Sensory Neuron Synapses Research Articles

A Novel Postsynaptic Mechanism for Heterosynaptic Sharing of Short-Term Plasticity

Postsynaptic release of Ca(2+) from intracellular stores is an important means of cellular signaling that mediates numerous forms of synaptic plasticity. Previous studies have identified a postsynaptic intracellular Ca(2+) requirement for a form of short-term plasticity, post-tetanic potentiation (PTP) at sensory neuron (SN)-motor neuron synapses in Aplysia. Here, we show that postsynaptic IP(3)-mediated Ca(2+) release in response to a presynaptic tetanus in an SN that induces PTP can confer transient plasticity onto a neighboring SN synapse receiving subthreshold activation. This heterosynaptic sharing of plasticity represents a dynamic, short-term synaptic enhancement of synaptic inputs onto a common postsynaptic target. Heterosynaptic sharing is blocked by postsynaptic disruption of Ca(2+)- and IP(3)-mediated signaling, and, conversely, it is mimicked by postsynaptic injection of nonhydrolyzable IP(3), and by photolysis of caged IP(3) in the MN. The molecular mechanism for heterosynaptic sharing involves metabotropic glutamate receptors and Homer-dependent interactions, indicating that Homer can facilitate the integration of Ca(2+)-dependent plasticity at neighboring postsynaptic sites and provides a postsynaptic mechanism for the spread of plasticity induced by presynaptic activation. Our results support a model in which postsynaptic summation of IP(3) signals from suprathreshold and subthreshold inputs results in molecular coincidence detection that gives rise to a novel form of heterosynaptic plasticity.

Direct enhancement of presynaptic calcium influx in presynaptic facilitation at Aplysia sensorimotor synapses

Regulation of synaptic transmission by modulation of the calcium influx that triggers transmitter release underlies different forms of synaptic plasticity, and thus could contribute to learning. In the mollusk Aplysia, the neuromodulator serotonin (5-HT) increases evoked transmitter release from sensory neurons and thereby contributes to dishabituation and sensitization of defensive reflexes. We combined electrophysiological recording with fluorescence measurements of intracellular calcium in sensory neuron synapses in culture to test whether direct up-modulation by 5-HT of calcium influx triggered by single action potentials contributes to facilitation of transmitter release. We observe increases in a previously undescribed calcium influx that are strongly correlated with increases in the amplitude of the evoked postsynaptic potentials and which cannot be accounted for by action potential prolongation. Our results suggest that direct modulation of a presynaptic calcium conductance that controls neurotransmitter release contributes to the presynaptic facilitation that underlies a simple form of learning.

Mechanosensory Gating of Proprioceptor Input to Modulatory Projection Neurons

Sensorimotor gating commonly occurs at sensory neuron synapses onto motor circuit neurons and motor neurons. Here, using the crab stomatogastric nervous system, we show that sensorimotor gating also occurs at the level of the projection neurons that activate motor circuits. We compared the influence of the gastro-pyloric receptor (GPR) muscle stretch-sensitive neuron on two projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), with and without a preceding activation of the mechanosensory ventral cardiac neurons (VCNs). MCN1 and CPN2 project from the paired commissural ganglia (CoGs) to the stomatogastric ganglion (STG), where they activate the gastric mill (chewing) motor circuit. When stimulated separately, the GPR and VCN neurons each elicit the gastric mill rhythm by coactivating MCN1 and CPN2. When GPR is instead stimulated during the VCN-gastric mill rhythm, it slows this rhythm. This effect results from a second GPR synapse onto MCN1 that presynaptically inhibits its STG terminals. Here, we show that, during the VCN-triggered rhythm, the GPR excitation of MCN1 and CPN2 in the CoGs is gated out, leaving only its influence in the STG. This gating effect appears to occur within the CoG and does not result from a ceiling effect on projection neuron firing frequency. Additionally, this gating action enables GPR to either activate rhythmic motor activity or act as a phasic sensorimotor feedback system. These results also indicate that the site of sensorimotor gating can occur at the level of the projection neurons that activate a motor circuit.

Aplysia synapse associated protein (APSAP): identification, characterization, and selective interactions with Shaker‐type potassium channels

The vertebrate post-synaptic density (PSD) is a region of high molecular complexity in which dynamic protein interactions modulate receptor localization and synaptic function. Members of the membrane-associated guanylate kinase (MAGUK) family of proteins represent a major structural and functional component of the vertebrate PSD. In order to investigate the expression and significance of orthologous PSD components associated with the Aplysia sensory neuron-motor neuron synapse, we have cloned an Aplysia Dlg-MAGUK protein, which we identify as Aplysia synapse associated protein (ApSAP). As revealed by western blot, RT-PCR, and immunocytochemical analyses, ApSAP is predominantly expressed in the CNS and is located in both sensory neuron and motor neurons. The overall amino acid sequence of ApSAP is 55-61% identical to Drosophila Dlg and mammalian Dlg-MAGUK proteins, but is more highly conserved within L27, PDZ, SH3, and guanylate kinase domains. Because these conserved domains mediate salient interactions with receptors and other PSD components of the vertebrate synapse, we performed a series of GST pull-down assays using recombinant C-terminal tail proteins from various Aplysia receptors and channels containing C-terminal PDZ binding sequences. We have found that ApSAP selectively binds to an Aplysia Shaker-type channel AKv1.1, but not to (i) NMDA receptor subunit AcNR1-1, (ii) potassium channel AKv5.1, (iii) receptor tyrosine kinase ApTrkl, (iv) glutamate receptor ApGluR1/4, (v) glutamate receptor ApGluR2/3, or (vi) glutamate receptor ApGluR7. These findings provide preliminary information regarding the expression and interactions of Dlg-MAGUK proteins of the Aplysia CNS, and will inform questions aimed at a functional analysis of how interactions in a protein network such as the PSD may regulate synaptic strength.

Parallel somatic and synaptic processing in the induction of intermediate-term and long-term synaptic facilitation in Aplysia.

The induction of different phases of memory depends on the amount and patterning of training, raising the question of whether specific training patterns engage different cellular mechanisms and whether these mechanisms operate in series or in parallel. We examined these questions by using a cellular model of memory formation: facilitation of the tail sensory neuron-motor neuron synapses by serotonin (5-hydroxytryptamine, 5-HT) in the CNS of Aplysia. We studied facilitation in two temporal domains: intermediate-term facilitation (1.5-3 h) and long-term facilitation (LTF, >24 h). Both forms can be induced by using several different temporal and spatial patterns of 5-HT, including (i) repeated, temporally spaced pulses of 5-HT to both the sensory neuron soma and the sensory neuron-motor neuron synapse, and (ii) temporally asymmetric exposure of 5-HT to the soma and synapse under conditions in which neither exposure alone induces LTF. We first examined the protein and RNA synthesis requirements for LTF induced by these two patterns and found that asymmetric (but not repeated) 5-HT application induced LTF that required postsynaptic protein and RNA synthesis. We next focused on the patterning and protein synthesis requirements for intermediate-term facilitation. We found that intermediate-term facilitation (i) is induced locally at the synapse, (ii) requires multiple pulses of 5-HT, and (iii) requires synaptic protein synthesis. Our findings show that different temporal and spatial patterns of 5-HT induce specific temporal phases of long-lasting facilitation in parallel by engaging different cellular and molecular mechanisms.

Regulation of synaptic depression rates in the cricket cercal sensory system.

To assess the roles of pre- and postsynaptic mechanisms in the regulation of depression, short-term synaptic depression was characterized at the synapses between sensory neurons and two interneurons in the cricket cercal sensory system. Changes in excitatory postsynaptic potential (EPSP) amplitude with repetitive stimulation at 5 and 20 Hz were quantified and fitted to the depletion model of transmitter release. The depression rates of different sensory neuron synapses on a single interneuron varied with the age of the sensory neurons such that old sensory neuron synapses depressed faster than young synapses. Although all synapses showed depression, short-term facilitation was selectively expressed only at sensory neuron synapses on one interneuron, the medial giant interneuron (MGI). These synapses showed concurrent facilitation and depression with high-frequency stimulation (100 Hz), whereas the synapses on another interneuron, 10-3, showed only depression at all stimulus frequencies. A previous study showed that the ability of a synapse to facilitate is correlated with the identity of the postsynaptic neuron. The present results indicate that depression and facilitation are regulated independently. Depression is regulated presynaptically in a manner related to sensory neuron age; whereas, facilitation is regulated by the postsynaptic target.

Activation of ATP P2X receptors elicits glutamate release from sensory neuron synapses.

Painful stimuli to the skin initiate action potentials in the peripheral terminals of dorsal root ganglion (DRG) neurons. These action potentials propagate to DRG central terminals in the dorsal horn of the spinal cord, evoking release of excitatory transmitters such as glutamate onto postsynaptic dorsal horn neurons. P2X receptors, a family of ligand-gated ion channels activated by the endogenous ligand ATP, are highly expressed by DRG neurons. Immunoreactivity to P2X receptors has been identified in the dorsal horn superficial laminae associated with nociceptive DRG central terminals, suggesting the presence of presynaptic P2X receptors. Here we have used a DRG-dorsal horn co-culture system to show that P2X receptors are localized at presynaptic sites on DRG neurons; that activation of these receptors results in increased frequency of spontaneous glutamate release; and that activation of P2X receptors at or near presynaptic DRG nerve terminals elicits action potentials that cause evoked glutamate release. Thus activation of P2X receptors at DRG central terminals can modify sensory signal throughput, and might even initiate sensory signals at central synapses without direct peripheral input. This putative central modulation and generation of sensory signals may be associated with physiological and pathological pain sensation, making presynaptic P2X receptors a possible target for pain therapy.

A Simplified Preparation for Relating Cellular Events to Behavior: Mechanisms Contributing to Habituation, Dishabituation, and Sensitization of theAplysiaGill-Withdrawal Reflex

To relate cellular events to behavior in a more rigorous fashion, we have developed a simplified preparation for studying the gill-withdrawal reflex of Aplysia, in which it is relatively easy to record the activity of individual neurons during simple forms of learning. Approximately 84% of the reflex in this preparation is mediated through the single motor neuron LDG1, so that changes in the firing of LDG1 can account for most of the changes in behavior. We have used this preparation to investigate cellular mechanisms contributing to habituation, dishabituation, and sensitization by recording evoked firing, the complex postsynaptic potential (PSP), and the monosynaptic component of the complex PSP in LDG1. Our results suggest that habituation is largely attributable to depression at sensory neuron synapses. By contrast, dishabituation and sensitization involve several mechanisms at different loci, including facilitation at sensory neuron synapses, enhancement in the periphery (perhaps attributable to post-tetanic potentiation at the neuromuscular junction), and both facilitation and inhibition of excitatory and inhibitory interneurons. Moreover, these different mechanisms contribute preferentially at different times after training, so that information processing in the neuronal circuit for the reflex is distributed not only in space but also in time. Nonetheless, our results also suggest that the neuronal circuit is not a highly distributed neural network. Rather, plasticity of the reflex can evidently be accounted for by several specific mechanisms and loci of plasticity in a defined neural circuit, including a limited number of neurons, some of which make a large contribution to the behavior.

Pairing-specific, activity-dependent presynaptic facilitation at Aplysia sensory-motor neuron synapses in isolated cell culture

Synapses made by Aplysia sensory neurons onto motor- and interneuron followers in the intact nervous system exhibit an associative form of synaptic facilitation that is thought to contribute to classical conditioning of the animal's gill and siphon withdrawal reflex (Hawkins et al., 1983; Walters and Byrne, 1983). Here we demonstrate that a similar associative facilitation can be induced between individual sensory and motor neurons isolated in culture. Pairing tetanic stimulation with either of two facilitatory transmitters, 5-HT or small cardioactive peptide, considerably prolongs facilitation compared to either tetanus or transmitter alone. When corrected for the depression that occurs simply in response to low-frequency testing, the facilitation produced by one pairing trial does not decay for more than 20 min after training. This facilitation requires the temporal pairing (0.5 sec forward interstimulus interval) of the two stimuli, tetanus and 5-HT. Delivering the same two stimuli in an unpaired fashion (1 min forward interval) fails to produce the long-lasting effect. Measurements of spontaneous transmitter release during either paired or unpaired training reveal no changes in unitary mEPSP or mEPSC ("mini") amplitude, indicating that the facilitation involves a presynaptic mechanism. While both forms of training dramatically increase the initial frequency of spontaneous release, mini frequency does not remain elevated as long as the evoked EPSP following paired training, nor does paired training specifically enhance spontaneous release frequency. Pairing-specific facilitation was not blocked by the protein kinase C inhibitor H7. In contrast, the same training procedure produced pairing-specific increases of sensory neuron excitability and action potential width, suggesting that cAMP-mediated processes are involved in the paired effect. Although Ca2+ influx is necessary for the associative effect (Abrams, 1985), we find that the facilitation does not require influx through L-type voltage-gated Ca2+ channels, since the effect was not blocked by the dihydropyridine antagonist nitrendipine. Together, these findings indicate that the mechanism underlying associative, activity-dependent facilitation is intrinsic to the sensory neuron synapse, that it is presynaptically mediated by processes unique to evoked synaptic transmission, and that it appears to involve a pairing-specific broadening of the presynaptic action potential, allowing enhanced Ca2+ influx through the dihydropyridine-insensitive channels responsible for release.

Analysis of synaptic depression contributing to habituation of gill-withdrawal reflex in Aplysia californica.

1. Repeated stimulation of the siphon skin results in short-term habituation of the reflex contractions of the gill (38). The habituation, in turn, is correlated with a depression of the excitatory postsynaptic potentials (EPSPs) in motor neurons from mechanoreceptor sensory neurons (SN) (7, 16). The present study was undertaken to examine the parametric features of the synaptic depression and gain insights into the mechanisms underlying the reduced transmitter release. 2. Single sensory neuron action potentials were repeatedly elicited with depolarizing current pulses while the amplitude of the resultant EPSPs in the motor neuron was monitored. Synaptic depression varies as a complex function of interstimulus interval (ISI). At an ISI of 1 s, depression is rapid and reaches a plateau at 36% of control. In contrast, the depression at an ISI of 100 s is less pronounced, showing a gradual decay to 65% of control with the 10th EPSP. Surprisingly, there are no significant differences in time course or magnitude of depression across a broad range of intermediate ISIs (3, 10, and 30 s), although depression at these ISIs is intermediate between the 1 and 100 s ISIs. 3. There is also a complex relationship between spike interval and the depression of the second of two EPSPs. Thus, depression of the second of two EPSPs or depression of a train of EPSPs is not a monotonic function of spike interval. Indeed, the data suggest that there may be a slight underlying facilitatory process with short spike intervals. 4. The results also indicate that the recovery of synaptic depression following a train of 10 stimuli is not constant. Shorter spike intervals produce more rapid recovery. 5. These data are inconsistent with a classical depletion model (33) for synaptic depression and indicate that either a single complex function of time and ISI or multiple functions underlie synaptic depression and its recovery at the sensory neuron synapse.