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Multiple biological signals and mechanisms regulate the eﬃcacy of synaptic transmission, thus providing rich combinatorial possibilities for modifying neural communication. These processes have been studied particularly well in invertebrate preparations, whose large neurons and relatively simple neural circuits permit detailed mechanistic and functional analyses.
1. Multiple Processes Regulate Synaptic Eﬃcacy
The processes regulating synaptic eﬃcacy vary along several dimensions, including (a) the nature of the change itself (e.g., an increase or decrease in eﬃcacy); (b) mechanisms of induction —that is, the nature of the neural signals that initiate the synaptic change; (c) mechanisms of expression, the proximal causes of synaptic changes; (d) mechanisms of maintenance, the steps involved in their persistence; and (e) time course, ranging from transient forms of regulation lasting only a few seconds, to relatively permanent long-term changes persisting a day or more.
2. Simple Forms Of Synaptic Regulation
2.1 Decreases And Increases Of Synaptic Eﬃcacy
Simple forms of synaptic regulation include decreases and increases in synaptic eﬃcacy. Synaptic depression is typically expressed as a decrease in the amplitude or other parameters of a postsynaptic potential (PSP) at a given synaptic connection (Fig. 1). Other things being equal, the larger the synaptic potential, the greater the inﬂuence a presynaptic neuron exerts on its postsynaptic target neuron. Hence, in synaptic depression, the presynaptic neuron exerts less control over its postsynaptic target. Facilitation, enhancement, augmentation, and potentiation all refer to an increase in synaptic strength, often expressed as an increase in PSP amplitude (Fig. 1).
Synaptic regulation has been studied most extensively at chemical synapses. However, changes in the eﬃcacy of electrical synapses can also occur.
2.2 Mechanisms Of Induction
A separate but related issue is the type and locus of cellular activity necessary to induce, or trigger, the change in synaptic eﬃcacy.
Homosynaptic changes arise from activity that occurs at the same (‘homo’) synapse that is modiﬁed. Thus, homosynaptic depression is a decrease in synaptic strength (‘depression’) that arises from activity at that synapse (hence, ‘homosynaptic’). In contrast, heterosynaptic changes arise from activity in neurons other than those being modiﬁed. Both are instances of non-associative induction mechanisms, in that only one type or locus of neural activity is involved in triggering the synaptic change.
In contrast, conjunctive or associative mechanisms of synaptic regulation depend on activity in more than one cell. Associate mechanisms are of particular interest from a theoretical perspective, because they involve a more advanced regulatory logic: changes in synaptic strength occur if and only if two conditions are met.
2.3 Regulation Of Synaptic Eﬃcacy In The Aplysia Siphon-Withdrawal Circuit
The following four examples illustrate these diﬀerent forms of induction as well as certain other aspects of synaptic regulation in the context of the gilland siphon-withdrawal circuit of the marine mollusk Aplysia (Fig. 2).
As detailed by Eric Kandel and colleagues (Kandel and Schwartz 1982, Kandel and Pittenger 1999), the Aplysia gilland siphon-withdrawal reﬂex to a tactile siphon stimulus is mediated in part by the LE siphon sensory neurons, which connect both directly and indirectly to LFS and other siphon and gill motor neurons. The strength of LE sensorimotor connections can be modiﬁed in several ways. Similar changes have also been found at connections of Aplysia tail mechanosensory neurons.
2.3.1 Homosynaptic Depression And Homosynaptic Facilitation. Low-frequency activation of LE siphon sensory neurons depresses transmitter release at sensorimotor connections and hence also depresses the postsynaptic response (Fig. 1; Castellucci and Kandel 1976). Functionally, homosynaptic depression contributes to behavioral habituation of the gill-and siphon-withdrawal reﬂex. The siphon is a small ﬂeshy spout that expels seawater from the mantle cavity. A tactile stimulus to the siphon causes both the gill and the siphon to withdraw. After repeated taps of the siphon, however, the amplitude and duration of the reﬂex diminish. Habituation is an important and ubiquitous form of learning that enables an organism to screen out stimuli of little consequence.
In addition to homosynaptic depression, Aplysia sensorimotor connections exhibit post-tetanic potentiation (PTP) in response to high-frequency activation, which is due at least in part to homosynaptic facilitation (Clark and Kandel 1984).
2.3.2 Heterosynaptic Facilitation And Heterosynaptic Inhibition. In Aplysia siphon-withdrawal circuits, strong noxious stimuli (e.g., tail shock) activate a group of facilitatory interneurons (e.g., L29 cells) that synapse onto LE siphon sensory cells. Activation of facilitatory interneurons modiﬁes sensory cells and increases their transmitter release via multiple mechanisms. Because this increase in synaptic eﬃcacy is regulated by an extrinsic input, it is an example of heterosynaptic facilitation (Fig. 1; Castellucci and Kandel 1976). Behaviorally, presynaptic facilitation contributes to sensitization of the siphon-withdrawal reﬂex, increasing its amplitude and duration. With very strong stimuli, heterosynaptic inhibition can also be induced.
2.3.3 aCTIVITY-DEPENDENT ENHANCEMENT OF PRESYNAPTIC FACILITATION. In this associative mechanism (also termed activity-dependent neuromodulation and activity-dependent extrinsic modulation), activity in sensory neurons enhances the heterosynaptic presynaptic facilitation normally evoked by facilitatory interneurons (Hawkins et al. 1983). The facilitation is greater than can be accounted for by a simple combination of a homosynaptic increase (PTP arising from sensory neuron activity) and heterosynaptic facilitation (arising from activation of facilitatory interneurons) and thus is associative. This mechanism is believed to contribute to classical conditioning of the siphon-withdrawal reﬂex, in which a siphon tap conditioned stimulus (CS) that activates siphon sensory neurons is paired with a tail shock unconditioned stimulus (US) that activates neuromodulatory facilitatory interneurons. Such pairings result in greater synaptic facilitation and a greater increase in the siphon-withdrawal response than that produced by unpaired CS and US presentations.
2.3.4 Hebbian Facilitation. Hebbian facilitation (Hebb 1949) also contributes to increases in synaptic eﬃcacy at LE-LFS connections (Glanzman 1995). The critical inductive signal for Hebbian facilitation is conjoint activity of the presynaptic sensory neuron and activity or large depolarization of the post- synaptic motor neuron. The associative properties presumably arise from a glutamate N-methyl-D -aspartate (NMDA)-type receptor, which acts as a molecular ‘AND-gate’ that requires both neurotransmitter binding and large postsynaptic depolarization (t o remove Mg block of the channel) in order for Ca2+ inﬂux to occur through the channel and trigger synaptic change. Behaviorally, Hebbian facilitation also contributes to classical conditioning of the reﬂex. The tactile siphon CS activates siphon sensory neurons, and the tail shock US activates and depolarizes siphon motor neurons, thereby providing the key conjunctive events.
2.4 Expression, Maintenance, And Time Course Of Synaptic Regulation
In principle, increases and decreases in synaptic eﬃcacy can result from either a presynaptic expression mechanism (e.g., a decrease in transmitter release from the presynaptic cell), or a postsynaptic expression mechanism (e.g., a decrease in the sensitivity or number of transmitter receptors on the postsynaptic target cell), or both.
Although the mechanisms for expression and for maintenance and duration of synaptic regulation are logically and biologically separable, they are often functionally related and will be considered together here. Four broad categories of mechanisms allow the regulation of synaptic eﬃcacy to span a wide range of temporal domains: (a) transient regulation, which is voltage- and Ca2+-dependent, lasting milliseconds to seconds; (b) short-term regulation, involving second- messenger mediated covalent modiﬁcations of preexisting proteins, typically lasting minutes to an hour; (c) intermediate-term regulation, dependent on protein synthesis, persisting several hours; and (d) long-term regulation, dependent on both RNA synthesis and protein synthesis, persisting for hours and days (Fig. 3).
2.4.1 Transient, Voltage- And Ca2+-Dependent Presynaptic Regulation. As demonstrated by studies at the squid giant synapse, which permits intracellular stimulation and recording from both presynaptic and postsynaptic elements, transmitter release is regulated by the voltage of the presynaptic terminal, which in turn inﬂuences the basal concentration of intracellular Ca2+, the ion responsible for transmitter release. Depolarization of the presynaptic terminal elevates intracellular Ca2+ concentrations and increases release, whereas hyperpolarization decreases intracellular Ca2+ concentrations and decreases release. Axo-axonic synapses (connections from one neuron onto presynaptic terminals of another) can depolarize or hyperpolarize presynaptic terminals, thereby regulating release and producing presynaptic facilitation or inhibition, respectively. This form of synaptic regulation is typically transient, as it is dependent on the continuation of the aﬀerent axoaxonic input, so it allows a moment-to-moment gating of information ﬂow. Homosy naptic spike activity can also inﬂuence residual Ca2+ levels and thus modify transmitter release relatively directly.
2.4.2 Short-Term Regulation Via Second-Messenger Mediated Covalent Modiﬁcations. Short-term regulation of synaptic eﬃcacy, lasting minutes to approximately an hour, typically involves second-messenger cascades and the subsequent covalent modiﬁcation of pre-existing proteins, often via kinase-dependent phosphorylation. For example, in short-term facilitation at Aplysia sensorimotor connections, facilitatory interneurons activate both the cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) in sensory cells. In the cAMP-dependent cascade, the facilitatory transmitter (e.g., serotonin), the ‘ﬁrst messenger,’ binds to a receptor coupled to G-protein that activates the enzyme adenylyl cyclase. Adenylyl cyclase activation converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP), the ‘second messenger.’ cAMP in turn activates PKA by binding to the regulatory subunits of the kinase, causing them to dissociate from the catalytic subunits. The freed catalytic subunits then phosphorylate the substrate proteins, invoking conformational and functional changes. Similarly, in the PKC cascade, serotonin activates phospholipase C via a separate G-protein coupled receptor, which in turn elevates the levels of the second messenger diacylglycerol and activates PKC. Together, these two second-messenger cascades produce a broadening of sensory neuron action potentials, enhanced Ca + inﬂux, and an enhancement of spike-release coupling, thereby increasing transmitter release.
Second-messenger cascades modify a variety of substrate proteins, allowing for coordinated regulation of multiple steps involved in synaptic transmission. Because substrate phosphorylation persists even after the brief input from facilitatory interneurons terminates, synaptic facilitation is maintained beyond the presence of sensitizing stimulus itself. However, biochemical mechanisms for substrate dephosphorylation also exist. Hence, covalent modiﬁcations of substrate proteins by themselves are not well suited for more permanent maintenance of synaptic changes.
2.4.3 Intermediate-Term And Long-Term Regulation Of Synaptic Eﬃcacy Involve Translation And Transcription. Intermediate-term (lasting hours) and long-term forms of synaptic regulation (lasting a day or more) typically require the recruitment of additional steps to those implicated in short-term regulation. The relationship among these diﬀerent forms of cellular memory has been particularly well studied at Aplysia sensorimotor connections. Unlike short-term facilitation, certain forms of intermediate-term facilitation require the synthesis of new proteins (translation of mRNA to protein). Long-term facilitation involves not only protein synthesis, but also the modiﬁcation of gene expression (transcription of DNA to RNA) (Montarolo et al. 1986). These forms of synaptic regulation are thus distinct on a mechanistic as well as phenomenological level.
With repeated sensitizing stimuli or their analogs (e.g., serotonin application) used to induce long-term facilitation, PKA phosphorylates not only substrate proteins involved in short-term facilitation at the synapse, but also the transcriptional activator cAMPresponse element binding protein (CREB) in the sensory neuron nucleus. CREB then binds to and activates the promoter element CRE (the cAMP response element), inducing gene expression. The new gene products involve not only ‘eﬀector’ molecules directly involved in the expression of long-term facilitation, but also additional transcription factors, such as C EBP, that regulate the expression of other genes, thereby producing a cascade of gene regulation. Interruption of this gene cascade at early time points by inhibitors of RNA or protein synthesis blocks subsequent long-term facilitation, but leaves short-term facilitation intact. In contrast, inhibition of RNA and protein synthesis at later time points has little eﬀect. Thus the dynamics of macromolecular synthesis in long-term facilitation mimic the time-sensitivity of long-term memory itself. cAMP and CREB have been implicated in neural substrates of memory in other systems, such as Drosophila.
Just as short-term facilitation involves the coordinated facilitation of several processes involved in transmitter release, long-term facilitation involves multiple eﬀector genes. Enhanced expression of the enzyme ubiquitin carboxyterminal hydrolase promotes the degradation of the inhibitory regulatory subunits of PKA, thereby making the kinase persistently active, even after cAMP levels return to baseline. Other gene products lead to the growth of new sensory neuron synaptic terminals (which does not occurn short-term facilitation), thus providing a stable, enduring substrate for long-term memory. Structural changes in sensory neurons are accompanied by corresponding growth in postsynaptic targets, providing further evidence for the coordinated regulation of synaptic eﬃcacy on multiple levels.
3. Functional Consequences Of Simple Synaptic Regulation
3.1 Modulation Of Input–Output Pathways
The functional consequences of simple forms of synaptic regulation can be relatively straightforward in circuits where there is a relatively uniform direction of information ﬂow from input to output. For example, as described above, decreases in synaptic strength at Aplysia sensorimotor connections contribute to decreases in the siphon-withdrawal response (e.g., habituation), and increases in synaptic strength contribute to increases in the siphon-withdrawal response (e.g., sensitization and classical conditioning).
3.2 Reconﬁguration Of Multifunctional Networks
In other instances, however, even relatively simple forms of synaptic change can have more complex functional consequences. Many neural networks (or their components) are multifunctional networks, participating in more than one behavior. Because the operation of such networks depends on interactions among multiple non-linear processes at the cellular, synaptic, and network levels, regulation of synaptic eﬃcacy can profoundly alter network operation, reconﬁguring the network into functionally distinct modes.
3.2.1 Escape Swimming In Tritonia. One interesting example is the reconﬁguration of a premotor network from a resting reﬂex withdrawal circuit into a rhythmic central pattern generator (CPG) swim circuit in the marine mollusk Tritonia (Katz and Frost 1996). None of the three major interneuron types in this circuit have endogenous bursting properties; the oscillation of the network instead arises from, and hence depends upon, their synaptic interactions. The dorsal swim interneurons (DSIs) are essential elements of the CPG circuit for escape swimming. DSI activity contributes directly to the generation of the swim motor program; in addition, it rapidly enhances the synaptic eﬃcacy and excitability of a second interneuron, C2. C2 has been implicated in the reconﬁguration mechanism; its activity disinhibits the DSI neurons, enabling rhythmic dorsal ﬂexions. Thus, the rapid enhancement of C2 contributions may functionally rewire the network, allowing the reconﬁguration from simple reﬂex withdrawal to rhythmic swimming to occur. Because the regulation of C2 synaptic eﬃcacy develops and decays dynamically with DSI activity, it may also play a role in the cycle-by-cycle generation of the swim rhythm itself.
3.2.2 Cpgs In Lobster Stomatogastric Ganglion. The gastric mill CPG and pyloric CPG in the lobster stomatogastric ganglion are also polymorphic networks that can be functionally reconﬁgured to produce fundamental variations in output under diﬀerent circumstances (Harris-Warwick and Marder 1991). Such alterations arise both from regulation of synaptic eﬃcacy in the circuit and from changes in the intrinsic properties of the component neurons. For example, stimulation of the anterior pyloric modulator (APM) neuron activates the pyloric motor pattern by enhancing the plateau potential capabilities of pyloric CPG neurons. The platean maintains neurons in a depolarized state and so allows them to ﬁre repetitively. As a consequence, a brief excitatory synaptic input that was previously ineﬀective can trigger a transition of the neuron to the tonically active plateau condition. In the plateau state, neurons also actively resist hyperpolarization, thus rendering inhibitory inputs less eﬀective. The combined enhancement of synaptic excitation and diminution of inhibition result in a more active network.
Modulatory transmitters produce bursting pacemaker potentials in the anterior burster (AB) neurons, the most important pacemaker of the pyloric CPG. Synaptic interactions within the stomatogastric ganglion are mediated partly by non-spiking release, which varies continuously with membrane potential. Hence, larger bursting potentials enhance transmitter release from AB terminals, increasing AB inhibition of certain postsynaptic CPG targets and removing them from the functional circuit, thereby altering the pyloric network and rhythm. Because diﬀerent modulatory amines exert diﬀerent eﬀects on AB bursts, the anatomical circuit can be sculpted in a variety of ways to yield a number of diﬀerent functional networks.
4. Qualitative Regulation Of Synaptic Eﬃcacy
The decreases and increases in synaptic eﬃcacy described above involve primarily quantitative modiﬁcations of synaptic strength, rendering synapses either more or less potent. However, regulation of synaptic eﬃcacy can also involve a number of qualitative changes in synaptic transmission, thus extending the range of adaptive mechanisms that the nervous system utilizes.
4.1 Regulation Of Transmitter Type
An important additional form of synaptic regulation involves changes in the type, rather than the amount, of transmitter that a neuron releases. Such changes can arise either from shifts in the relative release of diﬀerent co-transmitters resulting from diﬀerent rates and patterns of neural ﬁring, or from changes in transmitter synthesis (Simerly 1990). Release of one co-transmitter or another can produce qualitative changes in the overall functional eﬀect of synaptic transmission on a particular postsynaptic target (e.g., from excitation to inhibition), rather than simple quantitative changes in a pre-existing synaptic response. Regulation of the transmitter type can also eﬀectively add or remove a particular postsynaptic pathway from operation, thus allowing functional rewiring of the neural circuit without the need for anatomical reorganization. In principle, similar molecular switching might be accomplished by regulation of the relative number or sensitivities of diﬀerent postsynaptic receptors.
4.1.1 Lobster Stomatogastric Ganglion. Changes in co-transmitter release aﬀect the pyloric CPG in the lobster stomatogastric ganglion. The inferior ventricular nerve (IVN) cells evoke both excitatory and inhibitory responses in the AB neurons, the most important pacemaker of the pyloric CPG, as described above. At low frequencies of IVN activation, one (unknown) excitatory transmitter is released, and the frequency of the pyloric rhythm increases. In contrast, at higher frequencies, the inhibitory transmitter histamine is preferentially released, and the pyloric rhythm is disrupted. In a variation on this theme, the neuromodulatory transmitter dopamine changes the sign of the synaptic connection between certain identiﬁed neurons in the CPG from inhibitory to excitatory by reducing the eﬃcacy of the inhibitory synapse and uncovering a weak electrical connection.
4.1.2 Feeding In Aplysia. Diﬀerential release of cotransmitters may also provide for intrinsic self-regulation of a network. For example, the motor neurons in the feeding circuit of Aplysia use a variety of peptide co-transmitters that modulate neuromuscular transmission, extending the dynamic range over which the muscles can operate eﬀectively. High frequency activation of motor neuron B15 enhances the co-release of small cardioactive peptide B. This neuromodulatory peptide increases the amplitude and quickens relaxation time of the muscle, thereby enabling larger muscle contractions to be driven at a higher rate. Similarly, in the leech, the myogenic heart muscle is innervated by heart excitatory motor neurons that co-release acetylcholine and FMRFamide, a modulatory peptide that regulates amplitude and duration of heart beat. Amines also modulate the amplitude of potentials at many crustacean neuromuscular junctions.
4.2 Regulation Of Synaptic Plasticity: Metaplasticity
An additional form of synaptic regulation, recently termed ‘metaplasticity’ (Abraham and Bear 1996), is manifested as a modiﬁcation of the ability of the synapse to undergo changes in synaptic eﬃcacy, rather than as a change in synaptic eﬃcacy per se. For example, long-term potentiation (LTP) not only increases the population spike evoked in dentate granule cells of mammalian hippocampus by aﬀerent stimulation, it also reduces non-linear response properties so that synaptic eﬃcacy is less aﬀected by the recent history of activity at that synapse. LTP thus increases the ﬁdelity as well as the gain of the system (Berger and Sclabassi 1985). Regulation of more longlasting forms of synaptic plasticity also occurs at hippocampal synapses, involving, for example, shifts in the tendency of synapses to exhibit LTP or long-term depression (LTD).
Metaplasticity has recently been implicated at identiﬁed inhibitory synapses of the siphon-withdrawal circuit of Aplysia (Fischer et al. 1997). The inhibitory interneuron neuron L30 exhibits multiple forms of synaptic enhancement that control the gain of the siphon-withdrawal reﬂex, including frequency facilitation during initial phases of high-frequency activation and synaptic augmentation and potentiation that persist after activation. Tail shock reduces the baseline strength of L30 connections onto L29 cells; it also attenuates augmentation potentiation while leaving frequency facilitation relatively intact. Behaviorally, tail shock decreases the inhibition normally produced by cutaneous stimuli, perhaps by reducing augmentation of L30 inhibition, thus suggesting a functional role for metaplasticity within a deﬁned neural circuit.
5. Regulation Of Synaptic Eﬃcacy And Of Other Neural Properties
Although not the explicit subject of this research paper, changes in neural excitability and other intrinsic neural properties can occur in addition to modiﬁcations of synaptic eﬃcacy. Synaptic and non-synaptic changes can co-occur in the same neurons, and can be functionally and even mechanistically related. For example, both Aplysia mechanosensory cells and Hermissenda type B photoreceptors exhibit learning related increases in excitability (increased ﬁring to a given stimulus), as well as enhanced synaptic strength (Schuman and Clark 1994). These two modiﬁcations work in concert to increase both the number and the potency of the synaptic signals conveyed to their postsynaptic targets. Similarly, as described above, reconﬁguration of the Tritonia escape swim CPG and the pyloric CPG in lobster stomatogastric ganglion involve both synaptic and non-synaptic forms of neural regulation.
One important distinction between regulation of synaptic eﬃcacy and of other neural properties is the greater speciﬁcity that synaptic changes may exhibit. Because any given neuron may form preor postsynaptic contacts with thousands of other cells, the ability to modify only selected connections of a neuron confers a high degree of precision in the modiﬁcation of neural pathways. In contrast, intrinsic changes such as enhanced excitability are often expressed cell-wide or nearly cell-wide, enhancing responses to virtually all inputs and enhancing virtually all outputs. In Aplysia siphon sensory cells, several short-term forms of synaptic plasticity, including heterosynaptic facilitation, heterosynaptic inhibition, and Hebbian facilitation, exhibit synapse speciﬁcity (Clark and Kandel 1984, Clark 2001). Synapse speciﬁcity is perhaps not surprising in short-term regulation, which involves covalent modiﬁcations of pre-existing proteins. However, long-term heterosynaptic facilitation, which depends on regulation of transcriptional and translational events, is also in part synapse-speciﬁc (Clark 2001, Martin et al. 1997). Synapse-speciﬁc long-term facilitation raises the intriguing question of how new gene products are preferentially utilized at facilitated synapses, compared with other synapses of the same neuron. An emerging answer, consistent with comparable ﬁndings in hippocampal LTP, is that synapse-speciﬁc long-term facilitation involves synaptic tagging and depends in part on local protein synthesis. According to this hypothesis, relatively short-lived, local regulatory events at treated synapses allow the subsequent capture of new gene products that are shipped cell-wide, thus conferring the speciﬁcity. Irrespective of the particular mechanistic details, long-term synapse-speciﬁc facilitation provides an additional illustrative example of the rich interplay and elegance of the multiple biological processes involved in synaptic regulation.
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