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

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

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

Evidence for a developmental role for TLR4 in learning and memory

Toll-like receptors (TLRs) play essential roles in innate immunity and increasing evidence indicates that these receptors are expressed in neurons, astrocytes and microglia in the brain where they mediate responses to infection, stress and injury. Very little is known about the roles of TLRs in cognition. To test the hypothesis that TLR4 has a role in hippocampus-dependent spatial learning and memory, we used mice deficient for TLR4 and mice receiving chronic TLR4 antagonist infusion to the lateral ventricles in the brain. We found that developmental TLR4 deficiency enhances spatial reference memory acquisition and memory retention, impairs contextual fear-learning and enhances motor functions, traits that were correlated with CREB up-regulation in the hippocampus. TLR4 antagonist infusion into the cerebral ventricles of adult mice did not affect cognitive behavior, but instead affected anxiety responses. Our findings indicate a developmental role for TLR4 in shaping spatial reference memory, and fear learning and memory. Moreover, we show that central TLR4 inhibition using a TLR4 antagonist has no discernible physiological role in regulating spatial and contextual hippocampus-dependent cognitive behavior

Additional file 1: Table S1. of Mango (Mangifera indica L.) germplasm diversity based on single nucleotide polymorphisms derived from the transcriptome

Mango transcriptome annotation. (XLSX 16205 kb

Developmental TLR4 deficiency, but not pharmacological TLR4 antagonism, enhances retention of spatial reference memory.

<p>(<b>A</b>) TLR4^−/− (n = 19) and TLR4^+/+ (n = 24) mice were tested in probe trials at 24, 48, 72 and 96 hours following training for retention of spatial reference memory. Tests were done after all experimental groups exhibited loss of memory of the platform location. Mean distance from the platform was measured and used to indicate efficiency in locating the hidden platform. TLR4^−/− mice showed shorter mean distance from the platform at 24 and 48 hours after training compared with TLR4^+/+ mice, indicating a more accurate swim toward the platform quadrant (<b>B</b>) Mice (C57BL/6) were implanted with an osmotic pump containing either aCSF (n = 10) or a TLR4 antagonist (n = 10). The pump was connected via tubing to a cannula, which was positioned to the lateral ventricle. Following training in the MWM task, mice were tested in probe trials at 24 and 48 hours following training for retention of spatial reference memory. Both experimental groups exhibited similar performance during probe trials, as measured by mean distance from the platform.</p