Neuromodulation and the evolution of a simple motor system

Grounding the Attentional Boost Effect in Events and the Efficient Brain

Attention and memory for everyday experiences vary over time, wherein some moments are better attended and subsequently better remembered than others. These effects have been demonstrated in naturalistic viewing tasks with complex and relatively uncontrolled stimuli, as well as in more controlled laboratory tasks with simpler stimuli. For example, in the attentional boost effect (ABE), participants perform two tasks at once: memorizing a series of briefly presented stimuli (e.g., pictures of outdoor scenes) for a later memory test, and responding to other concurrently presented cues that meet pre-defined criteria (e.g., participants press a button for a blue target square and do nothing for a red distractor square). However, rather than increasing dual-task interference, attending to a target cue boosts, rather than impairs, subsequent memory for concurrently presented information. In this review we describe current data on the extent and limitations of the attentional boost effect and whether it may be related to activity in the locus coeruleus neuromodulatory system. We suggest that insight into the mechanisms that produce the attentional boost effect may be found in recent advances in the locus coeruleus literature and from understanding of how the neurocognitive system handles stability and change in everyday events. We consequently propose updates to an early account of the attentional boost effect, the dual-task interaction model, to better ground it in what is currently known about event cognition and the role that the LC plays in regulating brain states.

A tale of two leeches: Toward the understanding of the evolution and development of behavioral neural circuits

In the animal kingdom, behavioral traits encompass a broad spectrum of biological phenotypes that have critical roles in adaptive evolution, but an EvoDevo approach has not been broadly used to study behavior evolution. Here, we propose that, by integrating two leech model systems, each of which has already attained some success in its respective field, it is possible to take on behavioral traits with an EvoDevo approach. We first identify the developmental changes that may theoretically lead to behavioral evolution and explain why an EvoDevo study of behavior is challenging. Next, we discuss the pros and cons of the two leech model species, Hirudo, a classic model for invertebrate neurobiology, and Helobdella, an emerging model for clitellate developmental biology, as models for behavioral EvoDevo research. Given the limitations of each leech system, neither is particularly strong for behavioral EvoDevo. However, the two leech systems are complementary in their technical accessibilities, and they do exhibit some behavioral similarities and differences. By studying them in parallel and together with additional leech species such as Haementeria, it is possible to explore the different levels of behavioral development and evolution.

Evolution of central pattern generators and rhythmic behaviours

Comparisons of rhythmic movements and the central pattern generators (CPGs) that control them uncover principles about the evolution of behaviour and neural circuits. Over the course of evolutionary history, gradual evolution of behaviours and their neural circuitry within any lineage of animals has been a predominant occurrence. Small changes in gene regulation can lead to divergence of circuit organization and corresponding changes in behaviour. However, some behavioural divergence has resulted from large-scale rewiring of the neural network. Divergence of CPG circuits has also occurred without a corresponding change in behaviour. When analogous rhythmic behaviours have evolved independently, it has generally been with different neural mechanisms. Repeated evolution of particular rhythmic behaviours has occurred within some lineages due to parallel evolution or latent CPGs. Particular motor pattern generating mechanisms have also evolved independently in separate lineages. The evolution of CPGs and rhythmic behaviours shows that although most behaviours and neural circuits are highly conserved, the nature of the behaviour does not dictate the neural mechanism and that the presence of homologous neural components does not determine the behaviour. This suggests that although behaviour is generated by neural circuits, natural selection can act separately on these two levels of biological organization.

Neural mechanisms underlying the evolvability of behaviour

The complexity of nervous systems alters the evolvability of behaviour. Complex nervous systems are phylogenetically constrained; nevertheless particular species-specific behaviours have repeatedly evolved, suggesting a predisposition towards those behaviours. Independently evolved behaviours in animals that share a common neural architecture are generally produced by homologous neural structures, homologous neural pathways and even in the case of some invertebrates, homologous identified neurons. Such parallel evolution has been documented in the chromatic sensitivity of visual systems, motor behaviours and complex social behaviours such as pair-bonding. The appearance of homoplasious behaviours produced by homologous neural substrates suggests that there might be features of these nervous systems that favoured the repeated evolution of particular behaviours. Neuromodulation may be one such feature because it allows anatomically defined neural circuitry to be re-purposed. The developmental, genetic and physiological mechanisms that contribute to nervous system complexity may also bias the evolution of behaviour, thereby affecting the evolvability of species-specific behaviour.

Homologues of serotonergic central pattern generator neurons in related nudibranch molluscs with divergent behaviors

Homologues of a neuron that contributes to a species-specific behavior were identified and characterized in species lacking that behavior. The nudibranch Tritonia diomedea swims by flexing its body dorsally and ventrally. The dorsal swim interneurons (DSIs) are components of the central pattern generator (CPG) underlying this rhythmic motor pattern and also activate crawling. Homologues of the DSIs were identified in six nudibranchs that do not exhibit dorsal-ventral swimming: Tochuina tetraquetra, Melibe leonina, Dendronotus iris, D. frondosus, Armina californica, and Triopha catalinae. Homology was based upon shared features that distinguish the DSIs from all other neurons: (1) serotonin immunoreactivity, (2) location in the Cerebral serotonergic posterior (CeSP) cluster, and (3) axon projection to the contralateral pedal ganglion. The DSI homologues, named CeSP-A neurons, share additional features with the DSIs: irregular basal firing, synchronous inputs, electrical coupling, and reciprocal inhibition. Unlike the DSIs, the CeSP-A neurons were not rhythmically active in response to nerve stimulation. The CeSP-A neurons in Tochuina and Triopha also excited homologues of the Tritonia Pd5 neuron, a crawling efferent. Thus, the CeSP-A neurons and the DSIs may be part of a conserved network related to crawling that may have been co-opted into a rhythmic swim CPG in Tritonia.

Evolution and development of neural circuits in invertebrates

Developmental mechanisms can shed light on how evolutionary diversity has arisen. Invertebrate nervous systems offer a wealth of diverse structures and functions from which to relate development to evolution. Individual homologous neurons have been shown to have distinct roles in species with different behaviors. In addition, specific neurons have been lost or gained in some phylogenetic lineages. The ability to address the neural basis of behavior at the cellular level in invertebrates has facilitated discoveries showing that species-specific behavior can arise from differences in synaptic strength, in neuronal structure and in neuromodulation. The mechanisms involved in the development of neural circuits lead to these differences across species.

Species-specific modulation of pattern-generating circuits

Phylogenetic comparison can reveal general principles governing the organization and neuromodulation of neural networks. Suitable models for such an approach are the pyloric and gastric motor networks of the crustacean stomatogastric ganglion (STG). These networks, which have been well studied in several species, are extensively modulated by projection neurons originating in higher-order ganglia. Several of these have been identified in different decapod species, including the paired modulatory proctolin neuron (MPN) in the crab Cancer borealis [Nusbaum & Marder (1989) J. Neurosci., 9,1501-1599; Nusbaum & Marder (1989), J. Neurosci., 9, 1600-1607] and the apparently equivalent neuron pair, called GABA (gamma-aminobutyric acid) neurons 1 and 2 (GN1/2), in the lobster Homarus gammarus [Cournil et al. (1990) J. Neurocytol., 19, 478-493]. The morphologies of MPN and GN1/2 are similar, and both exhibit GABA-immunolabelling. However, unlike MPN, GN1/2 does not contain the peptide transmitter proctolin. Instead, GN1/2, but not MPN, is immunoreactive for the neuropeptides related to cholecystokinin (CCK) and FLRFamide. Nonetheless, GN1/2 excitation of the lobster pyloric rhythm is similar to the proctolin-mediated excitation of the crab pyloric rhythm by MPN. In contrast, GN1/2 and MPN both use GABA but produce opposite effects on the gastric mill rhythm. While MPN stimulation produces a GABA-mediated suppression of the gastric rhythm [Blitz & Nusbaum (1999) J. Neurosci., 19, 6774-6783], GN1/2 activates or enhances gastric rhythmicity. These results highlight the care needed when generalizing neuronal organization and function across related species. Here we show that the 'same' neuron in different species does not contain the same neurotransmitter complement, nor does it exert all of the same effects on its postsynaptic targets. Conversely, a different transmitter phenotype is not necessarily associated with a qualitative change in the way that a modulatory neuron influences target network activity.

The evolution of neuronal circuits underlying species-specific behavior

The nervous system is evolutionarily conservative compared to the peripheral appendages that it controls. However, species-specific behaviors may have arisen from very small changes in neuronal circuits. In particular, changes in neuromodulatory systems may allow multifunctional circuits to produce different sets of behaviors in closely related species. Recently, it was demonstrated that even species differences in complex social behavior may be attributed to a change in the promoter region of a single gene regulating a neuromodulatory action.

Intrinsic and extrinsic neuromodulation of motor circuits

Neuromodulation of motor circuits by extrinsic inputs provides enormous flexibility in the production of behavior. Recent work has shown that neurons intrinsic to central pattern-generating circuits can evoke neuromodulatory effects in addition to their neurotransmitting actions. Modulatory neurons often elicit a multitude of different effects attributable to actions at different receptors and/or through the release of co-transmitters. Differences in neuromodulation between species can account for differences in behavior. Modulation of neuromodulation may provide an additional level of flexibility to motor circuits.

Development of the olfactory and accessory lobes in the American lobster: an allometric analysis and its implications for the deutocerebral structure of decapods

The allometric changes characterizing the growth of the deutocerebrum (midbrain) of the American lobster (Homarus americanus) are studied using computerized three-dimensional reconstructions of serial brain sections. During the embryogenesis of the midbrain, the paired accessory lobes (higher order processing areas) appear later than the paired olfactory lobes (primary olfactory centers), but the former grow faster from their emergence until metamorphosis. The accessory lobes, as they enlarge, shift progressively from a medial to a posterior position in the lateral deutocerebrum. In early juvenile stages the accessory lobes are one of the largest neuropils of the brain. However, these lobes stop growing in adult animals, whereas the brain and olfactory lobes continue to enlarge, albeit at a slow rate. The overall shape of the brain and the relative proportions and locations of the deutocerebral neuropils and associated cell clusters of various lobster ontogenetic stages are similar to those of selected adult decapods. In addition, the relation between deutocerebral organization and brain size seem parallel during lobster development and across crustacean species. Measurements of the brains of 13 species of decapods (illustrated in Sandeman et al. [1993] J. Exp. Zool. 265:112, plus Homarus) indicate the following trends: Small brains possess olfactory lobes but no accessory lobes, larger brains possess accessory lobes that are medial and small relative to the olfactory lobes, and the largest brains contain relatively voluminous posterior accessory lobes. These observations indicate that some differences in the organization of the deutocerebrum are related to absolute brain size in crustaceans and suggest that ontogenetic scaling of proportions may apply to the deutocerebral neuropils of decapods. Peramorphosis and paedomorphosis in the evolution of the decapod brain are considered.

Replacement of an inherited stretch receptor by a newly evolved stretch receptor in hippid sand crabs

Primary sensory neurons that are motoneuron-like in morphology and often nonspiking (transmit afferent signals as graded depolarizations) characterize an unusual type of stretch receptor in decapod crustaceans. Nonspiking and spiking receptors occur in similar positions at homologous joints in different species and have been presumed to be homologous, the spiking one considered "primitive". To better understand the evolutionary origin of these stretch receptors and why some are nonspiking, we examined the spiking telson-uropod stretch receptors in the spiny sand crab Blepharipoda occidentalis (Albuneidae) and the squat lobster Munida quadrispina (Galatheidae) and compared them with the nonspking telson-uropod stretch receptor of the mole sand crab Emerita analoga (Hippidae). The position, morphology and responses to stretch of the sensory neurons, and the ultrastructure of the elastic strand portion of the receptor are similar in M. quadrispina and B. occidentalis, except that in B. occidentalis the receptor muscles are substantially smaller and the extracellular matrix of the elastic receptor strand is both more extensive and more organized, reminiscent of the ultrastructure of E. analoga's nonspiking receptor. We conclude that the spiking telson-uropod stretch receptors of albuneids and galatheids are homologous. The differences in the ultrastructure of their receptor strands imply that the efficiency of coupling receptor length change to deformation of the dendritic termini increases in the order M. quadrispina < B. occidentalis < E. analoga. The spiking and nonspiking telson-uropod stretch receptors differ anatomically in three major respects that appear to preclude their homology. (1) The receptor strands are on opposite sides of a conserved muscle. (2) The sensory somata are in different regions of the sixth abdominal ganglion: a lateral cluster of somata for the spiking sensory neurons and two medial clusters, one anterior, one posterior, for the nonspiking sensory neurons. (3) The neuropil projections of the sensory neurons are different. We conclude that the hippid's nonspiking telson-uropod stretch receptor evolved de novo and not by modification of the ancestral anomuran telson-uropod stretch receptor (which Hippidae have lost).

A good eye for arthropod evolution

Insect and crustacean lineages diverged over 500 Myr ago, and there are continuing uncertainties about whether they evolved from a common arthropod ancestor or, alternatively, they evolved independently from annelid worms. Despite the diversity of their limbs and lifestyles, the nervous systems of insects and crustaceans share many common features both in development and in function. Cellular and molecular embryology techniques reveal good evidence for homologies in the developing segmental ganglia. In the visual system, this seemingly common programme of insect and crustacean CNS development culminates in common adult neural function. Comparisons of the cellular anatomy and physiology of animals as diverse as flies and crayfishes indicate that the neural circuits in the lamina of their optic lobe have been inherited largely unchanged from a common ancestor with good compound eyes.