Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator of nervous systems in both invertebrates and vertebrates. It has been proposed for several decades that it impacts animal cognition and behavior. In spite of a completely distinct organization of the 5-HT systems across the animal kingdom, several lines of evidence suggest that the influences of 5-HT on behavior and cognition are evolutionary conserved. In this review, we have selected some behaviors classically evoked when addressing the roles of 5-HT on nervous system functions. In particular, we focus on the motor activity, arousal, sleep and circadian rhythm, feeding, social interactions and aggressiveness, anxiety, mood, learning and memory, or impulsive/compulsive dimension and behavioral flexibility. The roles of 5-HT, illustrated in both invertebrates and vertebrates, show that it is more able to potentiate or mitigate the neuronal responses necessary for the fine-tuning of most behaviors, rather than to trigger or halt a specific behavior. 5-HT is, therefore, the prototypical neuromodulator fundamentally involved in the adaptation of all organisms across the animal kingdom.

The crayfish that was afraid of the dark: We tend to assume that complex emotions, such as anxiety, only occur in mammals or other cognitively complex vertebrates. But a heightened sense of awareness and the avoidance of novel or dangerous environments could be helpful for any animal species. Fossat et al. show that crayfish exposed to a stressful electric field refuse to enter dark arms in a light/dark maze, even after the electric field has been removed. The animals calmed down when they were injected with an anxiolytic drug used to treat anxiety in humans, and they entered the dark as normal. The stressed animals had increased levels of the neurotransmitter serotonin in the brain, and injections of serotonin induced anxiety-like behavior in control animals. Thus, these invertebrates display a primitive form of anxiety that shares a mechanism with the more complex emotions displayed by vertebrates. Science , this issue p. 1293

In the animal kingdom, biogenic amines are widespread modulators of nervous system that frequently interact in the control of mood. Our previous investigations in crayfish (Procambarus clarkii) have established that stress induces changes in brain serotonin (5-HT) concentrations that are responsible for the appearance of anxiety-like behavior (ALB). The following study further analyzes the roles of 5-HT and of another biogenic amine, dopamine (DA), on crayfish response to stress. Results showed that the intensity of crayfish ALB depended on the intensity of stressful stimulation and was associated with increased concentrations of 5-HT in the brain. These 5-HT levels were significantly correlated, before, as well as after stress, with those of DA, which were approximately 3-5 times less abundant. However, whereas ALB intensity was clearly correlated with brain 5-HT concentrations, it was not significantly correlated with those of DA. Moreover, contrary to 5-HT, DA injections were not able to elicit stress response and ALB. Incidentally, 5-HT and DA levels were not modified by treatment with the anxiolytic chlordiazepoxide, confirming that ALB suppression by this ligand of GABA-A receptors acted downstream and independently from changes in crayfish bioamine levels. Our study also provided evidence that the anxiogenic effect of 5-HT injections can be prevented by a preliminary injection of 5-HT antagonists. Altogether, our results emphasize that the rises in the brain concentrations of 5-HT, but not of DA, play a role in controlling the induction and the intensity of crayfish ALB.

By acting on their ionotropic chloride channel receptors, GABA and glycine represent the major inhibitory transmitters of the central nervous system. Nevertheless, in various brain structures, depolarizing GABAergic/glycinergic postsynaptic potentials (dGPSPs) lead to dual inhibitory (shunting) and excitatory components, the functional consequences of which remain poorly acknowledged. Indeed, the extent to which each component prevails during dGPSP is unclear. Understanding the mechanisms predicting the dGPSP outcome on neural network activity is therefore a major issue in neurobiology. By combining electrophysiological recordings of spinal embryonic mouse motoneurons and modelling study, we demonstrate that increasing the chloride conductance (g_Cl) favors inhibition either during a single dGPSP or during trains in which g_Cl summates. Finally, based on this summation mechanism, the excitatory effect of EPSPs is overcome by dGPSPs in a frequency-dependent manner. These results reveal an important mechanism by which dGPSPs protect against the overexcitation of neural excitatory circuits.

Background: Vibrotactile stimulation is a promising venue in the field of prosthetics to retrain sensory feedback deficits following amputation. Discrimination is well established at the forearm level but not at the upper arm level. Moreover, the effects of combining vibration characteristics such as duration and intensity has never been investigated. Method: We conducted experiments on spatial discrimination (experiment 1) and tactile intensity perception (experiment 2), using 9 combinations of 3 intensities and 3 durations of vibror stimulations device. Those combinations were tested under 4 arrangements with an array of 6 vibrors. In both experiments, linear orientation aligned with the upper arm longitudinal axis were compared to circular orientation on the upper arm circumference. For both orientations, vibrors were placed either with 3cm space between the center of 2 vibrors or proportionally to the length or the circumference of the subject upper arm. Eleven heathy subjects underwent the 2 experiments and 7 amputees (humeral level) participated in the spatial discrimination task with the best arrangement found. Results: Experiment 1 revealed that circular arrangements elicited better scores than the linear ones. Arrangements with vibrors spaced proportionally elicited better scores (up to 75% correct) than those with 3 cm spacing. Experiment 2, showed that the perceived intensity of the vibration increases with the intensity of the vibrors’ activation, but also with their duration of activation. The 7 patients obtained high scores (up to 91.67% correct) with the circular proportional (CP) arrangement. Discussion: These results highlight that discrete and short vibrations can be well discriminated by healthy subjects and people with an upper limb amputation. These new characteristics of vibrations have great potential for future sensory substitution application in closed-loop prosthetic control.