Presynaptic Inhibition Of Transmitter Release Research Articles

Desensitization-resistant and -sensitive GPCR-mediated inhibition of GABA release occurs by Ca2+-dependent and -independent mechanisms at a hypothalamic synapse.

Whereas the activation of Gαi/o-coupled receptors commonly results in postsynaptic responses that show acute desensitization, the presynaptic inhibition of transmitter release caused by many Gαi/o-coupled receptors is maintained during agonist exposure. However, an exception has been noted where GABAB receptor (GABABR)-mediated inhibition of inhibitory postsynaptic currents (IPSCs) recorded in mouse proopiomelanocortin (POMC) neurons exhibit acute desensitization in ∼25% of experiments. To determine whether differential effector coupling confers sensitivity to desensitization, voltage-clamp recordings were made from POMC neurons to compare the mechanism by which μ-opioid receptors (MORs) and GABABRs inhibit transmitter release. Neither MOR- nor GABABR-mediated inhibition of release relied on the activation of presynaptic K(+) channels. Both receptors maintained the ability to inhibit release in the absence of external Ca(2+) or in the presence of ionomycin-induced Ca(2+) influx, indicating that inhibition of release can occur through a Ca(2+)-independent mechanism. Replacing Ca(2+) with Sr(2+) to disrupt G-protein-mediated inhibition of release occurring directly at the release machinery did not alter MOR- or GABAB -mediated inhibition of IPSCs, suggesting that reductions in evoked release can occur through the inhibition of Ca(2+) channels. Additionally, both receptors inhibited evoked IPSCs in the presence of selective blockers of N- or P/Q-type Ca(2+) channels. Altogether, the results show that MORs and GABABRs can inhibit transmitter release through the inhibition of calcium influx and by direct actions at the release machinery. Furthermore, since both the desensitizing and nondesensitizing presynaptic receptors are similarly coupled, differential effector coupling is unlikely responsible for differential desensitization of the inhibition of release.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity.

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

Altered atrial neurotransmitter release in transgenic p75−/− and gp130 KO mice

Heart rate is controlled by stimulatory sympathetic and inhibitory parasympathetic nerves innervating the sino-atrial node and cardiac conduction system. Sympathetic release of norepinephrine (NE) and parasympathetic release of acetylcholine (ACh) are controlled by the central nervous system, and by pre-synaptic inhibition of transmitter release within the atria. An increase in cardiac sympathetic transmission relative to parasympathetic transmission is pathological as it can lead to disturbances in heart rhythm, catecholaminergic toxicity and development of arrhythmias or fibrillation. Mice lacking the p75 neurotrophin receptor (p75−/−) have elevated atrial NE but a low heart rate suggesting autonomic dysregulation. Similarly, mice whose sympathetic neurons lack the gp130 cytokine receptor (gp130 KO) have a normal heart rate but enhanced bradycardia after vagal nerve stimulation. What is unclear is whether cardiac autonomic disturbances in these animals reflect systemic alterations in nerve activity or whether localized defects in neurotransmitter stores or release are involved. To examine local stimulus-evoked release of neurotransmitters, we have developed a novel method for simultaneous quantification of both NE and ACh after ex vivo atrial field stimulation. Using HPLC with electrochemical detection for NE, and HPLC with mass spectrometry for ACh, we found that following field stimulation NE release was impaired in p75−/− atria while ACh content and release was elevated in gp130 KO atria. Thus, alterations in localized transmitter release from atrial explants are consistent with in vivo deficits in heart rate control, suggesting peripheral alterations in autonomic transmission in these mice.

Astroglia: Not Just Glue

Astroglia: Not Just Glue

Differential Regulation of Synaptic Inputs by Constitutively Released Endocannabinoids and Exogenous Cannabinoids

Endocannabinoid release from a single neuron has been shown to cause presynaptic inhibition of transmitter release at many different sites. Here, we demonstrate that hypothalamic proopiomelanocortin (POMC) neurons release endocannabinoids continuously under basal conditions, unlike other release sites at which endocannabinoid production must be stimulated. The basal endocannabinoid release selectively inhibited GABA release onto POMC neurons, although exogenous administration of cannabinoid agonists also inhibited glutamate release. The CB1 cannabinoid receptor antagonist AM 251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] blocked endocannabinoid-mediated inhibition of GABA release without affecting excitatory synaptic currents, whereas the CB1 receptor agonist WIN 55,212-2 [R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol [1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl) methanone monomethanesulfonate] inhibited both inhibitory and excitatory synaptic currents in POMC neurons. These data demonstrate that endogenously released cannabinoids and exogenously applied CB1 receptor agonists can have markedly different effects on synaptic inputs. Furthermore, the data suggest a novel form of endocannabinoid-mediated retrograde inhibition, whereby the regulation of a subset of inputs requires either the removal of tonic presynaptic inhibition caused by endocannabinoids or the engagement of a mechanism that actively inhibits endocannabinoid production.

Presynaptic Inhibition via a Phospholipase C- and Phosphatidylinositol Bisphosphate-Dependent Regulation of Neuronal Ca2+Channels

Presynaptic inhibition of transmitter release is commonly mediated by a direct interaction between G protein betagamma subunits and voltage-activated Ca2+ channels. To search for an alternative pathway, the mechanisms by which presynaptic bradykinin receptors mediate an inhibition of noradrenaline release from rat superior cervical ganglion neurons were investigated. The peptide reduced noradrenaline release triggered by K+-depolarization but not that evoked by ATP, with Ca2+ channels being blocked by Cd2+. Bradykinin also reduced Ca2+ current amplitudes measured at neuronal somata, and this effect was pertussis toxin-insensitive, voltage-independent, and developed slowly within 1 min. The inhibition of Ca2+ currents was abolished by a phospholipase C inhibitor, but it was not altered by a phospholipase A2 inhibitor, by the depletion of intracellular Ca2+ stores, or by the inactivation of protein kinase C or Rho proteins. In whole-cell recordings, the reduction of Ca2+ currents was irreversible but became reversible when 4 mM ATP or 0.2 mM dioctanoyl phosphatidylinositol-4,5-bisphosphate was included in the pipette solution. In contrast, the effect of bradykinin was entirely reversible in perforated-patch recordings but became irreversible when the resynthesis of phosphatidylinositol-4,5-bisphosphate was blocked. Thus, the inhibition of Ca2+ currents by bradykinin involved a consumption of phosphatidylinositol-4,5-bisphosphate by phospholipase C but no downstream effectors of this enzyme. The reduction of noradrenaline release by bradykinin was also abolished by the inhibition of phospholipase C or of the resynthesis of phosphatidylinositol-4,5-bisphosphate. These results show that the presynaptic inhibition was mediated by a closure of voltage-gated Ca2+ channels through depletion of membrane phosphatidylinositol bisphosphates via phospholipase C.

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.

G protein {beta}{gamma} subunits mediate presynaptic inhibition of transmitter release from rat superior cervical ganglion neurones in culture.

The activation of presynaptic G protein-coupled receptors (GPCRs) is widely reported to inhibit transmitter release; however, the lack of accessibility of many presynaptic terminals has limited direct analysis of signalling mediators. We studied GPCR-mediated inhibition of fast cholinergic transmission between superior cervical ganglion neurones (SCGNs) in culture. The adrenoceptor agonist noradrenaline (NA) caused a dose-related reduction in evoked excitatory postsynaptic potentials (EPSPs). NA-induced EPSP decrease was accompanied by effects on the presynaptic action potential (AP), reducing AP duration and amplitude of the after-hyperpolarization (AHP), without affecting the pre- and postsynaptic membrane potential. All effects of NA were blocked by yohimbine and synaptic transmission was reduced by clonidine, consistent with an action at presynaptic alpha2-adrenoceptors. NA-induced inhibition of transmission was sensitive to pre-incubation of SCGNs with pertussis toxin (PTX), implicating the involvement of Galpha(i/o)betagamma subunits. Expression of Galpha transducin, an agent which sequesters G protein betagamma (Gbetagamma) subunits, in the presynaptic neurone caused a time-dependent attenuation of NA-induced inhibition. Injection of purified Gbetagamma subunits into the presynaptic neurone inhibited transmission, and also reduced the AHP amplitude. Furthermore, NA-induced inhibition was occluded by pre-injection of Gbetagamma subunits. The Ca(2+) channel blocker Cd(2+) mimicked NA effects on transmitter release. Cd(2+), NA and Gbetagamma subunits also inhibited somatic Ca(2+) current. In contrast to effects on AP-evoked transmitter release, NA had no clear action on AP-independent EPSPs induced by hypertonic solutions. These results demonstrate that Gbetagamma subunits functionally mediate inhibition of transmitter release by alpha2-adrenoceptors and represent important regulators of synaptic transmission at mammalian presynaptic terminals.

Neurogenic inflammation of the airways and skin

According to its original definition, the main feature of neurogenic inflammation is plasma leakage induced by the stimulation of peripheral sensory nerves. The plasma leakage from postcapillary venules is accompanied by increased blood flow due to dilatation of upstream arterioles, and by other phenomena, including leukocyte adhesion and migration. Neurogenic inflammation occurs in the airways, skin, and parts of the intestinal, urinary, and reproductive tract of man and animals, but varies markedly in its magnitude and extent. In skin, neurogenic inflammation is manifested as wheal and flare. Both phenomena are mediated by neuropeptides released from unmyelinated sensory nerve fibers via stimulation of capsaicin‐sensitive vanilloid receptors. Substance P is mainly responsible for the plasma leakage, acting via NK‐1 receptors present on target blood vessels, whereas calcitonin gene‐related peptide and substance P both induce vasodilatation. Sensory neuropeptides also trigger release of histamine from mast cells, which contributes substantially to plasma leakage in the skin, but less so in the airways. The increase in vascular permeability is due to a focal, transient, and fully reversible formation of gaps located between endothelial cell junctions. Neurogenic inflammation can be inhibited by preventing the stimulation of sensory nerves, by depleting them of their neuropeptide transmitters, by presynaptic inhibition of transmitter release, or by blocking neuropeptide receptors. Anti‐inflammatory drugs such as β‐adrenergic agonists and steroids can reduce neurogenic inflammation by stabilizing endothelial cells.

My close encounter with GABA B receptors

In this review, I summarize the sequence of events involved in characterizing the functional role of GABA B receptors in the CNS and their involvement in synaptic transmission. The story was launched with the realization that baclofen was a selective agonist of GABA B receptors. This lead to the discovery in the CNS that GABA B receptor activation could result in a presynaptic inhibition of transmitter release as well as a postsynaptic increase in potassium conductance. Based on this information, it was found that GABA also activated a potassium conductance. A role for GABA B receptors in synaptic transmission was suggested by the fact that activation of GABAergic interneurons could generate a slow IPSP mediated by an increase in potassium conductance. To link this slow IPSP to GABA B receptors required a selective GABA B antagonist. Phaclofen was the first antagonist developed and was found to antagonize the action of baclofen and the GABA A independent action of GABA. Most importantly, it blocked the slow IPSP. The properties of GABA A and GABA B IPSPs are remarkably different. GABA A IPSPs powerfully inhibit neurons and rapidly curtail excitatory inputs. This greatly enhances the precision of excitatory synaptic transmission. GABA B IPSPs are recruited with repetitive and synchronous activity and are postulated to modulate the rhythmic network activity of cortical tissue.

Differences in the mechanisms that increase noradrenaline efflux after administration of d-amphetamine: a dual-probe microdialysis study in rat frontal cortex and hypothalamus.

1. The extent to which impulse-independent release of noradrenaline and/or inhibition of its reuptake contribute to the response to d-amphetamine in vivo is unclear. Here, dual-probe microdialysis was used to investigate this question in the rat frontal cortex and hypothalamus. 2. After systemic administration of d-amphetamine (10 mg kg(-1)), or its local infusion (10 micro M), the increase in noradrenaline efflux in the hypothalamus was greater than in the frontal cortex. 3. In contrast, during local infusion of the noradrenaline reuptake inhibitor, BTS 54 354 (50 micro M), the noradrenaline response was similar in the frontal cortex and hypothalamus, even after systemic administration of the alpha(2)-antagonist, atipamezole, to block presynaptic inhibition of transmitter release and neuronal firing. 4. In the frontal cortex, but not the hypothalamus, the noradrenaline response to 10 micro M d-amphetamine was constrained by activation of alpha(2)-adrenoceptors. This suggests that, at this concentration, inhibition of reuptake of noradrenaline, following its impulse-dependent release, is evident in the frontal cortex, but that the noradrenaline response in the hypothalamus derives mostly from impulse-independent release (retrotransport). 5. Atipamezole did not affect the noradrenaline response to 100 micro M d-amphetamine in either brain region possibly because, at this higher concentration, retrotransport of noradrenaline masks any compensatory reduction in impulse-evoked release. 6. It is concluded that inhibition of reuptake and retrotransport make different contributions to the noradrenaline response to d-amphetamine in the frontal cortex and hypothalamus and that retrotransport increases with the concentration of d-amphetamine.

Presynaptic inhibition of olfactory receptor neurons in crustaceans.

Presynaptic inhibition of transmitter release from primary sensory afferents is a common strategy for regulating sensory input to the arthropod central nervous system. In the olfactory system, presynaptic inhibition of olfactory receptor neurons has been long suspected, but until recently could not be demonstrated directly because of the difficulty in recording from the afferent nerve terminals. A preparation using the isolated but intact brain of the spiny lobster in combination with voltage-sensitive dye staining has allowed stimulus-evoked responses of olfactory receptor axons to be recorded selectively with optical imaging methods. This approach has provided the first direct physiological evidence for presynaptic inhibition of olfactory receptor neurons. As in other arthropod sensory systems, the cellular mechanism underlying presynaptic afferent inhibition appears to be a reduction of action potential amplitude in the axon terminal. In the spiny lobster, two inhibitory transmitters, GABA and histamine, can independently mediate presynaptic inhibition. GABA- and histaminergic interneurons in the lobster olfactory lobe (the target of olfactory receptor neurons) constitute dual, functionally distinct inhibitory pathways that are likely to play different roles in regulating primary olfactory input to the CNS. Presynaptic inhibition in the vertebrate olfactory system is also mediated by dual inhibitory pathways, but via a different cellular mechanism. Thus, it is possible that presynaptic inhibition of primary olfactory afferents evolved independently in vertebrates and invertebrates to fill a common, fundamental role in processing olfactory information.

Effect of chronic clonidine treatment on transmitter release from sympathetic varicosities of the guinea-pig vas deferens.

1. Previous studies have demonstrated that chronic pre-synaptic inhibition of transmitter release by morphine evokes a counter-adaptive response in the sympathetic nerve terminals that manifests itself as an increase in transmitter release during acute withdrawal. In the present study we examined the possibility that other pre-synaptically acting drugs such as clonidine also evoke a counter-adaptive response in the sympathetic nerve terminals. 2. In chronically saline treated (CST) preparations, clonidine (0.5 microM) completely abolished evoked transmitter release from sympathetic varicosities bathed in an extracellular calcium concentration ([Ca(2+)](o)) of 2 mM. The inhibitory effect of clonidine was reduced by increasing [Ca(2+)](o) from 2 to 4 mM and the stimulation frequency from 0.1 to 1 Hz. 3. The nerve terminal impulse (NTI) was not affected by concentrations of clonidine that completely abolished evoked transmitter release. 4. Sympathetic varicosities developed a tolerance to clonidine (0.5 microM) following 7-9 days of chronic exposure to clonidine. 5. Acute withdrawal of preparations following chronic clonidine treatment (CCT) resulted in a significant (P < 0.005) enhancement of neurotransmitter release (3.75 times) above control levels observed in CST preparations. 6. The present findings demonstrate an enhancement of neurotransmitter release from sympathetic varicosities following acute withdrawal from chronic clonidine treatment.

Presynaptic localization of the carboxy-terminus epitopes of the μ opioid receptor splice variants MOR-1C and MOR-1D in the superficial laminae of the rat spinal cord

Opioids inhibit nociceptive transmission at the level of the spinal cord, possibly through inhibition of neurotransmitter release by presynaptic μ opioid receptors (MORs) thus preventing the activation of ascending pathways and the perception of pain. Most nociceptive primary afferents are unmyelinated fibers containing peptides such as substance P and/or calcitonin gene-related peptide. However, few terminals contain both substance P and MOR. Recently, we identified new carboxy-terminal MOR splice variants that are localized in the superficial laminae of the dorsal horn. We now report the precise cellular distribution of two of these MOR-1 variants, MOR-1C (exon 7/8/9 epitope) and MOR-1D (exon 8/9 epitope), at the ultrastructural level. In the superficial laminae of the dorsal horn, the majority of the labeling of MOR-1C and MOR-1D was found in unmyelinated axons. This distribution contrasts with that of MOR-1 (exon 4 epitope), in which labeling is equally found in dendrites and soma, as well as in axons. The presence of dense core vesicles in many of the MOR-1C-like immunoreactive terminals implies that this splice variant might be involved in presynaptic inhibition of transmitter release from peptide-containing afferents to the dorsal horn. Consistent with this finding, confocal microscopy analyses showed that many MOR-1C profiles in laminae I–II also contained calcitonin gene-related peptide, whereas fewer MOR-1 profiles contained either substance P or calcitonin gene-related peptide in this same region. From these findings we suggest that there are differential distributions of MOR-1 splice variants as well as distinct peptide colocalizations in the dorsal horn.

Receptors and transmission in the brain-gut axis: potential for novel therapies. IV. GABA(B) receptors in the brain-gastroesophageal axis.

GABA(B) receptors are inhibitory G protein-coupled receptors that are commonly associated with presynaptic inhibition of transmitter release in the central nervous system. In the brain-gastroesophageal axis, a role has recently been demonstrated for GABA(B) receptors on extrinsic afferent endings within the stomach and esophagus, where they reduce mechanosensitivity. This action is compounded by inhibition of communication centrally from these afferents in the brain stem and within central circuits. There is a final peripheral action on the motor pathway where GABA(B) receptors reduce output of acetylcholine from vagal preganglionic motoneurons. These potent, multiple actions of GABA(B) receptors may have therapeutic benefit by reducing the triggering of transient lower esophageal relaxations, which are the major cause of gastroesophageal reflux. An important clinical application is therefore emerging for this recent discovery.

Actions of cannabinoid receptor ligands on rat cultured sensory neurones: implications for antinociception

Cannabinoids modulate nociceptive processing in models of acute, inflammatory and neuropathic pain. We have investigated the location and function of cannabinoid receptors on cultured neonatal dorsal root ganglion (DRG) neurones and F-11 cells, a dorsal root ganglion×neuroblastoma hybridoma which displays several of the features of authentic DRG neurones. CB 1 receptor immunolabelling was observed on the cell bodies and as fine puncta on processes of both cultured DRG neurones and F-11 cells. Additionally, fluorescence-activated cell sorting (FACS) analysis provided evidence that both CB 1 and CB 2 receptors are expressed on populations of cells within the cultured DRG and F-11 cells. The cannabinoid receptor agonist (+)-WIN55212 (10 and 100 nM) inhibited the mean voltage-activated Ca 2+ current in DRG neurones by 21% and 30%, respectively. The isomer, (−)-WIN55212 (10 and 100 nM) produced significantly less inhibition of 6% and 10% respectively. The CB 1 selective receptor antagonist SR141716A (100 nM) enhanced the peak high voltage-activated Ca 2+ current by 24% and simultaneous application of SR141716A (100 nM) and (+)-WIN55212 (100 nM) resulted in a significant attenuation of the inhibition obtained with (+)-WIN55212 alone. These data give functional evidence for the hypothesis that the analgesic actions of cannabinoids may be mediated by presynaptic inhibition of transmitter release in sensory neurones.

Regulation of Central Synaptic Transmission by 5-HT1BAuto- and Heteroreceptors

Although 5-HT(1B) receptors are believed to be expressed on nerve terminals, their precise mode of action is not fully understood because of the lack of selective antagonists. The 5-HT(1B) receptor knockout mouse was used in the present investigation to assess the function of 5-HT(1B) receptors in the modulation of synaptic transmission in three areas of the central nervous system: the dorsal raphe, the ventral midbrain, and the nucleus accumbens. N-(3-Trifluoromethylphenyl)piperazine, a 5-HT(1B) receptor agonist, potently inhibited 5-HT(1A) receptor-mediated slow inhibitory postsynaptic potentials (IPSPs) in the dorsal raphe of wild-type but not knockout mice. Both synaptically released 5-HT and exogenous 5-HT caused a presynaptic inhibition that outlasted the postsynaptic hyperpolarization only in wild-type mice. In the ventral midbrain, 5-HT(1B) receptor-dependent inhibition of gamma-aminobutyric acid(B) IPSPs in dopamine neurons was present in wild-type animals and absent in knockout animals. Similar results were obtained in the nucleus accumbens measuring glutamate-mediated excitatory postsynaptic currents in medium spiny neurons. Finally, cocaine, which blocks 5-HT uptake, inhibited IPSPs in the dorsal raphe and the ventral midbrain of wild-type but not knockout mice, whereas cocaine produced comparable inhibition of excitatory postsynaptic currents in the nucleus accumbens of both types of animals. These results indicate that 5-HT(1B) receptors function as autoreceptors and heteroreceptors to exert presynaptic inhibition of transmitter release in the central nervous system. Furthermore, this study underscores the role played by presynaptic 5-HT(1B) receptors in mediating the effects of cocaine on synaptic transmission.

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 α-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 100 ms. 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.

Presynaptic inhibition and the participation of GABA(B) receptors at neuromuscular junctions of the crab Eriphia spinifrons.

Presynaptic inhibition exerted by the common inhibitor on the closer and opener muscles and by the specific inhibitor on the opener muscle was investigated in the crab Eriphia spinifrons. In the closer muscle, activation of GABA(B) receptors by baclofen reduced the mean quantal content of excitatory junctional currents by about 25%. Blocking GABA(B) receptors with CGP 55845 diminished presynaptic inhibition at a similar percentage. GABA(B) receptor-mediated presynaptic inhibition is linked to G proteins. Application of pertussis toxin eliminated about 25% of the inhibition exerted by the common inhibitory neuron. GABA(B) receptors participate in presynaptic inhibition at release boutons of the slow and the fast closer excitor at a similar percentage. In the opener muscle, presynaptic inhibition of transmitter release from the same endings of the opener excitor was about 15% stronger with the specific inhibitor than with the common inhibitor. About 10% of the presynaptic inhibition produced by either one of the two inhibitors could be abolished by blocking GABA(B) receptors. The amplitudes of the excitatory junctional currents in the opener were reduced in the presence of baclofen by about 25%, suggesting that synaptic terminals of the opener excitor are endowed with a similar percentage of GABA(B) receptors as terminals of the slow and the fast closer excitors. Baclofen had no effect on postsynaptic inhibition, indicating that GABA(B) receptors are not involved in postsynaptic neuromuscular inhibition.

Nitric oxide inhibits norepinephrine-induced hepatic vascular responses but potentiates hepatic glucose output.

We previously reported that sympathetic nerve-induced vasoconstriction in the intestine resulted in shear stress induced release of nitric oxide (NO) that led to presynaptic inhibition of transmitter release. In contrast, studies in the liver suggested a postsynaptic inhibition of vascular responses, thus leading to the hypothesis tested here that maintained catecholamine release in the liver would result in maintained metabolic catecholamine action in the face of inhibition of vascular responses. In rats, norepinephrine (NE) induced elevations in arterial glucose content were inhibited by NO synthase antagonism (N(omega)-nitro-L-arginine methyl ester (L-NAME), 10 mg/kg, intraportal) but potentiated by NO donor administration (3-morpholinosydnonimine (SIN-1), 0.2 mg/kg, intraportal). The potentiated effect of SIN-1 was abolished by indomethacin (7.5 mg/kg, intraportal). To confirm the hepatic site of metabolic effect, cats were used so that blood flow and hepatic glucose balance could be determined. SIN-1 potentiated NE-induced glucose output from the liver from 5.0 +/- 0.4 to 7.2 +/- 0.6 mg x min(-1) x kg(-1). The potentiation was blocked by methylene blue, a guanylate cyclase inhibitor. Contrary to the glucose response, L-NAME potentiated but SIN-1 attenuated NE-induced portal vasoconstriction. Thus NO is shown to produce differential modulation of vascular and metabolic effects of NE. Vasoconstriction of the hepatic vasculature is inhibited by NO, whereas the glycogenolytic response to NE is potentiated, responses that are probably mediated by prostaglandin.