Recovery of synapses in axotomized adult cat spinal motoneurons after reinnervation into muscle

 Peripheral axotomy of adult cat spinal motoneurons induces a marked loss of synaptic boutons from the cell bodies and dendritic trees. The aim of the present study was to analyze the recovery of synaptic contacts in axotomized motoneurons following reinnervation into muscle. Adult cat spinal motoneurons were first deprived of their muscular contacts for 12 weeks and, then, allowed to reinnervate their target muscle. Two years later, regenerated motoneurons were labeled with horseradish peroxidase to allow quantitative ultrastructural analyses of the synaptic covering of the cell bodies and dendrites. Presynaptic boutons were classified according to their size and the shape of their synaptic vesicles. Results show that a recovery of synaptic covering occurs in the axotomized neurons after muscle reinnervation, but it affects various bouton types to different degrees. The number of S-type boutons synapsing with the soma was 70% higher after reinnervation than at 12 weeks after axotomy, while the number of F-type boutons had increased by only 13%. Compared with the normal situation, the number of S-type boutons synapsing with the proximal dendrites increased from 82% at 12 weeks after axotomy to 180% in the reinnervated state. In conclusion, in adult cat spinal motoneurons, the reestablishment of muscular contact is followed by a normalization of some of the synaptological changes induced by a prolonged state of axotomy. In certain respects restitution is incomplete, but in others it results in overcompensation. Received: 10 December 1997 / Accepted: 30 July 1998     

Relation between structural and release parameters at the young rat sensorimotor connection

The present work was carried out on isolated spinal cords of young rats. The aim of this study was the combined morphological and electrophysiological investigation of sensorimotor connections labelled with horseradish peroxidase and the evaluation of the relationship between their structural and functional properties. Sensorimotor contacts were widely distributed along the postsynaptic cell: from the soma and juxtasomatic dendrites to distal dendrites. The number of contacting boutons in the connection of a single afferent fibre and an individual motoneuron was about 10. The amplitude fluctuation patterns of the unitary and the minimal excitatory postsynaptic potentials of the motoneurons fitted with predictions based on a binomial model. A close correspondence was found between the estimated number of binomial release sites, n, and the number of contacting boutons. The calculated size of the quantal potential was about 100 microV. The difference in the organization of sensorimotor connections of the young rat and the frog is discussed.     

Compensation in the number of presynaptic dense projections and synaptic vesicles in remaining parallel fibres following cerebellar lesions

Summary Our previous investigations demonstrated an increase in the size of remaining synaptic sites as an intermediate or possible alternative to sprouting plasticity. The total amount of postsynaptic contact area remained relatively constant for each target neuron even though there was a marked decrease in the number of sites on these neurons. In addition, enlarged boutons containing numerous synaptic vesicles were positioned adjacent to enlarged postsynaptic sites.The question posed by this study was to determine whether dense projections, parts of the presynaptic grids of the remaining parallel fibres, spread to cover the enlarged postsynaptic sites, or if the number of these densities increased on each site to maintain the structural organization of the presynaptic grid. In addition, the number of synaptic vesicles per bouton was quantitated to determine whether they compensated by increasing their number in relationship to the increased area of the presynaptic grid.The number of parallel fibre synapses on Purkinje cells was reduced by transection of a narrow bundle of parallel fibres accompanied by a small lesion undercutting the molecular layer to destroy granule cells contributing to this bundle. The number of presynaptic dense projections was quantitated in control and lesioned preparations (using ethanolic phosphotungstic acid staining) in order to determine their correlation to the area of each site. In addition, the average number of synaptic vesicles in boutons was compared to the average size of boutons and the average contact area of the synaptic sites. At 3 to 7 days following partial deafferentation of Purkinje cells in adult rats, the density of dense projections of parallel fibre synapses on Purkinje cell spines remained uniform. This occurred throughout a range of reduction in the number of synapses in conjunction with a reciprocal increase in the size of sites. The finding of a uniform density of these projections and an increase in the size of sites implies that each granule cell axon must gain dense projections. In addition, the remaining presynaptic boutons had a uniform density of synaptic vesicles even though the volume of the boutons and the area of the synaptic contact doubled. Thus, the number of synaptic vesicles gained in proportion to the total enlargement of the contact site and the bouton size.These results strongly suggest that deficits or losses in synaptic connections of parallel fibre on Purkinje cell spines produces a compensation in the total number of synaptic vesicles and presynaptic dense projections of the remaining boutons. An enlargement of the presynaptic grid occurs in concert with redistribution of the constant total area of membrane occupied by macromolecules (or insertion of new ones) on remaining postsynaptic sites. These compensations could be facilitating efficacy of neuronal connections after lesions or neuronal attrition by re-establishing available transmitter and release sites in proportion to the constant amount of receptor area.     

Morphometric synaptology of gracilo-diencephalic relay cells: an electron microscopic study in the cat using retrograde transport of horseradish peroxidase

Summary Gracilo-diencephalic relay cells were identified with the aid of retrograde transport of horseradish peroxidase. Section embedding permitted light-microscopic identification and selection of relay cells and adjacent unlabelled neurons for ultrastructural study. Electron-microscopic analysis of the boutons which contacted the perikaryal surface of the cells revealed a statistically significant positive relationship between bouton covering ratio and cell body size for both labelled and unlabelled neurons. Estimates of the density of boutons also showed a positive correlation between the size of the cell body and the density of terminals contacting its surface. The functional implications of these findings are discussed.     

The development of the inferior olivary complex in preweanling opossums

Summary Pre- and postsynaptic elements within the developing inferior olive (IO) of both control and experimental opossums were examined via electron microscopy. Electron dense boutons identified di-/mesencephalic, cerebellar and spinal afferents within the IO of 8–71 day old animals, which survived 4–48 hours following either midbrain hemisections or spinal transections.During its initial stage of development (3–22 days) the neuropil of the IO is segregated into fields of small diameter neurites or flocculent profiles. Within the fields of flocculent profiles, synaptic interactions are established, which are both infrequent and immature. Although some flocculent profiles are presynaptic, most are postsynaptic and emanate from olivary somata and dendrites. Synaptic contacts also occur with olivary somata, dendritic shafts, spines and dendritic varicosities. Clear round vesicles (crv's; 40 m) predominate within all boutons, normal ones as well as those which degenerate after di-/mesencephalic, cerebellar and spinal lesions; however, larger (70 m) dense cored vesicles (dcv's) are occasionally observed within some boutons. Degenerating terminals from all three sources primarily contact flocculent profiles and dendritic shafts.As the opossum matures (42 days) dramatic increases occur in the number and complexity of both pre- and postsynaptic elements. Marked variations are observed in the matrix density of dendritic shafts. Although all terminal boutons predominantly contain crv's, the number of dcv's within the population of presynaptic elements increases markedly. Concurently, olivary neurons are profusely studded with spines. Simple dendritic spines and spiny appendages as well as dendritic shafts are the most frequent postsynaptic structures within the principal nucleus (PO). Olivary somata and their spines, however, are postsynaptic to degenerating de-/mesencephalic afferents within the PO. Flocculent profiles, which persist within the accessory nuclei, and dendritic shafts are postsynaptic to degenerating spinal boutons.By 70 days of age synaptic contacts appear more mature and more nearly approximate those seen in the adult (King 1980). Few somatic contacts, opaque dendrites, dendritic varicosities, and flocculent profiles are evident within the PO. Dendritic shafts and spines are the principal postsynaptic structures. Many di-/mesencephalic and cerebellar afferents synapse within maturing synaptic clusters on spines between which a rare gap junction is observed. Other di-/mesencephalic and cerebellar endings in the PO as well as spinal endings in the accessory nuclei are presynaptic to dendritic shafts and spines external to synaptic clusters. This predilection for contacting more specific loci on olivary neurons provides good evidence for synaptic remodeling.As the olivary nuclei develop further, the incidence of gap junctions increases and pleomorphic vesicles appear within boutons. The glial investment of neuronal elements, including synaptic clusters, also becomes more extensive.In conclusion, early di-/mesencephalic, cerebellar and spinal synaptic contacts appear qualitatively uniform in their synaptic features and postsynaptic interactions. As olivary development proceeds, however, the distinguishing synaptic features of the nuclear complex become more apparent. Synaptic remodeling occurs as some midbrain and cerebellar terminals are localized within synaptic clusters. The ultrastructural features characteristic of the adult IO are finally achieved by 80 days of age.This research was supported by N.I.H. Research Grant NS-08798     

Combined morphological and electrophysiological description of connections between single primary afferent fibres and individual motoneurons in the frog spinal cord

In experiments on the isolated frog spinal cord the relationship between the statistical properties of the unitary EPSP and the number of synaptic contacts was determined when the primary afferent fibre used in evoking the EPSP and the motoneuron in which it was recorded were both stained with HRP. The size of the chemical component of the EPSP corresponds to the number of presynaptic boutons. Less obvious numerical correlation exists between the number of contact zones and the number of binomial units.     

An ultrastructural study of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal boutons in the motor nucleus of spinal cord segments L7-S1 in the adult cat

The distribution and fine structure of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive synaptic boutons and varicosities were studied in the motor nucleus of the spinal cord segments L7-S1 in the cat, using the peroxidase-antiperoxidase immunohistochemical technique and analysis of ultrathin serial sections. The 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive boutons had a similar ultrastructural appearance as judged from serial section analysis. The boutons could be classified into two types on the basis of their vesicular content, with one type containing a large number of small agranular vesicles together with only a few, if any large granular vesicles, while the other type contained a large number of large granular vesicles in addition to small agranular vesicles. The vesicles were spherical or spherical-to-pleomorphic. Postsynaptic dense bodies (Taxi bodies) were occasionally observed in relation to all three types of immunoreactive boutons, which almost invariably formed synaptic junctions with dendrites. Judged by the calibre of the postsynaptic dendrites, the boutons were preferentially distributed to the proximal dendritic domains of motoneurons. In one case, a substance P-immunoreactive bouton formed an axosomatic synaptic contact. In addition to synaptic boutons, 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal varicosities containing a large number of large granular and small agranular vesicles but lacking any form of conventional synaptic contact were observed. Such varicosities were either directly apposing surrounding neuronal elements or separated from the neurons by thin glial processes. The origin of the immunoreactive boutons was not traced, but it was thought likely that the main source of the boutons was neurons with their cell bodies located in the medullary raphe nuclei.     

Phenotype changes of inferior olive neurons following collateral reinnervation

Inferior olive neurons are able to enlarge or retract their axonic terminal fields in response to changes in the extension of their target domain. Following Purkinje cell loss, the retraction of target-deprived climbing fibres is accompanied by a size reduction in the inferior olive neuron cell bodies. Here, we asked whether perikaryal modifications also occur when inferior olivary neurons enlarge their terminal fields to innervate supernumerary targets. To achieve this aim, we carried out a morphometric analysis on the somatic compartment of inferior olive neurons in two experimental conditions known to induce an expansion of their terminal field, i.e. a subtotal 3-acetylpyridine inferior olive lesion in the adult and a unilateral transection of the inferior cerebellar peduncle in newborn rats. In both experimental conditions, the inferior olive neurons that survived the lesion showed a remarkable increase in cell body and nuclear size, although the latter change was less pronounced in the 3-acetylpyridine-treated animals. These results show that both developing and mature inferior olive neurons are capable of adjusting their perikaryal phenotype to match the modifications of their target size.     

GABA- and glycine-immunoreactive terminals contacting motoneurons in lamprey spinal cord

Double postembedding GABA- and glycine-immunostaining was performed on the lamprey (Lampetra fluviatilis) spinal cord after previous HRP labeling of motoneurons. Immunopositive boutons contacting motoneurons were counted and distinguished as GABA (39%), glycine (30%) and both GABA+glycine-immunopositive (31%). Densely-packed, flattened synaptic vesicles were only observed in glycine-immunopositive boutons while GABA-immunoreactive and GABA+glycine-immunoreactive boutons contained rounded or oval synaptic vesicles. Dense-core vesicles of different diameters were associated with conventional synaptic vesicles in 74% of GABA-only-immunopositive boutons, 50% of double GABA+glycine-immunopositive boutons, but were only observed in 9% of glycine-only-immunopositive boutons. The presence of terminals immunoreactive to either GABA or glycine contacting the motoneurons suggests that there is a morphological substrate for both GABAergic and glycinergic postsynaptic inhibition of motoneurons in the lamprey spinal cord.     

Phenylethanolamine N-methyltransferase-immunoreactive terminals synapse on adrenal preganglionic neurons in the rat spinal cord

Adrenergic neurons in the C1 region in the ventrolateral medulla oblongata send descending axons into spinal cord which terminate in thoracic and upper lumbar segments, overlapping the distribution of sympathetic preganglionic neurons. The present study was undertaken to determine whether adrenergic fibers synapse directly on preganglionic neurons which innervate the adrenal medulla and to examine the ultrastructure of these fibers during development. The ultrastructure and synaptology of adrenergic axons in the intermediolateral nucleus of mid-thoracic spinal cord were studied in 7-, 9-, 24-, 30-, 60-, and 90-day-old rats using immunocytochemical staining for phenylethanolamine N-methyltransferase, the epinephrine-synthesizing enzyme. Phenylethanolamine N-methyltransferase-immunoreactivity was observed in the cytoplasm of unmyelinated axonal varicosities and intervaricose segments in the neuropil of intermediolateral nucleus. Phenylethanolamine N-methyltransferase-immunoreactive synaptic boutons were filled with spherical electron-lucent vesicles and occasional larger dense-core vesicles. These boutons were observed to form symmetrical synaptic contacts with dendritic processes at all ages examined. Asymmetrical synapses on dendrites were also observed in adult rats. Axosomatic synaptic contacts were frequently observed in immature rats, but were never observed in adult rats. To determine whether adrenergic axons synapse on preganglionic neurons which project to the adrenal medulla, adrenal preganglionic neurons were retrogradely labeled with horseradish peroxidase and adrenergic axons were stained for phenylethanolamine N-methyltransferase-immunoreactivity. In young rats, phenylethanolamine N-methyltransferase-immunoreactive boutons were observed to form symmetrical axosomatic and axodendritic synaptic contacts with adrenal preganglionic neurons in intermediolateral nucleus. These contacts had already formed by postnatal day 7, the youngest age studied. In contrast, it was not possible to verify that adrenal preganglionic neurons receive adrenergic innervation in adult rats, since phenylethanolamine N-methyltransferase-immunoreactive boutons were only observed in contact with small diameter dendrites that were not retrogradely labeled by horseradish peroxidase. These studies demonstrate that adrenal preganglionic neurons receive adrenergic synapses prior to the first postnatal week. The initial synapses which form on preganglionic somata and proximal dendrites appear to reorganize late in development. It is suggested that these become more distally located as the dendritic tree matures. More generally, these observations suggest that adrenergic bulbospinal neurons are involved in central regulation of adrenal development and function.     

Ultrastructural observations in the frog spinal cord in relation to the generation of primary afferent depolarization

The spatial distribution of different types of synaptic boutons over the motoneuron cell body and dendrites was studied both in the normal and in the chronically deafferented frog spinal cord. In both preparations, endings with S-type (spherical) vesicles were more numerous than those with F (flat) vesicles, the $\text{S}\text{F}$ synapse ratio being approximately constant when measured at the soma, the proximal dendrites or the fine dendrites. However, the proportion of S boutons, as well as the proportion of degenerated boutons of afferent fibers in close apposition with other boutons, was higher on the fine dendrites in the lateral neuropile than on the motoneuron cell body and proximal dendrites, and in all cases significantly higher than the proportion of F boutons showing close appositions with other boutons. Degenerated boutons from descending fibers were found to synapse over the motoneuron cell body and proximal dendrites, but these boutons were rarely seen to be closely apposed to other boutons. The possible relation of these findings to the mechanisms generating primary afferent depolarization is discussed.     

Neuronal ultrastructure and somatostatin immunolocalization in the ciliary ganglion of chicken and quail

Summary Ciliary and choroid neurons of the avian ciliary ganglion innervate different targets in the eye bulb. By light microscopic immunocytochemistry, somatostatin (SOM) has been localized to a subset of ganglionic neurons believed to be, for the most part, choroid neurons. Although several studies have been published on the physiology, afferent and efferent innervation, and response to experimental injury of this population of cells, their morphological features are still unclear. This has led us to perform a fine structural and immunocytochemical study on the ciliary ganglia of adult chickens and quails to provide the first thorough characterization of the choroid neurons and to analyze whether or not they can be unequivocally identified by expression of SOM. Here, we show that standard and immuno-electron microscopy provide firm criteria for the distinction of ciliary and choroid neurons, whose populations overlap in cell size and territory of distribution. The satellite cell sheaths form compact myelin lamellae around ciliary neurons and flattened processes around choroid neurons. Moreover, ciliary neurons are innervated by a larger number of boutons than choroid neurons. Chicken ciliary neurons are invested by boutons only over one pole of the cell body, while their quail counterparts have an almost complete shell of presynaptic boutons over the entire cell body. Ciliary neurons form mixed synaptic junctions (chemical and electrical), while choroid neurons form only chemical synapses. Crest synapses are present in ciliary neurons of both species. Nematosomes occur in both ciliary and choroid neurons. Choroid neurons contain a larger complement of large dense core vesicles than ciliary neurons and their Golgi apparatuses are more prominent. In the light microscope, somatostatin-immunostaining appears noticeably different in the two species: mostly granular in the chicken and skein-shaped in the quail. Immuno-electron microscopy reveals that somatostatin-like immunoreactivity is localized to Golgi apparatus and large dense core vesicles. Somatostatin is expressed by all the choroid neurons, but not by the ciliary neurons. This neuropeptide is, therefore, a true cell population marker.     

Exogenous brain-derived neurotrophic factor regulates the synaptic composition of axonally lesioned and normal adult rat motoneurons

Brain-derived neurotrophic factor has previously been shown to promote survival and axonal regeneration in injured spinal motoneurons and, also, to modulate synaptic transmission and regulate the density of synaptic innervation in a variety of neurons. The present light and electron microscopic study demonstrates synaptotrophic effects of exogenously applied brain-derived neurotrophic factor on the synaptic composition of both normal and axonally lesioned adult rat spinal motoneurons. After L5-L6 ventral root avulsion, a massive loss of all types of boutons occurred on the somata of the lesioned motoneurons which persisted for at least 12 weeks postoperatively. We found that (i) intrathecal infusion of brain-derived neurotrophic factor during the first postoperative week did not prevent the synaptic detachment and activation of glial cells; (ii) prolonged treatment for four weeks restored synaptic covering and significantly reduced microglial reaction; (iii) the synaptotrophic effect remained significant for at least eight weeks after cessation of the treatment; (iv) brain-derived neurotrophic factor mainly supported F-type boutons with presumably inhibitory function, while it had little effect on S-type boutons associated with excitatory action; and (v) in normal unlesioned motoneurons, four weeks of treatment with brain-derived neurotrophic factor induced sprouting of F-type boutons, a loss of S-type boutons and motoneuron atrophy.The present data show that exogenous neurotrophins not only help to restore synaptic circuitry in axonally injured motoneurons, but also strongly influence the synaptic composition in normal motoneurons.     

Target-cell-dependent plasticity within the mossy fibre-CA3 circuit reveals compartmentalized regulation of presynaptic function at divergent release sites

Individual axons of central neurons innervate a large number of distinct postsynaptic targets belonging to divergent functional categories such as glutamatergic principal cells and inhibitory interneurons. While each bouton along a common axon should experience the same activity pattern in response to action potential firing within the parent presynaptic neuron, accumulating evidence suggests that neighbouring boutons contacting functionally distinct postsynaptic targets regulate their release properties independently, despite being separated by only a few microns. This target-cell-specific autonomy of presynaptic function can greatly expand the computational prowess of central axons to allow for precise coordination of large neuronal ensembles within a given circuit. An excellent example of target-cell-specific presynaptic mechanisms occurs in the CA3 hippocampus where mossy fibre (MF) axons of dentate gyrus granule cells target both principal cells and local circuit inhibitory interneurons via both anatomically and functionally specialized terminals. Of particular interest, mechanisms of both short- and long-term plasticity remain autonomous at these divergent release sites due to an anatomical and biochemical segregation of discrete molecular signalling cascades. Here we review roughly a decades worth of research on the MF–CA3 pathway to showcase the target-cell dependence of presynaptically expressed NMDA receptor-independent synaptic plasticity.     

Vesicular glutamate transporter 1 and vesicular glutamate transporter 2 synapses on cholinergic neurons in the sublenticular gray of the rat basal forebrain: a double-label electron microscopic study

The basal forebrain (BF) comprises morphologically and functionally heterogeneous cell populations, including cholinergic and non-cholinergic corticopetal neurons that are implicated in sleep–wake modulation, learning, memory and attention. Several studies suggest that glutamate may be among inputs affecting cholinergic corticopetal neurons but such inputs have not been demonstrated unequivocally. We examined glutamatergic axon terminals in the sublenticular substantia innominata in rats using double-immunolabeling for vesicular glutamate transporters (Vglut1 and Vglut2) and choline acetyltransferase (ChAT) at the electron microscopic level. In a total surface area of 30,000 μm², we classified the pre- and postsynaptic elements of 813 synaptic boutons. Vglut1 and Vglut2 boutons synapsed with cholinergic dendrites, and occasionally Vglut2 axon terminals also synapsed with cholinergic cell bodies. Vglut1 terminals formed synapses with unlabeled dendrites and spines with equal frequency, while Vglut2 boutons were mainly in synaptic contact with unlabeled dendritic shafts and occasionally with unlabeled spines. In general, Vglut1 boutons contacted more distal dendritic compartments than Vglut2 boutons. About 21% of all synaptic boutons (n=347) detected in tissue that was stained for Vglut1 and ChAT were positive for Vglut1, and 14% of the Vglut1 synapses were made on cholinergic profiles. From separate cases stained for Vglut2 and ChAT, 35% of all synaptic boutons (n=466) were positive for Vglut2, and 23% of the Vglut2 synapses were made on cholinergic profiles. On average, Vglut1 boutons were significantly smaller than Vglut2 synaptic boutons. The Vglut2 boutons that synapsed cholinergic profiles tended to be larger than the Vglut2 boutons that contacted unlabeled, non-cholinergic postsynaptic profiles. The presence of two different subtypes of Vgluts, the size differences of the Vglut synaptic boutons, and their preference for different postsynaptic targets suggest that the action of glutamate on BF neurons is complex and may arise from multiple afferent sources.     

Morphological changes in the rat neostriatum after unilateral 6-hydroxydopamine injections into the nigrostriatal pathway

Destruction of the dopamine-containing neurons in the rat substantia nigra results in morphological changes in the striatum which have been characterized at both the light and electron microscopic levels. After a unilateral 6-hydroxydopamine injection into the medial forebrain bundle, Golgi-impregnated medium-sized spiny neurons in the neostriatum ipsilateral to the injection had a lower density of spines on their dendrites than those on the contralateral side. A similar decrease in spine density was apparent from 12 days until at least 13.5 months after the lesion. A bilateral loss of spines occurred with increasing age regardless of the presence or absence of the nigrostriatal dopaminergic pathway. At the ultrastructural level, the general pattern of synaptic input to the Golgi-impregnated medium-sized spiny neurons was similar on both sides of the brain. The most obvious class of afferent boutons contacting these spiny neurons formed prominent asymmetrical synaptic specializations with the heads of the spines. The numbers of asymmetric synaptic profiles counted in random electron micrographs from the striata ipsilateral and contralateral to the lesion were not significantly different from each other. A small but significant increase in the length of asymmetric synaptic specialization profiles was, however, detected in the striata lacking a dopamine input.     

GABAergic innervation of corticotropin-releasing hormone (CRH)-secreting parvocellular neurons and its plasticity as demonstrated by quantitative immunoelectron microscopy

GABA has been identified as an important neurotransmitter in stress-related circuitry mediating inhibitory effects on neurosecretory neurons that comprise the central limb of the hypothalamo-pituitary-adrenocortical axis. Using combinations of pre-embedding immunostaining and postembedding immunogold methods at the ultrastructural level, direct synaptic contacts were revealed between GABA-containing terminals and neurosecretory cells that were immunoreactive for corticotropin-releasing hormone (CRH) in the hypothalamic paraventricular nucleus (PVN). The vast majority of axo-dendritic GABA synapses was symmetric (inhibitory) type, and 46% of all synaptic boutons in the medial parvocellular subdivision of the PVN were immunoreactive to GABA. Using the disector method, an unbiased stereological method on serial ultrathin sections, the total calculated number of synaptic contacts within the medial parvocellular subdivision of the PVN was 55.4 x 10(6)/mm(3). On CRH-positive profiles 20.1 x 10(6) GABAergic synaptic boutons were detected per mm(3) in control, colchicine-treated rats. In the medial parvocellular subdivision, 79% of GABAergic boutons terminated on CRH neurons. Following adrenalectomy, which increases the synthetic and secretory activities of CRH neurons, the number of GABAergic synapses that terminate on CRH-positive profiles was increased by 55%. GABA-containing boutons appeared to be swollen, while the contact surfaces of cellular membranes between GABAergic boutons and CRH-positive profiles were shorter in adrenalectomized animals than in controls.Our data provide ultrastructural evidence for direct inhibitory GABAergic control of stress-related CRH neurons and suggest a pivotal role of GABA-containing inputs in the functional plasticity of parvocellular neurosecretory neurons seen in response to adrenalectomy.     

Neurofibrillary tangle-related synaptic alterations of spinal motor neurons of P301L tau transgenic mice

We investigated axosomatic synapses of anterior horn cells of transgenic (TG) mice expressing mutant P301L human tau and non-transgenic (NTG) mice using electron microscopic methods to demonstrate the relationship between neurofibrillary tangles (NFTs) and synaptic alterations. Animals aged 3.5-8.5 months were used because at this age many motor neurons in TG mice have NFTs. We measured the perimeter of anterior horn cell perikarya, the number of boutons and total length of boutons in contact with the neuronal perikarya from the micrographs of NFT and non-NFT-bearing neurons. We also calculated the proportion of the perimeter covered by boutons, density of boutons and mean size of boutons. The density of synaptic boutons in contact with NFT-bearing neurons was significantly decreased compared to non-NFT-bearing neurons. These findings suggest that synaptic reduction occurs during neurofibrillary degeneration and is probably associated with NFT. In addition, synaptic boutons were detached from NFT-bearing neurons with the resulting space occupied by astrocytic processes, suggesting that astrocytes may be involved in the observed synaptic alterations.     

Fine structure and neurotransmitter cytochemistry of neurons in the rat ventral cochlear nucleus projecting to the ipsilateral dorsal cochlear nucleus

The neural tracer wheat germ agglutinin conjugated to horse radish peroxidase was injected into the rat dorsal cochlear nucleus and acoustic stria. Some labelled neurons in the ipsilateral ventral cochlear nucleus were found as a result. These neurons were studied at the ultrastructural level, and their axo-somatic synaptic profile and glycine immunoreactivity were determined. Most neurons were glycine negative and classified as type I multipolar neurons. The latter showed a different synaptic profile from that of neurons projecting to the contralateral inferior colliculus or cochlear nucleus. This suggests the presence of differing populations of multipolar cells based on their synaptic profile. Few labelled multipolar neurons of type II were found, which appeared glycine negative and, rarely, glycine positive. The latter show an ultrastructure and axo-somatic profile similar to that of glycinergic commissural neurons in the dorsal and ventral cochlear nucleus. In particular, about one-third of boutons contained round synaptic vesicles, which are believed to contain an excitatory neurotransmitter. The ultrastructural analysis of the synaptic boutons in the cochlear nucleus confirms the presence of numerous cases of colocalization of glycine and GABA where flat and pleomorphic synaptic vesicles are mixed. The present study is in accordance with previous tract-tracing light microscopic studies which have indicated that large glycinergic neurons in the ventral cochlear nucleus act as broad-band inhibitory neurons in microcircuits of the dorsal cochlear nucleus and contralateral cochlear nucleus.