Alterations in Frequency Coding and Activity Dependence of Excitability in Cultured Neurons ofDrosophilaMemory Mutants

Abstract

Mutants of the Drosophila dunce (dnc) and rutabaga (rut) genes, which encode a cAMP-specific phosphodiesterase and a calcium/calmodulin-responsive adenylyl cyclase, respectively, are deficient in short-term memory. Altered synaptic plasticity has been demonstrated at neuromuscular junctions in these mutants, but little is known about how their central neurons are affected. We examined this problem by using the "giant" neuron culture, which offers a unique opportunity to analyze mutational effects on neuronal activity and the underlying ionic currents in Drosophila. On the basis of instantaneous frequency and first latency of spikes evoked by current steps, four categories of firing patterns (tonic, adaptive, delayed, and interrupted) were identified in wild-type neurons, revealing interesting parallels to those commonly observed in vertebrate CNS neurons. The distinct firing patterns were correlated with expression of different ratios of 4-aminopyridine-and tetraethylammoniumsensitive K ϩ currents. Subsets of dnc and rut neurons displayed abnormal spontaneous spikes and altered firing patterns. Altered frequency coding in mutant neurons was demonstrated further by using stimulation protocols involving conditioning with previous activity. Abnormal spike activity and reduced K ϩ current remained in double-mutant neurons, suggesting that the opposite effects on cAMP metabolism by dnc and rut do not counterbalance the mutual functional defects. The aberrant spontaneous activity and altered frequency coding in different stimulus paradigms may present problems in the stability and reliability of neural circuits for information processing during certain behavioral tasks, raising the possibility of modulation in neuronal excitability as a cellular mechanism underlying learning and memory.

Reduced Delayed-Rectifier K+ Current in the Learning Mutant rutabaga

“…This is in agreement with the reduction of K + current reported previously for cleavage-arrested giant rutabaga neurons (Zhao and Wu 1997). The reduced current mainly included a non-inactivating component that was sustained throughout the 100 msec.…”

“…Moreover, neuronal K + current was shown to be greater in dunce than in wildtype neurons (Alshuaib and Mathew 1998). dunce and rutabaga were shown to display altered firing patterns in giant (cleavage-arrested) cultured neurons (Zhao and Wu 1997). However, neuronal K + current of normal embryonic cultures has not been investigated in the rutabaga mutant.…”

The Steroid Hormone 20-Hydroxyecdysone Enhances Neurite Growth ofDrosophilaMushroom Body Neurons Isolated during Metamorphosis

“…This structural fidelity suggests that pupal M B neurons have an endogenous program directing their morphogenetic development, analogous to that proposed for hippocampal pyramidal neurons (Banker and Cowan, 1979;Dotti et al, 1988). Previous studies of cultured Drosophila neurons of various stages from wild type and mutants allowed morphological and electrophysiological assessment, but without specific identification of the neurons beyond their regional origin (Salvaterra et al, 1987;Byerly and Leung, 1988;Solc and Aldrich, 1988;Wu, 1988;Li and Meinertzhagen, 1995;O'Dowd, 1995;Z hao and Wu, 1997). Primary cultures enriched for M B neurons have been established from brains of other insects (Kreissl and Bicker, 1992;Cayre et al, 1998), but the lack of tools for genetic manipulation limits the versatility of these systems.…”

The DISC locus in psychiatric illness

“…Mutations in Dunce cause learning and memory defects, which result from altered cAMP hydrolysis and associated elevated cAMP concentrations. 98 It has also been shown that Dunce mutants have altered axonal growth cone motility, 99 neuronal function 100,101 and synaptic plasticity. 102 The PDE4 inhibitors MEM1018 and MEM1091 influence NMDA receptor-dependent memory, 103 and other selective PDE4 inhibitors including rolipram have been shown to influence memory and cognition in several studies.…”