Treatment of Status Epilepticus: Physiology, Pharmacology, and Future Directions

The review presents retrospective, present views and future perspectives for the treatment of status epilepticus (SE). First, presynaptic, postsynaptic and extra-synaptic mechanisms underlying sustaining ongoing seizure activity, are highlighted. Next, mechanism-based choices of antiseizure medications capable of promptly arresting SE are introduced. Finally, challenges associated with translating the advances in laboratory research in clinical practice are discussed.

Evaluation of radiolabeled acetylcholine synthesis and release in rat striatum

Cholinergic transmission underlies higher brain functions such as cognition and movement. To elucidate the process whereby acetylcholine (ACh) release is maintained and regulated in the central nervous system, uptake of [³ H]choline and subsequent synthesis and release of [³ H]ACh were investigated in rat striatal segments. Incubation with [³ H]choline elicited efficient uptake via high-affinity choline transporter-1, resulting in accumulation of [³ H]choline and [³ H]ACh. However, following inhibition of ACh esterase (AChE), incubation with [³ H]choline led predominantly to the accumulation of [³ H]ACh. Electrical stimulation and KCl depolarization selectively released [³ H]ACh but not [³ H]choline. [³ H]ACh release gradually declined upon repetitive stimulation, whereas the release was reproducible under inhibition of AChE. [³ H]ACh release was abolished after treatment with vesamicol, an inhibitor of vesicular ACh transporter. These results suggest that releasable ACh is continually replenished from the cytosol to releasable pools of cholinergic vesicles to maintain cholinergic transmission. [³ H]ACh release evoked by electrical stimulation was abolished by tetrodotoxin, but that induced by KCl was largely resistant. ACh release was Ca²⁺ dependent and exhibited slightly different sensitivities to N- and P-type Ca²⁺ channel toxins (ω-conotoxin GVIA and ω-agatoxin IVA, respectively) between both stimuli. [³ H]ACh release was negatively regulated by M2 muscarinic and D2 dopaminergic receptors. The present results suggest that inhibition of AChE within cholinergic neurons and of presynaptic negative regulation of ACh release contributes to maintenance and facilitation of cholinergic transmission, providing a potentially useful clue for the development of therapies for cholinergic dysfunction-associated disorders, in addition to inhibition of synaptic cleft AChE.

Differential calcium channel-mediated dopaminergic modulation in the subthalamonigral synapse

Dopamine (DA) modulates basal ganglia (BG) activity for initiation and execution of goal-directed movements and habits. While most studies are aimed to striatal function, the cellular and molecular mechanisms underlying dopaminergic regulation in other nuclei of the BG are not well understood. Therefore, we set to analyze the dopaminergic modulation occurring in subthalamo-nigral synapse, in both pars compacta (SNc) and pars reticulata (SNr) neurons, because these synapses are important for the integration of information previously processed in striatum and globus pallidus. In this study, electrophysiological and pharmacological evidence of dopaminergic modulation on glutamate release through calcium channels is presented. Using paired pulse ratio (PPR) measurements and selective blockers of these ionic channels, together with agonists and antagonists of DA D₂ -like receptors, we found that blockade of the Ca_V 3 family occludes the presynaptic inhibition produced by the activation of DA receptors pharmacologically profiled as D₃ -type in the STh-SNc synapses. On the contrast, the blockade of Ca_V 2 channels, but not Ca_V 3, occlude with the effect of the D₃ agonist, PD 128907, in the STh-SNr synapse. The functional role of this differential distribution of calcium channels that modulate the release of glutamate in the SN implies a fine adjustment of firing for both classes of neurons. Dopaminergic neurons of the SNc establish a DA tone within the SN based on the excitatory/inhibitory inputs; such tone may contribute to processing information from subthalamic nucleus and could also be involved in pathological DA depletion that drives hyperexcitation of SNr neurons.

Acid-sensing ion channels regulate spontaneous inhibitory activity in the hippocampus: possible implications for epilepsy

Acid-sensing ion channels (ASICs) play an important role in numerous functions in the central and peripheral nervous systems ranging from memory and emotions to pain. The data correspond to a recent notion that each neuron and many glial cells of the mammalian brain express at least one member of the ASIC family. However, the mechanisms underlying the involvement of ASICs in neuronal activity are poorly understood. However, there are two exceptions, namely, the straightforward role of ASICs in proton-based synaptic transmission in certain brain areas and the role of the Ca(2+)-permeable ASIC1a subtype in ischaemic cell death. Using a novel orthosteric ASIC antagonist, we have found that ASICs specifically control the frequency of spontaneous inhibitory synaptic activity in the hippocampus. Inhibition of ASICs leads to a strong increase in the frequency of spontaneous inhibitory postsynaptic currents. This effect is presynaptic because it is fully reproducible in single synaptic boutons attached to isolated hippocampal neurons. In concert with this observation, inhibition of the ASIC current diminishes epileptic discharges in a low Mg(2+) model of epilepsy in hippocampal slices and significantly reduces kainate-induced discharges in the hippocampus in vivo Our results reveal a significant novel role for ASICs.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Role of presynaptic glutamate receptors in pain transmission at the spinal cord level

Nociceptive primary afferents release glutamate, activating postsynaptic glutamate receptors on spinal cord dorsal horn neurons. Glutamate receptors, both ionotropic and metabotropic, are also expressed on presynaptic terminals, where they regulate neurotransmitter release. During the last two decades, a wide number of studies have characterized the properties of presynaptic glutamatergic receptors, particularly those expressed on primary afferent fibers. This review describes the subunit composition, distribution and function of presynaptic glutamate ionotropic (AMPA, NMDA, kainate) and metabotropic receptors expressed in rodent spinal cord dorsal horn. The role of presynaptic receptors in modulating nociceptive information in experimental models of acute and chronic pain will be also discussed.

Acid-sensing ion channels 1a (ASIC1a) inhibit neuromuscular transmission in female mice

Acid-sensing ion channels (ASIC) open in response to extracellular acidosis. ASIC1a, a particular subtype of these channels, has been described to have a postsynaptic distribution in the brain, being involved not only in ischemia and epilepsy, but also in fear and psychiatric pathologies. High-frequency stimulation of skeletal motor nerve terminals (MNTs) can induce presynaptic pH changes in combination with an acidification of the synaptic cleft, known to contribute to muscle fatigue. Here, we studied the role of ASIC1a channels on neuromuscular transmission. We combined a behavioral wire hanging test with electrophysiology, pharmacological, and immunofluorescence techniques to compare wild-type and ASIC1a lacking mice (ASIC1a ^−/− knockout). Our results showed that 1) ASIC1a ^−/− female mice were weaker than wild type, presenting shorter times during the wire hanging test; 2) spontaneous neurotransmitter release was reduced by ASIC1a activation, suggesting a presynaptic location of these channels at individual MNTs; 3) ASIC1a-mediated effects were emulated by extracellular local application of acid saline solutions (pH = 6.0; HEPES/MES-based solution); and 4) immunofluorescence techniques revealed the presence of ASIC1a antigens on MNTs. These results suggest that ASIC1a channels might be involved in controlling neuromuscular transmission, muscle contraction and fatigue in female mice.

The cholinergic agonist carbachol increases the frequency of spontaneous GABAergic synaptic currents in dorsal raphe serotonergic neurons in the mouse

Dorsal raphe nucleus (DRN) serotonin (5-HT) neurons play an important role in feeding, mood control and stress responses. One important feature of their activity across the sleep-wake cycle is their reduced firing during rapid-eye-movement (REM) sleep which stands in stark contrast to the wake/REM-on discharge pattern of brainstem cholinergic neurons. A prominent model of REM sleep control posits a reciprocal interaction between these cell groups. 5-HT inhibits cholinergic neurons, and activation of nicotinic receptors can excite DRN 5-HT neurons but the cholinergic effect on inhibitory inputs is incompletely understood. Here, in vitro, in DRN brain slices prepared from GAD67-GFP knock-in mice, a brief (3 min) bath application of carbachol (50 μM) increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in GFP-negative, putative 5-HT neurons but did not affect miniature (tetrodotoxin-insensitive) IPSCs. Carbachol had no direct postsynaptic effect. Thus, carbachol likely increases the activity of local GABAergic neurons which synapse on 5-HT neurons. Removal of dorsal regions of the slice including the ventrolateral periaqueductal gray (vlPAG) region where GABAergic neurons projecting to the DRN have been identified, abolished the effect of carbachol on sIPSCs whereas the removal of ventral regions containing the oral region of the pontine reticular nucleus (PnO) did not. In addition, carbachol directly excited GFP-positive, GABAergic vlPAG neurons. Antagonism of both muscarinic and nicotinic receptors completely abolished the effects of carbachol. We suggest cholinergic neurons inhibit DRN 5-HT neurons when acetylcholine levels are lower i.e. during quiet wakefulness and the beginning of REM sleep periods, in part via excitation of muscarinic and nicotinic receptors located on local vlPAG and DRN GABAergic neurons. Higher firing rates or burst firing of cholinergic neurons associated with attentive wakefulness or phasic REM sleep periods leads to excitation of 5-HT neurons via the activation of nicotinic receptors located postsynaptically and presynaptically on excitatory afferents.

Transducin translocation contributes to rod survival and enhances synaptic transmission from rods to rod bipolar cells

In rod photoreceptors, several phototransduction components display light-dependent translocation between cellular compartments. Notably, the G protein transducin translocates from rod outer segments to inner segments/spherules in bright light, but the functional consequences of translocation remain unclear. We generated transgenic mice where light-induced transducin translocation is impaired. These mice exhibited slow photoreceptor degeneration, which was prevented if they were dark-reared. Physiological recordings showed that control and transgenic rods and rod bipolar cells displayed similar sensitivity in darkness. After bright light exposure, control rods were more strongly desensitized than transgenic rods. However, in rod bipolar cells, this effect was reversed; transgenic rod bipolar cells were more strongly desensitized than control. This sensitivity reversal indicates that transducin translocation in rods enhances signaling to rod bipolar cells. The enhancement could not be explained by modulation of inner segment conductances or the voltage sensitivity of the synaptic Ca(2+) current, suggesting interactions of transducin with the synaptic machinery.

Adenosine inhibits the excitatory synaptic inputs to Basal forebrain cholinergic, GABAergic, and parvalbumin neurons in mice

Coffee and tea contain the stimulants caffeine and theophylline. These compounds act as antagonists of adenosine receptors. Adenosine promotes sleep and its extracellular concentration rises in association with prolonged wakefulness, particularly in the basal forebrain (BF) region involved in activating the cerebral cortex. However, the effect of adenosine on identified BF neurons, especially non-cholinergic neurons, is incompletely understood. Here we used whole-cell patch-clamp recordings in mouse brain slices prepared from two validated transgenic mouse lines with fluorescent proteins expressed in GABAergic or parvalbumin (PV) neurons to determine the effect of adenosine. Whole-cell recordings were made from BF cholinergic neurons and from BF GABAergic and PV neurons with the size (>20 μm) and intrinsic membrane properties (prominent H-currents) corresponding to cortically projecting neurons. A brief (2 min) bath application of adenosine (100 μM) decreased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (EPSCs) in all groups of BF cholinergic, GABAergic, and PV neurons we recorded. In addition, adenosine decreased the frequency of miniature EPSCs in BF cholinergic neurons. Adenosine had no effect on the frequency of spontaneous inhibitory postsynaptic currents in cholinergic neurons or GABAergic neurons with large H-currents but reduced them in a group of GABAergic neurons with smaller H-currents. All effects of adenosine were blocked by a selective, adenosine A1 receptor antagonist, cyclopentyltheophylline (CPT, 1 μM). Adenosine had no postsynaptic effects. Taken together, our work suggests that adenosine promotes sleep by an A1 receptor-mediated inhibition of glutamatergic inputs to cortically projecting cholinergic and GABA/PV neurons. Conversely, caffeine and theophylline promote attentive wakefulness by inhibiting these A1 receptors in BF thereby promoting the high-frequency oscillations in the cortex required for attention and cognition.

Dopaminergic modulation of axon collaterals interconnecting spiny neurons of the rat striatum

Dopamine is a critical modulator of striatal function; its absence produces Parkinson's disease. Most cellular actions of dopamine are still unknown. This work describes the presynaptic actions of dopaminergic receptor agonists on GABAergic transmission between neostriatal projection neurons. Axon collaterals interconnect projection neurons, the main axons of which project to other basal ganglia nuclei. Most if not all of these projecting axons pass through the globus pallidus. Thus, we lesioned the intrinsic neurons of the globus pallidus and stimulated neostriatal efferent axons antidromically with a bipolar electrode located in this nucleus. This maneuver revealed a bicuculline-sensitive synaptic current while recording in spiny cells. D1 receptor agonists facilitated whereas D2 receptor agonists depressed this synaptic current. In contrast, a bicuculline-sensitive synaptic current evoked by field stimulation inside the neostriatum was not consistently modulated, in agreement with previous studies. The data are discussed in light of the most recent experimental and modeling results. The conclusion was that inhibition of spiny cells by axon collaterals of other spiny cells is quantitatively important; however, to be functionally important, this inhibition might be conditioned to the synchronized firing of spiny neurons. Finally, dopamine exerts a potentially important role regulating the extent of lateral inhibition.

Vanilloid-sensitive afferents activate neurons with prominent A-type potassium currents in nucleus tractus solitarius

Cranial visceral afferents innervate second-order nucleus tractus solitarius (NTS) neurons via myelinated (A-type) and unmyelinated (C-type) axons in the solitary tract (ST). A- and C-type afferents often evoke reflexes with distinct performance differences, especially with regard to their frequency-dependent properties. In horizontal brainstem slices, we used the vanilloid receptor 1 agonist capsaicin (CAP; 100 nm) to identify CAP-sensitive and CAP-resistant ST afferent pathways to second-order NTS neurons and tested whether these two groups of neurons had similar intrinsic potassium currents. ST stimulation evoked monosynaptic EPSCs identified by minimal synaptic jitter (<150 microsec) and divided into two groups: CAP-sensitive (n = 37) and CAP-resistant (n = 22). EPSCs in CAP-sensitive neurons had longer latencies (5.1 +/- 0.3 vs 3.6 +/- 0.3 msec; p = 0.001) but similar jitter (p = 0.57) compared with CAP-resistant neurons, respectively. Transient outward currents (TOCs) were significantly greater in CAP-sensitive than in CAP-resistant neurons. Steady-state currents were similar in both groups. 4-Aminopyridine or depolarized conditioning blocked the TOC, but tetraethylammonium had no effect. Voltage-dependent activation and inactivation of TOC were consistent with an A-type K+ current, I(KA). In current clamp, the activation of I(KA) reduced neuronal excitability and action potential responses to ST transmission. Our results suggest that the potassium-channel differences of second-order NTS neurons contribute to the differential processing of A- and C-type cranial visceral afferents beginning as early as this first central neuron. I(KA) can act as a frequency transmission filter and may represent a key target for the modulation of temporal processing of reflex responsiveness such as within the baroreflex arc.

Vanilloid receptors presynaptically modulate cranial visceral afferent synaptic transmission in nucleus tractus solitarius

Although the central terminals of cranial visceral afferents express vanilloid receptor 1 (VR1), little is known about their functional properties at this first synapse within the nucleus tractus solitarius (NTS). Here, we examined whether VR1 modulates afferent synaptic transmission. In horizontal brainstem slices, solitary tract (ST) activation evoked EPSCs. Monosynaptic EPSCs had low synaptic jitter (SD of latency to successive shocks) averaging 84.03 +/- 3.74 microsec (n = 72) and were completely blocked by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo[f]quinoxaline (NBQX). Sustained exposure to the VR1 agonist capsaicin (CAP; 100 nm) blocked ST EPSCs (CAP-sensitive) in some neurons but not others (CAP-resistant). CAP-sensitive EPSCs had longer latencies than CAP-resistant EPSCs (4.65 +/- 0.27 msec, n = 48 vs 3.53 +/- 0.28 msec, n = 24, respectively; p = 0.011), but they had similar jitter. CAP evoked two transient responses in CAP-sensitive neurons: a rapidly developing inward current (I(cap)) (108.1 +/- 22.9 pA; n = 21) and an increase in spontaneous synaptic activity. After 3-5 min in CAP, I(cap) subsided and ST EPSCs disappeared. NBQX completely blocked I(cap). The VR1 antagonist capsazepine (10-20 microm) attenuated CAP responses. Anatomically, second-order NTS neurons were identified by 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide transported from the cervical aortic depressor nerve (ADN) to stain central terminals. Neurons with fluorescent ADN contacts had CAP-sensitive EPSCs (n = 5) with latencies and jitter similar to those of unlabeled monosynaptic neurons. Thus, consistent with presynaptic VR1 localization, CAP selectively activates a subset of ST axons to release glutamate that acts on non-NMDA receptors. Because the CAP sensitivity of cranial afferents is exclusively associated with unmyelinated axons, VR1 identifies C-fiber afferent pathways within the brainstem.

Presynaptic modulation of cholinergic neurotransmission in the human proximal stomach

1. This study investigates whether the cholinergic neurones, innervating the human proximal stomach, can be modulated by nitric oxide (NO) or vasoactive intestinal polypeptide (VIP), or via presynaptic muscarinic, alpha(2)- or 5-hydroxytryptamine(4) (5-HT(4)-) receptors. 2. Circular muscle strips, without mucosa, were incubated with [(3)H]-choline to incorporate [(3)H]-acetylcholine into the cholinergic transmitter stores. The basal and electrically-induced release of tritium and [(3)H]-acetylcholine were analysed in a medium containing guanethidine (4 x 10(-6) M), hemicholinium-3 (10(-5) M), physostigmine (10(-5) M) and atropine (10(-6) M). Tissues were stimulated twice for 2 min (S(1) and S(2): 40 V, 1 ms, 4 Hz) and drugs were added before S(2). 3. The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3 x 10(-4) M) and the NO donor sodium nitroprusside (10(-5) M), as well as VIP (10(-7) M) did not influence the basal release nor the electrically-evoked release. 4. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically-evoked release of [(3)H]-acetylcholine, and this was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2 x 10(-6) M). 5. The 5-HT(4)-receptor agonist prucalopride (3 x 10(-7) M) significantly enhanced the electrically-evoked release of [(3)H]-acetylcholine, and the 5-HT(4)-receptor antagonist SB204070 (10(-9) M) prevented this. 6. When atropine (10(-6) M) was omitted from the medium and added before the second stimulation, it significantly increased the release of [(3)H]-acetylcholine. 7. These results suggest that the release of acetylcholine from the cholinergic neurones, innervating the circular muscle in the human proximal stomach, can be inhibited via presynaptic muscarinic auto-receptors and alpha(2)-adrenoceptors, and stimulated via presynaptic 5-HT(4)-receptors. No evidence for modulation by NO or VIP was obtained.

GABA enhances transmission at an excitatory glutamatergic synapse

GABA mediates both presynaptic and postsynaptic inhibition at many synapses. In contrast, we show that GABA enhances transmission at excitatory synapses between the lateral gastric and medial gastric motor neurons and the gastric mill 6a and 9 (gm6a, gm9) muscles and between the lateral pyloric motor neuron and pyloric 1 (p1) muscles in the stomach of the lobster Homarus americanus. Two-electrode current-clamp or voltage-clamp techniques were used to record from muscle fibers. The innervating nerves were stimulated to evoke excitatory junctional potentials (EJPs) or excitatory junctional currents. Bath application of GABA first decreased the amplitude of evoked EJPs in gm6a and gm9 muscles, but not the p1 muscle, by activating a postjunctional conductance increase that was blocked by picrotoxin. After longer GABA applications (5-15 min), the amplitudes of evoked EJPs increased in all three muscles. This increase persisted in the presence of picrotoxin. beta-(Aminomethyl)-4-chlorobenzenepropanoic acid (baclofen) was an effective agonist for the GABA-evoked enhancement but did not increase the postjunctional conductance. Muscimol activated a rapid postsynaptic conductance but did not enhance the amplitude of the nerve-evoked EJPs. GABA had no effect on iontophoretic responses to glutamate and decreased the coefficient of variation of nerve-evoked EJPs. In the presence or absence of tetrodotoxin, GABA increased the frequency but not the amplitude of miniature endplate potentials. These data suggest that GABA acts presynaptically via a GABA(B)-like receptor to increase the release of neurotransmitter.

Presynaptic P2X receptors facilitate inhibitory GABAergic transmission between cultured rat spinal cord dorsal horn neurons

The superficial layers of the spinal cord dorsal horn (DH) express P2X2, P2X4, and P2X6 subunits entering into the formation of ionotropic (P2X) receptors for ATP. Using a culture system of laminae I-III from neonatal rat DH, we show that ATP induced a fast nonselective cation current in 38% of the neurons (postsynaptic effect). ATP also increased the frequency of miniature IPSCs (mIPSCs) mediated by GABA(A) receptors or by glycine receptors in 22 and 9%, respectively, of the neurons tested (presynaptic effect) but had no effect on glutamatergic transmission. The presynaptic effect of ATP on GABAergic transmission was not significantly affected by thapsigargin (1 microM) but was completely dependent on Ca(2+) influx. Presynaptic and postsynaptic effects were inhibited by suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and reactive blue and were not reproduced by uridine 5'-triphosphate (UTP) or adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S), suggesting the implication of ionotropic P2X rather than of metabotropic P2Y receptors. alphabeta-methylene-ATP (100 microM) did not reproduce the effects of ATP. ATP reversibly increased the amplitude of electrically evoked GABAergic IPSCs and reduced paired-pulse inhibition or facilitation without affecting IPSC kinetics. This effect was preferentially, but not exclusively, observed in neurons coreleasing ATP and GABA. We conclude that in cultured DH neurons, the effects of ATP are mediated by P2X receptors having a pharmacological profile dominated by the P2X2 subunit. The presynaptic receptors might underlie a modulatory action of ATP on a subset of GABAergic interneurons involved in the spinal processing of nociceptive information.

ATP stimulates sympathetic transmitter release via presynaptic P2X purinoceptors

ATP is a fast transmitter in sympathetic ganglia and at the sympathoeffector junction. In primary cultures of dissociated rat superior cervical ganglion neurons, ATP elicits noradrenaline release in an entirely Ca2+-dependent manner. Nevertheless, ATP-evoked noradrenaline release was only partially reduced (by approximately 50%) when either Na+ or Ca2+ channels were blocked, which indicates that ATP receptors themselves mediated transmembrane Ca2+ entry. An "axonal" preparation was obtained by removing ganglia from explant cultures, which left a network of neurites behind; immunostaining for axonal and dendritic markers revealed that all of these neurites were axons. In this preparation, ATP raised intraaxonal Ca2+ and triggered noradrenaline release, and these actions were not altered when Ca2+ channels were blocked by Cd2+. Hence, Ca2+-permeable ATP-gated ion channels, i.e., P2X purinoceptors, are located at presynaptic sites and directly mediate Ca2+-dependent transmitter release. These presynaptic P2X receptors displayed a rank order of agonist potency of ATP >/= 2-methylthio-ATP > ATPgammaS >> alpha,beta-methylene-ATP approximately beta,gamma-methylene-L-ATP and were blocked by suramin or PPADS. ATP, 2-methylthio-ATP, and ATPgammaS also evoked inward currents measured at neuronal somata, but there these agonists were equipotent. Hence, presynaptic P2X receptors resemble the cloned P2X2 subtype, but they appear to differ from somatodendritic P2X receptors in terms of agonist sensitivity. Suramin reduced depolarization-evoked noradrenaline release by up to 20%, when autoinhibitory mechanisms were inactivated by pertussis toxin. These results indicate that presynaptic P2X purinoceptors mediate a positive, whereas G-protein-coupled P2Y purinoceptors mediate a negative, feedback modulation of sympathetic transmitter release.

NMDA receptor-mediated control of presynaptic calcium and neurotransmitter release

Before action potential-evoked Ca2+ transients, basal presynaptic Ca2+ concentration may profoundly affect the amplitude of subsequent neurotransmitter release. Reticulospinal axons of the lamprey spinal cord receive glutamatergic synaptic input. We have investigated the effect of this input on presynaptic Ca2+ concentrations and evoked release of neurotransmitter. Paired recordings were made between reticulospinal axons and the neurons that make axo-axonic synapses onto those axons. Both excitatory and inhibitory paired-cell responses were recorded in the axons. Excitatory synaptic inputs were blocked by the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and by the NMDA receptor antagonist 2-amino-5-phosphonopentanoate (AP-5; 50 microM). Application of NMDA evoked an increase in presynaptic Ca2+ in reticulospinal axons. Extracellular stimulation evoked Ca2+ transients in axons when applied either directly over the axon or lateral to the axons. Transients evoked by the two types of stimulation differed in magnitude and sensitivity to AP-5. Simultaneous microelectrode recordings from the axons during Ca2+ imaging revealed that stimulation of synaptic inputs directed to the axons evoked Ca2+ entry. By the use of paired-cell recordings between reticulospinal axons and their postsynaptic targets, NMDA receptor activation was shown to enhance evoked release of transmitter from the axons that received axoaxonic inputs. When the synaptic input to the axon was stimulated before eliciting an action potential in the axon, transmitter release from the axon was enhanced. We conclude that NMDA receptor-mediated input to reticulospinal axons increases basal Ca2+ within the axons and that this Ca2+ is sufficient to enhance release from the axons.

Neuronal nicotinic acetylcholine receptors are blocked by intracellular spermine in a voltage-dependent manner

A common feature of neuronal nicotinic acetylcholine receptors (nAChRs) is that they conduct inward current at negative membrane potentials but little outward current at positive membrane potentials, a property referred to as inward rectification. Physiologically, inward rectification serves important functions, and the main goal of our study was to investigate the mechanisms underlying the rectification of these receptors. We examined recombinant alpha3beta4 and alpha4beta2 neuronal nAChR subtypes expressed in Xenopus oocytes and native nAChRs expressed on superior cervical ganglion (SCG) neurons. Whole-cell ACh-evoked currents recorded from these receptors exhibited strong inward rectification. In contrast, we showed that single-channel currents from these neuronal nAChRs measured in outside-out patches outwardly rectify. On the basis of recent findings that spermine, a ubiquitous intracellular polyamine, confers rectification to glutamate receptors and inwardly rectifying potassium channels, we investigated whether spermine causes neuronal nAChRs to inwardly rectify. When spermine was added to the patch electrode in outside-out recordings, it caused a concentration- and voltage-dependent block of ACh-evoked single-channel currents. Using these single-channel data and physiological concentrations of intracellular spermine, we could account for the inward rectification of macroscopic whole-cell ACh-evoked conductance-voltage relationships. Therefore, we conclude that the voltage-dependent block by intracellular spermine underlies inward rectification of neuronal nAChRs. We also found that extracellular spermine blocks both alpha3beta4 and alpha4beta2 receptors; this finding points to a mechanism whereby increases in extracellular spermine, perhaps during pathological conditions, could selectively block these receptors.

Modulation of the firing activity of noradrenergic neurones in the rat locus coeruleus by the 5-hydroxtryptamine system

1. The aim of the present study was to investigate the putative modulation of locus coeruleus (LC) noradrenergic (NA) neurones by the 5-hydroxytryptaminergic (5-HT) system by use of in vivo extracellular unitary recordings and microiontophoresis in anaesthetized rats. To this end, the potent and selective 5-HT1A receptor antagonist WAY 100635 (N-[2-[4(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydroxychloride) was used. 2. In the dorsal hippocampus, both local (by microiontophoresis, 20 nA) and systemic (100 micrograms kg-1, i.v.) administration of WAY 100635 antagonized the suppressant effect of microiontophorectically-applied 5-HT on the firing activity of CA3 pyramidal neurones, indicating its antagonistic effect on postsynaptic 5-HT1A receptors. 3. WAY 100635 and 5-HT failed to modify the spontaneous firing activity of LC NA neurones when applied by microiontophoresis. However, the intravenous injection of WAY 100635 (100 micrograms kg-1) readily suppressed the spontaneous firing activity of LC NA neurones. 4. The lesion of 5-HT neurones with the neurotoxin 5,7-dihydroxytryptamine increased the spontaneous firing activity of LC NA neurones and abolished the suppressant effect of WAY 100635 on the firing activity of LC NA neurones. 5. In order to determine the nature of the 5-HT receptor subtypes mediating the suppressant effect of WAY 100635 on NA neurone firing activity, several 5-HT receptor antagonists were used. The selective 5-HT3 receptor antagonist BRL 46470A (10 and 100 micrograms kg-1, i.v.), the 5-HT1D receptor antagonist GR 127935 (100 micrograms kg-1, i.v.) and the 5-HT1A/1B receptor antagonist (-)-pindolol (15 mg kg-1, i.p.) did not prevent the suppressant effect of WAY 100635 on the firing activity of LC NA neurones. However, the suppressant effect of WAY 100635 was prevented by the non-selective 5-HT receptor antagonists spiperone (1 mg kg-1, i.v.) and metergoline (1 mg kg-1, i.v.), by the 5-HT2 receptor antagonist ritanserin (500 micrograms kg-1, i.v.). It was also prevented by the 5-HT1A receptor/alpha 1D-adrenoceptor antagonist BMY 7378 (1 mg kg-1, i.v.) and by the alpha 1-adrenoceptor antagonist prazosin (100 micrograms kg-1, i.v.). 6. These data support the notion that the 5-HT system tonically modulates NA neurotransmission since the lesion of 5-HT neurones enhanced the LC NA neurones firing activity and the suppressant effect of WAY 100635 on the firing activity of NA neurones was abolished by this lesion. However, the location of the 5-HT1A receptors involved in this complex circuitry remains to be elucidated. It is concluded that the suppressant effect of WAY 100635 on the firing activity of LC NA neurones is due to an enhancement of the function of 5-HT neurones via a presynaptic 5-HT1A receptor. In contrast, the postsynaptic 5-HT receptor mediating this effect of WAY 100635 on NA neurones appears to be of the 5-HT2A subtype.

Modulation of inhibitory transmission by dopamine in rat basal forebrain nuclei: activation of presynaptic D1-like dopaminergic receptors

The effects of dopamine (DA) on inhibitory transmission onto identified magnocellular neurons were examined in rat basal forebrain slices using whole-cell recording. IPSCs evoked by focal stimulation within basal forebrain nuclei were reversibly blocked by 10 microM bicuculline and had a decay time constant of 20.1 +/- 0.77 msec in the presence of 6-cyano-7-nitroquinoxalline-2,3-dione (5 mM). Bath application of DA reduced the amplitude of IPSCs up to 71.1 +/- 1.49% in a concentration-dependent manner between 0.003 and 1 mM (the IC50 value being 6.6 microM), without any effect on the holding current at -70 mV. DA (10 microM) reduced the frequency of miniature IPSCs (mIPSCs) recorded in the presence of TTX (0.5 microM), without affecting their mean amplitude, rise time, and decay time constant. Furthermore, the DA-induced effect on mIPSCs remained unaffected by 100 microM cadmium, suggesting a presynaptic mechanism independent of calcium influx. SKF 81297, a D1-like agonist, mimicked DA-induced effect on evoked IPSCs (IC50, 10.9 microM), whereas R(-)-TNPA or (-)-quinpirole, D2-like agonists (30 microM), had little or no effect on the amplitude of evoked IPSCs. R(+)-SCH 23390, a D1-like antagonist, antagonized the DA-induced effect on IPSCs (K(B) 0.82 microM), whereas S(-)-eticlopride, a D2-like antagonist, showed slight antagonism (K(B) 7.8 microM). Forskolin (10 microM) reduced the amplitude of evoked IPSCs to approximately 58% of the control and occluded the inhibitory effect of DA. These findings indicate that DA reduces inhibitory transmission onto magnocellular basal forebrain neurons by activating presynaptic D1-like receptors.

Peptidergic modulation of synaptic transmission in the parabrachial nucleus in vitro: importance of degradative enzymes in regulating synaptic efficacy

This study examined the effects of substance P (SP) and calcitonin gene-related peptide (CGRP) on synaptic transmission in a pontine slice containing the parabrachial nucleus (PBN). Stimulation of the ventral, external lateral portion of the PBN elicited glutamate-mediated EPSCs in cells recorded using the nystatin perforated-patch recording technique in the external lateral, external medial, and central lateral subnuclei of the PBN. Bath application of SP or CGRP dose-dependently and reversibly attenuated the evoked EPSC. The attenuation of the EPSC induced by both of these peptides was not accompanied by changes in input resistance of PBN cells over a wide voltage range, nor did these peptides alter the inward current induced by a brief bath application of AMPA. The combined application of subthreshold concentrations of these peptides revealed a synergistic interaction in reducing the evoked EPSC. The substance P neurokinin-1 receptor antagonist CGP49823 completely and reversibly blocked both the SP- and the CGRP-induced attenuation of the EPSC. However, the rat CGRP receptor antagonist human-CGRP8-37 did not block the actions of CGRP or SP on the EPSC. Using a metabolically stable analog of SP, SP (5-11), or an endopeptidase inhibitor, phosphoramidon, we were able to demonstrate that CGRP enhances the SP effect by inhibiting an SP endopeptidase. Application of phosphoramidon also revealed an endogenous SP "tone" apparently made effective by blockade of the endopeptidase. These results suggest that SP (and CGRP indirectly through an inhibition of the SP endopeptidase) acts on presynaptic NK-1 receptors to cause an inhibition of excitatory transmission in the PBN. These results indicate an important role of endopeptidases in regulating synaptic modulation by peptides.