Targeted expression of somatostatin in vasopressinergic magnocellular hypothalamic neurons of transgenic mice

Somatostatin (SS) is a neuropeptide synthesized in the mammalian brain. To identify the cis-acting elements responsible for neural-specific expression we produced transgenic mice by microinjecting a 15-kb fragment of the mouse SS gene that contains both exons, the intron, 12 kb of the 5' flanking region, and 2 kb of the 3' flanking region. As a reporter, a 30-bp oligonucleotide was inserted in the 5' untranslated region of the SS gene. Four pedigrees of transgenic mice with different chromosomal integrations were identified by Southern blots. In situ hybridizations revealed an unexpected expression of the transgene in the paraventricular, supraoptic, and retro-chiasmatic nuclei of the hypothalamus. Conversely, areas with high levels of endogenous SS mRNA, including the periventricular nucleus of the hypothalamus, reticular nucleus of the thalamus, and hippocampus were devoid of transgene-encoded SS expression. Immunocytochemistry with a pro-SS antibody labeled the identical ectopic hypothalamic and the normal somatostatinergic neurons. Double-staining immunocytochemistry showed colocalization of SS and vasopressin (AVP) in all groups of hypothalamic magnocellular neurons while there was no colocalization of SS and oxytocin. In dehydrated transgenic mice, hypothalamic AVP and transgenic SS mRNA increased 2.5- and 2.3-fold, respectively. Our results indicate that the 15-kb fragment of the SS gene does not provide all the necessary elements to activate its own transcription in somatostatinergic neurons. In addition, the expression of the transgene in vasopressinergic magnocellular neurons suggests that the SS gene fragment contains sequences that are recognized by transcriptional activating factors present in these cells.