Identification of adrenoceptor subtype-mediated changes in the density of synapses in the rat visual cortex

Both serotonin and noradrenaline affect synapse formation and maintenance in the CNS. Although we previously demonstrated that serotonin regulates synaptic density via activation of serotonin(2A) receptor, it was still unclear which receptor subtype mediates the function of noradrenaline. In the present study we tried to identify the noradrenaline receptor (adrenoceptor) subtype, which could regulate the density of synapses in the rat visual cortex. Selective antagonists and/or agonists of adrenoceptor subtypes were administered to six weeks old rats. Changes in the density of axodendritic synapses were quantitatively examined in lamina I, where noradrenaline rather than serotonin is known to regulate the density of synapses. The alpha1 adrenoceptor antagonists (prazosin and 2-{[b-(4-hydroxyphenyl)ethyl]aminomethyl}-1-tetralone) decreased the number of synapses in a dose-dependent manner. In contrast, administrations of the alpha1-agonist (methoxamine) increased the density of synapses. The beta1 adrenoceptor antagonist (atenolol) had no effect on the density of synapses. The alpha2-antagonist (rauwolscine) increased synaptic density, whereas the beta2-antagonist (ICI-118,551) decreased synaptic density. Simultaneous treatments with the alpha1-antagonist and alpha1-agonist caused the alpha1-agonist to competitively block the effect of the alpha1-antagonist and recover the density of synapses to the control values. In addition, the alpha1-antagonist/agonist appeared to show a reverse effect on the changes in synaptic density following alpha2- or beta2-antagonist treatment by acting via the alpha1 receptor. Moreover, decreased synaptic density when a selective noradrenergic neurotoxin (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) was counterbalanced by the alpha1-agonist. These data suggest that noradrenaline regulates the density of synapses in the rat visual cortex primarily via the alpha1 receptor subtype. Both serotonin(2A) and alpha1 receptors are known to couple with phospholipase C, which has been shown to increase intracellular calcium. It may help us to understand the underlying mechanisms for synaptic plasticity in the CNS.     

Mechanisms for formation and maintenance of synapses mediated by biogenic amines: pathogenesis and therapy of mental retardation and developmental disabilities by genetic and epigenetic factors

Fibers of the global projection system ramify tremendously and distribute in the diverse region of the brain. Biogenic amines in the global projection system have been shown to facilitate formation and maintenance of synapses in the developing and adult brain.In terms of serotonin 5-HT2A receptor was shown to mediate the function of serotonin. We raised specific antibodies against 5-HT2A receptor protein. Virtually all the neurons in the cerebral cortex expressed 5-HT2A receptor. By using the function of biogenic amines to facilitate synapse formation and maintenance a novel approach can be developed in the neuroscience. That is to perturb biogenic amines, to change synaptic density, and to examine changes in the ability of learning and memory. Removing serotonin and acetylcholine for a week, at the maximum 58% of synapses are decreased in the hippocampus. The animals losing synapses spent a longer latency compared to intact animals in Morris water maze. The level of biogenic amines in the developing brain has been known to decrease tremendously by genetic diseases such as phenylketonuria, Down syndrome and autism as well as environmental factors such as nutrition and stress. In those situations synapses in the brain are suggested to be decreased. Synaptic mechanism for mental retardation and developmental disability by the cascade appears to contribute for understanding pathophysiology and a new therapy.     

Structural maturation of synapses in the rat superior cervical ganglion continues beyond four weeks of age.

We have examined the morphology of preganglionic synapses in the rat superior cervical ganglion (SCG) at 10 days, 4 weeks and 1 year. Between 10 days and 4 weeks the mean thickness of the postsynaptic density (PSD) increased from 45.9 +/- 0.1 nm to 52.1 +/- 1.7 nm (P = 0.017), the mean length of the PSD (0.41 +/- 0.02 microns) did not change, and the distribution of synapses on the neuronal surface changed with a decrease in the proportion of somatic and an increase in the proportion of dendritic spine synapses. Since both synapse elimination and synapse formation are occurring during this period several mechanisms may contribute to these changes. However, between 4 weeks and 1 year, when there is no net change in the number of synapses, the mean length of the PSD increased to 0.53 +/- 0.02 microns (P = 0.001), there was no change in either the mean thickness of the PSD or the distribution of the synapses but the proportion of concave ('smile') synapses increased. A comparison with previous developmental studies of synapses in cerebral cortex of rat and chicken indicate that both the nature and the rate of synapse maturation can vary between different populations of synapses.     

Developmental maturation of synaptic vesicle cycling as a distinctive feature of central glutamatergic synapses

The formation of chemical synapses in the mammalian brain involves complex pre- and postsynaptic differentiation processes. Presynaptically, the progressive accumulation of synaptic vesicles is a hallmark of synapse maturation in the neocortex [J Neurocytol 12 (1983b) 697]. In this study, we analyzed the functional consequences of presynaptic vesicle-pool maturation at central glutamatergic and GABAergic synapses. Using (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide (FM1-43) staining of recycling synaptic vesicles, we demonstrate a pronounced developmental increase in presynaptic vesicle accumulation during differentiation of neocortical neurons in culture. Using electrophysiological methods to study functional synaptic maturation, we found an improved recovery from hypertonic solution-induced depletion. As supported by the FM1-43 staining results, this change is most likely caused by a developmental increase in the number of reserve-pool vesicles. In addition, assuming a rapid reuse of freshly recycled vesicles, a developmental maturation of the endocytosis process may also contribute. The observed presynaptic maturation process occurred selectively at glutamatergic synapses, while GABAergic synapses did not show similar developmental alterations. Furthermore, we used high-frequency stimulation (HFS) of glutamatergic and GABAergic synapses to reveal the physiological consequences of reserve-pool maturation. As expected, recovery from HFS-induced depletion was incomplete at immature glutamatergic synapses and strongly improved during synapse maturation. Again, GABAergic synapses did not show similar developmental changes. Taken together, our study characterizes the functional consequences of a pronounced accumulation of reserve-pool vesicles occurring selectively at glutamatergic synapses.     

Birth,growth and elimination of a single synapse

Synapses are functional units regulating information flows in the neuronal circuits. How synaptic junctions are formed and remodelled is a fundamental question in developmental neurobiology. In recent years, it has become possible to visualize the formation, maintenance and remodelling of a single synapse by using new imaging methods. These studies, identifying synaptic structures by lipophilic dye markers and genetically modified synaptic molecules with fluorescent proteins, provided new insights into synapse development and maturation. Experimental evidence indicates very rapid assembly of both presynaptic and postsynaptic marker proteins at newly formed synaptic junctions. Morphological expansion of the synaptic junctional membrane is tightly coupled to both efficacy of the presynaptic neurotransmitter release and postsynaptic receptor distribution. The elimination process of pre-existing synapses has also been reported, and evidence for persistent remodelling of synaptic junctions has been provided. Information regarding birth, maturation and elimination of a single synapse is accumulating and will influence our concepts about how neuronal circuits are organized and maintained.     

Neonatal synapse elimination in the rat submandibular ganglion: effect of retarded target growth

1. We have studied synapse elimination in the submandibular ganglion of neonatal rats to determine the effects of retarded target growth on synaptic development. Neurons of this ganglion provide parasympathetic innervation to the submandibular and sublingual salivary glands. 2. Ligating the main salivary ducts 2-4 days after birth at a point where nerve fibers were not damaged reduces gland weight by 55% during the 2nd wk after birth and 80% by adulthood. 3. In control animals, the average number of preganglionic inputs/neuron normally declines steadily during the first few weeks after birth, before stabilizing during the 5th wk at the control adult level. Between birth and adulthood, the number of ganglionic neurons increases by 150%. 4. Ganglia from duct-ligated animals showed an acceleration in the process of synapse elimination. Input number in experimental ganglia reached the control adult level during the 3rd wk after birth. This acceleration is confined solely to ganglia that innervate the underdeveloped glands. 5. The loss of inputs was not further enhanced by prolonged target atrophy. Thus average input numbers to neurons of 5th wk or adult experimental ganglia were not different from age-matched control values. 6. No differences from control values were seen in most cases for resting potentials, input resistances, or cell size. However, the increase in neuron number was retarded in experimental animals, and the number of synapses/neuronal profile was reduced in the adult animals. 7. Thus subnormal target growth leads to an acceleration in the process of synaptic elimination in neonatal rats. This acceleration may be mediated by alterations in the level of trophic factors emanating from the target.     

Synaptogenesis in the dorsal lateral geniculate nucleus of the rat

Summary Synapse formation and maturation were examined in the rat dorsal lateral geniculate nucleus (dLGN) from birth to adulthood. Examination of animals, whose ages were closely spaced in time, showed that the maturation of the synaptic organization of the nucleus takes place chiefly during the first 3 weeks of postnatal life. This period of maturation may be divided into 3 broad stages. During the first stage, which spans the first 4 days of life, there are only a few immature synapses scattered throughout the nucleus; occasionally aggregates of 3 or 4 synapses are encountered. Dendrodendritic synapses first appear at the end of this stage. The second stage, which lasts from the end of the first stage through day 8, is characterized by intensive synaptogenesis as well as extensive growth and degeneration. For the first time, large boutons resembling retinal terminals form multiple synaptic contacts with dendrites and dendritic protrusions; these synaptic arrangements are partially covered by glial processes.A feature characteristic of the developing dLGN during the first 2 postnatal weeks, and particularly during the second stage, is the presence of membrane specializations that resemble vacant postsynaptic densities. These specializations, which may be unapposed or opposite another neuronal process, decrease in frequency as the number of synapses increases. It is not known whether these densities are converted to synapses or whether they result from loss of presynaptic elements.The third stage in the process of synaptogenesis, which spans a period between days 10 and 20, is characterized by myelination and by the diminution of growth cones, degenerating profiles and vacant postsynaptic densities. There is also a very significant increase in the number and maturation of synapses including synaptic glomeruli. However, it is not until the end of this stage that synapses appear qualitatively indistinguishable from synaptic arrangements identified in adult animals.     

Age-related changes of catecholamines and their metabolites content in the visual system of the rat

The changes in the content of the catecholamines in each structure of the geniculate and extrageniculate visual system of the rat during the aging period (6-30 months) have been studied. Dopamine was found at lower levels than noradrenaline in all the structures. The dopamine and noradrenaline showed different developmental profiles. Dopamine and its metabolite levels decreased in the lateral geniculate and visual cortex and increased in superior colliculus and posterior thalamus. Noradrenaline and its metabolites increased in all structures during the aging period. However, 3-methoxy-4-hydroxyphenylglycol/noradrenaline and normetanephrine/noradrenaline ratios decreased in all structures except in superior colliculus. These results suggest age-related changes in the catecholamines in the visual system of the rat.     

Interplay between neuromodulator-induced switching of short-term plasticity at sensorimotor synapses in the neonatal rat spinal cord

In the present study, we investigated the modulation of short-term depression (STD) at synapses between sensory afferents and rat motoneurons by serotonin, dopamine and noradrenaline. STD was elicited with trains of 15 stimuli at 1, 5 and 10 Hz and investigated using whole-cell voltage-clamp recordings from identified motoneurons in the neonatal rat spinal cord in vitro . STD was differentially modulated by the amines. Dopamine was effective at all stimulation frequencies, whereas serotonin affected STD only during 5 and 10 Hz stimulus trains and noradrenaline during 1 and 5 Hz trains. Dopamine and serotonin homogenized the degree of depression observed with the different stimulation modalities, in contrast to noradrenaline, which amplified the rate differences. The different modulatory profiles observed with the amines were partly due to GABAergic interneuron activity. In the presence of GABA_A and GABA_B receptor antagonists, the rate and/or kinetics of STD did not vary with the stimulation frequency in contrast to the control condition, and noradrenaline failed to alter either synaptic amplitude or STD, suggesting indirect actions. Dopamine and serotonin strongly decreased STD and converted depression to facilitation at 5 and 10 Hz during the blockade of the GABAergic receptors in 50% of the neurons tested. Altogether, these results show that STD expressed at sensorimotor synapses in the neonatal rat not only is a function of the frequency of afferent firing but also closely depends on the neuromodulatory state of these connections, with a major contribution from GABAergic transmission.     

A new form of synaptic plasticity is transiently expressed in the developing rat visual cortex: a modulatory role for visual experience and brain-derived neurotrophic factor.

Synaptic plasticity has been implicated in the mechanisms contributing to the shaping of the cortical circuits responsible for the transmission of the visual input in the rat primary visual cortex. However, the degree of plasticity of the thalamocortical synapse may change during development, perhaps reflecting the degree of stabilization of the circuitry subserving it. We have chosen the ability of this synapse to be first depressed and then potentiated as a specific indicator of its plasticity. In this study we have investigated how this parameter changes during development and the factors controlling it. Extracellular field potentials in cortical layers 2/3 were evoked by stimulation of the white matter in rat primary visual cortex slices prepared at different postnatal ages. Low-frequency stimulation (900 pulses at 1 Hz) of the white matter was used to induce long-term depression of field potential amplitude, whereas long-term potentiation was evoked by high-frequency stimulation consisting of three trains at 100 Hz. We provide evidence that while it is possible to potentiate previously depressed synapses soon after eye opening (postnatal day 17) this synaptic characteristic decreases rapidly thereafter. The decrease in this form of cortical synaptic plasticity closely matches the stabilization of the cortical circuitry towards an adult pattern of connectivity and function. Depressed cortical synapses cannot be potentiated in normal rats at postnatal 23, but they can be potentiated in rats reared in the dark from postnatal days 17 to 29. Moreover, application of brain-derived neurotrophic factor, known to be expressed in an activity-dependent manner, was able to restore the ability of synapses to be potentiated after long-term depression, thus indicating its important modulatory role in brain development.     

Visualizing synapse formation in arborizing optic axons in vivo: dynamics and modulation by BDNF

Dynamic developmental changes in axon arbor morphology may directly reflect the formation, stabilization and elimination of synapses. We used dual-color imaging to study, in the live, developing animal, the relationship between axon arborization and synapse formation at the single cell level, and to examine the participation of brain-derived neurotrophic factor (BDNF) in synaptogenesis. Green fluorescent protein (GFP)-tagged synaptobrevin II served as a marker to visualize synaptic sites in individual fluorescently labeled Xenopus optic axons. Time-lapse confocal microscopy revealed that although most synapses remain stable, synapses are also formed and eliminated as axons branch and increase their complexity. Most new branches originated at GFP-labeled synaptic sites. Increasing BDNF levels significantly increased both axon arborization and synapse number, with BDNF increasing synapse number per axon terminal. The ability to visualize central synapses in real time provides insights about the dynamic mechanisms underlying synaptogenesis, and reveals BDNF as a modulator of synaptogenesis in vivo.     

Synaptic remodelling and astrocytic hypertrophy in rat cerebral cortex from early to late adulthood

Ultrastructural observation of the molecular layer of the parietal cortex of rats, aged 3, 6, 10 and 17 months, revealed various atypical synaptic profiles besides typical synapses. The atypical synapses were frequently in the vacinity of hypertrophied astrocytic profiles, and were sometimes completely surrounded by astrocytic processes. The presynaptic terminal contained either no vesicles or a few small distored vesicles. Vacant postynaptic terminals were occasionally seen. The total surface area of astrocytic profiles and the numbers of atypical synapses increased significantly between 3 and 10 months. The astrocytic acquistion of degerating terminals was repeatedly observed over this period. Since there was no decrease in total synaptic number at this age, the astrocytic phenomenon may represent a stage in a continuous cycle of synaptic loss and replacement in the normal brain. By 17 months, when total synapse numbers decrease, synaptic replacement may be less than optimal.     

帕金森病大鼠模型额叶皮质的代谢及形态改变

目的：探讨帕金森病中大脑额叶皮质的形态及代谢改变。方法：大鼠实验组于6-OHDA毁损后用BIOSPEC47／30磁共振波谱仪(4．7T)，采用点分辨波谱法对双侧额叶皮质行H-MRS检测，分析该区N-乙酰天门冬氨酸／肌酸(NAA／Cr)和胆碱／肌酸(Cho／Cr)比值的变化，并用电镜观察该区神经元及突触的形态变化。结果：实验组大鼠损毁侧额叶皮质．NA．A／Cr比值显著低于对侧，对照组两侧无显著差别；Cho／Cr比值在两组的两侧相比均无显著差别。电镜观察显示：损毁侧额叶皮质的突触数量较对侧减少，突触前、后膜和突触小泡结构异常，典型突触结构消失，树突棘出现大片低电子密度区，细胞器消失。结论：帕金森病大鼠模型损毁侧额叶皮质内存在神经元缺失或突触数量的减少，突触结构异常及功能异常。     

Long-term callosal lesions and learning of a black-white discrimination by one-eyed rats.

We know from our previous studies that mature rats with monocular enucleation at birth (OEBs), as well as animals enucleated at maturity (OETs), were unable to learn a black-white discrimination when they were trained after lesions of the visual cortex contralateral to the remaining eye. Since it is well known that synaptic reorganization takes place in the adult rat brain through reactive synaptogenesis following deafferentation, we wondered if long-term callosal lesions in OEBs and OETs would bring out such synaptic reorganization in the visual cortex and, consequently, affect the outcome of the discrimination mentioned above. In the present study, two experiments were carried out: in Experiment 1 the previous experiment was replicated in that OEBs and OETs of 3 months of age were trained on the discrimination 10 days following unilateral visual cortex lesions; in Experiment 2, effects of callosal lesions made 10 weeks earlier either at 3 weeks of age or 13 weeks of age were investigated. The results were: 1) the findings of the previous experiment were confirmed; 2) the long-term callosal lesions facilitated the acquisition of discrimination in OEBs but not in OETs; 3) the facilitative effects were more prominent in OEBs with callosal lesions at 3 weeks of age than in those at 13 weeks of age. The findings were discussed in relation to possible synaptic reorganization produced in the visual cortex ipsilateral to the remaining eye following callosal lesions made 10 weeks earlier and also in relation to reorganization of the uncrossed visual pathways resulting from monocular enucleation at birth.     

Effect of partial sensory deprivation on monoaminergic neuromodulators in striate cortex of adult cat

The role of monoaminergic neuromodulators in the reorganization of cortical topography following limited sensory deprivation in the adult cat was investigated. The total concentrations of dopamine, noradrenaline, serotonin and their major metabolites were measured in the visual cortex of both normal control and experimental animals using microbore high-performance liquid chromatography coupled with electrochemical detection. The experimental animals were subjected to a binocular retinal lesion corresponding to the central 10 degrees of vision and killed two weeks post-lesion. The sensory deprivation was confirmed in area 17 by measuring immediate-early gene zif-268 messenger RNA expression. Following the retinal lesion, the total concentrations of noradrenaline and dopamine were significantly higher in the non-deprived cortex of retinal lesion cats than in the deprived cortex of retinal lesion cats and the cortex of normal animals. This pattern follows the release of the excitatory neurotransmitter glutamate under the same conditions. Serotonin levels were significantly lower in the deprived cortex, and its metabolite 5-hydroxyindole-3-acetic acid was significantly higher in the non-deprived cortex than in deprived cortex and normal cortex.From these results, we suggest that the modulation of noradrenaline, dopamine and serotonin is regulated by visual afferent activity.     

Synaptic characteristics of identified pyramidal and multipolar non-pyramidal neurons in the visual cortex of young and adult rabbits. A quantitative Golgi-electron microscope study

The visual cortex of 20 day old rats and rabbits has been considered as mature on the basis of the observations that the dendritic arborization and the overall synaptic population have almost reached their adult stage in these animals. In the present study we have investigated the visual cortex of 20 day and 7 month old (adult) rabbits in order to determine whether this apparent adult appearance also holds for the synaptic organization of individual neurons. Neurons mainly located in layers III and IV of the primary visual cortex (area 17) were Golgi-impregnated, gold toned and deimpregnated and were then, after embedding in plastic, sectioned serially. The number and length of synaptic profiles, and the length of the neuronal boundaries were analysed in every tenth section. From these counts and measurements the size distribution of the synaptic discs, the number of synapses per 100 micron2 neuronal surface and the receptive surface expressed as the percentage of the total neuronal surface covered with synaptic contacts were estimated using stereological methods. At both ages studied, the density of synapses was significantly higher for the non-pyramidal neurons than for the pyramidal neurons. Differences in the amount of receptive surface were parallel to the differences observed for the number of synapses per 100 micron2. At day 20 the receptive surface of the non-pyramidal neurons was significantly larger than that of the pyramidal neurons. The receptive surface of the non-pyramidal neurons in the adult stage was not only larger than that of the pyramidal neurons in the adults, but also larger than that of the day 20 non-pyramidal neurons. From our results the following conclusions can be drawn: (1) The synaptic input received by the pyramidal neurons is mainly established at day 20 of postnatal life, i.e. prior to the establishment of adult visual behaviour. (2) The non-pyramidal neurons complete their maturation in a later stage than the pyramidal neurons. (3) Medium to large sized synaptic contacts are newly formed after day 20 and are mainly added to the synaptic population on dendrites of non-pyramidal neurons. (4) The specific increase in the number of synapses on non-pyramidal neurons is discussed in relation to intracortical inhibition which is thought to be important for the fine regulation of visual function during development.     

Quantitative assessment of cortical synaptic density in Alzheimer's disease

Significant progress has been made over the last decade in delineating the neuropathological and neurochemical changes in the brains of patients with Alzheimer's disease (AD). Less well studied are the actual synaptic connections of affected areas of the brain, such as the cerebral cortex. Because the final common pathway for neurotransmission involves synaptic integrity, we quantitatively assessed synaptic number and synapse size in lamina III and V of human frontal cortex (Brodmann area 9) in patients with AD and age-matched controls. Samples were also matched for postmortem interval, and artifacts associated with postmortem change were eliminated. We found a significant decrease in synaptic number per unit volume in both lamina, more marked in lamina III (-42%) than V (-29%). In both normal controls and AD brains, there was a negative correlation between synapse number and synapse size as indexed by the length of the postsynaptic density (PSD); cortical samples with fewer synapses had larger synapse size. This appeared to be a compensatory response, rather than a selective loss of small synapses, since the total amount of synaptic contact area per unit volume did not decline in lamina V (despite a 29% loss of synapses); in lamina III it was reduced 11% despite a 42% loss of synapses. The loss of synapses in AD is widespread and significant in frontal cortex; there is observable compensation by enlargement of synaptic size. This compensatory effort is overcome by the continuing loss of synapses in areas most affected by the degeneration.     

Comparison of synaptic changes in the precentral and postcentral cerebral cortex of aging humans: a quantitative ultrastructural study

Ultrastructural quantitative analysis was undertaken to determine whether any age-related synaptic changes occur in cortical layer 1 of the human precentral motor gyrus (Brodmann's area 4) and postcentral somatosensory gyrus (Brodmann's area 3). Immersion fixed, osmicated, uranyl acetate/lead citrate stained (OsUL) preparations of autopsied brains were taken from patients aged 45 to 84 years, with no prior history of neurological or intellectual abnormalities. In the precentral gyrus there was a significant decrease in the number of synapses, which was primarily due to a decrease in asymmetrical axospinous synapses. Symmetrical synapses remained constant in number, while axodendritic synapses showed a small increase with age. Accompanying the decline in synapse number was an increase in mean length of the postsynaptic contact zone. In the postcentral gyrus there were no significant changes in synaptic number or in any of the synaptic parameters measured. The results suggest that the motor cortex of the human brain is capable of synaptic plasticity in response to aging-induced synaptic loss. This plasticity is not apparent in the somatosensory cortex, where there is no age-related synapse loss.     

Synaptic development in the rabbit superior colliculus and visual cortex

Summary The development of synapses in the visual cortex (VC) and superior colliculus (SC) of the rabbit has been examined with the electron microscope. In both areas, the number of synapses reaches adult levels by 20–25 days of postnatal age, but the development in the visual cortex is delayed in comparison to that in the superior colliculus. When S synapses (spheroidal vesicles, asymmetric thickening) are compared with F synapses (flattened vesicles, symmetric thickening), even greater differences are seen. In both the VC and SC, S synapses develop earlier than F synapses, though there is considerable overlap. Of interest is the fact that synapses in the visual cortex seem to overshoot their adult levels late in development, suggesting that an excess of synapses may be formed in this system. Multiple synapses, probably of retinal origin, increase in the first 3 weeks of synaptic development in the SC, but never are present in significant proportions in the VC.Synapse formation most often is characterized by formation of a junction and a postsynaptic thickening, followed by acquisition of synaptic vesicles. After 15 days, there is only a small number of such non-vesicle synapses in either the SC or VC.     

大白鼠下丘脑腹内侧核突触的电镜观察

本文报告大白鼠下丘脑腹内侧核内突触的类型和突触亚显微结构的形态特征。共观察了1005个突触,其中轴-树突触占96.6%;轴-体突触占2.8%;轴-轴突触占0.5%。在下丘脑腹内侧核中还观察到1例嵴突触,以往未见报道。嵴突触的突触后成分来自树突,呈杵指状突起。突触后膜明显增厚,具有突触下致密小体。两个内含圆形清亮小泡的轴突并列于嵴的两侧壁上,构成嵴突触。在轴-体、轴-树突触中尚观察到并联突触(包括突触复合体)、切线突触及串联突触等复杂的联接形式。本文对某些复合突触的机能也作了讨论。