Long-term facilitation and low-frequency depression in a crayfish phasic motor axon

Sexual differentiation of identified motor terminals inDrosophila larvae

In Drosophila, we have found that some of the motor terminals in wandering third-instar larvae are sexually differentiated. In three out of the four body-wall muscle fibers that we examined, we found female terminals that produced a larger synaptic response than their male counterparts. The single motor terminal that innervates muscle fiber 5 produces an EPSP that is 69% larger in females than in males. This is due to greater release of transmitter from female than male synaptic terminals because the amplitude of spontaneous miniature EPSPs was similar in male and female muscle fibers. This sexual difference exists throughout the third-instar: it is seen in both early (foraging) and late (wandering) third-instar larvae. The sexual differentiation appears to be neuron specific and not muscle specific because the same axon produces Is terminals on muscle fibers 2 and 4, and both terminals produce larger EPSCs in females than males. Whereas, the Ib terminals innervating muscle fibers 2 and 4 are not sexually differentiated. The differences in transmitter release are not due to differences in the size of the motor terminals. For the terminal on muscle fiber 5 and the Is terminal on muscle fiber 4, there were no differences in terminal length, the number of branches, or the number of synaptic boutons in males compared to females. These sexual differences in neuromuscular synaptic physiology may be related to male-female differences in locomotion.

Activity-dependent plasticity of calcium clearance from crayfish motor axons

Previous studies of a crayfish explant culture demonstrated that regenerating motor axons with high impulse activity develop more rapid clearance of cytoplasmic free Ca(2+) than those with low impulse activity. We examined whether Ca(2+) clearance in mature axons also showed activity-dependent plasticity. We studied the phasic and tonic axons of the motor bundle innervating the crayfish closer muscle that display large differences in impulse activity. To compare their Ca(2+) regulation, we applied the Ca(2+) ionophore Br-23187 (1 microM) and measured the increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)) with fura-2. After 55 min of ionophore application, the increase in [Ca(2+)](i) in the phasic axons (1,326 +/- 192 nM) was significantly greater than in the tonic axons (359 +/- 148 nM). This resulted from stronger Ca(2+) clearance in the tonic axon rather than less Ca(2+) influx because blocking Ca(2+) clearance by Na/Ca exchange and mitochondria eliminated these differences in [Ca(2+)](i). Next we determined whether Ca(2+) clearance from the phasic axon could be strengthened by a prolonged increase in impulse activity. The phasic axon was stimulated in vivo at 5 Hz for 1 h/day for 5 days, and 1-3 days after stimulation, Ca(2+) clearance was again examined. After 55 min of Br-23187 (1 microM) exposure, the increase in [Ca(2+)](i) in the stimulated phasic axon was only 232 plus minus 123 nM, which was much less than in the control phasic axons and similar to that in the tonic axons. Thus Ca(2+)-clearance mechanisms adapt to changes in impulse activity both in growing and mature axons.

Visible evidence for differences in synaptic effectiveness with activity-dependent vesicular uptake and release of FM1-43

Activity-dependent uptake and release of the fluorescent probe FM1-43 were used to compare synaptic performance (rates of transmitter release and synaptic vesicle turnover) at different frequencies in phasic and tonic motor neurons innervating the crayfish leg extensor muscle and in the tonic motor neuron of the opener muscle. The phasic extensor motor neuron, which has a high quantal content of transmitter release, accumulated and released FM1-43 more rapidly than the tonic motor neuron, especially at low frequencies of stimulation. Individual bright spots appeared on the varicosities of the junctional terminals during stimulation in FM1-43; these spots corresponded to zones of immunostaining for the synaptic vesicle associated protein synaptotagmin, but they were larger and less numerous than synapses identified by electron microscopy and appear to represent one to several synapses with their associated clusters of synaptic vesicles. The number of bright spots observed on varicosities of the tonic terminal after stimulation at >/=20 Hz is generally similar to values for responding units (n) calculated from binomial distributions derived from quantal analysis. At frequencies of </=10 Hz, bright spots did not usually appear on tonic extensor varicosities, and the quantal release patterns were best fitted with Poisson distributions. Another tonic motor neuron, the excitor of the opener muscle, showed individual bright spots at lower frequencies of stimulation, consistent with its higher quantal output at these frequencies and corresponding with the binomial fits for quantal release distributions. In this axon, the number of distinctive bright spots increased with frequency in the 2- to 20-Hz range, indicating increased participation of synapses during frequency facilitation. In the tonic extensor neuron terminals, the brightness and the size of the individual spots increased with frequency, and new foci of dye uptake appeared at the edges of preexisting spots. Relative intensity change varied considerably among individual spots during dye loading at different frequencies. Similarly, individual spots on a single tonic terminal destained at different rates when stimulated after previous loading with FM1-43. These results suggest differential performance of individual synapses or small groups of synapses, some being more effective in transmitter release than others, as inferred from previous ultrastructural and quantal analysis studies. The large overall differences between phasic and tonic synapses suggest differential regulation of transmitter release at individual synapses in the two neurons.

Motoneuron reinnervation of phasic uropod muscles in crayfish

Motoneuron reinnervation of the lateral abductor and adductor muscles in the exopodite of the crayfish uropod was obtained by cross-tying the cut proximal ends of one set of uropod nerve roots to the cut distal ends of their contralateral homologues. In normal (nonsurgically treated) animals the abductor was innervated by two excitatory motoneurons and an inhibitor while two excitors innervated the adductor. For each muscle one of the excitors produced large excitatory junction potentials (EJPs) while the other evoked small EJPs. When stimulated repetitively only the smaller EJP in the adductor generated a facilitating response. Within less than 10 weeks postsurgery the muscles were each reinnervated by two excitatory motoneurons. While the abductor motoneurons generated synaptic potentials with similar amplitudes and time courses to those of normal animals, they differed from those in normal animals in that they facilitated when stimulated repetitively. In contrast to the large and small EJPs evoked in the normal animal, the two motoneurons that reinnervated the adductor muscle elicited similar amplitude EJPs, neither of which facilitated in response to repetitive stimulation.

Modulation of swimming behavior in the medicinal leech. IV. Serotonin-induced alteration of synaptic interactions between neurons of the swim circuit

Serotonin enhances the expression of swimming in the medicinal leech Hirudo medicinalis. These two reports examine the physiological causes underlying this modulation. The initial paper (Mangan et al. 1994) demonstrated that serotonin enhanced the participation of inhibitory swim motor neurons (MNs) in the generation of the swimming rhythm in the isolated nerve cord. In experiments reported here, we examined whether synaptic interactions between neurons of the swim circuit are altered by serotonin. Following exposure to 50 microM serotonin, pairwise intracellular recording revealed the presence of a time-dependent synaptic decrement. Synaptic decrement was characterized by: 1) a substantial decline in synaptic inhibition (half-decay time about 0.4 s) during constant presynaptic excitation; 2) a reduced half-time of recovery from synaptic inhibition; and 3) a strong dependence on the presynaptic neuron's membrane potential. We found little alteration in the physiology of synaptic transmission involving MNs following amine depletion in leech nerve cords. We propose that alterations in synaptic interactions resulting from exposure to elevated serotonin levels, coupled with the changes in MN cellular properties described earlier, are crucial to the increased efficacy of MNs in participating in generating and expressing the leech swimming rhythm.

Synaptic plasticity in a regenerated crayfish phasic motoneuron

Crustacean neuromuscular systems provide many advantages for the study of synaptic transmission and plasticity. The present study examines aspects of synaptic transmission in the phasic, fast closer excitor (FCE) motoneuron of regenerated crayfish claws. Excitatory postsynaptic potentials (EPSPs) fatigued rapidly and showed poor long-term facilitation (LTF) in the smallest of regenerating claws. EPSPs in larger regenerating claws fatigued less and showed pronounced facilitation. These observations were not the same as those previously made during primary development of this motoneuron (Lnenicka and Atwood, 1985a, J. Neuroscience 5:459-467). Hence, regeneration is not the recapitulation of primary development. In situ stimulation of the FCE is known to lead to long-lasting adaptation of synaptic performance. This adaptation is age dependent; it is expressed in young but not old animals. In the regenerated FCE of old animals, we observed a novel form of long-lasting adaptation to imposed activity: EPSPs showed large initial EPSPs and did not exhibit resistance to fatigue during maintained stimulation. This indicates that aged motoneurons can express adaptive changes to increased activity following axonal regeneration, but that the adaptive changes are the opposite to what is observed in nonregenerated motoneurons.

Activity-dependent induction of facilitation, depression, and post-tetanic potentiation at an insect central synapse

In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of greater than 300 Hz, and a tonic firing rate of 10-20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (greater than 10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.