John Koester

Further identification of neurons in the abdominal ganglion ofAplysia using behavioral criteria

This review is an updating of the paper by Frazier et at. 13 in which 30 individual cells and 8 cell clusters were identified in the abdominal ganglion of of Aplysia californica on the basis of morphological, physiological and pharmacological criteria. Since that time, a number of neurobiological studies have utilized the neurons of the abdominal ganglion for behavioral studies. Based on these new investigations, 32 additional cells and 2 additional cell clusters have been identified, bringing the total of identified cells in the ganglion to 62 individual cells and 10 cell clusters. Additional features of the previously identified cells have also emerged. Much of the new information concerns the role of identified cells in behavior. We have summarized the current list of identified cells in the abdominal ganglion emphasizing these behavioral features. We also review the known synaptic connections made by identified interneurons in the ganglion. Included are descriptions of 6 new interneurons that connect to other cells in the ganglion. A major conclusion from this survey is that behavioral criteria permit resolution between cells in identified cell clusters that could previously not be distinguished using other criteria.

Respiratory pumping: neuronal control of a centrally commanded behavior in Aplysia.

Abstract Respiratory pumping in Aplysia californica is a relatively stereotyped behavioral pattern with three components: (1) withdrawal of gill, siphon and mantle shelf; (2) closing of parapodia; (3) heart inhibition accompanied by a decrease in vasomotor tone. This phasic behavior is triggered by a central burst-generating network of interneurons in the abdominal ganglion. During respiratory pumping, motor neurons innervating the several effector organs receive a burst of either excitatory or inhibitory synaptic input which has previously been attributed to an unidentified central command cell called Interneuron II. Several of these motor cells are also concomitantly release from tonic synaptic input, which is opposite in sign to that which they receive from Interneuron II. This tonic input has been attributed to an unidentified cell called Interneuron XI. In this paper we identify and describe some of the neurons which contribute to the burst generating network; specifically, we focus on the neurons that produce the synaptic action attributed to Interneurons II and XI. The synaptic actions attributed to Interneuron XI are produced by a single, spontaneously active neuron, cell L24. This cell is a multi-action interneuron: it produces inhibitory synaptic potentials in some follower motor neurons, excitatory synaptic potentials in other follower cells, and a conjoint excitatory-inhibitory synaptic action onto gill motor neuron L7. At low frequency, L24 is excitatory to L7. With high frequency firing of L24, the synaptic potential produced in L7 converts from excitatory to inhibitory. In contrast to Interneuron XI, which is a single cell, the synaptic potentials previously attributed to Interneuron II are actually produced by a cluster of at least 3 respiratory command cells which we call L25, L26 and L27. Each of these cells accounts for only a limited portion of the synaptic input that drives the motor neurons during respiratory pumping. For most motor neurons innervated by both the respiratory command cells and Interneuron XI, the two synaptic inputs are opposite in sign. Mutually inhibitory connections between Interneuron XI and some of the central respiratory command cells ensure that the synaptic potentials from these two sources are constrained to occur at different times. Thus, centrally commanded synaptic inhibition or excitation of these motor neurons is made more effective by simultaneous disexcitation or disinhibition of Interneuron XI input. In addition to their role in generating respiratory pumping, L24 and L26 also contribute to the mediation of the defensive gill and siphon withdrawal reflex.

Adaptive changes in heart rate ofAplysia californica

SummaryVarious physiological stimuli and changes in state were found to modulate heart rate inAplysia californica. Heart rate increases following a meal, during spontaneous increases in activity, in response to an increase in temperature, to noxious or food stimuli, to moderate hypoxia and during eating. Heart rate decreases in response to extreme hypoxia and during eating in animals that are nearly satiated.The role of the abdominal ganglion in mediating these responses was investigated in chronic denervation experiments. Cutting the pericardial nerve had no effect on heart rate responses to hypoxia, noxious stimuli, or changes in temperature. The heart rate increase that follows eating was also unaffected. Denervation of the heart did reduce the heart rate increase that accompanies arousal produced by food stimuli.

Behavioral changes in aging Aplysia: A model system for studying the cellular basis of age-impaired learning, memory, and arousal

The marine mollusc Aplysia californica was used to examine the effects of age on simple forms of learning, memory, and arousal. We have found that aging impairs the long-term retention of habituation and prevents the acquisition of sensitization in the siphon withdrawal reflex. In addition, aging reduces arousal as evident in the heart rate component of the response to food stimuli. Our results are similar to the age-dependent alterations in the capacity for behavioral plasticity that have been reported in a variety of vertebrates, including man. These similarities suggest that the mechanisms underlying the effects of age on behavior and its modification may share common features across phyla and therefore might be studied to advantage in Aplysia whose central nervous system is especially accessible to cell biological approaches.

Time sharing of heart power: Cardiovascular adaptations to food-arousal inAplysia

Summary1.Food stimuli have been shown previously to elicit inAplysia an arousal state characterized by an increased tendency to bite and an increased strength of bitting. These overt behavioral changes have also been correlated with an increase in heart rate.2.In the present study, we show that systemic blood pressure increases two-fold during food arousal. Blood flow through the artery that supplies the head is also increased about two-fold. Pressure and flow both increase within 1–2 min of food presentation, and both decay back to their baseline values within about five minutes after removal of the food stimulus.3.Unlike the increase in heart rate during food arousal, the increases in blood pressure and blood flow are independent of the degree of satiation of the animal.4.The increase in blood pressure during food arousal is neurally mediated.5.During rhythmic biting, the vascular resistance of the abdominal and anterior aortae vary in an alternating pattern that is phase-locked to the biting cycle. This pattern of resistance changes is such that blood flow to digestive organs is maximal during the retraction phase of biting, whereas flow to the head is maximal during the protraction phase.6.The increase in resistance of the abdominal aorta during protraction is mediated by neuronal input to vasoconstrictor muscle.7.Strongly reducing blood flow to the head for even a few minutes significantly impairs the efficiency of biting movements.

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

SummaryThe 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 SCPB, FMRFamide, R15α1 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 (PAS)- 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 R15α1 peptide. The PAS 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 PAS neurons may prevent disturbances in blood flow due to bending or kinking of the arterial walls.

Evidence that hemolymph glucose in Aplysia californica is regulated but does not affect feeding behavior.

Hemolymph glucose increased following a meal of a commercially available dried seaweed (laver) in Aplysia californica (Aplysia). Glucose injected into the hemocoel did not affect meal size, bite latencies, swallowing rate, or 24-hr food intake. The authors found that injection of a homogenate of nerves containing a putative Aplysia insulin-like substance decreased hemolymph glucose. The nerve homogenate, however, did not affect feeding behavior. Injection of 2-deoxy-D-glucose was found to increase hemolymph glucose, an indication of gluco-privation, but instead of increasing feeding it either had no effect or, at high doses, debilitated animals and interfered with feeding. These studies suggest that glucose may be physiologically regulated in Aplysia, but it does not appear to play a role in the control of feeding behavior.

Different cholinergic synapses converging onto neurons inAplysia produce the same synaptic action

We have investigated neurons that receive inputs from more than one cholinergic interneuron, to see whether one cholinergic input can depolarize and the other hyperpolarize by virtue of activating different ACh receptors. We have examined synapses made in the abdominal ganglion of Aplysia californica by 3 cholinergic interneurons. Two of those interneurons have previously been shown to be cholinergic. Using both biochemical and pharmacological tests we have shown a third interneuron to be cholinergic. For 7 postsynaptic cells, we compared 6 new connections that we have identified from cholinergic interneurons, and 12 previously known connections. In each case we found that different cholinergic inputs onto any given cell produced the same synaptic action. This finding held even for L7, a cell known to have two types of ACh receptors. These data are consistent with the hypothesis that a postsynaptic cell does not segregate different types of ACh receptors. If generally true, this lack of segregation may help explain why a nervous system uses more than one neurotransmitter as well as why certain interneurons are needed in neural networks.