A Tyrosine-Hydroxylase Characterization of Dopaminergic Neurons in the Honey Bee Brain

Abstract

Dopamine (DA) plays a fundamental role in insect behavior as it acts both as a general modulator of behavior and as a value system in associative learning where it mediates the reinforcing properties of unconditioned stimuli (US). Here we aimed at characterizing the dopaminergic neurons in the central nervous system of the honey bee, an insect that serves as an established model for the study of learning and memory. We used tyrosine hydroxylase (TH) immunoreactivity (ir) to ensure that the neurons detected synthesize DA endogenously. We found three main dopaminergic clusters, C1–C3, which had been previously described; the C1 cluster is located in a small region adjacent to the esophagus (ES) and the antennal lobe (AL); the C2 cluster is situated above the C1 cluster, between the AL and the vertical lobe (VL) of the mushroom body (MB); the C3 cluster is located below the calyces (CA) of the MB. In addition, we found a novel dopaminergic cluster, C4, located above the dorsomedial border of the lobula, which innervates the visual neuropils of the bee brain. Additional smaller processes and clusters were found and are described. The profuse dopaminergic innervation of the entire bee brain and the specific connectivity of DA neurons, with visual, olfactory and gustatory circuits, provide a foundation for a deeper understanding of how these sensory modules are modulated by DA, and the DA-dependent value-based associations that occur during associative learning.

Tyrosine hydroxylase-immunoreactive neurons in the mushroom body of the field cricket, Gryllus bimaculatus

“…Putative dopaminergic neurons projecting to the mushroom body have been demonstrated in the cockroach , flies (Nӓssel and Elekes 1992;Mao and Davis 2009;Aso et al 2014), the honey bee (Schäfer and Rehder 1989;Tedjakumala et al 2017), the locust (Wendt and Homberg 1992), and the cricket (present study). Some of these neurons are very well conserved between species although some minor differences also exist, suggesting conservation and speciesspecific variations of their roles in the mushroom body.…”

“…In the field cricket, a cluster of TH-immunoreactive cells (TH-IP1 neurons) are located beneath the medial lobe, and they project their branches into the distal portions of the medial and γ lobes. Anatomically corresponding neurons are C1 neurons in the honey bee (Schäfer and Rehder 1989;Tedjakumala et al 2017), DIP1 neurons in the cockroach , DIP1 neurons in the locust (Wendt and Homberg 1992), and PAM neurons in flies (Nӓssel and Elekes 1992;Aso et al 2014). Another cell cluster (TH-IP2) in the field cricket may be homologous to C2 neurons in the honey bee (Schäfer and Rehder 1989;Tedjakumala et al 2017), DIP2 neurons in the cockroach , and DIP2 neurons in the locust (Wendt and Homberg 1992).…”

“…The TH-SLP neurons possess putatively dendritic fibers in the superior lateral protocerebral neuropil and project their axons into the distal vertical lobe. This is the most promising candidate for the one that conveys aversive signals (unconditioned stimulus, US) into the lobes to ) and the honey bee (a subset of C3 neurons) (Schäfer and Rehder 1989;Tedjakumala et al 2017). As in the field cricket and the fruit fly, dopaminergic neurons play important roles in aversive olfactory and visual learning in the honey bee (Vergoz et al 2007).…”

“…The most striking similarity is seen in neurons projecting to the lobes. According to the location of their cell bodies and axon projection pattern, the TH-SLP neurons in the field cricket display close resemblance to DCa1 neurons in the cockroach ), a subset of C3 neurons in the honey bee (Schäfer and Rehder 1989;Tedjakumala et al 2017), and PPL1 neurons in flies (Nӓssel and Elekes 1992;Aso et al 2014). These have cell bodies in the cell body rind ventro-lateral to the calyx and extend the fibers to the vertical and/or medial lobes.…”

Species- and size-related differences in dopamine-like immunoreactive clusters in the brain of Nasonia vitripennis and N. giraulti

“…Here, we investigate differences in numbers and morphology of dopaminergic neurons between normal-and small-sized N. vitripennis females and between normal-sized N. vitripennis and N. giraulti females. First, by analyzing immunoreactivity against dopamine, we will provide a comparison of dopaminergic clusters in the Nasonia brain with the clusters known from the fruit fly (Nässel and Elekes 1992;Mao and Davis 2009) and two hymenopteran species, the honey bee (Tedjakumala et al 2017) and Trichogramma evanescens (van der Woude and Smid 2017a). This comparison will be based mainly on the comparative location of the clusters within the cell body rind, relative to specific neuropils, as well as the putative projection targets of these clusters.…”