Exocytotic release from neuronal cell bodies, dendrites and nerve terminals in sympathetic ganglia of the rat, and its differential regulation

The Role of the Central Histaminergic System in Behavioral State Control

Histamine is a small monoamine signaling molecule that plays a role in many peripheral and central physiological processes, including the regulation of wakefulness. The tuberomammillary nucleus is the sole neuronal source of histamine in the brain, and histamine neurons are thought to promote wakefulness and vigilance maintenance - under certain environmental and/or behavioral contexts - through their diffuse innervation of the cortex and other wake-promoting brain circuits. Histamine neurons also contain a number of other putative neurotransmitters, although the functional role of these co-transmitters remains incompletely understood. Within the brain histamine operates through three receptor subtypes that are located on pre- and post-synaptic membranes. Some histamine receptors exhibit constitutive activity, and hence exist in an activated state even in the absence of histamine. Newer medications used to reduce sleepiness in narcolepsy patients in fact enhance histamine signaling by blunting the constitutive activity of these histamine receptors. In this chapter, we provide an overview of the central histamine system with an emphasis on its role in behavioral state regulation and how drugs targeting histamine receptors are used clinically to treat a wide range of sleep-wake disorders.

Multiple signalling modalities mediated by dendritic exocytosis of oxytocin and vasopressin

The mammalian hypothalamic magnocellular neurons of the supraoptic and paraventricular nuclei are among the best understood of all peptidergic neurons. Through their anatomical features, vasopressin- and oxytocin-containing neurons have revealed many important aspects of dendritic functions. Here, we review our understanding of the mechanisms of somato-dendritic peptide release, and the effects of autocrine, paracrine and hormone-like signalling on neuronal networks and behaviour.

The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse"

It is generally accepted that intestinal sensory vagal fibers are primary afferent, responding nonsynaptically to luminal stimuli. The gut also contains intrinsic primary afferent neurons (IPANs) that respond to luminal stimuli. A psychoactive Lactobacillus rhamnosus (JB-1) that affects brain function excites both vagal fibers and IPANs. We wondered whether, contrary to its primary afferent designation, the sensory vagus response to JB-1 might depend on IPAN to vagal fiber synaptic transmission. We recorded ex vivo single- and multiunit afferent action potentials from mesenteric nerves supplying mouse jejunal segments. Intramural synaptic blockade with Ca(2+) channel blockers reduced constitutive or JB-1-evoked vagal sensory discharge. Firing of 60% of spontaneously active units was reduced by synaptic blockade. Synaptic or nicotinic receptor blockade reduced firing in 60% of vagal sensory units that were stimulated by luminal JB-1. In control experiments, increasing or decreasing IPAN excitability, respectively increased or decreased nerve firing that was abolished by synaptic blockade or vagotomy. We conclude that >50% of vagal afferents function as interneurons for stimulation by JB-1, receiving input from an intramural functional "sensory synapse." This was supported by myenteric plexus nicotinic receptor immunohistochemistry. These data offer a novel therapeutic target to modify pathological gut-brain axis activity.-Perez-Burgos, A., Mao, Y.-K., Bienenstock, J., Kunze, W. A. The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse."

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Cycling of dense core vesicles involved in somatic exocytosis of serotonin by leech neurons

We studied the cycling of dense core vesicles producing somatic exocytosis of serotonin. Our experiments were made using electron microscopy and vesicle staining with fluorescent dye FM1-43 in Retzius neurons of the leech, which secrete serotonin from clusters of dense core vesicles in a frequency-dependent manner. Electron micrographs of neurons at rest or after 1 Hz stimulation showed two pools of dense core vesicles. A perinuclear pool near Golgi apparatuses, from which vesicles apparently form, and a peripheral pool with vesicle clusters at a distance from the plasma membrane. By contrast, after 20 Hz electrical stimulation 47% of the vesicle clusters were apposed to the plasma membrane, with some omega exocytosis structures. Dense core and small clear vesicles apparently originating from endocytosis were incorporated in multivesicular bodies. In another series of experiments, neurons were stimulated at 20 Hz while bathed in a solution containing peroxidase. Electron micrographs of these neurons contained gold particles coupled to anti-peroxidase antibodies in dense core vesicles and multivesicular bodies located near the plasma membrane. Cultured neurons depolarized with high potassium in the presence of FM1-43 displayed superficial fluorescent spots, each reflecting a vesicle cluster. A partial bleaching of the spots followed by another depolarization in the presence of FM1-43 produced restaining of some spots, other spots disappeared, some remained without restaining and new spots were formed. Several hours after electrical stimulation the FM1-43 spots accumulated at the center of the somata. This correlated with electron micrographs of multivesicular bodies releasing their contents near Golgi apparatuses. Our results suggest that dense core vesicle cycling related to somatic serotonin release involves two steps: the production of clear vesicles and multivesicular bodies after exocytosis, and the formation of new dense core vesicles in the perinuclear region.

Colocalization of FM1-43, Bassoon, and GnRH-1: GnRH-1 release from cell bodies and their neuroprocesses

Pulsatile release of GnRH-1 is critical for reproductive function. However, the cellular mechanism of GnRH-1 neurosecretion is still elusive. In this study, we examined the neurosecretory process of GnRH-1 neurons using time-lapse image acquisition followed by immunocytochemistry with confocal microscopy. To monitor exocytotic processes, cultured GnRH-1 neurons derived from monkey embryos were labeled with the lipophilic dye, FM1-43, or its fixable form FM1-43Fx, in the presence or absence of depolarization signals, and changes in vesicles labeled with FM1-43 were analyzed. The results show FM1-43 was taken up into the cell and labeled puncta in the soma and neuroprocesses in the absence of depolarization signals, indicating that GnRH-1 neurons were spontaneously active. Depolarization of GnRH-1 neurons with high K+ or veratridine challenge increased the intensity and size of puncta in both soma and neuroprocesses, and the veratridine-induced changes in puncta were blocked by tetrodotoxin, indicating that changes in the puncta intensity and size reflect neurosecretory activity. Subsequent double immunocytochemistry for GnRH-1 and the synaptic vesicle marker, vesicle-associated membrane protein, demonstrated that the FM1-43Fx-labeled puncta were synaptic vesicles with the GnRH-1 peptide. Additional double immunocytochemistry for GnRH-1 and the marker of the neurosecretory active zone, Bassoon, indicated that the FM1-43Fx-labeled puncta were located at the sites of neurosecretory active zones in GnRH-1 neurons. These results suggest that GnRH-1 neurons have the capacity to release the peptide from the soma and dendrites. Collectively, we hypothesize that soma-dendritic release of the peptide may be a mechanism of synchronized activity among GnRH-1 neurons.

Somatic ATP release from guinea pig sympathetic neurons does not require calcium-induced calcium release from internal stores

Prior studies indicated that a Ca(2+)-dependent release of ATP can be initiated from the soma of sympathetic neurons dissociated from guinea pig stellate ganglia. Previous studies also indicated that Ca(2+)-induced Ca(2+) release (CICR) can modulate membrane excitability in these same neurons. As Ca(2+) release from internal stores is thought to support somatodendritic transmitter release in other neurons, the present study investigated whether CICR is essential for somatic ATP release from dissociated sympathetic neurons. Caffeine increased intracellular Ca(2+) and activated two inward currents: a slow inward current (SIC) in 85% of cells, and multiple faster inward currents [asynchronous transient inward currents (ASTICs)] in 40% of cells voltage-clamped to negative potentials. Caffeine evoked both currents when cells were bathed in a Ca(2+)-deficient solution, indicating that both were initiated by Ca(2+) release from ryanodine-sensitive stores in the endoplasmic reticulum. Sodium influx contributed to generation of both SICs and ASTICs, but only ASTICs were inhibited by the presence of the P2X receptor blocker PPADs. Thus ASTICs, but not SICs, resulted from an ATP activation of P2X receptors. Ionomycin induced ASTICs in a Ca(2+)-containing solution, but not when it was applied in a Ca(2+)-deficient solution, demonstrating the key requirement for external Ca(2+) in initiating ASTICs by ionomycin. Pretreatment with drugs to deplete the internal stores of Ca(2+) did not block the ability of ionomycin or long depolarizing voltage steps to initiate ASTICs. Although a caffeine-induced release of Ca(2+) from internal stores can elicit both SICs and ASTICs in dissociated sympathetic neurons, CICR is not required for the somatic release of ATP.

Three-photon microscopy shows that somatic release can be a quantitatively significant component of serotonergic neurotransmission in the mammalian brain

Recent experiments on monoaminergic neurons have shown that neurotransmission can originate from somatic release. However, little is known about the quantity of monoamine available to be released through this extrasynaptic pathway or about the intracellular dynamics that mediate such release. Using three-photon microscopy, we directly imaged serotonin autofluorescence and investigated the total serotonin content, release competence, and release kinetics of somatic serotonergic vesicles in the dorsal raphe neurons of the rat. We found that the somata of primary cultured neurons contain a large number of serotonin-filled vesicles arranged in a perinuclear fashion. A similar distribution is also observed in fresh tissue slice preparations obtained from the rat dorsal raphe. We estimate that the soma of a cultured neuron on an average contains about 9 fmoles of serotonin in about 450 vesicles (or vesicle clusters) of < or =370 nm average diameter. A substantial fraction (>30%) of this serotonin is released with a time scale of several minutes by K(+)-induced depolarization or by para-chloroamphetamine treatment. The amount of releasable serotonin stored in the somatic vesicles is comparable to the total serotonin content of all the synaptic vesicles in a raphe neuron, indicating that somatic release can potentially play a major role in serotonergic neurotransmission in the mammalian brain.

Synaptic ultrastructural alterations anticipate the development of neuroaxonal dystrophy in sympathetic ganglia of aged and diabetic mice

Neuroaxonal dystrophy, a distinctive axonopathy characterized by marked enlargement of distal axons, is the hallmark pathologic alteration in aged and diabetic human prevertebral sympathetic ganglia and in corresponding rodent models. Neuroaxonal dystrophy is thought to represent the abnormal outcome of cycles of synaptic degeneration and regeneration; a systematic study of identified axon terminals in aged and diabetic prevertebral ganglia, however, has not previously been performed. We examined the initial changes that develop in presynaptic and postsynaptic elements in sympathetic ganglia of aged and diabetic mice and found numerous synaptic changes involving both presynaptic and postsynaptic elements. Early alterations in presynaptic axon terminal size, vesicle content, and morphology culminate in the development of anastomosing membranous tubulovesicular aggregates, accumulation of autophagosomes, and amorphous debris that form a continuum with progressively larger classically dystrophic swellings. Dendritic changes consist of the development of swellings composed of delicate tubulovesicular elements and mitochondriopathy characterized by increased numbers of small mitochondria and, exclusively in aged ganglia, megamitochondria. These results support the hypothesis that neuroaxonal dystrophy results from progressive changes in presynaptic axon terminals that likely involve membrane dynamics and which are accompanied by distinctive changes in postsynaptic dendritic elements.

Structure of peripheral synapses: autonomic ganglia

Final motor neurons in sympathetic and parasympathetic ganglia receive synaptic inputs from preganglionic neurons. Quantitative ultrastructural analyses have shown that the spatial distribution of these synapses is mostly sparse and random. Typically, only about 1%-2% of the neuronal surface is covered with synapses, with the rest of the neuronal surface being closely enclosed by Schwann cell processes. The number of synaptic inputs is correlated with the dendritic complexity of the target neuron, and the total number of synaptic contacts is related to the surface area of the post-synaptic neuron. Overall, most neurons receive fewer than 150 synaptic contacts, with individual preganglionic inputs providing between 10 and 50 synaptic contacts. This variation is probably one determinant of synaptic strength in autonomic ganglia. Many neurons in prevertebral sympathetic ganglia receive additional convergent synaptic inputs from intestinofugal neurons located in the enteric plexuses. The neurons support these additional inputs via larger dendritic arborisations together with a higher overall synaptic density. There is considerable neurochemical heterogeneity in presynaptic boutons. Some synapses apparently lack most of the proteins normally required for fast transmitter release and probably do not take part in conventional ganglionic transmission. Furthermore, most preganglionic boutons in the ganglionic neuropil do not form direct synaptic contacts with any neurons. Nevertheless, these boutons may well contribute to slow transmission processes that need not require conventional synaptic structures.

Exocytotic release of ATP and activation of P2X receptors in dissociated guinea pig stellate neurons

Activation of P2X receptors by a Ca(2+)- and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein-dependent release of ATP was measured using patch-clamp recordings from dissociated guinea pig stellate neurons. Asynchronous transient inward currents (ASTICs) were activated by depolarization or treatment with the Ca(2+) ionophore ionomycin (1.5 and 3 microM). During superfusion with a HEPES-buffered salt solution containing 2.5 mM Ca(2+), depolarizing voltage steps (-60 to 0 mV, 500 ms) evoked ASTICs on the decaying phase of a larger, transient inward current. Equimolar substitution of Ba(2+) for Ca(2+) augmented the postdepolarization frequency of ASTICs, while eliminating the larger transient current. Perfusion with an ionomycin-containing solution elicited a sustained activation of ASTICs, allowing quantitative analysis over a range of holding potentials. Under these conditions, increasing extracellular [Ca(2+)] to 5 mM increased ASTIC frequency, whereas no events were observed following replacement of Ca(2+) with Mg(2+), demonstrating a Ca(2+) requirement. ASTICs were Na(+) dependent, inwardly rectifying, and reversed near 0 mV. Treatment with the nonselective purinergic receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (10 microM) blocked all events under both conditions, whereas the ganglionic nicotinic antagonist hexamethonium (100 microM and 1 mM) had no effect. PPADS also blocked the macroscopic inward current evoked by exogenously applied ATP (300 microM). The presence of botulinum neurotoxin E (BoNT/E) in the whole-cell recording electrode significantly attenuated the ionomycin-induced ASTIC activity, whereas phorbol ester treatment potentiated this activity. These results suggest that ASTICs are mediated by vesicular release of ATP and activation of P2X receptors.

Synaptic and Extrasynaptic Secretion of Serotonin

Serotonin is a major modulator of behavior in vertebrates and invertebrates and deficiencies in the serotonergic system account for several behavioral disorders in humans. The small numbers of serotonergic central neurons of vertebrates and invertebrates produce their effects by use of two modes of secretion: from synaptic terminals, acting locally in "hard wired" circuits, and from extrasynaptic axonal and somatodendritic release sites in the absence of postsynaptic targets, producing paracrine effects. In this paper, we review the evidence of synaptic and extrasynaptic release of serotonin and the mechanisms underlying each secretion mode by combining evidence from vertebrates and invertebrates. Particular emphasis is given to somatic secretion of serotonin by central neurons. Most of the mechanisms of serotonin release have been elucidated in cultured synapses made by Retzius neurons from the central nervous system of the leech. Serotonin release from synaptic terminals occurs from clear and dense core vesicles at active zones upon depolarization. In general, synaptic serotonin release is similar to release of acetylcholine in the neuromuscular junction. The soma of Retzius neurons releases serotonin from clusters of dense core vesicles in the absence of active zones. This type of secretion is dependent of the stimulation frequency, on L-type calcium channel activation and on calcium-induced calcium release. The characteristics of somatic secretion of serotonin in Retzius neurons are similar to those of somatic secretion of dopamine and peptides by other neuron types. In general, somatic secretion by neurons is different from transmitter release from clear vesicles at synapses and similar to secretion by excitable endocrine cells.

Calcium-induced calcium release contributes to somatic secretion of serotonin in leech Retzius neurons

We analyzed the contribution of calcium (Ca2+)-induced Ca2+ release to somatic secretion in serotonergic Retzius neurons of the leech. Somatic secretion was studied by the incorporation of fluorescent dye FM1-43 upon electrical stimulation with trains of 10 impulses and by electron microscopy. Quantification of secretion with FM1-43 was made in cultured neurons to improve optical resolution. Stimulation in the presence of FM1-43 produced a frequency-dependent number of fluorescent spots. While a 1-Hz train produced 19.5+/-5.0 spots/soma, a 10-Hz train produced 146.7+/-20.2 spots/soma. Incubation with caffeine (10 mM) to induce Ca2+ release from intracellular stores without electrical stimulation and external Ca2+, produced 168+/-21.7 spots/soma. This staining was reduced by 49% if neurons were preincubated with the Ca2+- ATPase inhibitor thapsigargin (200 nM). Moreover, in neurons stimulated at 10 Hz in the presence of ryanodine (100 microM) to block Ca2+-induced Ca2+ release, FM1-43 staining was reduced by 42%. In electron micrographs of neurons at rest or stimulated at 1 Hz in the ganglion, endoplasmic reticulum lay between clusters of dense core vesicles and the plasma membrane. In contrast, in neurons stimulated at 20 Hz, the vesicle clusters were apposed to the plasma membrane and flanked by the endoplasmic reticulum. These results suggest that Ca2+-induced Ca2+ release produces vesicle mobilization and fusion in the soma of Retzius neurons, and supports the idea that neuronal somatic secretion shares common mechanisms with secretion by excitable endocrine cells.

Calcitonin gene-related peptide regulates gene transcription in primary afferent neurons

Although primary afferent neurons express receptors for calcitonin gene-related peptide (CGRP), understanding of the cellular effects of these receptors is limited. We determined that CGRP receptors regulate gene transcription in primary afferent neurons through a cyclic AMP (cAMP)-dependent pathway. CGRP increased cAMP in neonatal dorsal root ganglion (DRG) neurons in a concentration-dependent manner that was blocked by the receptor antagonist CGRP(8-37). The response to CGRP also occurred in adult DRG cells. In contrast, CGRP did not alter the concentration of free intracellular calcium in neonatal or adult DRG neurons. Immunohistochemical data showed that one downstream effect of the cAMP signaling pathway was phosphorylation of cAMP response element binding (CREB) protein, suggesting that CGRP regulates gene expression. This interpretation was supported by evidence that CGRP increased CRE-dependent gene transcription in neurons transiently transfected with a CRE-luciferase DNA reporter construct. The effect of CGRP on gene transcription was inhibited by H89, myristoylated-protein kinase A inhibitor(14-22)-amide and U0126, indicating that protein kinase A and mitogen-activated protein kinase/extracellular receptor kinase kinase are enzymes that mediate effects of CGRP on gene transcription. Therefore, CGRP receptors may regulate expression of proteins by primary afferent neurons during development and in response to tissue-damaging stimuli.

Pacemaker potentials are the physiologic basis of epileptiform activity in the buccal ganglia of Helix pomatia

Mechanisms of epileptic activity in nervous systems were studied using the identified neurons B1 through B4 in the buccal ganglia of the snail Helix pomatia as a model system. Activities were recorded with intracellular microelectrodes. Epileptiform activity was induced by bath application of an epileptogenic drug (pentylenetetrazol: 1 mM to 40 mM, or etomidate: 0.1 mM to 1.0 mM). Epileptiform potentials recorded from the somata of neurons consisted of paroxysmal depolarization shifts (PDSs). With increasing concentration of an epileptogenic drug, pacemaker potentials in neuron B3 developed into PDS. Simultaneously several types of chemical post-synaptic potentials were suppressed in amplitude. Since on the one hand epileptic seizures only appear when PDS are synchronized in many neurons and since on the other hand synaptic potentials were found to be suppressed during epileptic conditions, mechanisms underlying neuronal synchronization were studied. Evidence was found that, under epileptogenic conditions only, neurons were synchronized by an non-synaptic release of substances. Strong depolarizations accompanied by an increase in intracellular calcium concentration are known to induce an unspecific exocytosis. Thus, an unspecific exocytosis from the dendrites of PDS-generating neurons probably appears under epileptic conditions and synchronizes neighbouring neurons.

Ca(2+) and frequency dependence of exocytosis in isolated somata of magnocellular supraoptic neurones of the rat hypothalamus

In addition to action potential-evoked exocytotic release at neurohypophysial nerve terminals, the neurohormones arginine vasopressin (aVP) and oxytocin (OT) undergo Ca(2+)-dependent somatodendritic release within the supraoptic and paraventricular hypothalamic nuclei. However, the cellular and molecular mechanisms that underlie this release have not been elucidated. In the present study, the whole-cell patch-clamp technique was utilized in combination with high-time-resolved measurements of membrane capacitance (C(m)) and microfluorometric measurements of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) to examine the Ca(2+) and stimulus dependence of exocytosis in the somata of magnocellular neurosecretory cells (MNCs) isolated from rat supraoptic nucleus (SON). Single depolarizing steps (> or =20 ms) that evoked high-voltage-activated (HVA) Ca(2+) currents (I(Ca)) and elevations in intracellular Ca(2+) concentration were accompanied by an increase in C(m) in a majority (40/47) of SON neurones. The C(m) responses were composed of an initial Ca(2+)-independent, transient component and a subsequent, sustained phase of increased C(m) (termed DeltaC(m)) mediated by an influx of Ca(2+), and increased with corresponding prolongation of depolarizing step durations (20-200 ms). From this relationship we estimated the rate of vesicular release to be 1533 vesicles s(-1). Delivery of neurone-derived action potential waveforms (APWs) as stimulus templates elicited I(Ca) and also induced a DeltaC(m), provided APWs were applied in trains of greater than 13 Hz. A train of APWs modelled after the bursting pattern recorded from an OT-containing neurone during the milk ejection reflex was effective in supporting an exocytotic DeltaC(m) in isolated MNCs, indicating that the somata of SON neurones respond to physiological patterns of neuronal activity with Ca(2+)-dependent exocytotic activity.

Subcellular distribution of histamine, GABA and galanin in tuberomamillary neurons in vitro

Histamine acts as a neurotransmitter in the brain and regulates e.g. sleep, hibernation, vigilance, and release of several other transmitters. All histaminergic neurons are found in the tuberomamillary nucleus (TM), and send axons to almost all parts of the CNS. Despite the obvious importance of these neurons, their development, transmitter storage, and compartmentalization of cotransmitters are poorly known. Histaminergic neurons from fetal rat hypothalamus were studied in primary explant cultures and analyzed by confocal microscopy. Most histaminergic neurons were oval in shape, but round and triangular ones were also found. The average size of the 212 analyzed neurons was 19.2 microm (length), 12.5 microm (width) and 11.7 microm (thickness). The cells possessed two to five microtubule-associated protein (MAP2) positive processes, putative dendrites, and in general one MAP2-negative thin process, a putative axon. Granular histamine-immunoreactivity was found in the cell bodies, axons, and dendrites. In tuberomamillary neurons, most histamine-containing structures displayed immunoreactivity for vesicular monoamine transporter 2 (VMAT2), indicating that the two markers may coexist in the same structures. Lack of VMAT2 in some histamine-immunoreactive structures indicates that another transporter for histamine may exist. In the same neurons, gamma-aminobutyric acid (GABA)-immunoreactivity was found in structures, distinct from those containing histamine, indicating that the two transmitters may be differentially localized, regulated and released. Galanin-immunoreactivity in the cultured tuberomamillary neurons was partially located in the same structures as VMAT2. The results suggest that histamine and GABA, the two principal transmitters of tuberomamillary neurons, are not costored in the same structures in tuberomamillary neurons.

Paroxysmal depolarization shifts (PDS) induce non-synaptic responses in neighboured neurons (buccal ganglia, Helix pomatia)

A non-synaptic spread of excitation between neighboured neurons was studied in a model nervous system using epileptiform activity. The identified giant neuron B3 in the buccal ganglia of Helix pomatia reliably generated paroxysmal depolarization shifts (PDS) when treated with pentylenetetrazol or etomidate. Simultaneous recordings of neuron B3 and other neurons showed that each PDS in neuron B3 was accompanied by a depolarization in the other neurons. These related depolarizations (PDS-RD) appeared about 1 to 5 s after the beginning of PDS, their amplitude was up to 20 mV and their duration ca. 1 min. Reduction of extracellular calcium concentration or application of a "high Mg-low Ca" solution blocked PDS-RD. There were, however, no hints for synaptic contacts of the studied neurons. Occasional failures of spontaneous PDS in one neuron B3 of the B3-network of neurons, resulted in a failure of PDS-RD in the neighboured neurons. Block and induction of PDS in one neuron by injection of hyperpolarizing and depolarizing currents, respectively, blocked and induced PDS-RD in the neighboured neurons. As intracellular staining of neurons B1 and B3 showed that their dendritic arborizations were co-localized in the same region of the ganglion, a dendro-dendritic release of substances may cause PDS-RD. Since PDS-RD could themselves trigger PDS, PDS-RD may provide a new basic mechanism of synchronizing epileptic activity of neighboured neurons within an epileptic focus.

Somatic exocytosis of serotonin mediated by L-type calcium channels in cultured leech neurones

We studied somatic exocytosis of serotonin and its mediation by L-type calcium (Ca2+) channels in cultured Retzius neurones of the leech. Exocytosis was induced by trains of impulses at different frequencies or by depolarisation with 40 mM potassium (K+), and was quantified by use of the fluorescent dye FM 1-43. Stimulation increased the membrane fluorescence and produced a pattern of FM 1-43 fluorescent spots of 1.28 +/- 0.01 microm in diameter, provided that Ca2+ was present in the bathing fluid. Individual spots lost their stain during depolarisation with 40 mM K+. Electron micrographs showed clusters of dense core vesicles, some of which were in contact with the cell membrane. Presynaptic structures with clear vesicles were absent from the soma. The number of fluorescent spots per soma, but not their diameter or their fluorescence intensity, depended on the frequency of stimulation. Trains at 1 Hz produced 19.5 +/- 5 spots per soma, 77.9 +/- 13.9 spots per soma were produced at 10 Hz and 91.5 +/- 16.9 spots per soma at 20 Hz. Staining patterns were similar for neurones in culture and in situ. In the presence of the L-type Ca2+ channel blocker nimodipine (10 microM), a 20 Hz train produced only 22.9 +/- 6.4 spots per soma, representing a 75 % reduction compared to control cells (P < 0.05). Subsequent incubation with 10 mM caffeine to induce Ca2+ release from intracellular stores increased the number of spots to 73.22 +/- 12.5. Blockers of N-, P-, Q- or invertebrate Ca2+ channels did not affect somatic exocytosis. Our results suggest that somatic exocytosis by neurones shares common mechanisms with excitable endocrine cells.

Synaptic vesicle recycling at two classes of release sites in giant nerve terminals of the embryonic chicken ciliary ganglion

Rapid synaptic transmission in the embryonic chicken ciliary ganglion occurs through the activation of two distinct classes of nicotinic acetylcholine receptors (AChRs): those containing alpha3 subunits (alpha 3*-AChRs) and those containing alpha7 subunits (alpha 7*-AChRs). alpha3*-AChRs are found on ciliary neurons in clusters at synaptic sites on the cell body, whereas alpha7* -AChRs are found on somatic spines, which historically were thought not to have release sites in the embryo. However, Shoop et al. (Shoop et al. [1999] J. Neurosci. 19:692-704) recently described release sites having pre- and postsynaptic densities on somatic spines. We used transmission electron microscopy to compare the structure of synaptic sites on spines with those on the smooth surfaced part of the cell. We find that the two populations of sites are similar in active zone length, number of vesicles, and distance between vesicles and active zone. To study the functional properties of these sites, we examined their stimulation-dependent uptake and release of the extracellular tracer horseradish peroxidase (HRP). We found that each class of release sites both took up and released HRP in a stimulation- and calcium-dependent manner. The mean fraction of synaptic vesicles labeled with tracer was similar for the two populations, both after loading ( approximately 45%) and after unloading ( approximately 7%). Thus we detect no differences between these two anatomically distinct classes of release sites, other than their incidence: sites on spines occurred only 12% as often as those on the cell body. The release sites on somatic spines presumably underlie synaptic responses attributable to alpha7*-AChRs.

Distinct mechanisms underlying alpha1-adrenoceptor and P2x purinoceptor operated ATP release and contraction in the guinea-pig vas deferens

The temperature-dependence of ATP release and contraction response evoked by different agonists were investigated in superfused guinea-pig vas deferens. Alpha-adrenoceptor agonists, i.e. noradrenaline (300 microM), and alpha-methyl-noradrenaline (300 microM), increased the basal ATP outflow, measured by the luciferin-luciferase assay, and induced biphasic contractile response. Cooling the bath temperature to 12 degrees C almost completely inhibited ATP release and twitch contraction evoked by alpha-adrenoceptor agonists, whereas the phasic contraction remained unaffected. In contrast, twitch contraction and subsequent ATP release induced by beta,gamma-methylene-ATP, a selective P2 receptor agonist (100 microM), was not reduced by low temperature. The ectoATPase activity, measured by HPLC technique was not significantly different at 37 degrees C and 12 degrees C. Nifedipine (1 microM), the voltage sensitive Ca2+ channel blocker eliminated beta,gamma-methylene-ATP evoked twitch contraction but not ATP release. In conclusion, alpha-adrenoceptor and P2 receptor agonists utilize distinct mechanisms to elicit ATP release and contraction: alpha-adrenoceptor-mediated ATP release and contraction is temperature-dependent, indicating the involvement of a carrier-mediated process in it, whereas P2x purinoceptor evoked ATP release and twitch is mediated by a different mechanism.

Vesicular acetylcholine transporter in the rat nucleus accumbens shell: subcellular distribution and association with mu-opioid receptors

Cholinergic interneurons in the nucleus accumbens shell (AcbSh) are implicated in the reinforcing behaviors that develop in response to opiates active at mu-opioid receptors (MOR). We examined the electron microscopic immunocytochemical localization of the vesicular acetylcholine transporter (VAChT) and MOR to determine the functional sites for storage and release of acetylcholine (ACh), and potential interactions involving MOR in this region of rat brain. VAChT was primarily localized to membranes of small synaptic vesicles in axon terminals. Less than 10% of the VAChT-labeled terminals were MOR-immunoreactive. In contrast, 35% of the cholinergic terminals formed symmetric or punctate synapses with dendrites showing an extrasynaptic plasmalemmal distribution of MOR. Membranes of tubulovesicles in other selective dendrites were also VAChT-labeled, and almost half of these dendrites displayed plasmalemmal MOR immunoreactivity. The VAChT-labeled dendritic tubulovesicles often apposed unlabeled axon terminals that formed symmetric synapses. Our results indicate that in the AcbSh MOR agonists can modulate the release of ACh from vesicular storage sites in axon terminals as well as in dendrites where the released ACh may serve an autoregulatory function involving inhibitory afferents. These results also suggest, however, that many of the dendrites of spiny projection neurons in the AcbSh are dually influenced by ACh and opiates active at MOR, thus providing a cellular substrate for ACh in the reinforcement of opiates.

Capsaicin-evoked release of immunoreactive calcitonin gene-related peptide from rat trigeminal ganglion: evidence for intraganglionic neurotransmission

Chemically-mediated cross-excitation has been described between neurons within sensory ganglia. However, the identity and source of the chemical mediators is not known. Ca(2+)-dependent release of neurotransmitters from cultured sensory neurons in vitro has been observed, although neurite outgrowth may confound the ability to extrapolate findings from culture systems to in vivo conditions. Thus, the present studies evaluate the hypothesis of capsaicin-sensitive intraganglionic neuropeptide release from freshly prepared slices of rat sensory ganglia. The ganglionic slice preparation provides an advantage over neuronal cultures, because release may be assessed within minutes after tissue collection (minimizing phenotypic changes) and while maintaining gross anatomical relationships. Trigeminal ganglia (TGG) were quickly removed from male, Sprague--Dawley rats (175--200 g), chopped into 200 microm slices and placed into chambers within 3 min of collection. Chambers were perfused with buffer, and superfusates were collected and assayed for immunoreactive calcitonin gene-related peptide (iCGRP) release via radioimmunoassay. After about 90 min of baseline collection, tissue was treated with capsaicin followed by a washout period. Capsaicin (1--100 microM) evoked concentration-dependent increases in iCGRP release. A competitive capsaicin receptor antagonist, capsazepine, significantly inhibited capsaicin-evoked release of iCGRP. In addition, capsaicin-evoked release of iCGRP was dependent on the presence of extracellular calcium. Furthermore, capsaicin-evoked release from TGG slices was significantly greater than that from slices of equivalent weights of adjacent trigeminal nerve shown histologically to be free of neuronal somata. These data support the hypothesis that Ca(2+)-dependent exocytosis of neuropeptides may occur within the TGG in vivo and that the majority of this release derives from neuronal somata.

Stimulant-induced exocytosis from neuronal somata, dendrites, and newly formed synaptic nerve terminals in chronically decentralized sympathetic ganglia of the rat

Loss of preganglionic neurones underlies the autonomic failure of human multiple system atrophy. In rat sympathetic ganglia decentralization leads to new synapse formation. We explored whether these synapses are functional, and whether chronically decentralized neurones respond normally to activation, in terms of exocytosis. Potassium depolarization and cholinergic agonists were applied to freshly excised rat superior cervical sympathetic ganglia, preganglionically denervated with prevented reinnervation 5 months earlier. Ganglia were incubated and stimulated in the presence of tannic acid, which stabilizes released vesicle cores for subsequent electron microscopy. In denervated ganglia exocytosis was observed from newly formed synaptic nerve terminals, and from nonsynaptic surfaces of neurone somata and dendrites. The results demonstrated that the new intraganglionic synapses, which are mostly catecholaminergic, can function and that chronically decentralized sympathetic neurones remain capable of stimulant-induced exocytosis from somata and dendrites. The maximal release upon potassium depolarization did not differ significantly between denervated and contralateral ganglia. Relative to this, the exocytotic responses of decentralized somata and dendrites to nicotine resembled those of contralateral ganglia. Responses to muscarine were significantly less in denervated than in contralateral ganglia, indicating inhibition in dendrites. Responses to carbachol suggested interactions between nicotinic and excitatory muscarinic effects. Nerve terminals in denervated ganglia showed high basal release. Their responses to muscarine and carbachol resembled those of the decentralized neurones, from which most may originate. Their response to nicotine evidenced inhibition. Their actions, coupled with nonsynaptic effects of soma-dendritic exocytosis, might modulate responses of the decentralized neurone population to other surviving inputs. This modulation could be influential in disease-induced decentralization in man.

Lesions of the C1 catecholaminergic neurons of the ventrolateral medulla in rats using anti-DbetaH-saporin

Phenylethanolamine-N-methyltransferase (PNMT)-containing neurons in the rostral ventrolateral medulla (RVLM) are believed to play a role in cardiovascular regulation. To determine whether injection of anti-dopamine beta-hydroxylase (DbetaH)-saporin directly into the RVLM in rats could selectively destroy these cells and thereby provide an approach for evaluating their role in cardiovascular regulation, we studied rats 2 wk after unilateral injection of 21 ng anti-DbetaH-saporin into the RVLM. There was an approximately 90% reduction in the number of PNMT-positive neurons in the RVLM, although the number of non-C1, spinally projecting barosensitive neurons of this area was not altered. The A5 cell group was the only other population of DbetaH-containing cells that was significantly depleted. The depressor response evoked by injection of tyramine into the RVLM was abolished by prior injection of toxin. The pressor response evoked by injection of glutamate into the RVLM was attenuated ipsilateral to the toxin injection but was potentiated contralateral to the toxin injection. Thus anti-DbetaH-saporin can be used to make selective lesions of PNMT-containing cells, allowing for the evaluation of their role in cardiovascular regulation.

Somatic and prejunctional nicotinic receptors in cultured rat sympathetic neurones show different agonist profiles

1. The release of [3H]-noradrenaline ([3H]-NA) in response to nicotinic acetylcholine receptor (nAChR) agonists was compared with agonist-induced currents in cultured rat superior cervical ganglion (SCG) neurones. 2. [3H]-NA release in response to high concentrations of nicotinic agonists was reduced, but not fully inhibited, by the presence of either tetrodotoxin (TTX) or Cd2+ to block voltage-gated Na+ or Ca2+ channels, respectively. We used the component of transmitter release that remained in the presence of these substances (named TTX- or Cd2+-insensitive release) to pharmacologically characterize nAChRs in proximity to the sites of vesicular exocytosis (prejunctional receptors). Prejunctional nAChRs were activated by nicotinic agonists with a rank order of potency of dimethylphenylpiperazinium iodide (DMPP) > nicotine > cytisine > ACh, and with EC50 values ranging from 22 microM (DMPP) to 110 microM (ACh). 3. [3H]-NA release in response to low concentrations of nAChR agonists was fully inhibited by the presence of either TTX or Cd2+ (named TTX- or Cd2+-sensitive release). TTX-sensitive release was triggered by nicotinic agonists with a rank order of potency of DMPP > cytisine approximately nicotine approximately ACh, which due to its similarity to TTX-insensitive release indicates that it might also be triggered by prejunctional-type nAChRs. The EC50 values for TTX (Cd2+)-sensitive release were less than 10 microM for all four agonists. 4. By contrast to transmitter release, somatic nAChRs as seen by patch clamp recordings were most potently activated by cytisine, with a rank order of potency of cytisine > nicotine approximately DMPP > ACh. EC50 values for the induction of currents exceeded 20 microM for all four agonists. 5. The nicotinic antagonist mecamylamine potently inhibited all transmitter release in response to nicotine. alpha-Bungarotoxin (alpha-BuTX) was, on the other hand, without significant effect on nicotine-induced TTX-insensitive release. The competitive antagonist dihydro-beta-erythroidine (DHbetaE) caused rightward shifts of the dose-response curves for both TTX-sensitive and TTX-insensitive transmitter release as well as for currents in response to nicotine, with pA2 values ranging from 4.03 to 4.58. 6. Due to clear differences in the pharmacology of agonists we propose that nAChRs of distinct subunit composition are differentially targeted to somatic or axonal domains.

Galanin in ascending systems. Focus on coexistence with 5-hydroxytryptamine and noradrenaline

Galanin can be synthesized in several ascending systems including cholinergic forebrain neurons, serotonergic dorsal raphe neurons, and the noradrenergic locus coeruleus system. Recent immunohistochemical studies suggest that of these three systems, the locus coeruleus neurons express the highest levels of galanin and that in cortex and hippocampus galanin peptide can only be detected in the noradrenergic projections. Electrophysiologic studies show that galanin hyperpolarizes both serotonergic dorsal raphe neurons and noradrenergic locus coeruleus neurons at fairly high concentrations (10(6)-10(-7) M). In addition, galanin at low concentrations (10(-9) M) enhances the 5-HT- and noradrenaline-induced hyperpolarization. Consequently, a galanin antagonist could attenuate an inhibitory tone on both dorsal raphe and locus coeruleus neurons and thus perhaps exert antidepressant activity.