Avenues of research in dietary interventions to target tumor metabolism in osteosarcoma

Osteosarcoma (OS) is the most frequent primary bone cancer, affecting mostly children and adolescents. Although much progress has been made throughout the years towards treating primary OS, the 5-year survival rate for metastatic OS has remained at only 20% for the last 30 years. Therefore, more efficient treatments are needed. Recent studies have shown that tumor metabolism displays a unique behavior, and plays important roles in tumor growth and metastasis, making it an attractive potential target for novel therapies. While normal cells typically fuel the oxidative phosphorylation (OXPHOS) pathway with the products of glycolysis, cancer cells acquire a plastic metabolism, uncoupling these two pathways. This allows them to obtain building blocks for proliferation from glycolytic intermediates and ATP from OXPHOS. One way to target the metabolism of cancer cells is through dietary interventions. However, while some diets have shown anticancer effects against certain tumor types in preclinical studies, as of yet none have been tested to treat OS. Here we review the features of tumor metabolism, in general and about OS, and propose avenues of research in dietary intervention, discussing strategies that could potentially be effective to target OS metabolism.

Disulfiram reduces metastatic osteosarcoma tumor burden in an immunocompetent Balb/c or-thotopic mouse model

Introduction

The overall survival rate of patients with osteosarcoma (OS) and pulmonary metastases has remained stagnant at 15–30% for several decades. Disulfiram (DSF) is an FDA-approved aldehyde dehydrogenase inhibitor that reduces the metastatic phenotype of OS cells in vitro. Here we evaluate its in vivo efficacy, as compared to doxorubicin chemotherapy, in a previously-validated orthotopic model of metastatic OS.

Results

All treatment groups displayed a significantly reduced quantitative OS metastatic burden compared with controls. The metastatic burden of Lo DSF-treated animals was equivalent to the DXR group. Ninety-five percent of control animals displayed evidence of metastatic disease, which was significantly greater than all treatment groups.

Discussion

Disulfiram treatment resulted in a reduced burden of OS metastatic disease compared with controls. This was statistically-equivalent to doxorubicin. No additive effect was observed between these two therapies.

Materials and Methods

One-hundred twenty immunocompetent Balb/c mice received proximal tibia paraphyseal injections of 5 × 10⁵ K7M2 murine OS cells. Therapy began three weeks after injection: saline (control), low-dose disulfiram (Lo DSF), high-dose disulfiram (Hi DSF), doxorubicin (DXR), Lo DSF + DXR, and Hi DSF + DXR. Transfemoral amputations were performed at 4 weeks. Quantitative metastatic tumor burden was measured using near-infrared indocyanine green (ICG) angiography.

Lung cells support osteosarcoma cell migration and survival

Background

Osteosarcoma (OS) is the most common primary bone tumor, with a propensity to metastasize to the lungs. Five-year survival for metastatic OS is below 30%, and has not improved for several decades despite the introduction of multi-agent chemotherapy. Understanding OS cell migration to the lungs requires an evaluation of the lung microenvironment. Here we utilized an in vitro lung cell and OS cell co-culture model to explore the interactions between OS and lung cells, hypothesizing that lung cells would promote OS cell migration and survival. The impact of a novel anti-OS chemotherapy on OS migration and survival in the lung microenvironment was also examined.

Methods

Three human OS cell lines (SJSA-1, Saos-2, U-2) and two human lung cell lines (HULEC-5a, MRC-5) were cultured according to American Type Culture Collection recommendations. Human lung cell lines were cultured in growth medium for 72 h to create conditioned media. OS proliferation was evaluated in lung co-culture and conditioned media microenvironment, with a murine fibroblast cell line (NIH-3 T3) in fresh growth medium as controls. Migration and invasion were measured using a real-time cell analysis system. Real-time PCR was utilized to probe for Aldehyde Dehydrogenase (ALDH1) expression. Osteosarcoma cells were also transduced with a lentivirus encoding for GFP to permit morphologic analysis with fluorescence microscopy. The anti-OS efficacy of Disulfiram, an ALDH-inhibitor previously shown to inhibit OS cell proliferation and metastasis in vitro, was evaluated in each microenvironment.

Results

Lung-cell conditioned medium promoted osteosarcoma cell migration, with a significantly higher attractive effect on all three osteosarcoma cell lines compared to basic growth medium, 10% serum containing medium, and NIH-3 T3 conditioned medium (p <0.05). Lung cell conditioned medium induced cell morphologic changes, as demonstrated with GFP-labeled cells. OS cells cultured in lung cell conditioned medium had increased alkaline phosphatase staining.

Conclusions

Lung endothelial HULEC-5a cells are attractants for OS cell migration, proliferation, and survival. The SJSA-1 osteosarcoma cell line demonstrated greater metastatic potential than Saos-2 and U-2 cells. ALDH appears to be involved in the interaction between lung and OS cells, and ALP may be a valuable biomarker for monitoring functional OS changes during metastasis.

Sacroplasty for symptomatic sacral hemangioma: a novel treatment approach. A case report

Painful vertebral body hemangiomas have been successfully treated with vertebroplasty and kyphoplasty. Sacral hemangiomas are uncommon and as such painful sacral hemangiomas are rare entities. We report what we believe is only the second successful treatment of a painful sacral hemangioma with CT-guided sacroplasty. A 56-year-old woman with a history of right-sided total hip arthroplasty and lipoma excision presented to her orthopedic surgeon with persistent right-sided low back pain which radiated into her buttock and right groin and hindered her ability to walk and perform her activities of daily living. MRIs of the thoracic spine, lumbar spine and pelvis showed numerous lesions with imaging characteristics consistent with multiple hemangiomas including a 2.2×2.1 cm lesion involving the right sacrum adjacent to the right S1 neural foramen. Conservative measures including rest, physical therapy, oral analgesics and right-sided sacroiliac joint steroid injection did not provide significant relief. Given her lack of improvement and the fact that her pain localized to the right sacrum, the patient underwent CT-guided sacroplasty for treatment of a painful right sacral hemangioma. Under CT fluoroscopic guidance, a 10 gauge introducer needle was advanced through the soft tissues of the back to the margin of the lesion. Biopsy was then performed and after appropriate preparation, cement was then introduced through the needle using a separate cement filler cannula. Appropriate filling of the right sacral hemangioma was visualized using intermittent CT fluoroscopy. After injection of approximately 2.5 cc of cement, it was felt that there was near complete filling of the right sacral hemangioma. With satisfactory achievement of cement filling, the procedure was terminated. Pathology from biopsy taken at the time of the procedure was consistent with hemangioma. Image-guided sacroplasty with well-defined endpoints is an effective, minimally invasive and safe procedure. Patients with painful sacral hemangiomas can be treated with this technique with no significant complications.

Fixation of pathological humeral fractures by the cemented plate technique

Pathological fractures of the humerus are associated with pain, morbidity, loss of function and a diminished quality of life. We report our experience of stabilising these fractures using polymethylmethacrylate and non-locking plates. We undertook a retrospective review over 20 years of patients treated at a tertiary musculoskeletal oncology centre. Those who had undergone surgery for an impending or completed pathological humeral fracture with a diagnosis of metastatic disease or myeloma were identified from our database. There were 63 patients (43 men, 20 women) in the series with a mean age of 63 years (39 to 87). All had undergone intralesional curettage of the tumour followed by fixation with intramedullary polymethylmethacrylate and plating. Complications occurred in 14 patients (22.2%) and seven (11.1%) required re-operation. At the latest follow-up, 47 patients (74.6%) were deceased and 16 (25.4%) were living with a mean follow-up of 75 months (1 to 184). A total of 54 (86%) patients had no or mild pain and 50 (80%) required no or minimal assistance with activities of daily living. Of the 16 living patients none had pain and all could perform activities of daily living without assistance. Intralesional resection of the tumour, filling of the cavity with cement, and plate stabilisation of the pathological fracture gives immediate rigidity and allows an early return of function without the need for bony union. The patient's local disease burden is reduced, which may alleviate tumour-related pain and slow the progression of the disease. The cemented-plate technique provides a reliable option for the treatment of pathological fractures of the humerus.

Atm is a negative regulator of intestinal neoplasia

The ataxia telangiectasia-mutated (ATM) gene has been implicated as an early barrier to the growth and progression of incipient solid tumors. Here, we show that germ-line nullizygosity for the mouse Atm gene significantly increases the proliferative index, net growth rate and multiplicity of intestinal adenomas in two distinct models of familial colon cancer: Apc(Min/+) and Apc(1638N/+). These effects of Atm deficiency are quantitatively different from deficiency for either of the genomic stability genes Bloom's syndrome helicase or DNA ligase 4, and the effect of Atm loss on tumor multiplicity is largely independent of the effect of ionizing radiation. Furthermore, the loss of heterozygosity rates at the adenomatous polyposis coli (Apc) locus are unaffected by Atm loss. Taken together, these data implicate the Atm gene product as a barrier to dysplastic growth in the early stages of intestinal tumor progression, independent of its effects on genomic stability.

A newly identified buccal interneuron initiates and modulates feeding motor programs in aplysia

Despite considerable progress in characterizing the feeding central pattern generator (CPG) in Aplysia, the full complement of neurons that generate feeding motor programs has not yet been identified. The distribution of neuropeptide-containing neurons in the buccal and cerebral ganglia can be used as a tool to identify additional elements of the feeding circuitry by providing distinctions between otherwise morphologically indistinct neurons. For example, our recent study revealed a unique and potentially interesting unpaired PRQFVamide (PRQFVa)-containing neuron in the buccal ganglion. In this study, we describe the morphological and electrophysiological characterization of this novel neuron, which we designate as B50. We found that activation of B50 is capable of producing organized rhythmic output of the feeding CPG. The motor programs elicited by B50 exhibit some similarities as well as differences to motor programs elicited by the command-like cerebral-to-buccal interneuron CBI-2. In addition to activating the feeding CPG, B50 may act as a program modulator.

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.

PRQFVamide, a novel pentapeptide identified from the CNS and gut of Aplysia

We have purified a novel pentapeptide from the Aplysia nervous system using bioassay on gut contractions. The structure of the peptide is Pro-Arg-Gln-Phe-Val-amide (PRQFVa). The precursor for PRQFVa was found to code for 33 copies of PRQFVamide and four related pentapeptides. Peaks corresponding to the predicted masses of all five pentapeptides were detected in Aplysia neurons by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Northern analysis revealed that expression of the precursor is abundant in the abdominal ganglion, much less in the pedal and cerebral ganglia, and rarely seen in the buccal and pleural ganglia. PRQFVa-positive neurons, mapped by immunohistochemistry and in situ hybridization, were present in all the central ganglia. PRQFVa immunopositive processes were observed in the gut, particularly in association with the vasculature. Some arteries and other highly vascularized tissues, such as the gill and the kidney, also contain numerous PRQFVa immunopositive processes. Application of synthetic PRQFVa suppresses not only contractions of the gut but also contractions of vasculature. PRQFVa is expressed in some of the neurons within the feeding circuitry and application of synthetic PRQFVa was found to decrease the excitability of some (B4/5 and B31/32) but not all (B8) neurons of the buccal feeding circuit. Our findings suggest that PRQFVa may act as a modulator within the feeding system as well as in other systems of Aplysia.

Identification and characterization of the feeding circuit-activating peptides, a novel neuropeptide family of aplysia

We use a multidisciplinary approach to identify, map, and characterize the bioactivity of modulatory neuropeptides in the circuitry that generates feeding behavior in Aplysia. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of the cerebral-buccal connective (CBC), a nerve containing axons of many interneurons that control feeding behavior of Aplysia, was used to identify neuropeptides that may participate in generation and shaping of feeding motor programs. Using this functionally oriented search, we identified a novel family of peptides that we call the feeding circuit-activating peptides (FCAPs). Two peptides with masses identical to those observed in the CBCs (molecular weight 1387 and 1433) were purified from buccal ganglia and partially sequenced using mass spectrometry. The amino acid sequence was then used to clone the FCAP precursor, which encodes multiple copies of eight different FCAPs. The two FCAPs present in highest copy number correspond to those observed in the CBC. The distribution of FCAP expression was mapped using Northern analysis, whole-mount in situ hybridization, and immunocytochemistry. Consistent with our initial findings, FCAP-immunopositive axons were observed in the CBC. Furthermore, we found that FCAP was present in some cerebral-buccal and buccal-cerebral interneurons. As their name suggests, FCAPs are capable of initiating rhythmic feeding motor programs and are the first neuropeptides with such activity in this circuit. The actions of FCAPs suggest that these peptides may contribute to the induction and maintenance of food-induced arousal. FCAPs were also localized to several other neuronal systems, suggesting that FCAPs may play a role in the regulation of multiple behaviors.

Motor program selection in simple model systems

In recent years, studies of molluscan and crustacean feeding circuits have greatly expanded our knowledge of how the nervous system selects specific behaviors. Increasing use of neurobehavioral studies, and examination of the roles of identified command-like or influential neurons have narrowed the gap between knowledge of circuit connectivity and understanding of the normal behavioral functions of these circuits.

Cloning, expression and processing of the CP2 neuropeptide precursor of Aplysia

The cDNA sequence encoding the CP2 neuropeptide precursor is identified and encodes a single copy of the neuropeptide that is flanked by appropriate processing sites. The distribution of the CP2 precursor mRNA is described and matches the CP2-like immunoreactivity described previously. Single cell RT-PCR independently confirms the presence of CP2 precursor mRNA in selected neurons. MALDI-TOF MS is used to identify additional peptides derived from the CP2 precursor in neuronal somata and nerves, suggesting that the CP2 precursor may give rise to additional bioactive neuropeptides.

The enterins: a novel family of neuropeptides isolated from the enteric nervous system and CNS of Aplysia

To identify neuropeptides that have a broad spectrum of actions on the feeding system of Aplysia, we searched for bioactive peptides that are present in both the gut and the CNS. We identified a family of structurally related nonapeptides and decapeptides (enterins) that are present in the gut and CNS of Aplysia, and most of which share the HSFVamide sequence at the C terminus. The structure of the enterin precursor deduced from cDNA cloning predicts 35 copies of 20 different enterins. Northern analysis, in situ hybridization, and immunocytochemistry show that the enterins are abundantly present in the CNS and the gut of Aplysia. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry we characterized the enterin-precursor processing, demonstrated that all of the precursor-predicted enterins are present, and determined post-translational modifications of various enterins. Enterin-positive neuronal somata and processes were found in the gut, and enterins inhibited contractions of the gut. In the CNS, the cerebral and buccal ganglia, which control feeding, contained the enterins. Enterin was also present in the nerve that connects these two ganglia. Enterins reduced the firing of interneurons B4/5 during feeding motor programs. Such enterin-induced reduction of firing also occurred when excitability of B4/5 was tested directly. Because reduction of B4/5 activity corresponds to a switch from egestive to ingestive behaviors, enterin may contribute to such program switching. Furthermore, because enterins are present throughout the nervous system, they may also play a regulatory role in nonfeeding behaviors of Aplysia.

Neural mechanisms of motor program switching in Aplysia

The Aplysia multifunctional feeding central pattern generator (CPG) produces at least two types of motor programs, ingestion and egestion, that involve two sets of radula movements, protraction-retraction and opening-closing movements. In ingestion, the radula closes during retraction to pull food in, whereas in egestion, the radula closes during protraction to push inedible objects out. Thus, radula closure shifts the phase in which it occurs with respect to protraction-retraction in the two programs. To identify the central switching mechanisms, we compared activity of CPG neurons during the two types of motor programs elicited by a higher-order interneuron, cerebral-buccal interneuron-2 (CBI-2). Although CPG elements (B63, B34, and B64) that mediate the protraction-retraction sequence are active in both programs, two other CPG elements, B20 and B4/5, are preferentially active in egestive programs and play a major role in mediating CBI-2-elicited egestive programs. Both B20 and B4/5 control the phasing of radula closure motoneurons (B8 and B16) to ensure that, in egestive programs, these motoneurons fire and produce radula-closing movements only during protraction. Elsewhere, another higher-order interneuron, CBI-3, was shown to convert CBI-2-elicited egestion to ingestion. We show that CBI-3 switches the programs by suppressing the activity of B20 and B4/5. CBI-3, active only during protraction, accomplishes this through fast inhibition of B20 during protraction and slow inhibition of B4/5 during retraction. The slow inhibition is mimicked and occluded by APGWamide, a neuropeptide contained in CBI-3. Thus, fast conventional and slow peptidergic transmissions originating from the same interneuron act in concert to meet specific temporal requirements in pattern switching.

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.

Cerebrin prohormone processing, distribution and action in Aplysia californica

The isolation, characterization, and bioactivity in the feeding circuitry of a novel neuropeptide in the Aplysia californica central nervous system are reported. The 17-residue amidated peptide, NGGTADALYNLPDLEKIamide, has been termed cerebrin due to its primary location in the cerebral ganglion. Liquid chromatographic purification guided by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry allowed the isolation of the peptide with purity adequate for Edman sequencing. The cerebrin cDNA has been characterized and encodes an 86 amino acid prohormone that predicts cerebrin and one additional peptide. Mapping using in situ hybridization and immunocytochemistry showed that cerebrin containing neuronal somata are localized almost exclusively in the cerebral ganglion, mostly in the F- and C-clusters. Both immunostaining and mass spectrometry demonstrated the presence of cerebrin in the neurohemal region of the upper labial nerve. In addition, immunoreactive processes were detected in the neuropil of all of the ganglia, including the buccal ganglia, and in some interganglionic connectives, including the cerebral-buccal connective. This suggests that cerebrin may also function as a local signaling molecule. Cerebrin has a profound effect on the feeding motor pattern elicited by the command-like neuron CBI-2, dramatically shortening the duration of the radula protraction in a concentration-dependent manner, mimicking the motor-pattern alterations observed in food induced arousal states. These findings suggest that cerebrin may contribute to food-induced arousal in the animal. Cerebrin-like immunoreactivity is also present in Lymnaea stagnalis suggesting that cerebrin-like peptides may be widespread throughout gastropoda.

Targeting of peptidergic vesicles in cotransmitting terminals

In the present study, we examined the targeting of neuropeptide-containing vesicles in terminals of neurons that release both neuropeptides and classical transmitters. Single neurons were electrically stimulated with patterns of activity that were recorded in freely behaving animals. The amount of peptide release was measured biochemically using a radioimmunoassay, and the targeting of peptidergic vesicles was quantified using immunoelectronmicroscopy. Repeated electrical stimulation of single neurons produced a very large increase in peptide release. Peptide release is paralleled by a twofold increase in the number of peptidergic vesicles docked at the portion of the terminal membrane that is away from the target muscle. This is in stark contrast to cholinergic vesicles, which aggregate at, and are released from the conventional release sites in close apposition to the muscle. This differential targeting of cholinergic and peptidergic vesicles may play a significant role in the distinct release requirements and spatial and temporal characteristics of the actions of conventional and peptidergic transmitters.

Cerebral-abdominal interganglionic coordinating neurons in Aplysia

Three cerebral-abdominal interneurons (CAIs), CC2, CC3, and CC7, were identified in the cerebral ganglion C cluster. The cells send their axons to the abdominal ganglion via the pleural-abdominal connective. CC2 and CC3 are bilaterally symmetrical cells, whereas CC7 is a unilateral cell. CC3 is immunopositive for serotonin and may be the same cell (CB-1) previously described as located in the B cluster rather than the C cluster. We suggest that the full designation of CC3, be CC3(CB-1). All three cells respond to feeding-related inputs. Each CAI has a monosynaptic connection to at least one abdominal ganglion neuron involved in the control of various nonsomatic organs. The CAIs also exert widespread polysynaptic actions in the abdominal and head ganglia. The results suggest that the CAIs may act as interneurons that coordinate visceral responses mediated by the abdominal ganglion, with behaviors such as feeding and head withdrawal, that are controlled by neurons located in the head ganglia of the animal.

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.

Temporal pattern dependence of neuronal peptide transmitter release: models and experiments

In this paper we construct, on the basis of existing experimental data, a mathematical model of firing-elicited release of peptide transmitters from motor neuron B15 in the accessory radula closer neuromuscular system of Aplysia. The model consists of a slow "mobilizing" reaction and the fast release reaction itself. Experimentally, however, it was possible to measure only the mean, heavily averaged release, lacking fast kinetic information. Considered in the conventional way, the data were insufficient to completely specify the details of the model, in particular the relative properties of the slow and the unobservable fast reaction. We illustrate here, with our model and with additional experiments, how to approach such a problem by considering another dimension of release, namely its pattern dependence. The mean release is sensitive to the temporal pattern of firing, even to pattern on time scales much faster than the time scale on which the release is averaged. The mean release varies with the time scale and magnitude of the pattern, relative to the time scale and nonlinearity of the release reactions with which the pattern interacts. The type and magnitude of pattern dependence, especially when correlated systematically over a range of patterns, can therefore yield information about the properties of the release reactions. Thus, temporal pattern can be used as a probe of the release process, even of its fast, directly unobservable components. More generally, the analysis provides insights into the possible ways in which such pattern dependence, widespread especially in neuropeptide- and hormone-releasing systems, might arise from the properties of the underlying cellular reactions.

Intrinsic and extrinsic modulation of a single central pattern generating circuit

Intrinsic and extrinsic neuromodulation are both thought to be responsible for the flexibility of the neural circuits (central pattern generators) that control rhythmic behaviors. Because the two forms of modulation have been studied in different circuits, it has been difficult to compare them directly. We find that the central pattern generator for biting in Aplysia is modulated both extrinsically and intrinsically. Both forms of modulation increase the frequency of motor programs and shorten the duration of the protraction phase. Extrinsic modulation is mediated by the serotonergic metacerebral cell (MCC) neurons and is mimicked by application of serotonin. Intrinsic modulation is mediated by the cerebral peptide-2 (CP-2) containing CBI-2 interneurons and is mimicked by application of CP-2. Since the effects of CBI-2 and CP-2 occlude each other, the modulatory actions of CBI-2 may be mediated by CP-2 release. Although the effects of intrinsic and extrinsic modulation are similar, the neurons that mediate them are active predominantly at different times, suggesting a specialized role for each system. Metacerebral cell (MCC) activity predominates in the preparatory (appetitive) phase and thus precedes the activation of CBI-2 and biting motor programs. Once the CBI-2s are activated and the biting motor program is initiated, MCC activity declines precipitously. Hence extrinsic modulation prefacilitates biting, whereas intrinsic modulation occurs during biting. Since biting inhibits appetitive behavior, intrinsic modulation cannot be used to prefacilitate biting in the appetitive phase. Thus the sequential use of extrinsic and intrinsic modulation may provide a means for premodulation of biting without the concomitant disruption of appetitive behaviors.

Peptide profiling of cells with multiple gene products: combining immunochemistry and MALDI mass spectrometry with on-plate microextraction

Due to the intracellular chemical complexity and a wide range of transmitter concentrations, the detection of the complete set of peptide transmitters in a single cell is problematic. In the current study, a multidisciplinary approach combining single-cell MALDI-MS peptide profiling, northern analysis, in situ hybridization, and immunocytochemistry allows characterization of a more complete set of neurotransmitters than individual approaches in the Aplysia californica B1 and B2 motor neurons. Because different results were obtained using both in situ and immunohistochemical techniques compared to previous reports, MALDI-MS assays have been used to examine CP1-related gene products in these cells. However, MALDI with standard sample preparation does not detect the presence of the CP1 gene products. A novel on-plate microextraction approach using concentrated MALDI matrix 2,5-dihydroxybenzoic acid with a mixture of acetone and water as the solvent has been developed to allow the detection of trace-level gene expression products. Both neuropeptide precursors in the B1 and B2 neurons-the SCP and CP1 prohormones-end with large peptides that have multiple cysteine residues. For SCP, MALDI-MS verifies the presence of a novel 9325 Da SCP-related peptide. In the case of CP1, a disulfide-bonded homodimer is detected and the disulfide bonding pattern elucidated using MALDI-MS coupled with on-plate enzymatic digestion.

Glutamate is the fast excitatory neurotransmitter of small cardioactive peptide-containing Aplysia radula mechanoafferent neuron B21

B21 is a radula mechanoafferent neuron in the mollusc Aplysia which likely plays a crucial role in integrating environmental cues into the feeding motor program. To facilitate understanding B21's interactions with its postsynaptic followers, we sought to identify its neurotransmitter. We find that B21 makes a chemical synapse onto the follower neuron B8. Although B21-induced excitatory postsynaptic potentials (EPSPs) in B8 paradoxically diminish in amplitude with B8 hyperpolarization, we show that an inwardly rectifying current is responsible. We conclude that these B21-induced EPSPs are likely glutamatergic as they are blocked by the glutamate antagonist DNQX. Furthermore, B8 exhibits a depolarizing response to exogenous glutamate, which is antagonized by DNQX. Finally, exogenous glutamate occludes B21-evoked EPSPs in B8.

Multifunctional neuron CC6 in Aplysia exerts actions opposite to those of multifunctional neuron CC5

The controls of somatic and autonomic functions often appear to be organized into antagonistic systems. This issue was explored in the bilaterally paired C cluster neuron, CC6, which was found to have properties that suggested that it might function antagonistically to the previously identified multiaction neuron, CC5. Similar to CC5, CC6 is an interganglionic neuron that sends its sole axon to the ipsilateral and contralateral pedal and pleural ganglia. Synaptic inputs to CC6 were opposite to those of CC5. For example, CC6 receives inhibitory inputs from mechanical touch to the lips and tentacles and is excited by firing of C-PR, a neuron involved in the control of a head extension response. Also during rhythmic buccal mass movements CC6 receives synaptic inputs that are out of phase with those received by CC5. CC6 is inhibited during a fictive locomotor program, whereas CC5 is excited, but unlike CC5, the inputs to CC6 are not rhythmic. CC6 has extensive mono- and polysynaptic outputs to many identified and unidentified neurons located in various central ganglia. Firing of CC6 evoked ipsilateral contraction of the transverse muscles of the neck, whereas CC5 contracts longitudinal neck muscles. CC6 monosynaptically inhibits the pedal artery shortener neuron, whereas CC5 monosynaptically excites the pedal artery shortener neuron. Specific motor neurons in the pedal ganglion receive synaptic inputs of opposite sign from CC5 and CC6. Although the inputs and most of the effects of CC6 were opposite to those of CC5, both cells were found to produce polysynaptic excitation of the abdominal ganglion neuron RBhe, a cell whose activity excites the heart. CC5 and CC6 appear to be multifunctional neurons that form an antagonist pair.

Peptide cotransmitter release from motorneuron B16 in aplysia californica: costorage, corelease, and functional implications

Many neurons contain multiple peptide cotransmitters in addition to their classical transmitters. We are using the accessory radula closer neuromuscular system of Aplysia, which participates in feeding in these animals, to define the possible consequences of multiple modulators converging on single targets. How these modulators are released onto their targets is of critical importance in understanding the outcomes of their modulatory actions and their physiological role. Here we provide direct evidence that the partially antagonistic families of modulatory peptides, the myomodulins and buccalins, synthesized by motorneuron B16 are costored and coreleased in fixed ratios. We show that this release is calcium-dependent and independent of muscle contraction. Furthermore, we show that peptide release is initiated at the low end of the physiological range of motorneuron firing frequency and that it increases with increasing motorneuron firing frequency. The coordination of peptide release with the normal operating range of a neuron may be a general phenomenon and suggests that the release of peptide cotransmitters may exhibit similar types of regulation and plasticity as have been observed for classical transmitters. Stimulation paradigms that increase muscle contraction amplitude or frequency also increase peptide release from motor neuron B16. The net effect of the modulatory peptide cotransmitters released from motorneuron B16 would be to increase relaxation rate and therefore allow more frequent and/or larger contractions to occur without increased resistance to antagonist muscles. The end result of this modulation could be to maximize the efficiency of feeding.

Activation of a heterologously expressed octopamine receptor coupled only to adenylyl cyclase produces all the features of presynaptic facilitation in aplysia sensory neurons

Short-term behavioral sensitization of the gill-withdrawal reflex after tail stimuli in Aplysia leads to an enhancement of the connections between sensory and motor neurons of this reflex. Both behavioral sensitization and enhancement of the connection between sensory and motor neurons are importantly mediated by serotonin. Serotonin activates two types of receptors in the sensory neurons, one of which is coupled to the cAMP/protein kinase A (PKA) pathway and the other to the inositol triphosphate/protein kinase C (PKC) pathway. Here we describe a genetic approach to assessing the isolated contribution of the PKA pathway to short-term facilitation. We have cloned from Aplysia an octopamine receptor gene, Ap oa(1), that couples selectively to the cAMP/PKA pathway. We have ectopically expressed this receptor in Aplysia sensory neurons of the pleural ganglia, where it is not normally expressed. Activation of this receptor by octopamine stimulates all four presynaptic events involved in short-term synaptic facilitation that are normally produced by serotonin: (i) membrane depolarization; (ii) increased membrane excitability; (iii) increased spike duration; and (iv) presynaptic facilitation. These results indicate that the cAMP/PKA pathway alone is sufficient to produce all the features of presynaptic facilitation.

The neuromuscular transform: the dynamic, nonlinear link between motor neuron firing patterns and muscle contraction in rhythmic behaviors

The nervous system issues motor commands to muscles to generate behavior. All such commands must, however, pass through a filter that we call here the neuromuscular transform (NMT). The NMT transforms patterns of motor neuron firing to muscle contractions. This work is motivated by the fact that the NMT is far from being a straightforward, transparent link between motor neuron and muscle. The NMT is a dynamic, nonlinear, and modifiable filter. Consequently motor neuron firing translates to muscle contraction in a complex way. This complexity must be taken into account by the nervous system when issuing its motor commands, as well as by us when assessing their significance. This is the first of three papers in which we consider the properties and the functional role of the NMT. Physiologically, the motor neuron-muscle link comprises multiple steps of presynaptic and postsynaptic Ca(2+) elevation, transmitter release, and activation of the contractile machinery. The NMT formalizes all these into an overall input-output relation between patterns of motor neuron firing and shapes of muscle contractions. We develop here an analytic framework, essentially an elementary dynamical systems approach, with which we can study the global properties of the transformation. We analyze the principles that determine how different firing patterns are transformed to contractions, and different parameters of the former to parameters of the latter. The key properties of the NMT are its nonlinearity and its time dependence, relative to the time scale of the firing pattern. We then discuss issues of neuromuscular prediction, control, and coding. Does the firing pattern contain a code by means of which particular parameters of motor neuron firing control particular parameters of muscle contraction? What information must the motor neuron, and the nervous system generally, have about the periphery to be able to control it effectively? We focus here particularly on cyclical, rhythmic contractions which reveal the principles particularly clearly. Where possible, we illustrate the principles in an experimentally advantageous model system, the accessory radula closer (ARC)-opener neuromuscular system of Aplysia. In the following papers, we use the framework developed here to examine how the properties of the NMT govern functional performance in different rhythmic behaviors that the nervous system may command.

Optimization of rhythmic behaviors by modulation of the neuromuscular transform

We conclude our study of the properties and the functional role of the neuromuscular transform (NMT). The NMT is an input-output relation that formalizes the processes by which patterns of motor neuron firing are transformed to muscle contractions. Because the NMT acts as a dynamic, nonlinear, and modifiable filter, the transformation is complex. In the two preceding papers we developed a framework for analysis of the NMT and identified with it principles by which the NMT transforms different firing patterns to contractions. We then saw that, with fixed properties, the NMT significantly constrains the production of functional behavior. Many desirable behaviors are not possible with any firing pattern. Here we examine, theoretically as well as experimentally in the accessory radula closer (ARC) neuromuscular system of Aplysia, how this constraint is alleviated by making the properties of the NMT variable by neuromuscular plasticity and modulation. These processes dynamically tune the properties of the NMT to match the desired behavior, expanding the range of behaviors that can be produced. For specific illustration, we continue to focus on the relation between the speed of the NMT and the speed of cyclical, rhythmic behavior. Our analytic framework emphasizes the functional distinction between intrinsic plasticity or modulation of the NMT, dependent, like the contraction itself, on the motor neuron firing pattern, and extrinsic modulation, independent of it. The former is well suited to automatically optimizing the performance of a single behavior; the latter, to multiplying contraction shapes for multiple behaviors. In any case, to alleviate the constraint of the NMT, the plasticity and modulation must be peripheral. Such processes are likely to play a critical role wherever the nervous system must command, through the constraint of the NMT, a broad range of functional behaviors.

The neuromuscular transform constrains the production of functional rhythmic behaviors

We continue our study of the properties and the functional role of the neuromuscular transform (NMT). The NMT is an input-output relation that formalizes the processes by which patterns of motor neuron firing are transformed to muscle contractions. Because the NMT acts as a dynamic, nonlinear, and modifiable filter, the transformation is complex. In the preceding paper we developed a framework for analysis of the NMT and identified with it principles by which the NMT transforms different firing patterns to contractions. The ultimate question is functional, however. In sending different firing patterns through the NMT, the nervous system is seeking to command different functional behaviors, with specific contraction requirements. To what extent do the contractions that emerge from the NMT actually satisfy those requirements? In this paper we extend our analysis to address this issue. We define representative behavioral tasks and corresponding measures of performance, for a single neuromuscular unit, for two antagonistic units, and, in a real illustration, for the accessory radula closer (ARC)-opener neuromuscular system of Aplysia. We focus on cyclical, rhythmic behaviors which reveal the underlying principles particularly clearly. We find that, although every pattern of motor neuron firing produces some state of muscle contraction, only a few patterns produce functional behavior, and even fewer produce efficient functional behavior. The functional requirements thus dictate certain patterns to the nervous system. But many desirable functional behaviors are not possible with any pattern. We examine, in particular, how rhythmic behaviors degrade and disintegrate as the nervous system attempts to speed up their cycle frequency. This happens because, with fixed properties, the NMT produces only a limited range of contraction shapes that are kinetically well matched to the firing pattern only on certain time scales. Thus the properties of the NMT constrain and restrict the production of functional behaviors. In the following paper, we see how the constraint may be alleviated and the range of functional behaviors expanded by appropriately tuning the properties of the NMT through neuromuscular plasticity and modulation.

The Aplysia mytilus inhibitory peptide-related peptides: identification, cloning, processing, distribution, and action

Neuropeptides are a ubiquitous class of signaling molecules. In our attempt to understand the generation of feeding behavior in Aplysia, we have sought to identify and fully characterize the neuropeptides operating in this system. Preliminary evidence indicated that Mytilus inhibitory peptide (MIP)-like peptides are present and operating in the circuitry that generates feeding in Aplysia. MIPs were originally isolated from the bivalve mollusc Mytilus edulis, and related peptides have been identified in other invertebrate species, but no precursor has been identified. In this study, we describe the isolation and characterization of novel Aplysia MIP-related peptides (AMRPs) and their precursor. Several AMRPs appear to have some structural and functional features similar to vertebrate opioid peptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to confirm that all 14 AMRPs predicted by the precursor are processed in isolated neurons. Northern analysis, whole-mount in situ hybridization, and immunohistochemistry are used to map the abundant expression of these peptides in the CNS and peripheral tissues such as the digestive tract, vasculature, and the reproductive organs. Physiological studies demonstrate that the rank order of the inhibitory actions of these peptides is different for three target muscles. These results underscore the importance of using a multidisciplinary approach to identifying and characterizing the actions of neuropeptides in an effort to gain understanding of their role in systems of interest. The widespread distribution of the AMRPs indicates that they may be operating in many different systems of Aplysia.

Insulin prohormone processing, distribution, and relation to metabolism in Aplysia californica

The first Aplysia californica insulin gene is characterized and its proteolytic processing from prohormone to final peptides elucidated using a combination of biochemical and mass spectrometric methods. Aplysia insulin (AI) is one of the largest insulins found, with a molecular weight of 9146 Da, and an extended A chain compared with other invertebrate and vertebrate insulins. The AI prohormone produces a series of C peptides and also a unique N-terminally acetylated D peptide. AI-producing cells are restricted to the central region of the cerebral ganglia mostly within the F and C clusters, and AI is transported to neurohemal release sites located on the upper labial and anterior tentacular nerves. The expression of AI mRNA decreases when the animal is deprived of food, and injections of AI reduce hemolymph glucose levels, suggesting that the function of insulin-regulating metabolism has been conserved.

Modulation of radula opener muscles in Aplysia

We observed fibers immunoreactive (IR) to serotonin (5-HT), the myomodulins (MMs), and FMRFamide on the I7-I10 complex in the marine mollusk Aplysia californica. The I7-I10 muscle complex, which produces radula opening, is innervated primarily by one motor neuron, B48. B48 is MM-IR and synthesizes authentic MM(A). When B48 is stimulated in a physiological manner, cAMP levels are increased in opener muscles. cAMP increases also are seen when the MMs are applied to opener muscles but are not seen with application of the B48 primary neurotransmitter acetylcholine (ACh). Possible physiological sources of 5-HT and FMRFamide are discussed. When modulators are applied to resting opener muscles, changes in membrane potential are observed. Specifically, 5-HT, MM(B), and low concentrations of MM(A) all depolarize muscle fibers. This depolarization is generally not sufficient to elicit myogenic activity in the absence of neural activity under "rest" conditions. However, if opener muscles are stretched beyond rest length, stretch- and modulator-induced depolarizations can summate and elicit contractions. This only occurs, however, if "depolarizing" modulators are applied alone. Thus other modulators (i.e., FMRFamide and high concentrations of MM(A)) hyperpolarize opener muscle fibers and can prevent depolarizing modulators from eliciting myogenic activity. All modulators tested affected parameters of motor neuron-elicited contractions of opener muscles. MM(B) and 5-HT increased contraction size over the range of concentrations tested, whereas MM(A) potentiated contractions when it was applied at lower concentrations but decreased contraction size at higher concentrations. FMRFamide decreased contraction size at all concentrations and did not affect relaxation rate. Additionally, the MMs and 5-HT increased muscle relaxation rate, decreased contraction latency, and decreased the rate at which tension was developed during motor neuron-elicited muscle contractions. Thus these modulators dramatically affect the ability of opener muscles to follow activity in the opener motor neuron B48. The possible physiological significance of these findings is discussed.

Decrement of the response of a serotonergic modulatory neuron (the metacerebral cell) in Aplysia, during repeated presentation of appetitive (food) stimuli

Application of food (seaweed, SW) stimuli to the lips evokes a burst of metacerebral cell (MCC) spikes, and it was found in free-moving animals that repeated presentation of the stimulus was associated with a rapid decrement of the evoked responses, even in the absence of ingestion of the food. To aid in discriminating between mechanisms that may be responsible for this decrement, SW was applied repeatedly to the lip ipsilateral or contralateral to one of the paired MCCs, and then generalization of the response decrement was tested by applying a SW stimulus to the opposite (non-stimulated) receptive field. There was statistically significant generalization of response decrement and the amount of generalization appeared to be a function of whether the decrementing stimuli were presented on the side ipsilateral vs. contralateral to the recorded MCC. The overall data suggest that MCC response decrement to repeated food stimuli results in a process analogous to behavioral habituation, and the data are consistent with a simple neural model.

Actions of a pair of identified cerebral-buccal interneurons (CBI-8/9) in Aplysia that contain the peptide myomodulin

A combination of biocytin back-fills of the cerebral-buccal connectives and immunocytochemistry of the cerebral ganglion demonstrated that of the 13 bilateral pairs of cerebral-buccal interneurons in the cerebral ganglion, a subpopulation of 3 are immunopositive for the peptide myomodulin. The present paper describes the properties of two of these cells, which we have termed CBI-8 and CBI-9. CBI-8 and CBI-9 were found to be dye coupled and electrically coupled. The cells have virtually identical properties, and consequently we consider them to be "twin" pairs and refer to them as CBI-8/9. CBI-8/9 were identified by electrophysiological criteria and then labeled with dye. Labeled cells were found to be immunopositive for myomodulin, and, using high pressure liquid chromatography, the cells were shown to contain authentic myomodulin. CBI-8/9 were found to receive synaptic input after mechanical stimulation of the tentacles. They also received excitatory input from C-PR, a neuron involved in neck lengthening, and received a slow inhibitory input from CC5, a cell involved in neck shortening, suggesting that CBI-8/9 may be active during forward movements of the head or buccal mass. Firing of CBI-8 or CBI-9 resulted in the activation of a relatively small number of buccal neurons as evidenced by extracellular recordings from buccal nerves. Firing also produced local movements of the buccal mass, in particular a strong contraction of the I7 muscle, which mediates radula opening. CBI-8/9 were found to produce a slow depolarization and rhythmic activity of B48, the motor neuron for the I7 muscle. The data provide continuing evidence that the small population of cerebral buccal interneurons is composed of neurons that are highly diverse in their functional roles. CBI-8/9 may function as a type of premotor neuron, or perhaps as a peptidergic modulatory neuron, the functions of which are dependent on the coactivity of other neurons.

C-PR neuron of Aplysia has differential effects on "Feeding" cerebral interneurons, including myomodulin-positive CBI-12

Head lifting and other aspects of the appetitive central motive state that precedes consummatory feeding movements in Aplysia is promoted by excitation of the C-PR neuron. Food stimuli activate C-PR as well as a small population of cerebral-buccal interneurons (CBIs). We wished to determine if firing of C-PR produced differential effects on the various CBIs or perhaps affected all the CBIs uniformly as might be expected for a neuron involved in producing a broad undifferentiated arousal state. We found that when C-PR was fired, it produced a wide variety of effects on various CBIs. Firing of C-PR evoked excitatory input to a newly identified CBI (CBI-12) the soma of which is located in the M cluster near the previously identified CBI-2. CBI-12 shares certain properties with CBI-2, including a similar morphology and a capacity to drive rhythmic activity of the buccal-ganglion. Unlike CBI-2, CBI-12 exhibits myomodulin immunoreactivity. Furthermore when C-PR is fired, CBI-12 receives a polysynaptic voltage-dependent slow excitation, whereas, CBI-2 receives relatively little input. C-PR also polysynaptically excites other CBIs including CBI-1 and CBI-8/9 but produces inhibition in CBI-3. In addition, firing of C-PR inhibits plateau potentials in CBI-5/6. The data suggest that activity of C-PR may promote the activity of one subset of cerebral-buccal interneurons, perhaps those involved in ingestive behaviors that occur during the head-up posture. C-PR also inhibits some cerebral-buccal interneurons that may be involved in behaviors in which C-PR activity is not required or may even interfere with other feeding behaviors such as rejection or grazing, that occur with the head down.

A pair of reciprocally inhibitory histaminergic sensory neurons are activated within the same phase of ingestive motor programs in Aplysia

Previous studies have shown that each buccal ganglion in Aplysia contains two B52 neurons, one in each hemiganglion. We now show that there are two B52 neurons in a single buccal hemiganglion and four cells in an animal. We also show that the B52 neurons are histamine-immunoreactive and use reverse phase HPLC to show that the histamine-immunoreactive substance is authentic histamine. Previous studies have shown that the B52 neurons make numerous inhibitory synaptic connections with neurons active during the radula closing/retraction phase of ingestive motor programs. A computational model of the Aplysia feeding central pattern generator has, therefore, suggested that the B52 neurons play a role in terminating closing/retraction. Consistent with this idea we show that both B52 neurons fire at the beginning of radula opening/protraction. We also show that both B52 neurons are sensory neurons. They are depolarized when a flap of connective tissue adjacent to the buccal commissural arch is stretched. During ingestive feeding this is likely to occur at the peak of closing/retraction as opening/protraction begins. In the course of this study we compare the two ipsilateral B52 neurons and show that these cells are virtually indistinguishable; e.g., they use a common neurotransmitter, make the same synaptic connections, and are both sensory as well as premotor neurons. Nevertheless we show that the B52 neurons are reciprocally inhibitory. Our results, therefore, strikingly confirm theoretical predictions made by others that neurons that inhibit each other will not necessarily participate in antagonistic phases of behavior.

Mass spectrometric survey of interganglionically transported peptides in Aplysia

The major ganglionic connectives in Aplysia are assayed to determine putative neuropeptides. Matrix-assisted laser desorption/ionization mass spectrometry allows direct measurement of peptides in a nerve. Many previously characterized peptides are observed, including APGWamide, buccalins, small cardioactive peptides, and egg-laying hormone. Several unreported peptides are detected in specific nerves, suggesting they may have important physiological roles. Furthermore, novel processing products of the L5-67 precursor peptide and the APGWamide/cerebral peptide 1 prohormone are strongly suggested, and their interganglionic transport demonstrated.

Analyzing the functional consequences of transmitter complexity

Neurons and other cells are regulated by a great multiplicity of neurotransmitters, modulators, hormones and other chemical messengers, which, through complex networks of extensively diverging and converging pathways, exert a multiplicity of effects. How do we analyze the functioning of such a complex network? If the effects of a transmitter depend on the presence of many other transmitters, how can we predict what they will be? If multiple transmitters act through the same network, how can their actions be specific? If they converge on the same effects, are some of the transmitters redundant? Why are there so many transmitters? Such questions can be addressed using an analytical approach that examines, qualitatively or quantitatively, how the operation of the network globally maps a multidimensional input space of transmitters to a multidimensional output space of effects.

Ion currents and mechanisms of modulation in the radula opener muscles of Aplysia

Ion currents and mechanisms of modulation in the radula opener muscles of Aplysia. J. Neurophysiol. 78: 2372-2387, 1997. Numerous studies of plasticity in the feeding behavior of Aplysia have shown that substantial plasticity is due to peripheral neuromodulation of the feeding musculature. Extensive previous work focusing on the accessory radula closer (ARC) muscle has led to the realization that a major function of the modulation in that muscle may be to ensure efficient coordination between its contractions and those of its antagonist muscles. For a more complete understanding, therefore, we must study these muscles also. Here we have studied the radula opener muscles I7-I10. Using single isolated muscle fibers under voltage clamp, we have characterized ion currents gated by voltage and by the physiological contraction-inducing neurotransmitter acetylcholine (ACh) and the effects of the physiological modulators serotonin, myomodulins A and B, and FMRFamide. Our results explain significant aspects of the electrophysiological behavior of the whole opener muscles, as well as why the opener and ARC muscles behave similarly in many ways yet differently in some key respects. Opener muscles express four types of K currents: inward rectifier, A-type [IK(A)], delayed rectifier [IK(V)], and Ca2+-activated [IK(Ca)]. They also express an L-type Ca current [ICa] and a leakage current. ACh activates a positive-reversing cationic current [IACh(cat)] and a negative-reversing Cl current [IACh(Cl)]. The opener muscles differ from the ARC in that, in the openers, activation of IK(A) occurs approximately 9 mV more positive and there is much less IACh(Cl). In both muscles, IACh(cat) most likely serves to depolarize the muscle until ICa activates to supply Ca2+ for contraction, but further depolarization and spiking is opposed by coactivation of IK(A), IK(V), IK(Ca), and IACh(Cl). Thus the differences in IK(A) and IACh(Cl) may well be key factors that prevent spikes in the ARC but often allow them in the opener muscles. As in the ARC, the modulators enhance ICa and so potentiate contractions. They also activate a modulator-specific K current, which causes hyperpolarization and depression of contractions. Finally, in the opener muscles but not in the ARC, the modulators activate a depolarizing cationic current that may help phase-advance the contractions. Each modulator exerts these effects to different degrees and thus has a distinct effect on voltage and contraction size and shape. The overall effect then will depend on the specific combinations of modulators released in different behaviors. By understanding the modulation in the opener muscles, as well as in the ARC, we are now in a position to understand how the behavior of the two muscles is coordinated under a variety of circumstances.

Control of time-dependent biological processes by temporally patterned input

Temporal patterning of biological variables, in the form of oscillations and rhythms on many time scales, is ubiquitous. Altering the temporal pattern of an input variable greatly affects the output of many biological processes. We develop here a conceptual framework for a quantitative understanding of such pattern dependence, focusing particularly on nonlinear, saturable, time-dependent processes that abound in biophysics, biochemistry, and physiology. We show theoretically that pattern dependence is governed by the nonlinearity of the input-output transformation as well as its time constant. As a result, only patterns on certain time scales permit the expression of pattern dependence, and processes with different time constants can respond preferentially to different patterns. This has implications for temporal coding and decoding, and allows differential control of processes through pattern. We show how pattern dependence can be quantitatively predicted using only information from steady, unpatterned input. To apply our ideas, we analyze, in an experimental example, how muscle contraction depends on the pattern of motorneuron firing.

Ganglionic distribution of inputs and outputs of C-PR, a neuron involved in the generation of a food-induced arousal state in Aplysia

Cerebral neuron C-PR is thought to play an important role in the appetitive phase of feeding behavior of Aplysia. Here, we describe the organization of input and output pathways of C-PR. Intracellular dye fills of C-PR revealed extensive arborization of processes within the cerebral and the pedal ganglia. Numerous varicosities of varying sizes may provide points of synaptic inputs and outputs. Blocking polysynaptic transmission in the cerebral ganglion eliminated the sensory inputs to C-PR from stimuli applied to the rhinophores or tentacles, indicating that this input is probably mediated by cerebral interneurons. Identified cerebral mechanoafferent sensory neurons polysynaptically excite C-PR. Stimulation of the eyes and rhinophores with light depresses C-PR spike activity, and this effect also appears to be mediated by cerebral interneurons. C-PR has bilateral synaptic actions on numerous pedal ganglion neurons, and also has effects on cerebral neurons, including the MCC, Bn cells, CBIs and the contralateral C-PR. Although the somata of these cerebral neurons are physically close to C-PR, experiments using high divalent cation-containing solutions and cutting of various connectives indicated that the effects of C-PR on other cerebral ganglion neurons (specifically Bn cells and the MCC) are mediated by interneurons that project back to the cerebral ganglion via the pedal and pleural connectives. The indirect pathways of C-PR to other cerebral neurons may help to ensure that consummatory motor programs are not activated until the appropriate appetitive motor programs, mediated by the pedal ganglia, have begun to be expressed.

Giant protein kinases: domain interactions and structural basis of autoregulation

The myosin-associated giant protein kinases twitchin and titin are composed predominantly of fibronectin- and immunoglobulin-like modules. We report the crystal structures of two autoinhibited twitchin kinase fragments, one from Aplysia and a larger fragment from Caenorhabditis elegans containing an additional C-terminal immunoglobulin-like domain. The structure of the longer fragment shows that the immunoglobulin domain contacts the protein kinase domain on the opposite side from the catalytic cleft, laterally exposing potential myosin binding residues. Together, the structures reveal the cooperative interactions between the autoregulatory region and the residues from the catalytic domain involved in protein substrate binding, ATP binding, catalysis and the activation loop, and explain the differences between the observed autoinhibitory mechanism and the one found in the structure of calmodulin-dependent kinase I.

Costorage and corelease of modulatory peptide cotransmitters with partially antagonistic actions on the accessory radula closer muscle of Aplysia californica

Many neurons that contain a classical neurotransmitter also contain modulatory peptides, but it has been difficult to establish unequivocally that these peptides are functional cotransmitters. Here, we provide evidence for functional cotransmission in a neuromuscular system of Aplysia. Using immunocytochemical techniques, we localize members of two peptide families, the small cardioactive peptides (SCPs) and the buccalins (BUCs), to a single subset of dense-core vesicles in the terminals of the cholinergic motorneuron B15. We describe a new preparation and method for the direct detection of released peptides and show that the SCPs and BUCs are released when neuron B15 is intracellularly stimulated. Consistent with their subcellular localization, the SCPs and BUCs are released in a stoichiometric ratio that is constant across conditions that change the absolute amount of peptides released. Peptide release is calcium-dependent but does not require muscle contractions. Thus, the release cannot be attributed to a displacement of peptides that may be present in the extracellular space. In previous studies, we characterized the physiological firing patterns of neuron B15. Here, we simulate these firing patterns and show that peptide release occurs. Additionally, we find that significant quantities of material are released under behaviorally relevant conditions. We find that concentrations of released peptides in the muscle are in the concentration range in which exogenously applied peptides exert characterized modulatory actions on muscle contractions. Together, our findings provide strong support for the hypothesis that peptides contained in neuron B15 are functional cotransmitters.

Release of peptide cotransmitters in Aplysia: regulation and functional implications

To gain insights into the physiological role of cotransmission, we measured peptide release from cell B15, a motorneuron that utilizes ACh as its primary transmitter but also contains putative peptide cotransmitters, the small cardioactive peptides (SCPs) and the buccalins (BUCs). All stimulation parameters used were in the range in which B15 fires in freely moving animals. We stimulated neuron B15 in bursts and systematically varied the interburst interval, the intraburst frequency, and burst duration. Both peptides were preferentially released when B15 was stimulated at higher intra- or interburst frequencies or with longer burst durations. Across stimulation patterns, the amount of peptide released depended on the mean frequency of stimulation and was independent of the specific pattern of stimulation. The parameters of stimulation that produce a larger release of peptides correspond to those that evoke larger contractions. Large and frequent contractions are likely to fuse or summate, thus disrupting the rhythmic behavior mediated by the muscle innervated by motorneuron B15. Because the combined effect of the SCPs and BUCs is to accelerate the relaxation and shorten the duration of muscle contractions, these peptides reduce the probability of the disruptive fusion or summation of muscle contractions. Because these cotransmitters regulate an aspect of muscle contractions that is not controlled by acetylcholine (ACh), the primary transmitter of B15, we suggest that peptides and ACh form parallel but functionally distinct lines of transmission at the neuromuscular junction. Both types of transmission may be necessary to ensure that behavior remains efficient over a wide range of conditions.

Substrate specificity and inhibitor sensitivity of Ca2+/S100-dependent twitchin kinases

Myosin-associated giant protein kinases of the titin/witchin-like superfamily have previously been implicated in the regulation of muscle function, based on genetic and physiological studies. We find that recombinant constitutively active Caenorhabditis elegans and Aplysia twitchin kinase fragments differ in their catalytic activities and peptide-substrate specificities, as well as in their sensitivities to the naphthalene sulfonamide inhibitors 1-(5-chloronaphthalenesulfonyl)-1H-hexahydro-1,4-diazepine (ML-7) and 1-(5-iodonaphthalenesulfonyl)-1H-hexahydro-1,4-diazepine (ML-9). The constitutively active Aplysia twitchin kinase fragment has a remarkably high activity (Vmax > 100 mumol.min-1.mg-1) towards some substrate peptides. The autoinhibited forms of these twitchin kinases can be activated in a Ca(2+)-dependent manner by the dimeric form of the S100A1 protein (S100A1(2)). The twitchin kinase S100A1(2)-binding site can also bind Ca2+/calmodulin but neither kinase is activated by calmodulin. The data provide a functional basis for the ongoing crystallographic study of twitchin kinase fragments.

Synaptic mechanisms in invertebrate pattern generation

Although individual neurons can be intrinsically oscillatory and can be network pacemakers, motor patterns are often generated in a more distributed manner. Synaptic connections with other neurons are important because they either modify the rhythm of the pacemaker cell or are essential for pattern generation in the first place. Computational studies of half-center oscillators have made much progress in describing how neurons make transitions between active and inactive phases in these simple networks. In addition to characterizing phase transitions, recent studies have described the synaptic mechanisms that are important for the initiation and maintenance of activity in half-center oscillators.

A cerebral central pattern generator in Aplysia and its connections with buccal feeding circuitry

Different feeding-related behaviors in Aplysia require substantial variations in the coordination of movements of two separate body parts, the lips and buccal mass. The central pattern generators (CPGs) and motoneurons that control buccal mass movements reside largely in the buccal ganglion. It was previously thought that control of the cerebral neuronal circuitry and motoneurons that generate lip movements was coordinated directly by feedback from buccal interneurons. Here, we describe cerebral lip motoneuron C15, which drives rhythmic activity in the isolated cerebral ganglion. Other lip motoneurons are active during this program, so we define it as a cerebral motor program (CMP). The C15 in each cerebral hemiganglion drives the CMP in ipsilateral neurons only, suggesting there are independent CPGs in each hemiganglion. The cerebral and buccal CPGs interact at several points. For example, cerebral-to-buccal interneurons (CBIs), which can drive the buccal CPG, receive excitatory input when the cerebral CPG is active. Likewise, C15, which can drive the cerebral CPG, is excited when the buccal CPG is active. This excitation is simultaneous in both C15s, coupling the activity in the two hemiganglionic cerebral CPGs. Therefore, there are independent cerebral and buccal CPGs, which can produce distinct rhythms, but which interact at several points. Furthermore, the connections between the cerebral and buccal CPGs alter during different forms of motor program. We suggest that such alterations in the interactions between these CPGs might contribute to the generation of the various forms of coordination of lip and buccal mass movements that are necessary during different feeding-related behaviors.