Morphology and Physiology of the Serotonin-immunoreactive Putative Antennal Lobe Feedback Neuron in the Male Silkmoth Bombyx mori

Abstract

In the male silkmoth Bombyx mori, olfactory information is relayed from olfactory receptor neurons in the antennae to the antennal lobe, and then to a variety of protocerebral neuropils. Currently, very little is known about neuromodulators that may affect the dynamics of this olfactory neural network. Immunocytochemical studies have revealed the presence of a serotonin-immunoreactive (SI) neuron that, in several insect species, is thought to provide feedback to the antennal lobe. To date, no studies have revealed details of this neuron's physiology. Using intracellular recording and staining, the silkmoth SI neuron (in two individuals) was first characterized physiologically and then stained with Lucifer Yellow to reveal morphological details. Immunocytochemical methods were also used to confirm the presence of serotonin. The silkmoth SI neuron branched in many important brain neuropils such as the mushroom body, central body, lateral accessory lobe and antennal lobe. The SI neuron in both individuals fired spontaneous, long duration action potentials, and responded to mechanosensory stimuli to the antennae.

Visualization of modulatory effects of serotonin in the silkmoth antennal lobe

“…Taken together, these data suggest the possibility of non-synaptic release of serotonin. Serotonin could potentially diffuse throughout a glomerulus and bind to receptors in outer regions of the glomerulus where the SI neuron does not branch (Kent et al, 1987;Sun et al, 1993;Hill et al, 2002). Thus, serotonin released from the SI neuron could potentially bind to receptors on the sensory terminals (located in the outer regions of glomeruli).…”

“…A pair of unique serotonin-immunoreactive (SI) neurons that innervate both ALs has been identified in many insects (Schürmann and Klemm, 1984;Kent et al, 1987;Rehder et al, 1987;Homberg and Hildebrand, 1989;Breidbach, 1990;Salecker and Distler, 1990;Sun et al, 1993;Hill et al, 2002). The B. mori SI neuron innervates every glomerulus in the contralateral AL, fires longduration spontaneous action potentials and responds to mechanosensory stimuli to the antennae (Hill et al, 2002).…”

Serotonin modifies the sensitivity of the male silkmoth to pheromone

“…Furthermore, Hill et al (2002) showed that this SI neuron spikes spontaneously and responds to mechanosensory stimuli to the antennae. Due to the fact that, in M. sexta, its branchings in the AL contain mostly output synapses (Sun et al, 1993), the SI neuron may be involved in a feedback system from the protocerebrum to the AL.…”

“…In moths, serotonin may be released in the ALs by a single pair of serotonin-immunoreactive (SI) neurons with branches in every glomerulus of the AL as well as in higher order neuropil regions of the brain (Kent et al, 1987;Hill et al, 2002). Furthermore, Hill et al (2002) showed that this SI neuron spikes spontaneously and responds to mechanosensory stimuli to the antennae.…”

“…The effects of serotonin on the response to pheromone in moths may be related to the presence of a pair of unique serotonin-immunoreactive neurons that innervate both ALs and have been identified in B. mori (Hill et al, 2002) as well as in several other insects (Schurmann and Klemm, 1984;Kent et al, 1987;Rehder et al, 1987;Breidbach, 1990;Salecker and Distler, 1990).…”