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.

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.

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.

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.

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.

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.

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.

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.

Effect of a serotonergic extrinsic modulatory neuron (MCC) on radula mechanoafferent function in Aplysia.

The serotonergic metacerebral cells (MCCs) and homologous neurons in related mollusks have been extensively investigated within the context of feeding. Although previous work has indicated that the MCCs exert widespread actions, MCC modulation of sensory neurons has not been identified. We characterized interactions between the MCCs and a cell that is part of a recently described group of buccal radula mechanoafferents. The cell, B21, has a peripheral process in the tissue underlying the chitinous radula [the subradula tissue (SRT)]. Previous studies have shown that B21 can fire phasically during ingestive motor programs and provide excitatory drive to the circuitry active during radula closing/retraction. We now show that activity of B21 can be modulated by serotonin (5-HT) and the MCCs. Centrally, although a slow depolarization is typically recorded in B21 as a result of MCC stimulation, this depolarization does not cause B21 to spike. It can, however, increase B21 excitability enabling a pulse that was previously subthreshold to elicit an action potential in B21. B21 is in fact rhythmically depolarized during the radula closing/retraction phase of ingestive motor programs. Thus central effects of the MCCs on radula mechanoafferent activity are only likely to be apparent while B21 is receiving input from the feeding central pattern generator. Peripherally, radula mechanoafferent neurons can be activated 1) when a mechanical stimulus is applied to the biting surface of the SRT and 2) when the SRT contracts. MCC stimulation and 5-HT modulate B21 responses to both types of stimuli. For example, MCC stimulation and low concentrations of 5-HT cause subthreshold mechanical stimuli applied to the SRT to become suprathreshold. 5-HT and MCC stimulation also enhance SRT contractility. Peripheral effects of MCC activity are also likely to be phase dependent. For example, MCC stimulation does not cause B21 to respond to peripheral stimuli with an afterdischarge. Consequently, radula mechanoafferents are likely to be activated when food is present between the radula halves during radula closing/retraction but are not likely to continue to fire as opening/protraction is initiated. In a similar vein, MCC effects on the contractility of the SRT will only be apparent when contractions are elicited by motor neuron activity. SRT motor neurons are rhythmically activated during ingestive motor programs. Thus we have shown that radula mechanoafferent activity can be modulated by the MCCs and that this modulation is likely to occur in a phase-dependent manner.