Physiology and morphology of visual movement detector neurons in a crab (Decapoda: Brachyura).

Although visually elicited behaviors have been extensively studied in crabs, their investigation at the neurophysiological level is scant. The present study is a physiological and morphological description of intracellularly recorded and dye injected visual movement detector neurons that respond to the same stimulus that elicits the escape response in the crab Chasmagnathus granulatus. The neurons were investigated in intact animals. The response of movement detector neurons to the danger stimulus (an object moving above the animal) consists of a strong discharge of action potentials frequently superimposed on noisy graded potentials, whereas the response to stationary changes in illumination is weak or undetectable. The response to the moving stimulus is relatively independent of the background intensity and of the contrast between target and background. Repeated presentations of the moving stimulus produce rapid habituation of the neural response. Some of the neurons also respond to mechanical stimulation. These physiological results coincide with those from early studies on visual movement detector fibers of crustaceans achieved by extracellular recordings. However, there are no previous morphological studies of these neurons. Intracellular injection with Lucifer Yellow revealed that these neurons in Chasmagnathus arborize extensively in the internal medulla and in the lateral protocerebrum. They have their somata located in the cell body cluster laying beneath the internal medulla. Their axons project centripetally across the protocerebral tract.

Visual interneurons of the crab Chasmagnathus studied by intracellular recordings in vivo.

Comparative physiology of visual systems has become an important field of investigation. However, despite the fact that Crustacea represents a major phylogenetic group, research on the physiology of vision of these animals is scant and almost limited to the crayfish. We developed a preparation to study in vivo the visual nervous system of a semiterrestrial crab through intracellular recordings. The response to a pulse of light was investigated in 206 interneurons from 38 animals. Seventy-eight of these neurons could be classified by functional criteria as sustaining cells, dimming cells, nonspiking hyperpolarizing cells and nonspiking depolarizing cells. Quantitative description is provided for the first two groups and qualitative description is given for the last two. The remaining neurons presented a broad range of different types of phasic responses to light. Although semiterrestrial crabs are behaviorally more reactive to visual stimuli than the crayfish, the general physiological properties of identified lamina and medullary neurons of Chasmagnathus resemble those of the crayfish. The results described here represent the first attempt to study the visual system of crabs with intracellular recordings and constitute the beginning of a project aimed to investigate the neuronal functions underlying behavioral responses elicited by visual stimuli.

Two related forms of long-term habituation in the crab Chasmagnathus are differentially affected by scopolamine.

An opaque screen moving overhead elicits an escape response in the crab Chasmagnathus, which, after a few presentations, habituates for a long period (long-term habituation, LTH). Previous results distinguished two types of LTH: the (context-signal)-LTH yielded by spaced training, determined by an association between context and habituating stimulus, and cycloheximide sensitive: and the (signal)-LTH produced by massed training, context independent, and cycloheximide insensitive. Present experiments were aimed at studying the possible involvement of cholinergic mechanisms in one or both types of LTH, using the muscarinic antagonist, scopolamine (SCP). Results indicate that LTH acquired by spaced training (30 trials separated by 85 s) is blocked in a dose-dependent manner by posttraining SCP. Amnesia is shown with 100 ng SCP/g injected immediately before or after spaced training but not delayed 1-h posttraining. No effect of SCP on LTH acquired by massed training (300 trials separated by 4 s) is detected. Pretraining SCP induces a decrease in the response level at the initial trials of either a spaced or a massed training. It is concluded that the storage of (context-signal)-LTH may be selectively regulated by a muscarinic-cholinergic mechanism. However, the possibility that other cholinergic receptors would be involved in the consolidation of the (signal)-LTH is discussed.

Relationship between mouse uterine contractility, nitric oxide and prostaglandin production in early pregnancy.

Despite the evidence for a functional role of nitric oxide (NO) in the regulation of uterine contractility in several species, there is little information about the effects of this gas on the mouse uterus. The aims of this study were to investigate if the NO relaxation pathway is present in mouse pregnant uterus and the relationship with the uterotonic prostaglandins (PGs E and F2alpha) production. We evaluated the effect of the treatment with a competitive nitric oxide synthase (NOs) inhibitor: N(G)-monomethyl-L-arginine on the spontaneous contractile activity and prostaglandin production on two different days of pregnancy: second day of pregnancy (preimplantation stage) and on the afternoon of the fifth day of pregnancy (postimplantation stage). We found that only on the fifth day of pregnancy did the inhibitor induce a highly significant isometric developed tension (IDT) and that this effect was maintained throughout the experiment. In order to evaluate if the generation of NO was also different between the two days of pregnancy, NOs activity was measured. Total NOs activity was significantly elevated during the postimplantation stage. We studied the interaction between the NO and cyclooxygenase (COX) pathways on the fifth day of pregnancy, and the data show no stimulation of PGs production by endogenous NO. In summary, we found that NO participates in the control of uterine contractility on the fifth day (a postimplantation stage) and that in this condition the NO was not able to elicit an increase in PGs production.

Nitric oxide mediates platelet-activating factor stimulatory action on uterine prostaglandin production.

Accumulated evidence suggests that platelet-activating factor (PAF) may have a role in implantation by stimulating prostaglandin (PG) production. Since we had demonstrated that nitric oxide (NO) can increase uterine PGs, the aim of this study was to explore whether or not NO could mediate rat uterus responses to PAF on day 5 of gestation, when implantation takes place. Uterine motility was enhanced by PAF as compared to controls. This action was abolished by either the arginine analogue, N-monomethyl L-arginine (L-NMMA) or the cyclooxygenase inhibitor, indomethacin. On the other hand, NOS activity was detected in uterine strips and could be stimulated by PAF. The cyclooxygenase product PGE2 was also significantly stimulated by PAF. Inhibition of endogenous NO formation abolished the PAF effect on PG synthesis. Our results suggest that NO is an important intermediate in the interaction between PAF and PGs.

Role of nitric oxide on oxytocin-evoked contractions and prostaglandin synthesis in isolated pregnant rat uterus.

Uterine contractions elicited by oxytocin (OT), possibly linked with uterus prostaglandin (PG) release, are involved in the final pathway of labor. It is known that nitric oxide (NO) may contribute to the maintenance of uterine contractile quiescence during gestation. Therefore in this study the effect of the inhibition of NO synthase (NOS), with N-monomethyl L-arginine (L-NMMA), on the ability of OT to stimulate uterine contractions and PG synthesis was investigated in isolated rat uterus at days 13 and 21 of pregnancy. L-NMMA did not modify the frequency and the force of contractions elicited by OT at day 13. On day 21 the frequency of contractions evoked by OT were better sustained in the presence of L-NMMA. PGs were not affected by OT on day 13. OT stimulated PGF2alpha on day 21 when NOS had been inhibited with L-NMMA, but not in the absence of L-NMMA. NOS activity was stimulated by OT at day 21 of gestation. In summary these findings indicate that near term NO can regulate OT PGF2alpha induced contractions and PG synthesis in isolated pregnant rat uterus.