Electrophysiological and morphological analyses in two species of insects, Protophormia terraenovae and Ceratitis capitata

Chemoreception represents one of the most important sensory modality to guarantee animals survival, and it plays a fundamental role also in an insect life. In fact, the detection of food sources and proper sites of ovoposition, the identification of conspecifics for mating or aggregation, the recognition of prey or predators are all behaviors resulting from the activation of different processes after the exposure to the wide range of soluble and volatile chemical molecules in the environment. In this respect, insects present clearly separate senses of taste and olfaction comparable to those of vertebrates, and they represent an excellent experimental model to investigate the complexity of gustative and olfactory systems also thanks to the relative simple organization of their neuronal circuits, as well as the feasibility of the breeding. On the basis of these considerations, aim of my work was to give a contribution on our understanding of chemoreceptive mechanisms. In particular, I investigated two main aspects of chemoreception in insects, divided in two separate sections. Section 1: The spike generator in the labellar taste receptors of the blowfly is differently affected by 4-aminopyridine and 5-hydroxytryptamine In taste chemoreception of invertebrates the interaction of taste stimuli with specific membrane receptors and/or ion channels located in the apical membrane of taste receptor cells results in the generation of a receptor potential which, in turn, activates the ‘encoder’ region to produce action potentials which propagate to the CNS. This study investigates, in the labellar chemosensilla of the blowfly, Protophormia terraenovae, the voltage-gated K+ currents involved in the action potential repolarization and repetitive firing of the neurons by way of the Kv channel inhibitors, 4-aminopyridine and 5-hydroxytryptamine. The receptor potential and the spike activity were simultaneously recorded from the ‘salt’, ‘sugar’ and ‘deterrent’ cells, by means of the extracellular side-wall technique, in response to 150 mM NaCl, 100 mM sucrose and 1 mM quinine HCl, before, 0-10 min after apical administration of 4-AP (0.01–10 mM) or 5-HT (0.1–100 mM). The results show that the receptor potential in all three cells is neither affected by 4-AP nor by 5-HT. Instead, spike activity is significantly decreased, by way of blocking different Kv channel types: an inactivating A-type K+ current (KA) modulating repetitive firing of the cells and responsible for the after hyperpolarization, and a sustained K+ current that resembles the delayed rectifier (DKR) and contributes to action potential repolarization. Section 2: Morphological characterization of the antennal lobes in the Mediterranean fruit fly Ceratitis capitata The medfly (Ceratitis capitata Wied.) is one of the most important pest for horticulture, targeting a great variety of fruit and vegetables species worldwide. Due to its commercial relevance, many studies focused on the development and improvement of control strategies based on olfactory chemoreception. A complete knowledge of the anatomical and functional properties of the olfactory system is still lacking. Aim of this work is to give a morphological characterization based on the three-dimensional reconstruction of the antennal lobes (ALs) in adult medfly brains. In order to reach this goal, we performed unilateral antennal backfills of olfactory receptor neurons (ORNs) in adult males and females by means of the neuronal tracer neurobiotin, revealed by streptavidin-Cy3 or Avidin-Alexafluor 488 conjugated. In association with the anterograde staining, immunohistochemistry was applied in some brains. Confocal stacks acquired from whole-mount specimens were analyzed with the AMIRA software, using the Segmentation tool. Unilateral neurobiotin and immunohistochemical stainings successfully revealed the AL structure of the adult medfly in all the specimens tested. As in other insects, the ALs of C. capitata are organized in glomeruli, more tightly packed in the anterior part than the posterior one. Axons of ORNs innervate a bilateral pair of homologous glomeruli and form a commissure between the two ALs, which is a typical feature of Diptera. We counted systematically a number of 53 glomeruli in each AL studied, with few exception. Our results provide a basis for future investigations on the interactions with host plants of this important agricultural pest.