~ Pergamon 0306-4522(94)E0124-M Neuroscience Vol. 61, No. 3, pp. 565-573, 1994 Elsevier ScienceLtd Copyright © 1994IBRO Printed in Great Britain.All rights reserved 0306-4522/94$7.00+ 0.00 REGULATION OF 1,4,5-IP3, 1,3,4,5-IP4 AND IP 6 BINDING SITES FOLLOWING ENTORHINAL CORTEX LESIONS IN RAT BRAIN A. PARENT, J. POIRIER, A. BACCICHET and R. QUIRION* Douglas Hospital Research Centre and Dept. Psychiatry, Centre for Studies in Aging, McGill University, 6875 LaSalle Blvd., Verdun, Qu6bec, Canada H4H 1R3 Abstract--A lesion of the entorhinal cortex produces a loss of more than 80% of the synapses in the outer molecular layer of the hippocampus in the rat. However, this synaptic loss is transient. Beginning a few days after denervation, new synapses are formed, virtually replacing the lost inputs within two months. Synaptic remodelling induced by entorhinal cortex lesion is associated with specific modifications of various neurotransmitters, hormones and growth factors. Many of these substances act at membrane bound-receptors to induce the hydrolysis of phosphatidylinositols generating various inositol phosphates. Some of the key members of this family include inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakispho- sphate and inositol hexakisphosphate which are all associated with the maintenance Ca 2+ homeostasis. To investigate the potential roles and/or alterations of inositol phosphates in entorhinal cortex lesions-induced neuronal plasticity, we quantified specific receptor sites for inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate using their respective tritiated ligands, at different periods post-lesion corresponding to the degenerative and subsequent reinnervation phases. [3H]inositol 1,4,5-trisphosphate binding sites are maximally increased (30%) between two and eight days post-lesion in the hippocampal formation on both sides of the lesion. In the cortex, [3H]inositol 1,4,5-trisphosphate binding increased also bilaterally following the lesion. Changes in [3H]inositol 1,3,4,5-tetrakisphosphate binding are delayed and reduced (20% increase) in magnitude compared to these seen for [3H]inositol 1,4,5-trisphosphate binding. The maximal peak in [3H]inositol 1,3,4,5-tetrakisphos- phate binding is observed between eight and 14 days after the lesion in the hippocampal formation and the cortex. On the other hand, decreases in [3H]inositol hexakisphosphate binding (up to 30%) in the parietal cortex and the pyramidal cell layer of the hippocampal formation were observed between 14 and 30 days post-lesion. Taken together, these results suggest that inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate receptors can be regulated in vivo following entorhinal cortex lesions. A unique time course is observed for each inositol phosphate receptor site studied. This finding supports the hypothesis suggesting that each inositide is differentially involved in the process of neuronal plasticity observed following deafferentation in the entorhinal cortex of rats. The bilateral changes in unilaterally lesioned animals is consistent with secondary synapse turnover associated with deafferentation. Entorhinal cortex lesions (ECL) in rats is a well known model of synaptic plasticity. 7's'~2'44Following such a lesion, a severe loss of hippocampal input is observed. 37 The greater proportion of degenerating synapses appears two days post-lesion and represents up to 80% of the total number of synapses in the molecular layer of the dentate gyrus. 37 However, between day 8 and 30, the deafferented zone of the hippocampus is repopulated by newly formed synapses. Therefore, prominent hippocampal sprout- ing occurs, forming new synapses to replace most of those lost following the lesion. TM Synaptic remodelling in the CNS is generally *To whom correspondence should be addressed. Abbreviations: ECL, entorhinal cortex lesion; EDTA, ethylenediaminetetraacetic acid; IP, inositol phosphate; 1,4,5-IP3, inositol 1,4,5-trisphosphate; 1,3,4,5-IP4, inosi- tol 1,3,4,5-tetrakisphosphate; IP6, inositol hexakisphos- phate. associated with the specific modifications of a variety of neurotransmit~ers, hormones and growth factors. These neuronal factors acting on their specific recep- tors exert their effects, including neuronal survival, through various transduction mechanisms such as the activation of phospholipase C known to induce the hydrolysis of phosphatidylinositol and to produce various inositol phosphates. L14,34,36 The best known member of this family is the inositol 1,4,5-tri- sphosphate (1,4,5,-IP3). 1,4,5-IP 3 binds to intra- cellular membrane high affinity receptors to induce the release of high amounts of Ca 2÷ into the cyt0- SO1.6,13,14,68 1,4,5-IP 3 is rapidly phosphorylated by a specific 1,4,5-IP 3 3-kinase to form the inositol 1,3,4,5-tetrak- isphosphate (1,3,4,5-IP4). 27,36High affinity 1,3,4,5-IP4 binding sites have been characterized in a variety of tissues including the brain. ~°'tL47,s9 As compared to 1,4,5-IP 3 the functional relevance of 1,3,4,5-IP 4 is more controversial. 26-2sIt has been proposed that this 565