Putative excitatory and putative inhibitory inputs are localised in different dendritic domains in a Drosophila flight motoneuron

Abstract

Input-output computations of individual neurons may be affected by the three-dimensional structure of their dendrites and by the targeting of input synapses to specific parts of their dendrites. However, only few examples exist where dendritic architecture can be related to behaviorally relevant computations of a neuron. By combining genetic, immunohistochemical, and confocal laser scanning methods this study estimates the location of the spike initiating zone and the dendritic distribution patterns of putative synaptic inputs on an individually identified Drosophila flight motorneuron, MN5. MN5 is a monopolar neuron with more than 4000 dendritic branches. The site of spike initiation was estimated by mapping sodium channel immunolabel onto geometric reconstructions of MN5. Maps of putative excitatory cholinergic and of putative inhibitory GABAergic inputs on MN5 dendrites were created by charting tagged Dα7 nicotinic acetylcholine receptors and Rdl GABAA receptors onto MN5 dendritic surface reconstructions. Although these methods provided only an estimate of putative input synapse distributions, the data indicated that inhibitory and excitatory synapses were targeted preferentially to different dendritic domains of MN5, and thus, computed mostly separately. Most putative inhibitory inputs were close to spike initiation, which was consistent with sharp inhibition, as predicted previously based on recordings of motoneuron firing patterns during flight. By contrast, highest densities of putative excitatory inputs at more distant dendritic regions were consistent with the prediction that in response to different power demands during flight, tonic excitatory drive to flight motoneuron dendrites must be smoothly translated into different tonic firing frequencies.

Intra-neuronal Competition for Synaptic Partners Conserves the Amount of Dendritic Building Material

“…The main excitatory input is cholinergic through the Dα7 nicotinic acetylcholine receptor (Dα7nAChR) (Fayyazuddin et al, 2006), whereas the main inhibitory input is GABAergic to the Rdl GABA A receptor. We have previously shown (Kuehn and Duch, 2013) that more than 75% of all cholinergic inputs to the Dα7nAChR are targeted to the proximal dendritic domain of MN5 (Figure 1B, dotted red oval), whereas more than 75% of all GABAergic inputs to the Rdl GABA A R are targeted to dendrites originating from the distal part of the primary neurite (distal domain; Figure 1B, dotted blue ovals). Third, genetic tools allow selective manipulation of synaptic input to the GABAergic or to the cholinergic domain of MN5.…”

“…All MN5 dendrites (Ryglewski et al, 2014a) as well as cholinergic and GABAergic inputs form during pupal life (Kuehn and Duch, 2013). We employed targeted genetic manipulation to selectively alter cholinergic or GABAergic synaptic activity in different parts of the developing MN5 arbor.…”

“…The complex MN5 dendritic arbor with more than 6mmtotal length and more than 4,000 branches covers a diffuse motor neuropil with intermingled terminals of GABAergic and cholinergic synaptic terminals. We have previously shown that cholinergic inputs to the Dα7nAChR are targeted preferentially to the proximal dendritic domain of MN5, whereas GABAergic inputs to the Rdl GABA A R are targeted preferentially to the distal dendritic domain (Kuehn and Duch, 2013) (Figure 1). …”

Dendrites are dispensable for basic motoneuron function but essential for fine tuning of behavior

“…Therefore, additional physiological experiments were conducted. Normally MN5 receives excitatory drive via the Dα7 nicotinic ACh receptor and inhibitory GABAergic input via the Rdl GABA A receptor (21,30). Picospritzing the cholinergic agonist nicotine into the flight neuropil reliably evokes postsynaptic potentials as recorded from the motoneuron somata, both in controls (Fig.…”

“…1A) (17), for which firing patterns and function are well known and can be recorded in vivo during restrained flight (14,18). We have described the dendritic structure (19)(20)(21) and membrane properties (19,22,23) of one such neuron, MN5, in particular detail. MN5 is a unipolar neuron with its soma contralateral to the DLM target muscle ( Fig.…”