Serotonergic Neurons in the Brain and Gnathal Ganglion of Larval Spodoptera frugiperda

Abstract

The fall armyworm Spodoptera frugiperda (S. frugiperda) (Lepidoptera: Noctuidae) is a worldwide, disruptive, agricultural pest species. The larvae of S. frugiperda feed on seedling, leave, and kernel of crops with chewing mouthparts, resulting in reduced crop yields. Serotonin is an important biogenic amine acting as a neural circuit modulator known to mediate lots of behaviors including feeding in insects. In order to explore the serotonergic neural network in the nervous system of larval S. frugiperda, we performed immunohistochemical experiments to examine the neuropil structure of the brain and the gnathal ganglion with antisynapsin and to examine their serotonergic neurons with antiserotonin serum. Our data show that the brain of larval S. frugiperda contains three neuromeres: the tritocerebrum, the deutocerebrum, and the protocerebrum. The gnathal ganglion also contains three neuromeres: the mandibular neuromere, the maxillary neuromere, and the labial neuromere. There are about 40 serotonergic neurons in the brain and about 24 serotonergic neurons in the gnathal ganglion. Most of these neurons are wide-field neurons giving off processes in several neuropils of the brain and the gnathal ganglion. Serotonergic neuron processes are mainly present in the protocerebrum. A pair of serotonergic neurons associated with the deutocerebrum has arborizations in the contralateral antennal lobe and bilateral superior lateral protocerebra. In the gnathal ganglion, the serotonergic neuron processes are also widespread throughout the neuropil and some process projections extend to the tritocerebrum. These findings on the serotonergic neuron network in larval S. frugiperda allow us to explore the important roles of serotonin in feeding and find a potential approach to modulate the feeding behavior of the gluttonous pest and reduce its damage.

Keywords: Spodoptera frugiperda; brain; gnathal ganglion; immunoreactivity; neuropils; serotonin; taste.

FIGURE 1

Confocal images of the brain of Spodoptera frugiperda larvae. (A1–A5) Merged confocal image showing the neuropil (magenta) and the serotonin-immunoreactive neurons (green). (B1–B5) Confocal image showing the neuropils of the brain. (C1–C5) Confocal image showing the serotonin-immunoreactive neurons in the brain. α, alpha lobe; AL, antennal lobe; CA, calyx; CB, central body; LAL, lateral accessory lobe; OL, optic lobe; PB,: protocerebral bridge; PED, pedunculus; TR, tritocerebrum. Scale bars, 100 μm.

FIGURE 2

The average and relative volume of the neuropils, the average diameter and volume of cell bodies, and the distribution of the serotonergic neuronal processes and cell bodies in the brain. (A) Three-dimensional reconstructions of the neuropils in the brain in frontal view. (B,C) The average and relative volume of the neuropils. (D) Reconstructions of cell bodies in the brain. (E,F) The average diameter and volume of cell bodies in the brain. (G) Reconstructed skeleton trees of the thick neuronal processes showing their projection patterns in anterior view. (H) Reconstructed skeleton trees of commissures 1–6 in anterior view. PR-M, PR-L, PR-LD, PR-LA, DE-L, and TR-A are cell clusters. α, alpha lobe; AL, antennal lobe; β, belta lobe; CA, calyx; CB, central body; LAL, lateral accessory lobe; LOB, mushroom body lobes; OL, optic lobe; PB, protocerebral bridge; TR, tritocerebrum. Directions: a, anterior; d, dorsal; l, lateral; m, medial; p, posterior; v, ventral. Scale bar, 100 μm.

FIGURE 3

The projection area of PR-M neurons. (A–C) The confocal stack images of PR-M neurons. (D) The anterior view of three-dimensional reconstructions of PR-M neurons. (E) The lateral view of three-dimensional reconstructions of PR-M neurons. AVLP, anterior ventrolateral protocerebrum; CB, central body; IMP, inferior medial protocerebrum; SLP, superior lateral protocerebrum; SMP, superior medial protocerebrum; POTU, posterior optic tubercle.PR-M is cell cluster. Directions: a, anterior; d, dorsal; l, lateral; p, posterior; v, ventral. Scale bar, 100 μm.

FIGURE 4

The projection area of PR-L and LAL neurons. (A) The confocal stack images of PR-L neurons. (B,C) The anterior and lateral view of the three-dimensional reconstructions of the PR-L neurons. (D) The confocal stack images of LAL neuron. (E,F) The three-dimensional reconstructions of the LAL neurons in the anterior and lateral views. α, alpha lobe; β, belta lobe; CA, calyx; CB, central body; LAL, lateral accessory lobe; OL, optic lobe; POTU, posterior optic tubercle; PR-M and PR-L are cell clusters. Directions: a, anterior; d, dorsal; l, lateral; p, posterior; v, ventral. Scale bars, 100 μm.

FIGURE 5

The projection area of DE-L and TR neurons. (A,B) The confocal stack images of DE-L neurons. (C,D) The anterior and lateral views of the three-dimensional reconstructions of the DE-L neurons. (E) The confocal stack images of TR neuron. (F) The three-dimensional reconstructions of the TR neurons in anterior view. α, alpha lobe; AL, antennal lobe; β, belta lobe; CA, calyx; CB, central body; CeC, circumoesphageal connective; PED, pedunculus; SLP, superior lateral protocerebrum; PR-M and DE-L are cell clusters. Directions: a, anterior; d, dorsal; l, lateral; p, posterior; v, ventral. Scale bars, 100 μm.

FIGURE 6

Neuromeres, cell bodies, and processes of the serotonergic neuron in the gnathal ganglion. (A–C) Confocal images showing the distribution of the serotonergic neuron processes and cell bodies in the gnathal ganglion. (D) The confocal stack images of gnathal ganglion. (E) Three-dimensional reconstructions of main neuromeres in gnathal ganglion. (F) The average volume of main neuromeres. (G) The location of cell bodies in gnathal ganglion. (H,I) The average diameter and volume of cell bodies in gnathal ganglion. (J,K) Three-dimensional reconstructions of the serotonergic neurons in the cluster of gnathal ganglion in anterior and lateral view. GNG-AD, GNG-AV, GNG-M, GNG-L1, GNG-L2, and GNG-L3 are cell clusters. MdNe, mandibular neuromere, MxNe, maxillary neuromere, LbNe, labial neuromere. Directions: a, anterior; d, dorsal; l, lateral; m, medial; p, posterior; v, ventral. Scale bars, 100 μm.