Role of cAMP Cascade in Synaptic Stability and Plasticity: Ultrastructural and Physiological Analyses of Individual Synaptic Boutons inDrosophilaMemory Mutants

Abstract

Mutations of the genes rutabaga (rut) and dunce (dnc) affect the synthesis and degradation of cAMP, respectively, and disrupt learning in Drosophila. Combined ultrastructural analysis and focal electrophysiological recording in the larval neuromuscular junction revealed a loss of stability and fine tuning of synaptic structure and function in both mutants. Increased ratios of docked/undocked vesicles and poorly defined synaptic specializations characterized dnc synapses. In contrast, rut boutons possessed fewer, although larger, synapses with lower proportions of docked vesicles. At reduced Ca(2+) levels, decreased quantal content coupled with an increase in failure rate was seen in rut boutons and reduced pair-pulse facilitation were found in both rut and dnc mutants. At physiological Ca(2+) levels, strong enhancement, instead of depression, in evoked release was observed in some dnc and rut boutons during 10 Hz tetanus. Furthermore, increased variability of synaptic transmission, including fluctuation and asynchronicity of evoked release, paralleled an increase in synapse size variation in both dnc and rut boutons, which might impose problems for effective signal processing in the nervous system. Pharmacological and genetic studies indicated broader ranges of physiological alteration by dnc and rut mutations than either the acute effects of cAMP analogs or the available mutations that affect cAMP-dependent protein kinase (PKA) activity. This is consistent with previous reports of more severe learning defects in dnc and rut mutations than these PKA mutants and allows identification of the phenotypes involving long-term developmental regulation and those conferred by PKA.

Neuronal Activity and Adenylyl Cyclase in Environment-Dependent Plasticity of Axonal Outgrowth inDrosophila

“…The anti-HRP immunoreactive varicosities have been classified into four subtypes: type Ib (big boutons), type Is (small boutons), type II, and type III, on the basis of immunoreactivity, bouton size, types of synaptic vesicles contained, and electrophysiological responses (Johansen et al, 1989;Budnik et al, 1990;Kurdyak et al, 1994;Renger et al, 2000). These subtypes were not distinguished in this study.…”

“…The eag, Sh, and Hk genes encode different subunits of K ϩ channels (Kamb et al, 1987;Papazian et al, 1987;Warmke et al, 1991;Chouinard et al, 1995), and para encodes an Na ϩ channel subunit (Loughney et al, 1989). This activity-dependent enhancement has been suggested to be mediated by elevated cAMP levels in response to hyperneural activities, because dunce (dnc) mutants with reduced phosphodiesterase activity (Byers et al, 1981), and hence higher cAMP levels, also cause enhanced arborization (Zhong et al, 1992;Renger et al, 2000). It remains to be determined how increased neural activity leads to activation of the cAMP pathway, which might be achieved via activation of one or multiple forms of adenylyl cyclase or by inhibition of phosphodiesterase activity.…”

Distinct Frequency-Dependent Regulation of Nerve Terminal Excitability and Synaptic Transmission by IA and IK Potassium Channels Revealed by Drosophila Shaker and Shab Mutations

“…The major focus in the study of activity-dependent synaptic plasticity has been directed to the regulation of transmitter release machinery, including the studies using the Drosophila neuromuscular junction preparation (Kuromi and Kidokoro, 2000;Renger et al, 2000). Less attention has been directed to the control of presynaptic terminal excitability in this process.…”

Cholinergic Synaptic Transmission in AdultDrosophilaKenyon CellsIn Situ

“…Behavioral studies in the fruit fly Drosophila melanogaster, using an extensive library of mutant and transgenic animals, have resulted in identification of a number of genes and signal cascades that contribute to memory formation (Davis, 2004). In addition, physiological recordings in Drosophila, at the neuromuscular junction, from neurons in primary culture and in the CNS of embryos and larvae, have expanded our understanding of the molecular mechanisms regulating synaptic plasticity and neuronal excitability (Zhong and Wu, 1991;Broadie et al, 1997;Lee and O'Dowd, 2000;Renger et al, 2000;Yao and Wu, 2001;Hodges et al, 2002;Park et al, 2002;Rohrbough and Broadie, 2002;Baines, 2003;Hou et al, 2003;Rohrbough et al, 2003). However, limited electrophysiological access to neurons in central circuits in the adult fly has precluded exploration of the direct cellular links that mediate the translation of changes in gene expression to memory formation.…”