Acetylcholine and GABA mediate opposing actions on neuronal chloride channels in crayfish.

A central principle of neural integration is that excitatory and inhibitory neurotransmitters effect the opening of distinct classes of membrane ionic channels and that integration consists of the summation of the opposing ionic currents on the postsynaptic membrane. In tangential cells of crayfish optic lobes, a hyperpolarizing, biphasic synaptic potential is produced by the concurrent action of acetylcholine and gamma aminobutyric acid (GABA). Acetylcholine hyperpolarizes the cell and increases chlorine conductance. GABA depolarizes the cell by closing some of the same chloride channels. Therefore, in this case integration is achieved by the antagonistic actions of two transmitters on the same ionic channel.

Tachykinin-related peptide and GABA-mediated presynaptic inhibition of crayfish photoreceptors.

Off-axis illumination elicits lateral inhibition at the primary visual synapse in crustacea and insects. The evidence suggests that the inhibitory action is presynaptic (i.e., on the photoreceptor terminal) and that the amacrine neurons of the lamina ganglionaris (the first synaptic layer) may be part of the inhibitory pathway. The neurotransmitters and the synaptic mechanisms are unknown. We show by immunocytochemistry that GABA and a tachykinin-related peptide (TRP) are localized in the amacrine neurons of the crayfish lamina ganglionaris. Indirect evidence suggests that GABA and TRP may be colocalized in these neurons. The extensive processes of the amacrine neurons occupy lamina layers containing the terminals of photoreceptors. Application of exogenous GABA and TRP to photoreceptor terminals produces a short-latency, dose-dependent hyperpolarization with a decay time constant on the order of a few seconds. TRP also exhibits actions that evolve over several minutes. These include a reduction of the receptor potential (and the light-elicited current) by approximately 40% and potentiation of the action of GABA by approximately 100%. The mechanisms of TRP action in crayfish are not known, but a plausible pathway is a TRP-dependent elevation of intracellular Ca(2+) that reduces photoreceptor sensitivity in arthropods. Although the mechanisms are not established, the results indicate that in crayfish photoreceptors TRP displays actions on two time scales and can exert profound modulatory control over cell function.

Motion detection and adaptation in crayfish photoreceptors. A spatiotemporal analysis of linear movement sensitivity.

Impulse and sine wave responses of crayfish photoreceptors were examined to establish the limits and the parameters of linear behavior. These receptors exhibit simple low pass behavior which is well described by the transfer function of a linear resistor-capacitor cascade of three to five stages, each with the same time constant (tau). Additionally, variations in mean light intensity modify tau twofold and the contrast sensitivity by fourfold. The angular sensitivity profile is Gaussian and the acceptance angle (phi) increases 3.2-fold with dark adaptation. The responses to moving stripes of positive and negative contrast were measured over a 100-fold velocity range. The amplitude, phase, and waveform of these responses were predicted from the convolution of the receptor's impulse response and angular sensitivity profile. A theoretical calculation based on the convolution of a linear impulse response and a Gaussian sensitivity profile indicates that the sensitivity to variations in stimulus velocity is determined by the ratio phi/tau. These two parameters are sufficient to predict the velocity of the half-maximal response over a wide range of ambient illumination levels. Because phi and tau vary in parallel during light adaptation, it is inferred that many arthropods can maintain approximately constant velocity sensitivity during large shifts in mean illumination and receptor time constant. The results are discussed relative to other arthropod and vertebrate receptors and the strategies that have evolved for movement detection in varying ambient illumination.

Immunocytochemical studies of the distribution of acetylcholine in the crayfish brain.

A number of studies indicate that acetylcholine is an important transmitter in most crustacean primary afferents and in at least several central pathways. Little is known, however, regarding the structure or distribution of cholinergic pathways in the central nervous system. The recent introduction of antibodies to choline-protein conjugates provides a potentially powerful means for localizing putative cholinergic neurons and pathways in the nervous system. Acetylcholine was localized with immunocytochemical procedures in the axons and terminals of cephalic primary afferents and in interneurons of the crayfish brain. The most intensely reactive loci were the primary sensory neuropiles, which contain the terminals of the statocyst afferents (parolfactory lobes) and antennal afferents (antennal lobe). These results are generally in accord with previous findings based upon choline uptake and enzyme assay in lobster cephalic nerves. We also found evidence consistent with the presence of acetylcholine in the globular interneurons of the accessory lobe and in descending interneurons which originate in the dorsal medial and anterior clusters of the protocerebrum. The axons of several neurons in the circumesophageal connective (descending interneurons and primary afferents) are also reactive to the choline antibody.

Dye coupling and gap junctions between crustacean neurosecretory cells.

Neurosecretory cells in the X organ-sinus gland system of the crayfish were impaled and Lucifer Yellow was intracellularly iontophoresed. In some neurons the injected dye was transferred to neighboring neurons. The interneuronal dye transfer was between adjacent somata. Coupling was also observed between neurons and smaller cells, possibly glia. Gap junctions were identified by freeze-fracture in neuron somata and glial cells in the X organ and also in neurosecretory axons in the sinus gland.

Light input to crustacean neurosecretory cells.

Electrical activity was recorded intracellularly from neurosecretory cells in the crayfish eyestalk identified by lucifer yellow injection. The activity is most commonly enhanced by illumination of retinal fields. Increments in spontaneous activity as well as bursts in otherwise silent cells were the most common type of response. Occasionally light-induced inhibitory responses were recorded. At neuropil level, light pulses result in EPSPs with amplitudes dependent on intensity of light and the previous adaptation to darkness.

A quantitative correlation of contour sensitivity with dendritic density in an identified visual neuron.

Crayfish sustaining fibers are visual interneurons that are tonically excited by increases in illumination within their receptive fields. The receptive fields of sustaining fibers (SFs) are correlated with the two-dimensional position of their dendritic fields within the central neuropil. Dendritic branching patterns and contour sensitivity maps were obtained for SF 019 in several preparations. The contour sensitivity along the dorsal-ventral visual arc (which intersects 019's receptive field) is positively correlated with the variations in the spatial frequency of 019's dendritic arbor along the dorsal ventral midline of the neuropil. We conclude that an SF's receptive field is determined by the position and extent of its dendritic field and that sensitivity variations within the receptive field are principally determined by changes in its dendritic density.

A neuronal nicotinic acetylcholine receptor in crayfish neurons.

In warm-blooded vertebrates, neuronal nicotinic acetylcholine receptors (nAChRs) are distinguished from muscle endplate receptors by their ligand affinities and sensitivity to several toxins. In the crayfish optic lobe, synaptic and acetylcholine (ACh)-elicited responses are blocked by toxins (F-toxin and neosurugatoxin) selective for neuronal nAChRs and are insensitive to the alpha-neurotoxins selective for endplate nAChRs.