Electron microscopy demonstrating noise exposure alters synaptic vesicle size in the inferior colliculus of cat.

CONTEXT: White noise is known to have detrimental effects on different brain regions, especially auditory regions, including inferior colliculus. Although the basis for such alterations has been hypothesized to result from abnormalities in neurotransmitter release, the mechanism is unclear. The final step in neurotransmission is the docking and transient fusion of synaptic vesicles at the base of cup-shaped lipoprotein structures called porosomes at the presynaptic membrane and the consequent release of neurotransmitters. Earlier studies in cat brain document altered morphology of the secretory portal the porosome at nerve terminals in the inferior colliculus following white noise exposure. The current study was performed to test the hypothesis of possible changes to synaptic vesicle size in the colliculus, following white noise exposure.
MATERIAL AND METHODS: Electron microscopic morphometry of synaptic vesicles size in axo-dendritic synapses at the colliculus region of the cat brain was performed.
RESULTS: We report, for first time, decreased size of both docked and undocked vesicles in high-intensity white noise-exposed animals. In both control and experimental animals, docked vesicles are demonstrated to be smaller than undocked vesicles, suggesting fractional discharge of vesicular contents via porosome-mediated kiss-and-run mechanism.
CONCLUSION: These studies advance our understanding of neurotransmitter release and the impact of white noise on brain function.

INTRODUCTION

Noise pollution is an increasing problem in modern society, especially in highly populated localities such as major cities. Everyday traffic, loud music, equipment and appliances, all contribute to the exposure of people to harmful noise levels.[123] According to the World Health Organization, about 40% of Europeans from urban areas is systematically exposed to road traffic noise at levels exceeding 55 dB and more than 30% − to levels exceeding 55 dB at night. Moreover, nearly 20% are negatively impacted by continuous noise, exceeding much beyond the 65 dB.[4] Similarly, soldiers and the public in areas of conflict, primarily in low and mild-income countries, are exposed to prolonged, harmful levels of noise with damaging consequences.[567] According to the World Health Organization, about 40% of Europeans from urban areas is systematically exposed to road traffic noise at levels exceeding 55 dB and more than 30% − to levels exceeding 55 dB at night. Moreover, nearly 20% are negatively impacted by continuous noise, exceeding much beyond the 65 dB.[8] Noise, above 70 dB over a prolonged period of time may contribute to start to various pathologies, including hearing damage, cardiovascular disturbances or several mental or neurological disorders. According some data, even acute exposure to noise, particularly high-frequency noise, negatively impacts cardiovascular and hearing conditions, and provoke alterations in the development and physiology of auditory structures.[910] Besides, chronic noise interferes with the normal growth and function of some non-auditory regions of the brain, resulting in impaired language acquisition, memory performance, other cognitive activities, neurotransmission, emotions, etc.[111213] Numerous electrophysiological, molecular, neuroanatomical and neuroimaging studies have focused on the sensitivity of these regions to noise.[141516] However, much remains to be understood at the molecular level, regarding this impact on neurotransmission. The final steps in neurotransmission are the docking and fusion of synaptic vesicles at the presynaptic membrane, including the “kiss-and-run” mechanism mediated by the 15 nm cup-shaped lipoprotein porosome structures [Figure 1].[1718]

Figure 1

Electron micrograph of a neuronal porosome with a docked synaptic vesicle at its base. Central plug (white arrows) in the cup-shaped porosome complex (black arrowheads) measuring approximately 15 nm, and docked synaptic vesicles (SV) at the base of porosome in the presynaptic membrane. Scale bar = 10 nm. (Published in Journal Discoveries. Zhvania MG, Bikashvili TZ, Japaridze NJ, Lazrishvili II, Ksovreli M. White noise and neuronal porosome complex: transmission electron microscopic study. Discoveries 2014, Jul-Sep; 2(3): e25. DOI: 10.15190/d.2014.17)

In earlier electron microscopic (EM) studies, we documented altered porosome morphology in the presynaptic membrane of synaptosomes of the central amygdala in rats with chronic motor deficit.[19] In separate EM studies, we reported that chronic noise impacts the porosome structures present primarily in regions of auditory brain − medial geniculate body and inferior colliculus in cat brain.[2021] These results suggest possible alterations in synaptic vesicles and their consequent effect on neurotransmission and brain function. The current EM study was carried out to test this hypothesis. The goal was to assess, if chronic white noise, in parallel with the fine changes in porosome complex structure, affects docked and undocked synaptic vesicle size in excitatory synapses of the central nucleus of inferior colliculus (ICC) − the largest subdivision of auditory system, mesencephalic interface between lower brainstem auditory pathways, the auditory cortex and motor system.[2223]

At least two levels of afferent organization in ICC were identified.[2425] A tonotopic map define this space to different audible frequencies of sounds. Another “nucleotopic” organization subdivides the IC into three main patterns of anatomical zones (functional or synaptic domains) that receive projections from particular nuclei.[232526] Noise may affect different inputs, frequency domains or functional zones differentially within the ICC. In addition, different input axon terminals may originally have different vesicle sizes. In the present research, we are focused only on large buttons (more than 2 µm2 in area) of projection fibers, which made asymmetric excitatory synapses with spherical synaptic vesicles (∼60%) [Figure 2]. In opposite to it, in other subdivisions of IIC, excitatory buttons are mainly small or intermediate in size.[2728]

Figure 2

Axo-dendritic asymmetric synapse with large active zone in the inferior coliculus of rat brain. Large presynaptic profile contains numerous synaptic vesicles

Numerous studies of noise-induced effects highlight the importance of evaluating the noise-related experiments from male and female separately.[2930] Due to specific cochlear anatomy, the males are less protected from detrimental consequences of loud noise than females,[29] in the current research we are focused on male animals.

MATERIAL AND METHODS

Adult male cats (140-145 days old), were used (n = 10). The animals were housed 1/cage in a wire cages (38 × 30 × 25 cm) that ensured acoustic transparence. The room was well controlled with a light/dark cycle of 12:12 h (the temperature − 20°C −22°C, humidity − 55-60%, 12h light-dark period (on at 07.30 am and off at 7.30 pm). The animals had free access to food and water. Just before the experiments, the animals were randomly assigned to an experimental (n=5) and control groups (n=5). The exposure of animals to noise was described in our previous study.[23] Briefly experimental animals were exposed to 100 dB acoustic white noise in their home cage for one hour per day, for 10 consecutive days. The noise exposure was provided by two Paradigm Signature S1 P- Be loudspeakers (Paradigm Electronics Inc., Canada). Two speakers were mounted 55 cm above the floor of the cages. Each speaker primarily affected two cages. Sound levels were constantly monitored using two microphones, suspended in a line 45 cm above the cage. EM evaluation of experimental animals was done one day after the 10-day noise exposure. Control rats were kept in the same vivarium conditions but were not exposed to any loud noise. The animal maintenance and experimental procedures were conducted in accordance with European Union Directive on the protection of animals used for scientific research and were approved by The Committee of Animal Care at Ivane Beritashvili Center of Experimental Biomedicine and Committee on Ethics at Ilia State University.

Conventional EM technique, described in our earlier studies was used.[2324] Briefly after pentobarbital injection (100 mg/kg), control and experimental animals underwent trans cardiac perfusion with ice cold heparinized 0.9% NaCl, followed by 500 mL of 4% paraformaldehyde and 2.5% glutaraldehyde in 0.1 M phosphate buffer, pH 7.4, at a perfusion pressure of 120 mm Hg. The left hemispheric brain tissue blocks containing ICC were cut into 400-micron thick coronal slices using a cryostat. The slices were post-fixed in 1% osmium tetroxide for 2 h. The region of interest was identified with an optical microscope Leica MM AF, cut out from the coronal slices, dehydrated in graded series of ethanol and acetone, and embedded in araldite. From araldite blocks, 70–75 nm thick sections were prepared with an ultra-microtome Leica EM UC7. The sections were placed on 200-mesh copper grids, double-stained with uranyl-acetate and lead-citrate, and examined with JEM 1400 (JEOL, Japan). 120 sections from experimental animals and 120 sections from control animals (30 from each) were evaluated. Only large axon profiles (∼ 2 µm2 in area), which made asymmetric junctions and contained 25-40 spherical synaptic vesicles were included in data sets. Technical approach was described in our recent study.[3132] Briefly in order to determine the changes in the diameter in both docked and undocked synaptic vesicles, on electron micrographs the 250 axon endings from control animals and 250 endings from noise-exposed animals (50 endings from each cat) were outlined. The tracings of axon terminals were scanned, using the scan plug-in for Adobe Photoshop CS3 and saved as 150 dpi tiff files. The scans were imported into ImageJ software (version 1.44, The National Institute of Mental Health). The images of the axon terminals were enlarged onto the computer screen and each vesicle was sequentially marked, using the brush tool. The diameter of docked and undocked spherical synaptic vesicles was measured with “Image J” software (version 1.44, The National Institute of Mental Health) [Figure 1a, 1b]. To determine whether white noise impacted vesicle size, one-way ANOVA was performed. Multiple comparisons were made using the two-sample t-test. A P-value less than 0.05 was considered to be statistically significant. The results are presented as a mean ± standard error of the mean (SEM) and standard deviation (StDev).

RESULTS

The morphometric analysis of synaptic vesicles from EM micrographs of the central nucleus of IC, demonstrates decreased size of docked and undocked synaptic vesicles in both control and noise-exposed cats. Such decrease was more prominent in noise-exposed animals. Specifically, in control cats the loss of the diameter of docked synaptic vesicles over undocked vesicles was 5.7% (42.62 ± 0.68 nm vs. 45.04 ± 0.35 nm, P < 0.001). As to noise-exposed cats, such decrease constituted 11.3% (34.27 ± 0.69 nm vs. 38.13 ± 0.24nm, P < 0.001). The results suggest that due to continuous neurotransmitter release the majority of vesicles are unable to replenish vesicular

cargo via the neurotransmitter transporters. Significant difference was also observed when comparing the diameters of synaptic vesicles from different vesicle pools (docked and undocked) in control and noise-exposed cats. In particular, in noise-exposed animals there was a 19.6% drop in diameter in the docked synaptic vesicles over those in control (42.62 ± 0.68 nm vs. 34.27 ± 0.69 nm, P<0.001), while only a 15.3% decrease in undocked synaptic vesicles diameter was observed in white noise-exposed animals over control (45.04 ± 0.35nm vs. 38.13 ± 0.24 nm P<0.001). Therefore, in both groups of animals, docked synaptic vesicles show more prominent decrease in size than undocked synaptic vesicles [Figure 3].

Figure 3

Size of docked and undocked synaptic vesicles in inferior colliculus of the cat brain in control and white noise exposed animals, demonstrating the decrease in size of both docked and undocked vesicles as a result of white noise exposure. A - Bars represent percent difference in mean values of docked vesicles’ diameters in control animals vs. white noise exposed animals; it reaches 20%, P<0.001; B - Percent difference in mean values of undocked vesicles in control animals vs. white noise exposed animals is 25% and P<0.001. WN – White noise, ***P<0.001.

DISCUSSION

Present study is the part of our broad investigation of presynaptic architecture in different mammals under various physiological and pathological conditions.[1920213132] In the present study, we assess the size of synaptic vesicles in the subdivision of inferior colliculus, which receives the projections predominantly from cochlear nucleus and the part of lateral lemniscus.[232425] The majority of these projections (∼60%) have large axon terminals and made asymmetric excitatory synapses with spherical synaptic vesicles.[2627] Since both control and chronic HIWN-exposed animals demonstrate the decreased size of docked synaptic vesicles compared to undocked vesicles, the fractional discharge of vesicular content via porosome-mediated kiss-and-run mechanism of synaptic vesicle fusion and neurotransmitter release at large axon terminal is interfered.[1718] This observation is consistent with our earlier findings, demonstrating that the exposure of cats to chronic noise alters the main structural parameters of porosome complex, i.e. its diameter and depth.[20]

It is well established that secretory vesicle swelling is a requirement in cell secretion, including neurotransmitter release at the nerve terminal.[3334] Furthermore, recent studies using fluorescence correlation spectroscopy and cryogenic electron microscopy, show that glutamatergic synaptic vesicles reversibly increase their size upon filling with glutamate.[35363738] The increase in diameter usually corresponds to an increase in surface area and in volume.[34] The large size increase implies a large structural change in vesicles upon loading with neurotransmitters, and or ion and water transport. Studies report changes in both number and size of trafficking synaptic vesicles following stimulation.[353637] In earlier research, using EM, we demonstrate that continuous exposure of cats to white noise, results in pronounced alterations in the ultrastructure of large axo-dendritic terminals in two auditory subcortical regions − ventral portion of medial geniculate body and central nucleus of IC of the brain.[20] In addition, our unpublished data obtained on adult male rats subjected to chronic noise point to the modifications in the ultrastructure of large axon profiles in the CCI and ventral part of medial geniculate body (the part of auditory thalamus, which receives mainly auditory information). Among these subcortical areas involved in auditory processing, in both types of mammals ultrastructural modifications in synapses were mostly detected in the mesencephalic IC (7% of synaptic profiles in cat brain and 9% of synaptic profiles in rat brain). Such modifications included pathological alterations (clustering of synaptic vesicles and degeneration/vacuolization of presynaptic mitochondria) and non-pathological alterations (the appearance of large polymorphic synaptic vesicles, which are absent in control material, increased number of docking synaptic vesicles and expanded total length of presynaptic active zone) [ [Figure 4].[20] Present study confirms that chronic auditory stimulation results in the changes in fine architecture of subcortical auditory region − IC: in addition to above-mentioned ultrastructural changes of presynaptic regions, we show the depletion of synaptic vesicles in some large terminals forming axo-dendritic synapses. With high probability, such depletion could reflect the changes in neurotransmission because of continuous noise. Interestingly, small axon terminals of this region remained unchanged, suggesting their relative inactivity.

Figure 4

Electron micrograph of presynaptic terminal in the colliculus inferior region of the brain in control and white noise exposed cats. (A) Relatively uniform distribution of spherical synaptic vesicles in axo-dendritic synapse of control animal. Most part of synaptic vesicles is spherically shaped. (B) Postsynapic region with destroyed mitochondria (black arrows). Scale bar = 250 nm

Evaluation of changes to synaptic vesicles size undertaken in the current study using EM has advanced the understanding of the pathophysiology of white noise exposure on auditory brain processing regions, in addition to our understanding of fractional neurotransmitter release at the nerve terminal and on overall brain function.

Financial support was provided by Shota Rustaveli National Science Foundation of Georgia: Grant − PHDF-18-1136.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.