We aimed at the further characterization of rats in which SERT gene silencing was achieved by hippocampal injection of a lentiviral vector, carrying three si-RNA to block SERT mRNA at 66% of normal levels. Improved self-control and reduced restlessness were already demonstrated in these rats. Present further studies consisted of male adult rats, bilaterally inoculated within the hippocampus; control rats received lentivirus particles inactivated with heat. Both groups were maintained in isolation for 5 months, starting from inoculation. Neurochemical changes were studied by proton magnetic resonance spectroscopy (1H-MRS): we found increased hippocampal viability and bioenergetic potential; however, rats showed a behaviorally depressive pattern, also characterized by enhanced affiliation. Based on the extent of such effects, the whole lenti-SERT group was divided into two subgroups, termed intermediate- and extreme- phenotype profiles. While all rats had a widespread modification within dorsal/ventral striatum, amygdala, and hypothalamus, only the former subgroup showed an involvement of Raphé medialis, while, for the latter subgroup, an increase of SERT within hippocampus was unexpectedly caused. Within the less-affected "intermediate" rats, hippocampal 5-HT7 receptors were down-modulated, and also similarly within substantia nigra, septum, and neocortex. This picture demonstrates that additional rather than fewer neurobiological changes accompany a lower phenotypic expression. Overall, tapping hippocampal SERT affected the balance between habits versus strategies of coping by promoting morphogenetic processes indicative of a serotonergic fiber plasticity. Supplementary studies about serotonergic dynamics and neurogenesis within fronto-striatal circuits are needed.
Keywords: 5-HT7 receptor; Porsolt's forced swim test; dentate gyrus; elevated plus maze (EPM); in vivo gene silencing; magnetic resonance spectroscopy (1H-MRS); novelty seeking test; serotonin transporter (SERT); social interaction test (SIT).
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