Serotonin in Animal Cognition and Behavior

Abstract

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 organization of the 5-HT systems is completely different between species ranging from a limited number of cells in Aplysia or Drosophila (100 5-HT immunoreactive cells) to several thousand neurons in vertebrates. The variety and the heterogeneity of 5-HT neurons have been highlighted in vertebrates [7] as well as in invertebrates [8]. In general, the 5-HT systems in animals comprise 5-HT neurons, which share a particular shape made of thousands of varicosities, leading to the concept of volume transmission [9], and several 5-HT receptors (5-HTRs). In vertebrates, 5-HT neurons are localized in the midbrain raphe nuclei described as the B1-B9 cell groups [1,10]. The raphe pallidus, obscurus, and pontis form a caudal cluster (B1-B5), whereas the dorsal raphe and median raphe form a rostral cluster in the pons (B6-B9) [11]. The caudal raphe group projects to the spinal cord and the rostral group to the forebrain via an extensive and diffuse innervation. The dorsal raphe nucleus contains the highest number of 5-HT neurons and its anatomical sub-regions display some degrees of specific innervation of the forebrain [12][13][14]. The median and dorsal raphe nuclei receive excitatory and inhibitory inputs from most brain areas [15]. In crustaceans or insects, 5-HT neurons are widely present in each of the ventral cord ganglia, display large local ramifications that act in multiple neuropil areas, and some axonal branches form three pairs of rostrocaudal fibers [16][17][18][19]. Presumably, two of those fibers would project anteriorly and one posteriorly to the entire nervous system [20]. 5-HT neurons are often localized close to sensory integration input area in the brain of arthropods and some 5-HT cells in the abdominal ganglia of crayfish nerve cord are sensitive to the mechanical stimulation of abdominal segmental fringe hairs [21]. In cnidarians, 5-HT cells ...

Serotonergic Modulation of Locomotor Activity From Basal Vertebrates to Mammals

“…The role of the serotonergic (5-HT) system is well known to be an enigma (Jacobs and Fornal, 1995 ). 5-HT modulates animal cognition and behavior, including motor function, in a complex manner (for review Bacqué-Cazenave et al, 2020 ). Historically, the interest toward the study of 5-HT modulation of locomotor circuits followed the pioneering observations in Paris by Viala and Buser ( 1969 ), who showed that systemic injection of the 5-HT precursor 5-hydroxytryptophan (5-HTP) evoked fictive locomotor activity in rabbits (Viala and Buser, 1969 , 1971 ; see “5-HT Precursors” section).…”

Effects of Thryptophan Hydroxylase Blockade by P-Chlorophenylalanine on Contextual Memory Reconsolidation after Training of Different Intensity

“…It participates in a number of important physiological functions [81,82]. The participation of the serotonin system in animal cognition and behavior, in learning and memory attracts increasing attention [28,[83][84][85]. Therefore, the deregulation of the serotonergic system causes the pathogenesis of many psychiatric and neurological disorders [84,86].…”

“…There are a number of techniques for reducing the level of 5-HT. These are the use of 5-HT transporter blockers [85,90,94,95] and application of technologies associated with the exposure to 5-HT receptors [85,90,96,97]. Tryptophan hydroxylase, an enzyme that limits the rate of 5-HT synthesis, which regulates the activity of 5-HT in the brain, plays an important role [97][98][99][100][101].…”

“…Our experiments were conducted on a mollusk-a terrestrial snail. A characteristic feature of mollusks is that their defensive reflexes are mediated by serotonin [1][2][3][4][5]8,10,85,107]. Thus, it was found that anxious-like behavior in crayfish is controlled by serotonin [108], and in the snail, the formation of long-term sensitization is blocked by the neurotoxin 5,6-dihydroxytryptamine [27,109,110].…”

Hormonal Regulation of Mammalian Adult Neurogenesis: A Multifaceted Mechanism

“…5-HT regulates diverse brain functions such as autonomic nervous system reactivity, sleep cycles, and appetite [246,247]. Furthermore, 5-HT has a role in regulating various emotional behaviors such as anxiety, aggression, and affiliative behaviors [248]. Serotonergic projections originating from the brain stem raphe nuclei innervate nearly every part of the forebrain including the HYP, AMY, prefrontal cortex, and hippocampus where the 5-HT effects are mediated via 15 different 5-HT receptors [249].…”