The role of juvenile hormone and brain factor(s) in the regulation of plasma juvenile hormone esterase activity during the last larval stadium of the tobacco hornworm, Manduca sexta

Abstract

During the last (fifth) larval stadium of gate-1 Manduca sexta there were two peaks of juvenile hormone esterase activity in the plasma. The first peak occurred on day 3 preceding wandering and the second of a similar size on day 8 preceding pupation. The topical application of juvenoids (juvenile hormone I, epofenonane, and ( RS)-methoprene) to the abdomens of prewandering (day 0 to 3) and postwandering (day 5 and 7) larvae increased the plasma juvenile hormone esterase activity. Head ligation and starvation of prewandering larvae blocked the first peak and ligation of postwandering larvae blocked the second peak. Application of juvenoids could not rescue the effects of ligation and starvation in prewandering day 0 to 2 larvae, but ligation of day 1 and 2 larvae for 24 h followed by juvenoid treatment restored normal juvenile hormone esterase activity levels in ligated but not starved larvae. Juvenile hormone acted directly on the abdomen presumably on the fat body to increase juvenile hormone esterase activity throughout the last stadium but day-3 and -7 larvae were most responsive. Injection of head extracts of day-1, -2 and -3 fifth-instar larvae into the abdomens of head-ligated day-1 larvae did not increase the plasma juvenile hormone esterase activity, whereas it increased the α-naphthyl acetate esterase activity. Day-1 head extract decreased the plasma juvenile hormone esterase activity in unligated day-0, -1 and -6 larvae and in 24 h head-ligated, day-2 larvae that were treated with epofenonane. In day-1 unligated larvae treated with epofenonane one head equivalent reduced the juvenile hormone esterase activity by 48%. Injection of 4 head equivalents from day-1 larvae into larvae of the same age not treated with juvenoids completely inhibited the normal increase in juvenile hormone esterase activity seen between day 1 and 2. The inhibitory factor was specific in action not affecting the α-naphthyl acetate esterase activity, was dose-dependent, and found both in head and brain but not in whole body extracts. The inhibitory effect was present in head extracts prepared from day-1, -2 and -3 larvae. Head extracts also significantly decreased the juvenile hormone esterase activity in the whole body of normal as well as epofenonane-treated larvae without affecting the α-naphthyl acetate esterase activity.