Coincident induction of long-term facilitation in Aplysia: cooperativity between cell bodies and remote synapses.

Induction of long-term synaptic changes at one synapse can facilitate the induction of long-term plasticity at another synapse. Evidence is presented here that if Aplysia sensory neuron somata and their remote motor neuron synapses are simultaneously exposed to serotonin pulses insufficient to induce long-term facilitation (LTF) at either site alone, processes activated at these sites interact to induce LTF. This coincident induction of LTF requires that (i) the synaptic pulse occur within a brief temporal window of the somatic pulse, and (ii) local protein synthesis occur immediately at the synapse, followed by delayed protein synthesis at the soma.

Differential induction of long-term synaptic facilitation by spaced and massed applications of serotonin at sensory neuron synapses of Aplysia californica.

Serotonin (5HT)-induced facilitation of synaptic transmission from tail sensory neurons (SNs) to motor neurons (MNs) in the marine mollusc Aplysia provides a cellular model of short- and long-term memory for behavioral sensitization of the tail withdrawal reflex. Synaptic facilitation at these synapses occurs in three temporal phases: short-term (STF, lasting minutes), intermediate-term (ITF, lasting more than an hour), and long-term (LTF, lasting >24 hr). STF, ITF, and LTF differ in their induction requirements: A single brief exposure of 5HT induces STF, whereas five applications are required for ITF and LTF. Moreover, STF and LTF can be induced independently. Different forms of memory often show differential sensitivity to the pattern of training trials. To begin to explore this effect at a cellular level, we examined ITF and LTF induced by one of two patterns of 5HT application: a spaced pattern (five 5-min exposures with an interval of 15 min) or a massed pattern (one continuous 25-min application). The spaced and massed patterns both induced ITF; however, spaced 5HT application was significantly more reliable at inducing LTF than was massed application. Thus, whereas induction of ITF and LTF require similar amounts of 5HT, the cellular mechanisms underlying the induction of LTF are more sensitive to the pattern of the induction trials. In the massed group, further analysis revealed a relationship between the expression of ITF and the subsequent expression of LTF, suggesting that these two processes may be mechanistically related.

Passive properties of swimmeret motor neurons.

Four different functional types of motor neurons innervate each swimmeret: return-stroke excitors (RSEs), power-stroke excitors (PSEs), return-stroke inhibitors (RSIs), and power-stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to approximately 35 microm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of -59 mV, with 25th and 75th percentiles of -66.0 and -53 mV. The median input resistance was 6.4 M omega, with 25th and 75th percentiles of 3.4 and 13.7 M omega. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.

Tests of the motor neuron model of the local pattern-generating circuits in the swimmeret system.

The motor pattern that drives each crayfish swimmeret consists of alternating bursts of impulses in power-stroke (PS) and return-stroke (RS) motor neurons. A model of the neural circuit that generates this pattern focused on connections between motor neurons themselves (Heitler, 1978, 1981). The model predicts that synergist motor neurons are electrically coupled, whereas antagonists make mostly inhibitory synapses. We tested this model by observing the responses of motor neurons to pressure ejection of GABA and glutamate, transmitters that crayfish motor neurons release at neuromuscular junctions, and by measuring the strengths and delays of synapses between pairs of motor neurons. Both GABA and glutamate inhibited motor neurons. This inhibition persisted when synaptic transmitter release was blocked by high Mg2+. The effects of GABA were mimicked by muscimol, but not by baclofen or the GABAc receptor agonist cis-4-aminocrotonic acid, and they were not blocked by bicuculline. The effects of glutamate were mimicked by ibotenic acid. Picrotoxin partially blocked glutamate's inhibition of the motor pattern, but did not affect GABA responses. Most (87%) pairs of synergist motor neurons tested made weak, noninverting connections. Approximately half of these had synaptic delays of <2 msec, consistent with direct electrical or chemical synapses. Individual motor neurons were dye-coupled to between one and three other motor neurons, and to interneurons. Less than half (44%) of the pairs of antagonist motor neurons tested made synaptic connections. These connections were weak, had long latencies (>4 msec), and therefore were probably polysynaptic. We conclude that direct synapses between swimmeret motor neurons cannot account for alternation of PS and RS bursts.

Red pigment concentrating hormone is a modulator of the crayfish swimmeret system.

The crustacean red pigment concentrating hormone (RPCH) has been localized in neurons of the crayfish abdominal nerve cord and modulates the crayfish swimmeret rhythm. An antibody to RPCH labels a small set of cell bodies and axons in each abdominal ganglion. Physiological experiments in which RPCH was perfused into the ganglia of isolated nerve cords showed that RPCH modulated the swimmeret rhythm. In nerve cords that were spontaneously producing the swimmeret rhythm, RPCH lengthened both the period and the duration of bursts of action potentials, but did not alter the phase relationships between bursts in different segments. RPCH did not initiate the swimmeret rhythm in preparations that showed intermittent or no bursting activity. We believe that RPCH is released as a neurotransmitter in the lateral neuropil, where it exerts its effects on the local swimmeret circuits.