Release of peptide cotransmitters in Aplysia: regulation and functional implications

Neuromodulation Enables Temperature Robustness and Coupling Between Fast and Slow Oscillator Circuits

Acute temperature changes can disrupt neuronal activity and coordination with severe consequences for animal behavior and survival. Nonetheless, two rhythmic neuronal circuits in the crustacean stomatogastric ganglion (STG) and their coordination are maintained across a broad temperature range. However, it remains unclear how this temperature robustness is achieved. Here, we dissociate temperature effects on the rhythm generating circuits from those on upstream ganglia. We demonstrate that heat-activated factors extrinsic to the rhythm generators are essential to the slow gastric mill rhythm’s temperature robustness and contribute to the temperature response of the fast pyloric rhythm. The gastric mill rhythm crashed when its rhythm generator in the STG was heated. It was restored when upstream ganglia were heated and temperature-matched to the STG. This also increased the activity of the peptidergic modulatory projection neuron (MCN1), which innervates the gastric mill circuit. Correspondingly, MCN1’s neuropeptide transmitter stabilized the rhythm and maintained it over a broad temperature range. Extrinsic neuromodulation is thus essential for the oscillatory circuits in the STG and enables neural circuits to maintain function in temperature-compromised conditions. In contrast, integer coupling between pyloric and gastric mill rhythms was independent of whether extrinsic inputs and STG pattern generators were temperature-matched or not, demonstrating that the temperature robustness of the coupling is enabled by properties intrinsic to the rhythm generators. However, at near-crash temperature, integer coupling was maintained only in some animals while it was absent in others. This was true despite regular rhythmic activity in all animals, supporting that degenerate circuit properties result in idiosyncratic responses to environmental challenges.

Different microcircuit responses to comparable input from one versus both copies of an identified projection neuron

Neuronal inputs to microcircuits are often present as multiple copies of apparently equivalent neurons. Thus far, however, little is known regarding the relative influence on microcircuit output of activating all or only some copies of such an input. We examine this issue in the crab (Cancer borealis) stomatogastric ganglion, where the gastric mill (chewing) microcircuit is activated by modulatory commissural neuron 1 (MCN1), a bilaterally paired modulatory projection neuron. Both MCN1s contain the same co-transmitters, influence the same gastric mill microcircuit neurons, can drive the biphasic gastric mill rhythm, and are co-activated by all identified MCN1-activating pathways. Here, we determine whether the gastric mill microcircuit response is equivalent when stimulating one or both MCN1s under conditions where the pair are matched to collectively fire at the same overall rate and pattern as single MCN1 stimulation. The dual MCN1 stimulations elicited more consistently coordinated rhythms, and these rhythms exhibited longer phases and cycle periods. These different outcomes from single and dual MCN1 stimulation may have resulted from the relatively modest, and equivalent, firing rate of the gastric mill neuron LG (lateral gastric) during each matched set of stimulations. The LG neuron-mediated, ionotropic inhibition of the MCN1 axon terminals is the trigger for the transition from the retraction to protraction phase. This LG neuron influence on MCN1 was more effective during the dual stimulations, where each MCN1 firing rate was half that occurring during the matched single stimulations. Thus, equivalent individual- and co-activation of a class of modulatory projection neurons does not necessarily drive equivalent microcircuit output.

Circadian rhythm of airways caliber and its autonomic modulation

The autonomic nervous system (ANS) is one of the effector pathways for circadian variation of many physiological parameters. Autonomic tone and airways caliber have been reported to exhibit circadian variation in separate studies. A simultaneous investigation of heart rate variability (HRV) and airway caliber might ascertain how airway caliber is modulated by autonomic tone. This study was planned to identify the variations in airway caliber and autonomic function tone during a 24-hour span. A total of 56 healthy male subjects with almost similar daily routines were studied. Time domain, frequency domain and nonlinear analysis of R-R interval from 5 min electrocardiogram (ECG) was done seven times during the daytime wake span at 3-hour intervals starting at 05:00 h in the morning until 23:00 h in the night. Simultaneously peak expiratory flow rate (PEFR) was determined using a mini Wright's peak flow meter. Rhythmometric analysis was done for PEFR and HRV parameters. Significant circadian variation in low frequency (LF) and high frequency (HF) variance was identified in this group of healthy subjects. The circadian rhythm of LF variance was characterized by a gradual increase and corresponding reciprocal change in HF variance from morning until night. The LF/HF ratio and SD2/SD1 ratio reflecting sympatho-vagal balance showed low to high values from morning to evening. The acrophase of the PEFR temporal pattern is similar to that of LF power and almost opposite in phase to that of HF power. PEFR is positively correlated with LF power. The circadian rhythm of airway caliber co-varies with cardiac autonomic tone. It appears that the temporal pattern of cardiac autonomic tone precedes in time that of airways caliber, thereby suggesting the latter operates under the modulatory effect of the 24-hour pattern in sympatho-vagal balance.

General Principles of Neuronal Co-transmission: Insights From Multiple Model Systems

It is now accepted that neurons contain and release multiple transmitter substances. However, we still have only limited insight into the regulation and functional effects of this co-transmission. Given that there are 200 or more neurotransmitters, the chemical complexity of the nervous system is daunting. This is made more-so by the fact that their interacting effects can generate diverse non-linear and novel consequences. The relatively poor history of pharmacological approaches likely reflects the fact that manipulating a transmitter system will not necessarily mimic its roles within the normal chemical environment of the nervous system (e.g., when it acts in parallel with co-transmitters). In this article, co-transmission is discussed in a range of systems [from invertebrate and lower vertebrate models, up to the mammalian peripheral and central nervous system (CNS)] to highlight approaches used, degree of understanding, and open questions and future directions. Finally, we offer some outlines of what we consider to be the general principles of co-transmission, as well as what we think are the most pressing general aspects that need to be addressed to move forward in our understanding of co-transmission.

Peptide Cotransmitters as Dynamic, Intrinsic Modulators of Network Activity

Neurons can contain both neuropeptides and "classic" small molecule transmitters. Much progress has been made in studies designed to determine the functional significance of this arrangement in experiments conducted in invertebrates and in the vertebrate autonomic nervous system. In this review article, we describe some of this research. In particular, we review early studies that related peptide release to physiological firing patterns of neurons. Additionally, we discuss more recent experiments informed by this early work that have sought to determine the functional significance of peptide cotransmission in the situation where peptides are released from neurons that are part of (i.e., are intrinsic to) a behavior generating circuit in the CNS. In this situation, peptide release will presumably be tightly coupled to the manner in which a network is activated. For example, data obtained in early studies suggest that peptide release will be potentiated when behavior is executed rapidly and intervals between periods of neural activity are relatively short. Further, early studies demonstrated that when neural activity is maintained, there are progressive changes (e.g., increases) in the amount of peptide that is released (even in the absence of a change in neural activity). This suggests that intrinsic peptidergic modulators in the CNS are likely to exert effects that are manifested dynamically in an activity-dependent manner. This type of modulation is likely to differ markedly from the modulation that occurs when a peptide hormone is present at a relatively fixed concentration in the blood.

Multifaceted Expression of Peptidergic Modulation in the Feeding System of Aplysia

Neuropeptides are present in species throughout the animal kingdom and generally exert actions that are distinct from those of small molecule transmitters. It has, therefore, been of interest to define the unique behavioral role of this class of substances. Progress in this regard has been made in experimentally advantageous invertebrate preparations. We focus on one such system, the feeding circuit in the mollusc Aplysia. We review research conducted over several decades that played an important role in establishing that peptide cotransmitters are released under behaviorally relevant conditions. We describe how this was accomplished. For example, we describe techniques developed to purify novel peptides, localize them to identified neurons, and detect endogenous peptide release. We also describe physiological experiments that demonstrated that peptides are bioactive under behaviorally relevant conditions. The feeding system is like others in that peptides exert effects that are both convergent and divergent. Work in the feeding system clearly illustrates how this creates potential for behavioral flexibility. Finally, we discuss experiments that determined physiological consequences of one of the hallmark features of peptidergic modulation, its persistence. Research in the feeding system demonstrated that this persistence can change network state and play an important role in determining network output.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Circuit feedback increases activity level of a circuit input through interactions with intrinsic properties

Central pattern generator (CPG) motor circuits underlying rhythmic behaviors provide feedback to the projection neuron inputs that drive these circuits. This feedback elicits projection neuron bursting linked to CPG rhythms. The brief periodic interruptions in projection neuron activity in turn influence CPG output, gate sensory input, and enable coordination of multiple target CPGs. However, despite the importance of the projection neuron activity level for circuit output, it remains unknown whether feedback also regulates projection neuron intraburst firing rates. I addressed this issue using identified neurons in the stomatogastric nervous system of the crab, Cancer borealis, a small motor system controlling chewing and filtering of food. Mechanosensory input triggers long-lasting activation of two projection neurons to elicit a chewing rhythm, during which their activity is patterned by circuit feedback. Here I show that feedback increases the intraburst firing rate of only one of the two projection neurons (commissural projection neuron 2: CPN2). Furthermore, this is not a fixed property because the CPN2 intraburst firing rate is decreased instead of increased by feedback when a chewing rhythm is activated by a different modulatory input. I establish that a feedback pathway that does not impact the CPN2 activity level in the control state inhibits CPN2 sufficiently to trigger postinhibitory rebound following mechanosensory stimulation. The rebound increases the CPN2 intraburst firing rate above the rate due only to mechanosensory activation of CPN2. Thus in addition to patterning projection neuron activity, circuit feedback can adjust the intraburst firing rate, demonstrating a novel functional role for circuit feedback to central projection neurons.NEW & NOTEWORTHY Feedback from central pattern generator (CPG) circuits patterns activity of their projection neuron inputs. However, whether the intraburst firing rate between rhythmic feedback inhibition is also impacted by CPG feedback was not known. I establish that CPG feedback can alter the projection neuron intraburst firing rate through interactions with projection neuron intrinsic properties. The contribution of feedback to projection neuron activity level is specific to the modulatory condition, demonstrating a state dependence for this novel role of circuit feedback.

Synaptic transmission parallels neuromodulation in a central food-intake circuit

NeuromedinU is a potent regulator of food intake and activity in mammals. In Drosophila, neurons producing the homologous neuropeptide hugin regulate feeding and locomotion in a similar manner. Here, we use EM-based reconstruction to generate the entire connectome of hugin-producing neurons in the Drosophila larval CNS. We demonstrate that hugin neurons use synaptic transmission in addition to peptidergic neuromodulation and identify acetylcholine as a key transmitter. Hugin neuropeptide and acetylcholine are both necessary for the regulatory effect on feeding. We further show that subtypes of hugin neurons connect chemosensory to endocrine system by combinations of synaptic and peptide-receptor connections. Targets include endocrine neurons producing DH44, a CRH-like peptide, and insulin-like peptides. Homologs of these peptides are likewise downstream of neuromedinU, revealing striking parallels in flies and mammals. We propose that hugin neurons are part of an ancient physiological control system that has been conserved at functional and molecular level.

DOI:http://dx.doi.org/10.7554/eLife.16799.001

Network feedback regulates motor output across a range of modulatory neuron activity

Modulatory projection neurons alter network neuron synaptic and intrinsic properties to elicit multiple different outputs. Sensory and other inputs elicit a range of modulatory neuron activity that is further shaped by network feedback, yet little is known regarding how the impact of network feedback on modulatory neurons regulates network output across a physiological range of modulatory neuron activity. Identified network neurons, a fully described connectome, and a well-characterized, identified modulatory projection neuron enabled us to address this issue in the crab (Cancer borealis) stomatogastric nervous system. The modulatory neuron modulatory commissural neuron 1 (MCN1) activates and modulates two networks that generate rhythms via different cellular mechanisms and at distinct frequencies. MCN1 is activated at rates of 5-35 Hz in vivo and in vitro. Additionally, network feedback elicits MCN1 activity time-locked to motor activity. We asked how network activation, rhythm speed, and neuron activity levels are regulated by the presence or absence of network feedback across a physiological range of MCN1 activity rates. There were both similarities and differences in responses of the two networks to MCN1 activity. Many parameters in both networks were sensitive to network feedback effects on MCN1 activity. However, for most parameters, MCN1 activity rate did not determine the extent to which network output was altered by the addition of network feedback. These data demonstrate that the influence of network feedback on modulatory neuron activity is an important determinant of network output and feedback can be effective in shaping network output regardless of the extent of network modulation.

Mass spectrometry-based characterization of endogenous peptides and metabolites in small volume samples

Technologies to assay single cells and their extracellular microenvironments are valuable in elucidating biological function, but there are challenges. Sample volumes are low, the physicochemical parameters of the analytes vary widely, and the cellular environment is chemically complex. In addition, the inherent difficulty of isolating individual cells and handling small volume samples complicates many experimental protocols. Here we highlight a number of mass spectrometry (MS)-based measurement approaches for characterizing the chemical content of small volume analytes, with a focus on methods used to detect intracellular and extracellular metabolites and peptides from samples as small as individual cells. MS has become one of the most effective means for analyzing small biological samples due to its high sensitivity, low analyte consumption, compatibility with a wide array of sampling approaches, and ability to detect a large number of analytes with different properties without preselection. Having access to a flexible portfolio of MS-based methods allows quantitative, qualitative, untargeted, targeted, multiplexed, and spatially resolved investigations of single cells and their similarly scaled extracellular environments. Combining MS with on-line and off-line sample conditioning tools, such as microfluidic and capillary electrophoresis systems, significantly increases the analytical coverage of the sample's metabolome and peptidome, and improves individual analyte characterization/identification. Small volume assays help to reveal the causes and manifestations of biological and pathological variability, as well as the functional heterogeneity of individual cells within their microenvironments and within cellular populations. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

The parasympathetic nervous system as a regulator of mast cell function

Often considered as the archetype of neuroimmune communication, much of our understanding of the bidirectional relationship between the nervous and immune systems has come from the study of mast cell-nerve interaction. Mast cells play a role in resistance to infection and are extensively involved in inflammation and subsequent tissue repair. Thus, the relationship between mast cells and neurons enables the involvement of peripheral and central nervous systems in the regulation of host defense mechanisms and inflammation. Recently, with the identification of the cholinergic anti-inflammatory pathway, there has been increased interest in the role of the parasympathetic nervous system in regulating immune responses. Classical neurotransmitters and neuropeptides released from cholinergic and inhibitory NANC neurons can modulate mast cell activity, and there is good evidence for the existence of parasympathetic nerve-mast cell functional units in the skin, lung, and intestine that have the potential to regulate a range of physiological processes.

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.

Off the beaten path: drug addiction and the pontine laterodorsal tegmentum

Drug addiction is a multileveled behavior controlled by interactions among many diverse neuronal groups involving several neurotransmitter systems. The involvement of brainstem-sourced, cholinergic neurotransmission in the development of addiction and in the persistent physiological processes that drive this maladaptive behavior has not been widely investigated. The major cholinergic input to neurons in the midbrain which are instrumental in assessment of reward and assignment of salience to stimuli, including drugs of abuse, sources from acetylcholine- (ACh-) containing pontine neurons of the laterodorsal tegmentum (LDT). Excitatory LDT input, likely cholinergic, is critical in allowing behaviorally relevant neuronal firing patterns within midbrain reward circuitry. Via this control, the LDT is positioned to be importantly involved in development of compulsive, addictive patterns of behavior. The goal of this review is to present the anatomical, physiological, and behavioral evidence suggesting a role of the LDT in the neurobiology underlying addiction to drugs of abuse. Although focus is directed on the evidence supporting a vital participation of the cholinergic neurons of the LDT, data indicating a contribution of noncholinergic LDT neurons to processes underlying addiction are also reviewed. While sparse, available information of actions of drugs of abuse on LDT cells and the output of these neurons as well as their influence on addiction-related behavior are also presented. Taken together, data from studies presented in this review strongly support the position that the LDT is a major player in the neurobiology of drug addiction. Accordingly, the LDT may serve as a future treatment target for efficacious pharmaceutical combat of drug addiction.

Release of a single neurotransmitter from an identified interneuron coherently affects motor output on multiple time scales

Neurotransmitters can have diverse effects that occur over multiple time scales often making the consequences of neurotransmission difficult to predict. To explore the consequences of this diversity, we used the buccal ganglion of Aplysia to examine the effects of GABA release by a single interneuron, B40, on the intrinsic properties and motor output of the radula closure neuron B8. B40 induces a picrotoxin-sensitive fast IPSP lasting milliseconds in B8 and a slow EPSP lasting seconds. We found that the excitatory effects of this slow EPSP are also mediated by GABA. Together, these two GABAergic actions structure B8 firing in a pattern characteristic of ingestive programs. Furthermore, we found that repeated B40 stimulation induces a persistent increase in B8 excitability that was occluded in the presence of the GABA B receptor agonist baclofen, suggesting that GABA affects B8 excitability over multiple time scales. The phasing of B8 activity during the feeding motor programs determines the nature of the behavior elicited during that motor program. The persistent increase in B8 excitability induced by B40 biased the activity of B8 during feeding motor programs causing the motor programs to become more ingestive in nature. Thus, a single transmitter released from a single interneuron can have consequences for motor output that are expressed over multiple time scales. Importantly, despite the differences in their signs and temporal characteristics, the three actions of B40 are coherent in that they promote B8 firing patterns that are characteristic of ingestive motor outputs.

Organization of the serotonergic innervation of the feeding (buccal) musculature during the maturation of the pond snail Lymnaea stagnalis: a morphological and biochemical study

The serotonergic innervation of the buccal musculature responsible for feeding (radula protraction) was investigated during the maturation of the pond snail, Lymnaea stagnalis L., applying light and electron microscopic immunohistochemistry and biochemical approaches. According to epifluorescence and laser confocal microscopy, the first 5-HT-like-immunoreactive (5-HTLIR) processes appeared on the surface of the musculature at the postmetamorphic E80% embryonic stage. Until hatching, the innervation continued to increase in density, showing axon arborizations with projections into the deeper muscle levels. An adult-like pattern of 5-HTLIR innervation appeared at P2-P3 juvenile stages. At the ultrastructural level, close (16-20 nm) but mostly unspecialized neuromuscular contacts were formed by both unlabeled and 5-HTLIR axon profiles from the E80% embryonic stage. Labeled processes were also found located relatively far from the muscle cells. An HPLC assay showed a gradual increase of the 5-HT level in the buccal mass during development. The buccal mass was characterized by a single-component high-affinity 5-HT uptake system, and 5-HT release could be evoked by 100 mM K(+) and blocked in Ca(2+) -free medium. It is suggested that 5-HT plays a wide modulatory role in the peripheral feeding system and is also involved in the functional maturation of the muscle system.

Collection of peptides released from single neurons with particle-embedded monolithic capillaries followed by detection with matrix-assisted laser desorption/ionization mass spectrometry

Characterization of the stimulated release of neuropeptides from brain slices and individual cultured neurons requires efficient collection of the releasate from relatively large volumes of physiological saline. Here, several collection approaches are optimized using particle-embedded monolithic capillaries (PEMCs) with poly(stearyl methacrylate-co-ethylene glycol dimethacrylate) monolith acting as a "glue". Two distinct extraction particles, with either pyrrolidone (PY) or ethylenediamine (EDA) as the functional group on polystyrene backbone, have been embedded into capillaries having an inner diameter of 250 μm. The capillaries act as collection devices for sampling neuropeptide release; the collection protocols are described, and the extraction efficiency of the probes are characterized. Specifically, the binding of angiotensin II from a peptide mixture onto the PY and EDA columns was 16 and 28 pmol, respectively, in a volume of 20 μL of saline. The peptides released from these columns have been characterized via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with low femtomole detection limits. When the PEMC columns were positioned in close proximity to individual neurons and 50 mM KCl was used as the secretagogue, peptides released from individual identified cultured neurons isolated from Aplysia californica were collected and characterized.

Regulatory actions of neuropeptides and peptide hormones on the reproduction of molluscsThe present review is one of a series of occasional review articles that have been invited by the Editors and will feature the broad range of disciplines and expertise represented in our Editorial Advisory Board

Reproductive success of individual animals is essential for the survival of any species. Molluscs have adapted to a wide variety of environments (freshwater, brackish water, seawater, and terrestrial habits) and have evolved unique tactics for reproduction. Both of these features attract the academic interests of scientists. Because neuropeptides and peptide hormones play critical roles in neural and neurohormonal regulation of physiological functions and behaviors in this animal group, the regulatory actions of these messengers in reproduction have been extensively investigated. In this review, we will briefly summarize how peptidergic messengers are involved in various aspects of reproduction, using some peptides such as egg-laying hormone, caudo-dorsal cell hormone, APGWamide, and gonadotropin-releasing hormone as typical examples.

Peptidergic modulation of serotonin and nerve elicited responses of the salivary duct muscle in the snail, Helix pomatia

In the present study, the ability of a range of endogenous neuropeptides to modulate neuromuscular transmission was examined in the salivary duct neuromuscular preparation of the terrestrial snail, Helix pomatia. Immunohistochemical and physiological techniques were used to localize the neuropeptides (GSPYFVamide, CARP, FMRFamide and APGWamide) and to investigate whether contractions elicited by the stimulation of the salivary nerve or by exogenously applied 5-HT are subject to peptidergic modulation. All of the neuropeptides studied decreased the tonus by a direct action on the muscle fibers in a concentration dependent manner in a range of 10(-9) to 10(-6)M. Neuropeptides distinctly affected the 5-HT evoked contraction or relaxation and GSPYFVa and APGWa decreased also the amplitude of contractions elicited by the stimulation of the salivary nerve. All four neuropeptides facilitated the relaxation phase providing further evidence for the postsynaptic action of neuropeptides. Low Ca(2+)/high Mg(2+) saline abolished the nerve-elicited contractions, however the denervated muscle retained the ability to contract due to the mobilization of the Ca(2+) from intracellular stores. It was concluded, that peptides belonging to different peptide families exerted their effects through pre- and postsynaptic mechanisms. The modulatory effect of neuropeptides can be assigned to the partial co-localization of 5-HT and neuropeptides in the nerves innervating muscles of the salivary duct, as it was demonstrated by double-labeling immunohistochemistry. A double origin of the 5-HTergic innervation was demonstrated, including efferents originating from both the cerebral and visceral ganglia.

Increased parasympathetic tone as the underlying cause of asthma: a hypothesis

Asthma is a chronic inflammatory disease of airways that is characterized by increased responsiveness of the tracheo-bronchial tree to multiple number of stimuli. Immunological theory does not explain all features in asthma, for example hyper-reactivity of the airways. Neurogenic theory also fails to explain the pathogenesis of asthma comprehensively. Higher parasympathetic tone has been reported in asthmatics but has never been suggested as a major underlying cause of asthma. This article attempts to explain the occurrence of hyper-responsiveness, inflammatory/allergic reactions and broncho-constriction in asthma on a common basis of inherent higher parasympathetic tone in asthmatics. The higher background parasympathetic firing leads to increased nitric oxide (NO) production owing to its co-localization with acetylcholine (ACh) in inhibitory non-adrenergic and non-cholinergic (i-NANC) nerves. NO is a neurotransmitter of i-NANC system and it mediates bronchodilation. Increased NO release has been found to be responsible for hyper-responsiveness and increased inflammation in the airways. The authors suggest that an inherently higher background parasympathetic tone in concert with inflammation or a specific genetic background could modify the effects of NO on lung homeostasis in humans leading to increased susceptibility to an asthmatic state.

Neuropeptide signaling near and far: how localized and timed is the action of neuropeptides in brain circuits?

Neuropeptide signaling is functionally very diverse and one and the same neuropeptide may act as a circulating neurohormone, as a locally released neuromodulator or even as a cotransmitter of classical fast-acting neurotransmitters. Thus, neuropeptides are produced by a huge variety of neuron types in different parts of the nervous system. Within the central nervous system (CNS) there are numerous types of peptidergic interneurons, some with strictly localized and patterned branching morphologies, others with widespread and diffuse arborizations. From morphology alone it is often difficult to predict the sphere of influence of a peptidergic interneuron, especially since it has been shown that neuropeptides can diffuse over tens of micrometers within neuropils, and that peptides probably are released exclusively in perisynaptic (or non-synaptic) regions. This review addresses some questions related to peptidergic signaling in the insect CNS. How diverse are the spatial relations between peptidergic neurons and their target neurons and what determines the sphere of functional influence? At one extreme there is volume transmission and at the other targeted cotransmission at synapses. Also temporal aspects of peptidergic signaling are of interest: how transient are peptidergic messages? Factors important for these spatial and temporal aspects of peptidergic signaling are proximity between release sites and cognate receptors, distribution of peptidase activity that can terminate peptide action and colocalization of other neuroactive compounds in the presynaptic peptidergic neuron (and corresponding receptors in target neurons). Other factors such as expression of different channel types, receptor inactivation mechanisms and second messenger systems probably also contribute to the diversity in temporal properties of peptide signaling.

Activity-dependent peptidergic modulation of the plateau-generating neuron B64 in the feeding network of Aplysia

Many behaviors display various forms of activity-dependent plasticity. An example of such plasticity is the progressive shortening of the duration of protraction phase of feeding responses of Aplysia that occurs when feeding responses are repeatedly elicited. A similar protraction-duration shortening is observed in isolated ganglia of Aplysia when feeding-like motor programs are elicited through a prolonged stimulation of the command-like neuron CBI-2. Here, we investigate a cellular mechanism that may underlie this activity-dependent shortening of protraction duration of feeding motor programs. CBI-2 contains two neuropeptides, CP2 and FCAP. Previous work showed that CP2 shortens protraction duration of CBI-2 elicited programs. We show here that the same is true for FCAP. We also show that both CP2 and FCAP modulated the biophysical properties of a plateau-generating neuron, B64, that plays an important role in terminating the protraction phase of feeding motor programs. We find that prestimulation of CBI-2, as well as superfusion of CP2 and FCAP, lowered the threshold for activation of the plateau potential in B64. The threshold-lowering actions of CBI-2 prestimulation were occluded by superfusion of FCAP and CP2. Furthermore, at elevated temperature, conditions under which peptide release is prevented in Aplysia, prestimulation of CBI-2 does not lower the plateau-potential threshold, whereas superfusion of CP2 and FCAP does. Our findings are consistent with the hypothesis that peptides released from CBI-2 lower the threshold for activation of plateau potential in B64, thereby contributing to the shortening of protraction duration when CBI-2 is repeatedly activated.

Invertebrate central pattern generation moves along

Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.

Control of Feeding inAplysiaWith Ad Libitum Access to Food: Presence of Food Increases the Intervals Between Feeding Bouts

The patterning of feeding and the quantity eaten in Aplysia californica with ad libitum food access cannot be explained by the effects of three variables previously shown to control the patterning of consummatory feeding responses and the quantity eaten in animals hand-fed individual meals. Feeding in ad libitum conditions is regulated primarily by varying the time between feeding bouts rather than by modulating bout lengths or the efficacy of consummatory movements within a bout. Aplysia with steady-state food access are in a newly characterized feeding state in which they are relatively unresponsive to food. They eat very little (1-4% of the time), and the quantity eaten is unrelated to the quantity of food in the anterior gut. The steady state can be maintained by the presence of food, even if animals do not contact food. The chemosensory rhinophores signal the presence of food that maintains the steady state. Up to 24 h without food is needed for animals to recover from the inhibition of feeding by steady-state presence of food. Recovery from the steady state is partially governed by postingestion stimuli as shown by a faster recovery in animals that have not been in contact with food. Inhibition of feeding during the steady-state is mediated in part via humoral factors because bathing the cerebral and buccal ganglia in hemolymph from animals in the steady state inhibits the ability to elicit buccal motor programs via a cholinomimetic thought to simulate stimulation of the lips with food. After food deprivation that is sufficiently long so that the steady-state decays, animals eat a large meal the size and dynamics of which are consistent with regulation via the three variables previously identified. This large meal is modulated by pheromones secreted by conspecifics even in sexually immature Aplysia.

Profiling of neuropeptides released at the stomatogastric ganglion of the crab, Cancer borealis with mass spectrometry

Studies of release under physiological conditions provide more direct data about the identity of neuromodulatory signaling molecules than studies of tissue localization that cannot distinguish between processing precursors and biologically active neuropeptides. We have identified neuropeptides released by electrical stimulation of nerves that contain the axons of the modulatory projection neurons to the stomatogastric ganglion of the crab, Cancer borealis. Preparations were bathed in saline containing a cocktail of peptidase inhibitors to minimize peptide degradation. Both electrical stimulation of projection nerves and depolarization with high K+ saline were used to evoke release. Releasates were desalted and then identified by mass using MALDI-TOF (matrix-assisted laser desorption/ionization-time-of-flight) mass spectrometry. Both previously known and novel peptides were detected. Subsequent to electrical stimulation proctolin, Cancer borealis tachykinin-related peptide (CabTRP), FVNSRYa, carcinustatin-8, allatostatin-3 (AST-3), red pigment concentrating hormone, NRNFLRFa, AST-5, SGFYANRYa, TNRNFLRFa, AST-9, orcomyotropin-related peptide, corazonin, Ala13-orcokinin, and Ser9-Val13-orcokinin were detected. Some of these were also detected after high K+ depolarization. Release was calcium dependent. In summary, we have shown release of the neuropeptides thought to play an important neuromodulatory role in the stomatogastric ganglion, as well as numerous other candidate neuromodulators that remain to be identified.

Calcium/calmodulin-dependent nitric oxide synthase activity in the CNS of Aplysia californica: biochemical characterization and link to cGMP pathways

We characterized enzymatic activity of nitric oxide synthase (NOS) in the central nervous system of Aplysia californica, a popular experimental model in cellular and system neuroscience, and provided biochemical evidence for NO-cGMP signaling in molluscs. Aplysia NOS (ApNOS) activity, determined as citrulline formation, revealed its calcium-/calmodulin-(Ca/CaM) and NADPH dependence and it was inhibited by 50% with 5mM of W7 hydrochloride (a potent Ca/CaM-dependent phosphodiesterase inhibitor). A representative set of inhibitors for mammalian NOS isoforms also suppressed NOS activity in Aplysia. Specifically, the ApNOS was inhibited by 65-92% with 500 microM of L-NAME (a competitive NOS inhibitor) whereas d-NAME at the same concentration had no effect. S-Ethylisothiourea hydrobromide (5mM), a selective inhibitor of all NOS isoforms, suppressed ApNOS by 85%, l-N6-(1-iminoethyl)lysine dihydrochloride (L-NIL, 5mM), an iNOS inhibitor, by 78% and L-thiocitrulline (5mM) (an inhibitor of nNOS and iNOS) by greater than 95%. Polyclonal antibodies raised against rat nNOS hybridized with a putative purified ApNOS (160 kDa protein) from partially purified central nervous system homogenates in Western blot studies. Consistent with other studies, the activity of soluble guanylyl cyclase was stimulated as a result of NO interaction with its heme prosthetic group. The basal levels of cGMP were estimated by radioimmunoassay to be 44.47 fmol/microg of protein. Incubation of Aplysia CNS with the NO donors DEA/NONOate (diethylammonium (Z)-1-(N,N-diethylamino) diazen-1-ium-1,2-diolate - 1mM) or S-nitroso-N-acetylpenicillamine (1mM) and simultaneous phosphodiesterase inhibition with 3-isobutyl-1-methylxanthine (1mM) prior to the assay showed a 26-80 fold increase in basal cGMP levels. Addition of ODQ (1H-[1,2,4]oxadiazolo[4,3-a] quinoxaline-1-one - 1mM), a selective inhibitor of soluble guanylyl cyclase, completely abolished this effect. This confirms that NO may indeed function as a messenger in the molluscan CNS, and that cGMP acts as one of its effectors.

Temperature compensation of neuromuscular modulation in aplysia

Physiological systems that must operate over a range of temperatures often incorporate temperature-compensatory mechanisms to maintain their output within a relatively narrow, functional range of values. We analyze here an example in the accessory radula closer (ARC) neuromuscular system, a representative part of the feeding neuromusculature of the sea slug Aplysia. The ARC muscle's two motor neurons, B15 and B16, release, in addition to ACh that contracts the muscle, modulatory peptide cotransmitters that, through a complex network of effects in the muscle, shape the ACh-induced contractions. It is believed that this modulation is critical in optimizing the performance of the muscle for successful, efficient feeding behavior. However, previous work has shown that the release of the modulatory peptides from the motor neurons decreases dramatically with increasing temperature. From 15 to 25 degrees C, for example, release decreases 20-fold. Yet Aplysia live and feed successfully not only at 15 degrees C, but at 25 degrees C and probably at higher temperatures. Here, working with reduced B15/B16-ARC preparations in vitro as well as a mathematical model of the system, we have found a resolution of this apparent paradox. Although modulator release decreases 20-fold when the temperature is raised from 15 to 25 degrees C, the observed modulation of contraction shape does not decrease at all. Two mechanisms are responsible. First, further downstream within the modulatory network, the modulatory effects themselves-experimentally dissected by exogenous modulator application-have temperature dependencies opposite to that of modulator release, increasing with temperature. Second, the saturating curvature of the dose-response relations within the network diminishes the downstream impact of the decrease of modulator release. Thus two quite distinct mechanisms, one depending on the characteristics of the individual components of the network and the other emerging from the network's structure, combine to compensate for temperature changes to maintain the output of this physiological system.

Peptidergic contribution to posttetanic potentiation at a central synapse of aplysia

Posttetanic potentiation (PTP)-like phenomena appear to be mediated by a variety of mechanisms. Although neuropeptides are located in a large number of neurons and many neuropeptides, like PTP, can enhance synaptic transmission, there is a paucity of studies indicating that peptides may actually participate in PTP. Here, we utilize a single central synapse in the feeding circuit of Aplysia to investigate a possible peptidergic contribution to PTP in the CNS. The cholinergic command-like interneuron, cerebral-buccal interneuron 2 (CBI-2), contains two neuropeptides, feeding circuit activating peptide (FCAP) and cerebral peptide 2 (CP2). Previous studies showed that tetanic prestimulation or repeated stimulation of CBI-2, as well as perfusion of FCAP and CP2, increase the size of the cholinergic excitatory postsynaptic potentials (EPSPs) that CBI-2 evokes in the motoneurons B61/62 and shorten the latency to initiate B61/62 firing in response to CBI-2 stimulation. We used temperature-dependent suppression of peptide release and occlusion experiments to examine the possible contribution of FCAP and CP2 to PTP at the CBI-2 to B61/62 synapse. When peptide release was suppressed, perfusion of exogenous peptides increased the size of posttetanic EPSPs. In contrast, when peptide release was not suppressed, exogenous peptides did not enhance the size of posttetanic EPSPs, thus indicating occlusion. Temperature manipulation and occlusion experiments also indicated that peptides extend PTP duration. This peptide-dependent prolongation of PTP has functional consequences in that it extends the duration of time during which the latency to initiate B61/62 firing in response to CBI-2 stimulation is shortened.

Rapid Dopaminergic Signaling by Interneurons That Contain Markers for Catecholamines and GABA in the Feeding Circuitry of Aplysia

Consummatory feeding behaviors in Aplysia californica are controlled by a polymorphic central pattern generator (CPG) circuit. Previous investigations have demonstrated colocalization of markers for GABA and catecholamines within two interneurons, B20 and B65, that participate in configuring the functional output of this CPG. This study examined the contributions of GABA and dopamine (DA) to rapid synaptic signaling from B20 and B65 to follower cells that implement their specification of motor programs. Pharmacological tests did not substantiate the participation of GABA in the mediation of the excitatory postsynaptic potentials (EPSPs) from either B20 or B65. However, GABA and the GABAB receptor agonist baclofen were found to modify these signals in a target-specific manner. Several observations indicated that DA acts as the neurotransmitter mediating fast EPSPs from B20 to two radula closer motor neurons B8 and B16. In both motor neurons, application of DA produced depolarizing responses associated with decreased input resistance and increased excitation. B20-evoked EPSPs in both follower cells were occluded by exogenous dopamine and blocked by the DA antagonist sulpiride. While dopamine occlusion and sulpiride block of convergent signaling to B8 from B65 resembled that of B20, both of these actions were less potent on the rapid signaling from B65 to the multifunctional and widely acting interneuron B4/5. These findings indicate that dopamine mediates divergent (B20 to B16 and B8) and convergent (B20 and B65 to B8) rapid EPSPs from two influential CPG interneurons in which it is colocalized with GABA-like immunoreactivity.

Neuropeptide co-localisation in the lepidopteran frontal ganglion studied by electron-microscopic gold-labelling immunocytochemistry

An immunogold-labelling electron-microscopic study of the frontal ganglion of two noctuids, Lacanobia oleracea and Helicoverpa armigera, has been carried out with antisera directed against three neuropeptides; allatostatins of the Y/FXFGL-NH2 type, Manduca sexta allatostatin (Mas-AS) and M. sexta allatotropin. The ganglion of both noctuids has two pairs of large peptidergic neurones with many clusters of electron-dense granules, one pair being situated anteriorly and the other posteriorly. By means of a double-labelling ("flip-flop") technique, with different sizes of gold particles, all possible paired combinations of the three different types of peptide have been visualised within granules of the anterior neurones, leading to the conclusion that the three peptides are co-packaged and co-stored in these cells. Within the posterior neurones of L. oleracea, gold labelling of granules is only linked to the Y/FXFGL-NH2 allatostatin antisera and, in contrast to the anterior cells of this species in which double gold labelling results in a sparse accumulation of gold particles for any one peptide type, single labelling gives a more intense, uniform pattern of gold particles. In contrast to L. oleracea, the gold-labelling pattern seen in the posterior neurones of H. armigera reflects the co-localisation of allatostatins of the Y/FXFGL-NH2 type with Mas-AS in this species. Allatotropin is absent in the posterior neurones of both species.

Modeling neuromuscular modulation in Aplysia. III. Interaction of central motor commands and peripheral modulatory state for optimal behavior

Recent work in computational neuroethology has emphasized that "the brain has a body": successful adaptive behavior is not simply commanded by the nervous system, but emerges from interactions of nervous system, body, and environment. Here we continue our study of these issues in the accessory radula closer (ARC) neuromuscular system of Aplysia. The ARC muscle participates in the animal's feeding behaviors, a set of cyclical, rhythmic behaviors driven by a central pattern generator (CPG). Patterned firing of the ARC muscle's two motor neurons, B15 and B16, releases not only ACh to elicit the muscle's contractions but also peptide neuromodulators that then shape the contractions through a complex network of actions on the muscle. These actions are dynamically complex: some are fast, but some are slow, so that they are temporally uncoupled from the motor neuron firing pattern in the current cycle. Under these circumstances, how can the nervous system, through just the narrow channel of the firing patterns of the motor neurons, control the contractions, movements, and behavior in the periphery? In two earlier papers, we developed a realistic mathematical model of the B15/B16-ARC neuromuscular system and its modulation. Here we use this model to study the functional performance of the system in a realistic behavioral task. We run the model with two kinds of inputs: a simple set of regular motor neuron firing patterns that allows us to examine the entire space of patterns, and the real firing patterns of B15 and B16 previously recorded in a 2 1/2-h-long meal of 749 cycles in an intact feeding animal. These real patterns are extremely irregular. Our main conclusions are the following. 1) The modulation in the periphery is necessary for superior functional performance. 2) The components of the modulatory network interact in nonlinear, context- and task-dependent combinations for best performance overall, although not necessarily in any particular cycle. 3) Both the fast and the slow dynamics of the modulatory state make important contributions. 4) The nervous system controls different components of the periphery to different degrees. To some extent the periphery operates semiautonomously. However, the structure of the peripheral modulatory network ensures robust performance under all circumstances, even with the irregular motor neuron firing patterns and even when the parameters of the functional task are randomly varied from cycle to cycle to simulate a variable feeding environment. In the variable environment, regular firing patterns, which are fine-tuned to one particular task, fail to provide robust performance. We propose that the CPG generates the irregular firing patterns, which nevertheless are guaranteed to give robust performance overall through the actions of the peripheral modulatory network, as part of a trial-and-error feeding strategy in a variable, uncertain environment.

Neuromuscular modulation in Aplysia. II. Modulation of the neuromuscular transform in behavior

In this work we use mathematical modeling and complementary experiments to study the dynamics of modulation in the accessory radula closer (ARC) neuromuscular system of Aplysia. Here we join a dynamic model of the modulation from the preceding paper to a model of the basal neuromuscular transform (NMT). The resulting complete model of the NMT allows us to predict, test, and analyze the actual modulated contraction shapes in different types of feeding behavior, through entire quasi-realistic meals. The model reproduces a variety of published and new experimental observations. We find that components of the modulatory network act in interdependency and mutual complementarity, one or another playing a key role depending on the behavior and its past history. The history is remembered by slow dynamical components whose persistence prepares the system for future behavior of the same kind. The persistence becomes counterproductive, however, when the behavior suddenly changes. Superposition of fast dynamical components alleviates the problem under most, but not all, circumstances. In the quasi-realistic meals, the modulation improves functional performance on average, but degrades it after certain behavioral switches, when the model predicts sharp contraction transients. These are indeed seen in the real muscle. We propose that the real system does not switch the underlying motor neuron firing patterns abruptly, but relaxes them gradually, matching the relaxation of the peripheral modulatory state, through such behavioral transitions. We model food-induced arousal, a known phenomenon of this kind. The peripheral dynamics of the modulated NMT thus constrain the motor commands of the CNS.

Neuromuscular modulation in Aplysia. I. Dynamic model

Many physiological systems are regulated by complex networks of modulatory actions. Here we use mathematical modeling and complementary experiments to study the dynamic behavior of such a network in the accessory radula closer (ARC) neuromuscular system of Aplysia. The ARC muscle participates in several types of rhythmic consummatory feeding behavior. The muscle's motor neurons release acetylcholine to produce basal contractions, but also modulatory peptide cotransmitters that, through multiple cellular effects, shape the contractions to meet behavioral demands. We construct a dynamic model of the modulatory network and examine its operation as the motor neurons fire in realistic patterns that change gradually over an hour-long meal and abruptly with switches between the different feeding behaviors. The modulatory effects have very disparate dynamical time scales. Some react to the motor neuron firing only over many cycles of the behavior, but one key effect is fast enough to respond to each individual cycle. Switches between the behaviors are therefore followed by rapid relaxations along some modulatory dimensions but not others. The trajectory of the modulatory state is a transient throughout the meal, ranging widely over regions of the modulatory space not accessible in the steady state. There is a pronounced history-dependency: the modulatory state associated with a cycle of a particular behavior depends on when that cycle occurs and what behaviors preceded it. On average, nevertheless, each behavior is associated with a different modulatory state. In the following companion study, we add a model of the neuromuscular transform to reconstruct and evaluate the actual modulated contraction shapes.

Two neuropeptides colocalized in a command-like neuron use distinct mechanisms to enhance its fast synaptic connection

In many neurons more than one peptide is colocalized with a classical neurotransmitter. The functional consequence of such an arrangement has been rarely investigated. Here, within the feeding circuit of Aplysia, we investigate at a single synapse the actions of two modulatory neuropeptides that are present in a cholinergic interneuron. In combination with previous work, our study shows that the command-like neuron for feeding, CBI-2, contains two neuropeptides, feeding circuit activating peptide (FCAP) and cerebral peptide 2 (CP2). Previous studies showed that high-frequency prestimulation or repeated stimulation of CBI-2 increases the size of CBI-2 to B61/62 excitatory postsynaptic potentials (EPSPs) and shortens the latency of firing of neuron B61/62 in response to CBI-2 stimulation. We find that both FCAP and CP2 mimic these two effects. The variance method of quantal analysis indicates that FCAP increases the calculated quantal size (q) and CP2 increases the calculated quantal content (m) of EPSPs. Since the PSP amplitude represents the product of q and m, the joint action of the two peptides is expected to be cooperative. This observation suggests a possible functional implication for multiple neuropeptides colocalized with a classical neurotransmitter in one neuron.

Multiple presynaptic and postsynaptic sites of inhibitory modulation by myomodulin at ARC neuromuscular junctions of Aplysia

The functional activity of even simple cellular ensembles is often controlled by surprisingly complex networks of neuromodulators. One such network has been extensively studied in the accessory radula closer (ARC) neuromuscular system of Aplysia. The ARC muscle is innervated by two motor neurons, B15 and B16, which release modulatory peptide cotransmitters to shape ACh-mediated contractions of the muscle. Previous analysis has shown that key to the combinatorial ability of B15 and B16 to control multiple parameters of the contraction is an asymmetry in their peptide modulatory actions. B16, but not B15, releases myomodulin, which, among other actions, inhibits the contraction. Work in single ARC muscle fibers has identified a distinctive myomodulin-activated K current as a candidate postsynaptic mechanism of the inhibition. However, definitive evidence for this mechanism has been lacking. Here, working with the single fibers and then motor neuron-elicited excitatory junction potentials (EJPs) and contractions of the intact ARC muscle, we have confirmed two central predictions of the K-current hypothesis: the myomodulin inhibition of contraction is associated with a correspondingly large inhibition of the underlying depolarization, and the inhibition of both contraction and depolarization is blocked by 4-aminopyridine (4-AP), a potent and selective blocker of the myomodulin-activated K current. However, in the intact muscle, the experiments revealed a second, 4-AP-resistant component of myomodulin inhibition of both B15- and B16-elicited EJPs. This component resembles, and mutually occludes with, inhibition of the EJPs by another peptide modulator released from both B15 and B16, buccalin, which acts by a presynaptic mechanism, inhibition of ACh release from the motor neuron terminals. Direct measurements of peptide release showed that myomodulin also inhibits buccalin release from B15 terminals. At the level of contractions, nevertheless, the postsynaptic K-current mechanism is responsible for much of the myomodulin inhibition of peak contraction amplitude. The presynaptic mechanism, which is most evident during the initial build-up of the EJP waveform, underlies instead an increase of contraction latency.

Differential effects of serotonin enhance activity of an electrically coupled neural network

Networks of electrically coupled neurons play an important role in coordinating activity among widely distributed neurons in the CNS. Such networks are sensitive to neuromodulation; but how modulation of individual cells affects activity of the entire network is not well understood. In the CNS of the medicinal leech, the S interneuron (S-cell) forms a network of electrically coupled neurons where each S-cell is linked to its two neighboring S-cells by electrical synapses. An action potential initiated in one cell is carried the length of the animal along this S-cell chain. The S-cell network is of interest because it is crucial for sensitization and dishabituation of the whole-body shortening reflex, although it is not necessary for reflexive shortening itself. Mechanosensory stimuli that produce shortening will directly elicit a train of action potentials by the S-cell network. This activity reflects the sum of action potential initiations in several S interneurons within the chain. The activity was enhanced by serotonin (5HT) in terms of both the total number of action potentials initiated and the average frequency of these initiations. Increases in evoked activity were accompanied by differential changes in the rates of action potential initiation in individual S-cells. 5HT only weakly enhanced initiations in S-cells that made a large contribution to the network-level response, while initiations in other, less active, S-cells were strongly enhanced by 5HT. This neurotransmitter also modulated the pattern of how activity was distributed throughout the network. 5HT-induced changes in activity patterns of the S-cell network may represent an important component of learning-related neuroplasticity in the leech shortening reflex.

Colocalization of gamma-aminobutyric acid-like immunoreactivity and catecholamines in the feeding network of Aplysia californica

Functional consequences of neurotransmitter coexistence and cotransmission can be readily studied in certain experimentally favorable invertebrate motor systems. In this study, whole-mount histochemical methods were used to identify neurons in which gamma-aminobutyric acid (GABA)-like immunoreactivity (GABAli) was colocalized with catecholamine histofluorescence (CAh; FaGlu method) and tyrosine hydroxylase (TH)-like immunoreactivity (THli) in the feeding motor circuitry (buccal and cerebral ganglia) of the marine mollusc Aplysia californica. In agreement with previous reports, five neurons in the buccal ganglia were found to exhibit CAh. These included the paired B20 buccal-cerebral interneurons (BCIs), the paired B65 buccal interneurons, and an unpaired cell with projections to both cerebral-buccal connectives (CBCs). Experiments in which the FaGlu method was combined with the immunohistochemical detection of GABA revealed double labeling of all five of these neurons. An antibody generated against TH, the rate-limiting enzyme in the biosynthesis of catecholamines, was used to obtain an independent determination of GABA-CA colocalization. Biocytin backfills of the CBC performed in conjunction with TH immunohistochemistry revealed labeling of the rostral B20 cell pair and the unpaired CBI near the caudal surface of the right hemiganglion. THli was also present in a prominent bilateral pair of caudal neurons that were not stained with CBC backfills. On the basis of their position, size, shape, and lack of CBC projections, the lateral THli neurons were identified as B65. Double-labeling immunohistochemical experiments revealed GABAli in all five buccal THli neurons. Finally, GABAli was observed in individual B20 and B65 neurons that were identified using electrophysiological criteria and injected with a marker (neurobiotin). Similar methods were used to demonstrate that a previously identified catecholaminergic cerebral-buccal interneuron (CBI) designated CBI-1 contained THli but did not contain GABAli. Although numerous THli and GABAli neurons and fibers were present in the cerebral and buccal ganglia, additional instances of their colocalization were not observed. These findings indicate that GABA and a catecholamine (probably dopamine) are colocalized in a limited number of interneurons within the central pattern generator circuits that control feeding-related behaviors in Aplysia.

Colocalized neuropeptides activate a central pattern generator by acting on different circuit targets

In the presence of descending modulatory inputs, the stomatogastric ganglion (STG) of the lobster Homarus americanus generates a triphasic motor pattern, the pyloric rhythm. Red pigment-concentrating hormone (RPCH) and Cancer borealis tachykinin-related peptide (CabTRP) are colocalized in a pair of fibers that project into the neuropil of the STG. When the STG was isolated from anterior ganglia modulatory inputs, the lateral pyloric (LP) and pyloric (PY) neurons became silent, whereas the anterior burster (AB) and pyloric dilator (PD) neurons were rhythmically active at a low frequency. Exogenous application of 10(-6) m RPCH activated the LP neuron but not the PY neurons; 10(-6) m CabTRP activated the PY neurons but not the LP neuron. The actions of RPCH on the LP neuron and CabTRP on the PY neurons persisted when the rhythmic drive from the PD and AB neurons was removed, suggesting that the LP and PY neurons are direct targets for RPCH and CabTRP respectively. Coapplication of 10(-6) m RPCH and 10(-6) m CabTRP elicited triphasic motor patterns with phase relationships resembling those in a preparation with modulatory inputs intact. In summary, cotransmitters acting on different network targets act cooperatively to activate a complete central pattern-generating circuit.

Heightened seizure severity in somatostatin knockout mice

Patients and experimental models of temporal lobe epilepsy display loss of somatostatinergic neurons in the dentate gyrus. To determine if loss of the peptide somatostatin contributes to epileptic seizures we examined kainate-evoked seizures and kindling in somatostatin knockout mice. Somatostatin knockout mice were not observed to experience spontaneous seizures. Timm staining, acetylcholinesterase histochemistry, and immunocytochemistry for NPY, calbindin, calretinin, and parvalbumin revealed no compensatory changes or developmental abnormalities in the dentate gyrus of somatostatin knockout mice. Optical fractionator counting of Nissl-stained hilar neurons showed similar numbers of neurons in wild type and somatostatin knockout mice. Mice were treated systemically with kainic acid to evoke limbic seizures. Somatostatin knockout mice tended to have a shorter average latency to stage 5 seizures, their average maximal behavioral seizure score was higher, and they tended to be more likely to die than controls. In response to kindling by daily electrical stimulation of the perforant path, to more specifically challenge the dentate gyrus, mean afterdischarge duration in somatostatin knockout mice was slightly longer, but the number of treatments to five stage 4-5 seizures was similar to controls. Although we cannot exclude the possibility of undetected compensatory mechanisms in somatostatin knockout mice, these findings suggest that somatostatin may be mildly anticonvulsant, but its loss alone is unlikely to account for seizures in temporal lobe epilepsy.

Evidence that post-tetanic potentiation is mediated by neuropeptide release in Aplysia

Many neuromuscular and central synapses exhibit activity-dependent plasticity. The sustained high-frequency firing needed to elicit some forms of plasticity are similar to those often required to release neuropeptides. We wanted to determine if neuropeptide release could contribute to post-tetanic potentiation (PTP) and chose neuromuscular synapses in buccal muscle I3a to explore this issue. This muscle is innervated by two motor neurons (termed B3 and B38) that show PTP in response to tetanic stimulation. B3 and B38 use glutamate as their fast transmitter but express different modulatory neuropeptides. B3 expresses FMRFamide, a neuropeptide that only slightly increases its own excitatory junction potentials (EJPs). B38 expresses the small cardioactive peptide (SCP), a neuropeptide that dramatically increases its own EJPs. It was our hypothesis that SCP released from B38's terminals during tetanic stimulation mediated a component of PTP for B38. Because no antagonist to SCP currently exists, we used several indirect approaches to test this hypothesis. First, we studied the effects of increasing stimulation frequency during the tetanus or lowering temperature on PTP. Both of these changes are known to dramatically increase SCP release. We found that increasing the frequency of stimulation increased PTP for both neurons; however, the effects were larger for B38. Decreasing the temperature tended to reduce PTP for B3, while increasing PTP for B38. These results were consistent with known properties of SCP release from B38. Next we selectively superfused the neuromuscular synapses with exogenous SCP to determine if this would occlude the effects of SCP released from B38 during a tetanus. We found that exogenous SCP dramatically reduced PTP for B38 but had little effect on PTP for B3. Thus our results support the hypothesis that physiological stimulation of B38 elicits PTP that is predominantly dependent on the release of SCP from its own terminals. They also demonstrate that the mechanisms underlying PTP can be very different for two motor neurons innervating the same target muscle.

Presynaptic target of Ca2+ action on neuropeptide and acetylcholine release in Aplysia californica

1. When buccal neuron B2 of Aplysia californica is co-cultured with sensory neurons (SNs), slow peptidergic synapses are formed. When B2 is co-cultured with neurons B3 or B6, fast cholinergic synapses are formed. 2. Patch pipettes were used to voltage clamp pre- and postsynaptic neurons and to load the caged Ca2+ chelator o-nitrophenyl EGTA (NPE) and the Ca2+ indicator BTC into presynaptic neurons. The relationships between presynaptic [Ca2+]i and postsynaptic responses were compared between peptidergic and cholinergic synapses formed by cell B2. 3. Using variable intensity flashes, Ca2+ stoichiometries of peptide and acetylcholine (ACh) release were approximately 2 and 3, respectively. The difference did not reach statistical significance. 4. ACh quanta summate linearly postsynaptically. We also found a linear dose-response curve for peptide action, indicating a linear relationship between submaximal peptide concentration and response of the SN. 5. The minimum intracellular calcium concentrations ([Ca2+]i) for triggering peptidergic and cholinergic transmission were estimated to be about 5 and 10 microM, respectively. 6. By comparing normal postsynaptic responses to those evoked by photolysis of NPE, we estimate [Ca2+]i at the release trigger site elicited by a single action potential (AP) to be at least 10 microM for peptidergic synapses and probably higher for cholinergic synapses. 7. Cholinergic release is brief (half-width approximately 200 ms), even in response to a prolonged rise in [Ca2+]i, while some peptidergic release appears to persist for as long as [Ca2+]i remains elevated (for up to 10 s). This may reflect differences in sizes of reserve pools, or in replenishment rates of immediately releasable pools of vesicles. 8. Electron microscopy revealed that most synaptic contacts had at least one morphologically docked dense core vesicle that presumably contained peptide; these were often located within conventional active zones. 9. Both cholinergic and peptidergic vesicles are docked within active zones, but cholinergic vesicles may be located closer to Ca2+ channels than are peptidergic vesicles.

GABA enhances transmission at an excitatory glutamatergic synapse

GABA mediates both presynaptic and postsynaptic inhibition at many synapses. In contrast, we show that GABA enhances transmission at excitatory synapses between the lateral gastric and medial gastric motor neurons and the gastric mill 6a and 9 (gm6a, gm9) muscles and between the lateral pyloric motor neuron and pyloric 1 (p1) muscles in the stomach of the lobster Homarus americanus. Two-electrode current-clamp or voltage-clamp techniques were used to record from muscle fibers. The innervating nerves were stimulated to evoke excitatory junctional potentials (EJPs) or excitatory junctional currents. Bath application of GABA first decreased the amplitude of evoked EJPs in gm6a and gm9 muscles, but not the p1 muscle, by activating a postjunctional conductance increase that was blocked by picrotoxin. After longer GABA applications (5-15 min), the amplitudes of evoked EJPs increased in all three muscles. This increase persisted in the presence of picrotoxin. beta-(Aminomethyl)-4-chlorobenzenepropanoic acid (baclofen) was an effective agonist for the GABA-evoked enhancement but did not increase the postjunctional conductance. Muscimol activated a rapid postsynaptic conductance but did not enhance the amplitude of the nerve-evoked EJPs. GABA had no effect on iontophoretic responses to glutamate and decreased the coefficient of variation of nerve-evoked EJPs. In the presence or absence of tetrodotoxin, GABA increased the frequency but not the amplitude of miniature endplate potentials. These data suggest that GABA acts presynaptically via a GABA(B)-like receptor to increase the release of neurotransmitter.

Sonometric measurements of motor-neuron-evoked movements of an internal feeding structure (the radula) in Aplysia

In many systems used to study rhythmic motor programs, the structures that generate behavior are at least partially internal. In these systems, it is often difficult to directly monitor neurally evoked movements. As a consequence, although motor programs are relatively well characterized, it is generally less clear how neural activity is translated into functional movements. This is the case for the feeding system of the mollusk Aplysia. Here we used sonomicrometry to monitor neurally evoked movements of the food-grasping organ in Aplysia, the radula. Movements were evoked by intracellular stimulation of motor neurons that innervate radula muscles that have been extensively studied in reduced preparations. Nevertheless our results indicate that the movements and neural control of the radula are more complex than has been assumed. We demonstrate that motor neurons previously characterized as radula openers (B48) and closers (B8, B15, B16) additionally produce other movements. Moreover, we show that the size of the movement evoked by a motor neuron can depend on the preexisting state of the radula. Specifically, the motor neurons B15 and B16 produce large closing movements when the radula is partially open but produce relatively weak closing movements in a preparation at rest. Thus the efficacy of B15 and B16 as radula closers is context dependent.

Non-associative learning and serotonin induce similar bi-directional changes in excitability of a neuron critical for learning in the medicinal leech

In studies of the cellular basis of learning, much attention has focused on plasticity in synaptic transmission in terms of transmitter release and the number or responsiveness of neurotransmitter receptors. However, changes in postsynaptic excitability independent of receptors may also play an important role. Changes in excitability of a single interneuron in the leech, the S-cell, were measured during non-associative learning of the whole-body shortening reflex. This interneuron was chosen because it is known to be necessary for sensitization and full dishabituation of the shortening response. During sensitization, S-cell excitability increased, and this enhancement corresponded to facilitation of the shortening reflex and increased S-cell activity during the elicited response. During habituation training, there was a decrement in both the shortening reflex and the elicited S-cell activity, along with decreased S-cell excitability. Conversely, dishabituation facilitated both the shortening response and S-cell activity during shortening, with an accompanying increase in S-cell excitability. Bath application of 1-10 micrometer serotonin (5HT), a modulatory neurotransmitter that is critical for sensitization, for full dishabituation, and for associative learning, increased S-cell excitability. S-cell excitability also increased after stimulation of the serotonergic Retzius cells. However, focal application of serotonin onto the S-cell soma hyperpolarized the interneuron, and bath application of a lower dose of serotonin (0.1 micrometer) decreased excitability. The observed changes in postsynaptic excitability appear to contribute to non-associative learning, and modulatory neurotransmitters, such as serotonin, evidently help regulate excitability. Such changes in S-cell excitability may also be relevant for more complex, associative forms of learning.

Serotonergic and peptidergic modulation of the buccal mass protractor muscle (I2) in aplysia

Plasticity of Aplysia feeding has largely been measured by noting changes in radula protraction. On the basis of previous work, it has been suggested that peripheral modulation may contribute to behavioral plasticity. However, peripheral plasticity has not been demonstrated in the neuromuscular systems that participate in radula protraction. Therefore in this study we investigated whether contractions of a major radula protraction muscle (I2) are subject to modulation. We demonstrate, first, that an increase in the firing frequency of the cholinergic I2 motoneurons will increase the amplitude of the resulting muscle contraction but will not modulate its relaxation rate. We show, second, that neuronal processes on the I2 muscle are immunoreactive to myomodulin (MM), RFamide, and serotonin (5-HT), but not to small cardioactive peptide (SCP) or buccalin. The I2 motoneurons B31, B32, B61, and B62 are not immunoreactive to RFamide, 5-HT, SCP, or buccalin. However, all four cells are MM immunoreactive and are capable of synthesizing MMa. Third, we show that the bioactivity of the different modulators is somewhat different; while the MMs (i.e., MMa and MMb) and 5-HT increase I2 muscle relaxation rate, and potentiate muscle contraction amplitude, MMa, at high concentrations, depresses muscle contractions. Fourth, our data suggest that cAMP at least partially mediates effects of modulators on contraction amplitude and relaxation rate.

Activity-dependent release of endogenous brain-derived neurotrophic factor from primary sensory neurons detected by ELISA in situ

To define activity-dependent release of endogenous brain-derived neurotrophic factor (BDNF), we developed an in vitro model using primary sensory neurons and a modified ELISA, termed ELISA in situ. Dissociate cultures of nodose-petrosal ganglion cells from newborn rats were grown in wells precoated with anti-BDNF antibody to capture released BDNF, which was subsequently detected using conventional ELISA. Conventional ELISA alone was unable to detect any increase in BDNF concentration above control values following chronic depolarization with 40 mM KCl for 72 hr. However, ELISA in situ demonstrated a highly significant increase in BDNF release, from 65 pg/ml in control to 228 pg/ml in KCl-treated cultures. The efficacy of the in situ assay appears to be related primarily to rapid capture of released BDNF that prevents BDNF binding to the cultured cells. We therefore used this approach to compare BDNF release from cultures exposed for 30 min to either continuous depolarization with elevated KCl or patterned electrical field stimulation (50 biphasic rectangular pulses of 25 msec, at 20 Hz, every 5 sec). Short-term KCl depolarization was completely ineffective at evoking any detectable release of BDNF, whereas patterned electrical stimulation increased extracellular BDNF levels by 20-fold. In addition, the magnitude of BDNF release was dependent on stimulus pattern, with high-frequency bursts being most effective. These data indicate that the optimal stimulus profile for BDNF release resembles that of other neuroactive peptides. Moreover, our findings demonstrate that BDNF release can encode temporal features of presynaptic neuronal activity.