AMPA/kainate and NMDA-like glutamate receptors at the chromatophore neuromuscular junction of the squid: role in synaptic transmission and skin patterning.
Glutamate receptor types were examined at the chromatophore synapses of the squids Alloteuthis subulata and Loligo vulgaris, where nerve-induced muscle contraction causes chromatophore expansion. Immunoblotting with antibody raised against a squid AMPA receptor (sGluR) demonstrated that AMPA/kainate receptors are present in squid skin. Application of l-glutamate evoked chromatophore muscle contractions in both ventral and dorsal skins, while NMDA was only active on a subpopulation of dorsal chromatophores. In dorsal skin, neurotransmission was partly blocked by either AMPA/kainate receptor antagonists (CNQX and DNQX) or NMDA receptor antagonists (AP-5 and MK-801) or completely blocked by simultaneous application of both classes of antagonists. In isolated muscle fibres, ionophoretic application of l-glutamate evoked fast inward CNQX- and DNQX-sensitive currents with reversal potentials around +14 mV and a high conductance to Na+. In fibres from dorsal skin only, a slower outward glutamate-sensitive current appeared at positive holding potentials. At negative potentials, currents were potentiated by glycine or by removing external Mg2+ and were blocked by AP-5 and MK-801. Glutamate caused a fast, followed by a slow, transient increase in cytoplasmic Ca2+. The slow component was increased in amplitude and duration by glycine or by lowering external Mg2+ and decreased by AP-5 and MK-801. In cells from ventral skin, no 'NMDA-like responses' were detected. Thus, while AMPA/kainate receptors mediated fast excitatory synaptic transmission and rapid colour change over the whole skin, activation of both AMPA/kainate and NMDA-like receptors in a subpopulation of dorsal chromatophores prolonged the postsynaptically evoked Ca2+ elevation causing temporally extended colour displays with behavioural significance.
Lima
Pedro A
PA
Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK.
Nardi
Giovanna
G
Brown
Euan R
ER
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002856:Chromatophores / Q000378:metabolism*; D049832:Decapodiformes / Q000502:physiology*; D004558:Electric Stimulation; D004591:Electrophoresis, Polyacrylamide Gel; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D005456:Fluorescent Dyes; D016257:Fura-2; D018698:Glutamic Acid / Q000378:metabolism; D015151:Immunoblotting; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D007473:Ion Channels / Q000502:physiology; D007478:Iontophoresis; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D016202:N-Methylaspartate / Q000378:metabolism; D009469:Neuromuscular Junction / Q000378:metabolism*; D018091:Receptors, AMPA / Q000378:metabolism*; D018092:Kainic Acid Receptors / Q000378:metabolism*; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D012879:Skin Physiological Phenomena; D009435:Synaptic Transmission / Q000502:physiology*
D002118:Calcium; D005456:Fluorescent Dyes; D016257:Fura-2; D018698:Glutamic Acid; D007473:Ion Channels; D016202:N-Methylaspartate; D018091:Receptors, AMPA; D018092:Kainic Acid Receptors; D016194:Receptors, N-Methyl-D-Aspartate