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PubMed1986

Mechanism of long-term potentiation of transmitter release induced by adrenaline in bullfrog sympathetic ganglia.

A mechanism of the long-term potentiation of transmitter release induced by adrenaline (ALTP) was studied by recording intracellularly the fast excitatory postsynaptic potentials (fast EPSPs). The ALTP was produced during the blockade of K+ channels at the presynaptic terminals by tetraethylammonium (TEA). The synaptic delay, possibly reflecting a relative change in the duration of an action potential at the presynaptic terminal, was not changed during the course of the ALTP. By contrast, it was significantly lengthened by TEA and other K+ channel inhibitors (4-aminopyridine and Cs+) that markedly enhanced the evoked release of transmitter. The magnitude of facilitation of the fast EPSP, induced by a conditional stimulus to the preganglionic nerve, was decreased during the generation of the ALTP, but was unchanged during the potentiation of transmitter release caused by TEA. These results, together with theoretical considerations applying the residual Ca2+ hypothesis to the facilitation, suggest that the enhancement of transmitter release during the ALTP is not caused by an increased Ca2+ influx during a presynaptic impulse owing to the blockade of K+ channel or the modulation of Ca2+ channel, but presumably is induced by a rise in the basal level of free Ca2+ in the presynaptic terminal.

Animals↗
PubMed1982

Ca2+ and calmodulin-regulated endogenous tubulin kinase activity in presynaptic nerve terminal preparations.

Synaptosomal tubulin was shown to be the major substrate for a Ca2+-calmodulin regulated protein kinase in synaptosome soluble fractions as determined by two-dimensional gel electrophoresis and peptide mapping. Ca2+ activated this endogenous tubulin kinase system in presynaptic nerve terminal preparations. The Ca2+-dependent activation of the tubulin kinase system was mediated by the Ca2+ binding protein, calmodulin. Trifluoperazine, a known inhibitor of calmodulin, significantly blocked the calmodulin-stimulated [32P]phosphate incorporation into synaptic tubulin. This inhibition of endogenous tubulin phosphorylation could be reversed by addition of exogenous calmodulin to the reaction mixture. The concentrations of Ca2+ and calmodulin required to produce a half-maximal stimulation of the tubulin kinase were 0.8 microM and 0.3 microM respectively. Greater than 70% of soluble tubulin present in the nerve terminal was phosphorylated in less than 50 s by this kinase system. Evidence is presented indicating that the synaptic Ca2+-calmodulin tubulin kinase is a distinct enzyme system from the previously described cyclic AMP microtubule-associated kinase. The anticonvulsant phenytoin inhibited the Ca2+-calmodulin stimulated phosphorylation of tubulin, and alpha- and beta-tubulin were identified as major components of previously designated synaptic phosphoprotein bands of DPH-L and DPH-M. Existence of the kinase as a calmodulin-tubulin-kinase complex is suggested from kinetic studies. The Ca2+-calmodulin tubulin kinase is very labile and specialized isolation procedures were necessary to retain activity. The activation of the tubulin kinase by Ca2+ and calmodulin may play a role in the functional utilization of tubulin in the nerve terminal and may mediate some of the effects of Ca2+ on synaptic function.

Animals↗
PubMed1993

Spatial distribution of excitatory and inhibitory synapses on a Purkinje cell in a rat cerebellar culture.

1. The spatial distribution of excitatory and inhibitory synapses on cultured Purkinje cells was studied with fluorescence, scanning electron microscopy (SEM), and electrophysiological techniques. 2. Presynaptic terminals were identified with immunohistochemical staining of synaptophysin and the results were correlated with SEM micrographs. 3. Excitatory and inhibitory inputs onto the Purkinje cell were identified from the direction and pharmacology of the postsynaptic current. 4. The localization of the presynaptic terminals on the Purkinje cell was observed after electrophysiological identification by filling the presynaptic neuron with Lucifer yellow and the Purkinje cell with Texas red. 5. The axon and presynaptic terminals of excitatory and inhibitory inputs had a different spatial organization. Excitatory inputs from granule cells were exclusively localized on the dendrites of Purkinje cells, whereas inhibitory contacts were found on both the soma and dendrites. This result is similar to that described in vivo.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1971

Morphological and biochemical changes in rat synaptosome fractions during neonatal development.

A biochemical and quantitative morphologic study of presynaptic endings during postnatal development was carried out in subcellular fractions from cerebral cortex of 1, 4, 8, 12, and 18 day old and adult rats. Crude mitochondrial fractions were subfractionated in Ficoll gradients and all resulting fractions were examined in the electron microscope. Presynaptic terminals and other intact processes were counted. Protein content and enzyme activities were assayed in the fractions and in total brain homogenate. In the first and fourth day of life, most of the presynaptic terminals were found in two "light" fractions, between supernatant and 7.5% Ficoll, where they accounted, respectively, for 6 and 22% of all the processes. Progressively with age, more presynaptic terminals were found in the traditional "synaptosomal" fractions between 7.5 and 13% Ficoll. In that region of the gradient, 40, 54, 75, and 89% of the processes were presynaptic endings at 8, 12, and 18 postnatal days and in the adult animal, respectively. A similar shift from the lighter to the heavier fractions was observed in the distribution of choline acetyltransferase and acetylcholinesterase between days 8 and 12. The rate of increase of the specific activity of these two enzymes paralleled that of the percentage of the presynaptic endings after day 8. This study indicates that subcellular fractions can be used to study formation and maturation of synapses during postnatal development.

Acetylcholine↗
PubMed1989

Ultrastructural analysis of the development and maturation of synapses and subsynaptic structures in the ectostriatum of the zebra finch.

The development of synapses and subsynaptic features in the neuropil of the ectostriatum, a visual projection area in birds, was examined ultrastructurally at 5, 10, 20, and 100 days posthatching. The maturation of the synaptic complex is accompanied by a variety of different dynamic processes. The number of synapses in ectostriatum and the number of specific synaptic types vary with age as does the constellation of subsynaptic structures. At day 5, before eye opening, the total number of synapses is 16% of the adult value. These synapses, unlike synapses seen at maturity, have indistinct synaptic contact zones and generally are associated with few synaptic vesicles. Synapse number increases continuously until 20 days of age, paralleled by a steady increase of the observed brain volume. The largest increase in synapse number takes place during the time of eye opening (i.e., between 5 and 10 days). This increase is mainly due to an increase of asymmetric synapses, the most common type in the neuropil of ectostriatum (90% of the synapse population). At day 20 the number of synapses has reached its maximum and remains high in adulthood. Synapses on spines are more prominent in younger animals than in adults. The percentage of presynaptic terminals involved in synaptic contact with more than one postsynaptic element (multiple synapses) shows a significant reduction from 12% to 4% early in development (between days 10 and 20). Presynaptic terminal size and postsynaptic density (PSD) length increase until 20 days of age. From day 20 to adulthood the PSD shows a 10% reduction in contact length, and the presynaptic terminal further increases in size by 27%. Therefore, the pre- and postsynaptic structures described above continue to develop after the number of synapses remains constant.

Animals↗
PubMed2002

The role of neurotrophins in neurotransmitter release.

The neurotrophins (NTs) have recently been shown to elicit pronounced effects on quantal neurotransmitter release at both central and peripheral nervous system synapses. Due to their activity-dependent release, as well as the subcellular localization of both protein and receptor, NTs are ideally suited to modify the strength of neuronal connections by "fine-tuning" synaptic activity through direct actions at presynaptic terminals. Here, using BDNF as a prototypical example, the authors provide an update of recent evidence demonstrating that NTs enhance quantal neurotransmitter release at synapses through presynaptic mechanisms. The authors further propose that a potential target for NT actions at presynaptic terminals is the mechanism by which terminals retrieve synaptic vesicles after exocytosis. Depending on the temporal demands placed on synapses during high-frequency synaptic transmission, synapses may use two alternative modes of synaptic vesicle retrieval, the conventional slow endosomal recycling or a faster rapid retrieval at the active zone, referred to as "kiss-and-run." By modulating Ca2+ microdomains associated with voltage-gated Ca2+ channels at active zones, NTs may elicit a switch from the slow to the fast mode of endocytosis of vesicles at presynaptic terminals during high-frequency synaptic transmission, allowing more reliable information transfer and neuronal signaling in the central nervous system.

Animals↗
PubMed1993

Residual free calcium is not responsible for facilitation of neurotransmitter release.

An increase in internal free calcium ([Ca2+]i) in the presynaptic terminal is often assumed to directly produce facilitation of neurotransmitter release. Using a Ca(2+)-activated potassium conductance as a bioassay for free [Ca2+]i in the presynaptic terminal of the crayfish (Procambarus clarkii) opener neuromuscular junction, we now demonstrate that free [Ca2+]i has a decay time constant (tau) of 1-4 msec, whereas facilitation of neurotransmitter release has a decay tau of 7-43 msec. In addition, facilitation of neurotransmitter release can be markedly different at times when free [Ca2+]i values and presynaptic membrane voltages are equal. We conclude that free [Ca2+]i in the presynaptic terminal is not directly responsible for facilitation of neurotransmitter release. Our data suggest that facilitation results from bound Ca2+ or some long-lived consequence of bound Ca2+.

Animals↗
PubMed1971

Electron microscope histochemical evidence for a partial or total block of the tricarboxylic acid cycle in the mitochondria of presynaptic axon terminals.

Respiration-linked, massive accumulation of Sr(2+) is used to reveal the coupled oxidation of pyruvate, alpha-oxoglutarate, succinate, and malate by in situ mitochondria. All of these substrates were actively oxidized in the dendritic and perikaryal mitochondria, but no alpha-oxoglutarate or succinate utilization could be demonstrated in the mitochondria of the presynaptic axon terminals. A block at an early step of alpha-oxoglutarate and succinate oxidation is proposed to account for the negative histochemical results, since the positive reaction with pyruvate and malate proves that these mitochondria possess an intact respiratory chain and energy-coupling mechanism essential for Sr(2+) accumulation. This indicates that the mitochondria in the axon terminals would be able to generate energy for synaptic function with at least some of the respiratory substrates. With regard to the block in the tricarboxylic acid cycle, the oxaloacetate necessary for citrate formation is suggested to be provided by fixation of CO(2) into some of the pyruvate.

Axons↗
PubMed1990

Mechanisms underlying presynaptic inhibition through alpha 2-adrenoceptors in guinea-pig submucosal neurones.

1. Intracellular recordings were made from submucosal neurones of the guinea-pig ileum. The actions of noradrenaline, somatostatin and [Met5]enkephalin on nicotinic synaptic potentials (EPSPs) were studied. 2. In one series of experiments, agonists were applied by superfusion; noradrenaline (0.1-20 microM) decreased EPSP amplitude by 95-100% in all neurones. Similar application of somatostatin (1-100 nM) inhibited EPSPs in about half the neurones by a maximum of 40%. [Met5]enkephalin (0.1-10 microM) did not alter EPSPs. Idazoxan and yohimbine competitively antagonized the action of noradrenaline with dissociation equilibrium constants of 20 and 30 nM respectively. 3. In another series of experiments, noradrenaline and somatostatin were applied locally from a pipette so that they reached presynaptic terminals but not the cell bodies or axons of the presynaptic cell: noradrenaline inhibited EPSPs by 90% in all neurones but somatostatin had no effect. When applied locally to the cell bodies giving rise to the presynaptic fibres, both agonists inhibited EPSPs in half the neurones by 40%. 4. When noradrenaline was applied locally to presynaptic terminals, the latency to onset of noradrenaline to inhibit EPSPs was 45-160 ms; cadmium applied similarly depressed EPSPs in 5-50 ms. 5. Pertussis toxin pre-treatment only partially blocked presynaptic inhibition caused by noradrenaline but abolished the reduction of EPSP amplitude by somatostatin. 6. It is concluded that noradrenaline and somatostatin reduce the amplitude of the fast EPSP because they hyperpolarize cell bodies and prevent action potential initiation. Noradrenaline, but not somatostatin, has an additional action to inhibit acetylcholine release by acting at nerve terminal receptors. 7. The presynaptic inhibitory action of noradrenaline results from activation of alpha 2-adrenoceptors at nerve terminals but the mechanism(s) by which these presynaptic receptors act cannot be explained adequately by either activation of a potassium conductance and/or inhibition of a calcium conductance.

Action Potentials↗
PubMed2005

Nectin-like molecule-1/TSLL1/SynCAM3: a neural tissue-specific immunoglobulin-like cell-cell adhesion molecule localizing at non-junctional contact sites of presynaptic nerve terminals, axons and glia cell processes.

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules and comprise a family of four members. At the mossy fiber terminals of hippocampus, nectin-1 and nectin-3 localize at the presynaptic and postsynaptic sides of synaptic junctions, respectively, and their trans-interactions play a role in formation of synapses in cooperation with N-cadherin. Nectins are associated with the actin cytoskeleton through afadin, a nectin- and actin-filament-binding protein. Five nectin-like molecules (Necls) which have domain structures similar to those of nectins have been identified and here we characterize Necl-1/TSLL1/SynCAM3, from now on referred to as Necl-1. Tissue distribution analysis showed that Necl-1 was specifically expressed in the neural tissue. Immunofluorescence and immunoelectron microscopy revealed that Necl-1 localized at the contact sites among axons, their terminals, and glia cell processes that cooperatively formed synapses, axon bundles and myelinated axons. Necl-1 showed Ca2+-independent homophilic cell-cell adhesion activity. It furthermore showed Ca2+-independent heterophilic cell-cell adhesion activity with Necl-2/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1 from now on referred to as Necl-2, nectin-1 and nectin-3, but not with Necl-5 or nectin-2. The C-terminal cytoplasmic region of Necl-1 did not bind afadin but bound membrane-associated guanylate kinase subfamily members that contain the L27 domain, including Dlg3, Pals2 and CASK. These results indicate that Necl-1 is a neural-tissue-specific Ca2+-independent immunoglobulin-like cell-cell adhesion molecule which potentially has membrane-associated guanylate kinase subfamily member-binding activity and localizes at the non-junctional cell-cell contact sites.

Actins↗
PubMed1990

Calcium released by photolysis of DM-nitrophen stimulates transmitter release at squid giant synapse.

1. Transmitter release at the squid giant synapse was stimulated by photolytic release of Ca2+ from the 'caged' Ca2+ compound DM-nitrophen (Kaplan & Ellis-Davies, 1988) inserted into presynaptic terminals. 2. Competing binding reactions cause the amount of Ca2+ released by DM-nitrophen photolysis to depend on the concentrations of DM-nitrophen, total Ca2+, Mg+, ATP and native cytoplasmic Ca2+ buffer. Measurements of presynaptic [Ca2+] changes by co-injection of the fluorescent indicator dye Fura-2 show that DM-nitrophen photolysis causes a transient rise in Ca2+ followed by decay within about 150 ms to an increased steady-state level. 3. Rapid photolysis of Ca2(+)-loaded nitrophen within the presynaptic terminal was followed in less than a millisecond by depolarization of the postsynaptic membrane. As with action potential-evoked excitatory postsynaptic potentials (EPSPs), the light-evoked response was partially and reversibly blocked by 1-3 mM-kainic acid which desensitizes postsynaptic glutamate receptors. 4. Release was similar in magnitude and rate to normal action potential-mediated EPSPs. 5. The release of transmitter by photolysis of Ca2(+)-loaded DM-nitrophen was not affected by removal of Ca2+ from the saline or addition of tetrodotoxin. Photolysis of DM-nitrophen injected into presynaptic terminals without added Ca2+ did not stimulate release of transmitter nor did it interfere with normal action potential-mediated release. 6. Stimulation of presynaptic action potentials in Ca2(+)-free saline during the light-evoked response did not elicit increased release of transmitter if the ganglion was bathed in Ca2(+)-free saline, i.e. in the absence of Ca2+ influx. Increasing the intensity of the light or stimulating presynaptic action potentials in Ca2(+)-containing saline increased the release of transmitter. Therefore the failure of presynaptic voltage change to increase transmitter release resulting from release of caged Ca2+ was not due to saturation or inhibition of the release mechanism by light-released Ca2+. 7. Decreasing the temperature of the preparation increased the delay to onset of the light-evoked response and reduced its amplitude and rate of rise to an extent similar to that observed for action potential-evoked EPSPs.

Acetates↗
PubMed1975

The site of anoxic block in the spinal monosynaptic pathway.

Asphyxiation of the spinal cord for periods of 2-4 min leads to block of the monosynaptic pathway. At about the same time this blockage takes place, the afferent action potentials fail to invade the presynaptic terminals. Asphyxiation also interferes with the antidromic invasion of motoneurons, and the failure of the antidromic action potentials to invade the motoneuron dendrites coincides with the time of the disappearance of the orthodromic monosynaptic responses. During reoxygenation, both the presynaptic terminals and the dendrites recover their function, or rather their polarization, in a few seconds and yet synaptic transmission reappears only after several minutes. It is postulated that failure of synaptic transmission during asphyxia is due to depolarization of both the presynaptic terminals and the dendrites of the postsynaptic elements. However, repolarization of these elements during reoxygenation, is not sufficient to reestablish synaptic transmission, but recovery of some unidentified biochemical process is apparently necessary.

Animals↗
PubMed1994

Effects of peripheral axotomy on presynaptic axon terminals with GABA-like immunoreactivity.

The facial nerve was unilaterally crushed at its exit from the stylomastoid foramen in three 3-month old male rats. After 10 days survival, before the regenerating axons had reinnervated their target muscles, the facial nucleus was examined to determine central patterns of response in material prepared to demonstrate the presence of GABA-like immunoreactivity with postembedding procedures using gold-labeled secondary antibody. The uninjured nucleus served as a control. In both control and injured nuclei, the GABAergic terminals synapse with all parts of the motor neurons, except the axon, and exhibit diverse morphologies. GABAergic axon terminals vary in their size and in the electron density of their axoplasm and the majority of the terminals contain pleomorphic vesicle profiles that display a range in their packing density and size. In both control and injured facial nuclei, only approximately 40% of the axon terminal profiles with pleomorphic vesicles exhibit GABA immunoreactivity. A morphometric analysis of the synaptic vesicle profiles in the GABA-positive terminals reveals that following axotomy there is no change in the mean number of synaptic vesicle profiles per GABAergic terminal profile. However, the mean size of the synaptic vesicle profiles in these terminals shows an axotomy-induced 50% increase, without change in the shapes of the enlarged vesicle profiles. Also, the numerical density of gold particles associated with the GABA-positive terminals is consistently greater in the injured than the control axon terminals. In the control animals quantitative analysis of the relative distribution of all axon terminal profiles in the neuropil categorized by the shape of their vesicle profiles as round, pleomorphic, or flat is 57:37:6. Ten days after axotomy the ratio of these categories in the injured nucleus has shifted to 35:60:5. This study demonstrates that the functional state of a postsynaptic target can influence the morphology of vesicle profiles in presynaptic elements as well as patterns of its afferent input.

Animals↗
PubMed1999

A dynamically regulated 14-3-3, Slob, and Slowpoke potassium channel complex in Drosophila presynaptic nerve terminals.

Slob is a novel protein that binds to the carboxy-terminal domain of the Drosophila Slowpoke (dSlo) calcium-dependent potassium (K(Ca)) channel. A yeast two-hybrid screen with Slob as bait identifies the zeta isoform of 14-3-3 as a Slob-binding protein. Coimmunoprecipitation experiments from Drosophila heads and transfected cells confirm that 14-3-3 interacts with dSlo via Slob. All three proteins are colocalized presynaptically at Drosophila neuromuscular junctions. Two serine residues in Slob are required for 14-3-3 binding, and the binding is dynamically regulated in Drosophila by calcium/calmodulin-dependent kinase II (CaMKII) phosphorylation. 14-3-3 coexpression dramatically alters dSlo channel properties when wild-type Slob is present but not when a double serine mutant Slob that is incapable of binding 14-3-3 is present. The results provide evidence for a dSlo/Slob/14-3-3 regulatory protein complex.

14-3-3 Proteins↗
PubMedDate not supplied

Localized Ca2+ and calcium-activated potassium conductances in terminals of a barnacle photoreceptor.

Calcium channels are found in the presynaptic terminals of neurones, where they have a key role in synaptic transmission. They are also found in the somata of many cells, in dendrites and along a few axons. In no cell is the actual distribution of these channels known in detail, because there are no known toxins or other agents suitable for labelling calcium channels, and the current through these channels is usually too small to be quantified with extracellular electrodes. However, several experiments have suggested that the density of the channels is less in the axon than in the cell body or terminal region. Here we have used the indicator dye Arsenazo III in conjunction with an array of photodetectors to examine the spatial influx of calcium in the presynaptic terminal region of the giant barnacle, Balanus nubilus. In these cells, calcium entry occurs in a restricted region less than 50 micron in length, which corresponds closely to the region of synaptic contact with second-order cells. Outside this area the magnitude of calcium entry is reduced at least 50-fold. With reasonable assumptions it follows that the calcium channel density is equally localized. In addition, we demonstrate that these cells have a calcium-activated potassium conductance. Since calcium entry is restricted to the synaptic zone, this conductance must be effective only in this region.

Action Potentials↗
PubMed1987

Chronic ethanol consumption affects filipin-cholesterol complexes and intramembranous particles of synaptosomes of rat brain cortex.

To assess the effect of ethanol on the planar distribution of cholesterol as well as on the surface architecture of presynaptic terminals of rats, synaptosomes isolated from cerebral cortex of rats chronically exposed to alcohol were incubated with filipin, a cytochemical marker for beta-hydroxycholesterol, and analyzed using both conventional (qualitative and quantitative) and freeze-fracture electron microscopy. Synaptosomes incubated in the absence of filipin were used as cytochemical controls. Biochemical determination indicates a 12% increase of cholesterol in synaptosomal membranes from alcohol treated rats. This increase was confirmed by a significant increment in the number of filipin-cholesterol complexes. Synaptosomes of treated rats showed a reduction in the total number of synaptic vesicles (SV) as well as a decrease in the density and total number of intramembranous particles (IMP) per synaptosome. In control rats, most synaptosomal IMP were distributed in clusters whereas in those of rats exposed to alcohol they were distributed at random. These changes in distribution of IMP were also observed in presynaptic terminals analyzed "in situ." These findings indicate that ethanol acts on the presynaptic terminals. The variations in cholesterol content as well as in the density and distribution of IMP appear to be related to alcohol-induced changes in the physicochemical properties of components of the synaptosomal membrane.

Alcoholism↗
PubMed1999

Cellular and subcellular localization of a newly identified member of the protein 4.1 family, brain 4.1, in the cerebellum of adult and postnatally developing rats.

For obtaining a deeper insight into the properties of a newly characterized member of the protein 4.1 family, brain 4.1, the cellular and subcellular localization was investigated in the cerebellar cortex of adult and postnatally developing rats. Fluorescent immunohistochemical observations showed that brain 4.1 localized predominantly to glomeruli in the granular layer and throughout the molecular layer in adult rat cerebellar cortex. Analysis of subcellular localization of brain 4.1 by immuno-electron microscopy further demonstrated that presynaptic terminals of mossy fibers and parallel fibers, cytoplasm of granule cells and cytoplasm and/or processes of glial cells contained brain 4.1 while postsynaptic regions of the dendrites of granule cells and Purkinje cells, axons and myelin sheaths did not. Thus, one of the major subcellular destination of brain 4.1 was presynaptic terminal in the cerebellum. This was further supported by the fact that the immunostaining pattern of brain 4.1 in the cerebellum changed in a similar way to that of a synaptic terminal marker, synaptophysin during the postnatal development. Immunoblot analysis also demonstrated that contents of brain 4.1 isoforms varied in parallel with the changes of the immunostaining pattern. Biochemical analysis confirmed the presence of brain 4.1 at synaptic terminals, but there was no obvious correlation between each isoform and its subcellular localization. These results suggested that brain 4.1 is involved in the formation and maintenance of synapse as a membrane skeletal component at presynaptic terminals in the cerebellum.

Aging↗
PubMed1998

alpha2A-adrenergic receptors in the rat nucleus locus coeruleus: subcellular localization in catecholaminergic dendrites, astrocytes, and presynaptic axon terminals.

To define the anatomic substrates subserving the inhibitory actions of alpha2-adrenergic receptors (alpha2-ARs) in the locus coeruleus (LC), we used dual-label immunoelectron microscopy with antibodies directed against the A-subtype of alpha2-AR (alpha2A-AR) and the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Of the profiles containing peroxidase labeling for alpha2A-AR (alpha2A-AR-IR) in the LC (n=735), most were dendrites ( approximately 50%), glial processes ( approximately 30%), and axon terminals ( approximately 15%). alpha2A-AR-IR was also observed in unmyelinated axons and perikarya. Within dendrites, alpha2A-AR-IR was associated with nonsynaptic regions of the plasma membrane and subsurface cisternae. Approximately 60% of dendrites with alpha2A-AR-IR were dually labeled for TH. Fifty percent of the axon terminals contacting alpha2A-AR-immunoreactive dendrites formed asymmetric (excitatory) synaptic contacts. Axon terminals with alpha2A-AR-IR were not dually labeled for TH and generally formed asymmetric synapses with TH-immunoreactive dendrites that contained or lacked alpha2A-AR-IR. Astrocytic processes exhibiting alpha2A-AR-IR were closely apposed to TH-labeled dendrites. These results extend previous ultrastructural observations of alpha2A-ARs in the LC and suggest that the inhibitory actions of norepinephrine and epinephrine in this region may be mediated by postsynaptic alpha2A-ARs on catecholaminergic dendrites and presynaptic alpha2A-ARs on excitatory inputs to catecholaminergic dendrites. In addition, the localization of alpha2A-AR-IR in astrocytic processes apposed to TH-immunoreactive dendrites suggests a role for alpha2A-ARs in functional interactions between catecholaminergic dendrites and neighboring astrocytes.

Adrenergic alpha-Agonists↗