Glutamate is a transmitter that mediates inhibition at the rectifying electrical motor giant synapse in the crayfish

Rostral Ganglia Are Required for Induction But Not Expression of Crayfish Escape Reflex Habituation: Role of Higher Centers in Reprogramming Low-Level Circuits

It is widely assumed that learning results from alterations in the strength of synapses within the neural pathways that mediate a learned behavioral response and that these alterations are directly caused by training-induced activity of neurons connected by the changing synapses. Initial evidence for this view came from studies of habituation of defensive reflexes in several invertebrate species. However, more recent studies of habituation of the escape reflex in one of these species, the crayfish, have shown that habituation is substantially caused by tonic inhibitory input from cephalic ganglia; this descending inhibition suppresses the activity of neurons within the escape circuit, which reside in caudal ganglia. Such control by descending inhibition indicates that animals with encephalized nervous systems do not entirely abdicate to low-level circuitry the important decision of whether to habituate to stimuli that might warn of danger. Higher centers in fact play a major role in controlling the habituation of this potentially life-saving protective response. Another way for higher centers to control lower ones would be to induce alteration of the lower center's intrinsic properties. Here, we show that, whereas descending input from higher ganglia is needed to induce habituation, once established, habituation persists even after rostral ganglia are disconnected. This provides evidence that lower-level neural circuits can be reprogrammed through transient interaction with higher ganglia to decrease their intrinsic tendency to produce escape.

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.

GABAergic and glutamatergic inhibition of nonspiking local interneurons in the terminal abdominal ganglion of the crayfish

Nonspiking local interneurons in the terminal abdominal ganglion of the crayfish Procambarus clarkii receive inhibitory inputs from mainly glutamatergic spiking local interneurons and GABAergic nonspiking interneurons. In this study, the inhibitory responses of nonspiking interneurons to local application of glutamate and GABA into the neuropil were compared. Glutamate and GABA injection mediated the hyperpolarization of the nonspiking interneurons with an increase in membrane conductance. The glutamate-mediated membrane hyperpolarization was reversed by injection of 1 or 2 nA hyperpolarizing current. By contrast, more than 3 nA hyperpolarizing current was frequently necessary to reverse the GABA-mediated hyperpolarization. Bath application of a chloride channel blocker, 50 microM picrotoxin (PTX), reduced the glutamate-mediated hyperpolarization, but had no effect on the GABA-mediated hyperpolarization. The GABA-mediated hyperpolarization was not consistently affected by bath application of low chloride solution. These results suggest that the glutamate-mediated inhibition was related to the gating of a Cl(-) conductance, while the GABA-mediated inhibition was not. Electrical stimulation of sensory afferents innervating the exopodite elicited ipsps in uropod opener motor neurons. These sensory-evoked ipsps were also PTX-insensitive, suggesting GABAergic nonspiking interneurons could be the predominant premotor elements in organizing the uropod motor control system.

Distribution of glutamatergic immunoreactive neurons in the terminal abdominal ganglion of the crayfish

Using an antiserum directed against glutamate, we have analyzed the distribution of glutamate-like immunoreactive neurons in the terminal abdominal ganglion of the crayfish Procambarus clarkii. Approximately 160 central neurons (157 +/- 8; mean +/- SEM, n = 8) showed positive glutamate-like immunoreactivity, which represents approximately 25% of the total number of neurons in the terminal ganglion. Using a combination of intracellular staining with the marker Lucifer yellow and immunocytochemical staining has shown that most excitatory motor neurons are glutamatergic and that glutamate acts as an excitatory transmitter at peripheral neuromuscular junctions. Seven of 10 identified spiking local interneurons and only 2 of 19 identified ascending interneurons, showed positive immunoreactivity. Our observation that inhibitory spiking interneurons were immunopositive, whereas excitatory ascending interneurons were immunonegative, indicates that glutamate is likely to act as an inhibitory neurotransmitter within the central nervous system. Local pressure injection of L-glutamate into the neuropil of the ganglion caused a hyperpolarization of the membrane potentials of many interneurons. gamma-Aminobutyric acid (GABA)ergic posterolateral nonspiking interneurons and the bilateral nonspiking interneuron LDS showed no glutamate-like immunoreactivity, whereas non-GABAergic anterolateral III nonspiking interneurons showed glutamate-like immunoreactivity. Thus, not only GABA but also glutamate are used in parallel as inhibitory neurotransmitters at central synapses.

Presynaptic modulation of sensory neurons in the segmental ganglia of arthropods

The afferent terminals of arthropod sensory neurones receive abundant input synapses, usually closely intermingled with the sites of synaptic output. The majority of the input synapses use the neurotransmitter GABA, but in some afferents there is a significant glutamatergic or histaminergic component. GABA and histamine shunt afferent action potentials by increasing chloride conductance. Though glutamate can also have this effect in the arthropod central nervous system, its action on afferent terminals appears to be mediated by increases in potassium conductance or by the action of metabotropic receptors. The action of the presynaptic synapses on the afferents are many and varied. Even on the same afferent, they may have several distinct roles that can involve both tonic and phasic patterns of primary afferent depolarisation. Despite the ubiquity and importance of their effects however, the populations of neurones from which the presynaptic synapses are made, remain largely unidentified.

Neuromodulation of arthropod mechanosensory neurons

Arthropod mechanosensory afferents have long been known to receive efferent synaptic connections onto their centrally located axon terminals. These connections cause presynaptic inhibition by attenuating the action potentials arriving at the axon terminals, thus reducing the synaptic potentials in the postsynaptic neurons. This type of inhibition can specifically reduce the excitation of selected postsynaptic neurons while leaving others unaffected. However, recent research has demonstrated that sensory signals detected by arthropod mechanosensory neurons can also be synaptically modulated before they ever arrive at the axon terminals. In arachnids and crustaceans, wide and complex networks of synapses on all parts of the afferent neurons, including the somata and dendrites, provide mechanisms to inhibit or enhance the responses to mechanical stimuli as they are being detected. This modulation will affect the signal transmission to all axonal branches and postsynaptic cells of the affected receptor neuron. In addition to the increased complexity of mechanosensory information transmission produced by these synapses, a variety of circulating neuroactive substances also modulate these neurons by acting on their postsynaptic receptors.