Afferent–target interactions during olivocerebellar development: transcommissural reinnervation indicates interdependence of Purkinje cell maturation and climbing fibre synapse elimination Ann M. Lohof, 1 Jean Mariani 1 and Rachel M. Sherrard 2 1 Laboratoire De ´ veloppement et Vieillissement du Syste ` me Nerveux, UMR 7102 Neurobiologie des Processus Adaptatifs, CNRS et Universite ´ Pierre et Marie Curie, Case courrier 14, 9 quai Saint-Bernard, 75005 Paris, France 2 Developmental Neuroplasticity Laboratory, School of Anatomy and Human Biology, University of Western Australia, Perth, Australia Keywords: cerebellum, plasticity, rat, synaptic regression, synaptogenesis Abstract We have used a model of postlesional reinnervation to observe the interactions between synaptic partners during neosynaptogenesis to determine how the developmental states of the pre- and postsynaptic cells influence circuit maturation. After unilateral transection of the neonatal rat olivocerebellar pathway (pedunculotomy), axons from the remaining ipsilateral inferior olive grow into the denervated hemicerebellum and develop climbing fibre (CF) terminal arbors on Purkinje cells (PCs) at a later stage of development than normal. However, the significance of delayed CF-PC interactions on subsequent circuit maturation remains poorly defined. To examine this question, we recorded CF-induced currents in PCs and analysed PC morphology during the first two postnatal weeks in control animals and following left unilateral inferior cerebellar pedunculotomy on postnatal day (P)3. Our results show that transcommissural olivary axons multiply-reinnervate PCs in the denervated hemisphere over 4 days following pedunculotomy. Each PC received fewer CFs than did age-matched controls and the maximal multi-reinnervation was reached on P7, 2 days later than in controls. Consequently, the onset of CF synapse elimination in reinnervated PCs was delayed, but then proceeded in parallel with controls so that all PCs were monoinnervated by P15. Furthermore, reinnervated PCs had delayed dendritic maturation and subsequent dendritic abnormalities consistent with the role of CF innervation in PC dendritic growth. Thus, within the olivocerebellar system, our data suggest that target neurons depend upon sufficient afferent investment arriving at the correct time for their normal development, and maturation of the target neuron regulates afferent selection and therefore circuit maturation. Introduction Correct nervous system function requires precise neuronal circuitry, which emerges through remodelling of initial broad projections that form during development (Katz & Shatz, 1996). Control of this process lies in bidirectional cellular interactions between synaptic partners. Not only are target cells critically dependent on correct afferent investment for normal structural (Mooney et al., 1992) and physiological (Wiesel & Hubel, 1965; Sherman et al., 1972) maturation, but refinement of axon terminal arbors also requires normal target activity (Thompson, 1983; Rabacchi et al., 1992; Hashimoto & Kano, 2003). While the need for appropriate afferent– target interaction is evident, the temporal specificity of these interactions remains unknown. Denervation–reinnervation experiments provide an opportunity to compare afferent–target interactions during normal synaptogenesis and when it has been delayed. In the peripheral nervous system, the timing of cellular interactions does not appear to be critical. At the neuromuscular junction, for example, regenerating motor axons multiply-reinnervate old synaptic sites and then undergo synapse elimination, recapitulating aspects of normal development within a mature system (van Mier & Lichtman, 1994). However, in the central nervous system, the age-dependency of afferent–target interaction is less clear. A model to examine this question is the olivocerebellar system lesioned during the first postnatal week, in which reinnervating axons synapse upon appropriate target Purkinje cells (PCs; Sugihara et al., 2003), but at a slightly later developmental stage. Olivocerebellar axons cross the midline in the medulla, enter the cerebellum via the inferior cerebellar peduncle and form climbing fibres (CFs) that synapse contralaterally on approximately six PCs, located within a parasagittal microzone (Sugihara et al., 2001). Following unilateral CF transection early in development, the contralateral inferior olive degenerates and new axons, arising from the remaining inferior olive, grow into the denervated hemicerebellum (Zagrebelsky et al., 1997). These transcommissural axons develop normal CF arborizations within parasagittal microzones and form functional synapses on PCs (Sugihara et al., 2003). This normal structure and function suggests recapitulation of normal developmen- tal processes. A major component of olivocerebellar development comprises extensive early contacts producing transient PC multi- innervation that is refined to monoinnervation during the first two postnatal weeks (Crepel et al., 1976; Mariani & Changeux, 1981). During the same period, the target PC undergoes dramatic Correspondence: Dr Ann M Lohof, as 1 above. E-mail: ann.lohof@snv.jussieu.fr Received 4 August 2005, revised 21 September 2005, accepted 13 October 2005 European Journal of Neuroscience, Vol. 22, pp. 2681–2688, 2005 ª Federation of European Neuroscience Societies doi:10.1111/j.1460-9568.2005.04493.x