We examine the role of synaptic activity in the development of identified Drosophila embryonic motorneurons. Synaptic activity was blocked by both pan-neuronal expression of tetanus toxin light chain (TeTxLC) and by reduction of acetylcholine (ACh) using a temperature-sensitive allele of choline acetyltransferase (Cha^ts2). In the absence of synaptic activity, aCC and RP2 motorneurons develop with an apparently normal morphology and retain their capacity to form synapses. However, blockade of synaptic transmission results in significant changes in the electrical phenotype of these neurons. Specifically, increases are seen in both voltage-gated inward Na⁺ and voltage-gated outward K⁺ currents. Voltage-gated Ca²⁺currents do not change. The changes in conductances appear to promote neuron excitability. In the absence of synaptic activity, the number of action potentials fired by a depolarizing ramp (−60 to +60 mV) is increased and, in addition, the amplitude of the initial action potential fired is also significantly larger. Silencing synaptic input to just aCC, without affecting inputs to other neurons, demonstrates that the capability to respond to changing levels of synaptic excitation is intrinsic to these neurons. The alteration to electrical properties are not permanent, being reversed by restoration of normal synaptic function. Whereas our data suggest that synaptic activity makes little or no contribution to the initial formation of embryonic neural circuits, the electrical development of neurons that constitute these circuits seems to depend on a process that requires synaptic activity.

In this study, we describe the development of electrical properties of Drosophila embryonic central neurons in vivo. Using whole-cell voltage clamp, we describe the onset of expression of specific voltage- and ligand-gated ionic currents and the first appearance of endogenous and synaptic activity. The first currents occur during midembryogenesis [late stage 16, 13–14 hr after egg laying (AEL)] and consist of a delayed outward potassium current (I_K) and an acetylcholine-gated inward cation current (I_ACh). As development proceeds, other voltage-activated currents arise sequentially. An inward calcium current (I_Ca) is first observed at 15 hr AEL, an inward sodium current (I_Na) at 16 hr AEL, and a rapidly inactivating outward potassium current (I_A) at 17 hr AEL. The inward calcium current is composed of at least two individual and separable components that exhibit small temporal differences in their development. Endogenous activity is first apparent at 15 hr AEL and consists of small events (peak amplitude, 5 pA) that probably result from the random opening of relatively few numbers of ion channels. At 16 hr AEL, discrete (10–15 msec duration) currents that exhibit larger amplitude (25 pA maximum) and rapid activation but slower inactivation first appear. We identify these latter currents as EPSCs, an indication that functional synaptic transmission is occurring. In the neurons from which we record, action potentials first occur at 17 hr AEL. This study is the first to record from Drosophila embryonic central neurons in vivo and makes possible future work to define the factors that shape the electrical properties of neurons during development.

Dynamic changes in synaptic connectivity and strength, which occur during both embryonic development and learning, have the tendency to destabilize neural circuits. To overcome this, neurons have developed a diversity of homeostatic mechanisms to maintain firing within physiologically defined limits. In this study, we show that activity-dependent control of mRNA for a specific voltage-gated Na⁺channel [encoded byparalytic(para)] contributes to the regulation of membrane excitability inDrosophilamotoneurons. Quantification ofparamRNA, by real-time reverse-transcription PCR, shows that levels are significantly decreased in CNSs in which synaptic excitation is elevated, whereas, conversely, they are significantly increased when synaptic vesicle release is blocked. Quantification of mRNA encoding the translational repressorpumilio(pum) reveals a reciprocal regulation to that seen forpara. Pumilio is sufficient to influenceparamRNA. Thus,paramRNA is significantly elevated in a loss-of-function allele ofpum(pum^bemused), whereas expression of a full-lengthpumtransgene is sufficient to reduceparamRNA. In the absence ofpum, increased synaptic excitation fails to reduceparamRNA, showing that Pum is also necessary for activity-dependent regulation ofparamRNA. Analysis of voltage-gated Na⁺current (I_Na) mediated byparain two identified motoneurons (termed aCC and RP2) reveals that removal ofpumis sufficient to increase one of two separableI_Nacomponents (persistentI_Na), whereas overexpression of apumtransgene is sufficient to suppress both components (transient and persistent). We show, through use of anemone toxin (ATX II), that alteration in persistentI_Nais sufficient to regulate membrane excitability in these two motoneurons.

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene and is characterized by progressive muscle wasting. A number of Duchenne patients also present with mental retardation. The dystrophin protein is part of the highly conserved dystrophin-associated glycoprotein complex (DGC) which accumulates at the neuromuscular junction (NMJ) and at a variety of synapses in the peripheral and central nervous systems. Many years of research into the roles of the DGC in muscle have revealed its structural function in stabilizing the sarcolemma. In addition, the DGC also acts as a scaffold for various signaling pathways. Here, we discuss recent advances in understanding DGC roles in the nervous system, gained from studies in both vertebrate and invertebrate model systems. From these studies, it has become clear that the DGC is important for the maturation of neurotransmitter receptor complexes and for the regulation of neurotransmitter release at the NMJ and central synapses. Furthermore, roles for the DGC have been established in consolidation of long-term spatial and recognition memory. The challenges ahead include the integration of the behavioral and mechanistic studies and the use of this information to identify therapeutic targets.