Synaptic connections in the buccal ganglia of Aplysia mediated by an identified neuron containing a CCK/gastrin-like peptide co-localized with acetylcholine

Distribution, cellular localization, and colocalization of several peptide neurotransmitters in the central nervous system of Aplysia

Neuropeptides are widely used as neurotransmitters in vertebrates and invertebrates. In vertebrates, a detailed understanding of their functions as transmitters has been hampered by the complexity of the nervous system. The marine mollusk Aplysia, with a simpler nervous system and many large, identified neurons, presents several advantages for addressing this question and has been used to examine the roles of tens of peptides in behavior. To screen for other peptides that might also play roles in behavior, we observed immunoreactivity in individual neurons in the central nervous system of adult Aplysia with antisera raised against the Aplysia peptide FMRFamide and two mammalian peptides that are also found in Aplysia, cholecystokinin (CCK) and neuropeptide Y (NPY), as well as serotonin (5HT). In addition, we observed staining of individual neurons with antisera raised against mammalian somatostatin (SOM) and peptide histidine isoleucine (PHI). However, genomic analysis has shown that these two peptides are not expressed in the Aplysia nervous system, and we have therefore labeled the unknown peptides stained by these two antibodies as XSOM and XPHI There was an area at the anterior end of the cerebral ganglion that had staining by antisera raised against many different transmitters, suggesting that this may be a modulatory region of the nervous system. There was also staining for XSOM and, in some cases, FMRFamide in the bag cell cluster of the abdominal ganglion. In addition, these and other studies have revealed a fairly high degree of colocalization of different neuropeptides in individual neurons, suggesting that the peptides do not just act independently but can also interact in different combinations to produce complex functions. The simple nervous system of Aplysia is advantageous for further testing these ideas.

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.

Outputs of radula mechanoafferent neurons in Aplysia are modulated by motor neurons, interneurons, and sensory neurons

The gain of sensory inputs into the nervous system can be modulated so that the nature and intensity of afferent input is variable. Sometimes the variability is a function of other sensory inputs or of the state of motor systems that generate behavior. A form of sensory modulation was investigated in the Aplysia feeding system at the level of a radula mechanoafferent neuron (B21) that provides chemical synaptic input to a group of motor neurons (B8a/b, B15) that control closure and retraction movements of the radula, a food grasping structure. B21 has been shown to receive both excitatory and inhibitory synaptic inputs from a variety of neuron types. The current study investigated the morphological basis of these heterosynaptic inputs, whether the inputs could serve to modulate the chemical synaptic outputs of B21, and whether the neurons producing the heterosynaptic inputs were periodically active during feeding motor programs that might modulate B21 outputs in a phase-specific manner. Four cell types making monosynaptic connections to B21 were found capable of heterosynaptically modulating the chemical synaptic output of B21 to motor neurons B8a and B15. These included the following: 1) other sensory neurons, e.g. , B22; 2) interneurons, e.g., B19; 3) motor neurons, e.g., B82; and 4) multifunction neurons that have sensory, motor, and interneuronal functions, e.g., B4/5. Each cell type was phasically active in one or more feeding motor programs driven by command-like interneurons, including an egestive motor program driven by CBI-1 and an ingestive motor program driven by CBI-2. Moreover, the phase of activity differed for each of the modulator cells. During the motor programs, shifts in B21 membrane potential were related to the activity patterns of some of the modulator cells. Inhibitory chemical synapses mediated the modulation produced by B4/5, whereas excitatory and/or electrical synapses were involved in the other instances. The data indicate that modulation is due to block of action potential invasion into synaptic release regions or to alterations of transmitter release as a function of the presynaptic membrane potential. The results indicate that just as the motor system of Aplysia can be modulated by intrinsic mechanisms that can enhance its efficiency, the properties of primary sensory cells can be modified by diverse inputs from mediating circuitry. Such modulation could serve to optimize sensory cells for the different roles they might play.

Distribution of myomodulin-like and buccalin-like immunoreactivities in the central nervous system and peripheral tissues of the mollusc, Clione limacina

The distribution of the myomodulin-like and buccalin-like immunoreactivities in the central nervous system and peripheral tissues associated with feeding was examined in the pteropod mollusc Clione limacina by using wholemount immunohistochemical techniques. Immunoreactive neurons and cell clusters were located in all central ganglia except the pleural ganglia, with approximately 50 central neurons reactive to myomodulin antiserum and 60 central neurons reactive to buccalin antiserum. All central ganglia contained a dense network of myomodulin- and buccalin-immunoreactive processes in their neuropil regions and connectives. In the periphery, the primary attention was focused on the tissues associated with feeding, especially feeding structures unique to Clione, such as hook sacs and buccal cones, which are used for prey capture and acquisition. All of these feeding structures contained myomodulin-immunoreactive and buccalin-immunoreactive fibers, with each peptide family showing specific innervation fields that were common in buccal cones and were totally different in the hook sacs. The specific central and peripheral distribution of myomodulin-like and buccalin-like immunoreactivities as well as specific effects of the exogenous peptides on identified neurons involved in the control of feeding behavior and swimming suggest that neuropeptides from myomodulin and buccalin families act as neurotransmitters or neuromodulators in a variety of central circuits and in the peripheral neuromuscular systems associated with feeding in Clione limacina.

Immunocytological and biochemical localization and biological activity of the newly sequenced cerebral peptide 2 in Aplysia

Cerebral peptide 2 (CP2), a 41 amino acid neuropeptide, was identified because it was transported from the cerebral ganglia of Aplysia to other central ganglia. Immunocytology indicates that CP2 is distributed widely in the CNS and peripheral tissues of Aplysia. Most CP2-immunoreactive neurons were found in the cerebral ganglia and extensively overlap with the distribution of cerebral peptide 1 (CP1). HPLC analyses confirm that individual cerebral neurons synthesize both CP1 and CP2. In other ganglia, CP1 and CP2 are localized predominantly to different neurons. CP2-immunoreactive fibers and varicosities are present in the neuropil of all ganglia but were found surrounding cell bodies and axon hillocks most often in the buccal and abdominal ganglia. Thus, the effects of CP2 on neurons in these ganglia were determined using intracellular recording. In the buccal ganglia, CP2 evokes rhythmic activity in many motor neurons that seems similar to that observed during ingestion; however, only one identified neuron was found to be depolarized directly. By contrast, in the abdominal ganglion, many neurons are depolarized directly by CP2. A number of these have been shown to be part of the circuit that regulates respiratory pumping. Injection of CP2 into freely behaving Aplysia increases the rate of respiratory pumping and causes other changes in behavior. CP2 is stable in hemolymph, which raises the possibility that it may act as a hormone. Thus, CP2 is a bioactive neuropeptide that is present in many neurons and likely functions as a transmitter or a hormone.

Distribution of buccalin-like immunoreactivity in the central nervous system and peripheral tissues of Aplysia californica

The neuropeptide buccalin A was originally purified and sequenced from a nerve-muscle system used in feeding-related behaviors of Aplysia californica in which it has been proposed that it acts as a modulatory cotransmitter. The distribution of buccalin-like immunoreactivity in the central ganglia and in peripheral tissues of Aplysia californica was examined by whole mount immunohistochemical techniques. Immunoreactive material was located in specific cell bodies and clusters of neurons in each of the ganglia. Immunoreactive fibers were present in each of the connectives between ganglia, in tracts coursing through the ganglia, and in the majority of the peripheral nerves. Most fibers were smooth in contour, but some had regularly spaced swellings. Varicosities containing immunoreactive material were located on specific neuronal somata and on certain tissues associated with the feeding, circulatory, digestive, and reproductive systems. The specific and widespread distribution of buccalin-like immunoreactivity supports the hypothesis that members of the buccalin peptide family act as neuromodulators or neurotransmitters in a variety of central and peripheral circuits in Aplysia.

Peptidergic motoneurons in the buccal ganglia of Aplysia californica: immunocytochemical, morphological, and physiological characterizations

We used physiological recordings, intracellular dye injections and immunocytochemistry to further identify and characterize neurons in the buccal ganglia of Aplysia californica expressing Small Cardioactive Peptide-like immunoreactivity (SCP-LI). Neurons were identified based upon soma size and position, input from premotor cells B4 and B5, axonal projections, muscle innervation patterns, and neuromuscular synaptic properties. SCP-LI was observed in several large ventral neurons including B6, B7, B9, B10, and B11, groups of s1 and s2 cluster cells, at least one cell located at a branch point of buccal nerve n2, and the previously characterized neurons B1, B2 and B15. B6, B7, B9, B10 and B11 are motoneurons to intrinsic muscles of the buccal mass, each displaying a unique innervation pattern and neuromuscular plasticity. Combined, these motoneurons innervate all major intrinsic buccal muscles (I1/I3, I2, I4, I5, I6). Correspondingly, SCP-LI processes were observed on all of these muscles. Innervation of multiple nonhomologous buccal muscles by individual motoneurons having extremely plastic neuromuscular synapses, represents a unique form of neuromuscular organization which is prevalent in this system. Our results show numerous SCPergic buccal motoneurons with widespread ganglionic processes and buccal muscle innervation, and support extensive use of SCPs in the control of feeding musculature.

Evolutionary aspects of transmitter molecules, their receptors and channels

Classical transmitters are present in all phyla that have been studied; however, our detailed understanding of the process of neurotransmission in these phyla is patchy and has centred on those neurotransmitter receptor mechanisms which are amenable to study with the tools available at the time, for example, high-affinity ligands, tissues with high density of receptor protein, suitable electrophysiological recording systems. Studies also clearly show that many neurones exhibit co-localization of classical transmitters and neuropeptides. However, the physiological implications of this co-localization have yet to be elucidated in the vast majority of examples. The application of molecular biological techniques to the study of neurotransmitter receptors (to date mainly in vertebrates) is contributing to our understanding of the evolution of these proteins. Striking similarities in the structure of ligand-gated receptors have been revealed. Thus, although ligand-gated receptors differ markedly in terms of the endogenous ligands they recognize and the ion channels that they gate, the structural similarities suggest a strong evolutionary relationship. Pharmacological differences also exist between receptors that recognize the same neurotransmitter but in different phyla, and this may also be exploited to further the understanding of structure-function relationships for receptors. Thus, for instance, some invertebrate GABA receptors are similar to mammalian GABAA receptors but lack a modulatory site operated by benzodiazepines. Knowledge of the structure and subunit composition of these receptors and comparison with those that have already been elucidated for the mammalian nervous system might indicate the functional importance of certain amino acid residues or receptor subunits. These differences could also be exploited in the development of new agents to control agrochemical pests and parasites of medical importance. The study of the pharmacology of receptor proteins for neurotransmitters in invertebrates, together with the application of biochemical and molecular biological techniques to elucidate the structure of these molecules, is now gathering momentum. For certain receptors, e.g. the nicotinic receptor, we can expect to have fundamental information on the function of this receptor at the molecular level in both invertebrates and vertebrates in the near future.