Blockade of pilocarpine- or kainate-induced mossy fiber sprouting by cycloheximide does not prevent subsequent epileptogenesis in rats Beatriz M. Longo, Luiz E.A.M. Mello* Department of Physiology, Universidade Federal de Sa ˜o Paulo-EPM, Rua Botucatu 862, Sa ˜o Paulo, 04023-900 SP, Brazil Received 5 March 1997; revised version received 1 April 1997; accepted 1 April 1997 Abstract Post-injury sprouting of hippocampal mossy fibers has been suggested to be a causal mechanism underlying the development of temporal lobe epilepsy. However, this hypothesis rests entirely on indirect correlational evidence. Here we demonstrate that cycloheximide, a protein synthesis inhibitor, blocked pilocarpine- and kainate-induced mossy fiber sprouting in rats, but did not prevent the subsequent development of spontaneous seizures or affect their frequency. These results provide direct evidence against a causal role for mossy fiber sprouting in temporal lobe epileptogenesis.  1997 Elsevier Science Ireland Ltd. Keywords: Epilepsy; Spontaneous seizures; Hippocampus; Pilocarpine; Kainic acid; Protein synthesis inhibitor; Mossy fiber sprouting When rats are given pilocarpine (PILO), a muscarinic cholinergic agonist, they present status epilepticus (SE), and after an average latent period of 2–3 weeks they show spontaneous recurrent epileptic seizures (SRS) [11, 13]. Histological analysis of the hippocampal complex of these rats has revealed physiological and anatomical altera- tions [9,13] similar to those found in both, patients with temporal lobe epilepsy (TLE) [1,8], and other animals mod- els of epilepsy [5,20]. The aberrant reorganization of the dentate mossy fiber system has been touted as one of the leading basis for the hyperexcitability and seizures [1,5, 8,19,20]. This view however, has been challenged by some authors. Accordingly, mossy fiber sprouting would be a reorganization towards ‘homeostasis’, and inhibitory in nature (thus blocking seizures) rather than excitatory [16,18]. In fact, even though most authors tend to believe that mossy fiber sprouting leads to hyperexcitability, two major points arise from the work of these same authors: (1) SRS may develop prior to mossy fiber sprouting; (2) not every animal with mossy fiber sprouting has SRS [18,24]. Conversely, electrical recordings of epileptic tissue from TLE patients and animals models support the notion that mossy fiber sprouting is or can be a basis for seizures [1,5,8,9,20,21]. Arguments in the field have led researchers to show what would be the final proof that mossy fiber sprouting contribute to seizures: mossy fibers, which are granule cell axons, synapse onto granule cell dendrites, thus making a powerful monosynaptic recurrent excitatory circuit [10,15]. A more direct approach for defining the relationship between mossy fiber sprouting and seizures would depend on being able to block mossy fiber sprouting. The use of anti-NGF, c-fos knockout mice or barbiturates have not successfully blocked mossy fiber sprouting and/or asso- ciated seizures in different models of TLE [7,14,22]. Thus, the failure of more specific approaches have led us to use a general inhibitor of protein synthesis, cyclohexi- mide (CHX), in an attempt to block mossy fiber sprouting and studying its role in different models of TLE. Adult, male Wistar EPM-1 rats were injected with CHX (1 mg/kg, s.c.), 30 min later followed by methyl-scopola- mine (1 mg/kg, i.p.), and after an additional 30 min by PILO (320 mg/kg, i.p.). Animals developed SE approximately 30 min after PILO administration. Ninety minutes after SE onset, animals were injected with thionembutal (25 mg/ kg, i.p.), to reduce the otherwise high mortality rate in this period. Control animals were injected with all drugs used in the experimental group except CHX. Incidence of SE in the two groups was absolutely identical, and latencies for SE Neuroscience Letters 226 (1997) 163–166 0304-3940/97/$17.00  1997 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(97)00267-X * Corresponding author. Tel./fax: +55 11 5715651; e-mail: epmnf@eu.ansp.br