membranes which were incubated with 40 microM N-bromosuccinimide for 60 min at 25 degrees. The occupancy of 5-HT3 receptor binding sites by various 5-HT3 agonists and antagonists (phenylbiguanide, ondansetron, granisetron, MDL 72222) prevented, at least partially, any subsequent reduction in [3H]zacopride specific binding by N-bromosuccinimide treatment. However, neither m-chloro-phenylbiguanide, among the agonists, nor zacopride, among the antagonists, were able to prevent the effect of N-bromosuccinimide, suggesting that variations might exist in the molecular mechanisms implicated in the binding of 5-HT3 ligands to the recognition site on 5-HT3 receptors. Nevertheless, these data support the suggestion that tryptophan residue(s) are probably involved in the binding of agonists and antagonists onto 5-HT3 receptors in NG108-15 cell membranes.

Sleep changes induced by the local application of 5,7-dihydroxytryptamine into the nodose ganglia and aortic denervation in the rat

The effects of a bilateral microinjection of 5,7-dihydroxytryptamine (5,7-DHT) into the nodose ganglia and aortic denervation on the daily amounts of sleep/wake states were studied in rats. Both lesions produced an increase in paradoxical sleep and provoked the onset of paradoxical sleep episodes without slow-wave-sleep transition ("narcolepsy-like" paradoxical sleep episodes). The increase in paradoxical sleep observed after 5,7-DHT injection was more important than that of the aortic denervation. In addition, both 5,7-DHT-treated and aortic-denervated animals exhibited a delayed decrease in slow-wave sleep associated with an increase in wakefulness. These results show that the peripheral messages coming from aortic serotonergic afferent fibres to the nucleus tractus solitarius play a modulatory role in the daily expression of paradoxical sleep in rats.

[3H]5-methyl-urapidil labels 5-HT1A receptors and alpha 1-adrenoceptors in the rat CNS. In vitro binding and autoradiographic studies

The tritiated derivative of the potent antihypertensive agent, 5-methyl-urapidil, was used as a radioligand in binding studies with rat brain membranes and tissue sections. Serotonin and prazosin inhibited [3H]5-methyl-urapidil binding to membranes from the rat hippocampus, cerebral cortex and brainstem biphasically, leading to the definition of serotonin high-affinity and prazosin high-affinity [3H]5-methyl-urapidil binding sites. Comparison of these serotonin-sensitive [3H]5-methyl-urapidil binding sites with the 5-HT1A sites labelled by [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) revealed striking similarities regarding pharmacological properties, respective densities and regional distribution. On the other hand, the prazosin-sensitive [3H]5-methyl-urapidil binding sites should correspond to the alpha 1A-subtype of adrenoceptors which has recently been defined. Detailed autoradiographic investigations allowed the detection of 5-HT1A sites labelled by both [3H]5-methyl-urapidil and [3H]8-OH-DPAT in the posterior raphe nuclei (pallidus and obscurus) which are possibly involved in the hypotensive action of 5-methyl-urapidil. These data demonstrate that [3H]5-methyl-urapidil is a useful radioligand for the visualization and quantification of both 5-HT1A serotonin receptors and alpha 1A-adrenoceptors in the central nervous system.

The GTP-insensitive component of high-affinity [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding in the rat hippocampus corresponds to an oxidized state of the 5-hydroxytryptamine1A receptor

Previous studies on central 5-hydroxytryptamine1A (5-HT1A) receptors have consistently shown the existence of a GTP-insensitive component of agonist binding, i.e., binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) that persists in the presence of 0.1 mM GTP or guanylylimidodiphosphate (GppNHp). The molecular basis for this apparent heterogeneity was investigated pharmacologically and biochemically in the present study. The GppNHp-insensitive component of [3H]8-OH-DPAT binding increased spontaneously by exposure of rat hippocampal membranes or their 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate-soluble extracts to air; it was reduced by preincubation of solubilized 5-HT1A binding sites in the presence of dithiothreitol and, in contrast, reversibly increased by preincubation in the presence of various oxidizing reagents like sodium tetrathionate or hydrogen peroxide. In addition, exposure of hippocampal soluble extracts to short-cross-linking reagents specific for thiols produced an irreversible increase in the proportion of GppNHp-insensitive over total [3H]8-OH-DPAT binding. The pharmacological properties of this GppNHp-insensitive component of [3H]8-OH-DPAT binding were similar to those of 5-HT1A sites in the absence of nucleotide. Sucrose gradient sedimentation of solubilized 5-HT1A binding sites treated by dithiothreitol or sodium tetrathionate showed that oxidation prevented the dissociation by GTP of the complex formed by the 5-HT1A receptor binding subunit (R[5-HT1A]) and a guanine nucleotide-binding protein (G protein). Moreover, the oxidation of -SH groups by sodium tetrathionate did not prevent the inactivation of [3H]8-OH-DPAT specific binding by N-ethylmaleimide, in contrast to that expected from an interaction of both reagents with the same -SH groups on the R[5-HT1A]-G protein complex. These data suggest that the appearance of GTP-insensitive [3H]8-OH-DPAT specific binding occurs as a result of the (spontaneous) oxidation of essential -SH groups (different from those preferentially inactivated by N-ethylmaleimide) on the R[5-HT1A]-G protein complex.

The 5-HT1A receptor: an overview of recent advances

Progress in the field of neuronal receptor research has accelerated during the last few years due to developments in pharmacology and molecular biology. This is particularly true in the case of the serotonin 5-HT1A receptor. In 1983 the very selective, high affinity 5-HT1A agonist 8-OH-DPAT was developed which allowed the pharmacology and distribution of the 5-HT1A receptor in the central nervous system of the rat and man to be extensively characterized. By 1987, the gene encoding this receptor protein was cloned and sequenced, allowing not only elucidation of its structure, but also better insight into the nature of its coupling to transmembrane signal transduction systems. Thus in a short period of time considerable knowledge has accumulated on how serotonin exerts its functions in the central nervous system via the 5-HT1A receptor. In the present review we will briefly discuss some of the latest developments regarding the 5-HT1A receptor.