Thrombin induces long-term potentiation of reactivity to afferent stimulation and facilitates epileptic seizures in rat hippocampal slices: toward understanding the functional consequences of cerebrovascular insults

Abstract

The effects of thrombin, a blood coagulation serine protease, were studied in rat hippocampal slices, in an attempt to comprehend its devastating effects when released into the brain after stroke and head trauma. Thrombin acting through its receptor, protease-activated receptor 1 (PAR1), produced a long-lasting enhancement of the reactivity of CA1 neurons to afferent stimulation, an effect that saturated the ability of the tissue to undergo tetanus-induced long-term potentiation. This effect was mediated by activation of a PAR1 receptor, because it was shared by a PAR1 agonist, and was blocked by its selective antagonist. An independent effect of thrombin involved the lowering of the threshold for generating epileptic seizures in CA3 region of the hippocampus. Thus, the experiments in a slice mimicked epileptic and cognitive dysfunction induced by thrombin in the brain, and suggest that these effects are mediated by activation of the PAR1 receptor.

Figures

Figure 1.

Thrombin induces long-term potentiation of reactivity to afferent stimulation and saturates tetanus-induced LTP. A, B, Application of thrombin (1 U/ml; A) as well as the PAR1 receptor agonist, PAR1-AP (B), produces a gradual increase in population EPSP in stratum radiatum CA1, without affecting population presynaptic volleys (top traces). C, Thrombin-induced LTP is blocked by the PAR1 receptor antagonist SCH79797, which does not affect tetanic LTP. D, Thrombin induces LTP in a naive pathway without affecting an already saturated pathway, which was previously exposed to three tetanic stimulations. E, A short (10 min) application of thrombin produces a saturating LTP; a tetanic stimulation to the same pathway, adjusted to submaximal level (downward arrow), is able to produce a short-term but not a long-term potentiation. F, Once thrombin-induced LTP has been established, it cannot be blocked by later application of SCH79797, nor could it be further potentiated by a second sustained application of the drug, applied to an adjusted response level (downward arrow). G, Low concentration of PAR1-AP amplifies the ability of a short tetanic stimulation to produce LTP. Short tetanic stimulation applied before PAR1-AP was able to produce a short-lasting but no long-lasting potentiation. PAR1-AP at a low concentration (0.5 μm) did not affect responses to the stimulation, but was able to convert a short into a long-lasting potentiation to short stimulation applied later. H, Thrombin-induced LTP is partially blocked by application of the NR2B antagonist, ifenprodil, as is the potentiation produced by the tetanic stimulation. I, The NMDA antagonist APV totally blocks thrombin-induced LTP, but after washout of the drug, tetanic LTP is completely normal. Averaged EPSP slopes are plotted versus time. Representative traces at indicated times (a, b) are shown on top of each section. Upward arrows indicate the time of HFS.

Figure 2.

Thrombin facilitates the onset of seizure-like activity in hippocampal neurons. A, Thrombin as well as PAR1-AP produces a sustained increase in spontaneous spike activity of single neurons in region CA3. Such an effect is not seen in CA1 region (see below, D). B, Increasing extracellular concentrations of K+ by 4 mm or exposure to 100 μm glutamate does not affect spontaneous activity, but a larger increase, by 15 mm KCl or 500 μm glutamate, produces synchronized seizure-like activity. C, Thrombin or PAR1-AP facilitates the onset of seizure-like activity in the presence of below-threshold concentrations of K+ or glutamate (100 μm). Those seizure-like activities are blocked by PAR1 receptor antagonist, but not by NR2B or NMDA antagonists. D, The onset of seizure-like activity in CA1 depends on its anatomical connections with CA3; in intact hippocampus (middle trace), seizure activity is seen, but is absent when the connection between CA3 and CA1 is severed (bottom trace).