1. The effects of synapses established by smooth dendritic neurones on pyramidal and spiny stellate cells were studied in areas 17 and 18 of the cat visual cortex in vitro. Paired intracellular recordings with biocytin‐filled electrodes and subsequent light and electron microscopic analysis were used to determine the sites of synaptic interaction. 2. All smooth dendritic cells established type II synapses previously shown to be made by terminals containing GABA, therefore the studied cells are probably GABAergic. Three classes of presynaptic cell could be defined, based on their efferent synaptic target preference determined from random samples of unlabelled postsynaptic cells. (a) Basket cells (n = 6) innervated mainly somata (49.9 +/‐ 13.8%) and dendritic shafts (45.2 +/‐ 10.7%) and, to a lesser extent, dendritic spines (4.9 +/‐ 4.6%). (b) Dendrite‐targeting cells (n = 5) established synapses predominantly on dendritic shafts (84.3 +/‐ 9.4%) and less frequently on dendritic spines (11.2 +/‐ 6.7%) or somata (4.5 +/‐ 4.7%). (c) Double bouquet cells (n = 4) preferred dendritic spines (69.2 +/‐ 4.2%) to dendritic shafts (30.8 +/‐ 4.2%) as postsynaptic targets and avoided somata. 3. Interneurones formed 5240 +/‐ 1600 (range, 2830‐9690) synaptic junctions in the slices. Based on the density of synapses made by single interneurones and the volume density of GABAergic synapses, it was calculated that an average interneurone provides 0.66 +/‐ 0.20% of the GABAergic synapses in its axonal field. 4. The location of synaptic junctions on individual, identified postsynaptic cells reflected the overall postsynaptic target distribution of the same GABAergic neurone. The number of synaptic junctions between pairs of neurones could not be predicted from light microscopic examination. The number of electron microscopically verified synaptic sites was generally smaller for the dendritic domain and larger for the somatic domain than expected from light microscopy. All presynaptic cells established multiple synaptic junctions on their postsynaptic target cells. A basket cell innervated a pyramidal cell via fifteen release sites; the numbers of synapses formed by three dendrite‐targeting cells on pyramidal cells were seventeen and eight respectively, and three on a spiny stellate cell; the interaction between a double bouquet cell and a postsynaptic pyramidal cell was mediated by ten synaptic junctions. 5. All three types of interneurone (n = 6; 2 for each type of cell) elicited short‐latency IPSPs with fast rise time (10‐90%; 2.59 +/‐ 1.02 ms) and short duration (at half‐amplitude, 15.82 +/‐ 5.24 ms), similar to those mediated by GABAA receptors. 6. Average amplitudes of unitary IPSPs (n = 6) were 845 +/‐ 796 microV (range, 134‐2265 microV). Variability of IPSP amplitude was moderate, the average ratio of IPSP and baseline noise variance was 1.54 +/‐ 0.96. High frequency activation of single presynaptic dendrite‐targeting cells led to an initial summation followed by use‐dependent depression of the averaged postsynaptic response. Double bouquet cell‐evoked IPSPs, recorded in the soma, had a smaller amplitude than those evoked by the other two cell types. In all connections, transmission failures were rare or absent, particularly when mediated by a high number of release sites. 7. The results demonstrate that different types of neocortical GABAergic neurones innervate distinct domains on the surface of their postsynaptic target cells. Nevertheless, all three types of cell studied here elicit fast IPSPs and provide GABAergic input through multiple synaptic release sites with few, if any, failures of transmission.

A sensitivity of the circadian clock to light/dark cycles ensures that biological rhythms maintain optimal phase relationships with the external day. In animals, the circadian clock neuron network (CCNN) driving sleep/activity rhythms receives light input from multiple photoreceptors, but how these photoreceptors modulate CCNN components is not well understood. Here we show that the Hofbauer-Buchner eyelets differentially modulate two classes of ventral lateral neurons (LNvs) within the Drosophila CCNN. The eyelets antagonize Cryptochrome (CRY)- and compound-eye-based photoreception in the large LNvs while synergizing CRY-mediated photoreception in the small LNvs. Furthermore, we show that the large LNvs interact with subsets of “evening cells” to adjust the timing of the evening peak of activity in a day length-dependent manner. Our work identifies a peptidergic connection between the large LNvs and a group of evening cells that is critical for the seasonal adjustment of circadian rhythms. SIGNIFICANCE STATEMENT In animals, circadian clocks have evolved to orchestrate the timing of behavior and metabolism. Consistent timing requires the entrainment these clocks to the solar day, a process that is critical for an organism9s health. Light cycles are the most important external cue for the entrainment of circadian clocks, and the circadian system uses multiple photoreceptors to link timekeeping to the light/dark cycle. How light information from these photorecptors is integrated into the circadian clock neuron network to support entrainment is not understood. Our results establish that input from the HB eyelets differentially impacts the physiology of neuronal subgroups. This input pathway, together with input from the compound eyes, precisely times the activity of flies under long summer days. Our results provide a mechanistic model of light transduction and integration into the circadian system, identifying new and unexpected network motifs within the circadian clock neuron network.

Parkinson's disease (PD) is more commonly associated with its motor symptoms and the related degeneration of dopamine (DA) neurons. However, it is becoming increasingly clear that PD patients also display a wide range of non-motor symptoms, including memory deficits and disruptions of their sleep-wake cycles. These have a large impact on their quality of life, and often precede the onset of motor symptoms, but their etiology is poorly understood. The fruit fly Drosophila has already been successfully used to model PD, and has been used extensively to study relevant non-motor behaviours in other contexts, but little attention has yet been paid to modelling non-motor symptoms of PD in this genetically tractable organism. We examined memory performance and circadian rhythms in flies with loss-of-function mutations in two PD genes: PINK1 and parkin. We found learning and memory abnormalities in both mutant genotypes, as well as a weakening of circadian rhythms that is underpinned by electrophysiological changes in clock neurons. Our study paves the way for further work that may help us understand the mechanisms underlying these neglected aspects of PD, thus identifying new targets for treatments to address these non-motor problems specifically and perhaps even to halt disease progression in its prodromal phase.

How do the pioneer networks in the axial core of the vertebrate nervous system first develop? Fundamental to understanding any full-scale neuronal network is knowledge of the constituent neurons, their properties, synaptic interconnections, and normal activity. Our novel strategy uses basic developmental rules to generate model networks that retain individual neuron and synapse resolution and are capable of reproducing correct, whole animal responses. We apply our developmental strategy to youngXenopustadpoles, whose brainstem and spinal cord share a core vertebrate plan, but at a tractable complexity. Following detailed anatomical and physiological measurements to complete a descriptive library of each type of spinal neuron, we build models of their axon growth controlled by simple chemical gradients and physical barriers. By adding dendrites and allowing probabilistic formation of synaptic connections, we reconstruct network connectivity among up to 2000 neurons. When the resulting “network” is populated by model neurons and synapses, with properties based on physiology, it can respond to sensory stimulation by mimicking tadpole swimming behavior. This functioning model represents the most complete reconstruction of a vertebrate neuronal network that can reproduce the complex, rhythmic behavior of a whole animal. The findings validate our novel developmental strategy for generating realistic networks with individual neuron- and synapse-level resolution. We use it to demonstrate how early functional neuronal connectivity and behavior may in life result from simple developmental “rules,” which lay out a scaffold for the vertebrate CNS without specific neuron-to-neuron recognition.

A central question in behavioural control is how central pattern generators (CPGs) for locomotion are activated. This paper disputes the key role generally accredited to octopamine in activating the CPG for insect flight. In deafferented locusts, fictive flight was initiated by bath application of the muscarinic agonist pilocarpine, the acetylcholine analogue carbachol, and the acetylcholinesterase blocker eserine, but not by nicotine. Furthermore, in addition to octopamine, various other amines including dopamine, tyramine and histamine all induced fictive flight, but not serotonin or the amine-precursor amino acid tyrosine. However, flight initiation was not reversibly blocked by aminergic antagonists, and was still readily elicited by both natural stimulation (wind) and pilocarpine in reserpinized, amine-depleted locusts. By contrast, the muscarinic antagonists atropine and scopolamine reversibly blocked flight initiated by wind, cholinergic agonists, octopamine, and by selective stimulation of a flight-initiating interneurone (TCG). The short delay from TCG stimulation to flight onset suggests that TCG acts directly on the flight CPG, and accordingly that TCG, or its follower cell within the flight generating circuit, is cholinergic. We conclude that acetylcholine acting via muscarinic receptors is the key neurotransmitter in the mechanism underlying the natural activation of the locust flight CPG. Amines are not essential for this, but must be considered as potential neuromodulators for facilitating flight release and tuning the motor pattern. We speculate that muscarinic activation coupled to aminergic facilitation may be a general feature of behavioural control in insects for ensuring conditional recruitment of individual motor programs in accordance with momentary adaptive requirements.