Analysis of the regulatory domains in the IGR responsible for hypothalamic OT gene expression. A, The OT constructs used in this analysis were based on the OTIII.EGFP.IGR3.6 construct. Two domains in the 3.6 kb mouse IGR (A, B) were excised and linked to the OTIII.EGFP.Δ-IGR construct (Fig. 4), which contained no IGR sequence (see Materials and Methods). B, Results from the biolistic transfection of rat hypothalamic cultures using the OT constructs shown in A. Cultures were assayed by IHC using fluorescent microscopy, and EGFP-expressing neurons per filter were counted. Data are expressed as averages ± SEMs. Note that a robust enhancer activity is located in the 178 bp segment A domain, and that significant enhancer activity is also present in the 430 bp B domain (see Results). 3.6, OTIII.EGFP.IGR2.1; A, OTIII.EGFP.IGR178; B, OTIII.EGFP.IGR430; A+B, OTIII.EGFP.IGR430 plus OTIII.EGFP.IGR178; Δ, OTIII.EGFP.ΔIGR.