Development of spinocerebellar afferents in the clawed toad, Xenopus laevis

The development of spinocerebellar projections in the clawed toad, Xenopus laevis, was studied with horseradish peroxidase as an anterograde and retrograde tracer. Early in development cells of origin of spinocerebellar projections were found, contralaterally, in or close to the medial motor column. In older tadpoles ipsilaterally projecting spinal neurons were also labeled from the cerebellum. These are virtually indistinguishable from the large primary motoneurons that occupy a very similar position in the spinal cord. Most of the labeled spinal cells were found in the thoracic spinal cord; they lie halfway between the brachial and lumbar secondary motor columns. Surprisingly, no primary spinocerebellar projection arising from dorsal root spinal ganglion cells could be demonstrated in X. laevis tadpoles and adult toads. Therefore, fibers in the cerebellum that were labeled anterogradely from the spinal cord can be expected to originate exclusively from the secondary spinocerebellar tract cells. These fibers appear to cross the cerebellum in or at the border of the granular layer. The present data suggest that in X. laevis early in the development of the cerebellum a distinct secondary spinocerebellar projection is already present, originating in neurons that can be compared with the "spinal border cells" in mammals. The relative sparseness of this secondary spinocerebellar projection and the apparent absence of primary spinocerebellar afferents probably indicate that spinocerebellar pathways are only of minor importance in X. laevis. The possibility remains, however, that the expansion of the secondary spinocerebellar pathway only starts when metamorphosis has been completed.     

Direct comparison of projections from the central amygdaloid region and nucleus accumbens shell

Certain neurochemical and connectional characteristics common to extended amygdala and the nucleus accumbens shell suggest that the two represent a single functional-anatomical continuum. If this is so, it follows that the outputs of the two structures should be substantially similar. To address this, projections from the caudomedial shell and central nucleus of the amygdala, a key extended amygdala structure, were demonstrated in Sprague-Dawley rats with different anterograde axonal tracers processed separately to exhibit distinguishable brown and blue-black precipitates. The caudomedial shell projection is strong in the ventral pallidum and along the medial forebrain bundle through the lateral preopticohypothalamic continuum into the ventral tegmental area, distal to which it thins abruptly. The central nucleus projects strongly to the bed nucleus of the stria terminalis and the sublenticular extended amygdala, but substantially to the lateral hypothalamus only at levels behind the rostral part of the entopeduncular nucleus. Innervation of the ventral tegmental area by the central amygdala is minimal, but the lateral one-third of the substantia nigra, pars compacta and an adjacent lateral part of the retrorubral field receive substantial central amygdala input. Central amygdaloid projections are robust in caudal brainstem sites, such as the reticular formation, parabrachial nucleus, nucleus of the solitary tract and dorsal vagal complex, all of which receive little input from the accumbens. The substantial differences in the output systems of the caudomedial shell of accumbens and central amygdala suggest that the two represent distinct functional-anatomical systems.     

Glutamatergic inputs from the pedunculopontine nucleus to midbrain dopaminergic neurons in primates: Phaseolus vulgaris-leucoagglutinin anterograde labeling combined with postembedding glutamate and GABA immunohistochemistry

To verify the possibility that the pedunculopontine nucleus is a source of glutamatergic terminals in contact with midbrain dopaminergic neurons in the squirrel monkey, we used the anterograde transport of Phaseolus vulgaris-leucoagglutinin in combination with preembedding immunohistochemistry for tyrosine hydroxylase and for calbindin D-28k and postembedding immunocytochemistry for glutamate and for γ-aminobutyric acid. Following tracer injections in the pedunculopontine nucleus, numerous anterogradely labeled fibers emerged from the injection sites to innervate densely the pars compacta of the substantia nigra and ventral tegmental area. The major type of labeled fibers were thin with multiple collaterals and varicosities that established intimate contacts with midbrain dopaminergic neurons. At the electron microscopic level, the anterogradely labeled boutons were medium sized (maximum diameter between 0.9 μm and 2.5 μm) and contained numerous round vesicles and mitochondria. Postembedding immunocytochemistry revealed that 40–60% of anterogradely labeled terminals were enriched in glutamate and formed asymmetric synapses with dendritic shafts of substantia nigra and ventral tegmental area neurons. In triple-immunostained sections, some of the postsynaptic targets to these terminals were found to be dopaminergic. In addition, 30–40% of the anterogradely labeled terminals in both regions displayed immunoreactivity for γ-aminobutyric acid and, in some cases, formed symmetric synapses with dendritic shafts. In conclusion, our results provide the first ultrastructural evidence for the existence of synaptic contacts between glutamate-enriched terminals from the pedunculopontine nucleus and midbrain dopaminergic neurons in primates. Our results also show that the pedunculopontine nucleus is a potential source of γ-aminobutyric acid input to this region. These findings suggest that the pedunculopontine nucleus may play an important role in the modulation of the activity of midbrain dopaminergic cells by releasing glutamate or γ-aminobutyric acid as neurotransmitter. © 1996 Wiley-Liss, Inc.     

Connections of the superior paraolivary nucleus of the rat: projections to the inferior colliculus

GABAergic neurotransmission contributes to shaping the response properties of inferior colliculus (IC) neurons. In rodents, the superior paraolivary nucleus (SPON) is a prominent and well-defined cell group of the superior olivary complex that sends significant but often neglected GABAergic projections to the IC. To investigate the trajectory, distribution and morphology of these projections, we injected the neuroanatomical tracer biotinylated dextran amine into the SPON of albino rats. Our results demonstrate that: (1) the SPON innervates densely all three subdivisions of the ipsilateral IC: central nucleus (CNIC), dorsal cortex (DCIC) and external cortex (ECIC). The SPON also sends a sparse projection to the contralateral DCIC via the commissure of the IC. (2) SPON axons are relatively thick (diameter >1.2 μm), ascend to the midbrain tectum in the medial aspect of the lateral lemniscus, and, for the most part, do not innervate the nuclei of the lateral lemniscus. (3) SPON fibers ramify profusely within the IC and bear abundant en passant and terminal boutons. (4) The axons of neurons in discrete regions of the SPON form two laminar terminal plexuses in the ipsilateral IC: a medial plexus that spans the CNIC and DCIC parallel to the known fibrodendritic laminae of the CNIC, and a lateral plexus located in the ECIC and oriented more or less parallel to the surface of the IC. (5) The projection from SPON to the ipsilateral IC is topographic: medial SPON neurons innervate the ventromedial region of the CNIC and DCIC and the ventrolateral region of the ECIC, whereas more laterally situated SPON neurons innervate more dorsolateral regions of the CNIC and DCIC and more dorsomedial regions of the ECIC. Thus, SPON fibers follow a pattern of distribution within the IC similar to that previously reported for intracollicular and corticocollicular projections.     

大鼠三叉神经脊束核内Ⅱ型囊泡膜谷氨酸转运体样阳性终末向外侧臂旁核的投射

新近发现的Ⅰ型(VGLUT1)和Ⅱ型(VGLUT2)囊泡膜谷氨酸转运体已被作为谷氨酸能神经终末的标识物,在中枢神经系统呈互补分布,显示二者可能存在功能差异。以往的研究也已经证实接受口面部浅感觉信息的三叉神经脊束核,特别是尾侧亚核(Vc)向外侧臂旁核(LPB)发出丰富的谷氨酸能投射纤维,但该投射纤维内究竟是哪一型的囊泡膜谷氨酸转运体承担谷氨酸的转运功能,到目前为止尚未见报道。本实验选用SD大鼠,综合运用顺行束路追踪[将霍乱毒素B亚单位-CTb分别注入一侧Vc、三叉神经脊束核极间亚核(Vi)、吻侧亚核(Vo)]和免疫荧光组织化学相结合的三重标记技术,在激光共聚焦显微镜下对Vc、Vi和Vo内向LPB投射纤维内所含的囊泡膜谷氨酸转运体(VGLUTs)的情况进行了研究。结果显示:(1)将CTb分别注入大鼠一侧Vc、Vi或Vo后,在同侧的LPB内可观察到许多CTb顺行标记纤维,其中以来自Vc的标记纤维最多,Vi次之,Vo最少;(2)在LPB内可见部分CTb顺标终末同时表达VGLUT2样免疫阳性,但未见与VGLUT1样阳性终末的共存;(3)可观察到部分CTb/VGLUT2双标终末散在分布于NeuN标记的神经元的胞体周围,并与之形成密切接触。以上结果提示:大鼠口面部浅感觉、特别是伤害性信息从三叉神经脊束核向LPB传递的过程中,谷氨酸发挥着重要作用,负责其内谷氨酸转运功能的主要是VGLUT2。     

AXONAL PROJECTIONS FROM RESPIRATORY CENTRES TOWARDS THE ROSTRAL VENTROLATERAL MEDULLA IN THE RAT

1. Efferent pathways from brainstem respiratory centres towards bulbospinal tyrosine hydroxylase immunoreactive neurons were identified in the rat using a combination of electrophysiology, retrograde and anterograde tract-tracing, and immunohistochemistry. 2. Varicose axons originating from respiratory centres were found in close apposition to bulbospinal tyrosine hydroxylase immunoreactive neurons in the ventrolateral medulla. 3. These findings support the idea that respiratory rhythms in sympathetic nerves may be due to a synaptic connection between brainstem respiratory neurons and bulbospinal tyrosine hydroxylase immunoreactive neurons of the C1 cell group.     

Biocytin-derived MRI contrast agent for longitudinal brain connectivity studies

To investigate the connectivity of brain networks noninvasively and dynamically, we have developed a new strategy to functionalize neuronal tracers and designed a biocompatible probe that can be visualized in vivo using magnetic resonance imaging (MRI). Furthermore, the multimodal design used allows combined ex vivo studies with microscopic spatial resolution by conventional histochemical techniques. We present data on the functionalization of biocytin, a well-known neuronal tract tracer, and demonstrate the validity of the approach by showing brain networks of cortical connectivity in live rats under MRI, together with the corresponding microscopic details, such as fibers and neuronal morphology under light microscopy. We further demonstrate that the developed molecule is the first MRI-visible probe to preferentially trace retrograde connections. Our study offers a new platform for the development of multimodal molecular imaging tools of broad interest in neuroscience, that capture in vivo the dynamics of large scale neural networks together with their microscopic characteristics, thereby spanning several organizational levels.     

Green laser light (532nm) activates a chloride current in the C1 neuron of Helix aspersa

Five hundred and thirty-two nanometers laser light evokes neuron-specific electrical responses in identified neurons of Helix ganglia. Such responses are intensity-dependent over the range 25-1500 mW, readily reversible and repeatable. Detailed experiments on the C1 neuron, which is inhibited by 532 nm light, showed that inhibition results from a selective increase in transmembrane Cl(-) ion conductance. Experiments with calcium-sensitive microelectrodes suggest that the response does not result from an increase in [Ca(2+)](i). The change in Cl(-) ion conductance probably occurs in the extensive plasmalemma infoldings of the proximal axon.