Effects of the endogeneous cannabinoid, anandamide, on neuronal activity in rat hippocampal slices

Septal modulation of excitatory transmission in hippocampus

Application of the acetylcholinesterase inhibitor physostigmine to conventional hippocampal slices caused a significant reduction of field excitatory postsynaptic potentials (EPSPs) elicited by single pulse stimulation to the medial perforant path. Similar but smaller effects were obtained in the lateral perforant path and other excitatory pathways within hippocampus. The reductions were blocked by atropine, were not accompanied by evident changes in the EPSP waveform, and were eliminated by lesions to the cholinergic septo-hippocampal projections. Antidromic responses to mossy fiber stimulation, recorded in stratum granulosum, were not affected by the drug. However, paired-pulse facilitation was reliably increased, indicating that the depressed synaptic responses were secondary to reductions in transmitter release. The absence of cholinergic axo-axonic connections in the molecular layer suggests that physostigmine reduces presynaptic release by increasing retrograde signaling from the granule cells. In accord with this, an antagonist of the CB1 cannabinoid receptor eliminated the effects of physostigmine on synaptic responses, while an antagonist of the presynaptically located m2 muscarinic acetylcholine receptor did not. This is in contrast to previously reported effects involving application of cholinergic agonists, in which presynaptic inhibition likely results from direct activation of presynaptically located muscarinic receptors. In summary, it is proposed that the cholinergic inputs from the septum to the middle molecular layer modulate, via endocannabinoid release, the potency of the primary excitatory afferent of hippocampus.

Long-term plasticity of endocannabinoid signaling induced by developmental febrile seizures

Febrile (fever-induced) seizures are the most common form of childhood seizures, affecting 3%-5% of infants and young children. Here we show that the activity-dependent, retrograde inhibition of GABA release by endogenous cannabinoids is persistently enhanced in the rat hippocampus following a single episode of experimental prolonged febrile seizures during early postnatal development. The potentiation of endocannabinoid signaling results from an increase in the number of presynaptic cannabinoid type 1 receptors associated with cholecystokinin-containing perisomatic inhibitory inputs, without an effect on the endocannabinoid-mediated inhibition of glutamate release. These results demonstrate a selective, long-term increase in the gain of endocannabinoid-mediated retrograde signaling at GABAergic synapses in a model of a human neurological disease.

Functional localization of cannabinoid receptors and endogenous cannabinoid production in distinct neuron populations of the hippocampus

The possible localization of cannabinoid (CB) receptors to glutamatergic and GABAergic synaptic terminals impinging upon GABAergic interneurons in the CA1 region of the rat hippocampus was examined using the electrophysiological measurement of neurotransmitter release in brain slices. Whereas activation of cannabinoid receptors via the application of the cannabinoid agonist WIN55,212-2 significantly and dose-dependently reduced evoked IPSCs recorded from interneurons possessing somata located in the stratum radiatum (S.R.) and stratum oriens (S.O.) lamellae, evoked glutamatergic EPSCs were unaffected in both neuronal populations. However, in agreement with previous reports, WIN55,212-2 significantly reduced EPSCs recorded from CA1 pyramidal neurons. Additional experiments confirmed that the effects of WIN55,212-2 on IPSCs were presynaptic and that they could be blocked by the CB1 receptor antagonist SR141716A. The involvement of endogenous cannabinoids in the presynaptic inhibition of GABA release was also examined in the interneurons and pyramidal cells using a depolarization-induced suppression of inhibition (DSI) paradigm. DSI was observed in CA1 pyramidal neurons under control conditions, and its incidence was greatly increased by the cholinergic agonist carbachol. However, DSI was not observed in the S.R. or S.O. interneuron populations, in either the presence or absence of carbachol. Whereas DSI was not present in these interneurons, the inhibitory inputs to these cells were modulated by the synthetic cannabinoid WIN55,212-2. These data support the hypothesis that cannabinoid receptors are located on inhibitory, but not excitatory, axon terminals impinging upon hippocampal interneurons, and that CA1 pyramidal neurons, and not interneurons, are capable of generating endogenous cannabinoids during prolonged states of depolarization.

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

Effects of the synthetic cannabinoid nabilone on spatial learning and hippocampal neurotransmission

Cannabinoids, the active components of marijuana, affect memory and hippocampal neurotransmission. It has been claimed that nabilone, a synthetic cannabinoid endowed with antiemetic properties, has a peculiar profile of actions. We studied the effects of the drug on spatial learning and in vitro hippocampal CA1 electrophysiology in the rat. Nabilone (0.1, 0.5, and 1.0 mg/kg ip) does not impair place learning in a water maze task, whereas Delta(8)-tetrahydrocannabinol (Delta(8)-THC) disrupts this function. At concentrations ranging from 1 nM to 10 microM nabilone does not influence basal glutamatergic neurotransmission, which is decreased by Delta(8)-THC. Although cannabinoids have been consistently reported to affect synaptic plasticity, nabilone 1 microM does not change paired-pulse facilitation, long-term potentiation and the magnitude of long-term depression. However, the time course of the latter phenomenon is significantly changed by the drug, the depression being lower than in control experiments from 7 to 35 min postinduction. Altogether, our data indicate that there might be differences in the effects of agonists for central cannabinoid receptors, which could help to understand the pharmacology of this class of molecules. The results also suggest that amnesia induced by cannabinoids be possibly related to their effects on hippocampal neurotransmission. The study supports the use of nabilone in conditions the course of which is complicated by cognitive impairment.

Distinct cannabinoid sensitive receptors regulate hippocampal excitation and inhibition

One of the well-known effects of cannabinoids is the impairment of cognitive processes, including short-term memory formation, by altering hippocampal and neocortical functions reflected in network activity. Acting on presynaptically located G protein-coupled receptors in the hippocampus, cannabinoids modulate the release of neurotransmitter molecules. CB1 cannabinoid receptors, so far the only cloned cannabinoid receptor type in the CNS, are selectively expressed on the axon terminals of a subset of GABAergic inhibitory interneurons containing the neuropeptide cholecystokinin. Activation of CB1 receptors reduces GABA release from presynaptic terminals, thereby increasing the excitability of principal cells. Novel, non-CB1 cannabinoid sensitive receptors are present on the hippocampal excitatory axon terminals, which suppress glutamate release. These cannabinoid receptors have distinct pharmacological features compared to CB1, i.e. WIN 55212-2 is an order of magnitude less potent in reducing glutamatergic transmission than in inhibiting GABAergic postsynaptic currents, and the novel receptor binds vanilloid receptor ligands. Thus, at least two different cannabinoid sensitive presynaptic receptors regulate network activity in the hippocampus, CB1 via the GABAergic interneurons, and a new receptor via a direct action on pyramidal cell axon terminals.

Cannabinoids inhibit striatonigral GABAergic neurotransmission in the mouse

The substantia nigra pars reticulata (SNR) belongs to the brain regions with the highest density of CB(1) cannabinoid receptors. Anatomical studies indicate that the great majority of CB(1) receptors in the SNR are localized on terminals of GABAergic axons arriving from the caudate-putamen (striatonigral axons). The aim of the present experiments was to clarify the role of CB(1) receptors on terminals of striatonigral axons. Oblique sagittal slices, including the caudate-putamen and the substantia nigra, were prepared from brains of young mice. Electrical stimulation in the caudate-putamen elicited GABAergic inhibitory postsynaptic currents (IPSCs) in the SNR, which were studied by patch-clamp techniques. The long latency of IPSCs (14+/-1 ms) suggests that striatonigral axons were indeed activated within the caudate-putamen. The synthetic CB(1)/CB(2) cannabinoid receptor agonist WIN55212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate; 10(-5) M) decreased the amplitude of IPSCs by 93+/-1%. CP55940 ((-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol; 10(-5) M), another CB(1)/CB(2) receptor agonist, also reduced IPSC amplitude, by 76+/-4%. The CB(1) cannabinoid receptor antagonist SR141716A (N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide; 10(-6) M) prevented the inhibition produced by WIN55212-2 (10(-5) M). Depolarization of SNR neurons led to suppression of IPSCs; this suppression was prevented by SR141716A (10(-6) M). Three observations indicate that the agonists inhibited neurotransmission presynaptically. (1) CP55940 (10(-5) M) enhanced the ratio of amplitudes of two IPSCs which were elicited by two electrical stimuli 100 ms apart (paired pulses). (2) WIN55212-2 (10(-5) M) did not change the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. (3) WIN55212-2 (10(-5) M) also had no effect on currents elicited in SNR neurons by ejection of the GABA(A) receptor agonist muscimol from a pipet. In summary, we have established a method which allows selective examination of GABAergic neurotransmission between striatonigral axons and SNR neurons. Using this method, the function of CB(1) cannabinoid receptors on terminals of striatonigral axons was unequivocally clarified. Activation of these receptors causes strong presynaptic inhibition of GABAergic neurotransmission between striatonigral axons and SNR neurons. This effect may be one explanation of the catalepsy observed in animals after cannabinoid administration. Endocannabinoids released from SNR neurons can modulate striatonigral neurotransmission by inhibiting GABA release from terminals of striatonigral axons.

The endocannabinoid system: function in survival of the embryo, the newborn and the neuron

Since the identification and cloning of the first cannabinoid (CB1) receptor and the subsequent discovery of the endogenous cannabinoid ligands (endocannabinoids), anandamide, 2-arachidonoyl glycerol (2-AG) and noladin ether, a intensive search for their function in health and disease has been launched. The endocannabinoids in the central nervous system bind Gi/o coupled CB1 receptors that modulate adenylyl cyclase, ion channels and extracellular signal-regulated kinases. The present review discusses the nature of endocannabinoid (anandamide and 2-AG) neurotransmission, the activity of cannabinoids and the possibility that some of these activities are mediated via a receptor, yet to be discovered, which is distinct from the brain specific CB1 receptor. Three physiological functions in which the endocannabinoids play a critical role are also discussed: embryonal implantation, feeding and appetite, and neuroprotection.

Evidence for a physiological role of endocannabinoids in the modulation of seizure threshold and severity

The anticonvulsant effect of cannabinoids has been shown to be mediated through activation of the cannabinoid CB(1) receptor. This study was initiated to evaluate the effects of endogenously occurring cannabinoids (endocannabinoids) on seizure severity and threshold. The anticonvulsant effect of the endocannabinoid, arachidonylethanolamine (anandamide), was evaluated in the maximal electroshock seizure model using male CF-1 mice and was found to be a fully efficacious anticonvulsant (ED(50)=50 mg/kg i.p.). The metabolically stable analog of anandamide, (R)-(20-cyano-16,16-dimetyldocosa-cis-5,8,11,14-tetraenoyl)-1'-hydroxy-2'-propylamine (O-1812), was also determined to be a potent anticonvulsant in the maximal electroshock model (ED(50)=1.5 mg/kg i.p.). Furthermore, pretreatment with the cannabinoid CB(1) receptor specific antagonist N-(piperidin-1-yl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A) completely abolished the anticonvulsant effect of anandamide as well as O-1812 (P< or =0.01, Fisher exact test), indicating a cannabinoid CB(1) receptor-mediated anticonvulsant mechanism for both endocannabinoid compounds. Additionally, the influence of cannabinoid CB(1) receptor endogenous tone on maximal seizure threshold was assessed using SR141716A alone. Our data show that SR141716A (10 mg/kg i.p.) significantly reduced maximal seizure threshold (CC(50)=14.27 mA) compared to vehicle-treated animals (CC(50)=17.57 mA) (potency ratio=1.23, lower confidence limit=1.06, upper confidence limit=1.43), indicating the presence of an endogenous cannabinoid tone that modulates seizure activity. These data demonstrate that anandamide and its analog, O-1812, are anticonvulsant in a whole animal model and further implicate the cannabinoid CB(1) receptor as a major endogenous site of seizure modulation.

Pharmacological separation of cannabinoid sensitive receptors on hippocampal excitatory and inhibitory fibers

Our earlier studies demonstrated that in the hippocampus, cannabinoids suppress inhibitory synaptic transmission via CB(1) cannabinoid receptors, whereas a novel cannabinoid-sensitive receptor modulates excitatory synapses (Katona, I. et al., Journal of Neuroscience 19 (1999) 4544; Hájos, N. et al., European Journal of Neuroscience 12 (2000) 3239; Hájos, N. et al., Neuroscience 106 (2001) 1). The novel receptor does not correspond to CB(2), since this receptor type is not expressed in the brain (Munro, S. et al., Nature 365 (1993) 61). Recent binding experiments revealed that the synthetic cannabinoid WIN 55,212-2 binds with lower affinity to brain membranes of CB(1) receptor-knockout mice indicating that pharmacological differences exist between these two types of cannabinoid receptors in the hippocampus (Breivogel et al., Molecular Pharmacology 60 (2001) 155). To analyze this difference in detail, we first determined the EC(50) values of WIN 55,212-2 for excitatory and inhibitory transmission in rat hippocampal slices using whole-cell patch-clamp recordings. The estimated EC(50) value for inhibitory postsynaptic currents (IPSC) evoked by electrical stimulation in CA1 pyramidal cells was 0.24 microM, whereas for excitatory postsynaptic currents (EPSC) it was 2.01 microM, respectively. The cannabinoid antagonist, AM251, blocked the WIN 55,212-2-induced inhibition of evoked IPSCs, but not of EPSCs, providing evidence for its selectivity for CB(1). We then tested the hypothesis of whether the cannabinoid effect on hippocampal excitatory neurotransmission is mediated via receptors with an affinity for vanilloid ligands. Co-application of the vanilloid receptor antagonist capsazepine (10 microM) with cannabinoids (WIN55,212-2 or CP55,940) prevented the reduction of EPSCs, but not of IPSCs. The amplitude of evoked EPSCs was also suppressed by superfusion of the vanilloid receptor agonist capsaicin (10 microM), an effect which could also be antagonized by capsazepine. In contrast, capsaicin did not change the amplitude of evoked IPSCs. These results demonstrate that WIN 55,212-2 is an order of magnitude more potent in reducing GABAergic currents via CB(1) than in inhibiting glutamatergic transmission via the new CB receptor. The sensitivity of the new CB receptor (and EPSCs) to vanilloid ligands, but not to the cannabinoid antagonist AM251, represents another pharmacological tool to distinguish the two receptors, since CB(1) (and its effect on IPSCs) is not modulated by vanilloids, but is antagonized by AM251.

Putative neuroprotective actions of N-acyl-ethanolamines

N-Acyl-ethanolamines (NAEs) and their precursors, N-acyl-ethanolamine phospholipids (NAPEs), are present in the mammalian brain at levels of a few hundred picomoles/gram tissue and a few nanomoles/gram tissue, respectively. NAE-containing arachidonic acid is called anandamide, and it has attracted particular attention since it is a partial agonist for the cannabinoid receptors, for which 2-arachidonoylglycerol is the full agonist. In addition, anandamide may also activate the vanilloid receptor. Anandamide usually amounts to 1-10% of NAEs, as the vast majority of N-acyl groups are saturated and monounsaturated fatty acids. Formation of NAPE and NAE is catalyzed by an N-acyltransferase and an NAPE-hydrolyzing phospholipase D, respectively, two enzymes that have been characterized only preliminary. Interestingly, NAPEs and NAEs accumulate in the brain in response to neurodegenerative insults at a time when other phospholipids are subjected to rapid degradation. This is an important biosynthetic aspect of NAPE and NAE, as NAEs may be neuroprotective by a number of different mechanisms involving both receptor activation and non-receptor-mediated effects, e.g. by binding to cannabinoid receptors and interfering with ceramide turnover, respectively.

Identification of the endogenous cannabinoid system through integrative pharmacological approaches

Scientific progress in the biological sciences increasingly relies on an integration of behavioral, pharmacological, cellular, and molecular approaches, particularly in translating basic research observations into therapeutic potential. The strength of in vivo model systems lies in the direct assessment of physiological function. However, they only allow indirect evidence for mechanism of action. Frequently, in vitro models provide just the opposite. A combination of both in vitro and in vivo approaches are often essential for establishing the underlying mechanisms of a specific pharmacological effect. In recent times, an endogenous cannabinoid system has been characterized due to the combined efforts of chemists, pharmacologists, molecular and cellular biologists, and biochemists. This endogenous cannabinoid system is providing a basis for systematically addressing the pharmacological controversies surrounding marijuana. The description of this endogenous cannabinoid system and the strategies for establishing the physiological function of this system are the subjects of this article.

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.

Effect of cannabinoids on synaptic transmission in the rat hippocampal slice is temperature-dependent

We have previously reported that the synthetic cannabinoid R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate (WIN55,212-2) causes a selective inhibition of paired pulse depression of population spikes recorded from the CA1 region of rat hippocampal slices maintained at 28-30 degrees C. We now show that this effect is highly temperature-dependent and that WIN55,212-2 actually increases paired pulse depression of population spikes recorded from slices maintained at 35 degrees C. This temperature dependence was found to correlate with the effects of the known gamma-amino butyric acid (GABA)-uptake inhibitors, nipecotic acid and guvacine, which were without effect at 28-30 degrees C, but increased paired pulse depression at 35 degrees C. The results show that the effects of cannabinoids on synaptic transmission in the hippocampal slice are highly temperature-dependent and it is suggested that this is due to the presence of increased GABA uptake at higher temperatures.

Molecular biology of cannabinoid receptors

During the last decade, research on the molecular biology and genetics of cannabinoid receptors has led to a remarkable progress in understanding of the endogenous cannabinoid system, which functions in a plethora of physiological processes in the animal. At present, two types of cannabinoid receptors have been cloned from many vertebrates, and three endogenous ligands (the endocannabinoids arachidonoyl ethanolamide, 2-arachidonoyl glycerol and 2-arachidonoyl-glycerol ether) have been characterized. Cannabinoid receptor type 1 (CB(1)) is expressed predominantly in the central and peripheral nervous system, while cannabinoid receptor type 2 (CB(2)) is present almost exclusively in immune cells. Cannabinoid receptors have not yet been cloned from invertebrates, but binding proteins for endocannabinoids, endocannabinoids and metabolic enzyme activity have been described in a variety of invertebrates except for molting invertebrates such as Caenorhabditis elegans and Drosophila. In the central nervous system of mammals, there is strong evidence emerging that the CB(1) and its ligands comprise a neuromodulatory system functionally interacting with other neurotransmitter systems. Furthermore, the presynaptic localization of CB(1) together with the results obtained from electrophysiological experiments strengthen the notion that in cerebellum and hippocampus and possibly in other regions of the central nervous system, endocannabinoids may act as retrograde messengers to suppress neurotransmitter release at the presynaptic site. Many recent studies using genetically modified mouse lines which lack CB(1) and/or CB(2) finally could show the importance of cannabinoid receptors in animal physiology and will contribute to unravel the full complexity of the cannabinoid system.

Endocannabinoids in the central nervous system--an overview

Many aspects of the physiology and pharmacology of anandamide (arachidonoyl ethanol amide), the first endogenous cannabinoid ligand ("endocannabinoid") isolated from pig brain, have been studied since its discovery in 1992. Ethanol amides from other fatty acids have also been identified as endocannabinoids with similar in vivo and in vitro pharmacological properties. 2-Arachidonoyl glycerol and noladin ether (2-arachidonyl glyceryl ether), isolated in 1995 and 2001, respectively, so far, display pharmacological properties in the central nervous system, similar to those of anandamide. The endocannabinoids are widely distributed in brain, they are synthesized and released upon neuronal stimulation, undergo reuptake and are hydrolyzed intracellularly by fatty acid amide hydrolase (FAAH). For therapeutic purposes, inhibitors of FAAH may provide more specific cannabinoid activities than direct agonists, and several such molecules have already been developed. Pharmacological effects of the endocannabinoids are very similar, yet not identical, to those of the plant-derived and synthetic cannabinoid receptor ligands. In addition to pharmacokinetic explanations, direct or indirect interactions with other receptors have been considered to explain some of these differences, including activities at serotonin and GABA receptors. Binding affinities for other receptors such as the vanilloid receptor, have to be taken into account in order to fully understand endocannabinoid physiology. Moreover, possible interactions with receptors for the lysophosphatidic acids deserve attention in future studies. Endocannabinoids have been implicated in a variety of physiological functions. The areas of central activities include pain reduction, motor regulation, learning/memory, and reward. Finally, the role of the endocannabinoid system in appetite stimulation in the adult organism, and perhaps more importantly, its critical involvement in milk ingestion and survival of the newborn, may not only further our understanding of the physiology of food intake and growth, but may also find therapeutic applications in wasting disease and infant's "failure to thrive".

Novel cannabinoid-sensitive receptor mediates inhibition of glutamatergic synaptic transmission in the hippocampus

Psychoactive effects of cannabinoids are thought to be mediated, at least in part, by suppression of both glutamate and GABA release via CB1 cannabinoid receptor. Two types of cannabinoid receptor (CB1 and CB2) have been cloned so far. The CB1 receptors are abundantly expressed in the nervous system, whereas CB2 receptors are limited to lymphoid organs (Matsuda et al., 1990; Munro et al., 1993). Immunocytochemical and electrophysiological studies revealed that in the hippocampus CB1 receptors are expressed on axon terminals of GABAergic inhibitory interneurons (Tsou et al., 1999; Katona et al., 1999) and activation of these receptors decreases GABA release (Hájos et al., 2000). Other physiological studies pointed out the involvement of CB1 receptors in the modulation of hippocampal glutamatergic synaptic transmission and long-term potentiation (Stella et al., 1997; Misner and Sullivan, 1999), but anatomical studies could not confirm the existence of CB1 receptors on glutamatergic terminals. Here we examined cannabinoid actions on both glutamatergic and GABAergic synaptic transmission in the hippocampus of wild type (CB1+/+) and CB1 receptor knockout mice (CB1-/-). The synthetic cannabinoid agonist WIN55,212-2 reduced the amplitudes of excitatory postsynaptic currents in both wild type and CB1-/- mice, while inhibitory postsynaptic currents were decreased only in wild type mice, but not in CB1-/- animals. Our findings are consistent with a CB1 cannabinoid receptor-dependent modulation of GABAergic postsynaptic currents, but a novel cannabinoid-sensitive receptor must be responsible for the inhibition of glutamatergic neurotransmission.

Evidence for presynaptic cannabinoid CB(1) receptor-mediated inhibition of noradrenaline release in the guinea pig lung

Using neurochemical method, evidence was obtained that cannabinoid CB(1) receptors are localized on noradrenergic terminals and their stimulation by WIN-55,212-2 reduces the release of [3H]noradrenaline evoked by axonal activity in a frequency-dependent manner. At stimulation rates of 1 and 3 Hz, there was significant inhibition of noradrenaline release, with IC(50) of WIN-55,212-2 41.5+/-2.6 and 320.5+/-28.2 nM, for 1 and 3 Hz, respectively. Cannabinoid CB(1) receptor antagonist SR 141716A completely prevented WIN-55,212-2 from reducing the release. The release of noradrenaline is negatively modulated by presynaptic alpha(2)-adrenoceptors. Because BRL-44408, an alpha(2B)-adrenoceptor, and prazosin, an alpha(1)- and alpha(2B)-adrenoceptor antagonist, both increased the release of [(3)H]noradrenaline, it seems likely that the alpha(2B) subtype is responsible for the negative feedback modulation of noradrenaline release. In the presence of alpha(2)-adrenoceptor antagonism, cannabinoid CB(1) receptor activation by WIN-55,212-2 was much more effective in inhibiting the release of [(3)H]noradrenaline. Using a specific antibody against the C-terminus of the rat cannabinoid CB(1) receptor and also against neuropeptide Y, ultrastructural evidence was obtained that cannabinoid CB(1) receptors are exclusively localized on neuropeptide Y-positive noradrenergic varicosities. Since the sympathetic innervation of the human airway smooth muscle is sparse, and mainly the circulating adrenaline relaxes the airways via activation of beta(2)-adrenoceptor localized on the smooth muscle, it is suggested that inhibition of noradrenaline release by cannabinoids, and the subsequent bronchospasm, may be limited to those cases when noradrenaline released from sympathetic varicosities is involved in airway relaxation.

High-affinity inhibition of glutamate release from corticostriatal synapses by omega-agatoxin TK

To know which Ca(2+) channel type is the most important for neurotransmitter release at corticostriatal synapses of the rat, we tested Ca(2+) channel antagonists on the paired pulse ratio. omega-Agatoxin TK was the most effective Ca(2+) channel antagonist (IC(50)=127 nM; maximal effect=211% (with >1 microM) and Hill coefficient=1.2), suggesting a single site of action and a Q-type channel profile. Corresponding parameters for Cd(2+) were 13 microM, 178% and 1.2. The block of L-type Ca(2+) channels had little impact on transmission, but we also tested facilitation of L-type Ca(2+) channels. The L-type Ca(2+) channel agonist, s-(-)-1,4 dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644 (5 microM)), produced a 45% reduction of the paired pulse ratio, suggesting that even if L-type channels do not participate in the release process, they may participate in its modulation.

Modulation of transmitter release via presynaptic cannabinoid receptors

Cannabis (marijuana) is not only a frequently abused drug but also has the potential for the development of useful agents for the treatment of emesis, anorexia and multiple sclerosis. In this article, the effects of modulation of transmitter release by cannabinoids in both the CNS and the PNS of various species, including humans, will be discussed. Cannabinoids inhibit neurotransmitter release via specific presynaptic cannabinoid CB1 receptors. Studies using either the CB1 receptor antagonist and inverse agonist SR141716 or CB1-receptor-deficient mice suggest that numerous presynaptic cannabinoid receptors are tonically activated by endogenous cannabinoids and/or are constitutively active. CB1-receptor-mediated inhibition of transmitter release might explain, for example, reinforcing properties and memory impairment caused by cannabinoids.

Cannabinoids and neuroprotection

Cannabinoid compounds are endowed with pharmacological properties that make them interesting candidates for therapeutic development. These properties have been known since antiquity. However, in the last decade extremely important advances in the understanding of the physiology, pharmacology, and molecular biology of the cannabinoid system have given this field of research fresh impetus and have renewed the interest in the possible clinical exploitation of these compounds. In the present review we summarize the effects elicited, at the cellular level, by cannabinoids acting through receptor-dependent and receptor-independent mechanisms. These data suggest different ways by which cannabinoids may act as neuroprotective agents (prevention of excitotoxicity by inhibition of glutamate release, antioxidant effects, anti-inflammatory actions, etc.). The experimental evidence supporting these hypotheses are presented and discussed with regard to both preclinical and clinical studies in disease states such as cerebral ischemia, brain trauma, and Multiple Sclerosis.

Endocannabinoids Part II: pathological CNS conditions involving the endocannabinoid system and their possible treatment with endocannabinoid-based drugs

Changes in the levels of either the cannabinoid CB(1) receptors or of their endogenous ligands, anandamide and 2-arachidonoylglycerol, appear to be casual or consequential in many neurological disorders. Several examples of how such diseases may be treated by substances capable of selectively manipulating endocannabinoid levels and action are presented, using animal models of neuropathological conditions, such as motor disorders, multiple sclerosis, neuronal damage, chronic and inflammatory pain, anorexia, cachexia and motivational disturbances. These examples indicate that new therapeutic agents, lacking the undesirable psychotropic side effects of Cannabis, may be developed from current studies on the endocannabinoid system.

Are cannabinoid receptor knockout mice animal models for schizophrenia?

Schizophrenia is a devastating psychiatric disorder with a high prevalence worldwide. There is therefore a need for animal models allowing the development of new therapeutic interventions and reliable diagnostic tests. In the temporal domain, cannabinoid receptor gene (CB1) knockout mice exhibit behavioural alterations, which parallel symptoms in schizophrenia, cannabis intoxication and dopamine D2 activation. While a specific nucleotide homology between CB1 and D2 accounts for the pathophysiology, pre-inserted spirochaetal DNA on the polyadenylation signal of CB1 reveals the aetiology of schizophrenia. If, in analogy to thalassaemia, mutations occur within this 3' regulatory domain, the genetic expression of CB1 is disrupted and sequential information lost in time. CB1, previously unrecognized as a candidate gene, thus unifies the different aspects of schizophrenic psychosis: cannabis-induced model psychosis, disrupted information processing, spatio-temporal distortions and other psychotic symptoms, disturbed neuronal migration, schizophrenic brain disorder, familial transmission, and prenatal infection by Borrelia burgdorferi.

Presynaptic mechanisms underlying cannabinoid inhibition of excitatory synaptic transmission in rat striatal neurons

The striatum is a crucial site of action for the motor effects of cannabinoids (CBs). However, the electrophysiological consequences of activation of CB receptors on the striatal neurons have not been established. Here we report for the first time that the cannabimimetic aminoalkylindole WIN 55,212-2 and the endogenous cannabinoid anandamide substantially depress corticostriatal glutamatergic synaptic transmission onto striatal neurons in the brain slice preparation. The selective CB1 receptor antagonist SR 141716 effectively reversed this inhibition. WIN 55,212-2 significantly increased the paired-pulse facilitation of synaptically evoked EPSCs, while having no effect on the sensitivity of postsynaptic neurons to [alpha]-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. WIN 55,212-2 also reduced the frequency of spontaneous, action potential-dependent EPSCs (sEPSCs) without altering their amplitude distribution. Superfusion of WIN 55,212-2 elicited a membrane hyperpolarization accompanied by a decrease in input resistance. Both effects were blocked by intracellular caesium. In contrast, intracellular caesium failed to affect WIN 55,212-2-mediated synaptic inhibition. The WIN 55,212-2-mediated synaptic inhibition was blocked by the Gi/o protein inhibitor pertussis toxin (PTX), but not by the GABA(A) receptor antagonist bicuculline or GABA(B) receptor antagonist SCH 50911. Pretreatment with the N-type Ca2+ channel antagonist [omega]-conotoxin GVIA selectively abolished the WIN-55,212-2-mediated synaptic inhibition. These results suggest that cannabinoids depress the corticostriatal glutamatergic synaptic transmission through the activation of presynaptic CB1 receptors to inhibit N-type Ca2+ channel activity, which in turn reduces glutamate release. The presynaptic action of cannabinoids is mediated by a PTX-sensitive Gi/o protein-coupled signalling pathway.

GABAergic interneurons are the targets of cannabinoid actions in the human hippocampus

Cannabinoids have been shown to disrupt memory processes in mammals including humans. Although the CB1 neuronal cannabinoid receptor was identified several years ago, neuronal network mechanisms mediating cannabinoid effects are still controversial in animals, and even more obscure in humans. In the present study, the localization of CB1 receptors was investigated at the cellular and subcellular levels in the human hippocampus, using control post mortem and epileptic lobectomy tissue. The latter tissue was also used for [3H]GABA release experiments, testing the predictions of the anatomical data. Detectable expression of CB1 was confined to interneurons, most of which were found to be cholecystokinin-containing basket cells. CB1-positive cell bodies showed immunostaining in their perinuclear cytoplasm, but not in their somadendritic plasmamembrane. CB1-immunoreactive axon terminals densely covered the entire hippocampus, forming symmetrical synapses characteristic of GABAergic boutons. Human temporal lobectomy samples were used in the release experiments, as they were similar to the controls regarding cellular and subcellular distribution of CB1 receptors. We found that the CB1 receptor agonist, WIN 55,212-2, strongly reduced [3H]GABA release, and this effect was fully prevented by the specific CB1 receptor antagonist SR 141716A. This unique expression pattern and the presynaptic modulation of GABA release suggests a conserved role for CB1 receptors in controlling inhibitory networks of the hippocampus that are responsible for the generation and maintenance of fast and slow oscillatory patterns. Therefore, a likely mechanism by which cannabinoids may impair memory and associational processes is an alteration of the fine-tuning of synchronized, rhythmic population events.

Accumulation of the anandamide precursor and other N-acylethanolamine phospholipids in infant rat models of in vivo necrotic and apoptotic neuronal death

It has been demonstrated that the endogenous cannabinoid receptor ligand, anandamide, and other N-acylethanolamines (NAEs), accumulate during neuronal injury in vitro, a process that may be linked to the neuroprotective effects of NAEs. The crucial step for generation of NAEs is the synthesis of the corresponding precursors, N-acylethanolamine phospholipids (NAPEs). However, it is unknown whether this key event for NAE formation is regulated differently in the context of insults causing necrotic or apoptotic neuronal death. To address this question, we monitored a range of cortical NAPE species in three infant rat models of in vivo neurodegeneration: (i) necrosis caused by intrastriatal injection of NMDA (25 nmol); (ii) apoptosis induced by systemic administration of the NMDA-receptor antagonist (+)MK-801 (3 x 0.5 mg/kg, i.p.); and (iii) apoptosis following focal necrosis triggered by concussive head trauma. A marked increase of all NAPE species was observed in both hemispheres 4 and 24 h after NMDA-induced injury, with a relatively larger increase in N-stearoyl-containing NAPE species. Thus, the percentage of the anandamide precursor fell from 1.1 to 0.5 mol %. In contrast, administration of (+)MK-801 did not alter cortical NAPE levels. Concussion head trauma resulted in a similar but less pronounced upregulation of NAPE levels at both 4 and 24 h as compared to NMDA injections. Increased levels of NAPE 24 h post-trauma possibly reflect that necrosis is still ongoing at this time point. Consequently, our data suggest that excitotoxic necrotic mechanisms of neurodegeneration, as opposed to apoptotic neurodegeneration, have a profound effect on in vivo NAE precursor homeostasis.

N-Acylethanolamines and precursor phospholipids - relation to cell injury

The present review focuses on the relationship between formation of N-acylethanolamine phospholipids (NAPEs) and N-acyletransferase (NAEs) catalyzed by N-acyltranferase and NAPE-hydrolyzing phospholipase D, respectively, and cell injury in tissues like brain, heart, and testis. A number of mechanisms are proposed by which these two groups of lipids may have cytoprotective properties. The mechanisms may involve activation of cannabinoid receptors, as well as non-receptor-mediated effects such as stabilization of membrane bilayers, antioxidant mechanisms, inhibition of calcium leakage from mitochondria, and direct inhibition of ceramidase. Anandamide (20:4-NAE) is formed as a minor component along with other NAEs during cell injury. Whether 20:4-NAE has a separate physiological role is at present not known, but some data suggest that 20:4-NAE may be formed, e.g. in the uterus, by a more selective mechanism without being accompanied by a vast majority of saturated and monounsaturated NAEs.

Cannabinoid receptor mediated inhibition of excitatory synaptic transmission in the rat hippocampal slice is developmentally regulated

The cannabinoid (CB) receptor agonist WIN55,212-2 (500 nM) had no effect on the first of a pair of population spikes evoked in the CA1 region of hippocampal slices prepared from young adult (4 - 6 weeks old) rats, despite powerfully reducing paired-pulse depression. In contrast WIN55,212-2 caused a substantial depression of the single population spike (reduced to 43% control) and the field EPSP (reduced to 72% of control) recorded in slices prepared from neonatal (10 - 13 days old) rats. This effect was stereoselective and blocked by the CB(1) receptor antagonist AM281 (500 nM). The results indicate that activation of CB(1) receptors inhibits excitatory synaptic transmission in neonatal, but not adult rat hippocampus. This developmental regulation of CB(1) receptor mediated control of excitatory transmission may help explain some, but not all, of the previous discrepancies in the literature.

Cannabinoids inhibit hippocampal GABAergic transmission and network oscillations

Using a new antibody developed against the C-terminus of the cannabinoid receptor (CB1), the immunostaining in the hippocampus revealed additional axon terminals relative to the pattern reported previously with an N-terminus antibody. Due to a greater sensitivity of this antibody, a large proportion of boutons in the dendritic layers displaying symmetrical (GABAergic) synapses were also strongly immunoreactive for CB1 receptors, as were axon terminals of perisomatic inhibitory cells containing cholecystokinin. Asymmetrical (glutamatergic) synapses, however, were always negative for CB1. To investigate the effect of presynaptic CB1 receptor activation on hippocampal inhibition, we recorded inhibitory postsynaptic currents (IPSCs) from principal cells. Bath application of CB1 receptor agonists (WIN55,212-2 and CP55,940) suppressed IPSCs evoked by local electrical stimulation, which could be prevented or reversed by the CB1 receptor antagonist SR141716A. Action potential-driven IPSCs, evoked by pharmacological stimulation of a subset of interneurons, were also decreased by CB1 receptor activation. We also examined the effects of CB1 receptor agonists on Ca2+-independent miniature IPSCs (mIPSC). Both agonists were without significant effect on the frequency or amplitude of mIPSCs. Synchronous gamma oscillations induced by kainic acid in the CA3 region of hippocampal slices were reversibly reduced in amplitude by the CB1 receptor agonist CP 55,940, which is consistent with an action on IPSCs. We used CB1-/- knock-out mice to confirm the specificity of the antibody and of the agonist (WIN55,212-2) action. We conclude that activation of presynaptic CB1 receptors decreases Ca2+-dependent GABA release, and thereby reduces the power of hippocampal network oscillations.

Functional expression of cell surface cannabinoid CB(1) receptors on presynaptic inhibitory terminals in cultured rat hippocampal neurons

At present, little is known about the mechanisms by which cannabinoids exert their effects on the central nervous system. In this study, fluorescence imaging and electrophysiological techniques were used to investigate the functional relationship between cell surface cannabinoid type 1 (CB(1)) receptors and GABAergic synaptic transmission in cultured hippocampal neurons. CB(1) receptors were labelled on living neurons using a polyclonal antibody directed against the N-terminal 77 amino acid residues of the rat cloned CB(1) receptor. Highly punctate CB(1) receptor labelling was observed on fine axons and at axonal growth cones, with little somatic labelling. The majority of these sites were associated with synaptic terminals, identified either with immunohistochemical markers or by using the styryl dye FM1-43 to label synaptic vesicles that had undergone active turnover. Dual labelling of neurons for CB(1) receptors with either the inhibitory neurotransmitter GABA or its synthesising enzyme glutamate decarboxylase, demonstrated a strong correspondence. The immunocytochemical data was supported by functional studies using whole-cell patch-clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs). The cannabinoid agonist WIN55,212-2 (100nM) markedly inhibited (by 77+/-6.3%) the frequency of pharmacologically-isolated GABAergic mIPSCs. The effects of WIN55,212-2 were blocked in the presence of the selective CB(1) receptor antagonist SR141716A (100nM).In conclusion, the present data show that cell surface CB(1) receptors are expressed at presynaptic GABAergic terminals, where their activation inhibits GABA release. Their presence on growth cones could indicate a role in the targeting of inhibitory connections during development.

Determination of the phospholipid precursor of anandamide and other N-acylethanolamine phospholipids before and after sodium azide-induced toxicity in cultured neocortical neurons

Phospholipase D-mediated hydrolysis of N-acylethanolamine phospholipids (NAPEs) releases anandamide and other N-acylethanolamines, resulting in different actions at cellular targets in the CNS. Recently, we have demonstrated that these N-acyl lipids accumulate in cultured neocortical neurons subjected to sodium azide-induced cell injury. We here extend the information on the NAPE response, reporting on the composition of N-acylspecies of NAPE, employing a new methodological approach of HPLC-coupled electrospray ionization mass spectrometry. Exposure to sodium azide (5 mM) increased the total amount of NAPE threefold over control levels; however, no alteration of the relative composition of NAPE species was detected. The anandamide precursor (20 : 4-NAPE) constituted only 0.1% of all NAPEs detected in the neurons. Total NAPE species in control cells amounted to 956-1,060 pmol/10(7) cells. Moreover, we detected the presence of an unknown NAPE species with molecular weight identical to 20 : 4-NAPE. This may suggest the presence of a putative stereoisomer of the anandamide precursor with at least one trans-configured double bond in the N-arachidonoyl moiety. These results show that with the present method, neuronal NAPE species can be identified and quantified with respect to N-acyl composition, including a trans-isomer of the anandamide precursor. The anandamide precursor is up-regulated to the same extent as other NAPEs upon neuronal injury.

Cannabinoids inhibit excitatory neurotransmission in the substantia nigra pars reticulata

The substantia nigra pars reticulata belongs to the brain regions with the highest density of CB(1) cannabinoid receptors. Since the level of CB(1) receptor messenger RNA is very low in the pars reticulata, most of the receptors are probably localized on terminals of afferent axons. The hypothesis was tested that terminals of glutamatergic afferents of substantia nigra pars reticulata neurons possess CB(1) cannnabinoid receptors, the activation of which presynaptically modulates neurotransmission. Rat midbrain slices were superfused and the electrophysiological properties of substantia nigra pars reticulata neurons were studied with the patch-clamp technique. Focal electrical stimulation in the presence of bicuculline evoked excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate glutamate receptors. The excitatory postsynaptic currents were reduced by the metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 10(-4)M). The mixed CB(1)/CB(2) cannabinoid receptor agonists R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2, 3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone (WIN55212-2; 10(-8)-10(-5)M) and (-)-cis-3-[2-hydroxy-4-(1, 1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP55940; 10(-6)M) also produced inhibition. The maximal inhibition by WIN55212-2 was 54+/-6%. The CB(1) cannabinoid antagonist N-piperidino-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR141716A; 10(-6)M) prevented the effect of WIN55212-2, but had no effect when superfused alone. WIN55212-2 (10(-6)M) increased the amplitude ratio of two excitatory postsynaptic currents evoked with an interstimulus interval of 100ms. Currents evoked by short ejection of glutamate on to the surface of the slices were not changed by WIN55212-2. The results show that activation of CB(1) cannabinoid receptors inhibits glutamatergic synaptic transmission between afferent axons and neurons in the substantia nigra pars reticulata. The lack of effect of the cannabinoids on glutamate-evoked currents and the increase of the paired-pulse ratio indicate that the mechanism of action is presynaptic inhibition of transmitter release.

Cellular and molecular mechanisms underlying learning and memory impairments produced by cannabinoids

Why does smoking marijuana impair learning and memory? Behavioral studies suggest that a disruption of normal hippocampal function contributes to these deficits. In vitro experiments find that cannabinoid receptor activation reduces neurotransmitter release below the levels required to trigger long-term changes in synaptic strength in the hippocampus. Cannabinoids reduce glutamate release through a G-protein-mediated inhibition of the calcium channels responsible for neurotransmitter release from hippocampal neurons. These mechanisms likely play a role in the learning and memory impairments produced by cannabinoids and by endogenous cannabinoid receptor ligands.

GABAergic presynaptic inhibition of rat neostriatal afferents is mediated by Q-type Ca(2+) channels

Population spikes associated with the paired pulse facilitation paradigm have been successfully used to measure presynaptic inhibition in several systems. In the present work, this paradigm was used to evaluate the action of baclofen on neostriatal glutamatergic transmission. Baclofen enhanced synaptic facilitation with an EC(50)=0.57 microM and a maximal effect of 457%. Selective antagonists for N-, P- and Q-type Ca(2+)-channels enhanced paired pulse facilitation; suggesting that these channel types participate in the release of transmitter. Nevertheless, neither 1 microM omega-conotoxin GVIA, nor 20 nM omega-agatoxinTK occluded the action of baclofen. Baclofen's action was occluded only by 400 nM omega-agatoxinTK. These data suggest that Q-type Ca(2+)-channels mediate gamma-aminobutyric acid(B) presynaptic inhibition of neostriatal afferents.