The morphology,innervation and neural control of the anterior arterial system of Aplysia californica

Summary The morphology, innervation, and neural control of the anterior arterial system of Aplysia californica were investigated. Immunocytochemical and histochemical techniques generated positive reactions in the anterior arterial system for several neuroactive substances, including SCP_B, FMRFamide, R151 peptide, dopamine and serotonin. Three neurons were found to innervate the rostral portions of the anterior arterial tree. One is the identified peptidergic neuron R15 in the abdominal ganglion, and the other two are a pair of previously unidentified neurons, one in each pedal ganglion, named pedal arterial shorteners (P_AS)- The endogeneously bursting neuron R15 was found to innervate the proximal anterior aorta. It also innervates a branch of the distal anterior aorta, the left pedal-parapodial artery. Activity in R15 causes constriction of the left pedal-parapodial artery. This effect is presumed to direct hemolymph towards the genital groove and penis on the right side in vivo. This vasoconstrictor action of R15 is mimicked by the R151 peptide. The P_AS neuron pair causes longitudinal contraction of the rostral anterior aorta and the pedal-parapodial arteries. In vivo, the pair is active during behaviors involving head withdrawal and turning. By adjusting the length of the arteries during postural changes, the P_AS neurons may prevent disturbances in blood flow due to bending or kinking of the arterial walls.     

Effects of various environmental conditions on growth and reproduction of the sea hareAplysia oculifera (Adams and Reeve, 1850)

We studied the influences of food type, food quantity, water currents, starvation and light on growth and reproduction of the sea hareaplysia oculifera (Adams and Reeve, 1850) under laboratory conditions. Out of five species of algae served as food,Enteromorpha intestinalis promoted the fastest growth ofA. oculifera, Ulva spp. slower growth,Cladophora sp. allowed maintenance spp. slower growth,Cladophora sp. allowed maintenance of steady body mass, and the brown algaeColpomenia sp. andPadina pavonia were rejected by the sea hares. When sea hares were exposed to four levels of water currents, growth rates decreased as water currents increased. Sea hares fed on 50% ration grew slower than those fed on 100% ration (ad libitum). During 10 days of starvation sea hares lost weight, but when subsequently fed 100% ration they recovered and grew at a rate similar to those fed continuously with 100% ration. Under shade and under natural sunlight sea hares grew at the same rates. Whenever growth rates decreased, sea hares began to spawn at a smaller body size.A. oculifera demonstrated physiological plasticity that adapted them to varied and unpredictable environmental conditions. At different conditions of food availability they applied different tactics of resource allocation between growth and reproduction.     

Ultrastructural localization of egg-laying prohormone-related peptides in the atrial gland of Aplysia californica

The atrial gland is an exocrine organ that secretes into the oviduct of Aplysia californica and expresses three homologous genes belonging to the egglaying hormone gene family. Although post-translational processing of the egg-laying hormone precursor in the neuroendocrine bag cells has been examined in detail, relatively little is known about the post-translational processing of egg-laying hormone-related gene products in the atrial gland. A combination of morphologic techniques that included light-microscopic histology and immunocytochemistry, transmission electron microscopy, and immuno-electron microscopy were used to localize egg-laying hormone-related peptides in the atrial gland and to evaluate the characteristic morphology of their secretory cells. Results of these studies showed that there were at least three major types of secretory cells in the atrial gland (types 1–3). Significantly, of these three cell types, only type 1 was immunoreactive to antisera against egg-laying hormone-related precursor peptides. The immunoreactivity studies established that all three egg-laying hormone-related precursor genes are expressed in type-1 cells and indicated that the processing of these precursors also occurs within the secretory granules of this cell type. Evidence was also obtained that proteolytic processing of the egg-laying hormone-related precursors differed significantly from that observed in the bag cells. In contrast to the bag cells, the NH₂-terminal and COOH-terminal products of the egg-laying hormone-related precursors of the atrial gland were not sorted into different types of vesicles.     

Structure-activity relationship of the neurotransmitter alpha-bag cell peptide on Aplysia LUQ neurons: Implications regarding its inactivation in the extracellular space

Alpha-bag cell peptide [α-BCP (Ala-Pro-Arg-Leu-Arg-Phe-Tyr-Ser-Leu)] is a neurotransmitter that mediates bag cell-induced inhibition of left-upper-quadrant (LUQ) neurons L₂, L₃, L₄, and L₆ in the abdominal ganglion of Aplysia. Our recent biochemical studies have shown that α-BCP[1–9] is cleaved into α-BCP[1–2], [3–9], [1–5], [6–9], and [7–9] by a combination of three distinct peptidase activities located within the extracellular spaces of the CNS: A diaminopeptidase-IV (DAP-IV)-like enzyme cleaves α-BCP[1–9] at the 2–3 peptide bond; a neutral metalloendopeptidase (NEP)-like enzyme cleaves either α-BCP[1–9] or α-BCP[3–9] at the 5–6 bond; an aminopeptidase M-II (APM-II)-like enzyme cleaves α-BCP[6–9] at the 6–7 bond, but cleaves neither α-BCP[1–9], nor the other ganglionic peptidase products. To further understand the manner in which α-BCP is inactivated after release, that is loses its electro-physiological activity, we studied its structure-activity relationship by recording intracellularly from LUQ neurons in isolated abdominal ganglia that were arterially perfused with peptides dissolved in artificial sea water. The effects of α-BCP[1–9] and 15 of its fragments ([1–8], [1–7], [1–6], [1–5], [2–9], [3–9], [3–8], [6–9], [7–9], [8–9], [6–7], [6–8], [1–2], Phe, Tyr) indicated that the sequence Phe⁶-Tyr⁷ was both necessary and sufficient to produce LUQ inhibitory activity. The combined results of our electrophysiological and biochemical studies strongly suggest that α-BCP[1–9] is inactivated by the serial actions of the NEP-like and APM-II-like peptidases; that is, the NEP-like enzyme yields an electro-physiologically active product, α-BCP[6–9], that is cleaved by the APM-II-like enzyme to yield inactive α-BCP[7–9]. Furthermore, because α-BCP[6–9] is more active than α-BCP[1–9], cleavage by the NEP-like enzyme potentiates α-BCP's activity. © 1992 John Wiley & Sons, Inc.     

Identification of an abdominal ganglion neuron antagonizing L7-driven gill contraction in Aplysia

Gill motor neuron L7-induced longitudinal shortening of the gill in Aplysia kurodai and A. juliana was suppressed when extracellular stimuli were applied to a restricted dorsal central region of the abdominal ganglion. We found a neuron there which antagonized the L7-driven contraction. Since the contraction was suppressed when the identified neuron was activated simultaneously with L7, we refer to the newly found neuron as “Anti-L7”. Anti-L7 did not change the L7 impulse generation in the abdominal ganglion. No direct synaptic connection from L7 to Anti-L7 was detected. A fluorescent dye injected into the soma of Anti-L7 revealed that the neuron sent axonal branches to the branchial nerve. These results may show that Anti-L7 antagonizes L7 at the periphery inside the gill, rather than in the abdominal ganglion. EJPs induced by L7 were unaffected by Anti-L7. Activation of Anti-L7 alone did not induce any change in tone or membrane potential of the gill musculature. The suppressive effect of Anti-L7 lasts many seconds after the cessation of a train of Anti-L7 impulses. The results may suggest that the suppression is mediated through an inhibitory neuromodulatory mechanism without inhibition of L7 itself. Accepted: 1 April 1999     

Parallel processing in an identified neural circuit: the Aplysia californica gill-withdrawal response model system

The response of the gill of Aplysia calfornica Cooper to weak to moderate tactile stimulation of the siphon, the gill-withdrawal response or GWR, has been an important model system for work aimed at understanding the relationship between neural plasticity and simple forms of non-associative and associative learning. Interest in the GWR has been based largely on the hypothesis that the response could be explained adequately by parallel monosynaptic reflex arcs between six parietovisceral ganglion (PVG) gill motor neurons (GMNs) and a cluster of sensory neurons termed the LE cluster. This hypothesis, the Kupfermann-Kandel model, made clear, falsifiable predictions that have stimulated experimental work for many years. Here, we review tests of three predictions of the Kupfermann-Kandel model: (1) that the GWR is a simple, reflexive behaviour graded with stimulus intensity; (2) that central nervous system (CNS) pathways are necessary and sufficient for the GWR; and (3) that activity in six identified GMNs is sufficient to account for the GWR. The available data suggest that (1) a variety of action patterns occur in the context of the GWR; (2) the PVG is not necessary and the diffuse peripheral nervous system (PNS) is sufficient to mediate these action patterns; and (3) the role of any individual GMN in the behaviour varies. Both the control of gill-withdrawal responses, and plasticity in these responses, are broadly distributed across both PNS and CNS pathways. The Kupfermann-Kandel model is inconsistent with the available data and therefore stands rejected. There is, no known causal connection or correlation between the observed plasticity at the identified synapses in this system and behavioural changes during non-associative and associative learning paradigms. Critical examination of these well-studied central pathways suggests that they represent a 'wetware' neural network, architecturally similar to the neural network models of the widely used 'Perceptron' and/or 'Back-propagation' type. Such models may offer a more biologically realistic representation of nervous system organisation than has been thought. In this model, the six parallel GMNs of the CNS correspond to a hidden layer within one module of the gill-control system. That is, the gill-control system appears to be organised as a distributed system with several parallel modules, some of which are neural networks in their own right. A new model is presented here which predicts that the six GMNs serve as components of a 'push-pull' gain control system, along with known but largely unidentified inhibitory motor neurons from the PVG. This 'push-pull' gain control system sets the responsiveness of the peripheral gill motor system. Neither causal nor correlational links between specific forms of neural plasticity and behavioural plasticity have been demonstrated in the GWR model system. However, the GWR model system does provide an opportunity to observe and describe directly the physiological and biochemical mechanisms of distributed representation and parallel processing in a largely identifiable 'wetware' neural network.     

Ingestion motor programs of Aplysia are modulated by short-term synaptic enhancement in cerebral-buccal interneuron pathways

In the sea slug Aplysia, buccal synapses of cerebral-buccal interneurons (CBIs) CBI-2 and CBI-12 exhibit short-term synaptic enhancement (STE), including frequency-dependant facilitation and augmentation/post-tetanic potentiation (AUG/PTP). The STE that results from driving CBI-2 or CBI-12 is associated with significantly decreased latency to burst onset in buccal premotor neurons and motor neurons, increased cycle frequency of ingestion buccal motor programs (iBMPs) and increased intraburst firing frequency of buccal neurons during iBMPs. Tests of paired-pulse facilitation during AUG/PTP suggest that the locus for this plasticity is presynaptic. The AUG/PTP is not elicited by heterosynaptic pathways, indicating that its origin is homosynaptic. At low CBI-2 and CBI-12 firing frequencies, STE is likely to contribute to iBMP initiation, while at higher firing frequencies, STE is correlated with increased cycle frequency of iBMPs. Thus, STE is an important component of the mechanisms whereby cerebral neurons regulate cyclic feeding programs and likely contributes to observed variations in behavioral responses, including feeding arousal. Electronic Publication