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

The NO scavenger PTIO induces fictive feeding when applied to the isolated buccal ganglia.

<p>Fictive feeding was monitored via extracellular recordings from the radula nerve (RN) and from buccal nerve 2 (BN2). Activity in RN is a correlate of radula closing, whereas activity in BN2 is a correlate of retraction. Activity representative of radula retraction was counted as a single burst of fictive feeding. <b>A</b>) In ASW, no fictive feeding was seen, although a single unit in BN2 fired. <b>B</b>) Application of PTIO (at the start of the trace) elicited repeated bursts of fictive feeding. Recordings similar to those shown were observed in 7 separate isolated buccal ganglia preparations. <b>C</b>) Expansion of the boxed area in part B shows overlap between firing in BN2 and RN, indicating that PTIO induced ingestion-like activity. <b>D</b>) Means and standard errors of the number of fictive feeding bursts recorded in 10 min in ASW and after the application of PTIO. PTIO caused a significant increase in fictive feeding (<i>p</i> = 0.02 <i>t</i>(6) = 2.78; two-tailed paired <i>t</i>-test).</p

Effects of NO blockers and donors on other neurons.

<p><b>A</b>) L-NAME had no effect on neuron B8 in 6 of 6 preparations. L-NAME also had no effect on B4 in 5 of 5 preparations (not shown). <b>B</b>) Treatment of an isolated MCC neuron in culture with L-NAME had no effect in 7 of 7 preparations. <b>C</b>) In an isolated, cultured C-PR neuron application of L-NAME caused a depolarization in 5 of 5 preparations. Mean amplitude of depolarization: 13.3±1.07 (SE) mV; Mean latency to spiking: 7.11±1.8 (SE) min. The dashed line marks the −60 mV resting potential.</p

Block of NO depolarizes C-PR in situ.

<p><b>A</b>) Application of the NO scavenger PTIO in an isolated cerebral ganglion preparation depolarized C-PR and caused an increase in EPSPs. <b>B</b>) In the presence of TTX, PTIO still depolarizes C-PR (N = 5 cells in 3 preparation), indicating that part of the effect is direct.</p

Block of NO opens an inward current.

<p><b>A</b>) Effect of PTIO and of L-NAME on currents recorded during the last few hundred milliseconds of a voltage clamp experiment performed in TTX. Only currents recorded in response to voltage steps to −90, −60 and −10 mV are shown. Note that both PTIO and L-NAME induce inward currents at −90 and −60 mV, with the currents at −90 mV larger than those at −60 mV. Also note the reversal of the currents at −10 mV. <b>B</b>) Mean and standard errors (hidden by the points) of current amplitudes recorded during the last 500 msec of voltage pulses with and without PTIO or L-NAME (N = 5 for each group). <b>C</b>) The difference in current between values recorded with and without PTIO or L-NAME at the various voltage steps. The data were combined from experiments using the two blockers. Means and standard errors are shown.</p