A chromosome-level genome for the nudibranch gastropod Berghia stephanieae helps parse clade-specific gene expression in novel and conserved phenotypes.

BACKGROUND: How novel phenotypes originate from conserved genes, processes, and tissues remains a major question in biology. Research that sets out to answer this question often focuses on the conserved genes and processes involved, an approach that explicitly excludes the impact of genetic elements that may be classified as clade-specific, even though many of these genes are known to be important for many novel, or clade-restricted, phenotypes. This is especially true for understudied phyla such as mollusks, where limited genomic and functional biology resources for members of this phylum have long hindered assessments of genetic homology and function. To address this gap, we constructed a chromosome-level genome for the gastropod Berghia stephanieae (Valdés, 2005) to investigate the expression of clade-specific genes across both novel and conserved tissue types in this species. RESULTS: The final assembled and filtered Berghia genome is comparable to other high-quality mollusk genomes in terms of size (1.05 Gb) and number of predicted genes (24,960 genes) and is highly contiguous. The proportion of upregulated, clade-specific genes varied across tissues, but with no clear trend between the proportion of clade-specific genes and the novelty of the tissue. However, more complex tissue like the brain had the highest total number of upregulated, clade-specific genes, though the ratio of upregulated clade-specific genes to the total number of upregulated genes was low. CONCLUSIONS: Our results, when combined with previous research on the impact of novel genes on phenotypic evolution, highlight the fact that the complexity of the novel tissue or behavior, the type of novelty, and the developmental timing of evolutionary modifications will all influence how novel and conserved genes interact to generate diversity.

Egg-laying hormone expression in identified neurons across developmental stages and reproductive states of the nudibranch Berghia stephanieae.

Neuropeptides play essential roles in coordinating reproduction. Egg-laying hormone (ELH) is conserved in genetic sequence and behavioral function across molluscs, where neuronal clusters secrete ELH to modulate and induce egg-laying. Here we investigated ELH in the nudibranch mollusc, Berghia stephanieae. ELH preprohormone gene orthologs, which showed clade-specific differences at the C-terminus of the predicted bioactive peptide, were identified in brain transcriptomes across several nudipleuran species, including B. stephanieae. ELH shares deep homology with the corticotropin-releasing hormone gene family, which has roles broadly in stress response. Injection of synthesized B. stephanieae ELH peptide into mature individuals induced egg-laying. ELH gene expression in the brain and body was mapped using in-situ hybridization chain reaction. Across the adult brain, 300-400 neurons expressed ELH. Twenty-one different cell types were identified in adults, three of which were located unilaterally on the right side, which corresponds to the location of the reproductive organs. Ten cell types were present in pre-reproductive juvenile stages. An asymmetric cluster of approximately 100 small neurons appeared in the right pedal ganglion of late-stage juveniles. Additional neurons in the pleural and pedal ganglia expressed ELH only in adults that were actively laying eggs and sub-adults that were on the verge of doing so, implicating their direct role in reproduction. Outside the brain, ELH was expressed on sensory appendages, including in presumptive sensory neurons. Its widespread expression in the nudibranch B. stephanieae suggests that ELH plays a role beyond reproduction in gastropod molluscs.

Perspectives of Physicians with Experience in Nursing Home Care on Telehealth Use During the COVID-19 Public Health Emergency.

BACKGROUND: Despite expanded access to telehealth services for Medicare beneficiaries in nursing homes (NHs) during the COVID-19 public health emergency, information on physicians' perspectives on the feasibility and challenges of telehealth provision for NH residents is lacking. OBJECTIVE: To examine physicians' perspectives on the appropriateness and challenges of providing telehealth in NHs. PARTICIPANTS: Medical directors or attending physicians in NHs. APPROACH: We conducted 35 semistructured interviews with members of the American Medical Directors Association from January 18 through January 29, 2021. Outcomes of the thematic analysis reflected perspectives of physicians experienced in NH care on telehealth use. MAIN MEASURES: The extent to which participants used telehealth in NHs, the perceived value of telehealth for NH residents, and barriers to telehealth provision. KEY RESULTS: Participants included 7 (20.0%) internists, 8 (22.9%) family physicians, and 18 (51.4%) geriatricians. Five common themes emerged: (1) direct care is needed to adequately care for residents in NHs; (2) telehealth may allow physicians to reach NH residents more flexibly during offsite hours and other scenarios when physicians cannot easily reach patients; (3) NH staff and other organizational resources are critical to the success of telehealth, but staff time is a major barrier to telehealth provision; (4) appropriateness of telehealth in NHs may be limited to certain resident populations and/or services; (5) conflicting views about whether telehealth use will be sustained over time in NHs. Subthemes included the role of resident-physician relationships in facilitating telehealth and the appropriateness of telehealth for residents with cognitive impairment. CONCLUSIONS: Participants had mixed views on the effectiveness of telehealth in NHs. Staff resources to facilitate telehealth and the limitations of telehealth for NH residents were the most raised issues. These findings suggest that physicians in NHs may not view telehealth as a suitable substitute for most in-person services.

State-dependent, visually guided behaviors in the nudibranch Berghia stephanieae.

Nudibranch mollusks have structurally simple eyes whose behavioral roles have not been established. We tested the effects of visual stimuli on the behavior of the nudibranch Berghia stephanieae under different food and hunger conditions. In an arena that was half-shaded, animals spent most of their time in the dark, where they also decreased their speed and made more changes in heading. These behavioral differences between the light and dark were less evident in uniformly illuminated or darkened arenas, suggesting that they were not caused by the level of illumination. Berghia stephanieae responded to distant visual targets; animals approached a black stripe that was at least 15 deg wide on a white background. They did not approach a stripe that was lighter than the background but approached a stripe that was isoluminant with the background, suggesting the detection of spatial information. Animals traveled in convoluted paths in a featureless arena but straightened their paths when a visual target was present even if they did not approach it, suggesting that visual cues were used for navigation. Individuals were less responsive to visual stimuli when food deprived or in the presence of food odor. Thus, B. stephanieae exhibits visually guided behaviors that are influenced by odors and hunger state.

Behavior-related gene regulatory networks: A new level of organization in the brain.

Neuronal networks are the standard heuristic model today for describing brain activity associated with animal behavior. Recent studies have revealed an extensive role for a completely distinct layer of networked activities in the brain-the gene regulatory network (GRN)-that orchestrates expression levels of hundreds to thousands of genes in a behavior-related manner. We examine emerging insights into the relationships between these two types of networks and discuss their interplay in spatial as well as temporal dimensions, across multiple scales of organization. We discuss properties expected of behavior-related GRNs by drawing inspiration from the rich literature on GRNs related to animal development, comparing and contrasting these two broad classes of GRNs as they relate to their respective phenotypic manifestations. Developmental GRNs also represent a third layer of network biology, playing out over a third timescale, which is believed to play a crucial mediatory role between neuronal networks and behavioral GRNs. We end with a special emphasis on social behavior, discuss whether unique GRN organization and cis-regulatory architecture underlies this special class of behavior, and review literature that suggests an affirmative answer.

Bursting emerges from the complementary roles of neurons in a four-cell network.

Reciprocally inhibitory modules that form half-center oscillators require mechanisms for escaping or being released from inhibition. The central pattern generator underlying swimming by the nudibranch mollusc, Dendronotus iris, is composed of only four neurons that are organized into two competing modules of a half-center oscillator. In this system, bursting activity in left-right alternation is an emergent property of the network as a whole; none of the neurons produces bursts on its own. We found that the unique synaptic actions and membrane properties of the two neurons in each module (Si2 and the contralateral Si3) play complementary roles in generating stable bursting in this network oscillator. Although Si2 and Si3 each inhibits its contralateral counterpart, Si2 plays a dominant role in evoking fast and strong inhibition of the other module, the termination of which initiates postinhibitory rebound in the Si3 of that module by activating a hyperpolarization-activated inward current. Within each module, the synaptic actions and membrane properties of the two neurons complement each other: Si3 excites Si2, which then feeds back slow inhibition to Si3, terminating the burst. Using dynamic clamp, we showed that the magnitude of the slow inhibition sets the period of the oscillator. Thus, the synaptic actions of Si2 provide the hyperpolarization needed for the other module to rebound stably, whereas the membrane properties of Si3 in each module cause it to rebound first and excite Si2 to maintain the burst until terminated by the slow inhibition from Si2, which releases the other module to become active.NEW & NOTEWORTHY Half-center oscillators composed of reciprocally inhibitory neurons have been posited for over a century to underlie the production of rhythmic movements. The Dendronotus swim central pattern generator may be the simplest such circuit with only two pairs of bilaterally represented neurons. This study completes the description of the mechanism by which this network oscillator functions, showing how stable rhythmic activity arises from the complementary membrane and synaptic properties of the two neurons in the competing modules.

Clinical factors associated with recent medical care visits in nursing homes: a multi-site cross-sectional study.

OBJECTIVES: We examined which resident-level clinical factors influence the provision of a recent medical care visit in nursing homes (NHs). DESIGN: Multi-site cross-sectional. SETTING AND PARTICIPANTS: We extracted data on 3,556 NH residents from 18 NH facilities in Ontario, Canada, who received at minimum, an admission and first-quarterly assessment with the Resident Assessment Instrument Minimum Data Set (MDS) 2.0 between November 1, 2009, and October 31, 2017. METHODS: We conducted a secondary analysis of routinely collected MDS 2.0 data. The provision of a recent medical care visit by a physician (or authorized clinician) was assessed in the 14-day period preceding a resident's first-quarterly MDS 2.0 assessment. We utilized best-subset multivariable logistic regression to model the adjusted associations between resident-level clinical factors and a recent medical care visit. RESULTS: Two thousand eight hundred fifty nine (80.4%) NH residents had one or more medical care visits prior to their first-quarterly MDS 2.0 assessment. Six clinically relevant factors were identified to be associated with recent medical care visits in the final model: exhibiting wandering behaviours (OR = 1.34, 95% CI 1.09 - 1.63), presence of a pressure ulcer (OR = 1.37, 95% CI 1.05 - 1.78), a urinary tract infection (UTI) (OR = 1.52, 95% CI 1.06 - 2.18), end-stage disease (OR = 9.70, 95% CI 1.32 - 71.02), new medication use (OR = 1.31, 95% CI 1.09 - 1.57), and analgesic use (OR = 1.24, 95% CI 1.03 - 1.49). CONCLUSIONS AND IMPLICATIONS: Our findings suggest that resident-level clinical factors drive the provision of medical care visits following NH admission. Clinical factors associated with medical care visits align with the minimum competencies expected of physicians in NH practice, including managing safety risks, infections, medications, and death. Ensuring that NH physicians have opportunities to acquire and strengthen these competencies may be transformative to meet the ongoing needs of NH residents.

Command or Obey? Homologous Neurons Differ in Hierarchical Position for the Generation of Homologous Behaviors.

In motor systems, higher-order neurons provide commands to lower-level central pattern generators (CPGs) that autonomously produce rhythmic motor patterns. Such hierarchical organization is often thought to be inherent in the anatomical position of the neurons. Here, however, we report that a neuron that is member of a CPG in one species acts as a higher-order neuron in another species. In the nudibranch mollusc, Melibe leonina, swim interneuron 1 (Si1) is in the CPG underlying swimming, firing rhythmic bursts of action potentials as part of the swim motor pattern. We found that its homolog in another nudibranch, Dendronotus iris, serves as a neuromodulatory command neuron for the CPG of a homologous swimming behavior. In Dendronotus, Si1 fired irregularly throughout the swim motor pattern. The burst and spike frequencies of Dendronotus swim CPG neurons correlated with Si1 firing frequency. Si1 activity was both necessary and sufficient for the initiation and maintenance of the swim motor pattern. Each Si1 was electrically coupled to all of the CPG neurons and made monosynaptic excitatory synapses with both Si3s. Si1 also bilaterally potentiated the excitatory synapse from Si3 to Si2. "Virtual neuromodulation" of both Si3-to-Si2 synapses using dynamic clamp combined with depolarization of both Si3s mimicked the effects of Si1 stimulation on the swim motor pattern. Thus, in Dendronotus, Si1 is a command neuron that turns on, maintains, and accelerates the motor pattern through synaptic and neuromodulatory actions, thereby differing from its homolog in Melibe in its functional position in the motor hierarchy.SIGNIFICANCE STATEMENT Cross-species comparisons of motor system organization can provide fundamental insights into their function and origin. Central pattern generators (CPGs) are lower in the functional hierarchy than the neurons that initiate and modulate their activity. This functional hierarchy is often reflected in neuroanatomical organization. This paper definitively shows that an identified cerebral ganglion neuron that is a member of a CPG underlying swimming in one nudibranch species serves as a command neuron for the same behavior in another species. We describe and test the synaptic and neuromodulatory mechanisms by which the command neuron initiates and accelerates rhythmic motor patterns. Thus, the functional position of neurons in a motor hierarchy can shift from one level to another over evolutionary time.

Variations on a theme: species differences in synaptic connectivity do not predict central pattern generator activity.

A fundamental question in comparative neuroethology is the extent to which synaptic wiring determines behavior vs. the extent to which it is constrained by phylogeny. We investigated this by examining the connectivity and activity of homologous neurons in different species. Melibe leonina and Dendronotus iris (Mollusca, Gastropoda, Nudibranchia) have homologous neurons and exhibit homologous swimming behaviors consisting of alternating left-right (LR) whole body flexions. Yet, a homologous interneuron (Si1) differs between the two species in its participation in the swim motor pattern (SMP) and synaptic connectivity. In this study we examined Si1 homologs in two additional nudibranchs: Flabellina iodinea, which evolved LR swimming independently of Melibe and Dendronotus, and Tritonia diomedea, which swims with dorsal-ventral (DV) body flexions. In Flabellina, the contralateral Si1s exhibit alternating rhythmic bursting activity during the SMP and are members of the swim central pattern generator (CPG), as in Melibe The Si1 homologs in Tritonia do not burst rhythmically during the DV SMP but are inhibited and receive bilaterally synchronous synaptic input. In both Flabellina and Tritonia, the Si1 homologs exhibit reciprocal inhibition, as in Melibe However, in Flabellina the inhibition is polysynaptic, whereas in Tritonia it is monosynaptic, as in Melibe In all species, the contralateral Si1s are electrically coupled. These results suggest that Flabellina and Melibe convergently evolved a swim CPG that contains Si1; however, they differ in monosynaptic connections. Connectivity is more similar between Tritonia and Melibe, which exhibit different swimming behaviors. Thus connectivity between homologous neurons varies independently of both behavior and phylogeny.NEW & NOTEWORTHY This research shows that the synaptic connectivity between homologous neurons exhibits species-specific variations on a basic theme. The neurons vary in the extent of electrical coupling and reciprocal inhibition. They also exhibit different patterns of activity during rhythmic motor behaviors that are not predicted by their circuitry. The circuitry does not map onto the phylogeny in a predictable fashion either. Thus neither neuronal homology nor species behavior is predictive of neural circuit connectivity.

Depression Predicts Functional Outcome in Geriatric Inpatient Rehabilitation.

OBJECTIVE: To evaluate the effect of depression on functional recovery in geriatric patients who have completed an inpatient rehabilitation program. DESIGN: Prospective cohort study. SETTING: Inpatient rehabilitation unit of a university-affiliated geriatric hospital. PARTICIPANTS: Convenience sample of patients (N=65; mean age, 81.6y; 25 men) admitted to rehabilitation over a 10-month period. Patients >60 years of age who were proficient in English and capable of providing informed consent were eligible to participate in the study. INTERVENTIONS: Depression was assessed using both the Geriatric Depression Scale-short form (GDS-15) and the Patient Health Questionnaire (9-item screen for depression) (PHQ-9). Measures of well-established predictors of rehabilitation outcome, which may interact with depression, were also obtained, and multiple regression linear modeling was used to evaluate the relation between depression and functional outcome over and above the contribution of these other factors. MAIN OUTCOME MEASURE: FIM (Functional Independence Measure) at discharge from the rehabilitation program. RESULTS: Depression, as assessed by the GDS-15, but not the PHQ-9, was predictive of functional outcome (standardized beta=-.151, P=.030) after controlling for other significant predictors, which included baseline disability, pain, cognition, and educational level. Participation in recreational, but not physio- or occupational, therapy additionally contributed to a small amount of variance in the functional outcome. CONCLUSIONS: Our findings suggest that self-report of depression is an independent predictor of functional outcome in high-tolerance, short-duration geriatric rehabilitation. Routine assessment of depressive symptoms in older adults using an instrument (eg, GDS-15) may help identify those at risk for poorer outcomes in rehabilitation.

Best Practices for Chiropractic Care for Older Adults: A Systematic Review and Consensus Update.

OBJECTIVE: The purpose of this study was to update evidence-based recommendations on the best practices for chiropractic care of older adults. METHODS: The project consisted of a systematic literature review and a consensus process. The following were searched from October 2009 through January 2016: MEDLINE, Index to Chiropractic Literature, CINAHL (Cumulative Index to Nursing and Allied Health Literature), AMED (Allied and Complementary Medicine Database), Alt HealthWatch, Cochrane Database of Systematic Reviews, and Cochrane Registry of Controlled Trials. Search terms were: (manipulation, spinal OR manipulation, chiropractic OR chiropract*) AND (geriatric OR "older adult*"). Two reviewers independently screened articles and abstracts using inclusion and exclusion criteria. The systematic review informed the project steering committee, which revised the previous recommendations. A multidisciplinary panel of experts representing expertise in practice, research, and teaching in a variety of health professions serving older adults rated the revised recommendations. The RAND Corporation/University of California, Los Angeles methodology for a modified Delphi consensus process was used. RESULTS: A total of 199 articles were found; after exclusion criteria were applied, 6 articles about effectiveness or efficacy and 6 on safety were added. The Delphi process was conducted from April to June 2016. Of the 37 Delphi panelists, 31 were DCs and 6 were other health care professionals. Three Delphi rounds were conducted to reach consensus on all 45 statements. As a result, statements regarding the safety of manipulation were strengthened and additional statements were added recommending that DCs advise patients on exercise and that manipulation and mobilization contribute to general positive outcomes beyond pain reduction only. CONCLUSIONS: This document provides a summary of evidence-informed best practices for doctors of chiropractic for the evaluation, management, and manual treatment of older adult patients.

Evaluation of a novel risk assessment method for self-harm associated with Borderline Personality Disorder.

OBJECTIVES: Borderline personality disorder (BPD) is associated with frequent self-harm and suicidal behaviours. This study compared physician-assessed self-harm risk and intervention choice according to a (i) standard risk assessment and (ii) BPD-specific risk assessment methods. METHODS: Forty-five junior and senior mental health physicians were assigned to standard or BPD-specific risk training groups. The assessment utilized a BPD case vignette containing four scenarios describing high/low lethality self-harm and chronic/new patterns of self-harm behaviour. Participants chose from among four interventions, each corresponding to a risk category. RESULTS: Standard and BPD-specific groups were alike in their assessment of self-harm risk. Divergence occurred on intervention choice for assessments of low lethality, chronic risk ( p<.01) and high lethality, chronic risk ( p<.005). Overall, psychiatrists were more likely than their junior colleagues to correctly assess risk and management options. CONCLUSIONS: Although standard and BPD-specific methods are well aligned for assessing self harm-associated risk, BPD-specific training raised awareness of BPD-appropriate interventions, particularly in the context of chronic patterns of self-harm behaviour. Wider dissemination of BPD-specific risk training may enhance the confidence of mental health clinicians in identifying the nature of self-harm risk as well as the most clinically appropriate interventions for clients with BPD.

The central pattern generator underlying swimming in Dendronotus iris: a simple half-center network oscillator with a twist.

The nudibranch mollusc, Dendronotus iris, swims by rhythmically flexing its body from left to right. We identified a bilaterally represented interneuron, Si3, that provides strong excitatory drive to the previously identified Si2, forming a half-center oscillator, which functions as the central pattern generator (CPG) underlying swimming. As with Si2, Si3 inhibited its contralateral counterpart and exhibited rhythmic bursts in left-right alternation during the swim motor pattern. Si3 burst almost synchronously with the contralateral Si2 and was coactive with the efferent impulse activity in the contralateral body wall nerve. Perturbation of bursting in either Si3 or Si2 by current injection halted or phase-shifted the swim motor pattern, suggesting that they are both critical CPG members. Neither Si2 nor Si3 exhibited endogenous bursting properties when activated alone; activation of all four neurons was necessary to initiate and maintain the swim motor pattern. Si3 made a strong excitatory synapse onto the contralateral Si2 to which it is also electrically coupled. When Si3 was firing tonically but not exhibiting bursting, artificial enhancement of the Si3-to-Si2 synapse using dynamic clamp caused all four neurons to burst. In contrast, negation of the Si3-to-Si2 synapse by dynamic clamp blocked ongoing swim motor patterns. Together, these results suggest that the Dendronotus swim CPG is organized as a "twisted" half-center oscillator in which each "half" is composed of two excitatory-coupled neurons from both sides of the brain, each of which inhibits its contralateral counterpart. Consisting of only four neurons, this is perhaps the simplest known network oscillator for locomotion.

Identification of genes related to learning and memory in the brain transcriptome of the mollusc, Hermissenda crassicornis.

The sea slug Hermissenda crassicornis (Mollusca, Gastropoda, Nudibranchia) has been studied extensively in associative learning paradigms. However, lack of genetic information previously hindered molecular-level investigations. Here, the Hermissenda brain transcriptome was sequenced and assembled de novo, producing 165,743 total transcripts. Orthologs of 95 genes implicated in learning were identified. These included genes for a serotonin receptor and a GABA-B receptor subunit that had not been previously described in molluscs, as well as an adenylyl cyclase gene not previously described in gastropods. This study illustrates the Hermissenda transcriptome's potential as an important genetic tool in future learning and memory research.

Phylogenetic and individual variation in gastropod central pattern generators.

Gastropod molluscs provide a unique opportunity to explore the neural basis of rhythmic behaviors because of the accessibility of their nervous systems and the number of species that have been examined. Detailed comparisons of the central pattern generators (CPGs) underlying rhythmic feeding and swimming behaviors highlight the presence and effects of variation in neural circuits both across and within species. The feeding motor pattern of the snail, Lymnaea, is stereotyped, whereas the feeding motor pattern in the sea hare, Aplysia, is variable. However, the Aplysia motor pattern is regularized with operant conditioning or by mimicking learning using the dynamic clamp to change properties of CPG neurons. Swimming evolved repeatedly in marine gastropods. Distinct neural mechanisms underlie dissimilar forms of swimming, with homologous neurons playing different roles. However, even similar swimming behaviors in different species can be produced by distinct neural mechanisms, resulting from different synaptic connectivity of homologous neurons. Within a species, there can be variation in the strength and even valence of synapses, which does not have functional relevance under normal conditions, but can cause some individuals to be more susceptible to lesion of the circuit. This inter- and intra-species variation provides novel insights into CPG function and plasticity.

Homology and homoplasy of swimming behaviors and neural circuits in the Nudipleura (Mollusca, Gastropoda, Opisthobranchia).

How neural circuit evolution relates to behavioral evolution is not well understood. Here the relationship between neural circuits and behavior is explored with respect to the swimming behaviors of the Nudipleura (Mollusca, Gastropoda, Opithobranchia). Nudipleura is a diverse monophyletic clade of sea slugs among which only a small percentage of species can swim. Swimming falls into a limited number of categories, the most prevalent of which are rhythmic left-right body flexions (LR) and rhythmic dorsal-ventral body flexions (DV). The phylogenetic distribution of these behaviors suggests a high degree of homoplasy. The central pattern generator (CPG) underlying DV swimming has been well characterized in Tritonia diomedea and in Pleurobranchaea californica. The CPG for LR swimming has been elucidated in Melibe leonina and Dendronotus iris, which are more closely related. The CPGs for the categorically distinct DV and LR swimming behaviors consist of nonoverlapping sets of homologous identified neurons, whereas the categorically similar behaviors share some homologous identified neurons, although the exact composition of neurons and synapses in the neural circuits differ. The roles played by homologous identified neurons in categorically distinct behaviors differ. However, homologous identified neurons also play different roles even in the swim CPGs of the two LR swimming species. Individual neurons can be multifunctional within a species. Some of those functions are shared across species, whereas others are not. The pattern of use and reuse of homologous neurons in various forms of swimming and other behaviors further demonstrates that the composition of neural circuits influences the evolution of behaviors.

Parallel evolution of serotonergic neuromodulation underlies independent evolution of rhythmic motor behavior.

Neuromodulation can dynamically alter neuronal and synaptic properties, thereby changing the behavioral output of a neural circuit. It is therefore conceivable that natural selection might act upon neuromodulation as a mechanism for sculpting the behavioral repertoire of a species. Here we report that the presence of neuromodulation is correlated with the production of a behavior that most likely evolved independently in two species: Tritonia diomedea and Pleurobranchaea californica (Mollusca, Gastropoda, Opisthobranchia, Nudipleura). Individuals of both species exhibit escape swimming behaviors consisting of repeated dorsal-ventral whole-body flexions. The central pattern generator (CPG) circuits underlying these behaviors contain homologous identified neurons: DSI and C2 in Tritonia and As and A1 in Pleurobranchaea. Homologs of these neurons also can be found in Hermissenda crassicornis where they are named CPT and C2, respectively. However, members of this species do not exhibit an analogous swimming behavior. In Tritonia and Pleurobranchaea, but not in Hermissenda, the serotonergic DSI homologs modulated the strength of synapses made by C2 homologs. Furthermore, the serotonin receptor antagonist methysergide blocked this neuromodulation and the swimming behavior. Additionally, in Pleurobranchaea, the robustness of swimming correlated with the extent of the synaptic modulation. Finally, injection of serotonin induced the swimming behavior in Tritonia and Pleurobranchaea, but not in Hermissenda. This suggests that the analogous swimming behaviors of Tritonia and Pleurobranchaea share a common dependence on serotonergic neuromodulation. Thus, neuromodulation may provide a mechanism that enables species to acquire analogous behaviors independently using homologous neural circuit components.

Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusk Melibe leonina.

The central pattern generator (CPG) underlying the rhythmic swimming behavior of the nudibranch Melibe leonina (Mollusca, Gastropoda, Heterobranchia) has been described as a simple half-center oscillator consisting of two reciprocally inhibitory pairs of interneurons called swim interneuron 1 (Si1) and swim interneuron 2 (Si2). In this study, we identified two additional pairs of interneurons that are part of the swim CPG: swim interneuron 3 (Si3) and swim interneuron 4 (Si4). The somata of Si3 and Si4 were both located in the pedal ganglion, near that of Si2, and both had axons that projected through the pedal commissure to the contralateral pedal ganglion. These neurons fulfilled the criteria for inclusion as members of the swim CPG: 1) they fired at a fixed phase in relation to Si1 and Si2, 2) brief changes in their activity reset the motor pattern, 3) prolonged changes in their activity altered the periodicity of the motor pattern, 4) they had monosynaptic connections with each other and with Si1 and Si2, and 5) their synaptic actions helped explain the phasing of the motor pattern. The results of this study show that the motor pattern has more complex internal dynamics than a simple left/right alternation of firing; the CPG circuit appears to be composed of two kernels of reciprocally inhibitory neurons, one consisting of Si1, Si2, and the contralateral Si4 and the other consisting of Si3. These two kernels interact with each other to produce a stable rhythmic motor pattern.