Roles of NO signaling in long-term memory formation in visual learning in an insect

Critical roles of nicotinic acetylcholine receptors in olfactory memory formation and retrieval in crickets

Acetylcholine (ACh) is a major excitatory neurotransmitter in the insect central nervous system, and insect neurons express several types of ACh receptors (AChRs). AChRs are classified into two subgroups, muscarinic AChRs and nicotinic AChRs (nAChRs). nAChRs are also divided into two subgroups by sensitivity to α-bungarotoxin (α-BGT). The cricket Gryllus bimaculatus is one of the useful insects for studying the molecular mechanisms in olfactory learning and memory. However, the roles of nAChRs in olfactory learning and memory of the cricket are still unknown. In the present study, to investigate whether nAChRs are involved in cricket olfactory learning and memory, we tested the effects of two different AChR antagonists on long-term memory (LTM) formation and retrieval in a behavioral assay. The two AChR antagonists that we used are mecamylamine (MEC), an α-BGT-insensitive nAChR antagonist, and methyllycaconitine (MLA), an α-BGT-sensitive nAChR antagonist. In crickets, multiple-trial olfactory conditioning induced 1-day memory (LTM), whereas single-trial olfactory conditioning induced 1-h memory (mid-term memory, MTM) but not 1-day memory. Crickets injected with MEC 20 min before the retention test at 1 day after the multiple-trial conditioning exhibited no memory retrieval. This indicates that α-BGT-insensitive nAChRs participate in memory retrieval. In addition, crickets injected with MLA before the multiple-trial conditioning exhibited MTM but not LTM, indicating that α-BGT-sensitive nAChRs participate in the formation of LTM. Moreover, injection of nicotine (an nAChR agonist) before the single-trial conditioning induced LTM. Finally, the nitric oxide (NO)-cGMP signaling pathway is known to participate in the formation of LTM in crickets, and we conducted co-injection experiments with an agonist or inhibitor of the nAChR and an activator or inhibitor of the NO-cGMP signaling pathway. The results suggest that nAChR works upstream of the NO-cGMP signaling system in the LTM formation process.

Altered whole-brain gray matter volume in form-deprivation myopia rats based on voxel-based morphometry: A pilot study

Background

Myopia is one of the major public health problems worldwide. However, the exact pathogenesis of myopia remains unclear. This study proposes using voxel-based morphometry (VBM) to investigate potential morphological alterations in gray matter volume (GMV) in form-deprivation myopia (FDM) rats.

Methods

A total of 14 rats with FDM (FDM group) and 15 normal controls (NC group) underwent high-resolution magnetic resonance imaging (MRI). Original T2 brain images were analyzed using VBM method to identify group differences in GMV. Following MRI examination, all rats were perfused with formalin, and immunohistochemical analysis of NeuN and c-fos levels was performed on the visual cortex.

Results

In the FDM group, compared to the NC group, significantly decreased GMVs were found in the left primary visual cortex, left secondary visual cortex, right subiculum, right cornu ammonis, right entorhinal cortex and bilateral molecular layer of the cerebellum. Additionally, significantly increased GMVs were found in the right dentate gyrus, parasubiculum, and olfactory bulb.

Conclusions

Our study revealed a positive correlation between mGMV and the expression of c-fos and NeuN in the visual cortex, suggesting a molecular relationship between cortical activity and macroscopic measurement of visual cortex structural plasticity. These findings may help elucidate the potential neural pathogenesis of FDM and its relationship to changes in specific brain regions.

Nitric oxide effects on Rhodnius prolixus's immune responses, gut microbiota and Trypanosoma cruzi development

The immune system of Rhodnius prolixus comprehends the synthesis of different effectors that modulate the intestinal microbiota population and the life cycle of the parasite Trypanosoma cruzi inside the vector midgut. One of these immune responses is the production of reactive nitrogen species (RNS) derived by the action of nitric oxide synthase (NOS). Therefore, we investigated the effects of L-arginine, the substrate for nitric oxide (NO) production and N_ω-Nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of NOS, added in the insect blood meal. We analyzed the impact of these treatments on the immune responses and development of intestinal bacteria and parasites on R. prolixus nymphs. The L-arginine treatment in R. prolixus nymphs induced a higher NOS gene expression in the fat body and increased NO production, but reduced catalase and antimicrobial activities in the midgut. As expected, L-NAME treatment reduced NOS gene expression in the fat body. In addition, L-NAME treatment diminished catalase activity in the hemolymph and posterior midgut reduced phenoloxidase activity in the anterior midgut and increased the antimicrobial activity in the hemolymph. Both treatments caused a reduction in the cultivatable intestinal microbiota, especially in insects treated with L-NAME. However, T. cruzi development in the insect's digestive tract was suppressed after L-arginine treatment and the opposite was observed with L-NAME, which resulted in higher parasite counts. Therefore, we conclude that induction and inhibition of NOS and NO production are associated with other R. prolixus humoral immune responses, such as catalase, phenoloxidase, and antibacterial activities in different insect organs. These alterations reflect on intestinal microbiota and T. cruzi development.

Signaling Pathways for Long-Term Memory Formation in the Cricket

Unraveling the molecular mechanisms underlying memory formation in insects and a comparison with those of mammals will contribute to a further understanding of the evolution of higher-brain functions. As it is for mammals, insect memory can be divided into at least two distinct phases: protein-independent short-term memory and protein-dependent long-term memory (LTM). We have been investigating the signaling pathway of LTM formation by behavioral-pharmacological experiments using the cricket Gryllus bimaculatus, whose olfactory learning and memory abilities are among the highest in insect species. Our studies revealed that the NO-cGMP signaling pathway, CaMKII and PKA play crucial roles in LTM formation in crickets. These LTM formation signaling pathways in crickets share a number of attributes with those of mammals, and thus we conclude that insects, with relatively simple brain structures and neural circuitry, will also be beneficial in exploratory experiments to predict the molecular mechanisms underlying memory formation in mammals.

Roles of Octopamine and Dopamine Neurons for Mediating Appetitive and Aversive Signals in Pavlovian Conditioning in Crickets

Revealing neural systems that mediate appetite and aversive signals in associative learning is critical for understanding the brain mechanisms controlling adaptive behavior in animals. In mammals, it has been shown that some classes of dopamine neurons in the midbrain mediate prediction error signals that govern the learning process, whereas other classes of dopamine neurons control execution of learned actions. In this review, based on the results of our studies on Pavlovian conditioning in the cricket Gryllus bimaculatus and by referring to the findings in honey bees and fruit-flies, we argue that comparable aminergic systems exist in the insect brain. We found that administrations of octopamine (the invertebrate counterpart of noradrenaline) and dopamine receptor antagonists impair conditioning to associate an olfactory or visual conditioned stimulus (CS) with water or sodium chloride solution (appetitive or aversive unconditioned stimulus, US), respectively, suggesting that specific octopamine and dopamine neurons mediate appetitive and aversive signals, respectively, in conditioning in crickets. These findings differ from findings in fruit-flies. In fruit-flies, appetitive and aversive signals are mediated by different dopamine neuron subsets, suggesting diversity in neurotransmitters mediating appetitive signals in insects. We also found evidences of “blocking” and “auto-blocking” phenomena, which suggested that the prediction error, the discrepancy between actual US and predicted US, governs the conditioning in crickets and that octopamine neurons mediate prediction error signals for appetitive US. Our studies also showed that activations of octopamine and dopamine neurons are needed for the execution of an appetitive conditioned response (CR) and an aversive CR, respectively, and we, thus, proposed that these neurons mediate US prediction signals that drive appetitive and aversive CRs. Our findings suggest that the basic principles of functioning of aminergic systems in associative learning, i.e., to transmit prediction error signals for conditioning and to convey US prediction signals for execution of CR, are conserved among insects and mammals, on account of the fact that the organization of the insect brain is much simpler than that of the mammalian brain. Further investigation of aminergic systems that govern associative learning in insects should lead to a better understanding of commonalities and diversities of computational rules underlying associative learning in animals.

Communication or Toxicity: What Is the Effect of Cycloheximide on Leaf-Cutting Ant Workers?

Leaf-cutting ants are insects that use plant material to grow fungus from which they feed. These fungus-growing ants perform various behavioral activities to establish an environment conducive to the fungus. Among these behaviors are activities that can serve to detect materials harmful to the colony, such as licking, scraping, chopping, etc. However, there are substances that may not be detected as harmful to the fungus on first contact. Cycloheximide (CHX) is one such substance, described as a fungicide that inhibits the synthesis of proteins in eukaryotes, although its mechanism of action remains unclear. The present study aimed at evaluating the behavioral changes of worker ants, time carrying orange pellets, waste deposition and mortality, when subjected to seven days of CHX-incorporated pellets and another seven days of CHX-free pellets. The fungicide adversely and structurally affected the leaf-cutter ant colonies. Their behavior went through changes, such as an increase in pellet-licking frequencies and cleaning among the ants from the third day onward. Moreover, there was an increase in time carrying orange pellets, as well as in the mortality of workers during CHX incorporation.

Losing without Fighting - Simple Aversive Stimulation Induces Submissiveness Typical for Social Defeat via the Action of Nitric Oxide, but Only When Preceded by an Aggression Priming Stimulus

Losing a fight (social defeat) induces submissiveness and behavioral depression in many animals, but the mechanisms are unclear. Here we investigate how the social defeat syndrome can be established as a result of experiencing aversive stimuli and the roles of neuromodulators in the process. While biogenic amines and nitric oxide (NO) are associated with reduced aggression in mammals and insects, their specific actions during conflict are unknown. Although the social defeat syndrome normally results from complex interactions, we could induce it in male crickets simply by applying aversive stimuli (AS) in an aggressive context. Aggressive crickets became immediately submissive and behaved like losers after experiencing two brief AS (light wind puffs to the cerci), but only when preceded by a priming stimulus (PS, stroking the antenna with another male antenna). Notably, submissiveness was not induced when the PS preceded the AS by more than 1 min, or when the PS followed the AS, or using a female antenna as the preceding stimulus. These findings suggest that any potentially detrimental stimulus can acquire the attribute of an aversive agonistic signal when experienced in an aggressive context. Crickets, it seems, need only to evaluate their net sensory impact rather than the qualities of a variety of complex agonistic signals. Selective drug treatments revealed that NO, but not serotonin, dopamine or octopamine, is necessary to establish the submissive status following pairing of the priming and aversive stimuli. Moreover, treatment with an NO donor also induced the social defeat syndrome, but only when combined with the PS. This confirms our hypothesis that aversive agonistic experiences accumulated by crickets during fighting invoke social defeat via the action of NO and illustrates that a relatively simple mechanism underlies the seemingly complex social decision to flee. The simple stimulus regime described here for inducing social defeat opens new avenues for investigating the cellular control of subordinate behavior and post-conflict depression.

Nitric oxide/cGMP/PKG signaling pathway activated by M1-type muscarinic acetylcholine receptor cascade inhibits Na+-activated K+ currents in Kenyon cells

The interneurons of the mushroom body, known as Kenyon cells, are essential for the long-term memory of olfactory associative learning in some insects. Some studies have reported that nitric oxide (NO) is strongly related to this long-term memory in Kenyon cells. However, the target molecules and upstream and downstream NO signaling cascades are not completely understood. Here we analyzed the effect of the NO signaling cascade on Na(+)-activated K(+) (KNa) channel activity in Kenyon cells of crickets (Gryllus bimaculatus). We found that two different NO donors, S-nitrosoglutathione (GSNO) and S-nitroso-N-acetyl-dl-penicillamine (SNAP), strongly suppressed KNa channel currents. Additionally, this inhibitory effect of GSNO on KNa channel activity was diminished by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylate cyclase (sGC), and KT5823, an inhibitor of protein kinase G (PKG). Next, we analyzed the role of ACh in the NO signaling cascade. ACh strongly suppressed KNa channel currents, similar to NO donors. Furthermore, this inhibitory effect of ACh was blocked by pirenzepine, an M1 muscarinic ACh receptor antagonist, but not by 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP) and mecamylamine, an M3 muscarinic ACh receptor antagonist and a nicotinic ACh receptor antagonist, respectively. The ACh-induced inhibition of KNa channel currents was also diminished by the PLC inhibitor U73122 and the calmodulin antagonist W-7. Finally, we found that ACh inhibition was blocked by the nitric oxide synthase (NOS) inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME). These results suggested that the ACh signaling cascade promotes NO production by activating NOS and NO inhibits KNa channel currents via the sGC/cGMP/PKG signaling cascade in Kenyon cells.

Toward elucidating diversity of neural mechanisms underlying insect learning

Insects are widely used as models to study neural mechanisms of learning and memory. Our recent studies on crickets, together with reports on other insect species, suggest that some fundamental differences exist in neural and molecular mechanisms of learning and memory among different species of insects, particularly between crickets and fruit flies. First, we suggested that in crickets octopamine (OA) and dopamine (DA) neurons convey reward and punishment signals, respectively, in associated learning. On the other hand, it has been reported that in fruit flies different sets of DA neurons convey reward or punishment signals. Secondly, we have suggested that in crickets OA and DA neurons participate in the retrieval of appetitive and aversive memories, respectively, while this is not the case in fruit flies. Thirdly, cyclic AMP signaling is critical for short-term memory formation in fruit flies, but not in crickets. Finally, nitric oxide-cyclic GMP signaling and calcium-calmodulin signaling are critical for long-term memory (LTM) formation in crickets, but such roles have not been reported in fruit flies. Not all of these differences can be ascribed to different experimental methods used in studies. We thus suggest that there are unexpected diversities in basic mechanisms of learning and memory among different insect species, especially between crickets and fruit flies. Studies on a larger number of insect species will help clarify the diversity of learning and memory mechanisms in relation to functional adaptation to the environment and evolutionary history.

Nitric oxide modulates a swimmeret beating rhythm in the crayfish

The modulatory effects of nitric oxide (NO) and cAMP on the rhythmic beating activity of the swimmeret motor neurones in the crayfish were examined. Swimmerets are paired appendages located on the ventral side of each abdominal segment that show rhythmic beating activity during forward swimming, postural righting behaviour and egg ventilation in gravid females. In isolated abdominal nerve cord preparations, swimmeret motor neurones are usually silent or show a continuous low-frequency spiking activity. Application of carbachol, a cholinergic agonist, elicited rhythmic bursts of motor neurone spikes. The co-application of L-arginine, the substrate for NO synthesis with carbachol increased the burst frequency of the motor neurones. The co-application of the NO donor SNAP with carbachol also increased the burst frequency of the motor neurones. By contrast, co-application of a NOS inhibitor, L-NAME, with carbachol decreased beating frequency of the motor neurones. These results indicate that NO may act as a neuromodulator to facilitate swimmeret beating activity. The facilitatory effect of L-arginine was cancelled by co-application of the soluble guanylate cyclase (sGC) inhibitor ODQ suggesting that NO acts by activating sGC to promote the production of cGMP. Application of L-arginine alone or membrane-permeable cGMP analogue 8-Br-cGMP alone did not elicit rhythmic activity of motor neurones, but co-application of 8-Br-cGMP with carbachol increased bursting frequency of the motor neurones. Furthermore, application of the membrane-permeable cAMP analogue CPT-cAMP alone produced rhythmic bursting of swimmeret motor neurones, and the bursting frequency elicited by CPT-cAMP was increased by co-application with L-arginine. Co-application of the adenylate cyclase inhibitor SQ22536 ceased rhythmic bursts of motor neurone spikes elicited by carbachol. These results suggest that a cAMP system enables the rhythmic bursts of motor neurone spikes and that a NO-cGMP signaling pathway increases cAMP activity to facilitate swimmeret beating.

Roles of calcium/calmodulin-dependent kinase II in long-term memory formation in crickets

Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a key molecule in many systems of learning and memory in vertebrates, but roles of CaMKII in invertebrates have not been characterized in detail. We have suggested that serial activation of NO/cGMP signaling, cyclic nucleotide-gated channel, Ca²⁺/CaM and cAMP signaling participates in long-term memory (LTM) formation in olfactory conditioning in crickets, and here we show participation of CaMKII in LTM formation and propose its site of action in the biochemical cascades. Crickets subjected to 3-trial conditioning to associate an odor with reward exhibited memory that lasts for a few days, which is characterized as protein synthesis-dependent LTM. In contrast, animals subjected to 1-trial conditioning exhibited memory that lasts for only several hours (mid-term memory, MTM). Injection of a CaMKII inhibitor prior to 3-trial conditioning impaired 1-day memory retention but not 1-hour memory retention, suggesting that CaMKII participates in LTM formation but not in MTM formation. Animals injected with a cGMP analogue, calcium ionophore or cAMP analogue prior to 1-trial conditioning exhibited 1-day retention, and co-injection of a CaMKII inhibitor impaired induction of LTM by the cGMP analogue or that by the calcium ionophore but not that by the cAMP analogue, suggesting that CaMKII is downstream of cGMP production and Ca²⁺ influx and upstream of cAMP production in biochemical cascades for LTM formation. Animals injected with an adenylyl cyclase (AC) activator prior to 1-trial conditioning exhibited 1-day retention. Interestingly, a CaMKII inhibitor impaired LTM induction by the AC activator, although AC is expected to be a downstream target of CaMKII. The results suggest that CaMKII interacts with AC to facilitate cAMP production for LTM formation. We propose that CaMKII serves as a key molecule for interplay between Ca²⁺ signaling and cAMP signaling for LTM formation, a new role of CaMKII in learning and memory.

Ionic channel mechanisms mediating the intrinsic excitability of Kenyon cells in the mushroom body of the cricket brain

Intrinsic neurons within the mushroom body of the insect brain, called Kenyon cells, play an important role in olfactory associative learning. In this study, we examined the ionic mechanisms mediating the intrinsic excitability of Kenyon cells in the cricket Gryllus bimaculatus. A perforated whole-cell clamp study using β-escin indicated the existence of several inward and outward currents. Three types of inward currents (INaf, INaP, and ICa) were identified. The transient sodium current (INaf) activated at -40 mV, peaked at -26 mV, and half-inactivated at -46.7 mV. The persistent sodium current (INaP) activated at -51 mV, peaked at -23 mV, and half-inactivated at -30.7 mV. Tetrodotoxin (TTX; 1 μM) completely blocked both INaf and INaP, but 10nM TTX blocked INaf more potently than INaP. Cd(2+) (50 μM) potently blocked INaP with little effect on INaf. Riluzole (>20 μM) nonselectively blocked both INaP and INaf. The voltage-dependent calcium current (ICa) activated at -30 mV, peaked at -11.3 mV, and half-inactivated at -34 mV. The Ca(2+) channel blocker verapamil (100 μM) blocked ICa in a use-dependent manner. Cell-attached patch-clamp recordings showed the presence of a large-conductance Ca(2+)-activated K(+) (BK) channel, and the activity of this channel was decreased by removing the extracellular Ca(2+) or adding verapamil or nifedipine, and increased by adding the Ca(2+) agonist Bay K8644, indicating that Ca(2+) entry via the L-type Ca(2+) channel regulates BK channel activity. Under the current-clamp condition, membrane depolarization generated membrane oscillations in the presence of 10nM TTX or 100 μM riluzole in the bath solution. These membrane oscillations disappeared with 1 μM TTX, 50 μM Cd(2+), replacement of external Na(+) with choline, and blockage of Na(+)-activated K(+) current (IKNa) with 50 μM quinidine, indicating that membrane oscillations are primarily mediated by INaP in cooperation with IKNa. The plateau potentials observed either in Ca(2+)-free medium or in the presence of verapamil were eliminated by blocking INaP with 50 μM Cd(2+). Taken together, these results indicate that INaP and IKNa participate in the generation of membrane oscillations and that INaP additionally participates in the generation of plateau potentials and initiation of spontaneous action potentials. ICa, through L-type Ca(2+) channels, was also found to play a role in the rapid membrane repolarization of action potentials by functional coupling with BK channels.