T. F. Freund

Relationship of neuronal vulnerability and calcium binding protein immunoreactivity in ischemia

SummaryThe relationship between neuronal calcium binding protein content (calbindin D28K: CaBP and parvalbumin : PV) and vulnerability to ischemia was studied in different regions of the rat brain using the four vessel occlusion model of complete forebrain ischemia. The areas studied, i.e. the hippocampal formation, neocortex, neostriatum and reticular thalamic nucleus (RTN), show a characteristic pattern of CaBP and PV distribution, and are involved in ischemic damage to different degrees. In the hippocampal formation CaBP is present in dentate granule cells and in a subpopulation of the CA1 pyramidal cells, the latter being the most and the former the least vulnerable to ischemia. Non-pyramidal cells containing CaBP in these regions survive ischemia, whereas PV-containing non-pyramidal cells in the CA1 region are occasionally lost. Hilar somatostatin-containing cells and CA3 pyramidal cells contain neither PV nor CaBP. Nevertheless, the latter are resistant to ischemia and the former is the first population of cells that undergoes degeneration. Supragranular pyramidal neurons containing CaBP are the most vulnerable cell group in the sensory neocortex. In the RTN the degenerating neurons contain both PV and CaBP. In the neostriatum, ischemic damage involves both CaBP-positive and negative medium spiny neurons, although the degeneration always starts in the dorsolateral neostriatum containing relatively few CaBP-positive cells. The giant cholinergic interneurons of the striatum contain neither CaBP nor PV, and they are the most resistant cell type in this area. These examples suggest the lack of a consistent and systematic relationship between neuronal CaBP or PV content and ischemic vulnerability. It appears that some populations of cells containing CaBP or PV are more predisposed to ischemic cell death than neurons lacking these proteins. These neurons may express high levels of calcium binding proteins because their normal activity may involve a high rate of calcium uptake and/or intraneuronal release.

Synaptic input and local output of dopaminergic neurons in grafts that functionally reinnervate the host neostriatum

SummaryIn adult rats with a unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopamine pathway, grafts of embryonic ventral mesencephalon can establish extensive efferent connections with the previously denervated host neostriatum and can compensate for motor and sensorimotor asymmetries induced by the lesion. The object of this study was to examine the afferent synaptic inputs to grafted dopaminergic neurons, implanted into a cortical cavity overlying the previously denervated caudate-putamen, using electron microscopic immunocytochemistry. The dopaminergic neurons of the grafts in the same animals had previously been shown to re-innervate the host neostriatum, to form synaptic connections therein and to attenuate the lesion-induced motor asymmetry that occured in response to amphetamine (Freund et al. 1985). In the light microscope, the grafts were found to contain numerous tyrosine hydroxylase-immunoreactive perikarya, dendrites, axons and axonal swellings which had distinct distributions. In addition axons and axonal swellings that were immunoreactive for either substance P or glutamate decarboxylase were present. Electron microscopic analysis of the boutons contacting tyrosine hydroxylase-immunoreactive neurons in the grafts revealed the presence of at least five distinct types of afferent synaptic boutons based on their immunochemistry, morphology, or types of membrane specialization. One type was itself immunoreactive for tyrosine hydroxylase; such synapses are extremely rare in the intact substantia nigra, none were found in the contralateral substantia nigrae or the substantia nigra of a control rat. Three of the remaining types had ultrastructural features that were similar to synaptic terminals that were immunoreactive for substance P or glutamate decarboxylase. These synapses were similar to the types of synapses found contacting dopaminergic neurons in the substantia nigra contralateral to the graft or the substantia nigra of a control rat. The results demonstrate that, in the absence of the normal extrinsic afferent inputs, the intracortical mesencephalic grafts have a well-developed local synaptic circuitry. It is suggested that local circuit regulation of dopaminergic neurons within the graft may, at least in part, be responsible for the maintenance of a normal or close to normal functional activity.

Ischemia-induced changes in the electrical activity of the hippocampus.

SummaryConsequences of transient (15–20 min) ischemia on the neuronal activity of the dentate gyrus and hippocampal CA 1 region were investigated in chronically implanted Sprague-Dawley rats. Forebrain ischemia was produced by occlusion of the carotids for 15 or 20 min, following cauterization of the vertebral arteries. Following the release of the carotids, both spontaneous and evoked activity showed a steady but partial recovery, reaching a maximum 12 to 24 h after the ischemic insult. From this plateau, both the power of rhythmic slow activity recorded during walking and the power of slow delta activity obtained during alert immobility decreased monotonically, with large changes occuring between postischemic days 2 and 4. The changes in spontaneous activity were accompanied by a decrease and eventual disappearance of the Schaffer collateral evoked responses in CA 1. Perforant path volleys were less efficient in activating the granule cells following ischemia compared to baseline levels. This decreased responsiveness was paralleled by a relative impairment of paired pulse depression. Neurophysiological signs of spontaneous or evoked neuronal hyperexcitability were not observed at any time point during the 8 postischemic days. Neuronal damage in the CA 1 region varied from moderate to complete loss of pyramidal cells. In addition, degenerating neurons were also observed in the hilus of the dentate gyrus. These findings do not support the “overwork” version of the excitoxic hypothesis of delayed neuronal damage and indicate that the cause of ischemic cell death should be sought in factors other than neuronal hyperactivity.

Restoration and deterioration of function by brain grafts in the septohippocampal system.

Publisher Summary Several features make the septohippocampal system useful, especially for the study of physiological mechanisms of graft-host interactions, such as the hippocampus possesses electrical rhythms, which vary with ongoing behavior in a specific manner, and the excitability changes within the hippocampus are easy to monitor. This chapter discusses the possible mechanisms of electrophysiological recovery in the damaged septohippocampal system following transplantation of solid pieces or suspensions of fetal brain tissue into the lesion cavity or into the hippocampus, and the epileptogenic properties of hippocampal grafts. The experiments, using the septohippocampal model to study the mechanisms of action of brain grafts, suggest that a likely mechanism of restoration of physiological activity of the deafferented hippocampus is a passive bridging action of the graft between the host septum and hippocampus. The chapter demonstrates the reciprocal physiological and anatomical connections between the graft and host, and shows that hippocampal grafts can serve as an implanted epileptic focus, which may further worsen the function of the already damaged brain. However, experiments are required that determine why under certain conditions the grafts produce epileptic activity, while under seemingly similar conditions they lead to the restoration of physiological function of damaged brain circuitries.