Cannabinoid function in learning, memory and plasticity

Marijuana and its psychoactive constituents induce a multitude of effects on brain function. These include deficits in memory formation, but care needs to be exercised since many human studies are flawed by multiple drug abuse, small sample sizes, sample selection and sensitivity of psychological tests for subtle differences. The most robust finding with respect to memory is a deficit in working and short-term memory. This requires intact hippocampus and prefrontal cortex, two brain regions richly expressing CB1 receptors. Animal studies, which enable a more controlled drug regime and more constant behavioural testing, have confirmed human results and suggest, with respect to hippocampus, that exogenous cannabinoid treatment selectively affects encoding processes. This may be different in other brain areas, for instance the amygdala, where a predominant involvement in memory consolidation and forgetting has been firmly established. While cannabinoid receptor agonists impair memory formation, antagonists reverse these deficits or act as memory enhancers. These results are in good agreement with data obtained from electrophysiological recordings, which reveal reduction in neural plasticity following cannabinoid treatment, and increased plasticity following antagonist exposure. The mixed receptor properties of the pharmacological tool, however, make it difficult to define the exact role of any CB1 receptor population in memory processes with any certainty. This makes it all the more important that behavioural studies use selective administration of drugs to specific brain areas, rather than global administration to whole animals. The emerging role of the endogenous cannabinoid system in the hippocampus may be to facilitate the induction of long-term potentiation/the encoding of information. Administration of exogenous selective CB1 agonists may therefore disrupt hippocampus-dependent learning and memory by 'increasing the noise', rather than 'decreasing the signal' at potentiated inputs.

Activation of a K-252b-Sensitive Protein Kinase is Necessary for a Post-Synaptic Phase of Long-Term Potentiation in Area CA1 of Rat Hippocampus

K-252b, a potent inhibitor of protein kinases blocked a late phase of long-term potentiation (LTP) in area CA1 of rat hippocampal slices, resulting in decremental LTP. It also prevented the slowly developing increase in sensitivity of CA1 neurons to iontophoretically administered alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) which was seen in control slices that exhibit nondecremental LTP. However, K-252b applied 60 - 180 min after the induction of LTP had no effect on the potentiated synaptic and AMPA-induced responses. A K-252b-sensitive protein kinase may therefore be involved in a slowly developing postsynaptic component of LTP.

Functions of cannabinoid receptors in the hippocampus

Marijuana smoking is recognised to impair human cognition and learning, but the mechanisms by which this occurs are not well characterised. This article focuses exclusively on the hippocampus to review the effects of cannabinoids on hippocampal function and evaluate the evidence that hippocampal cannabinoid receptors play a role in learning and formation of memory. Activation of cannabinoid receptors inhibits release of a variety of neurotransmitters, and modulates a number of intrinsic membrane conductances. Suppression of inhibitory GABAergic synaptic transmission has been repeatedly described, but whether there is also control of excitatory glutamatergic transmission is more controversial. The recognition that the commonly used WIN55,212-2 also acts via non-cannabinoid receptors may help resolve this issue. The involvement of endocannabinoids in depolarisation induced suppression of inhibition (DSI) and the demonstration that activation of metabotropic glutamate receptors can stimulate endocannabinoid release have provided the first insights into the physiological roles of the cannabinoids. Cannabinoids have consistently been reported to inhibit high frequency stimulation induced synaptic long-term potentiation but the experimental design of most behavioural experiments have meant it is not possible to categorically demonstrate a role for hippocampal cannabinoid receptors in learning and memory.

The endogenous cannabinoid anandamide activates vanilloid receptors in the rat hippocampal slice

We have previously reported that the synthetic cannabinoid receptor agonist WIN55,212-2 causes a selective reduction in paired-pulse depression of population spikes in the CA1 region of the rat hippocampal slice. This effect is consistent with the observation that activation of cannabinoid receptors inhibits GABA release in the hippocampus. We have now investigated the actions of the putative endogenous cannabinoids 2-arachidonoyl-glycerol (2-AG) and anandamide in this system. 2-AG mimicked the effect of WIN55,212-2 by selectively reducing paired-pulse depression at concentrations of 1-30 microM. In contrast, anandamide caused a selective increase in paired-pulse depression at concentrations of 1-30 microM. This effect was mimicked by the vanilloid receptor agonists capsaicin and resiniferatoxin, and blocked by the vanilloid receptor antagonist capsazepine, but not by the cannabinoid receptor antagonist AM281. These results are the first to demonstrate a clear functional vanilloid receptor-mediated effect in the hippocampus, and further, that anandamide but not 2-AG acts at these receptors to increase paired-pulse depression of population spikes.

Carbenoxolone depresses spontaneous epileptiform activity in the CA1 region of rat hippocampal slices

An important contributor to the generation of epileptiform activity is the synchronization of burst firing in a group of neurons. The aim of this study was to investigate whether gap junctions are involved in this synchrony using an in vitro model of epileptiform activity. Hippocampal slices (400 microm) were prepared from female Sprague-Dawley rats (120-170 g). The perfusion of slices with a medium containing no added magnesium and 4-aminopyridine (50 microM) resulted in the generation of spontaneous bursts of population spikes of a fast frequency along with less frequent negative-going bursts. The frequency of the bursts produced was consistent over a 3h period. Carbenoxolone (100 microM), a gap junction blocker and mineralocorticoid agonist, perfused for 75 min, reduced the frequency of both types of spontaneous burst activity. Perfusion of spironolactone (1 microM), a mineralocorticosteroid antagonist, for 15 min prior to and during carbenoxolone perfusion did not alter the ability of carbenoxolone to depress the frequency of spontaneous activity. The incubation of hippocampal slices in carbenoxolone prior to recording increased the time taken for the spontaneous activity to start on change to the zero magnesium/4-aminopyridine medium and decreased the total number of spontaneous bursts over the first 60 min period. The ability of carbenoxolone to delay induction of epileptiform activity and reduce established epileptiform activity suggests that gap junctions contribute to the synchronization of neuronal firing in this model.

Developmental regulation of hippocampal excitatory synaptic transmission by metabotropic glutamate receptors

The aims of this study were, to use agonists selective for the 3 mGlu receptor groups to identify developmental changes in their effects, and to assess the usefulness of proposed selective antagonists as pharmacological tools. Hippocampal slices (400 microm) were prepared from neonate (9 - 14 days) and young adult (5 - 7 weeks) Sprague-Dawley rats. Field excitatory postsynaptic potentials (fEPSP) were recorded from CA1. DHPG (100 microM), a group I agonist, produced a slowly developing enhancement of fEPSP slope in slices from adults. In slices from neonates, DHPG (75 microM) depressed fEPSP slope. DCG-IV (500 nM), a group II agonist, did not affect the fEPSP recorded from slices from adults whereas perfusion in neonate slices produced a sustained depression. The group III agonist L-AP4 (50 microM) was ineffective in adult slices but depressed fEPSP slope in slices prepared from neonates. DHPG-induced depression of fEPSP slope was inhibited by 4-CPG (400 microM), a group I antagonist, but was unaffected by MCCG (500 microM) and MAP4 (500 microM), group II and III receptor antagonists respectively. MCCG but not MAP4 antagonized the effects of DCG-IV with 4-CPG producing variable effects. The effect of L-AP4 was unaffected by MCCG, blocked by MAP4, and enhanced by 4-CPG. The results show that the effects of the agonists for all groups of mGlu receptors are developmentally regulated. Furthermore, MCCG and MAP4 behave as effective and selective antagonists for group II and group III mGlu receptors respectively, whereas the usefulness of 4-CPG as a group I antagonist may be limited.

Correlation between cannabinoid mediated effects on paired pulse depression and induction of long term potentiation in the rat hippocampal slice

Cannabinoids cause an increase in synaptic transmission via gamma-aminobutyric acid (GABA) receptors and this may be the mechanism by which activation of CB1 receptors blocks the induction of long-term potentiation (LTP). To test this hypothesis, we used paired pulse depression (PPD) of CA1 population spike responses recorded in the rat hippocampal slice as an index of GABA-ergic feedback inhibition, to establish whether the effects of a stereoselective CB1 receptor agonist on GABA-ergic transmission and LTP were correlated. The active isomer, WIN55212-2, blocked the induction of LTP and suppressed PPD over the concentration range 250 nM-5 microM, whereas the inactive isomer, WIN55212-3, was inactive at 5 microM. The effects of 5 microM WIN55212-2 on both LTP and PPD were completely blocked by the CB1 receptor antagonist SR141716A (5 microM). The results show that the effects are correlated in that both suppression of PPD and blockade of induction of LTP are probably mediated by CBI receptors. However, the suppression in PPD suggests that WIN55212-2 caused a decrease in GABA-ergic feedback transmission which would be expected to facilitate, rather than block, the induction of LTP. We therefore conclude that the blockade of LTP by cannabinoids is not via upregulation of GABA-ergic synaptic transmission.

Arachidonic acid metabolites and the synaptic potentiation evoked by activation of metabotropic glutamate receptors

We have previously shown that coapplication of arachidonic acid (10 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 50 microM) evokes an enhancement of synaptic transmission in the CA1 region of the rat hippocampal slice. Here we have investigated whether the metabolites of arachidonic acid are implicated in this potentiation. Inclusion of the cyclo-oxygenase inhibitor indomethacin (10 microM) did not block the potentiation induced by coapplication of arachidonic acid and ACPD. However, the presence of either the cyclo-, lipo- and epoxygenase inhibitor 5,8,11,14-eicosatetraynoic acid (ETYA, 20 microM), or the lipoxygenase inhibitor nordihydroguaiaretic acid (10 microM), prevented the long-lasting enhancement. The results suggest that the lipoxygenase and epoxygenase metabolites of arachidonic acid may be involved in the induction of this form of synaptic potentiation.

Action of delta-9-tetrahydrocannabinol on GABA(A) receptor-mediated responses in a grease-gap recording preparation of the rat hippocampal slice

We have investigated the effects of delta-9-tetrahydrocannabinol (delta-9-THC) on gamma-aminobutyric acid (GABA) receptor-mediated responses in a grease-gap recording preparation of the rat hippocampus. GABA, and the selective GABA(A) receptor agonist muscimol, evoked depolarizing responses with EC50 values of 8.5 mM and 17.0 microM, respectively. Responses to both of these agonists were selectively reduced by the non-competitive GABA(A) antagonist picrotoxin (5 microM), but were unaffected by the GABA(B) antagonist 2-hydroxysaclofen (500 microM). Responses evoked by the selective GABA(B) receptor agonist baclofen were not sufficiently large to analyse. The GABA uptake inhibitor, nipecotic acid (500 microM), potentiated responses to GABA, but not to muscimol. Similarly, 10-1000 nM delta-9-THC had no significant effect on the response to muscimol, whereas 1000 nM delta-9-THC significantly increased the response to GABA. Since GABA is the substrate of an avid uptake system, but muscimol is not, the results are consistent with the suggestion that delta-9-THC inhibits the uptake of GABA in the hippocampus.

Melatonin blocks the induction of long-term potentiation in an N-methyl-D-aspartate independent manner

Perfusion of 100 microM melatonin had no effect on low frequency synaptic transmission, but prevented the induction of tetanically induced long-term potentiation (LTP) when recorded in the dendritic region of the CA1 in rat hippocampal slices. Perfusion of 100 microM melatonin in this preparation had no effect on the multiple population spikes recorded in Mg2+-free medium, and, in grease-gap recordings from the CA1-subiculum slice, 100 microM melatonin had no effect on depolarisations evoked by N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). This suggests that melatonin has the ability to prevent the formation of LTP, and that this effect is not mediated by blockade of NMDA receptors.

Repeated electroconvulsive stimulation impairs synaptic plasticity in the dentate gyrus in vivo but has no effect in CA1 in vitro

Repeated electroconvulsive stimulation (ECS) spaced at 48 h intervals significantly increased the synaptic response in the dentate gyrus in vivo, as measured by input/output curves, and reduced the degree of long-term potentiation (LTP) obtained following high frequency stimulation. An identical course of ECS had no effect on synaptic responses recorded in the stratum radiatum of CA1 in vitro and did not impair high frequency-induced LTP. These results suggest that either ECS has a selective effect on the sub-fields of the hippocampus or that in vitro recording techniques are unsuitable for detecting the increase in synaptic efficacy produced by the treatments.

Interactions between arachidonic acid and metabotropic glutamate receptors in the induction of synaptic potentiation in the rat hippocampal slice

Perfusion of neither the metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), nor arachidonic acid caused any long-term enhancement of synaptic transmission in the CA1 region of the rat hippocampal slice. However, co-perfusion of ACPD (50 microM) and arachidonic acid (10 microM) for 5 min induced a rapidly evoked and long-lasting enhancement of synaptic transmission. This enhancement persisted in the presence of D(-)-2-amino-5-phosphonopentanoic acid (40 microM) and is therefore independent of NMDA receptor activation. The potentiation was mimicked by perfusion of the phospholipase A2 activator melittin (10 micrograms/ml) for 5 or 10 min, or exogenous phospholipase A2 (1 microgram/ml) for 5 min, immediately before ACPD application. We propose a role for arachidonic acid in the induction of synaptic potentiation, possibly as a retrograde transmitter substance.

Indirect potentiation of synaptic transmission by metabotropic glutamate receptors in the rat hippocampal slice

The role that the metabotropic glutamate receptor plays in synaptic transmission is complex due to the multiple subtypes involved, which initiate a number of intracellular mechanisms. Here we have investigated the role of the metabotropic glutamate receptor in the induction of long-term potentiation (LTP). We have shown that, providing the CA3 region remains attached to the slice, it is possible to induce potentiation by bath perfusion of the metabotropic receptor agonist (1S,3R) 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) alone. The extent of the potentiation observed showed a strong negative correlation with the age of the animal from which the slices were prepared. Perfusion of ACPD was associated with an increase in the excitability of antidromically activated CA3 neurones, the appearance of spontaneous burst firing within the CA3 region, and an increased fibre volley recorded in the CA1 region. Blockade of N-methyl-D-aspartate (NMDA) receptors prevented all these effects. We suggest that the ACPD-induced potentiation of CA1 fEPSPs is an indirect effect caused by spontaneous burst firing and/or increased excitatory drive from CA3 neurones.

Prevention by the cannabinoid antagonist, SR141716A, of cannabinoid-mediated blockade of long-term potentiation in the rat hippocampal slice

Incubation of rat hippocampal slices in the presence of the synthetic cannabinoid (-)-11-OH-delta 8-dimethylheptyl tetrahydrocannabinol (HU-210) (100 nM) prevented the induction of long-term potentiation (LTP). Slices co-incubated with both HU-210 (100 nM) and the cannabinoid antagonist, SR141716A (100 nM), exhibited tetanically induced LTP, comparable to control slices. Intriguingly, coincubation with HU-210 and SR141716A prevented the induction of the early, short-term phase of LTP.

Potentiation of synaptic transmission in the rat hippocampal slice by exogenous L-glutamate and selective L-glutamate receptor subtype agonists

We have investigated the effects of administration of exogenous glutamate receptor agonists on the amplitude of field excitatory post-synaptic potentials (fEPSPs) evoked in the CA1 region of the rat hippocampal slice by stimulation of the Schaffer collateral-commissural fibres. L-Glutamate applied by iontophoresis or by bath perfusion (50 microM for 5 min) evoked a slowly rising increase in the amplitude of the fESPS which persisted for over 90 min. L-Glutamate induced potentiation was blocked by either D(-)-2-amino-5-phosphonopentanoic acid (40 microM) or by (RS)-alpha-methyl-4-carboxyphenylglycine (500 microM). In slices in which synaptic long-term potentiation had been saturated, iontophoretically applied L-glutamate did not induce further potentiation, but reset the fEPSP amplitude back to control levels. Iontophoretic administration of N-methyl-D-aspartate (NMDA) evoked a transient potentiation which decayed back to control levels within 90 min whereas bath perfusion of NMDA (50 microM) evoked a persistent depression. Bath perfusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 50 microM) evoked no persistent effects. Bath administration of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 50 or 100 microM) caused a short term depression of the fEPSP and no significant persistent effects. Perfusion of 100 microM ACPD in medium containing 1 microM picrotoxin caused a much smaller short term depression of the fEPSP and this was followed by a gradually developing and persistent potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

The action of synthetic cannabinoids on the induction of long-term potentiation in the rat hippocampal slice

Incubation of rat hippocampal slices with the synthetic cannabinoid (-)-11-OH-delta 8-dimethylheptyl tetrahydrocannabinol (HU-210) (100 nM) prevented the induction of long-term potentiation of field excitatory postsynaptic potentials recorded in the CA1 region. However, in slices incubated with its non-psychoactive (+)-isomer HU-211 (100 nM), which is reported to be an NMDA receptor antagonist, high frequency stimulation evoked a long-lasting potentiation, comparable to control slices.

Origin of post-depolarization hyperpolarizations in a grease-gap recording preparation

In grease-gap recording preparations, depolarizing responses evoked by agonists are often followed by a hyperpolarization (post-depolarization hyperpolarization). We have investigated the origin of these post-depolarization hyperpolarizations in the rat CA1-subiculum slice. They were evoked by L-glutamate, N-methyl-d-aspartate or alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate in approximately 80% of the slices tested. The post-depolarization hyperpolarizations evoked by perfusion of N-methyl-d-aspartate through the CA1 compartment of the chamber, persisted when N-methyl-d-aspartate receptors in the subicular compartment were blocked with D-2-amino-5-phosphonopentanoate. Carbachol only blocked the post-depolarization hyperpolarizations evoked by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate at concentrations which also blocked the depolarization. The post-depolarization hyperpolarization was selectively blocked by perfusion with Ca(2+)-free medium and by administration of ouabain, and showed a marked sensitivity to temperature. It is concluded that the post-depolarization hyperpolarizations observed in this preparation are not a consequence of diffusion of the agonist through the slice. The evidence is, however, consistent with them being generated by activation of the electrogenic Na(+)-K+ pump, although we cannot exclude an additional contribution from Ca(2+)- or voltage-dependent K+ currents.

Possible NMDA antagonist properties of drugs that affect high pressure neurological syndrome

1. Previous studies have suggested that a series of drugs modelled on part of the strychnine molecule interfere with the development of high pressure neurological syndrome (HPNS) and it was presumed that this effect was via an action on inhibitory glycinergic transmission. We have now used the rat hippocampal slice preparation to examine the possibility that some of these drugs might instead have an action at the strychnine-insensitive (SI) glycine binding site associated with the NMDA receptor. 2. D-2-Amino-5-phosphonovalerate (AP5) and 7-chlorokynurenate (7CK) had no significant effect on the height of the population spike recorded from the CA1 region in 1 mM Mg2+ medium, but both blocked the multiple population spikes recorded in Mg(2+)-free medium. The effect of 7CK, but not AP5, was reversed by 200 microM D-serine which is consistent with the known antagonist action of 7CK at the SI-glycine site. 3. A derivative of benzimidazole, which shows the clearest structural similarities to known SI-glycine site antagonists and ameliorates HPNS, mirrored the effects of 7CK although it was considerably less potent. 4. Gramine, which exacerbates HPNS, significantly increased the number of population spikes evoked in Mg(2+)-free medium. 5. Mephenesin, which is the most potent known drug in ameliorating HPNS, had no significant effect on the response recorded in 1 mM Mg2+ and significantly reduced the number of population spikes recorded in Mg(2+)-free medium, but this effect was only partially reversed by the addition of D-serine. 6. The results are consistent with the benzimidazole derivative, but not gramine, being an antagonist at the SI-glycine receptor. The results with mephenesin are equivocal but leave open the possibility that some of the drugs which are effective against HPNS act via an effect on excitatory NMDA receptor mediated transmission, rather than on inhibitory glycine-mediated transmission.

Co-administration of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid and arachidonic acid potentiates synaptic transmission in rat hippocampal slices

Perfusion of the 1S,3R isomer of trans-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD, 50 microM), or arachidonic acid (10 microM), for 5 min produced only depression of the field excitatory postsynaptic potential recorded in the CA1 region of rat hippocampal slices from which the CA3 region had been removed. However, perfusion of t-ACPD and arachidonic acid in combination induced a rapid potentiation of the response which in 4/6 slices was maintained for at least 90 min.

Long-term potentiation of NMDA receptor-mediated synaptic transmission in the hippocampus

Neurotransmission at most excitatory synapses in the brain operates through two types of glutamate receptor termed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors; these mediate the fast and slow components of excitatory postsynaptic potentials respectively. Activation of NMDA receptors can also lead to a long-lasting modification in synaptic efficiency at glutamatergic synapses; this is exemplified in the CA1 region of the hippocampus, where NMDA receptors mediate the induction of long-term potentiation (LTP). It is believed that in this region LTP is maintained by a specific increase in the AMPA receptor-mediated component of synaptic transmission. We now report, however, that a pharmacologically isolated NMDA receptor-mediated synaptic response can undergo robust, synapse-specific LTP. This finding has implications for neuropathologies such as epilepsy and neurodegeneration, in which excessive NMDA receptor activation has been implicated. It adds fundamentally to theories of synaptic plasticity because NMDA receptor activation may, in addition to causing increased synaptic efficiency, directly alter the plasticity of synapses.

Paired-pulse depression of monosynaptic GABA-mediated inhibitory postsynaptic responses in rat hippocampus

1. Intracellular recording techniques were used to characterize monosynaptic inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in rat hippocampal slices and to study the mechanism of paired-pulse depression of these synaptic responses. This was achieved by stimulation in stratum radiatum close (less than 0.5 mm) to an intracellularly recorded CA1 neurone after pharmacological blockade of all excitatory synaptic transmission. 2. Under these conditions, low-frequency stimulation (0.033 Hz) evoked a pure biphasic IPSP, which had a short and constant latency to onset. This IPSP was blocked by tetrodotoxin (1 microM) suggesting that it resulted from the electrical stimulation of the axons and/or cell bodies of a monosynaptic inhibitory pathway. 3. Picrotoxin (100 microM) abolished the early component of the biphasic IPSP/C. It left an intact, pure late IPSP/C (IPSP/CB) which had a latency to onset of 29 +/- 2 ms, latency to peak of 139 +/- 4 ms, a duration of 723 +/- 135 (range 390-1730) ms and a reversal potential of -93 +/- 2 mV. The duration was highly dependent on the stimulus intensity whereas the latency to onset was largely independent of the stimulus intensity. The IPSP/CB was reduced or abolished by 1 mM-phaclofen. 4. Phaclofen (1 mM) and 2-hydroxy-saclofen (0.1-1.0 mM) reversibly depressed (60-100%) the late component of the biphasic IPSP/C and, where maximally effective, left a pure, early IPSP/C (IPSP/CA). The IPSP/CA had a latency to onset of 3 ms or less, a latency to peak of 17 +/- 1 ms, a duration of 225 +/- 3 ms and a reversal potential of -75 +/- 2 mV. 5. Two shocks of identical strength were applied in close succession to characterize, and to study the mechanisms underlying, frequency-dependent depression of inhibitory synaptic responses. Paired-pulse depression was seen for both phases of the biphasic IPSP/C and of the pure IPSP/CB, recorded in the presence of picrotoxin. Paired-pulse depression was not accompanied by changes in the reversal potential of either component, indicating that it was caused by a reduction in the two synaptic conductances. Paired-pulse depression was greater when high stimulus intensities were employed. 6. Paired stimuli were applied at separation intervals of between 5 ms and 10 s to determine the temporal profile of frequency-dependent depression. Paired-pulse depression of both IPSCA and IPSCB was most pronounced at an interstimulus interval of 100-125 ms and ceased to occur at intervals greater than 5 10s.(ABSTRACT TRUNCATED AT 400 WORDS)

Antitussive agents as N-methylaspartate antagonists: further studies

The relative potencies of ketamine and the morphinan derivatives dextrorphan, dextromethorphan, and levorphanol as antagonists of the excitatory actions of N-methylaspartate on rat spinal neurones in vivo were examined, both following their microelectrophoretic administration and, with the exception of levorphanol, after intravenous injection. Applied microelectrophoretically, dextrorphan was a more potent N-methylaspartate antagonist than ketamine, levorphanol, or dextromethorphan. After systemic administration, however, dextrorphan was rather less potent than ketamine in this respect, whereas dextromethorphan remained less potent than either ketamine or dextrorphan. Noscapine, an antitussive that lacks anticonvulsant activity, failed to reduce selectively responses to N-methylaspartate as did papaverine, an isoquinoline structurally related to noscapine, and triprolidine, an antihistamine commonly found in proprietary cough medicines. The results are discussed with particular reference to the potential of the compounds tested as anticonvulsant and neuroprotective agents in vivo.

Role of excitatory amino acid receptors in synaptic transmission in area CA1 of rat hippocampus

The new antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), which blocks responses to kainate and quisqualate, has been used in conjunction with D-2-amino-5-phosphonovalerate (APV), which blocks selectively responses to N-methyl-D-aspartate (NMDA), to determine the role of excitatory amino acid receptors in synaptic transmission. An excitatory postsynaptic potential (EPSP)-inhibitory postsynaptic potential (IPSP) sequence was evoked in CA1 neurons by stimulation of the Schaffer collateral-commissural pathway in rat hippocampal slices. CNQX (10 microM) substantially reduced the EPSP without having any effect on input resistance or membrane potential. The IPSP was also reduced provided that the stimulating electrode was place approximately 1 mm from the recording electrode. The EPSP that remained in the presence of CNQX had characteristics of an NMDA receptor-mediated potential; it had a slow timecourse, summated at high frequencies, was blocked reversibly by APV, increased greatly in size in Mg2+-free medium, and showed an anomalous voltage dependence in Mg2+-containing medium. In the presence of CNQX, an APV-sensitive polysynaptic GABAergic IPSP could be evoked, indicating that NMDA receptors can mediate suprathreshold EPSPS in inhibitory interneurons. It is suggested that either NMDA or non-NMDA receptors can, under different circumstances, mediate the synaptic excitation of pyramidal neurons and inhibitory interneurons in area CA1 of the hippocampus.

Temporally distinct pre- and post-synaptic mechanisms maintain long-term potentiation

Long-term potentiation (LTP) in the hippocampus is widely studied as the mechanisms involved in its induction and maintenance are believed to underlie fundamental properties of learning and memory in vertebrates. Most synapses that exhibit LTP use an excitatory amino-acid neurotransmitter that acts on two types of receptor, the N-methyl-D-aspartate (NMDA) and quisqualate receptors. The quisqualate receptor mediates the fast synaptic response evoked by low-frequency stimulation, whereas the NMDA receptor system is activated transiently by tetanic stimulation, leading to the induction of LTP. The events responsible for maintaining LTP once it is established are not known. We now demonstrate that the sensitivity of CA1 neurons in hippocampal slices to ionophoretically-applied quisqualate receptor ligands slowly increases following the induction of LTP. This provides direct evidence for a functional post-synaptic change and suggests that pre-synaptic mechanisms also contribute, but in a temporally distinct manner, to the maintenance of LTP.

A comparison between the in vivo and in vitro activity of five potent and competitive NMDA antagonists

1. Phosphonate analogues of glutamate have been tested and compared as N-methyl-D-aspartate (NMDA) antagonists in electrophysiological and binding experiments. The compounds tested were three established NMDA antagonists: D-2-amino-5-phosphonopentanoate (D-AP5), DL-2-amino-7-phosphonoheptanoate (DL-AP7), 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), and two novel putative NMDA antagonists: 3-(2-carboxypiperidin-4-yl)propyl-1-phosphonate (CPPP) and 3-(2-carboxy-piperidin-4-yl)methyl-1-phosphonate (CPMP). 2. When administered electrophoretically to rat spinal neurones in vivo, these compounds were found to be selective NMDA antagonists with little effect on excitations evoked by quisqualate and kainate. CPMP and CPPP were approximately equipotent with CPP and about 5 times more potent than D-AP5. 3. Following systemic administration, 2-5 mg kg-1 i.v. of CPP, CPMP and CPPP reduced NMDA-evoked excitations by 70-100% whereas 50-100 mg kg-1 of D-AP5 and DL-AP7 produced a similar effect. The onset of the effects required 20-30 min and lasted more than six hours. 4. On bath application to cortical wedges, the IC50 values (microM) for antagonism of 40 microM NMDA were: CPP, 0.64 +/- 0.06 (mean +/- s.e.mean; n greater than 4); CPMP, 1.65 +/- 0.13; CPPP 0.89 +/- 0.09; D-AP5, 3.7 +/- 0.32; DL-AP7, 11.1 +/- 2.1; and DL-AP4 and DL-AP6 were inactive at 100 microM. 5. In binding studies with [3H]-CPP, the Ki values (nM) were: CPP, 446 + 150 (mean + s.e.mean; n > 3); CPMP, 183 + 74 and CPPP, 179 +/- 13 whereas against NMDA (lO microM)-stimulated [3H]- TCP (thienylcyclohexylpiperidine) binding the ICjo values (microM) for CPMP and CPPP respectively were 5.6 + 2.7 and 4.5 + 2.2. 6. Systemic administration of CPPP and CPMP, at doses sufficient to antagonize NMDA, also reduced cardiovascular responses to 5-hydroxytryptamine (Bezold-Jarisch reflex). This illustrates a role for NMDA receptors in central cardiovascular control. 7. The results indicate the systemic doses of piperidine and piperazine analogues of D-AP5 which may be used for assessing the role ofNMDA receptors in central synaptic function.

Ketamine blocks an NMDA receptor-mediated component of synaptic transmission in rat hippocampus in a voltage-dependent manner

We have examined the voltage dependence of the effects of ketamine on synaptic currents in hippocampal CA1 neurons in vitro under conditions where there is a large N-methyl-D-aspartate (NMDA) receptor mediated component of the response. Ketamine reduced inward currents to a greater extent than outward currents of a corresponding size. D-2-Amino-5-phosphonovalerate (APV) substantially reduced the residual outward currents recorded in ketamine, but had only a small effect on the residual inward ones. It is concluded that in this system the action of ketamine in blocking synaptically evoked NMDA receptor-mediated currents shows some voltage dependence.

Quinoxalinediones: potent competitive non-NMDA glutamate receptor antagonists

The N-methyl-D-aspartate (NMDA)-subtype of glutamate receptors has been well described as a result of the early appearance of NMDA antagonists, but no potent antagonist for the "non-NMDA" glutamate receptors has been available. Quinoxalinediones have now been found to be potent and competitive antagonists at non-NMDA glutamate receptors. These compounds will be useful in the determination of the structure-activity relations of quisqualate and kainate receptors and the role of such receptors in synaptic transmission in the mammalian brain.

Differences in results from in vivo and in vitro studies on the use-dependency of N-methylaspartate antagonism by MK-801 and other phencyclidine receptor ligands

We have used microelectrophoretic and intravenous administration of drugs to rat spinal cord neurones in vivo and bath application to rat cortical wedges in vitro to evaluate MK-801 and other phencyclidine (PCP) receptor ligands as N-methylaspartate (NMA) antagonists, paying particular regard to the possible use-dependent nature of their action. MK-801, 0.1-0.5 mg/kg, was a selective and long-lasting NMA antagonist. We were unable to demonstrate significant use-dependent onset of antagonism of NMA by any of the drugs in vivo. Recovery, however, for MK-801 was use-dependent. In vitro there was a gradation with MK-801 being very use-dependent, followed by (PCP), cyclazocine and ketamine, the last showing little or no use-dependence. Results of experiments modulating the in vitro environment suggest that a significant difference between the in vitro and in vivo systems was temperature. Raising the temperature of the wedge chamber from 23 to 33 degrees C reduced the use-dependence of MK-801, and lowering the temperature to 13 degrees C increased the use-dependence of PCP. The mechanism of action of PCP receptor ligands is discussed in the light of these results.

Evidence for involvement of N-methylaspartate receptors in 'wind-up' of class 2 neurones in the dorsal horn of the rat

We have examined the effects of ketamine and kynurenate on the initial response and frequency dependent potentiation of response (wind-up) of class 2 neurones of the rat dorsal horn induced by repeated electrical stimulation of their receptive fields. Iontophoretic kynurenate reduced both the initial response and the wind-up. Iontophoretic or intravenous ketamine had no consistent effect on the initial response but consistently reduced wind-up. N-Methylaspartate receptors therefore appear to contribute to the wind-up, but not the initial response, of class 2 neurones in the rat.

Non-pharmacological effects of the use of microelectrophoresis and pressure ejection of drugs in combination

Pressure ejection of physiological saline from multibarrel micropipette assemblies has been shown to selectively reduce responses of rat spinal cord neurones to electrophoretic ejection of kainate, N-methyl-aspartate and 4-methyl-homoibotenate, but not of quisqualate or L-glutamate. Reduction of response to an excitatory amino acid therefore appears to be correlated with the absence of an active transport system for that amino acid.

Is Metaphit a phencyclidine antagonist? Studies with ketamine, phencyclidine and N-methylaspartate

The dissociative anaesthetics, phencyclidine and ketamine, block excitation of central neurones by N-methylaspartate. Using the technique of microelectrophoresis on rat spinal neurones in vivo Metaphit, a phencyclidine receptor acylating agent, was tested to see whether it would antagonise this effect of dissociative anaesthetics. The predominant effect of Metaphit was, however, to reduce N-methylaspartate induced excitation. It is concluded that Metaphit has mixed agonist/antagonist effects at the phencyclidine receptor.

2-Methyl-3,3-diphenyl-3-propanolamine (2-MDP) selectively antagonises N-methyl-aspartate (NMA)

Using electrophoretic application to rat central neurones in vivo, and bath application to frog spinal cord in vitro, 2-methyl-3,3-diphenyl-3-propanolamine was found to be a selective antagonist of N-methyl-DL-aspartate, but not of quisqualate or kainate. In this respect the (-) isomer proved to be about three times more potent than the (+) in both preparations.

Sympathetic modulation of cold-receptive neurones in the trigeminal system of the rat

The effects of cervical sympathetic electrical stimulation on the activity of cold-receptive neurones in the trigeminal system of the rat have been studied. Sympathetic stimulation caused excitation and/or suppression of cold-receptive cells in the trigeminal nucleus caudalis. The responses were complex but within any one cell low-frequency stimulation (below 5 Hz) usually (81% of cells) caused excitation, while high-frequency stimulation (above 10 Hz) caused excitation and suppression. The temperature of the cutaneous receptive fields was 'clamped' with a thermode. Measurements of surface and intradermal temperatures suggested that the temperature fall caused by sympathetic stimulation was not sufficient to account for the increased firing rate observed. The suppression was blocked by a low dose of phentolamine; it was mimicked by carotid occlusion and may be a consequence of vasoconstriction. The excitation was resistant to beta-blockade; it was best mimicked by the alpha-agonist phenylephrine. Similar frequency-dependent excitations and suppressions, and responses to pharmacological agonists, were obtained in recordings from cold-receptive primary afferent neurones in the trigeminal ganglion. The excitation may be a complex secondary effect, or there may be a direct alpha-receptor-mediated excitation of cold-receptive primary afferent fibres by the sympathetic system.

A narrow escape from splenectomy

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Sensory processing in a thermal afferent pathway

Extracellular recordings were made from cold-receptive afferent fibers in the trigeminal ganglion of rats anesthetized with halothane. By applying a standardized series of steady or changing temperatures to the receptive fields, we recorded the static and dynamic responses of the afferents. Comparable recordings were made from neurons in the marginal layer of the caudal trigeminal nucleus onto which the cold fibers synapse. The static and dynamic responses of the afferent fibers were reproduced faithfully by the second-order neurons, but at a much higher level of activity. Ganglionectomy silenced the second-order cells. Their continuous high level of activity appears to depend on the tonic input from the afferent fibers and not on any intrinsic circuits in the medulla.

In a small boat round the world

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Evidence for peripheral, but not central modulation of trigeminal cold receptive cells in the rat

The effects of locus coeruleus (LC), periaqueductal grey (PAG) and segmental stimulation (all of which are known to inhibit convergent nociceptive cells), were tested on the activity of cold receptive cells in the trigeminal system of the rat. LC and PAG stimulation from sites which inhibited convergent nociceptive cells had no effect on cells with cold receptive input in the trigeminal nucleus caudalis. Electrical or mechanical segmental stimulation caused suppression of activity in cold receptive trigeminal nucleus neurons. Recording from the trigeminal ganglion showed this suppression to be a property of the primary afferent cold receptors themselves and therefore it is not analogous to the proposed mechanism for the segmental inhibition of convergent nociceptive neurons.

Facial sensitivity to rates of temperature change: neurophysiological and psychophysical evidence from cats and humans

The dynamic responses in a thermal afferent pathway to rates of temperature change have been studied in anaesthetized cats. Recordings were made in the caudal trigeminal nucleus from neurones with a synaptic input from facial cold receptors. Five rates of cooling and warming ranging from 0.05 degrees C/sec to 1 degree C/sec were applied to the receptive fields of the neurones. Several measures of the dynamic response were computed but the most representative was the maximum rate during cooling or the minimum rate during warming. During cooling the maximum rate increased with increasing cooling rates between 0.05 degrees C/sec and 0.25 degrees C/sec, but did not increase at faster rates. Minimum activity during warming reached near zero at rates of 0.25 degrees C/sec and faster. The total number of impulses generated during cooling or absent during warming was unrelated to rate of temperature change. The same thermal stimuli were applied to the cheeks of human subjects. They were able to sense cooling or warming changes at 0.05 degrees C/sec. They could also distinguish the faster of two cooling changes when these were slow, but not when they were fast. Warming rates could not be distinguished, except from an adapting temperature of 35 degrees C, when warm receptors would have been activated. There was good agreement between the responses of the cat neurones and the human sensations. Slow rates of cooling could be detected or distinguished. Fast rates appeared to saturate the neuronal and sensory mechanisms.

Evaluation of a monoclonal antibody-based immunoradiometric assay for prostatic acid phosphatase

This report evaluates a new immunoradiometric assay for prostatic acid phosphatase in serum, based on a dual monoclonal antibody reaction system (Hybritech-TANDEM). A solidphase antibody binds the acid phosphatase molecule and a second monoclonal antibody to a different antigenic site serves as the 125I-radiolabel. The method was tested on 67 patients with various stages of prostatic carcinoma and 134 patients without the disease. It also was compared with a conventional polyclonal radioimmunoassay (NEN) and an enzymatic activity method (duPont aca). The upper limit for the TANDEM assay on nondiseased male patients was found to be 2.0 microgram/L. Based on this upper limit of normal, the diagnostic sensitivity of the method for all cases of prostatic carcinoma was 60%. We could not distinguish the enzyme released in abnormal amounts due to benign prostatic hypertrophy and certain nonprostatic malignant diseases from that of prostatic carcinoma. The diagnostic specificity was calculated at 95%. For the clinically undetectable Stage 1 disease, sensitivity was 44% (four abnormal values out of nine cases). The TANDEM procedure is simple to use and reproducible.

The development of the peripheral trigeminal innervation in Xenopus embryos

The development of the ophthalmic, maxillary and mandibular nerves has been followed in Xenopus laevis embryos from the first emergence of growth cones from the trigeminal ganglia until the establishment of functional innervation of the skin or cement gland. The course of each main nerve is highly predictable and follows pre-existing openings between blocks of other tissues. The development of the mandibulary nerve was observed most easily. Like that of the other trigeminal nerves it falls into three stages: (1) A pioneer neurite emerges and a nerve forms as other, later neurites fasciculate with this. (2) On reaching the inside surface of the cement gland the neurites separate and penetrate holes in the basal lamina. (3) The neurites grow between the cells they will innervate and form free nerve endings. The scanning EM observations have been confirmed by electrical recordings from trigeminal neurones. The role of pioneer fibres and substrate guidance are discussed.