Neurons Controlling Aplysia Feeding Inhibit Themselves by Continuous NO Production

Neural activity can be affected by nitric oxide (NO) produced by spiking neurons. Can neural activity also be affected by NO produced in neurons in the absence of spiking?Applying an NO scavenger to quiescent Aplysia buccal ganglia initiated fictive feeding, indicating that NO production at rest inhibits feeding. The inhibition is in part via effects on neurons B31/B32, neurons initiating food consumption. Applying NO scavengers or nitric oxide synthase (NOS) blockers to B31/B32 neurons cultured in isolation caused inactive neurons to depolarize and fire, indicating that B31/B32 produce NO tonically without action potentials, and tonic NO production contributes to the B31/B32 resting potentials. Guanylyl cyclase blockers also caused depolarization and firing, indicating that the cGMP second messenger cascade, presumably activated by the tonic presence of NO, contributes to the B31/B32 resting potential. Blocking NO while voltage-clamping revealed an inward leak current, indicating that NO prevents this current from depolarizing the neuron. Blocking nitrergic transmission had no effect on a number of other cultured, isolated neurons. However, treatment with NO blockers did excite cerebral ganglion neuron C-PR, a command-like neuron initiating food-finding behavior, both in situ, and when the neuron was cultured in isolation, indicating that this neuron also inhibits itself by producing NO at rest.Self-inhibitory, tonic NO production is a novel mechanism for the modulation of neural activity. Localization of this mechanism to critical neurons in different ganglia controlling different aspects of a behavior provides a mechanism by which a humeral signal affecting background NO production, such as the NO precursor L-arginine, could control multiple aspects of the behavior

Variables controlling entry into and exit from the steady-state, one of two modes of feeding in Aplysia.

Aplysia feeding is a model system for examining the neural mechanisms by which changes in motivational state control behavior. When food is intermittently present, Aplysia eat large meals controlled by a balance between food stimuli exciting feeding and gut stimuli inhibiting feeding. However, when food is continuously present animals are in a state in which feeding is relatively inhibited and animals eat little. We examined which stimuli provided by food and feeding initiate steady-state inhibition of feeding, and which stimuli maintain the inhibition.Multiple stimuli were found to control entry into the steady-state inhibition, and its maintenance. The major variable governing entry into the steady-state is fill of the gut with bulk provided by food, but this stimulus cannot alone cause entry into the steady-state. Food odor and nutritional stimuli such as increased hemolymph glucose and L-arginine concentrations also contribute to inhibition of feeding leading to entry into the steady-state. Although food odor can alone cause some inhibition of feeding, it does not amplify the effect of gut fill. By contrast, neither increased hemolymph glucose nor L-arginine alone inhibits feeding in hungry animals, but both amplify the inhibitory effects of food odor, and increased glucose also amplifies the effect of gut fill. The major variable maintaining the steady-state is the continued presence of food odor, which can alone maintain the steady-state for 48-72 hrs. Neither increased glucose nor L-arginine can alone preserve the steady-state, although they partially preserve it. Glucose and arginine partially extend the effect of food odor after 72 hrs.These findings show that control of Aplysia feeding is more complex than was previously thought, in that multiple inhibitory factors interact in its control

Purification and characterization of the gonad lectin of Aplysia depilans

AbstractExtracts of gonads and fertilized eggs of Aplysia depilans contain a D-galacturonic and D-galactose-binding lectin. This lectin reacts strongly with rabbit and human erythrocytes independent of ABO blood groups, weakly with dog, mouse, rat, and chick erythrocytes and not at all or very weakly with sheep erythrocytes. Purification of the gonad lectin was easily achieved, with a high yield, by heating to 70°C, precipitation with ammonium sulfate and affinity chromatography on Sepharose 4B. The purified lectin was found to be a glucoprotein of molecular mass around 55–60 kDa; it stimulates mitogenesis of human peripheral lymphocytes

Habituation of excitatory effects of food odor reveals the inhibitory effect.

<p>Animals were pre-exposed to food odor for 24 (<i>N</i> = 13), 2 (<i>N</i> = 27) or 1 (<i>N</i> = 18) hours prior to being given access to food. The percent feeding was then measured over the next 4 hours. During the first hour, there was no significant difference in feeding between animals fed after a 24 hr exposure to seaweed odor and animals fed after a 2 hr pre-exposure to the odor (<i>p</i> = 0.54, <i>t</i>(38) = 0.61; two-tailed <i>t</i>-test)). By contrast, there was a significant difference in feeding between animals pre-exposed to food odor for 1 hour and the combined data from 2 and 24 hour pre-exposures to seaweed odor (<i>p</i> = 0.01, <i>t</i>(56 = 2.60; two-tailed <i>t</i>-test).</p

L-arginine does not amplify the inhibitory effect of gut fill.

<p>In this experiment, 3 groups of animals were examined. One (<i>N</i> = 8) was injected with ASW 10 min before a 4 hr feeding. A second (<i>N</i> = 12) was treated with 2.5 ml of gel injected into the crop before the 4 hr feeding. A third (<i>N</i> = 14) was treated with gel, plus an injection that raised the hemolymph concentration of L-arginine to 250 µM. Standard errors are shown. A) Percent time spent feeding in half-hour intervals over the 4 hr test. Data on animals treated with ASW are not shown. B) The total time spent feeding over the 4 hr test in the three groups. There was a significant difference between the three groups (<i>p</i>(2,31) = 0.036; <i>F</i> = 3.69; one-way analysis of variance). A post-hoc test (Student-Newman-Kuels) showed that at α = 0.05, animals treated with ASW ate significantly more than did animals in the other two groups, with no significant difference between animals treated with gel alone, or with gel and L-arginine.</p

Glucose amplifies the inhibitory effects of food odor and of gel fill.

<p>Animals were treated with glucose 1 hr before testing feeding<b>. </b><i>A</i>) There was no significant difference between the effects of glucose or of ASW on entry into the steady-state. <i>B</i>) Glucose amplified the inhibitory effects of food odor. During the first 2 hours of the meal, animals treated with glucose ate significantly less than did controls (<i>p</i> = 0.05, <i>t</i>(27) = 2.02). There was no significant difference in the subsequent two hours (<i>p</i> = 0.60, <i>t</i>(27) = 0.54). <i>C</i>) Glucose treatment also amplified the inhibitory effect of gut fill at the start of the meal. During the first half hour, glucose-treated animals ate significantly less (<i>p</i> = 0.007, <i>t</i>(44) = 2.79 than did controls, with no significant difference (<i>p</i> = 0.99, <i>t</i>(44) = 0.008); all tests are two-tailed <i>t</i>-tests) in the subsequent period. The dashed line shows steady-state feeding. Standard errors are shown.</p