Structural requirements for stimulation by thyrotropin-releasing hormone of α-MSH release from rainbow trout (Oncorhynchus mykiss) pituitary fragments in vitro

Juvenile rainbow trout (Oncorhynchus mykiss) pituitary glands were isolated and the neurointermediate lobes (NILs) were removed from the partes distalis. NILs were surgically fragmented, pooled, and superfused in vitro with culture media, then treated with 3-min pulses of native thyrotropin-releasing hormone (TRH) or TRH analog; 10-min fractions were collected and stored (−20 °C) for subsequent analysis of α-melanocyte-stimulating hormone (α-MSH) content by specific radioimmunoassay. After 1–2 h of superfusion, α-MSH release from the NIL remained relatively constant; α-MSH-like immunoreactivity was not detected in eluate from the partes distalis in a series of parallel experiments. Native TRH stimulated acute releases of α-MSH from the NILs with a minimum effective dose of 10−9 M and an estimated ED50 of 1.73 × 10−9 M on the basis of increasing dose–response experiments; decreasing dose–response data provide an estimated minimum effective dose and ED50 of 10−9 and 1.57 × 10−9 M, respectively. No up- or do...