Presynaptic Terminal Size Research Articles

Parvalbumin and Somatostatin: Biomarkers for Two Parallel Tectothalamic Pathways in the Auditory Midbrain.

The inferior colliculus (IC) represents a crucial relay station in the auditory pathway, located in the midbrain's tectum and primarily projecting to the thalamus. Despite the identification of distinct cell classes based on various biomarkers in the IC, their specific contributions to the organization of auditory tectothalamic pathways have remained poorly understood. In this study, we demonstrate that IC neurons expressing parvalbumin (ICPV+) or somatostatin (ICSOM+) represent two minimally overlapping cell classes throughout the three IC subdivisions in mice of both sexes. Strikingly, regardless of their location within the IC, these neurons predominantly project to the primary and secondary auditory thalamic nuclei, respectively. Cell class-specific input tracing suggested that ICPV+ neurons primarily receive auditory inputs, whereas ICSOM+ neurons receive significantly more inputs from the periaqueductal gray and the superior colliculus (SC), which are sensorimotor regions critically involved in innate behaviors. Furthermore, ICPV+ neurons exhibit significant heterogeneity in both intrinsic electrophysiological properties and presynaptic terminal size compared with ICSOM+ neurons. Notably, approximately one-quarter of ICPV+ neurons are inhibitory neurons, whereas all ICSOM+ neurons are excitatory neurons. Collectively, our findings suggest that parvalbumin and somatostatin expression in the IC can serve as biomarkers for two functionally distinct, parallel tectothalamic pathways. This discovery suggests an alternative way to define tectothalamic pathways and highlights the potential usefulness of Cre mice in understanding the multifaceted roles of the IC at the circuit level.

DSCAM gene triplication causes excessive GABAergic synapses in the neocortex in Down syndrome mouse models.

Down syndrome (DS) is caused by the trisomy of human chromosome 21 (HSA21). A major challenge in DS research is to identify the HSA21 genes that cause specific symptoms. Down syndrome cell adhesion molecule (DSCAM) is encoded by a HSA21 gene. Previous studies have shown that the protein level of the Drosophila homolog of DSCAM determines the size of presynaptic terminals. However, whether the triplication of DSCAM contributes to presynaptic development in DS remains unknown. Here, we show that DSCAM levels regulate GABAergic synapses formed on neocortical pyramidal neurons (PyNs). In the Ts65Dn mouse model for DS, where DSCAM is overexpressed due to DSCAM triplication, GABAergic innervation of PyNs by basket and chandelier interneurons is increased. Genetic normalization of DSCAM expression rescues the excessive GABAergic innervations and the increased inhibition of PyNs. Conversely, loss of DSCAM impairs GABAergic synapse development and function. These findings demonstrate excessive GABAergic innervation and synaptic transmission in the neocortex of DS mouse models and identify DSCAM overexpression as the cause. They also implicate dysregulated DSCAM levels as a potential pathogenic driver in related neurological disorders.

Sex Differences in the Spatial Behavior Functions of Adult-Born Neurons in Rats.

Adult neurogenesis modifies hippocampal circuits and behavior, but removing newborn neurons does not consistently alter spatial processing, a core function of the hippocampus. Additionally, little is known about sex differences in neurogenesis since few studies have compared males and females. Since adult-born neurons regulate the stress response, we hypothesized that spatial functions may be more prominent under aversive conditions and may differ between males and females given sex differences in stress responding. We therefore trained intact and neurogenesis-deficient rats in the spatial water maze at temperatures that vary in their degree of aversiveness. In the standard water maze, ablating neurogenesis did not alter spatial learning in either sex. However, in cold water, ablating neurogenesis had divergent sex-dependent effects: relative to intact rats, male neurogenesis-deficient rats were slower to escape the maze and female neurogenesis-deficient rats were faster. Neurogenesis promoted temperature-related changes in search strategy in females, but it promoted search strategy stability in males. Females displayed greater recruitment (Fos expression) of the dorsal hippocampus than males, particularly in cold water. However, blocking neurogenesis did not alter Fos expression in either sex. Finally, morphologic analyses revealed greater experience-dependent plasticity in males. Adult-born neurons in males and females had similar morphology at baseline but training increased spine density and reduced presynaptic terminal size, specifically in males. Collectively, these findings indicate that adult-born neurons contribute to spatial learning in stressful conditions and they provide new evidence for sex differences in their behavioral functions.

Similarity and Diversity of Presynaptic Molecules at Neuromuscular Junctions and Central Synapses.

Synaptic transmission is essential for controlling motor functions and maintaining brain functions such as walking, breathing, cognition, learning, and memory. Neurotransmitter release is regulated by presynaptic molecules assembled in active zones of presynaptic terminals. The size of presynaptic terminals varies, but the size of a single active zone and the types of presynaptic molecules are highly conserved among neuromuscular junctions (NMJs) and central synapses. Three parameters play an important role in the determination of neurotransmitter release properties at NMJs and central excitatory/inhibitory synapses: the number of presynaptic molecular clusters, the protein families of the presynaptic molecules, and the distance between presynaptic molecules and voltage-gated calcium channels. In addition, dysfunction of presynaptic molecules causes clinical symptoms such as motor and cognitive decline in patients with various neurological disorders and during aging. This review focuses on the molecular mechanisms responsible for the functional similarities and differences between excitatory and inhibitory synapses in the peripheral and central nervous systems, and summarizes recent findings regarding presynaptic molecules assembled in the active zone. Furthermore, we discuss the relationship between functional alterations of presynaptic molecules and dysfunction of NMJs or central synapses in diseases and during aging.

Evaluation of medial division of the medial geniculate (MGM) and posterior intralaminar nucleus (PIN) inputs to the rat auditory cortex, amygdala, and striatum.

The medial division of the medial geniculate (MGM) and the posterior intralaminar nucleus (PIN) are association nuclei of the auditory thalamus. We made tracer injections in these nuclei to evaluate/compare their presynaptic terminal and postsynaptic target features in auditory cortex, amygdala and striatum, at the light and electron microscopic levels. Cortical labeling was concentrated in Layer 1 but in other layers distribution was location-dependent. In cortical areas designated dorsal, primary and ventral (AuD, Au1, AuV) terminals deep to Layer 1 were concentrated in infragranular layers and sparser in the supragranular and middle layers. In ectorhinal cortex (Ect), distributions below Layer 1 changed with concentrations in supragranular and middle layers. In temporal association cortex (TeA) terminal distributions below Layer 1 was intermediate between AuV/1/D and Ect. In amygdala and striatum, terminal concentrations were higher in striatum but not as dense as in cortical Layer 1. Ultrastructurally, presynaptic terminal size was similar in amygdala, striatum or cortex and in all cortical layers. Postsynaptically MGM/PIN terminals everywhere synapsed on spines or small distal dendrites but as a population the postsynaptic structures in cortex were larger than those in the striatum. In addition, primary cortical targets of terminals were larger in primary cortex than in area Ect. Thus, although postsynaptic size may play some role in changes in synaptic influence between areas it appears that terminal size is not a variable used for that purpose. In auditory cortex, cortical subdivision-dependent changes in the terminal distribution between cortical layers may also play a role.

Pituitary adenylate cyclase activating polypeptide induces long-term, transcription-dependent plasticity and remodeling at autonomic synapses

Pituitary adenylate cyclase activating polypeptide (PACAP) is a multifunctional neuropeptide, widely expressed in the nervous system (Vaudry et al., 2009; Starr and Margiotta, 2016). At neuronal synapses where transmission is mediated by nicotinic acetylcholine receptors (nAChRs) transient PACAP exposure increases the frequency and amplitude (FS and AS) of spontaneous excitatory postsynaptic currents (sEPSCs) within minutes. This short-term (ST) plasticity requires high-affinity PACAP receptor (PAC1R) signaling via adenylate cyclase (AC), cyclic AMP (cAMP), Protein kinase A (PKA) and obligatory nAChR-dependent stimulation of nitric oxide (NO) synthesis to retrogradely increase presynaptic ACh release (Pugh et al., 2010; Jayakar et al., 2014). Remarkably, synaptic changes persist 48h after transient PACAP exposure, featuring a similar increase in FS and an even larger increase in AS. Pharmacological studies reveal that this long-term (LT) plasticity requires PACAP/PAC1R signaling via AC and cAMP, but unlike ST plasticity, Phospholipase-C and new gene transcription are also necessary, whereas PKA, nAChR, impulse and NO synthase (NOS1) activities are dispensable. In accord with the increases in FS and AS characterizing LT plasticity, miniature EPSC (mEPSC) frequency, ACh release (quantal content), and mEPSC amplitude (quantal size) all increased in parallel. Consistent with these functional changes, imaging studies reveal that LT, but not ST, PACAP-induced plasticity is accompanied by increases in presynaptic terminal size, postsynaptic nAChR cluster size and density, and the size and density of co-localized pre- and post-synpatic sites. Thus PACAP/PAC1R signaling induces mechanistically distinct forms of synaptic plasticity, with a ST form arising from acute, membrane-delimited processes, and a LT form arising from transcription-dependent alterations in the function and structural arrangement of pre- and post-synaptic components.

Differential electrophysiological and morphological alterations of thalamostriatal and corticostriatal projections in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal genetic disorder characterized by cell death of medium-sized spiny neurons (MSNs) in the striatum, traditionally attributed to excessive glutamate inputs and/or receptor sensitivity. While changes in corticostriatal projections have typically been studied in mouse models of HD, morphological and functional alterations in thalamostriatal projections have received less attention. In this study, an adeno-associated virus expressing channelrhodopsin-2 under the calcium/calmodulin-dependent protein kinase IIα promoter was injected into the sensorimotor cortex or the thalamic centromedian-parafascicular nuclear complex in the R6/2 mouse model of HD, to permit selective activation of corticostriatal or thalamostriatal projections, respectively. In symptomatic R6/2 mice, peak amplitudes and areas of corticostriatal glutamate AMPA and NMDA receptor-mediated responses were reduced. In contrast, although peak amplitudes of AMPA and NMDA receptor-mediated thalamostriatal responses also were reduced, the areas remained unchanged due to an increase in response decay times. Blockade of glutamate reuptake further increased response areas and slowed rise and decay times of NMDA responses. These effects appeared more pronounced at thalamostriatal synapses of R6/2 mice, suggesting increased activation of extrasynaptic NMDA receptors. In addition, the probability of glutamate release was higher at thalamostriatal than corticostriatal synapses, particularly in R6/2 mice. Morphological studies indicated that the density of all excitatory synaptic contacts onto MSNs was reduced, which matches the basic electrophysiological findings of reduced amplitudes. There was a consistent reduction in the area of spines but little change in presynaptic terminal size, indicating that the postsynaptic spine may be more significantly affected than presynaptic terminals. These results highlight the significant and differential contribution of the thalamostriatal projection to glutamate excitotoxicity in HD.

Cyfip1 Regulates Presynaptic Activity during Development.

Several developmental brain disorders have been associated with gene duplications and deletions that serve to increase or decrease levels of encoded proteins. Cyfip1 is one such protein, but the role it plays in brain development is poorly understood. We asked whether decreased Cyfip1 levels altered the function of developing synapses. The data show that synapses with reduced Cyfip1 are larger and release neurotransmitter more rapidly. These effects are due to Cyfip1's role in actin polymerization and are reversed by expression of a Cyfip1 mutant protein retaining actin regulatory function or by inhibiting Rac1. Thus, Cyfip1 has a more prominent early role regulating presynaptic activity during a stage of development when activity helps to define neural pathways.

αβγ-Synuclein triple knockout mice reveal age-dependent neuronal dysfunction

Synucleins are a vertebrate-specific family of abundant neuronal proteins. They comprise three closely related members, α-, β-, and γ-synuclein. α-Synuclein has been the focus of intense attention since mutations in it were identified as a cause for familial Parkinson's disease. Despite their disease relevance, the normal physiological function of synucleins has remained elusive. To address this, we generated and characterized αβγ-synuclein knockout mice, which lack all members of this protein family. Deletion of synucleins causes alterations in synaptic structure and transmission, age-dependent neuronal dysfunction, as well as diminished survival. Abrogation of synuclein expression decreased excitatory synapse size by ∼30% both in vivo and in vitro, revealing that synucleins are important determinants of presynaptic terminal size. Young synuclein null mice show improved basic transmission, whereas older mice show a pronounced decrement. The late onset phenotypes in synuclein null mice were not due to a loss of synapses or neurons but rather reflect specific changes in synaptic protein composition and axonal structure. Our results demonstrate that synucleins contribute importantly to the long-term operation of the nervous system and that alterations in their physiological function could contribute to the development of Parkinson's disease.

Diffusional Trapping of GluR1 AMPA Receptors by Input-Specific Synaptic Activity

Synaptic activity regulates the postsynaptic accumulation of AMPA receptors over timescales ranging from minutes to days. Indeed, the regulated trafficking and mobility of GluR1 AMPA receptors underlies many forms of synaptic potentiation at glutamatergic synapses throughout the brain. However, the basis for synapse-specific accumulation of GluR1 is unknown. Here we report that synaptic activity locally immobilizes GluR1 AMPA receptors at individual synapses. Using single-molecule tracking together with the silencing of individual presynaptic boutons, we demonstrate that local synaptic activity reduces diffusional exchange of GluR1 between synaptic and extraynaptic domains, resulting in postsynaptic accumulation of GluR1. At neighboring inactive synapses, GluR1 is highly mobile with individual receptors frequently escaping the synapse. Within the synapse, spontaneous activity confines the diffusional movement of GluR1 to restricted subregions of the postsynaptic membrane. Thus, local activity restricts GluR1 mobility on a submicron scale, defining an input-specific mechanism for regulating AMPA receptor composition and abundance.

Reduction of synaptophysin immunoreactivity in the prefrontal cortex of subjects with schizophrenia. Regional and diagnostic specificity.

Multiple lines of evidence indicate that the prefrontal cortex is a site of dysfunction in schizophrenia. However, the apparent absence of gross structural abnormalities in this area suggests that the pathophysiological characteristics of schizophrenia may involve more subtle disturbances in prefrontal cortical circuitry, such as alterations in synaptic connectivity and transmission. In this study, immunoreactivity for synaptophysin, an integral membrane protein of small synaptic vesicles, was used to assess the integrity of cortical synaptic circuitry in schizophrenia. Using immunocytochemical techniques and adjusted optical density measurements, we examined synaptophysin immunoreactivity in prefrontal cortical areas 9 and 46 and in area 17 (the primary visual cortex) from 10 pairs of case subjects with schizophrenia and control subjects matched on a pairwise basis for age, sex, race, and postmortem interval, and in 5 matched pairs of nonschizophrenic psychiatric case subjects and normal control subjects. Compared with levels found in matched control subjects, synaptophysin immunoreactivity in areas 46 and 9 was significantly decreased (P < .001 and P < .008, respectively) across all cortical layers in the case subjects with schizophrenia. In contrast, no differences were observed in area 17. In addition, levels of synaptophysin immunoreactivity in areas 46, 9, and 17 did not differ between 5 nonschizophrenic psychiatric case subjects and their matched controls, suggesting that decreased synaptophysin levels in the prefrontal cortex of patients with schizophrenia may be specific to that disorder. Additional studies are required to determine if the decrease in levels of synaptophysin immunoreactivity is caused by a decrease in the number or size of presynaptic terminals, a decrease in the number of synaptic vesicles per terminal, or a decrease in the expression of synaptophysin. However, all of these potential explanations are consistent with a disturbance in synaptic transmission in the prefrontal cortex of patients with schizophrenia.

Reduction of synaptophysin immunoreactivity in the prefrontal cortex of subjects with schizophrenia. Regional and diagnostic specificity.

Multiple lines of evidence indicate that the prefrontal cortex is a site of dysfunction in schizophrenia. However, the apparent absence of gross structural abnormalities in this area suggests that the pathophysiological characteristics of schizophrenia may involve more subtle disturbances in prefrontal cortical circuitry, such as alterations in synaptic connectivity and transmission. In this study, immunoreactivity for synaptophysin, an integral membrane protein of small synaptic vesicles, was used to assess the integrity of cortical synaptic circuitry in schizophrenia. Using immunocytochemical techniques and adjusted optical density measurements, we examined synaptophysin immunoreactivity in prefrontal cortical areas 9 and 46 and in area 17 (the primary visual cortex) from 10 pairs of case subjects with schizophrenia and control subjects. matched on a pairwise basis for age, sex, race, and postmortem interval, and in 5 matched pairs of nonschizophrenic psychiatric case subjects and normal control subjects. Compared with levels found in matched control subjects, synaptophysin immunoreactivity in areas 46 and 9 was significantly decreased (P < .001 and P < .008, respectively) across all cortical layers in the case subjects with schizophrenia. In contrast, no differences were observed in area 17. In addition, levels of synaptophysin immunoreactivity in areas 46, 9, and 17 did not differ between 5 nonschizophrenic psychiatric case subjects and their matched controls, suggesting that decreased synaptophysin levels in the prefrontal cortex of patients with schizophrenia may be specific to that disorder. Additional studies are required to determine if the decrease in levels of synaptophysin immunoreactivity is caused by a decrease in the number or size of presynaptic terminals, a decrease in the number of synaptic vesicle per terminal, or a decrease in the expression of synaptophysin. However, all of these potential explanations are consistent with a disturbance in synaptic transmission in the prefrontal cortex of patients with schizophrenia.

Ultrastructural evidence for synaptic interactions between thalamocortical axons and subplate neurons.

Thalamic axons are known to accumulate in the subplate for a protracted period prior to invading the cortical plate and contacting their ultimate targets, the neurons of layer 4. We have examined the synaptic contacts made by visual and somatosensory thalamic axons during the transition period in which axons begin to leave the subplate and invade the cortical plate in the ferret. We first determined when geniculocortical axons leave the subplate and begin to grow into layer 4 of the visual cortex by injecting 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine (Dil) into the lateral geniculate nucleus (LGN). By birth most LGN axons are still confined to the subplate. Over the next 10 days LGN axons grow into layer 4, but many axons retain axonal branches within the subplate. To establish whether thalamic axons make synaptic contacts within the subplate, the anterograde tracer PHA-L was injected into thalamic nuclei of neonatal ferrets between postnatal day 3 and 12 to label thalamic axons at the electron microscope level. The analysis of the PHA-L injections confirmed the Dil data regarding the timing of ingrowth of thalamic axons into the cortical plate. At the electron microscope level, PHA-L-labelled axons were found to form synaptic contacts in the subplate. The thalamic axon terminals were presynaptic primarily to dendritic shafts and dendritic spines. Between postnatal days 12 and 20 labelled synapses were also observed within layer 4 of the cortex. The ultrastructural appearance of the synapses did not differ significantly in the subplate and cortical plate, with regard to type of postsynaptic profiles, length of postsynaptic density or presynaptic terminal size. These observations provide direct evidence that thalamocortical axons make synaptic contacts with subplate neurons, the only cell type within the subplate possessing mature dendrites and dendritic spines; they also suggest that functional interactions between thalamic axons and subplate neurons could play a role in the establishment of appropriate thalamocortical connections.

Potentiation of nerve growth factor-induced alterations in cholinergic fibre length and presynaptic terminal size in cortex of lesioned rats by the monosialoganglioside GM1

The effect of monosialoganglioside GM1 and/or nerve growth factor treatment on the cholinergic innervation of the rat cortex was studied using both light- and electron-microscopic techniques assisted by image analysis. Adult male Wistar rats were unilaterally decorticated and received continuous infusions, via minipump, of vehicle, GM1 (1.5 mg/day) and/or nerve growth factor (12 μg/day) into the cerebroventricular space. Treatments were initiated immediately post-lesion and ended after seven days. Thirty days post-lesion (i.e. 23 days after the end of drug administration) brains were processed for choline acetyltransferase immunocytochemistry for either light- or electron-microscopic analysis. At this timepoint choline acetyltransferase-immunoreactive neurons in the ipsilateral nucleus basalis magnocellularis were significantly reduced in size especially in the mid portion of this nucleus, in lesion vehicle-treated rats. Moreover, decreases in choline acetyltransferase immunoreactive fibre length (ranging from 31 to 50%) and varicosity number (ranging from 26 to 39%) occurred in all cortical layers within a portion of the remaining cortex of these animals. Monosialoganglioside GM1 or nerve growth factor treatment equally attenuated deficits in nucleus basalis magnocellularis cell size and cortical choline acetyltransferase immunoreactive fibre length. However, nerve growth factor, but not monosialoganglioside GM1 treatment also increased choline acetyltransferase-immunoreactive varicosity number above control levels. In lesioned rats which received both nerve growth factor and the monosialoganglioside GM1, the mean cross-sectional area of nucleus basalis magnocellularis cholinergic neurons did not differ significantly from control values. By contrast, cortical choline acetyltransferase-immunoreactive fibre length and varicosity number were significantly increased above control values and that induced by nerve growth factor treatment alone. Quantitative electron-microscopic analysis showed that cholinergic boutons in cortical layer V were considerably shrunken in lesioned vehicle-treated rats and that GM1 treatment failed to significantly attenuate this deficit. However, exogenous nerve growth factor provoked a significant increase (35% above control values) in cortical cholinergic presynaptic terminal size which was even further augmented by concurrent GM1 treatment (69% above control values). This trophic factor-induced increase in bouton size was confirmed using serial electron microscopy and computer-assisted three-dimen-sional reconstruction of the cholinergic varicosities. The number of synaptic contacts in cortical layer V was also found to be significantly reduced (45% of control values) in lesioned vehicle-treated rats but was maintained at control levels by exogenous GM1 treatment. In addition, a significant increase (95% above control levels) in the number of choline acetyltransferase-immunoreactive boutons with synaptic differentiations was noted in lesioned nerve growth factor-treated rats. Concurrent GM1 and nerve growth factor treatment did not cause a further increase in synaptic number. Cortical cholinergic innervation and the size of choline acetyltransferase-immunoreactive nucleus basalis magnocellularis neurons were not altered in unlesioned rats by GM1, nerve growth factor or nerve growth factor and GM1 treatment. These results demonstrate that GM1 can attenuate deficits in cortical cholinergic innervation following injury and can further augment nerve growth factor induced synaptic remodelling in the injured adult rat brain.

Analysis of repolarization of presynaptic motor terminals in Drosophila larvae using potassium-channel-blocking drugs and mutations.

In Drosophila melanogaster muscles and neuronal cell bodies at least four different potassium currents have been identified whose activity shapes the electrical properties of these cells. Potassium currents also control repolarization of presynaptic terminals and, therefore, exert a major effect on transmitter release and synaptic plasticity. However, because of the small size of presynaptic terminals in Drosophila, it has not been possible to analyze the potassium currents they express. As a first approach to characterizing the ionic currents present at presynaptic motor terminals of Drosophila larvae, we recorded synaptic currents at the neuromuscular junction. From the alterations in evoked synaptic currents caused by various drugs and by mutations known to affect potassium currents in other tissues, we suggest that the repolarizing mechanism in presynaptic terminals consists of at least four distinct currents. One is affected by aminopyridines or Sh mutations, a second component is affected by the slo mutation, a third is sensitive to quinidine and one or more additional components are blocked by tetraethylammonium. Depolarization depends on a presynaptic calcium current, which displays only slight voltage-dependent inactivation. Because the mechanism of repolarization exerts a major effect on synaptic activity, this analysis provides a framework for further genetic and molecular dissection of the basic processes involved in the regulation of transmitter release.

Synaptic morphometry and synapse‐to‐neuron ratios in the superior colliculus of albino rats

The superior colliculus of mammals is generally divided into seven layers on the basis of the distribution of myelinated fibers, which are densely packed in layers III, V, and VII but sparse in the other layers. The laminar distribution of afferents and efferents allows, in addition, for the distinction of a superficial visual zone (layers I-III) and a deeper multimodal and premotor zone (layers IV-VII). Collicular neurons, however, do not show a lamination pattern, but are rather homogeneously distributed with only gradual transitions (Albers et al.: J. Comp. Neurol. 274:357-370, '88). The present study analyses whether the distribution of collicular synapses is correlated with the laminar organization of collicular axons or rather with the more homogeneous distribution of collicular neurons. For this purpose, the size and density of synaptic terminals and contacts as well as synapse-to-neuron ratios were determined in all collicular layers of albino rats by means of quantitative analysis of electron microscopic pictures. The size of presynaptic terminals and contacts does not differ significantly between individual collicular layers. On average, presynaptic terminal diameter is 1,079 nm, and synaptic contact size 338 nm, while 23% of all contacts are of the symmetrical type with pleiomorphic vesicles. The average numerical synaptic density is 422 million per mm3. This value is significantly higher in layers I and II (on average 670 million per mm3) than in layers III-VII (on average 370 million per mm3). The synapse-to-neuron (S/N) ratios calculated show that collicular neurons have on average 6,120 synaptic contacts on their receptive surface. The S/N ratio is lowest in layer III (4,330), while this ratio is highest in layers I and VII (i.e., 8,970 and 8,560 respectively). Layer II has a significantly higher S/N ratio than layer III (i.e., 8,060 and 4,330, respectively). Our results show that the size of synaptic terminals and contacts is not correlated with the different connectivity patterns of the distinct collicular layers. However, the density of synapses as well as the synapse-to-neuron ratios show a certain degree of laminar differentiation. In particular the superficial visual zone appears to be inhomogeneous in this respect, since layers I and II have a significantly higher density of synapses and higher S/N ratios than layer III. The deeper collicular zone is more homogeneously organized with synaptic densities similar to that of layer III and gradually increasing synapse-to-neuron ratios from layer IV to layer VII.

Monocular deprivation alters the development of synaptic structure in the ectostiatum of the zebra finch

The effects of monocular deprivation, beginning at hatching, were examined in the neuropil of ectostriatum, a visual telencephalic projection area in birds. The volume of ectostriatum, the number of synapses and subsynaptic features like the presynaptic terminal size and the length of the postsynaptic contact zone were quantified in juvenile (20 d) and adult (100 d) zebra finches. Monocular deprivation affects almost all of the parameters mentioned above in juvenile birds, but only one (i.e., the size of presynaptic terminals) is permanently altered in adulthood. Both hemispheres are affected in juvenile birds with respect to the volume of ectostriatum, the length of synaptic contact zones and the presynaptic terminal size when compared to normal values. In normal birds the number of synaptic contacts is established at 20 days and remains fairly constant at 100 days. In experimental birds there is an increase in synapse number during this time period. However, no interhemispheric differences or differences compared to normal animals could be identified. The presynaptic terminals in experimental birds are smaller compared to normal values in young (25% for the deprived side; 19% for the non-deprived side) and adult (13% for the deprived side; 11% for the non-deprived side) animals. The only permanent effect caused by monocular deprivation in the ectostriatum is characterized by smaller presynaptic terminals. It is surprising that the tectofugal pathway that is believed to be mostly ipsilateral is not very vulnerable to monocular deprivation in adult animals. It is even more surprising that the deprivation effects are seen on both sides of the brain. The implications of this plasticity will be discussed in this paper.

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.

Ultrastructural effects of monocular deprivation in the neuropil of nucleus rotundus in the zebra finch: a quantitative electron microscopic study

Ultrastructural effects of monocular deprivation starting at hatching have been studied in the neuropil of nucleus rotundus, the thalamic visual relay station of the tectofugal pathway in birds. Synaptic density, presynaptic terminal size, and length of postsynaptic density (PSD) have been quantified in juvenile (20-day) and adult (100-day) zebra finches. These parameters are mature in 20-day-old zebra finches when reared under normal conditions. Alterations obtained by monocular deprivation were: (1) The synaptic density increases by 35% in the nucleus rotundus of both sides of the brain above normal values in juvenile birds. In adult birds only the deprived side maintains this hypertrophy of synaptic density (33%), the non-deprived side returns to normal values. (2) The presynaptic terminal size remains small in the deprived nucleus of 20-day- and 100-day-old animals, whereas the non-deprived nucleus is not affected. (3) By the age of 20 days the length of PSD in deprived and non-deprived nuclei is not reduced as much as in normally reared zebra finches. By the age of 100 days, however, the PSDs of both sides of monocularly deprived birds show a further reduction of their median length and do not differ from PSDs of zebra finches reared under normal conditions.

Posthatching development of synapses in the neuropil of nucleus rotundus of the zebra finch: a quantitative electron microscopic study.

The development and maturation of synapses in the nucleus rotundus of the zebra finch were examined at 1, 5, 10, 20, and 100 days posthatching. Quantitative ultrastructural techniques were applied to investigate synaptic density, size of presynaptic terminals, and length of postsynaptic thickenings. During development there is a steady increases in the number of synapses and an enlargement of the presynaptic terminals. The length of the postsynaptic thickenings, however, decreases significantly during posthatching development. All three parameters reach adult values within 20 days of age. A close relationship was found between the enlargement of the presynaptic terminals and the increase in the number of contact zones up to the 20th day. The general feature of this developmental progress is in good agreement with biochemical and, to a certain extent, behavioral studies.