Abstract
Olfactory sensory neurons connect to the antennal lobe of the fly to create the primary units for processing odor cues, the glomeruli. Unique amongst antennal-lobe neurons is an identified wide-field serotonergic neuron, the contralaterally-projecting, serotonin-immunoreactive deutocerebral neuron (CSDn). The CSDn spreads its termini all over the contralateral antennal lobe, suggesting a diffuse neuromodulatory role. A closer examination, however, reveals a restricted pattern of the CSDn arborization in some glomeruli. We show that sensory neuron-derived Eph interacts with Ephrin in the CSDn, to regulate these arborizations. Behavioural analysis of animals with altered Eph-ephrin signaling and with consequent arborization defects suggests that neuromodulation requires local glomerular-specific patterning of the CSDn termini. Our results show the importance of developmental regulation of terminal arborization of even the diffuse modulatory neurons to allow them to route sensory-inputs according to the behavioural contexts.
Highlights
Serotonin, 5-hydroxytryptamine (5-HT), an evolutionarily ancient monoamine, plays diverse roles in the brain [1,2,3]
It is estimated that there is one serotonergic neuron per million in the mammalian central nervous system, yet, when axon terminals are examined in the rat cortex, as many as 1/500 are serotonergic [2], suggesting that a small set of neurons may act through their broad arborization pattern to play roles in modulating many brain circuits
In the central nervous systems, across taxa, serotonergic neurons are often small in number but connect to and act upon multiple brain circuits through their wide-field arborization pattern
Summary
5-hydroxytryptamine (5-HT), an evolutionarily ancient monoamine, plays diverse roles in the brain [1,2,3]. An iconic preparation in which this has been carried out is the circuit that drives pyloric rhythm in the crab/lobster stomatogastric system [12,13]. Such studies have led to the view that understanding the function of brain circuits requires a characterization of intrinsic dynamic properties of constituent neurons and their connectivity and an understanding of how specific neurotransmitters and neuromodulators impinge on the circuit [14]
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