The properties of the giant cerebral serotonin-containing neurons of the opisthobranch mollusc Aplysia californica were studied and were compared to the existing data on the giant serotonin-containing neurons (metacerebral cells) of pulmonate mulluscs. Among the properties examined were: axonal distribution, synaptic input and output, pharmacological responses, biophysical characteristics, and plasticity. With only minor exceptions, the properties of the serotonin-containing neurons of Aplysia and of pulmonate molluscs were remarkably similar, and it was concluded that these identified neurons are true homologues. The establishment of the homology of the metacerebral cells of Aplysia to the metacerebral cells of pulmonate molluscs extends the known distribution of these neurons to a second major subclass (Opisthobranchiata) of molluscs. Since pulmonate and opisthobranch molluscs differ substantially in behavioral and anatomical features, the study of the metacerebral cells of these two groups may promote the understanding of the evolutionary adaptation of the nervous system to different environmental pressures.

Acetylcholine↗

PubMed1975

Potentiation of muscle contraction: a possible modulatory function of an identified serotonergic cell in Aplysia.

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Animals↗

PubMed1975

Stimulus controllability and the latent inhibition effect.

The effects of stimulus controllability on latent inhibition was examined in instrumental and classical conditioning. Rats allowed control over stimulus termination prior to avoidance conditioning exhibited no retardation of avoidance learning whereas subjects yoked to these rats with respect to pattern and amount of stimulus exposure but given no control over stimulus termination were retarded. However, groups of rats differentiated only by control over stimulus termination, prior to classical conditioning, were equally and severely retarded in the acquisition of a classically conditioned response as indexed by the CER technique. The results were discussed in terms of a learning hypothesis of latent inhibition.

Animals↗

PubMed1974

Effects of septal lesions on latent inhibition and habituation of the orienting response in rats.

In Experiment I, the latent inhibition effect; i.e., retardation of conditioned response acquisition when a previously presented but not reinforced stimulus is subsequently used as the conditioned stimulus, was investigated in normal and septal-lesioned rats. Normal animals subjected to 30 nonreinforced CS presentations prior to CS-shock pairings, were retarded in active avoidance acquisition compared to normal animals given no nonreinforced CS presentations. The same manipulation failed to produce such a difference in septal-lesioned rats. In Experiment II, while drinking, thirsty normal and septal-lesioned rats were exposed to the same stimulus which served as CS in Experiment I. The course of suppression of on-going drinking was used as a measure of orienting response habituation. Since no differences in habituation were found between normal and septal-lesioned subjects, the deficiency of septal-lesioned rats in latent inhibition cannot be due to impairment of orienting response habituation. These results, in addition to other septal deficits, were discussed with respect to a theory of septal deficiency which suggests that septal lesions result in an impairment in situations which involve a discontinuity of response and reinforecement.

Animals↗

PubMed1974

Latent inhibition: a review and a new hypothesis.

A review of research regarding the deleterious effects of stimulus preexposure upon subsequent learning is presented. The investigations are discussed with respect both to the generality of this phenomenon and variables controlling its magnitude. Complementary experiments are suggested. Theoretical interpretations are critically reviewed. It is concluded that some of the hypotheses are too narrow to explain the existing data, and some of them lead to predictions which are contrary to the existing data. A new hypothesis is proposed suggesting that during the stimulus preerxposure phase the subject learns a "zero correlation" between the stimulus, or his own behavior and other environmental contingencies. When new learning contingencies are imposed the subject must first unlearn the "zero correlation" learned during preexposure. This process of unlearning retards the learning of the new contingency.

Acoustic Stimulation↗

PubMedDate not supplied

Sequence of small cardioactive peptide A: a second member of a class of neuropeptides in Aplysia.

Previous studies have shown that the nervous system and other tissues of molluscs contain a number of peptides that potently excite molluscan hearts. Two such peptides, termed small cardioactive peptides A and B (SCPA and SCPB) are present in large quantities in the nervous system of Aplysia. These peptides are widely distributed within the CNS and peripheral tissues and have been found to be potent modulators of synaptic transmission in Aplysia. SCPB has previously been purified from nervous tissue and sequenced. In this paper, we report the purification of SCPA and propose its sequence. This sequence was confirmed by comparing the chromatographic properties of native SCPA (labelled in organ culture) with a synthetic peptide that has the proposed sequence. A significant proportion of the sequence of the two SCPs is conserved, indicating that they are members of the same peptide class, a finding that is consistent with the recent observation that the two peptide sequences are present in a single precursor.

Amino Acid Sequence↗

PubMedDate not supplied

Structure, bioactivity, and cellular localization of myomodulin B: a novel Aplysia peptide.

Important insights into mechanisms by which neuromuscular activity can be modulated have been gained by the study of experimentally advantageous preparations such as the ARC neuromuscular system of Aplysia. Previous studies have indicated that one source of modulatory input to the ARC muscle is its own two motor neurons, B15 and B16. Both of these neurons synthesize multiple peptide cotransmitters in addition to their primary neurotransmitter acetylcholine (ACh). Peptides present in the ARC motor neurons include SCPA, SCPB, buccalin A and B, and myomodulin A. We have now purified a novel neuropeptide, myomodulin B, which is structurally similar to myomodulin A. Myomodulin B is present in two identified Aplysia neurons that contain myomodulin A; the ARC motor neuron B16 and the abdominal neuron L10. Ratios of myomodulin A to myomodulin B are approximately 6:1 in both cells. Like myomodulin A, myomodulin B potentiates ARC neuromuscular activity; it acts postsynaptically, and increases the size and relaxation rate of muscle contractions elicited either by motor neuron stimulation or by direct application of ACh to the ARC. When myomodulin A is applied to the ARC in high doses (e.g., at about 10(-7) M), it decreases the size of motor neuron-elicited muscle contractions. This inhibitory effect is never seen with myomodulin B. Thus, despite the structural similarity between the two myomodulins, there exists what may be an important difference in their bioactivity.

Amino Acid Sequence↗

PubMedDate not supplied

Atherogenic diet-induced modification of the subfraction distribution of high density lipoproteins in monkeys.

High density lipoproteins (HDL) were isolated from the plasma of adult male African green monkeys fed control or atherogenic diets and were then subfractionated in a density gradient by centrifugation at 50,000 rpm, 20 degrees C for 18 hours in the Beckman VTi50 vertical rotor. Material from the gradient was pooled into six subfractions for further analysis. With increasing density across the gradient, the subfractions had progressively decreasing flotation rates, molecular weights, apo A-I/apo A-II mass ratios, lipid percentages of mass, and average sizes. Apoprotein content per particle was reasonably constant compared to lipid content. The physical and chemical properties of the HDL subfractions suggested that the gradient separated three subclasses of HDL equivalent to HDL 2b, HDL 2a, and HDL3 of human beings. When values among animals were compared, the atherogenic diet shifted the relationship between total plasma cholesterol concentration and total HDL concentration from a positive to a negative correlation. The subfractions most affected were those of d less than 1.10 g/ml (HDL2b), while the 1.10 less than d less than 1.13 g/ml (HDL2a) subfractions were affected to a lesser degree; the more responsive the animal to dietary cholesterol, the lower was the mass concentrations of these subfractions. This pattern suggested that diet-induced changes in HDL2 subfraction particle concentrations may be related to the development of atherosclerosis in African green monkeys; parallel changes in particle composition were not identified.

Animals↗