Neuronal response of the hippocampal formation to injury: blood flow, glucose metabolism, and protein synthesis

Acute cognitive impairment after lateral fluid percussion brain injury recovers by 1 month: evaluation by conditioned fear response

Conditioned fear associates a contextual environment and cue stimulus to a foot shock in a single training trial, where fear expressed to the trained context or cue indicates cognitive performance. Lesion, aspiration or inactivation of the hippocampus and amygdala impair conditioned fear to the trained context and cue, respectively. Moreover, only bilateral experimental manipulations, in contrast to unilateral, abolish cognitive performance. In a model of unilateral brain injury, we sought to test whether a single lateral fluid percussion brain injury impairs cognitive performance in conditioned fear. Brain-injured mice were evaluated for anterograde cognitive deficits, with the hypothesis that acute injury-induced impairments improve over time. Male C57BL/6J mice were brain-injured, trained at 5 or 27 days post-injury, and tested 48h later for recall of the association between the conditioned stimuli (trained context or cue) and the unconditioned stimulus (foot shock) by quantifying fear-associated freezing behavior. A significant anterograde hippocampal-dependent cognitive deficit was observed at 7 days in brain-injured compared to sham. Cued fear conditioning could not detect amygdala-dependent cognitive deficits after injury and stereological estimation of amygdala neuron number corroborated this finding. The absence of injury-related freezing in a novel context substantiated injury-induced hippocampal-dependent cognitive dysfunction, rather than generalized fear. Variations in the training and testing paradigms demonstrated a cognitive deficit in consolidation, rather than acquisition or recall. By 1-month post-injury, cognitive function recovered in brain-injured mice. Hence, the acute injury-induced cognitive impairment may persist while transient pathophysiological sequelae are underway, and improve as global dysfunction subsides.

Focal cerebral hyperemia in postconcussive amnesia

Transient amnesia caused by minor head injury is commonly encountered in daily neurosurgical practice, but the mechanism of such amnesia has not been extensively studied. We measured the regional cerebral blood flow (rCBF) of patients with postconcussive amnesia with Xe/CT CBF to examine whether a focal disturbance of CBF exists. The Xe/CT CBF study was performed in eight patients with closed head injury without organic cerebral lesion while they were suffering from posttraumatic amnesia (concussion group). The time interval between accident and CBF measurement was less than 2 h in three patients, 5-6 h in two, 8-9 h in two, and 18 in one. The results were compared with those of nine normal volunteers and eight other age-matched patients who recovered without any neurological deficit despite the presence of hemorrhagic regions (mild hemorrhage group). The rCBF of the concussion group was significantly elevated in the bilateral mesial temporal cortex in comparison to the normal group. The rCBF in the mild hemorrhage group was lower than that of normal controls in all regions. The analysis of right-left difference in CBF indicated that there was significant asymmetry (right > left) in the frontal and temporal cortex in the concussion group, but not in the normal and mild hemorrhage group. This Xe/CT CBF study in acute stages of cerebral concussion, in which patients were amnestic, detected focal cerebral hyperemia. Such hyperemia in regions closely related to human memory function may be the result of vasoparalysis or the compensatory activation of memory circuits after denervation injury.

Functional cerebral activity during regeneration from entorhinal lesions in the rat

The consequences of an unilateral electrolytic entorhinal lesion on the functional activity in all major anatomically defined brain regions were evaluated in the rat. The 14C-2 deoxyglucose method served as a tool to quantify alterations of local cerebral glucose utilization (LCGU) ipsilateral and contralateral to the lesion at 4 days, 2 weeks, or 3 months after stereotaxic surgery. Apart from a few minor increases in the contralateral hemisphere, the predominant pattern consisted of reductions in the range of 10-40% in the ipsilateral hemisphere. Ipsilaterally, in extrahippocampal areas, LCGU had regained control levels at 2 weeks postlesion in contrast to hippocampal regions, where reductions were more pronounced than in other brain areas and partially persisted for up to 3 months. Interestingly, the termination zones of entorhinal fibers in the dentate gyrus did not regain control levels within 3 months. We conclude from the data that functional recovery of denervated primary target areas does not occur within 3 months after entorhinal lesions and that altered functional activity may be found beyond the primary target areas predominantly during the acute recovery period after the lesion. The data suggest that sprouting fibers do not reestablish a fully functional neuronal network during the recovery period.

Ischemia as an excitotoxic lesion: protection against hippocampal nerve cell loss by denervation

There are several indications for an involvement of neuroexcitatory mechanisms in ischemic neuron damage. Since we forwarded the hypothesis in 1982 that the transmitter glutamate is playing a key role, several lines of evidence have substantiated this: there is a pronounced transmitter release induced by ischemia and there is uptake of Ca++ via NMDA-operated calcium channels. Under certain circumstances postischemic neuron death can be impaired by administration of either NMDA-antagonists or calcium blockers. Further proof for the induction of harmful excitatory mechanisms by ischemia has been obtained by preischemic denervation of the vulnerable nerve cells. After transient cerebral ischemia in rats or gerbils, there are signs of irreversible damage (eosinophilia) of neurons in the dentate hilus (somatostatin-positive cells) after 2-3 hours and of hippocampal pyramidal neurons after 2-3 days (delayed neuron death). In the first case, removal of the (main) input to hilus cells by degranulation (colchicine selectively eliminates granule cells) protects these. In the case of pyramidal neurons removal of Schaffer collaterals/commisurals or input from the entorhinal cortex have a protective effect. Recently, we have measured glutamate and calcium in CA1 of denervated rats during 10 min of ischemia, and it turns out that there is almost no extracellular glutamate release or lowering of calcium in contrast to ischemic animals with intact innervation. Also in the postischemic period there are indications of a continuation of the damaging processes induced by ischemia. Besides the well known postischemic hypoperfusion, a prolonged release of glutamate has been reported, as well as burst firing in some models.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of radiotracer concentration in brain gray matter using positron emission tomography: MRI-based correction for partial volume effects

Accuracy in in vivo quantitation of brain function with positron emission tomography (PET) has often been limited by partial volume effects. This limitation becomes prominent in studies of aging and degenerative brain diseases where partial volume effects vary with different degrees of atrophy. The present study describes how the actual gray matter (GM) tracer concentration can be estimated using an algorithm that relates the regional fraction of GM to partial volume effects. The regional fraction of GM was determined by magnetic resonance imaging (MRI). The procedure is designated as GM PET. In computer simulations and phantom studies, the GM PET algorithm permitted a 100% recovery of the actual tracer concentration in neocortical GM and hippocampus, irrespective of the GM volume. GM PET was applied in a test case of temporal lobe epilepsy revealing an increase in radiotracer activity in GM that was undetected in the PET image before correction for partial volume effects. In computer simulations, errors in the segmentation of GM and errors in registration of PET and MRI images resulted in less than 15% inaccuracy in the GM PET image. In conclusion, GM PET permits accurate determination of the actual radiotracer concentration in human brain GM in vivo. The method differentiates whether a change in the apparent radiotracer concentration reflects solely an alteration in GM volume or rather a change in radiotracer concentration per unit volume of GM.

Septo-hippocampal deafferentation protects CA1 neurons against ischemic injury

Excessive synaptic excitation caused by transient cerebral ischemia has been proposed to explain the greater vulnerability of specific neuronal populations to ischemic injury. We tested this hypothesis in rats by cutting, alone or in combination, the afferent fibers that travel in the fimbria/fornix, the perforant, or the Schäffer collateral pathways and innervate the right CA1 hippocampus. Seven to twelve days later the animals were subjected to 30 min of reversible forebrain ischemia. Irreversible damage to the CA1 neurons was assessed with the light microscope after 70-120 h of cerebral reperfusion. The left, unlesioned hippocampus served as a control. Simultaneous cutting of the 3 major afferent pathways significantly reduced CA1 neuronal damage compared to the unlesioned side (P less than 0.001) or to sham-lesioned controls (P less than 0.001). Selective lesions of the fimbria/fornix but not the perforant or the Schäffer collateral pathways also protected against ischemic CA1 damage. These data indicate that afferent fiber input modulates hippocampal damage caused by ischemia, but they are inconsistent with the hypothesis that excitatory afferent fibers, travelling in either the perforant or the Schäffer collateral pathways alone, play a major role. Neurotransmitters, other than those activating excitatory amino acid receptors or yet-to-be-identified synaptic events, may be invoked to explain the spatial and temporal sensitivity of hippocampal CA1 neurons to ischemia.

The connections between the suprachiasmatic, ventrolateral geniculate and raphe nuclei studied by uptake of [14C]2-deoxyglucose

The [14C]2-deoxyglucose method was used to investigate the role of the ventrolateral geniculate and raphe nuclei in the control of the metabolism of the suprachiasmatic nuclei in adult female Wistar rats anaesthetized with alphaxalone. Three to seven days before the [14C]2-deoxyglucose studies a stimulating electrode was implanted or a lesion was made in the ventrolateral geniculate nucleus, or the ascending projection from the raphe nuclei was severed. Stimulation of the ventrolateral geniculate nucleus (biphasic rectangular pulses, 30 s on and 30 s off, 50 Hz, 500 microA pulse amplitude and 1 ms pulse duration) led to a significant increase in the relative metabolic activity of the ipsilateral suprachiasmatic nucleus and a smaller increase in the relative metabolic activity of the contralateral suprachiasmatic nucleus. The stimulus also increased significantly the relative metabolic activities of mainly the ipsilateral hypothalamus, midbrain central gray and reticular formation, all of which are too remote from the ventrolateral geniculate nucleus to be affected by current spread. In animals in which the ventrolateral geniculate nucleus had been lesioned, the relative metabolic activity of the suprachiasmatic nuclei was not significantly different from normal. In animals in which the ascending projection from the raphe nuclei had been severed, there was a slight, though significant increase in the relative metabolic activity of the suprachiasmatic nucleus of one side. These results, together with the effects of stimulating the suprachiasmatic nuclei [R. C. Maxwell and G. Fink, Neuroscience 23, 241-263 (1987)], show that the connections between the ventrolateral geniculate, raphe nuclei and suprachiasmatic nuclei are "metabolically functional", but that the integrity of the ventrolateral geniculate nucleus is not essential for maintaining the relative metabolic activity of the suprachiasmatic nuclei. The raphe nuclei may reduce the relative metabolic activity of the suprachiasmatic nucleus.

Focal protein synthesis inhibition in a model of neonatal hypoxic-ischemic brain injury

Cerebral hypoxia-ischemia was produced in 1-week-old rats by exposing them to 8% O2-92% N2 commencing 4 to 6 h after unilateral carotid artery ligation. Protein synthesis rates were measured in cerebral cortex, caudate-putamen, thalamus, and lateral septal nuclei during 2 h of hypoxia and at three times (15 min, 10 h, and 18 h) after a 3.5-h hypoxic exposure. Protein synthesis was inhibited in the ipsilateral but not contralateral forebrain during the brief hypoxic exposure, and in both hemispheres during early recovery after a 3.5 h exposure, which was sufficient to produce brain injury in the ipsilateral hemisphere. At 10 h after hypoxia, protein synthesis rates in the contralateral forebrain had recovered to control values, but in vulnerable structures of the ipsilateral forebrain, recovery of protein synthesis was transient and incomplete (cerebral cortex) or remained at the low values found immediately after hypoxia (caudate-putamen). The early development of abnormal patterns of protein synthesis in vulnerable brain regions during hypoxia and its persistence in some rats during recovery when irreversible cell injury becomes manifest suggests a possible role for abnormal protein metabolism in the evolution of irreversible brain cell damage.

RNA content of normal and axotomized retinal ganglion cells of rat and goldfish

The responses of rat and goldfish retinal ganglion cells to axotomy were examined by a quantitative cytochemical method for RNA and by morphometric measurement 1-60 (rat) and 3-90 (goldfish) days after interruption of one optic nerve or tract intracranially. Unoperated control animals were studied also. The RNA content of axotomized neurons of rat fell 7-60 days postoperatively. Additionally, atrophy of the axotomized somas occurred. Over time, neuronal atrophy approximately paralleled the loss of RNA, and mean cell area and RNA content were reduced by about 25% 60 days after axotomy. Incorporation of 3H-uridine by axotomized neurons declined also. Axotomized retinal ganglion cells of goldfish behaved differently from those of the rat and showed increases in RNA content, most conspicuously 14-60 days postoperatively. Enlargement of axotomized fish neurons occurred but was less proportionately than concomitant increases in RNA content. The nonaxotomized ganglion cells of goldfish displayed statistically significant increases in size and RNA content 14-49 days after unilateral optic nerve or tract lesions. In contrast, alterations in rat retinal ganglion cells contralateral to interruption of one optic nerve were of limited and questionable significance. The contrasting reactions to axotomy by the retinal ganglion cells of these two vertebrates, one of which regenerates optic axons and one of which does not, may support the proposition that the somal response to axon injury has an important bearing upon the success or failure of CNS regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Axon reaction in hypoglossal and dorsal motor vagal neurons of adult rat: incorporation of [3H]leucine

Pairs of adult rats received [3H]leucine (i.p., 5 microCi/g body weight) 0.25, 1, and 16 h before killing and zero (unoperated control animals) and 1 to 164 days after unilateral cervical vagotomy and hypoglossal neurotomy. Grain counts and morphometric measurements were made on axotomized and uninjured neurons in histoautoradiographs of the medullary nuclei. Axotomized hypoglossal neurons, which largely survive the injury, both enlarged and incorporated increased amounts of tritiated leucine at each labeling interval, 3 through 28 days postoperatively. In the vagal dorsal motor nucleus (DMN), axotomized cells, which frequently die after neurotomy, enlarged slightly through 28 days postoperatively, then atrophied; DMN neurons increased amino acid uptake for a shorter period (days 7 through 14) than hypoglossal neurons. This increase achieved statistical significance only when the labeling intervals were 0.25 or 1.0 h. Neurons of the DMN contralateral to vagotomy also enlarged. Axotomized DMN neurons did not sustain increased protein synthesis as long as their hypoglossal counterparts and seemed to fail to increase synthesis of structural proteins with long half-lives (16-h labeling interval). The frequently necrobiotic response of axotomized DMN neurons may relate to these phenomena. From these and earlier results, we conclude that axon reaction appears to differ fundamentally in peripheral and central neurons. This difference may have significance for research on regeneration in the central nervous system.

Multiple-radionuclide autoradiography in evaluation of cerebral function

We have developed the mathematical model and experimental technique for quantitative simultaneous multiple-radionuclide autoradiography. The technique is an extension of previously described dual-tracer methods and offers the advantage of a general approach in which any two or more radionuclides with different half-lives can be used to measure multiple parameters of cerebral function. The model allows the selection of nuclide doses and exposure times to balance cross-contamination of the multiple images with nuclide requirements. The technique was validated with homogeneous tissue sections containing combinations of 123I, 124I , 131I, 18F, 99mTc , 68Ga, and 14C, and then used in qualitative and quantitative investigations of interrelationships of local cerebral blood flow, glucose metabolism, and protein synthesis in normal and several pathological conditions in rats.