The effect of heat stress on the activity of A1 and A2 neurosecretory neurons of Morimus funereus (Coleoptera: Cerambycidae) larvae

A study on the effect of heat stress on the activity of the medial A1 and A2 neurosecretory neurons (NSN), and the activity of midgut a-amylase and protease was performed on the fifth larval instar of Morimus funereus. Exposure of the larvae to 35°C led to a decrease in the activity of both A1 and A2 NSN, the former being more marked than the latter. These neurons responded differently to heat stress in terms of neurosecretory material synthesis and release. The changes in the size of both A1 NSN and their nuclei and protease activity in the larvae exposed to elevated temperature were significantly correlated, which sug­ gests that A1 NSN play a role in the regulation of the activity of this enzyme during heat stress. The significant correlations between the size of A2 NSN and protease activity in the controls, and the size of both A2 NSN and their nuclei and amylase activity in larvae exposed to heat stress, similarly may mean that A2 NSN regulate both digestive enzyme activities depending on environmental tem­ perature. INTRODUCTION Temperature is one of the most important environ­ mental factors affecting the performance and ecological distribution of organisms. High temperatures affect mor­ tality, body size (Alpatov, 1930; Ochieng-Odero, 1992; Matsuki et al., 1994), flight ability (Shirai, 1993), oxygen consumption (Gray et al., 1991), juvenile hormone syn­ thesis (Cusson et al., 1990), and the activities of digestive enzymes (Applebaum et al., 1964; Ivanovic et al., 1975a, b), glycogen phosphorylase (Chen & Denlinger, 1990), glycolytic enzymes and the respiratory chain and detoxifi­ cation enzymes (Burnell et al., 1991). Elevated temperatures also disrupt the capability of insects to regu­ late water and Clions (Colburn, 1983) and their toler­ ance to other stressors (Riskallan, 1984; DaLage et al., 1988). High temperature disturbs the synthesis and release of neurosecretory material (NSM) from neurosecretory neu­ rons (NSN), and thus the hormonal equilibrium to a greater or lesser extent (Wiglesworth, 1952), which can interrupt or even stop development and metamorphosis (O’Kasha, 1964). Studies on acclimatization and acclimation in Morimus funereus larvae revealed seasonal changes in the activity of some digestive enzymes (protease and amylase), ther­ moresistance, haemolymph ion and trehalose concentra­ tion, qualitative and quantitative free amino acid composition and the level of glycogen in the fat body (Ivanovic, 1969; Ivanovic et al., 1975a; Ivanovic & Jankovic-Hladni, 1991; Jankovic-Hladni et al., 1992). These studies included investigations of the changes in proto­ cerebral NSN activity based on cytological parameters, and showed an interrelationship between the changes in some NSN of the medial group and in the proteolytic and amylolytic activity in the midgut of the larvae. The control of the activity of the digestive enzymes in insects has been little studied, although it is generally accepted that hormones play a dominant role in protein metabolism, i.e. in the synthesis of digestive enzymes. Indirect evidences suggest that the digestive enzyme activity in M. funereus larvae is under neurohormonal regulation (Ivanovic et al., 1978; Ivanovic et al., 1998). The aim of this study is to compare changes in protocere­ bral NSN and in digestive enzyme activity in larvae sub­ jected to heat stress. In addition, the possible role of the different morphological types or subtypes of neurosecre­ tory neurons in the synthesis of neurohormones that regu­ late proteolytic and amylolytic activities in M. funereus larvae was determined. MATERIAL AND METHODS Experimental animals Adult specimens of M. funereus (Coleoptera: Cerambycidae) collected from oak trees growing in a mixed deciduous forest on Fruska Gora Mt. were placed in separate cages for mating and egg laying and kept in a chamber at 23°C. In order to accurately determine each larval instar, the larvae were reared individually under controlled conditions on an artificial diet used for rearing DrosophUa (Roberts, 1986), at a constant temperature of 23°C, relative humidity of 70% and in the dark. Under these con­ ditions, the fifth larval instar lasts for 15 days. Seven day old 5th instar larvae, which had been reared at 23°C, were divided into three groups each consisting of five individuals: (a) larvae kept at 23°C for 15 days (control); (b) those kept initially at 23°C then exposed to 35°C from day 7 to 15 and (c) those kept initially at 23°C and then exposed to 35°C from day 7to11 and then transfered to23°C for 4 days. All larvae were killed at 8 a.m. * Corresponding author: e-mail:jeliva@ibbi.ibiss.bg.ac.yu