Activity patterns of neurosecretory cells releasing pheromonotropic neuropeptides in the moth Bombyx mori

Abstract

Short-and long-term firing patterns of neurosecretory cells releasing pheromonotropic neuropeptides in the silkworm moth Bombyx mori were examined. The cells showed three types of rhythmic changes in firing activity. Bursting activities with an interval of several seconds were synchronized with rhythmic abdominal motions for calling behavior. A slow f luctuation in firing activity over a period of several minutes depended on cyclic alternations of the f low of hemolymph. The electrical activity displayed a diel rhythm that related to light͞dark cycles of the environment and sex pheromone titers in the pheromone gland. In addition to a transient inhibition of firing caused by a tactile or light stimulus, a long-term permanent inhibition was induced by mating with a fertile male. Thus, the insect neurosecretory system is highly coordinated with physiology and behavior in Bombyx mori and is inf luenced by external stimuli.Female moths extrude the pheromone gland and emit sex pheromone to attract males. The calling behavior and sex pheromone production exhibit a diel periodicity with peak pheromone titers occurring during the photophase or scotophase. This diel periodicity of sex pheromone production in many species of moths is under the control of pheromonotropic neuropeptides, such as a pheromone biosynthesisactivating neuropeptide (PBAN) and PBAN-like factors (1, 2). PBANs have been identified from three species of moths (3-5). Analyses of cDNAs encoding PBANs in Bombyx mori and Helicoverpa zea showed that the neuropeptide is generated along with four additional family neuropeptides from a common precursor polyprotein that is translated from a single mRNA (6-8). In B. mori, the four neuropeptides, including a hormone inducing embryonic diapause, share a conserved pentapeptide amide at the C-terminal and have substantial pheromonotropic activity (8). Six pairs of somata of neurosecretory cells expressing the gene for the precursor protein are aggregated into three clusters localized at the ventral surface of the anterior, medial, and posterior neuromeres of the suboesophageal ganglion (SOG) (9). Surgical ablation of the anterior and medial clusters of somata at an early pupal stage greatly impaired pheromone production at the adult stage, whereas the same operation on the posterior cluster impaired diapause induction rather than pheromone production, thereby suggesting functional specialization of the three classes of neurosecretory cells (10). Intracellular dye injection revealed complete structures of individual neurosecretory cells, including a unique axonal pathway and neurohemal terminals (11). Five axons from two anterior and three medial cells project to the corpus cardiacum (CC) after passing through a branch of the maxillary nerve, nervus corporis cardiaci ventralis (NCC-V), that runs beneath the cuticle of the head. Many varicose terminal branches in the CC and associated nerves of the CC indicate that pheromonotropic neuropeptides synthesized in the somata are transferred to neurohemal areas and...

Combinative stimulation inactivates sex pheromone production in the silkworm moth,Bombyx mori

“…Alternatively, the restrainable stimulus of the abdomen could be sensed by mechanical receptors scattered over the abdominal wall and muscles. Recently, it was shown that mating with a pr-male or a tactile stimulation of abdominal tip inhibits the activity of neurosecretory cells releasing PBAN (Ichikawa, 1998), which supports our results. All values do not differ statistically.…”

Serotonin modifies the sensitivity of the male silkmoth to pheromone

“…mori female pheromone release also shows a circadian rhythm: the release of pheromone increases at the beginning of photophase to reach a peak 6·h later; this peak lasts for 2·h before decreasing until the beginning of scotophase (Ichikawa, 1998). The circadian variation of the male's sensitivity to pheromone allows the male to locate more efficiently the female during its pheromone release peak window.…”

Circadian Firing Activities of Neurosecretory Cells Releasing Pheromonotropic Neuropeptides in the Silkmoth, Bombyx mori

“…2 and 4), thereby indicating that sex pheromone production is under the control of a circadian pacemaker. It is often difficult to find a robust circadian rhythm in pheromone production by measuring pheromone contents in the gland because of a slow decline of pheromone titers after a stop of PBAN secretion (decapitation or mating) (Ichikawa, 1995(Ichikawa, , 1998. It should be noted that the proportion of females expressing a circadian oscillation of PBAN cells in DD was significantly low (10%); one reason being that complete darkness may be repelled by many female moths of Bombyx mori to perform calling behavior, because an abrupt termination of illumination of 100 lx at a mid-photophase stopped firings of PBAN cells and such a silent state often continued for as long as the darkness continued (unpublished observation).…”

“…Long-term chronic recordings from the maxillary nerve branch revealed complex firing activity rhythms of PBAN producing cells that are closely related to calling behavior, circulation of haemolymph, and diurnal changes in pheromone titers in a light/dark cycle (Ichikawa, 1998). Because electrical signals from neurosecretory cells can be sampled from a single female moth with a higher temporal resolution, analyses of the diurnal firing rhythms of PBAN cells may facilitate understanding of circadian control of pheromone production of individual moths and the nature of a circadian oscillator of the adult silkmoth.…”