Associatively reduced withdrawal from shadows in Hermissenda: a direct behavioral analog of photoreceptor responses to brief light steps

The multifaceted effects of flavonoids on neuroplasticity

 There has been a significant increase in the incidence of multiple neurodegenerative and terminal diseases in the human population with life expectancy increasing in the current times. This highlights the urgent need for a more comprehensive understanding of how different aspects of lifestyle, in particular diet, may affect neural functioning and consequently cognitive performance as well as in enhancing overall health. Flavonoids, found in a variety of fruits, vegetables, and derived beverages, provide a new avenue of research that shows a promising influence on different aspects of brain function. However, despite the promising evidence, most bioactive compounds lack strong clinical research efficacy. In the current scoping review, we highlight the effects of Flavonoids on cognition and neural plasticity across vertebrates and invertebrates with special emphasis on the studies conducted in the pond snail, Lymnaea stagnalis, which has emerged to be a functionally dynamic model for studies on learning and memory. In conclusion, we suggest future research directions and discuss the social, cultural, and ethnic dependencies of bioactive compounds that influence how these compounds are used and accepted globally. Bridging the gap between preclinical and clinical studies about the effects of bioactive natural compounds on brain health will surely lead to lifestyle choices such as dietary Flavonoids being used complementarily rather than as replacements to classical drugs bringing about a healthier future.

Invertebrates as models of learning and memory: investigating neural and molecular mechanisms

In this Commentary, we shed light on the use of invertebrates as model organisms for understanding the causal and conserved mechanisms of learning and memory. We provide a condensed chronicle of the contribution offered by mollusks to the studies on how and where the nervous system encodes and stores memory and describe the rich cognitive capabilities of some insect species, including attention and concept learning. We also discuss the use of planarians for investigating the dynamics of memory during brain regeneration and highlight the role of stressful stimuli in forming memories. Furthermore, we focus on the increasing evidence that invertebrates display some forms of emotions, which provides new opportunities for unveiling the neural and molecular mechanisms underlying the complex interaction between stress, emotions and cognition. In doing so, we highlight experimental challenges and suggest future directions that we expect the field to take in the coming years, particularly regarding what we, as humans, need to know for preventing and/or delaying memory loss. This article has an associated ECR Spotlight interview with Veronica Rivi.

Random Noise Paradoxically Improves Light-Intensity Encoding in Hermissenda Photoreceptor Network

Neurons are notoriously noisy devices. Although the traditional view posits that noise degrades system performance, recent evidence suggests that noise may instead enhance neural information processing under certain conditions. Here we report that random channel and synaptic noise improve the ability of a biologically realistic computational model of the Hermissenda eye to encode light intensity. The model was created in GENESIS and is based on a previous model used to examine effects of changes in type B photoreceptor excitability, synaptic strength, and network architecture. The network consists of two type A and three type B multicompartmental photoreceptors. Each compartment contains a population of Hodgkin–Huxley-type ion channels and each cell is stimulated via artificial light currents. We found that the addition of random channel and synaptic noise yielded a significant improvement in the accuracy of the network's encoding of light intensity across eight light levels spanning 3.5 log units ( P < 0.001, modified Levene test). The benefits of noise remained after controlling for several consequences of randomness in the model. Additionally, improvements were not confined to perithreshold stimulus intensities. Finally, the effects of noise are not present in individual neurons, but rather are an emergent property of the synaptically connected network that is independent of stochastic resonance. These results suggest that noise plays a constructive role in neural information processing, a concept that could have important implications for understanding neural information processing or designing neural interface devices.

Post-light potentiation at type B to A photoreceptor connections in Hermissenda

We investigated whether the long (approximately 30-s) or short (approximately 3-s) light stimuli that have been used during behavioral training would induce post-light potentiation (PLP) at the type B to A photoreceptor connections of the isolated nervous system of Hermissenda. We found that a single approximately 30-s light step induced PLP at these connections relative to both pre-light baseline and seawater control preparations, as did a series of nine short (approximately 3-s) light steps. In addition, a 30-s step of depolarization-elicited type B cell activity induced potentiation comparable to that induced by a approximately 30-s light step, indicating that light-elicited type B cell activity contributes to the induction of PLP. By contrast, even though a series of short (3-s) light steps induced potentiation, short steps of depolarization-evoked type B cell activity did not. Hence, light-evoked processes other than type B cell depolarization or activity (e.g., perhaps intracellular Ca2+ release) also contribute to the induction of PLP. Further results suggest that these other light-evoked processes interact nonadditively with type B cell activity to generate PLP. Some but not all instances of synaptic potentiation were accompanied by various changes in parameters of type B cell action potentials and afterhyperpolarizing potentials, suggesting diverse underlying mechanisms, including increases in neurotransmitter release. Given that the type A cells have been proposed to polysynaptically excite the motor neurons that drive phototaxis, a light-evoked potentiation of synaptic strength at the inhibitory type B to A photoreceptor connections may play a mechanistic role in light-elicited nonassociative learning.

Neurophysiological substrates of context conditioning in Hermissenda suggest a temporally invariant form of activity-dependent neuronal facilitation

The neurophysiological basis for context conditioning is conceptually problematic because neurophysiological descriptions of activity-dependent (associative) forms of neuronal plasticity uniformly assume that a specific temporal relationship between signals is necessary for memory induction. In the present experiments, this problem is addressed empirically by presenting, as a temporally diffuse contextual signal, a stimulus that results in known neural modifications following punctate (temporally contiguous) pairings with an aversive unconditioned stimulus. Hermissenda were trained to discriminate between adjoining contexts that were distinguished only in that one was lit and one was dark. Thirty unsignaled rotations were presented during each of three 15-min sessions in one of the two (lit or dark) contexts. Prior to training, animals displayed a slight preference for the lit context. After exposure to unsignaled rotation, animal's preferences shifted strongly to the dark context if unsignaled rotations were presented in the light, and tended (nonsignificantly) to the lit context if unsignaled rotations were presented in the dark. The B photoreceptors of the Hermissenda eye undergo several forms of activity-dependent facilitation (e.g., an increase in neuronal input resistance and evoked spike frequency) following pairings of punctate light (CS) and presynaptic vestibular stimulation (US). Similar facilitation in the B photoreceptor was observed following in vitro training that mimicked context conditioning in which presynaptic vestibular stimulation was presented repetitively during a continuous 7.5-min light. Subsequently, Ca(2+)-imaging experiments were conducted with Fura-2AM. It was determined that intracellular Ca(2+), the CS-induced second messenger critical for the induction of activity-dependent facilitation, was elevated in the B photoreceptor throughout the 7.5-min light presentation. These results indicate that activity-dependent facilitation within similar neural structures can underlie learning about both temporally diffuse contextual stimuli and temporally punctate CS-US pairings. These results suggest that a common mechanism may underlie learning about diffuse contextual stimuli as well as punctate-conditioned stimuli, provided that the stimuli are processed similarly in each type of conditioning arrangement. Consequently, the expression of different responses to contextual and discrete stimuli are likely to reflect a higher property of the neural network, and do not necessarily arise from unique underlying mechanisms.

Phospholipases and arachidonic acid contribute independently to sensory transduction and associative neuronal facilitation in Hermissenda type B photoreceptors

During contiguous pairings of light and rotation, B photoreceptors in the Hermissenda eye undergo an increase in excitability that contributes to a modification of several light-elicited behaviors. This excitability increase requires a light-induced rise in intracellular Ca2+ in the photoreceptor concomitant with transmitter binding to G protein-coupled receptors as a result of presynaptic vestibular hair cell stimulation. Phospholipases and arachidonic acid (ArA) are here reported to be involved in independent signal transduction pathways that underlie both receptor function and activity-dependent facilitation of the B photoreceptor. 4-Bromophenacyl bromide (BPB), an inhibitor of phospholipases A2 (PLA2) and C (PLC), blocked the generation of light-induced depolarizing generator potentials, but had no affect on the inhibitory postsynaptic potential (IPSP) in the B cell that results from hair cell stimulation. Quinacrine, which predominantly blocks the activity of PLA2 in neurons, had no affect on either the light response or the IPSP, but did block increases in excitability (i.e. increased input resistance and elicited spike rate) of the B cell that results from pairings of light and presynaptic vestibular stimulation (i.e., in vitro associative conditioning). Neither nordihydroquararetic acid (NDGA), which inhibits metabolism of ArA by cyclooxygenase, nor indomethacin, which inhibits lipoxygenase metabolism of ArA, affected the light response or IPSP, but both blocked the increases in excitability in the B cell that accompanied in vitro conditioning. In combination with earlier results, these data suggest that ArA activates PKC in a synergistic fashion with Ca2+ and diacylglycerol in the B cell, and suggest that PLA2-induced ArA release, though not necessary for transduction of light or the hair cell-induced IPSP in the B cell, is a critical component of the convergence of signals that precipitates associative facilitation in this system.

Long-term transformation of an inhibitory into an excitatory GABAergic synaptic response

For a constant membrane potential, a predominantly inhibitory GABAergic synaptic response is shown to undergo long-term transformation into an excitatory response after pairing of exogenous gamma-aminobutyric acid (GABA) with postsynaptic depolarization or pairing of pre- and postsynaptic stimulation. Current- and voltage-clamp experiments suggest that this synaptic transformation is due to a shift from a net increase of conductance to a net decrease of conductance in response to GABA. GABA-induced elevation of intracellular calcium is prolonged after the same stimulus pairing and may, therefore, contribute to this synaptic transformation via Ca(2+)-activated phosphorylation pathways. This synaptic transformation, which does not follow unpaired stimulus presentations, occurs in a neuronal compartment spatially separated from the soma, which also changes during stimulus pairing.

The interstimulus interval and classical conditioning in the marine snail Hermissenda crassicornis

We examined the role of the interstimulus interval for the conditioned association between light and rotation stimuli in the marine snail Hermissenda. This interval between the conditioned stimulus (CS) and the unconditioned stimulus (US) is an important and widespread property of vertebrate associative learning. We demonstrated that with a forward CS-US delay of 1.0 s we were able to produce significant 24-h retention of an associative memory after 50 training trials. Other paired treatments providing intervals of 1.5 s, 0.5 s, simultaneous, and backward arrangements did not support retention at 24 h.

Singleness of action in the interactions of feeding with other behaviors in Hermissenda crassicornis

Singleness of action in the interactions of feeding behavior with other behaviors was studied in Hermissenda crassicornis. In Experiment 1 withdrawal of the oral veil from a tactile stimulus was inhibited during feeding elicited by application of a mussel homogenate to hungry animals. Rolling-over behavior was also inhibited during feeding. When animals were satiated, thereby abolishing the feeding response to mussel homogenate, withdrawal occurred in the presence of the food stimulus; however, rolling-over behavior was still inhibited. In Experiments 2 and 3 it was demonstrated that spontaneous locomotion was also inhibited in hungry animals by food stimuli; however, satiated animals ignored the food stimulus and continued to locomote. Both biting behavior and the suppressive effect of food stimuli on locomotion were also observed in dissected anterior ends of Hermissenda, suggesting that the mechanisms underlying the interaction of feeding with other behaviors may be accessible to study in the nervous system of reduced preparations.