Ontogeny of serotonin-immunoreactive neurons in juvenile Aplysia california: implications for the development of learning

Expression of serotonin (5-HT) during CNS development of the cephalopod mollusk, Idiosepius notoides

Cephalopods are unique among mollusks in exhibiting an elaborate central nervous system (CNS) and remarkable cognitive abilities. Despite a profound knowledge of the neuroanatomy and neurotransmitter distribution in their adult CNS, little is known about the expression of neurotransmitters during cephalopod development. Here, we identify the first serotonin-immunoreactive (5-HT-ir) neurons during ontogeny and describe the establishment of the 5-HT system in the pygmy squid, Idiosepius notoides. Neurons that are located dorsally to each optic lobe are the first to express 5-HT, albeit only when the lobular neuropils are already quite elaborated. Later, 5-HT is expressed in almost all lobes, with most 5-HT-ir cell somata appearing in the subesophageal mass. Further lobes with numerous 5-HT-ir cell somata are the subvertical and posterior basal lobes and the optic and superior buccal lobes. Hatching squids possess more 5-HT-ir neurons, although the proportions between the individual brain lobes remain the same. The majority of 5-HT-ir cell somata appears to be retained in the adult CNS. The overall distribution of 5-HT-ir elements within the CNS of adult I. notoides resembles that of adult Octopus vulgaris and Sepia officinalis. The superior frontal lobe of all three species possesses few or no 5-HT-ir cell somata, whereas the superior buccal lobe comprises many cell somata. The absence of 5-HT-ir cell somata in the inferior buccal lobes of cephalopods and the buccal ganglia of gastropods may constitute immunochemical evidence of their homology. This integrative work forms the basis for future studies comparing molluscan, lophotrochozoan, ecdysozoan, and vertebrate brains.

Evolving concepts of arousal: insights from simple model systems

Arousal states strongly influence behavioral decisions. In general, arousal promotes activity and enhances responsiveness to sensory stimuli. Earlier work has emphasized general, or nonspecific, effects of arousal on multiple classes of behaviors. However, contemporary work indicates that arousal has quite specific effects on behavior. Here we review studies of arousal-related circuitry in molluscan model systems. Neural substrates for both general and specific effects of arousal have been identified. Based on the scope of their actions, we can distinguish two major classes of arousal elements: localized versus general. Actions of localized arousal elements are often limited to one class of behavior, and may thereby mediate specific effects of arousal. In contrast, general arousal elements may influence multiple classes of behaviors, and mediate both specific and nonspecific effects of arousal. One common way in which general arousal elements influence multiple behaviors is by acting on localized arousal elements of distinct networks. Often, effects on distinct networks have different time courses that may facilitate formation of specific behavioral sequences. This review highlights prominent roles of serotonergic systems in arousal that are conserved in gastropod molluscs despite extreme diversification of body forms, diet and ecological niches. The studies also indicate that the serotonergic elements can act as either localized or general arousal elements. We discuss the implications of these findings across animals.

Neural analog of arousal: persistent conditional activation of a feeding modulator by serotonergic initiators of locomotion

We investigated how a neural analog of a form of arousal induced by a mildly noxious stimulus can promote two antagonistic responses, locomotion and feeding. Two pairs of cerebral serotonergic interneurons in Aplysia, CC9 and CC10, were persistently activated by transient noxious stimuli. Direct stimulation of CC9-10 activated locomotor activity that outlasted the stimulation and enhanced subsequent nerve-evoked locomotor programs. Thus, CC9-10 function both as initiators and as modulators of the locomotor network. CC9-10 also interacted with the feeding circuit but in a fundamentally different manner. CC9-10 did not directly trigger feeding activity or activate feeding command or pattern generating interneurons. CC9-10 did, however, elicit slow EPSPs in serotonergic cells that modulate feeding responses, the metacerebral cells (MCCs). CC9-10 persistently enhanced MCC excitability, but did not activate the MCCs directly. Previous work has demonstrated that the MCCs are activated during food ingestion via a sensory neuron C2. Interestingly, we found that CC9-10 stimulation converted subthreshold C2 mediated excitation of the MCC into suprathreshold excitation. Transient noxious stimuli also enhanced MCC excitability, and this was largely mediated by CC9-10. To summarize, CC9-10 exert actions on the feeding network, but their functional effects appear to be conditional on the presence of food-related inputs to the MCCs. A potential advantage of this arrangement is that it may prevent conflicting responses from being directly evoked by noxious stimuli while also facilitating the ability of food-related stimuli to generate feeding responses in the aftermath of noxious stimulation.

Comparative mapping of serotonin-immunoreactive neurons in the central nervous systems of nudibranch molluscs

The serotonergic systems in nudibranch molluscs were compared by mapping the locations of serotonin-immunoreactive (5-HT-ir) neurons in 11 species representing all four suborders of the nudibranch clade: Dendronotoidea (Tritonia diomedea, Tochuina tetraquetra, Dendronotus iris, Dendronotus frondosus, and Melibe leonina), Aeolidoidea (Hermissenda crassicornis and Flabellina trophina), Arminoidea (Dirona albolineata, Janolus fuscus, and Armina californica), and Doridoidea (Triopha catalinae). A nomenclature is proposed to standardize reports of cell location in species with differing brain morphologies. Certain patterns of 5-HT immunoreactivity were found to be consistent for all species, such as the presence of 5-HT-ir neurons in the pedal and cerebral ganglia. Also, particular clusters of 5-HT-ir neurons in the anterior and posterior regions of the dorsal surface of the cerebral ganglion were always present. However, there were interspecies differences in the number of 5-HT-ir neurons in each cluster, and some clusters even exhibited strong intraspecies variability that was only weakly correlated with brain size. Phylogenetic analysis suggests that the presence of particular classes of 5-HT-ir neurons exhibits a great deal of homoplasy. The conserved features of the nudibranch serotonergic system presumably represent the shared ancestral structure, whereas the derived characters suggest substantial independent evolutionary changes in the number and presence of serotonergic neurons. Although a number of studies have demonstrated phylogenetic variability of peptidergic systems, this study suggests that serotonergic systems may also exhibit a high degree of homoplasy in some groups of organisms.

Serotonergic Modulation in Aplysia. I. Distributed Serotonergic Network Persistently Activated by Sensitizing Stimuli

A common feature of arousing stimuli used as reinforcement in animal models of learning is that they promote memory formation through widespread effects in the CNS. In the marine mollusk Aplysia, sensitization is typically induced by tail-shock, an aversive reinforcer that triggers a state of defensive arousal characterized by escape locomotion and increased heart rate. Serotonin (5-HT) contributes importantly to sensitization of defensive reflexes as well as to the regulation of locomotion and heart rate. Although specific serotonergic neurons increase their firing after tail-shock, it remains unclear whether this effect is restricted to these neurons or whether tail-shock recruits a more global serotonergic system. In this study, we recorded from serotonergic neurons throughout the CNS, which were prelabeled with 5,7-dihydroxytryptamine, during an in vitro analog of sensitization training, tail-nerve shock. We found that most of the serotonergic neurons that we recorded from (80%) increased their firing rate for several minutes after nerve shock. Most serotonergic neurons in the pedal and abdominal ganglion were also excited by 5-HT and by intracellular activation of the two serotonergic neurons CB1/CC3. This interconnectivity between serotonergic neurons might contribute to spread excitation within a large proportion of the serotonergic system during sensitization training. It is also possible that serotonergic neurons could be activated by 5-HT present in the hemolymph via a neuro-humoral positive feedback mechanism. Overall, these data indicate that sensitization training activates a large proportion of Aplysia serotonergic neurons and that this form of learning occurs in a context of increased serotonergic tone.

Quantitation of contacts among sensory, motor, and serotonergic neurons in the pedal ganglion of aplysia

Present models of long-term sensitization in Aplysia californica indicate that the enhanced behavioral response is due, at least in part, to outgrowth of sensory neurons mediating defensive withdrawal reflexes. Presumably, this outgrowth strengthens pre-existing connections by formation of new synapses with follower neurons. However, the relationship between the number of sensorimotor contacts and the physiological strength of the connection has never been examined in intact ganglia. As a first step in addressing this issue, we used confocal microscopy to examine sites of contact between sensory and motor neurons in naive animals. Our results revealed relatively few contacts between physiologically connected cells. In addition, the number of contact sites was proportional to the amplitude of the EPSP elicited in the follower motor neuron by direct stimulation of the sensory neuron. This is the first time such a correlation has been observed in the central nervous system. Serotonin is the neurotransmitter most closely examined for its role in modulating synaptic strength at the sensorimotor synapse. However, the structural relationship of serotonergic processes and sensorimotor synapses has never been examined. Surprisingly, serotonergic processes usually made contact with sensory and motor neurons at sites located relatively distant from the sensorimotor synapse. This result implies that heterosynaptic regulation is due to nondirected release of serotonin into the neuropil.

Evolution of learning in three aplysiid species: differences in heterosynaptic plasticity contrast with conservation in serotonergic pathways

We investigated the neurobiological basis of variation in sensitization between three aplysiid species: Aplysia californica, Phyllaplysia taylori and Dolabrifera dolabrifera. We tested two different forms of sensitization induced by a noxious tail shock: local sensitization, expressed near the site of shock, and general sensitization, tested at remote sites. Aplysia showed both local and general sensitization, whereas Phyllaplysia demonstrated only local sensitization, and Dolabrifera lacked both forms of learning. We then investigated a neurobiological correlate of sensitization, heterosynaptic modulation of sensory neuron excitability by tail-nerve stimulation. We found (1) an increase in sensory neuron (SN) excitability after both ipsilateral and contralateral nerve stimulation in Aplysia, (2) a smaller and shorter-lasting increase in Phyllaplysia, and (3) no effect in Dolabrifera. Because sensitization in Aplysia is strongly correlated with serotonergic (5-HT) neuromodulation, we hypothesized that the observed interspecific variation in sensitization and SN neuromodulation might be correlated with variation in the anatomy and/or functional response of the serotonergic system. However, using immunohistochemistry, we found that all three species showed a similar pattern of 5-HT innervation. Furthermore, they also showed comparable 5-HT release evoked by tail-nerve shock, as measured with chronoamperometry. These observations indicate that interspecific variation in learning is correlated with differences in SN heterosynaptic plasticity within a background of evolutionary conservation in the 5-HT neuromodulatory pathway. We thus hypothesize that evolutionary changes in learning phenotype do not involve modifications of the 5-HT pathway per se, but rather, changes in the response of SNs to the activation of this or other neuromodulatory pathways upon noxious stimulation.

Serotonin release evoked by tail nerve stimulation in the CNS of aplysia: characterization and relationship to heterosynaptic plasticity

Considerable experimental evidence suggests that serotonin (5-HT) at sensory neuron-->motor neuron (SN-->MN) synapses, as well as other neuronal sites, contributes importantly to simple forms of learning such as sensitization and classical conditioning in Aplysia. However, the actual release of 5-HT in the CNS induced by sensitizing stimuli such as tail shock has not been directly demonstrated. In this study, we addressed this question by (1) immunohistochemically labeling central 5-HT processes and (2) directly measuring with chronoamperometry the release of 5-HT induced by pedal tail nerve (P9) shock onto tail SNs in the pleural ganglion and their synapses onto tail MNs in the pedal ganglion. We found that numerous 5-HT-immunoreactive fibers surround both the SN cell bodies in the pleural ganglion and SN axons in the pedal ganglion. Chronoamperometric detection of 5-HT performed with carbon fiber electrodes implanted in the vicinity of tail SN somata and synapses revealed an electrochemical 5-HT signal lasting approximately 40 sec after a brief shock of P9. 5-HT release was restricted to discrete subregions (modulatory fields) of the CNS, including the vicinity of tail SN soma and synapses ipsilateral to the stimulation. Increasing P9 shock frequency augmented the amplitude of the 5-HT signal and, in parallel, increased SN excitability and SN synaptic transmission onto tail MNs. However, the relationship between the amount of 5-HT release and the two forms of SN plasticity was not uniform: SN excitability increased in a graded manner with increased 5-HT release, whereas synaptic facilitation exhibited a highly nonlinear relationship. The development of chronoamperometric techniques in Aplysia now paves the way for a more complete understanding of the contribution of the serotonergic modulatory pathway to memory processing in this system.

Immunocytochemical localization of serotonin in embryos, larvae and adults of the lancelet, Branchiostoma floridae

Serotonin (5-hydroxytryptamine) is a biogenic amine distributed throughout the metazoans and has an old evolutionary history. It is involved as a developmental signal in the early morphogenesis of both invertebrates and vertebrates, whereas in adults it acts mainly as a neurotransmitter and gastrointestinal hormone. In vertebrates, serotonin regulates the morphogenesis of the central nervous system and the specification of serotonergic as well as dopaminergic neurons. The present study uses, as an experimental model, an invertebrate chordate, the lancelet Branchiostomafloridae, characterized by its remarkable homologies with vertebrates that allows the 'bauplan' of the probable ancestor of vertebrates to be outlined. In particular, the involvement of serotonin as a developmental signal in embryos and larvae, as well as a neurotransmitter and gastrointestinal hormone in adult specimens of Branchiostoma floridae, gives further support to a common origin of cephalocordates and vertebrates.

Serotonin levels as a function of age in Aplysia californica

The neurotransmitter serotonin (5-HT) plays an important role in a number of behaviors in Aplysia californica some of which have been shown to vary with age. We were thus interested in examining the age-dependence of 5-HT in A. californica. Because animals of the same age can have very different weights, and weight alone is reliably known for wild-caught animals, we also examined the variation of 5-HT with weight. Serotonin was measured in the ring and abdominal ganglia combined, in lab-reared animals from 3 to 12 months post-hatch across a wide weight range. Serotonin increased rapidly from 4 to 6 months, and more slowly from 6 to 13 months. Serotonin scaled by soluble ganglion protein increased from 3 to 6-7 months, reached a maximum, and then decreased again. Serotonin, but not scaled 5-HT, increased significantly with weight across the whole weight range. Animals of the same weight, but different ages, had different 5-HT levels, as did young animals of the same age but different weight. Serotonin varied significantly with both age and weight, with the age-dependence being the more significant.

Developmental expression of the octopamine phenotype in lobsters, Homarus americanus

We have used immunocytochemical methods to examine the sequence of appearance of octopamine-immunoreactive neurons during development, and to try to correlate that appearance with the emergence of behavioral or physiological capabilities. The first octopamine neurons express their transmitter phenotype at approximately 43% of embryonic development. The last cells show immunostaining at the 3rd larval stage. In the wild, therefore, immunoreactivity in cells appears over a 9-12 month period. In contrast, serotonin-immunoreactive neurons stain early in embryonic development and the last serotonin-immunoreactive cells appear at about the same time the first octopamine-immunoreactive neurons show staining. The pattern of appearance of octopamine-immunoreactive cells is cell type-specific. A pair of brain cells and the descending interneurons stain first. Additional brain cell staining is seen throughout embryonic development. The ascending interneurons appear next, and a general anterior-posterior gradient typifies their emergence over a relatively short portion of embryonic life (E 48-62%). The neurosecretory cell staining appears last, is segment-specific, begins at about 62% development, and continues to the 3rd larval stage. The emergence of immunostaining for amine neurotransmitters within groups of identified neurons at precise times in development may specify possible functional units. With at least one group of cells, this possibility seems plausible: the three pairs of claw octopamine neurosecretory cells show immunostaining as a unit.