The effect of monoamine depleting drugs upon the synaptic bars in the inner ear of the bullfrog (Rana catesbeiana)

Passive diffusion as a mechanism underlying ribbon synapse vesicle release and resupply

Synaptic ribbons are presynaptic protein structures found at many synapses that convey graded, "analog" sensory signals in the visual, auditory, and vestibular pathways. Ribbons, typically anchored to the presynaptic membrane and surrounded by tethered synaptic vesicles, are thought to regulate or facilitate vesicle delivery to the presynaptic membrane. No direct evidence exists, however, to indicate how vesicles interact with the ribbon or, once attached, move along the ribbon's surface to reach the presynaptic release sites at its base. To address these questions, we have created, validated, and tested a passive vesicle diffusion model of retinal rod bipolar cell ribbon synapses. We used axial (bright-field) electron tomography in the scanning transmission electron microscopy to obtain 3D structures of rat rod bipolar cell terminals in 1-μm-thick sections of retinal tissue at an isotropic spatial resolution of ∼3 nm. The resulting structures were then incorporated with previously published estimates of vesicle diffusion dynamics into numerical simulations that accurately reproduced electrophysiologically measured vesicle release/replenishment rates and vesicle pool sizes. The simulations suggest that, under physiologically realistic conditions, diffusion of vesicles crowded on the ribbon surface gives rise to a flow field that enhances delivery of vesicles to the presynaptic membrane without requiring an active transport mechanism. Numerical simulations of ribbon-vesicle interactions predict that transient binding and unbinding of multiple tethers to each synaptic vesicle may achieve sufficiently tight association of vesicles to the ribbon while permitting the fast diffusion along the ribbon that is required to sustain high release rates.

Plasticity of retinal ribbon synapses

Ribbon synapses differ from conventional chemical synapses in that they contain, within the cloud of synaptic vesicles (SV's), a specialized synaptic body, most often termed synaptic ribbon (SR). This body assumes various forms. Reconstructions reveal that what appear as rod- or ribbon-like profiles in sections are in fact rectangular or horseshoe-shaped plates. Moreover, spherical, T-shaped, table-shaped, and highly pleomorphic bodies may be present. In mammals, ribbon synapses are present in afferent synapses of photoreceptors, bipolar nerve cells, and hair cells of both the organ of Corti and the vestibular organ. Synaptic ribbons (SR's) are also found in the intrinsic cells of the third eye, the pineal gland, and in the lateral line system. The precise function of SR's is enigmatic. The prevailing concept is that SR's function as conveyor belts to channel SV's to the presynaptic membrane for neurotransmitter release by means of exocytosis. The present article reviews the evidence that speaks for a plasticity of these organelles in the retina and the third eye, as reflected in changes in number, size, shape, location, and grouping pattern. SR plasticity is especially pronounced in the mammalian and submammalian pineal gland and in cones and bipolar cells of teleost fishes. Here, SR number and size wax and wane according to the environmental lighting conditions. In the pineal SR numbers increase at night and decrease during the day. In teleost cones, SR's are in their prime during daytime and decrease or disappear at night, when transmitter release is enhanced. In addition to numerical changes, SR's may also exhibit changes in size, shape, grouping pattern, and location. In the mammalian retina of adults, in contrast to the developing retina, the reported signs of SR plasticity are subtle and not always consistent. They may reflect changes in function or may represent signs of degradation. To distinguish between the-two, more detailed studies under selected experimental conditions are required. Probably the strongest evidence for SR plasticity in the mammalian retina is that in hibernating squirrels SR's leave the synaptic site and accumulate in areas as far as 5 microns from the synapse. Changes in shape include the occurrence of club-shaped SR's and round SR's or synaptic spheres (SS's). SS's may represent a special type of synaptic body, yet belonging to the family of SR's, or may be related to the catabolism of SR's. SR number is regulated by Ca2+ in teleost cones, whereas in the mammalian pineal gland cGMP is involved. An interesting biochemical feature of ribbon synapses is that they lack synapsins. The presently reviewed results suggest to us that SR's do not primarily function as conveyor belts, but are devices to immobilize SV's in inactive ribbon synapses.

The correlated blanching of synaptic bodies and reduction in afferent firing rates caused by transmitter-depleting agents in the frog semicircular canal

Synaptic bodies (SBs) associated with rings of synaptic vesicles and well-defined, pre- and post-synaptic membrane structures are indicators of maturity in most hair cell-afferent nerve junctions. The role of the SBs remains elusive despite several experiments showing that they may be involved in storage of neurotransmitter. Our results demonstrate that SBs of the adult posterior semicircular canal (SCC) cristae hair cells become less electron dense following incubation of the SCC with the transmitter-depleting drug tetrabenazine (TBZ). Objective quantification and comparison of the densities of the SBs in untreated and TBZ-treated frog SCC demonstrated that TBZ significantly decreased the electron density of SBs. This reduction in electron density was accompanied by a reduction in firing rates of afferent fibers innervating the posterior SCC. A second transmitter-depleting drug, guanethidine, previously shown to reduce the electron density of hair cell SBs, also reduced the firing rates of afferent fibers innervating the posterior SCC. In contrast, the electron density of dense granules (DG), similar in size and shape to synaptic bodies (SB) in hair cells, did not change after incubation in TBZ, thus indicating that granules and SBs are not similar in regard to their electron density. The role of SBs in synaptic transmission and the transmitter, if any, stored in the SBs remain unknown. Nonetheless, the association of the lessening of electron density with a reduction in afferent firing rate provides impetus for the further investigation of the SB's role in neurotransmission.

Pre- and postsynaptic excitatory action of glutamate agonists on frog vestibular receptors

In order to investigate the localization and the type(s) of excitatory amino acid receptors in the frog vestibular system, the exogenous amino acid agonists Quisqualic acid, Kainic acid and N-methyl-D-aspartic acid were tested on the sensory organ of semicircular canals. Intracellular recordings of the resting discharge from single afferents showed that these agonists exerted a complex excitatory action consisting in a rapid and brief increase in frequency of both EPSPs and spikes, followed by a slower and longer lasting membrane depolarization. The progressive impairment of natural transmitter release achieved by adding Mg2+ or Co2+ in the bath caused a dose-dependent decrease of the agonist-induced afferent discharge, without substantially affecting axonal depolarization. These results suggest that the exogenous amino acid agonists act both pre- and postsynaptically on the vestibular organs. Quisqualic acid and kainic acid were much more potent than N-methyl-D-aspartic acid in inducing excitatory effects, suggesting that the amino acid receptors located on both hair cells and afferent endings are mainly of the non-NMDA type. The present findings, while not excluding that an excitatory amino acid may be the afferent transmitter, highlight its possible function as a presynaptic modulator of the afferent transmission in the frog vestibular system.

Synaptic bodies in the different rows of outer hair cells in the guinea pig cochlea

The afferent synapses of the outer hair cells (OHCs) of the cochlea are peculiar, insofar as some of them contain special synaptic bodies (SBs) forming the so-called ribbon synapses. These SB-containing synapses are highly variable in number, exhibiting interspecies and intraspecies differences. As quantitative data on the incidence of SBs in the different rows of OHCs are lacking and as some of the above differences may have a circadian basis, in the present study SBs were counted in guinea pigs killed at different times of the day and night. In the second turn of the cochlea, synapses with perpendicular and parallel SBs were distinguished. Perpendicular SBs, but not parallel SBs, were significantly higher in number in the third row. Circadian changes were observed for perpendicular SBs, which were higher in number in the evening than in the morning. Circadian changes were absent in the parallel SBs of the OHCs and in the SBs of the inner hair cells. These results show that the afferent synapses of OHCs are rather complex, structurally as well as temporally; the functional significance of this finding remains to be elucidated.

Neurochemical evidence for afferent GABAergic and efferent cholinergic neurotransmission in the frog vestibule

Glutamate decarboxylase and choline acetyltransferase activities with magnitudes similar to those of their homologous enzymes in frog nervous tissue were found in homogenates of the frog labyrinth. Transection of the vestibular nerve resulted in a gradual diminution of choline acetyltransferase activity until it reached an 88% decrease 6 weeks after surgery. In contrast, glutamate decarboxylase activity did not suffer any alteration at any time after nerve excision. The presence of their enzymes of synthesis is evidence of the neurotransmitter participation of GABA and acetylcholine in the frog vestibule; the observed decrease of choline acetyltransferase following vestibule nerve excision supports the efferent synaptic bouton localization of choline acetyltransferase. The suggestion that glutamate decarboxylase is located in a cell type (or compartment) that may well be the hair cell is supported by the fact that this enzyme does not suffer any modification after surgery. These results are in accordance with an efferent cholinergic neurotransmission and a putative afferent role of GABA in the frog vestibule.

Serial section reconstruction of the neural poles of hair cells in the human organ of Corti. I. Inner hair cells

Study of the anatomy of the cochlea, and in particular the morphology of synaptic relationships between hair cells and cochlear neurons, is essential for elucidation of the mechanisms of transduction of mechanical acoustic signals into electrical neural events. Because considerable gaps remain in our understanding of the microscopic anatomy of these synapses, particularly in the human, a reconstruction of neural pole of inner hair cells of the human organ of Corti was performed. The data are based on 526 serial sections from the basal turn (10 mm region) and 356 serial sections from the middle turn (26 mm region). This provided complete data on 3 and partial data on 5 inner hair cells. Afferent terminals on inner hair cells were variable in size, ranging 1 to 20 micrometers in diameter. Branching of large fibers to produce multiple terminals innervating from 1 to 3 inner hair cells was common. Each inner hair cell received approximately 6 to 8 different nerve terminals. In addition, each terminal possessed a variable number of synaptic contacts. Junctional membrane specialization consisted of synapses, desmosomes, coated vesicles and arrays of microtubules and membrane cisternae. Specialization at synapses consisted of asymmetrical membrane thickening. At inner hair cells the postsynaptic membrane was thicker than the presynaptic membrane. Eighty-three percent of synapses had presynaptic bodies. Vesiculated efferent terminals synapsed on afferent fibers at the base of inner hair cells, but never directly on the inner hair cell. These anatomical data demonstrate distinct differences between the human and animal inner ear, which are important in the interpretation of neurophysiological data in animals and the formulation of hypotheses that involve assumptions crossing species.

Topographical relationships of synaptic ribbons in the pineal system of the vole (Microtus agrestis)

In the pineal system of the vole (Microtus agrestis) both the superficial and the deep pineal exhibit a high percentage of synaptic ribbons lying in intimate contact with the cell membrane of the pinealocytes. At the sites of contact, densities resembling the presynaptic dense projections of synapses are arranged between the ribbons and the cell membrane. Opposite the sites of contact various elements were found. The quantitative estimation revealed that in the superficial pineal about 40% and in the deep pineal about 60% of the membrane-contacting ribbons are located opposite glial cells: in both organ parts about 18% of the membrane-contacting ribbons were found opposite adjacent pinealocytes. The location of ribbons at the perivascular space was almost exclusively found in the superficial pineal, while the cerebrospinal fluid-contacting area in the deep pineal exhibited ribbons in intimate contact with the lumen of the third ventricle. The heterogeneity of the topographical relationships would seem to indicate a diffuse functional effect of the synaptic ribbons in the mammalian pineal gland.

Ultrastructure of synapses in the lateral line canal organ

The ultrastructure of afferent and efferent synapses on hair cells in the lateral line canal organ of hair cells in the lateral line canal organ of the fish Lota lota was studied, utilizing various fixation and staining techniques. New information was obtained about membrane-associated material, such as the presynaptic body and postsynaptic densities, by examining glutaral-dehyde-fixed material not subjected to osmication. Contrast was instead obtained either by section staining with uranyl acetate and lead citrate or by block impregnation with phosphotungstic acid (PTA). PTA-staining enhances the postsynaptic membrane of both the afferent and efferent synapses. It also stains the presynaptic dense projections at the efferent synapse and in a differential fashion, the afferent synaptic body. Secretion staining reveals a substructure in the feed of the afferent synaptic body, faintly seen but masked in osmicated tissue.

Ultrastructural obsevations on synaptic ribbons in the pineal organ of the goldfish

Synaptic ribbons in the pineal organ of the goldfish were examined electron microscopically with particular attention to their topography. These structures were formed of parallel membranes, which were poorly preserved with OsO4 fixation and could be extracted from thin sections with pronase indicating their proteinaceous nature. Synaptic ribbons were closely apposed to the plasma membrane bordering dendrites of ganglion cells, but were also related to processes of both photoreceptor and supportive cells. Their close proximity to invaginations of the plasma membrane and portions of the endoplasmic reticulum suggest that they are involved in the turnover of cytoplasmic membranes. Tubular and spherical organelles of unknown function are also described.

Ultrastructural analysis of the action of reserpine on the brain neuroendocrine system of the wax moth, Galleria mellonella L., Lepidoptera

This study concerns the influence of reserpine on the fine structure of peptidergic neurosecretory cells in the pars intercerebralis of Galleria mellonella, and of neurons containing smaller dense-cored vesicles (presumed to be aminergic) localized in the same area of the brain. The drug, administered in doses of 125 microgram and 250 microgram per g of insect body weight, reduces both the amount and the electron opacity of the dense-cored vesicles with a diameter of 60--80 nm in the neuronal perikarya as well as their terminals. Simultaneously, this treatment evokes an abnormal accumulation of secretory granules within the perikarya of peptidergic neurosecretory cells belonging to three types. This accumulation of secretory material is followed by some changes in the fine structure of these cells. One (fourth) type of neurosecretory cells seems to be insensitive to reserpine action. Participation of the aminergic system in the regulation of the secretory activity of some populations of peptidergic neurosecretory neurons of the insect brain is postulated.

Destruction of afferent nerve terminals in the inner ear of frog by aminooxyactic acid

The drug aminooxyacetic acid, which inhibits GABA-transaminase, destroys the afferent nerve ending in the inner ear of the frog. The efferent nerve endings and the sensory cells are not affected.

Inhibition by efferent nerve fibres: action on hair cells and afferent synaptic transmission in the lateral line canal organ of the burbot Lota lota

1. Intracellular recordings were made from morphologically identified hair cells in the lateral line canal organs of the burbot Lota lota. 2. I.p.s.p.s were recorded from hair cells when the efferent fibres were excited by electrical stimulation of the lateral line nerve. The i.p.s.p.s were abolished when the fish was injected with immobilizing concentration of Flaxedil which is known to block the efferent synapses. 3. The i.p.s.p.s are accompanied by a decrease in the resistance of the hair cell membrane and an increase in the intracellular receptor potential. 4. Spontaneous and mechanically evoked e.p.s.p.s which were recorded intracellularly from the post-synaptic afferent nerve terminals were reduced in amplitude for the duration of the i.p.s.p.

The effect of 6-hydroxydopamine on the rabbit cochlea

6-OH-DA is an isomer of noradrenalin which is selectively taken up by adrenergic axons. Noradrenalin stores are displaced by 6-OH-DA and at a certain intraneuronal concentration, degeneration of the terminals occurs and results in a chemical sympathectomy. The effects of 6-OH-DA on the rabbit cochlea were studied with fluorescence and electron microscopy after systemic administration of the substance and after local perfusion of the cochlea. Doses of 25 to 200 mg/kg were used. After intravenous injection there was an initial accumulation of 6-OH-DA in noradrenalin storage vesicles. A dose of 50 mg/kg 6-OH-DA initiated marked signs of degeneration in adrenergic nerve terminals but did not cause breakdown of their cell membranes. Higher doses did not seem to increase the damage. Local perfusion with 6-OH-DA gave rise to extensive degeneration of adrenergic nerve terminals and after 7 days all terminals had disappeared. These findings indicate the presence of a blood-perilymph barrier to 6-OH-DA. Some degeneration was also evident in cholinergic axons of the inner spiral bundle and of the tunnel spiral bundle.