Insights into a molecular switch that gates sensory neuron synapses during habituation in Aplysia

Tony D. Gover, Thomas W. Abrams

I think this paper spends so much space and time only to deny depletion hypothesis.
The effects of calpain and Arf sound very interesting and promissing.


1. Habituation of reflex responses in vertebrates and invertebrates: a simple form of neural plasticity
In addition to habituation, the repeated stimulus may lead to a strengthening of the response, known as sensitization.
Their observations led them to propose the ‘‘dual-process theory” in which the behavioral change actually reflects the
contribution of these two independent processes.

Whereas short-term depression at these synapses appears to be mediated entirely by presynaptic changes (Castellucci & Kandel, 1974; Manseau, Fan, Hueftlein, Sossin, & Castellucci, 2001), long-term depression that is initiated by extensive stimulation or blocks of stimuli delivered over hours appears to depend upon signals initiated by postsynaptic glutamate receptors, including Ca2+ influx through NMDA receptors (Lin & Glanzman, 1996; Mata, Chen, Cai, & Glanzman, 2008) (see also Ezzeddine & Glanzman, 2003).

2. Mechanisms of synaptic depression in other systems

2.1. Depletion of the readily releasable pool of vesicles as a mechanism of depression
Historically, the earliest proposed mechanism for synaptic depression was depletion of the neurotransmitter that is available for release. Vesicles at synaptic release sites that are available for immediate release are considered to constitute a readily releasable pool (Betz, 1970; Gingrich & Byrne, 1985; Stevens & Tsujimoto, 1995); these vesicles may already be docked at exocytosis sites.

2.2. Use-dependent inactivation of presynaptic Ca2+ channels

2.3. Desensitization of postsynaptic receptors

2.4. Autoreceptors and retrograde signaling
At parallel fiber-to-Purkinje cell synapses in cerebellum, high frequency presynaptic activity causes suppression of transmitter release, which is mediated by release of endocannabinoids from the postsynaptic Purkinje cell; the endocannabinoids act on receptors on presynaptic terminals, resulting in reduced Ca²⁺ influx (Brown, Brenowitz, & Regehr, 2003).

3. Proposed mechanisms for synaptic depression at the sensory neuron-to-motor neuron synapse in Aplysia
3.1. Short-term homosynaptic depression at sensory neuron-to-motor neuron synapses involves a presynaptic mechanism, rather than alteration in postsynaptic responses

3.2. Ca2+ current inactivation
With repeated stimulation of the sensory neuron, the action potential duration decreased, suggesting that Ca2+ influx decreased with each successive action potential. Klein et al. (1980) also measured the Ca²⁺ current in
the soma of the presynaptic sensory neuron under voltage clamp.
With repeated depolarizing voltage steps, they observed that the Ca²⁺ current decreased in parallel with the decline in the amplitude of the EPSP recorded in the motor neuron.
Gingrich and Byrne (1985) also reached the conclusion that Ca²⁺ current inactivation alone during the normal duration ac-tion potential does not account for depression at this synapse.

Armitage and Siegelbaum (1998) used fluorescent Ca²⁺ indicators to directly measure Ca²⁺ influx at presynaptic varicosities of sensory neurons through the dihydropyridine-insensitive Ca²⁺ channels. No change in Ca²⁺ influx through these channels that initiate release was observed during synaptic depression.

3.3. Depletion of the readily releasable pool of vesicles as the primary mechanism for synaptic depression during habituation
Thus, depletion of the readily releasable pool of vesicles can contribute to depression, but probably only with relatively prolonged presynaptic activity.
Recent evidence implicated the Ca²⁺-activated protease calpain both in modulation of depression at the sensory neuron-to-motor neuron synapse and in serotonin-induced facilitation after depression (Khoutorsky & Spira, 2005).
This intriguing observation has been interpreted to indicate that this protease acts to untether vesicles in the reserve
pool, and thereby initiates mobilization of vesicles to the readily releasable pool. Although this is a plausible model, another possible function for calpain is the proteolytic cleavage of protein kinase C (PKC), converting the enzyme to the persistently active PKM form (Sutton, Bagnall, Sharma, Shobe, & Carew, 2004). PKC is known to participate in facilitation of depressed sensory neuron-to-motor neuron synapses by serotonin (Ghirardi et al., 1992; Manseau
et al., 2001), and also acts to prevent the development of depression during bursts of activity (see below). Thus, calpain cleavage of PKC could reduce synaptic depression.

3.4. Switching of release sites to a silent state as a mechanism of synaptic depression

Gover et al. (2002) considered five mechanisms of synaptic depression:
(1) vesicle depletion, which by default must be dependent on vesicular release (Fig. 3);
(2) reduction in the probability of release of individual releasable vesicles after a release event;
(3) reduction in the probability of release of individual releasable vesicles after an action potential, but independent of a release event;
(4) silencing of release sites after a release event, and
(5) silencing of release sites, after an action potential, but independent of a release event (Fig. 4).

These data suggest that the probability of release does not change gradually; rather individual release sites are simply silenced, or switched off, during development of depression.
Furthermore, Royer et al. (2000) argued that given their data, if p were nonuniform across release sites, then homosynaptic depression due to silencing of release sites would be independent of release [which is consistent with the conclusions of Gover et al. (2002)].

3.5. Other evidence suggesting depletion of vesicles is not the major mechanism for synaptic depression at Aplysia sensory neuron synapses


4. Evidence from other systems suggesting that vesicle depletion is not the primary mechanism responsible for synaptic depression
Rather depression with relatively few stimuli or with low frequency stimulation cannot be explained primarily by depletion.

5. Ca²⁺ influx initiates the silencing of release sites during depression at sensory neuron-to-motor neuron synapse in Aplysia
In the absence of Ca²⁺ influx no detectable synaptic depressin developed. Thus the switching off of sensory neuron release sites during repetitive presynaptic activity appears to be initiated by presynaptic Ca²⁺ influx.

6. The switch that silences sensory neuron-to-motor neuron synapses during depression in Aplysia involves the small G protein Arf
One possibility is that because facilitation of depressed synapse involves protein kinase C (Ghirardi et al., 1992; Manseau et al., 2001), the switching of synapses to a silent state is mediated by a protein phosphatase. Indeed, some types of long-term depression involve protein phosphatases (...).

... a general inhibitor of Arf signaling powerfuly inhibited transmission at sensory neuron-to-motor neuron synapses and occluded the development of synaptic depression. Similarly, a binding domain of an Arf effector protein acted as a dominant negative and substantially reduced the depression that developed with single action pontentials. Reciprocally, we found that constitutively active Arf6, when injected into presynaptic sensory neurons, prevented the development of synaptic depression.

7. Bursts of sensory neuron activity prevent the development of homosynaptic depression"burst-dependent protection" from synaptic depression
Burst-dependent protection from synaptic depression involves Ca²⁺-dependent activation of protein kinase C (PKC). Chelating Ca²⁺ in the presynaptic sensory neuron with intracellular EGTA, which is not a sufficiently rapid buffer to block transmitter release, eliminates burst-dependent protection.