Influence of long-term experimental orthostatic body position on innervation density in extremity vessels

The aim of this study was to quantitate the density of nerve terminals as well as their synaptic vesicle population in the adventitia of saphenous (SV and SA) and brachial veins and arteries (BV and BA) obtained from rats maintained in a horizontal control or a tilted position. Adult animals were kept individually in tube-like cages in a 45 degrees head-up position. After 2 wk, both tilted and control animals were anesthetized, and the whole body was perfused with fixative solution at physiological pressure. Vessels segments were then excised for electron microscopy and immunohistochemistry. The nerve terminal density (NTD) of SA was 8.20 +/- 1.46 nerve terminals/100 microm(2) cross section of adventitia and that of SV was 4.53 +/- 0.61 nerve terminals/100 microm(2) cross section of adventitia in control rats. Tilting caused a significant increase in NTD of both SA (70%) and SV (52%). The synaptic microvesicle density (SyVD) was larger in SA than SV in control rats (30.48 +/- 4.41 vs. 13.38 +/- 2.61 synaptic vesicles/10 terminal sections), but tilting resulted in more pronounced changes in SyVD of SV (95%) than SA (54%). No significant changes in NTD and SyVD of BA were found after tilt (-3.6% relative to 4.99 +/- 0.33 compared with 0.4% relative to 24.89 +/- 3.7, respectively). Whereas NTD of BV exhibited a tendency to increase (3.73 +/- 0.86 vs. 2.31 +/- 0.29 nerve terminals/100 microm(2) cross section of adventitia), SyVD did not change significantly (18.96 +/- 2.74 vs. 22.85 +/- 3.17 synaptic vesicles/10 terminal sections). A large number of nerve terminals of all vessels were tyrosine hydroxylase immunoreactive (containing norepinephrine). These findings support the hypothesis that long-term gravitational load causes adaptive morphological and functional remodeling of sympathetic innervation in blood vessels of the extremities.     

Occurrence of uranaffin-positive synaptic vesicles in both adrenergic and non-adrenergic nerves of the rat anococcygeus muscle

Summary The uranaffin reaction in rat anococcygeus muscle, which receives a dual innervation of both adrenergic and non-cholinergic, non-adrenergic nerves was examined. Dense reaction product was observed in the vesicular membranes and/or the cores of some synaptic vesicles in the adrenergic nerve terminals. Occasional vesicles were filled up with dense reaction product. In the prominent population of small clear vesicles, however, no dense reaction product was observed. The number of small granular vesicles in the adrenergic nerve terminals was markedly increased after the administration of 5-hydroxydopamine (5-OHDA). These granular vesicles were moderately stained with uranaffin deposit on the cores but their limiting membranes possessed no uranaffin deposit at all.In the non-adrenergic nerve terminals, on the other hand, uranaffin deposit of variable density was observed on the cores of large granular vesicles but never on their limiting membranes or on the small clear vesicles. There was no change in the axon profiles after the administration of 5-OHDA.The possible occurrence of purines in the cores of large granular vesicles in the non-adrenergic nerves is discussed.     

High-frequency stimuli preferentially release large dense-core vesicles located in the proximity of nonspecialized zones of the presynaptic membrane in sympathetic ganglia

We characterized the effect of a brief high-frequency stimulus on the number, distribution, and optical density of large dense-core vesicles (LDCVs) in the nerve terminals of the rat superior cervical ganglia. From 4.21+/-0.37 LDCVs/bouton detected in control nerve terminals, a stimulus of 40 Hz for 1 min released 41% of LDCVs, decreasing their number to 2.48+/-0.14 LDCVs/bouton (p=0.0009). In control ganglia, most dense vesicles were located close to the plasma membrane (at 相似文献   

The effects of niamid and reserpine on the nerve endings of the pineal gland

Summary Kitten pineal glands were studied cytochemically under normal conditions, after reserpine injection, and after niamid administration. Adrenergic nerve elements were in perivascular spaces while cholinergic terminals were adjacent to pinealocytes, often times in synaptic contact. BA reactions are primarily in dotted vesicles of adrenergic terminals with some reaction in granular vesicles. Positive reaction occurs along neurotubules and membranous structures of adrenergig nerve fibers and terminals indicating membrane-bounded BA's. Niamid increased the number and density of dotted vesicles, and some granular vesicles are increased in density and size. Reserpine produced a loss reaction in dotted vesicles and a loss of vesicle matrix, producing elliptical vesicles. There is loss of reaction of the dotted vesicles, but occasionally, the positive granular reaction remains. Cholinergic terminals demonstrate no changes with either niamid or reserpine. These findings indicate BAs are stored in reserpine sensitive dotted vesicles and membraneous structures. The findings also show that the dotted vesicle matrix is reserpine sensitive and is necessary for storage of the BA's. Possibly biogenic amines cannot be stored or synthesized in terminals unless the matrix of the dotted vesicle is intact.Supported by: HEW Grant No. NS-10326. The University of Texas Medical School at Houston. — Special appreciation to Mrs. Charlotte Smith for her valuable technical assistance. Appreciation to Ciba-Geigy Corporation for supply of Serpasil (reserpine).     

Immunocytochemical localization of TRH and autoradiographic determination of 3H-TRH-binding sites in the arcuate nucleus-median eminence of the rat

Immunocytochemical and autoradiographic localization of thyrotropin-releasing hormone (TRH)- and 3H-TRH-binding sites was studied in the arcuate nucleus-median eminence region of the rat. TRH-like immunoreactivity was found in dense granular vesicles (90-140 nm in diameter) in TRH-like immunoreactive nerve fibers and terminals. In the median eminence, the immunoreactive terminals were observed to be in direct contact with the perivascular basal lamina of the portal vessel and to form synaptoid contacts with tanycytes. In the arcuate nucleus, the immunoreactive terminals were often found to form axosomatic and axo-axonic, and/or axo-dendritic synapses. The uptake of tritiated TRH into the nerve fibers and terminals of the median eminence was also observed by autoradiography and the distribution and localization of silver grains in them were analyzed quantitatively by circle analysis. Thirty minutes after intraventricular infusion of 3H-TRH, radioactive labeling occurred in type-2 and 3-nerve fibers and terminals containing dense granular vesicles in the median eminence. It is therefore suggested that the neurons labeled after 3H-TRH infusion possess certain functions as physiological recognition sites or receptors for TRH.     

Histochemical and ultrastructural study on the innervation of the byssus glands of Mytilus galloprovincialis

The aluminium-formaldehyde (ALFA) histofluorescence method reveals an extensive plexus of brilliant greenish monoaminergic elements in the glandular zones of the Mytilus foot, while only scanty nerve fibres are acetylcholinesterase-positive. By electron microscopy, bundles of nerve fibres can be seen i) in close connection with the intrinsic musculature located in the connective septa among the glands, and ii) near the cell bodies and necks of all the byssus glands. The nerve fibres show varicosities containing three types of vesicles: small clear (50-60 nm), small granular (80-90 nm), and large granular (160-200 nm). The regions of close apposition between nerve terminals and muscle or gland cells generally do not show typical pre- or postsynaptic specializations. Along the pedal groove, mainly in the proximal two thirds of the foot, peripheral bipolar neurons can be detected, both by fluorescence and electron microscopy.     

Auerbach's plexus of mammals and man: Electron microscopic identification of three different types of neuronal processes in myenteric ganglia of the large intestine from rhesus monkeys,guinea-pigs and man

Summary Ganglia from Auerbach's plexus of the large intestine (caecum, appendix vermiformis, colon transversum and rectum) in man, rhesus monkey and guinea-pig are composed of nerve cells and their processes, typical Schwann cells and a vast neuropil. The neuropil consists of dendrites and axons of intrinsic nerve cell perikarya and axons of extrinsic neurons. Axonal profiles in large nerve fibre bundles are of uniform size and appearance, embedded in infoldings of Schwann cell cytoplasm and contain occasional large granular vesicles, mitochondria and neurotubules. Preterminal axons widen into vesicle filled varicosities, some of which establish synaptic contact with intrinsic nerve cell bodies.At least three different types of neuronal processes can be distinguished in the myenteric neuropil according to the size, appearance and commutual proportion of vesicles present in axonal varicosities, and their ability to accumulate exogenous 5- and 6-hydroxydopamine and 5-hydroxydopa: 1. Axonal enlargements containing a major population of small electron lucent synaptic vesicles (350–600 Å in diameter) together with a small number of membrane-bound, opaque granules (800–1,100 Å). These profiles have been identified as cholinergic axons. The boutons establish synaptic contacts with dendritic processes of intrinsic nerve cell bodies; membrane specializations are found at the preand postsynaptic sites. 2. Axonal beads of sometimes very large diameter, containing an approximately equal amount of large granular vesicles (850–1,600 Å) and small, electron lucent or faintly opaque vesicles (400–600 Å). The granular core of the large vesicles is of medium electron density and may either fill the entire vesicle or is separated from the limiting membrane by a more or less clear interspace. The fibres probably belong to intrinsic neurons, and because of the similarity of the large, membrane-bound vesicles with neurosecretory elementary granules, they have been designated p-type fibres (polypeptide fibres). The granular core of the vesicles in these fibres becomes more electron dense after treatment with 5-OH-dopa. The accumulation of an amine precursor analogue in combination with a possible storage of a polypeptide substance (or an ATP-like substance) resembles the situation in several diffusely distributed endocrine cell systems. 3. Varicosities of axons equipped with small (400–600 Å) empty or sometimes granular vesicles, medium sized (500–900 Å) vesicles with highly electron dense cores and occasional large (900–1,300 Å) granular vesicles. Pretreatment with 5-OH-dopamine increases the electron density in almost all medium-sized granular vesicles and some of the large granular vesicles; an osmiophilic core develops in some small vesicles. 6-hydroxydopamine results in degenerative changes in the varicosities of this type of neurons. Concomitantly, both catecholamine analogues markedly reduce neuronal noradrenaline in the large intestine, as demonstrated by fluorescence histochemistry and in fluorimetric determinations. The ultrastructural features of these varicosities and their reaction to 5- and 6-OH-dopamine indicate that they belong to adrenergic, sympathetic nerves. No membrane specializations could be detected at sites of close contact of the adrenergic boutons with dendrites and cell bodies of intrinsic nerve cells.Supported by grants from the Deutsche Forschungsgemeinschaft.Supported by a grant from Albert Pahlsson's Foundation, Sweden. The work was carried out within a research organization sponsored by the Swedish Medical Research Council (projects No. B70-14X-1007-05B, B70-14X-712-05, and B70-14X-56-06).     

THE DISTRIBUTION OF LABELED NOREPINEPHRINE WITHIN SYMPATHETIC NERVE TERMINALS STUDIED WITH ELECTRON MICROSCOPE RADIOAUTOGRAPHY

The distribution of radioactivity in association with sympathetic nerve terminals and intraneuronal organelles 30 min after the administration of tritiated norepinephrine (NE-³H) was studied by electron microscope radioautography with recently developed quantitative methods of analysis reported in the accompanying paper (Salpeter et al., 1969). Nerves from the pineal body and the adrenal capsule were examined. It was found that nerve terminals containing vesicles were heavily labeled. (These terminals were not necessarily in contact with some innervated structure.) There was no selective labeling of either the intraneuronal mitochondria or the relatively small population of large (~1000 A) dense core granules. Small vesicles (~500 A), some of which have a dense internal granule, could not be analysed separately because they are closely packed and occupy ~60% of the volume in terminals. Because of the extensive distribution of these small granular and agranular vesicles in the radioactive terminals, they remain the most likely site for norepinephrine binding. Yet although the vesicles were uniformly distributed within the nerve terminals, it appears that the radioactivity was not. There appeared to be a somewhat higher concentration of radioactivity at the periphery of the nerve terminals than in the center. The usefulness of the method of analysis used in this study for determining the location of bound H³NE pools in the nerve is discussed.     

Vasoactive intestinal polypeptide (VIP)- and neuropeptide Y (NPY)-containing nerve fibres in the penile cavernous tissue of green monkeys (Cercopithecus aethiops)

Summary The cavernous body of green monkeys contains many unmyelinated and few myelinated axons. The unmyelinated axons form terminals in the adventitia of the arteries, between trabecular muscle cells, in the interstitium, and close to endothelium cells of the sinuses. All terminals displayed predominantly small clear vesicles and very few large granular vesicles; small granular vesicles were not seen. However, in rabbit penises, terminals with many large granular vesicles are prominent. Immunohistochemistry (PAP technique) showed a dense network of VIP- and NPY-reactive fibres around the arteries and around trabecular muscles. The density of nerve fibres was particularly high around the subendothelial cushions of the helicine arteries. Double staining for NPY and VIP revealed that both peptides were colocalized. Immunocytochemistry (preembedding PAP technique) showed VIP- and NPY-reactivity in terminals with small clear vesicles; the reaction product was bound to the cytoplasmic face of different membrane types. Although the intracellular localization of the reaction product is probably due to artefactual displacement during preparation, the uniformity of the terminals questions the view that large and small granular vesicles in all species characterize peptidergic and noradrenergic terminals, respectively. The essential findings can be summarized as (1) a high degree of uniformity of nerve terminals, (2) colocalization of VIP and NPY, (3) heavy innervation of the subendothelial cushions of the helicine arteries, and (4) possible innervation of endothelial cells.     

Ultrastructural evidence of indirect and direct autonomic innervation of human Leydig cells: comparison of neonatal,childhood and pubertal ages

Summary Recent physiological studies have indicated an autonomic influence on the secretion of testosterone from Leydig cells in humans and laboratory animals. Furthermore, a few studies have shown enhanced autonomic control of Leydig cell function in immature, relative to mature, laboratory animals. In the current ultrastructural study of the human testicular interstitium the morphology of autonomic components is described from neonatal, childhood and pubertal ages. Autonomic nerve fibers and varicosities with neurotransmitter vesicles are described in proximity to Leydig cells. The observed autonomic terminals are classified by vesicle morphology into three general types: (1) Type I with predominately small agranular vesicles (30–60 nm) and occasional larger granular vesicles (100 nm). This type is morphologically consistent with being cholinergic. (2) Type II with predominately small granular vesicles (30–60 nm), as well as sporadic large granular vesicles. These are morphologically consistent with adrenergic terminals. (3) Type III which exhibit numerous large granular vesicles of mixed size. Evidence of autonomic terminals is encountered most frequently in childhood biopsies, age 3 to 10 years. The neonatal specimen (4 months) is noteworthy in that many of the Schwann cells appear immature and no adrenergic terminals are observed. In contrast, terminals morphologically consistent with being adrenergic are common in the childhood series of biopsies. Although the vast majority of the autonomic terminals are associated with Leydig cells indirectly as boutons en passant, separated by approximately 150 nm to more than a m, evidence of direct contact (20 nm) of autonomic terminals with Leydig cells is presented. These findings provide morphological evidence of frequent indirect and rare direct contact of autonomic nerve terminals with Leydig cells in man.     

On the intracerebral locus for androgenization of female rats.

Comparison of control and androgenized rats has revealed that many nerve terminals ending on arcuate nerve cells of rats treated neonatally with testosterone phenylpropionate show a decrease in dense-core vesicles, and an increase in the number of clear vesicles and of vesicles that exhibit various degrees of electron density. The same changes were observed in nerve terminals ending on preoptic neurons of rats androgenized neonatally. These findings are discussed in view of the intracerebral locus for androgenization of female rats.     

Ultrastructure of the sympathetic paravertebral ganglia in rat embryogenesis. I. Cytogenesis and cell differentiation

The development of sympathetic paravertebral ganglia was studied in rat embryos by electron microscopy. The main attention was paid to the initial stages of ganglion formation. The first aggregations of presumptive ganglionic cells were observed in 12 day-old embryos. Single preganglionic terminals appeared in contact with cell bodies sometime later. The appearance of large granular vesicles in the cytoplasm is the first ultrastructural feature of the beginning of neural differentiation of cells. Small granulated cells observed from the 12th day of gestation and neuroblasts differentiate earlier than glial cells. In the ganglia of late fetuses nerve cells varied in the electron density of the cytoplasm, in the degree of distention of rough endoplasmic reticulum and in vacuolization of mitochondria.     

Synaptology of three peptidergic neuron types in the central nucleus of the rat amygdala

The synaptology of neurotensin (NT)-, somatostatin (SS)- and vasoactive intestinal polypeptide (VIP)-immunoreactive neurons was studied in the central nucleus of the rat amygdala (CNA). Three types of axon terminals formed synaptic contacts with peptide-immunoreactive neurons in the CNA: Type A terminals containing many round or oval vesicles; Type B terminals containing many pleomorphic vesicles; and Type C terminals containing fewer, pleomorphic vesicles. Peptide-immunoreactive terminals were type A. All three types of terminals formed symmetrical axosomatic and asymmetrical axodendritic contacts. However, type B and peptide-immunoreactive terminals frequently formed symmetrical axodendritic synaptic contacts. VIP-immunoreactive terminals also formed asymmetrical axodendritic contacts. SS- and NT-immunoreactive terminals commonly formed symmetrical contacts on SS- and NT-immunoreactive cell bodies, respectively. VIP-immunoreactive axon terminals were postsynaptic to nonreactive terminals. Type B terminals appeared more frequently on VIP neurons than on NT or SS neurons.     

Innervation of the ureterovesical junction musculature of man

The ultrastructure of the innervation of the human ureterovesical junction was studied. Three different nerve terminals were distinguished among the smooth muscle cells. 1. Nerve processes containing predominantly small granular vesicles (40--60 nm in diameter). 2. Other nerve fibres contained predominantly small round agranular vesicles (30--50 nm in diameter). 3. Processes with large granulated vesicles (80--120 nm in diameter). The first type may be adrenergic, the second cholinergic and the third may originate from the local nerve cells. The gap between the nerve fibres and muscle cells was 300 to 500 nm wide and no synaptic thickenings were observed. This suggests that the transmitter may influence several muscle cells, and the different nerve fibres may directly innervate the smooth muscle cells.     

Fine structure of the autonomic nerves of the rabbit myometrium

Summary The fine structure of the preterminal nerve fibers of the rabbit myometrial smooth muscle was studied using potassium permanganate fixation or glutaraldehyde fixation with postosmification. The preterminal fibers were mostly formed by 2–10 axons enveloped by Schwann cells. Two kinds of axons and axon terminals were found. (1) Adrenergic axons, which contained many small, granular vesicles (diameter 300–600 Å) and large granular vesicles (diameter 700–1200 Å) which represented ca. 2% of the total count of the vesicles. (2) Nonadrenergic axons, which contained small agranular vesicles (diameter 300–600 Å) and large granular vesicles (diameter 700–1200 Å). Both types of axons formed preterminal varicosities along their course. The real terminal varicosities, representing the anatomical end of the axons, were usually larger than the preterminal ones and showed close contact to the plasma membranes of the smooth muscle cells. Both adrenergic and nonadrenergic terminals were found close to the smooth muscle cells, but a gap of at least 2000 Å was always present between the two cell membranes. The axons and preterminal varicosities of both types of nerves were in intimate contact with each other within the preterminal nerve fiber. Axo-axonal interactions between the two types of axons are possible in the rabbit myometrium. The relative proportion of the nonadrenergic axons from the total was about one fourth.     

Ultrastructural changes in the neurohypophysis of the aged male rat

Summary The neurohypophyses (neural lobes of the pituitary) of young adult (3–6 months) and aged (12–30 months) male rats were studied by means of electron microscopy. Stereomorphometric analyses were performed to determine the size, number and relative volume of the hormone-containing vesicles. The principal observations included: 1) the conspicuous depletion in aged rats of the granular hormone-containing vesicles from the axon terminals and the Herring bodies, with a decrease in the relative volume fraction of the vesicles from 4.8 Vv % in the control animals to 1.1 Vv % in the aged rats; 2) a change in the size-distribution of the hormone vesicles; 3) an increase in the extracellular space around the nerve terminals, axons and capillaries; and 4) lipid accumulations and signs of activation in the pituicytes. The possible physiological significance of the findings is discussed in the light of several regulatory functions known to be altered during the process of ageing.     

Nerve terminals and epithelial cell variety in the human lacrimal gland

Summary Pieces of tissue taken from three human lacrimal glands were examined electron microscopically. Secretory cells varied according to the electron density and structure of their secretion granules. Secretory cells were arbitrarily categorized as light, medium or dark based on their granule content. Acini were composed of two or all three categories of cells together with myoepithelial cells and lymphocytes.A minority of interstitial nerve fibres contained small granular vesicles characteristic of adrenergic, sympathetic terminals. The appearance of most interstitial and all parenchymal fibres was consistent with that of cholinergic, parasympathetic terminals. Parenchymal terminals were confined to ducts, terminal tubule areas and to serous (dark) cells. A large proportion of parenchymal terminals lay adjacent to myoepithelial cells in the ducts and terminal tubule regions but terminals observed among serous cells were rarely in contact with myoepithelial cells. A possible parasympathetic control of serous secretion, granule production and duct contraction and the autonomy of mucous cells is discussed.The author is indebted to Dr. J. Egeberg of the Anatomy Department B, University of Copenhagen for the provision of embedded material.     

Fine structure of islet-cell innervation in the pancreas of normal and alloxan-treated rats

Summary The innervation of the islets of Langerhans of normal albino rats and of albino rats treated with several daily doses of 125 mg/kg of alloxan was studied by electron microscopy. In the normal rat, nerve endings containing either agranular vesicles (200–400 Å) alone or in combination with large granular vesicles (500–800 Å) were found on both alpha and beta cells. Infrequently a third type of nerve ending containing small granular synaptic vesicles could be observed. Bundles of unmyelinated axons were also seen, as were typical autonomic ganglion cells. Similar normal neural elements were noted in rats treated with alloxan. However, islets of alloxan-treated animals also possess large elliptical profiles which appear to be dystrophic nerve terminals. These structures most frequently contact degranulated beta cells. Islets of Langerhans fixed with zinc iodide-osmium (ZIO) reported to specifically impregnate synaptic vesicles were also studied. Synaptic vesicles of normal axons and nerve endings as well as of the dystrophic structures were filled with ZIO reactive material. These studies suggest that alloxan may induce autonomic nerve ending changes in the rat endocrine pancreas. This may result from neuronal hyperactivity in an attempt to secrete insulin from the post-alloxan insulin-depleted beta cell.     

Localization of substance P and leucine enkephalin in the nerve terminals of the guinea pig paracervical ganglion

Summary Immunoreactivities (IR) of substance P and leucine enkephalin have been demonstrated in the guinea-pig paracervical ganglion by an immunogold electron microscope method. Both substance P-IR and leucine enkephalin-IR were detected in large synaptic vesicles with electron-dense cores. The former neuropeptide was detected in nerve terminals and varicosities comprised mainly of large vesicles with electron-dense cores; the latter was detected in nerve terminals and varicosities that also included small, clear synaptic vesicles. In a minority of nerve terminals and varicosities coexistence of both immunoreactivities could be demonstrated within vesicles with an electron-dense core. Also present in these nerve terminals and varicosities were small, clear synaptic vesicles, though these were unreactive.     

Presynaptic effects of anandamide and WIN55,212-2 on glutamatergic nerve endings isolated from rat hippocampus

We examined the effects of the endocannabinoide-anandamide (AEA), the synthetic cannabinoid, WIN55,212-2, and the active phorbol ester, 4-beta-phorbol 12-myristate 13-acetate (4-beta-PMA), on the release of [(3)H]d-Aspartate ([(3)H]d-ASP) from rat hippocampal synaptosomes. Release was evoked with three different stimuli: (1) KCl-induced membrane depolarization, which activates voltage-dependent Ca(2+) channels and causes limited neurotransmitter exocytosis, presumably from ready-releasable vesicles docked in the active zone; (2) exposure to the Ca(2+) ionophore-A23187, which causes more extensive transmitter release, presumably from intracellular reserve vesicles; and (3) K(+) channel blockade by 4-aminopyridine (4-AP), which generates repetitive depolarization that stimulates release from both ready-releasable and reserve vesicles. AEA produced concentration-dependent inhibition of [(3)H]d-ASP release stimulated with 15 mM KCl (E(max)=47.4+/-2.8; EC(50)=0.8 microM) but potentiated the release induced by 4-AP (1mM) (+22.0+/-1.3% at 1 microM) and by A23187 (1 microM) (+98.0+/-5.9% at 1 microM). AEA's enhancement of the [(3)H]d-ASP release induced by the Ca(2+) ionophore was mimicked by 4-beta-PMA, which is known to activate protein kinase C (PKC), and the increases produced by both compounds were completely reversed by synaptosome treatment with staurosporine (1 microM), a potent PKC blocker. In contrast, WIN55,212-2 inhibited the release of [(3)H]d-ASP evoked by KCl (E(max)=47.1+/-2.8; EC(50)=0.9 microM) and that produced by 4-AP (-26.0+/-1.5% at 1 microM) and had no significant effect of the release induced by Ca(2+) ionophore treatment. AEA thus appears to exert a dual effect on hippocampal glutamatergic nerve terminals. It inhibits release from ready-releasable vesicles and potentiates the release observed during high-frequency stimulation, which also involves the reserve vesicles. The latter effect is mediated by PKC. These findings reveal novel effects of AEA on glutamatergic nerve terminals and demonstrate that the effects of endogenous and synthetic cannabinoids are not always identical.