Inhibitory glutamate receptors in spider peripheral mechanosensory neurons.
Most mechanosensory neurons are inhibited by GABAergic efferent neurons. This inhibition is often presynaptic and mediated by ionotropic GABA receptors at the axon terminals. GABA receptor activation opens Cl- channels, leading to membrane depolarization and an increase in membrane conductance. In many invertebrate preparations, efferent neurons that innervate mechanosensory afferents contain glutamate in addition to GABA, suggesting that the sensory neurons are also modulated by glutamate. However, the effects of glutamate on these neurons are not well understood. Peripheral parts of the spider (Cupiennius salei) mechanosensory neurons are surrounded by efferent fibers immunoreactive to antibodies against GABA and glutamate. GABA and its analogue muscimol were shown to effectively inhibit spider mechanosensory neurons innervating lyriform slit sensilla VS-3 that detects cuticular strains in the leg. Here, we show that glutamate also inhibits the VS-3 neurons, but its effects are different from those of GABA or muscimol, suggesting that it acts on a different group of receptors. GABA and muscimol always depolarized these neurons and the inhibitory effect was strongly correlated with the amount of depolarization. In contrast, glutamate inhibited the VS-3 neurons even when it did not depolarize them. In addition, while glutamate inhibited both the axonal action potentials elicited with electrical stimulation and dendritic action potentials produced by mechanical stimulation, muscimol only inhibited the axonal action potentials. Therefore, the inhibitory glutamate receptors in the VS-3 neurons are distinct from and differently distributed than the GABA receptors, providing a subtle control of the neurons' sensitivity in varying behavioural situations.