Physiological and structural evidence for hippocampal involvement in persistent seizure susceptibility after traumatic brain injury

Epilepsy is a common outcome of traumatic brain injury (TBI), but the mechanisms of posttraumatic epileptogenesis are poorly understood. One clue is the occurrence of selective hippocampal cell death after fluid-percussion TBI in rats, consistent with the reported reduction of hippocampal volume bilaterally in humans after TBI and resembling hippocampal sclerosis, a hallmark of temporal-lobe epilepsy. Other features of temporal-lobe epilepsy, such as long-term seizure susceptibility, persistent hyperexcitability in the dentate gyrus (DG), and mossy fiber synaptic reorganization, however, have not been examined after TBI. To determine whether TBI induces these changes, we used a well studied model of TBI by weight drop on somatosensory cortex in adult rats. First, we confirmed an early and selective cell loss in the hilus of the DG and area CA3 of hippocampus, ipsilateral to the impact. Second, we found persistently enhanced susceptibility to pentylenetetrazole-induced convulsions 15 weeks after TBI. Third, by applying GABA(A) antagonists during field-potential and optical recordings in hippocampal slices 3 and 15 weeks after TBI, we unmasked a persistent, abnormal APV-sensitive hyperexcitability that was bilateral and localized to the granule cell and molecular layers of the DG. Finally, using Timm histochemistry, we detected progressive sprouting of mossy fibers into the inner molecular layers of the DG bilaterally 2-27 weeks after TBI. These findings are consistent with the development of posttraumatic epilepsy in an animal model of impact head injury, showing a striking similarity to the enduring behavioral, functional, and structural alterations associated with temporal-lobe epilepsy.

Bidirectional synaptic plasticity correlated with the magnitude of dendritic calcium transients above a threshold

The magnitude of postsynaptic Ca(2+) transients is thought to affect activity-dependent synaptic plasticity associated with learning and memory. Large Ca(2+) transients have been implicated in the induction of long-term potentiation (LTP), while smaller Ca(2+) transients have been associated with long-term depression (LTD). However, a direct relationship has not been demonstrated between Ca(2+) measurements and direction of synaptic plasticity in the same cells, using one induction protocol. Here, we used glutamate iontophoresis to induce Ca(2+) transients in hippocampal CA1 neurons injected with the Ca(2+)-indicator fura-2. Test stimulation of one or two synaptic pathways before and after iontophoresis showed that the direction of synaptic plasticity correlated with glutamate-induced Ca(2+) levels above a threshold, below which no plasticity occurred (approximately 180 nM). Relatively low Ca(2+) levels (180-500 nM) typically led to LTD of synaptic transmission and higher levels (>500 nM) often led to LTP. Failure to show plasticity correlated with Ca(2+) levels in two distinct ranges: <180 nM and approximately 450-600 nM, while only LTD occurred between these ranges. Our data support a class of models in which failure of Ca(2+) transients to affect transmission may arise either from insufficient Ca(2+) to affect Ca(2+)-sensitive proteins regulating synaptic strength through opposing activities or from higher Ca(2+) levels that reset activities of such proteins without affecting the net balance of activities. Our estimates of the threshold Ca(2+) level for LTD (approximately 180 nM) and for the transition from LTD to LTP (approximately 540 nM) may assist in constraining the molecular details of such models.

Ca2+ signaling in gerbil CA3 hippocampal neurons following transient in vivo ischemia

Using the gerbil model of post-ischemic neuron death in the hippocampal CA1 region, it was recently shown that there is a strong down-regulation of voltage-gated Ca2+ influx in neurons examined at 2 days after the ischemic insult (Connor, J.A., Razani-Boroujerdi, S., Greenwood, A.C., Cormier, R.J., Petrozzino, J.J. and Lin, R.C., Reduced voltage-dependent Ca2+ signaling in CA1 neurons after brief ischemia in gerbils, J. Neurophysiol., 81 (1999) 299-306). The aim of the present study was to determine whether a similar change occurs in pyramidal neurons of the CA3 region that are relatively resistant to transient ischemia. In vitro intracellular recordings and fluorometric Ca2+ measurements were made from CA3 neurons in coronal slices prepared from controls and 1 or 2 days following in vivo ischemia. In slices from control and post-ischemic animals, the electrophysiological properties of CA3 neurons were consistent with significant voltage-gated Ca2+ influx, leading to spike frequency adaptation. Quantitative results indicated no significant difference in Ca2+ transients evoked by action potential trains. This Ca2+ signaling was compared with responses in CA1 neurons from the same preparations, which showed substantially diminished Ca2+ influx at 2 days post-ischemia. These findings suggest that diminished Ca2+-signaling is not a general feature of pyramidal neurons following ischemia, but is characteristic of neurons destined to die.

Testing the fit of a quantal model of neurotransmission

Many studies of synaptic transmission have assumed a parametric model to estimate the mean quantal content and size or the effect upon them of manipulations such as the induction of long-term potentiation. Classical tests of fit usually assume that model parameters have been selected independently of the data. Therefore, their use is problematic after parameters have been estimated. We hypothesized that Monte Carlo (MC) simulations of a quantal model could provide a table of parameter-independent critical values with which to test the fit after parameter estimation, emulating Lilliefors's tests. However, when we tested this hypothesis within a conventional quantal model, the empirical distributions of two conventional goodness-of-fit statistics were affected by the values of the quantal parameters, falsifying the hypothesis. Notably, the tests' critical values increased when the combined variances of the noise and quantal-size distributions were reduced, increasing the distinctness of quantal peaks. Our results support two conclusions. First, tests that use a predetermined critical value to assess the fit of a quantal model after parameter estimation may operate at a differing unknown level of significance for each experiment. Second, a MC test enables a valid assessment of the fit of a quantal model after parameter estimation.

Reduced voltage-dependent Ca2+ signaling in CA1 neurons after brief ischemia in gerbils

An initial overload of intracellular Ca2+ plays a critical role in the delayed death of hippocampal CA1 neurons that die a few days after transient ischemia. Without direct evidence, the prevailing hypothesis has been that Ca2+ overload may recur until cell death. Here, we report the first measurements of intracellular Ca2+ in living CA1 neurons within brain slices prepared 1, 2, and 3 days after transient (5 min) ischemia. With no sign of ongoing Ca2+ overload, voltage-dependent Ca2+ transients were actually reduced after 2-3 days of reperfusion. Resting Ca2+ levels and recovery rate after loading were similar to neurons receiving no ischemic insult. The tetrodotoxin-insensitive Ca spike, normally generated by these neurons, was absent at 2 days postischemia, as was a large fraction of Ca2+-dependent spike train adaptation. These surprising findings may lead to a new perspective on delayed neuronal death and intervention.

Quantal mechanism of long-term potentiation in hippocampal mossy-fiber synapses

1. The quantal mechanism underlying the expression of long-term potentiation (LTP) was studied in the mossy-fiber (mf) synapses of the rat hippocampus. Whole-cell recordings were used to measure the excitatory postsynaptic currents (EPSCs) before and after LTP induction in brain slices maintained at 31 +/- 1 degrees C. 2. Evoked EPSCs were recorded from 473 CA3 pyramidal neurons. The mf synapses were stimulated using paired pulses (40-ms interpulse interval) repeated every 2-10 s. At least 400 pairs of mf responses were obtained before and during the expression of LTP, which was produced by high-frequency (100 Hz) mf stimulation. Sufficiently stationary data were obtained from five neurons that exhibited LTP and that also satisfied strict criteria and procedures that are necessary for eliciting and identifying unitary mf responses. 3. Three independent lines of evidence implicated a presynaptic component to the mechanism underlying mf LTP. The first was based on a graphical version of the classical method of variance. The graphical variance (GV) method was evaluated by clamping the cell at two different holding potentials during paired-pulse facilitation (PPF). The results indicated that the GV method can distinguish changes in mean quantal content m and mean quantal size q in rat mf synapses. The same analysis, when applied to PPF before and after LTP induction, indicated that both result from an increase in m. 4. The second line of evidence was based on the classical method of failures. Consistent with the inference that mf LTP is due to an increase in m, there was a statistically significant reduction in the number of quantal release failures.(ABSTRACT TRUNCATED AT 250 WORDS)