Retrograde thalamic degeneration: observations using a modification of the Fink-Heimer silver impregnation technique

Inflammatory Responses in the Secondary Thalamic Injury After Cortical Ischemic Stroke

Stroke is one of the major causes of chronic disability worldwide and increasing efforts have focused on studying brain repair and recovery after stroke. Following stroke, the primary injury site can disrupt functional connections in nearby and remotely connected brain regions, resulting in the development of secondary injuries that may impede long-term functional recovery. In particular, secondary degenerative injury occurs in the connected ipsilesional thalamus following a cortical stroke. Although secondary thalamic injury was first described decades ago, the underlying mechanisms still remain unclear. We performed a systematic literature review using the NCBI PubMed database for studies that focused on the secondary thalamic degeneration after cortical ischemic stroke. In this review, we discussed emerging studies that characterized the pathological changes in the secondary degenerative thalamus after stroke; these included excitotoxicity, apoptosis, amyloid beta protein accumulation, blood-brain-barrier breakdown, and inflammatory responses. In particular, we highlighted key findings of the dynamic inflammatory responses in the secondary thalamic injury and discussed the involvement of several cell types in this process. We also discussed studies that investigated the effects of blocking secondary thalamic injury on inflammatory responses and stroke outcome. Targeting secondary injuries after stroke may alleviate network-wide deficits, and ultimately promote stroke recovery.

A polysynaptic feedback circuit in rat visual cortex

Feedback connections from extrastriate cortex to primary visual cortex (V1) in the primate may provide "top-down" information that plays a role in visual attention and object recognition. Our work in a rodent model of corticocortical circuitry demonstrates that feedback pathways synapse preferentially with pyramidal cells in V1 () and favor excitation over inhibition in cortical microcircuits (). To investigate the polysynaptic circuits activated by feedback inputs, we studied chains of neurons postsynaptic to feedback connections using a combination of axonal tract tracing and anterograde degeneration. This approach enabled independent labeling of local collaterals of forward-projecting neurons in V1 and feedback connections from extrastriate lateromedial (LM) visual area to V1. Postsynaptic targets were identified in the electron microscope after retrograde transport of biotinylated dextran amine (BDA) to identify dendrites of forward-projecting neurons (i.e., from V1 to LM) and postembedding immunogold labeling to identify GABAergic interneurons. The results show that feedback connections provide strong monosynaptic input to forward-projecting neurons in V1. These neurons in turn make local connections that preferentially form synapses with other pyramidal cells ( approximately 97%), many of which were identified as forward-projecting neurons. This indicates that feedback pathways provide input directly to neurons which make the reciprocal forward connection, and that feedback-recipient forward-projecting neurons are strongly interconnected. The function of these excitatory networks within V1 may be to amplify feedback activity and provide a circuit for modulation of striate cortical activity by top-down influences.

Neuronal network disturbance after focal ischemia in rats

We studied functional disturbances following left middle cerebral artery occlusion in rats. Neuronal function was evaluated by [14C]2-deoxyglucose autoradiography 1 day after occlusion. We analyzed the mechanisms of change in glucose utilization outside the infarct using Fink-Heimer silver impregnation, axonal transport of wheat germ agglutinin-conjugated-horseradish peroxidase, and succinate dehydrogenase histochemistry. One day after occlusion, glucose utilization was remarkably reduced in the areas surrounding the infarct. There were many silver grains indicating degeneration of the synaptic terminals in the cortical areas surrounding the infarct and the ipsilateral cingulate cortex. Moreover, in the left thalamus where the left middle cerebral artery supplied no blood, glucose utilization significantly decreased compared with sham-operated rats. In the left thalamus, massive silver staining of degenerated synaptic terminals and decreases in succinate dehydrogenase activity were observed 4 and 5 days after occlusion. The absence of succinate dehydrogenase staining may reflect early changes in retrograde degeneration of thalamic neurons after ischemic injury of the thalamocortical pathway. Terminal degeneration even affected areas remote from the infarct: there were silver grains in the contralateral hemisphere transcallosally connected to the infarct and in the ipsilateral substantia nigra. Axonal transport study showed disruption of the corticospinal tract by subcortical ischemia; the transcallosal pathways in the cortex surrounding the infarct were preserved. The relation between neural function and the neuronal network in the area surrounding the focal cerebral infarct is discussed with regard to ischemic penumbra and diaschisis.

MPTP produces a pattern of nigrostriatal degeneration which coincides with the mosaic organization of the caudate nucleus

In the normal dog we have found that cholinesterase and tyrosine hydroxylase (TH) histochemistry define a mosaic structure of the caudate nucleus that is similar to that described in other species. To determine if nigrostriatal afferents interlocked with this mosaic we injected dogs with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a toxin specific to dopaminergic nigrostriatal cells. Alternate sections in the caudate nucleus stained for acetylcholinesterase, TH, and terminal degeneration revealed that the areas of densest degeneration were localized to the matrix, thereby outlining areas of much lighter degeneration which were coincident with the patches. This pattern of terminal degeneration suggests the existence of subcomponents of the dopaminergic nigrostriatal pathway, at least one of which might be selectively vulnerable to MPTP.

MPTP produces a mosaic-like pattern of terminal degeneration in the caudate nucleus of dog

This paper describes the spatial pattern of terminal degeneration in the caudate nucleus and putamen of the dog following an injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Marked inhomogeneities in the filed of terminal degeneration were found in the head of the caudate nucleus, which are reminiscent of the fundamental patch/matrix organization of the striatum. Using MPTP as a selective lesioning method, our results provide further evidence that the dopaminergic, nigrostriatal projection has a heterogeneous organization.

Degeneration of hippocampal fibers and spatial memory deficit in the aged rat

Old and young Fisher 344 rats were compared for their ability to learn a delayed alternation task. The old animals displayed significant impairment of alternation learning, and were slower than the young animals. The brains of these animals were examined using a silver degeneration stain, and among old rats there was conspicuous degeneration. The greatest density of degenerating fibers was seen in the hippocampus and in anatomically related tracts, but there was substantial fiber staining in the corpus callosum, anterior commissure, and internal capsule. Examination of the young brains revealed only an occasional fiber. There were no signs of cortical atrophy in the old animals. The histopathology of the aged animals' hippocampus and fiber tracts supports the possibility that the delayed alternation impairment shown by these animals was a result of age related degenerative changes.

Thalamic role in spatial memory

The contribution of structures in the limbic diencephalon to spatial memory function was investigated. Rats with lesions of either the anteroventral thalamus, anteromedial thalamus, dorsomedial thalamus or mammillary bodies were compared in their ability to perform a delayed alternation task. The results indicate the ablation of the thalamic nuclei did not impair delayed-alternation memory, but there was impairment following damage to the mammillary bodies. Placement of the discrete lesions was verified using Nissl sections and by tracing the pattern of projections using a silver degeneration stain. The results suggest that individual thalamic nuclei are not essential in the storage and/or retrieval of spatial memory. The data are discussed in terms of spatial deficits resulting from damage to the hippocampus proper or to the pathways connecting it to other brain structures.

Thalamic projections of the dorsomedial prefrontal cortex in the rhesus monkey (Macaca mulatta)

Cortical aspirations were made of the dorsomedial prefrontal sector in the rhesus monkey and the resultant anterograde and retrograde degeneration plotted. Retrograde changes were mapped using both cresyl violet and modified Fink-Heimer techniques. Following large dorsomedial lesions, small numbers of degenerating fibers were traced through the medial part of the magnocellular ventral anterior nucleus (VAmc) and the ventral part of the internal medullary lamina surrounding the anterior nuclei. Degenerating fibers were also traced medially through the inferior thalamic peduncle and rostrodorsally through the basal thalamic region into the mediodorsal nucleus. All pathways converged on areas in the dorsal part of the parvocellular mediodorsal nucleus (MDpc) containing fine-grain dust and argyrophilic neurons, reactions usually associated with retrograde degenerative changes. Sparse fiber degeneration was also noted in the ventral part of the magnocellular mediodorsal nucleus (MDmc). After a longer survival period, slight to moderate cell loss and gliosis were seen in MDpc along with an increase in the number of degenerating fibers passing through the medial part of VAmc. Rostral dorsomedial lesions resulted in small numbers of degenerating fibers in the medial part of VAmc; no degenerating fibers appeared in the basal thalamic region. Fine-grain dust was noted in the dorsal part of MDpc along with sparse preterminal degeneration in the dorsal and ventral parts of MDpc and MDmc respectively. Following caudal dorsomedial lesions, fiber degeneration was traced through the medial part of VAmc and through the inferior thalamic peduncle and basal thalamic regions to areas of degenerating preterminals in the dorsal part of MDpc. Sparse fiber degeneration was also noted in MDmc. No evidence of cell loss and gliosis or increased numbers of degenerated fibers was noted following longer survival periods. Dorsolateral and orbital lesions resulted in large areas of fine-grain dust, argyrophilic neurons, and severe cell loss in MDpc and MDmc respectively. A large combined dorsolateral and orbital lesion made 32 days prior to sacrifice of the animal resulted in coarse fiber degeneration in the medial part of VAmc and in the anterior nuclear capsule. Severe cell loss and fiber degeneration were evident in the entire MD. These results suggest that the rostral dorsomedial prefrontal sector receives projections from MDpc while the caudal sector does not. This projection courses dorsally and rostrally through the ventral part of the capsule surrounding the anterior nuclei and into the medial part of VAmc. The entire dorsomedial sector projects sparsely to both MDpc and MDmc. The projection from the rostral medial surface passes through the medial part of VAmc while that from the caudal surface reaches MD both by the dorsal approach through VAmc and through a ventral approach via the inferior thalamic peduncle and the basal thalamic region.

A study of so-called 'retrograde fine-grain' degeneration in the thalamus

Following lesions of the neocortex a uniform dust-like silver deposit may appear in certain thalamic nuclei stained with the Nauta or Fink-Heimer methods. Because this deposit lacks the characteristic morphology of axons undergoing Wallerian degeneration and often appears in regions of retrograde cell degeneration, it has been interpreted as a retrograde reaction7,15. The present study, however, employing reduced silver and autoradiographic techniques, suggests that this silver deposit is an anterograde rather than a retrograde phenomenon. In cats with lesions of the posterior cingulate and presubicular cortex the Fink-Heimer method showed the fine-grain silver deposit in the lateralis dorsalis (LD) and the anteroventral nucleus (AV). The 'dust' was densest in the dorsomedial zones of the anteroventral nucleus. After the survival times used (6-8 days) no retrograde perikaryal changes were seen in these nuclei. In other cats, 3 weeks after injections of radioactive proline in the posterior cingulate cortex, the distribution of the label in the LD and AV nuclei was almost identical to the distribution of the 'dust' in the Fink-Heimer sections. Since the autoradiographic method reveals anterograde transport, it appears that both methods are demonstrating corticofugal axons. Thus it is suggested that the 'fine-grain' degeneration arises from an anterograde not a retrograde reaction.

Afferents to prefrontal cortex from the thalamic mediodorsal nucleus in the rhesus monkey

Thalamic afferents to Macaque prefrontal cortex from the mediodorsal nucleus were examined by techniques specific for anterograde degeneration and axoplasmic transport. The sampling procedure employed permits establishing the extent of topographic projections to cortex from subcortical foci for the same brain which was surveyed subsequently in tracing specific neuronal connections by electron microscopy. Topographic and general laminar distribution of thalamic terminals are presented in terms of 3 subareas of prefrontal cortex. The dorsolateral and ventral (orbital) surfaces of prefrontal cortex receive respectively projections from the lateral and medial subdivision of the mediodorsal nucleus. In addition, the medial wall of the frontal lobe, including the dorsomedial part of the lateral convexity, heretofore regarded as athalamic, receives input from the caudal-dorsomedial aspect of the mediodorsal nucleus. Preliminary evidence suggests that axons from the mediodorsal nucleus terminate in the head of caudate nucleus, as Sachs-81 described 65 years ago in the first orthograde study of thalamo-prefrontal cortex connections.