Functional regeneration of the meso-cortico-limbic dopaminergic system as a model to study novel neuroreparative strategies

Dopaminergic neurons in the VTA send projections to different forebrain structures, forming a complex crucial neuromodulatory system. Prefrontal, orbitofrontal and cingulated cortices receive the most marked innervation from the VTA; projections from the VTA to the medial prefrontal cortex (mPFC) constitute a portion of the mesocortical dopamine system. Much of this connectivity is bidirectional. Furthermore, the mesocortical dopamine system is involved in a great variety of brain functions, such as working memory, attention selection and memory retrieval, because of its interconnections with brain areas processing external information as well as internal information (Miller et al., 2002). Dopaminergic afferents from the mesencephalon seem to have a crucial role in the normal development of the PFC as well as in the regulation of neuronal activity in this brain area (Lewis et al., 1998). In fact, an abnormal maturation and alterations of the mesocortical projections to the PFC have been suggested to be involved in the development of psychostimulant-induced sensitization and in the pathophysiology of several disorders, such as schizophrenia, addiction or attention- deficit/hyperactivity disorder. The analysis of the formation and development of functional connections in the Central Nervous System needs suitable model systems to understand the basic, functional mechanisms (Hofmann et al., 2004). We have reconstructed parts of the mesocorticolimbic dopaminergic system using the model of organotypic co-cultures of tissue slices from the VTA/SN-complex and the PFC. In fact, organotypic brain slices are closer to the in vivo situation than cell cultures and they represent the most intact culture system for studying cortex function in isolation (Gähwiler et al., 1997; Karpiak and Plenz, 2002; Stewart and Plenz, 2008); furthermore, by combining organotypic cultures with microelectrode array (MEA) recordings, it is possible to simultaneously and repeatedly perform extracellular, non-invasive, multi-site recordings from electrodes covering the whole preparation, and to monitor long- term processes of neurite outgrowth and development and synapse formation at a functional level (Egert et al., 1998; Beggs and Plenz, 2003 and 2004; Hofmann et al., 2004). The aim of the present study was to characterize the developmental features of the VTA/SN-PFC co-cultures maintained on multielectrode array platforms, in order to study the functionality of the neuronal projections which have been demonstrated to grow in vitro between the two areas of the cultures (Franke et al., 2003; Heine et al., 2007). Furthermore, we validated the system as an useful pharmacological tool to study the effects of different neuromodulatory substances.