Electrically evoked release of [(3)H]noradrenaline from mouse cultured sympathetic neurons: release-modulating heteroreceptors

Cannabinoids for the treatment of schizophrenia? A balanced neurochemical framework for both adverse and therapeutic effects of cannabis use

Recent studies have found that cannabinoids may improve neuropsychological performance, ameliorate negative symptoms, and have antipsychotic properties for a subgroup of the schizophrenia population. These findings are in contrast to the longstanding history of adverse consequences of cannabis use, predominantly on the positive symptoms, and a balanced neurochemical basis for these opposing views is lacking. This paper details a review of the neurobiological substrates of schizophrenia and the neurochemical effects of cannabis use in the normal population, in both cortical (in particular prefrontal) and subcortical brain regions. The aim of this paper is to provide a holistic neurochemical framework in which to understand how cannabinoids may impair, or indeed, serve to ameliorate the positive and negative symptoms as well as cognitive impairment. Directions in which future research can proceed to resolve the discrepancies are briefly discussed.

Cluster headache associated with moyamoya

A 34-year-old man with right-sided cluster headache presented with a stroke from right-sided moyamoya. Following surgery on the right, both moyamoya and cluster headache remitted, but eighteen months later a cluster attack and symptoms of cerebral ischemia from moyamoya recurred on the left. Again, following surgery on the left, both moyamoya symptoms and cluster attacks disappeared. Cluster headache secondary to moyamoya has not previously been described.

Characterization of prostanoid receptors present on adrenergic neurons innervating the porcine uterine longitudinal muscle

The cyclooxygenase-prostanoid pathway regulates myometrial contractility through activation of prostanoid receptors on uterine smooth muscles. However, the possible expression of prostanoid receptors on autonomic nerves cannot be excluded completely. The aim of the present study was to clarify the presence of neural prostanoid receptors on adrenergic nerves in the porcine uterine longitudinal muscle. In [(3)H]-noradrenaline-loaded longitudinal muscle strips of porcine uterus, electrical field stimulation (EFS) evoked [(3)H]-noradrenaline release in a stimulation frequency-dependent manner. The EFS-evoked release was completely abolished in Ca(2+)-free (EGTA, 1mM) incubation medium and by tetrodotoxin or omega-conotoxin GVIA, suggesting that [(3)H]-noradrenaline was released from neural components. The EFS-evoked [(3)H]-noradrenaline release was significantly enhanced by treatment with indomethacin. In the presence of indomethacin, PGE(2) and PGF(2alpha), but not PGD(2), inhibited the EFS-evoked [(3)H]-noradrenaline release. Of synthetic prostanoid receptor agonists examined, both U46619 (TP) and sulprostone (EP(1)/EP(3)) decreased the EFS-evoked [(3)H]-noradrenaline release in a concentration-dependent manner, while fluprostenol (FP), BW245C (DP) and butaprost (EP(2)) were almost ineffective. SQ29548 (TP receptor antagonist) blocked the effect of U46619, but SC19220 (EP(1) receptor antagonist) did not change the inhibition by sulprostone or PGE(2). Double immunofluorescence staining using protein gene product 9.5, tyrosine hydroxylase, EP(3) receptor and TP receptor antibodies suggested the localization of EP(3) or TP receptors on adrenergic nerves in the porcine uterus. These results indicated that neural EP(3) and TP receptors are present on adrenergic nerves of the porcine uterine longitudinal muscle. Endogenous prostanoid produced by cyclooxygenase can regulate noradrenaline release in an inhibitory manner through activation of these neural prostanoid receptors.

Therapeutic use of release-modifying drugs

Presynaptic inhibitory or facilitatory autoreceptors are targets for the endogenous neurotransmitter of the respective neuron, and also for exogenous agonists, partial agonists and antagonists which can produce pharmacological actions through changes in transmitter release. In addition, presynaptic inhibitory or facilitatory heteroreceptors can also be acted upon by exogenous agonists, partial agonists or antagonists to induce changes in transmitter release with useful therapeutic effects. This article summarizes drugs that are known or likely to produce their therapeutic effects through presynaptic modulation of neurotransmitter release. Included are drugs acting on alpha and beta adrenoceptors, dopamine receptors, angiotensin, opioid, cannabinoid, and nicotinic acetylcholine receptors. Also discussed are changes in presynaptic receptor mechanisms produced by drugs that inhibit transmitter re-uptake.

Regulation of neural KCNQ channels: signalling pathways, structural motifs and functional implications

Neural M-type (KCNQ/Kv7) K(+) channels control somatic excitability, bursting and neurotransmitter release throughout the nervous system. Their activity is regulated by multiple signalling pathways. In superior cervical ganglion sympathetic neurons, muscarinic M(1), angiotensin II AT(1), bradykinin B(2) and purinergic P2Y agonists suppress M current (I(M)). Probes of PLC activity show agonists of all four receptors to induce robust PIP(2) hydrolysis. We have grouped these receptors into two related modes of action. One mode involves depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in the membrane, whose interaction with the channels is thought necessary for their function. The other involves IP(3)-mediated intracellular Ca(2+) signals that stimulate PIP(2) synthesis, preventing its depletion, and suppress I(M) via calmodulin. Carbon-fibre amperometry can evaluate the effect of M channel activity on release of neurotransmitter. Consistent with the dominant role of M current in control of neuronal discharge, M channel openers, or blockers, reduced or augmented the evoked release of noradrenaline neurotransmitter from superior cervical ganglion (SCG) neurons, respectively. We seek to localize the subdomains on the channels critical to their regulation by PIP(2). Based on single-channel recordings from chimeras between high-PIP(2) affinity KCNQ3 and low-PIP(2) affinity KCNQ4 channels, we focus on a 57-residue domain within the carboxy-terminus that is a possible PIP(2) binding site. Homology modelling of this domain using the published structure of IRK1 channels as a template predicts a structure very similar to an analogous region in IRK1 channels, and shows a cluster of basic residues in the KCNQ2 domain to correspond to those implicated in PIP(2) regulation of Kir channels. We discuss some important issues dealing with these topics.

Short-term exposure to somatostatin or muscarinic agonists reduce acetylcholine-induced 3H-MPP+ release from bovine adrenal medullary cells

The aim of this work was to investigate the effect of a short-term exposure to somatostatin (SS), its receptors (SSTR) selective agonists as well as muscarinic receptors agonists upon acetylcholine-induced release of (3)H-MPP(+ )from bovine adrenal medullary cells. Acetylcholine (ACH, 100, 500 microM) was found to increase the release of (3)H-MPP(+ )by these cells (to 175 and 171% of basal release, respectively). ACH-elicited (3)H-MPP(+) release was significantly reduced by hexamethonium (100 microM) and atropine (100 microM), selective nicotinic and muscarinic antagonists, respectively. Previous exposure to any of two muscarinic agonists, oxotremorine or pilocarpine, led to a significant reduction of (3)H-MPP(+) release in response to 100 microM ACH, to about a maximum of 51% and 78% of control, respectively. Somatostatin (SS, 0.01-0.1 microM), previously applied to the preparation, depressed ACH-elicited (3)H-MPP(+ )release by 25-27%, but only when a 500 microM ACH concentration was used. The inhibition exerted by SS upon ACH-evoked (3)H-MPP(+) release appeared to be mediated by its SSTR: (1) SSTR2, 3 and 4 subtype agonists mimicked the effects seen with SS, and (2) the SSTR non-selective antagonist, cyclo-SS, counteracted the SS inhibitory effect. When SS was tested in the presence of any of the muscarinic agonists, oxotremorine or pilocarpine, its inhibitory effect on 500 microM ACH-induced (3)H-MPP(+) release was no longer detectable. These results, showing a somewhat similar effect of short-term exposure to SS and muscarinic agonists over ACH-induced release of (3)H-MPP(+), as well as the loss of effect of SS by the presence of the muscarinic agonists, suggest that these compounds may share signalling pathways.

Effects of cannabinoid receptor agonists on rat gastric acid secretion: discrepancy between in vitro and in vivo data

The effects of the cannabinoid (CB)-receptor agonists WIN55,212-2 and HU-210 and the selective CB(1)-receptor antagonist SR141716A were tested on in vitro and in vivo acid secretion assays from the rat. In the isolated gastric fundus from immature rats, WIN55,212-2 (0.001-30 microM), HU-210 (0.001-10 microM), or SR141716A (0.1-10 microM) did not change the basal acid output or acid responses to histamine, pentagastrin, or electrical field stimulation. HU-210 (0.3 micromol/kg, intravenously) inhibited the acid response to pentagastrin in anesthetized adult, young, or immature rats with lumen-perfused stomachs; moreover, HU-210 reduced vagally induced acid secretion in adult animals, its antisecretory effect being reversed by SR141716A (0.65 micromol/kg, intravenously). In vitro and in vivo data indicate that CB(1) receptors are not located on parietal cells but, rather, on vagal pathways (possibly at preganglionic sites) supplying the gastric mucosa. The lack of effect of CB-receptor ligands in vitro cannot be ascribed to the use of immature rats, since HU-210 inhibited stimulated acid secretion in vivo, irrespective of the animal age.

Effects of cannabinoids on neurotransmission

The CB1 cannabinoid receptor is widely distributed in the central and peripheral nervous system. Within the neuron, the CB1 receptor is often localised in axon terminals, and its activation leads to inhibition of transmitter release. The consequence is inhibition of neurotransmission via a presynaptic mechanism. Inhibition of glutamatergic, GABAergic, glycinergic, cholinergic, noradrenergic and serotonergic neurotransmission has been observed in many regions of the central nervous system. In the peripheral nervous system, CB1 receptor-mediated inhibition of adrenergic, cholinergic and sensory neuroeffector transmission has been frequently observed. It is characteristic for the ubiquitous operation of CB1 receptor-mediated presynaptic inhibition that antagonistic components of functional systems (for example, the excitatory and inhibitory inputs of the same neuron) are simultaneously inhibited by cannabinoids. Inhibition of voltage-dependent calcium channels, activation of potassium channels and direct interference with the synaptic vesicle release mechanism are all implicated in the cannabinoid-evoked inhibition of transmitter release. Many presynaptic CB1 receptors are subject to an endogenous tone, i.e. they are constitutively active and/or are continuously activated by endocannabinoids. Compared with the abundant data on presynaptic inhibition by cannabinoids, there are only a few examples for cannabinoid action on the somadendritic parts of neurons in situ.

Presynaptic inhibition of transmitter release from rat sympathetic neurons by bradykinin

Bradykinin is known to stimulate neurons in rat sympathetic ganglia and to enhance transmitter release from their axons by interfering with the autoinhibitory feedback, actions that involve protein kinase C. Here, bradykinin caused a transient increase in the release of previously incorporated [3H] noradrenaline from primary cultures of dissociated rat sympathetic neurons. When this effect was abolished by tetrodotoxin, bradykinin caused an inhibition of tritium overflow triggered by depolarizing K+ concentrations. This inhibition was additive to that caused by the alpha2-adrenergic agonist UK 14304, desensitized within 12 min, was insensitive to pertussis toxin, and was enhanced when protein kinase C was inactivated. The effect was half maximal at 4 nm and antagonized competitively by the B2 receptor antagonist Hoe 140. The cyclooxygenase inhibitor indomethacin and the angiotensin converting enzyme inhibitor captopril did not alter the inhibition by bradykinin. The M-type K+ channel opener retigabine attenuated the secretagogue action of bradykinin, but left its inhibitory action unaltered. In whole-cell patch-clamp recordings, bradykinin reduced voltage-activated Ca2+ currents in a pertussis toxin-insensitive manner, and this action was additive to the inhibition by UK 14304. These results demonstrate that bradykinin inhibits noradrenaline release from rat sympathetic neurons via presynaptic B2 receptors. This effect does not involve cyclooxygenase products, M-type K+ channels, or protein kinase C, but rather an inhibition of voltage-gated Ca2+ channels.

3H-noradrenaline release from mouse iris?ciliary body: role of presynaptic muscarinic heteroreceptors

Sympathetic neurotransmitter release and its modulation by presynaptic muscarinic heteroreceptors were studied in mouse iris-ciliary bodies. Tissue preparations were preincubated with (3)H-noradrenaline and then superfused and stimulated electrically. Firstly, experimental conditions were defined, allowing study of presynaptic sympathetic inhibition in mouse iris-ciliary body. If tissue was stimulated four times with 36 pulses/3 Hz, tritium overflow peaks were reliably and reproducibly measured. As expected, these stimulation conditions led to marked alpha(2)-autoinhibition as indicated by the release-enhancing effect of the alpha(2)-antagonists phentolamine and rauwolscine. To ensure autoinhibition-free (3)H-noradrenaline release, which is optimal for studying presynaptic sympathetic inhibition, alpha(2)-receptors were blocked in all subsequent experiments. Under these conditions, evoked tritium overflow was almost completely abolished in the presence of the sodium channel blocker tetrodotoxin, indicating a neuronal origin of (3)H-noradrenaline release. Secondly, muscarinic inhibition of (3)H-noradrenaline release was characterized using the conditions described above (36 pulses/3 Hz; phentolamine 1 muM and rauwolscine 1 muM throughout). The muscarinic receptor agonist oxotremorine M decreased evoked tritium overflow in a concentration-dependent manner with an IC(50) of 0.33 muM and maximal inhibition of 51%. The concentration-response curve of oxotremorine M was shifted to the right by the muscarinic antagonists ipratropium and methoctramine, whereas pirenzepine was ineffective. The observed rank order of antagonist potencies, ipratropium > methoctramine > pirenzepine, which is typical for the M(2) subtype, indicates that presynaptic muscarinic receptors on sympathetic axons of mouse iris-ciliary bodies are predominantly M(2). Finally, inhibition of (3)H-noradrenaline release by endogenously secreted acetylcholine was investigated. Longer pulse trains, 120 pulses/3 Hz and 600 pulses/5 Hz, were used and the cholinesterase inhibitor physostigmine was added to the superfusion medium to increase synaptic levels of endogenous acetylcholine. Under these conditions, ipratropium approximately doubled the evoked overflow of tritium, indicating that endogenously released acetylcholine can activate presynaptic muscarinic heteroreceptors. In conclusion, the present experiments establish measurement of the electrically induced release of (3)H-noradrenaline from mouse iris-ciliary bodies. As in other species, noradrenaline release in this preparation was subject to presynaptic muscarinic inhibition. Our results also indicate that the presynaptic muscarinic receptors on sympathetic axons in mouse iris-ciliary body are predominantly M(2). Moreover, these receptors can be activated by both exogenous agonists and endogenously released acetylcholine and, hence, may operate physiologically in the interplay between the parasympathetic and sympathetic nervous system.

Activation of M1 muscarinic receptors triggers transmitter release from rat sympathetic neurons through an inhibition of M-type K+ channels

Acetylcholine has long been known to excite sympathetic neurons via M1 muscarinic receptors through an inhibition of M-currents. Nevertheless, it remained controversial whether activation of muscarinic receptors is also sufficient to trigger noradrenaline release from sympathetic neurons. In primary cultures of rat superior cervical ganglia, the muscarinic agonist oxotremorine M inhibited M-currents with half-maximal effects at 1 microM and induced the release of previously incorporated [3H]noradrenaline with half-maximal effects at 10 microM. This latter action was not affected by the nicotinic antagonist mecamylamine which, however, abolished currents through nicotinic receptors elicited by high oxotremorine M concentrations. Ablation of the signalling cascades linked to inhibitory G proteins by pertussis toxin potentiated the release stimulating effect of oxotremorine M, and the half-maximal concentration required to stimulate noradrenaline release was decreased to 3 microM. Pirenzepine antagonized the inhibition of M-currents and the induction of release by oxotremorine M with identical apparent affinity, and both effects were abolished by the muscarinic toxin 7. These results indicate that one muscarinic receptor subtype, namely M1, mediates these two effects. Retigabine, which enhances M-currents, abolished the release induced by oxotremorine M, but left electrically induced release unaltered. Moreover, retigabine shifted the voltage-dependent activation of M-currents by about 20 mV to more negative potentials and caused 20 mV hyperpolarisations of the membrane potential. In the absence of retigabine, oxotremorine M depolarised the neurons and elicited action potential discharges in 8 of 23 neurons; in its presence, oxotremorine M still caused equal depolarisations, but always failed to trigger action potentials. Action potential waveforms caused by current injection were not affected by retigabine. These results indicate that the inhibition of M-currents is the basis for the stimulation of transmitter release from sympathetic neurons via M1 muscarinic receptors.

Cannabinoid modulation of peripheral autonomic and sensory neurotransmission

Cannabinoids are cell membrane-derived signalling molecules that are released from nerves, blood cells and endothelial cells, and have diverse biological effects. They act at two distinct types of G-protein-coupled receptors, cannabinoid CB(1) and CB(2) receptors. Cannabinoid CB(1) receptors are highly localised in the central nervous system and are also found in some peripheral tissues, and cannabinoid CB(2) receptors are found outside the central nervous system, in particular in association with immune tissues. Novel actions of cannabinoids at non-CB(1) non-CB(2) cannabinoid-like receptors and vanilloid VR1 receptors have also recently been described. There is growing evidence that, among other roles, cannabinoids can act at prejunctional sites to modulate peripheral autonomic and sensory neurotransmission, and the present article is aimed at providing an overview of this. Inhibitory cannabinoid CB(1) receptors are expressed on the peripheral terminals of autonomic and sensory nerves. The role of cannabinoid receptor ligands in modulation of sensory neurotransmission is complex, as certain of these (anandamide, an "endocannabinoid", and N-arachidonoyl-dopamine, an "endovanilloid") also activate vanilloid VR1 receptors (coexpressed with cannabinoid CB(1) receptors), which excites sensory nerves and causes a release of sensory neurotransmitter. The fact that the activities of anandamide and N-arachidonoyl-dopamine span two distinct receptor families raises important questions about cannabinoid/vanilloid nomenclature, and as both compounds are structurally related to the archetypal vanilloid capsaicin, all three are arguably members of the same family of signalling molecules. Anandamide is released from nerves, but unlike classical neurotransmitters, it is not stored in and released from nerve vesicles, but is released on demand from the nerve cell membrane. In the central nervous system, cannabinoids function as retrograde signalling molecules, inhibiting via presynaptic cannabinoid CB(1) receptors the release of classical transmitter following release from the postsynaptic cell. At the neuroeffector junction, it is more likely that cannabinoids are released from prejunctional sites, as the neuroeffector junction is wide in some peripheral tissues and cannabinoids are rapidly taken up and inactivated. Understanding the actions of cannabinoids as modulators of peripheral neurotransmission is relevant to a variety of biological systems and possibly their disorders.

Heterogeneity of presynaptic muscarinic receptors mediating inhibition of sympathetic transmitter release: a study with M2- and M4-receptor-deficient mice

1 Presynaptic muscarinic receptors modulate sympathetic transmitter release. The goal of the present study was to identify the muscarinic receptor subtype(s) mediating inhibition of sympathetic transmitter release in mouse atria, urinary bladder and vas deferens. To address this question, electrically evoked noradrenaline release was assessed using tissue preparations from NMRI, M(2)- and M(4)-knockout, and the corresponding M(2)- and M(4)-wildtype mice, after preincubation with (3)H-noradrenaline. 2 The muscarinic agonist carbachol decreased evoked tritium overflow (20 pulses/50 Hz) in each tissue and strain investigated. After deletion of the M(2)-receptor the maximal inhibition by carbachol was significantly reduced (by 41-72%), but not abolished, in all tissues. After deletion of the M(4)-receptor a moderate and significant reduction of the maximal inhibition by carbachol (by 28%) was observed only in the vas deferens. 3 Experiments with the muscarinic antagonists methoctramine and pirenzepine confirmed that the presynaptic muscarinic receptors were predominantly M(2) in atria and bladder and probably a mixture of M(2) and M(4) in the vas deferens. 4 Experiments in the urinary bladder with the cholinesterase inhibitor physostigmine and the muscarinic antagonist ipratropium demonstrated that endogenously released acetylcholine predominantly acted through M(2)-receptors to inhibit noradrenaline release. However, the results do not exclude a minor contribution of M(4)-receptors to this endogenous inhibition. 5 In conclusion, our results clearly indicate that the release-inhibiting muscarinic receptors on postganglionic sympathetic axons in mouse atria, bladder and vas deferens represent mixtures of M(2)- and non-M(2)-receptors. The non-M(2)-receptors remain unknown in atria and the bladder, and may represent primarily M(4)-receptors in the vas deferens. These results reveal an unexpected heterogeneity among the muscarinic receptors mediating inhibition of noradrenaline release.

Presynaptic effects of moxonidine in isolated buffer perfused rat hearts: role of imidazoline-1 receptors and alpha2-adrenoceptors

Numerous studies support the concept that centrally acting antihypertensive drugs, such as imidazolines, mediate sympathoinhibition not only via activation of central nervous alpha2-adrenoceptors (alpha2-AR) but also via imidazoline-1 receptors (I1-R). An additional presynaptic involvement in sympathetic neurotransmission of imidazolines, via I1-R independent of alpha2-AR, is still controversial and remains to be clarified in the heart. Concentration response curves on endogenous norepinephrine (NE) overflow evoked by stimulation of epicardial postganglionic sympathetic nerves in isolated buffer-perfused rat hearts were performed for brimonidine, moxonidine, rauwolscine, 2-endo-amino-3-exo-isopropylbicyclo[2.2.1]heptane (AGN192403), and efaroxan. To unmask an I1-R-mediated effect of moxonidine, hearts were pre-exposed in additional experiments with brimonidine or rauwolscine with or without AGN192403 or efaroxan, respectively. Moxonidine reduced stimulated NE overflow (log EC50: -6.15 +/- 0.14). AGN192403, a selective ligand at I1-R, had no influence on the dose-response curve of moxonidine (log EC50: -6.01 +/- 0.25). After pre-exposure to brimonidine [ stimulation 1 (S1) + stimulation 2 (S2); 10(-5) M], the inhibitory action of moxonidine was potentiated compared with control (32.0 +/- 2.8 versus 73.13 +/- 3.0%) and completely abolished with AGN192403 or efaroxan. This effect was also totally inhibited by pre-exposure with indomethacin (10(-7) M) and tricyclodecan-9-yl-xanthogenate (D-609), an inhibitor of phosphatidylcholine-selective phospholipase C (PC-PLC; 10(-7) M). Conversely, moxonidine was without modulating efficacy under alpha2-AR-blockade by rauwolscine. In summary, we demonstrate that moxonidine reduces NE release independently of I1-R, demonstrating the prominent effect of alpha2-AR in cardiac tissue under basal conditions. We also demonstrate that I1-R are involved in NE release under conditions of alpha2-AR-stimulation involving both a pathway of prostaglandins and PC-PLC.

Lack of prejunctional modulation of noradrenaline release by endogenous nitric oxide in guinea pig pulmonary artery

The regulation of noradrenaline (NA) release by endogenous endothelium-derived compounds was investigated in the isolated guinea pig pulmonary artery preloaded with [3H]NA. The radioactivity uptake, the basal and electrical field stimulation (10 Hz, 2 ms, 360 shocks) evoked release of [(3)H]NA was similar in arteries with intact endothelium and after removal of the endothelium. The wide selectivity nitric oxide (NO) synthase inhibitor N-omega-nitro-L-arginine (100 microM) did not affect significantly the basal and stimulation-evoked release of [(3)H]NA in control and endothelium-denuded preparations. These results indicate that neither endogenous NO nor other compounds derived from the endothelium have substantial influence on the NA outflow from sympathetic nerves innervating the pulmonary artery.

Cannabinoid receptors and their ligands

There are at least two types of cannabinoid receptors, CB(1) and CB(2), both coupled to G proteins. CB(1) receptors exist primarily on central and peripheral neurons, one of their functions being to modulate neurotransmitter release. CB(2) receptors are present mainly on immune cells. Their roles are proving more difficult to establish but seem to include the modulation of cytokine release. Endogenous agonists for cannabinoid receptors (endocannabinoids) have also been discovered, the most important being arachidonoyl ethanolamide (anandamide), 2-arachidonoyl glycerol and 2-arachidonyl glyceryl ether. Other endocannabinoids and cannabinoid receptor types may also exist. Although anandamide can act through CB(1) and CB(2) receptors, it is also a vanilloid receptor agonist and some of its metabolites may possess yet other important modes of action. The discovery of the system of cannabinoid receptors and endocannabinoids that constitutes the "endocannabinoid system" has prompted the development of CB(1)- and CB(2)-selective agonists and antagonists/inverse agonists. CB(1)/CB(2) agonists are already used clinically, as anti-emetics or to stimulate appetite. Potential therapeutic uses of cannabinoid receptor agonists include the management of multiple sclerosis/spinal cord injury, pain, inflammatory disorders, glaucoma, bronchial asthma, vasodilation that accompanies advanced cirrhosis, and cancer. Following their release onto cannabinoid receptors, endocannabinoids are removed from the extracellular space by membrane transport and then degraded by intracellular enzymic hydrolysis. Inhibitors of both these processes have been developed. Such inhibitors have therapeutic potential as animal data suggest that released endocannabinoids mediate reductions both in inflammatory pain and in the spasticity and tremor of multiple sclerosis. So too have CB(1) receptor antagonists, for example for the suppression of appetite and the management of cognitive dysfunction or schizophrenia.

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.

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.

Cannabinoid receptor agonists inhibit depolarization-induced calcium influx in cerebellar granule neurons

Neuronal cannabinoid receptors (CB(1)) are coupled to inhibition of voltage-sensitive Ca(2+) channels (VSCCs) in several cell types. The purpose of these studies was to characterize the interaction between endogenous CB(1) receptors and VSCCs in cerebellar granule neurons (CGN). Ca(2+) transients were evoked by KCl-induced depolarization and imaged using fura-2. The CB(1) receptor agonists CP55940, Win 55212-2 and N-arachidonylethanolamine (anandamide) produced concentration-related decreases in peak amplitude of the Ca(2+) response and total Ca(2+) influx. Pre-treatment of CGN with pertussis toxin abolished agonist-mediated inhibition. The inhibitory effect of Win 55212-2 on Ca(2+) influx was additive with inhibition produced by omega-agatoxin IVA and nifedipine but not with omega-conotoxin GVIA, indicating that N-type VSCCs are the primary effector. Paradoxically, the CB(1) receptor antagonist, SR141716, also inhibited KCl-induced Ca(2+) influx into CGN in a concentration-related manner. SR141716 inhibition was pertussis toxin-insensitive and was not additive with the inhibition produced by Win 55212-2. Confocal imaging of CGN in primary culture demonstrate a high density of CB(1) receptor expression on CGN plasma membranes, including the neuritic processes. These data demonstrate that the CB(1) receptor is highly expressed by CGN and agonists serve as potent and efficacious inhibitory modulators of Ca(2+) influx through N-type VSCC.

Stimulation of mouse cultured sympathetic neurons by uracil but not adenine nucleotides

Cultured neurons from the paravertebral sympathetic chain of rats possess excitatory P2X as well as excitatory uracil nucleotide-sensitive P2Y receptors. Preliminary observations had indicated that the analogous neurons of mice lacked P2X receptors. This difference was now investigated. Thoracolumbar sympathetic neurons from one- to three-day-old mice were cultured for seven days. When the neurons were preincubated with [3H]noradrenaline and then superfused, ATP failed to cause any change in tritium outflow. UTP (3-300 microM) and UDP (30-100 microM), in contrast, caused marked increases, and so did nicotine (3-100 microM). The effect of UTP was not changed by suramin but abolished by tetrodotoxin and in the absence of calcium. The effect of nicotine was antagonized by hexamethonium and also abolished by tetrodotoxin and in the absence of calcium. Pre-exposure to UDP prevented the effect of UTP. In neurons studied by means of whole-cell patch-clamp techniques under current clamp, ATP lacked any effect. UTP (100 microM), UDP (100 microM) and nicotine (10 microM) caused depolarization accompanied by action potentials. Pre-exposure to UDP prevented the effect of UTP. In neurons studied under voltage clamp, ATP, UTP and UDP failed to cause any detectable current. Nicotine (10 microM), in contrast, elicited inward currents. Neither UTP nor UDP reduced the M-type potassium outward current. These results demonstrate a pronounced difference between cultured sympathetic neurons from the mouse and the rat paravertebral chain. Neurons from both species possess the nicotinic acetylcholine receptor. Neurons from both species also possess uracil nucleotide-sensitive P2Y receptors which, when activated, mediate depolarization, action potential firing and noradrenaline release; these effects are not due to inhibition of M-type potassium channels. Only the rat but not the mouse neurons, however, possess P2X receptors which, when activated, mediate cation entry, depolarization, action potential generation and transmitter release. The absence of functional P2X receptors makes the mouse neurons suitable for further study of the uracil nucleotide-sensitive P2Y receptors.