David L. Glanzman

Habituation revisited: an updated and revised description of the behavioral characteristics of habituation.

The most commonly cited descriptions of the behavioral characteristics of habituation come from two papers published almost 40 years ago [Groves, P. M., & Thompson, R. F. (1970). Habituation: A dual-process theory. Psychological Review, 77, 419-450; Thompson, R. F., & Spencer, W. A. (1966). Habituation: A model phenomenon for the study of neuronal substrates of behavior. Psychological Review, 73, 16-43]. In August 2007, the authors of this review, who study habituation in a wide range of species and paradigms, met to discuss their work on habituation and to revisit and refine the characteristics of habituation. This review offers a re-evaluation of the characteristics of habituation in light of these discussions. We made substantial changes to only a few of the characteristics, usually to add new information and expand upon the description rather than to substantially alter the original point. One additional characteristic, relating to long-term habituation, was added. This article thus provides a modern summary of the characteristics defining habituation, and can serve as a convenient primer for those whose research involves stimulus repetition.

Learning in Aplysia: looking at synaptic plasticity from both sides

Until recently, learning and memory in invertebrate organisms was believed to be mediated by relatively simple presynaptic mechanisms. By contrast, learning and memory in vertebrate organisms is generally thought to be mediated, at least in part, by postsynaptic mechanisms. But new experimental evidence from research using a model invertebrate organism, the marine snail Aplysia, indicates that this apparent distinction between invertebrate and vertebrate synaptic mechanisms of learning is invalid: learning in Aplysia cannot be explained in terms of exclusively presynaptic mechanisms. NMDA-receptor-dependent LTP appears to be necessary for classical conditioning in Aplysia. Furthermore, modulation of trafficking of postsynaptic ionotropic glutamate receptors underlies behavioral sensitization in this snail. Exclusively presynaptic processes appear to support only relatively brief memory in Aplysia. More persistent memory is likely to be mediated by postsynaptic processes, or by presynaptic processes whose expression depends upon retrograde signals.

Hebbian Induction of Long-Term Potentiation of Aplysia Sensorimotor Synapses: Partial Requirement for Activation of an NMDA-Related Receptor

Long-term potentiation (LTP) of Aplysia sensorimotor synapses in cell culture can be induced by pairing sensory neuron activity with depolarization of the motorneuron. This pairing-induced LTP is prevented by perfusion with D, L-2-amino-5-phosphononovalerate (APV), a selective antagonist for the N-methy-D-aspartate (NMDA) subclass of glutamate receptors. Repeated pairing of presynaptic activity with postsynaptic depolarization induces LTP comprising both APV-sensitive and APV-insensitive components. Infusing BAPTA, a selective Ca2+ chelator, into the postsynaptic motorneuron completely blocks pairing-induced LTP. These results demonstrate that Aplysia sensorimotor synapses are capable of hebbian LTP-similar to that exhibited by synapses in the mammalian hippocampus - and suggest a role for this type of synaptic plasticity in classical conditioning of the defensive withdrawal reflex of Aplysia.

The cellular basis of classical conditioning in Aplysia californica — it's less simple than you think

Classical conditioning of the withdrawal reflex of the marine snail Aplysia californica can be used as an important model system for investigating the neurobiology of associative learning. It results when weak tactile stimulation of the snails mantle shelf or siphon is repeatedly paired with strong electrical shocks to the animals tail. This learned behavioral change is thought to be mediated by a presynaptic neuronal mechanism-activity-dependent presynaptic facilitation of the connections between sensory and motor neurons in the CNS of Aplysia. Recent evidence suggests, however, that another type of synaptic plasticity-Hebbian potentiation of the sensorimotor connections-might contribute to classical conditioning in Aplysia. Additional evidence indicates that this relatively simple form of learning is likely to be mediated by multiple neuronal mechanisms.

Serotonin facilitates AMPA-type responses in isolated siphon motor neurons of Aplysia in culture

1 Serotonin (5‐HT) facilitates the connections between sensory and motor neurons in Aplysia during behavioural sensitization. The effect of 5‐HT on sensorimotor synapses is believed to be primarily presynaptic. Here we tested whether 5‐HT can have an exclusively postsynaptic facilitatory effect. 2 Siphon motor neurons were individually dissociated from the abdominal ganglion of Aplysia and placed into cell culture. Brief pulses of glutamate, the putative sensory neuron transmitter, were focally applied (0.1 Hz) to solitary motor neurons in culture, and the glutamate‐evoked postsynaptic potentials (Glu‐PSPs) were recorded. 3 When 5‐HT was perfused over the motor neuron for 10 min, the amplitude of the Glu‐PSPs was significantly increased. The 5‐HT‐induced enhancement of the Glu‐PSPs persisted for at least 40 min after washout. 4 Prior injection into the motor neuron of the calcium chelator BAPTA, GDP‐β‐S or GTP‐γ‐S blocked the 5‐HT‐induced facilitation of the Glu‐PSPs. However, the facilitation was not blocked when APV, an NMDA receptor antagonist, was applied together with the 5‐HT. 5 The enhancement of the Glu‐PSPs by 5‐HT was reversed by the AMPA receptor antagonist DNQX, indicating that 5‐HT increased the functional expression of AMPA‐type receptors in the motor neuron. 6 The presence of botulinum toxin in the motor neuron blocked the 5‐HT‐induced enhancement of the Glu‐PSPs. As botulinum toxin prevents exocytosis we hypothesize that during sensitization 5‐HT causes the insertion of additional AMPA‐type receptors into the postsynaptic membrane of sensorimotor synapses via exocytosis. This postsynaptic mechanism may contribute to facilitation of the synapses.

Synaptic Facilitation and Behavioral Dishabituation in Aplysia: Dependence on Release of Ca2+ from Postsynaptic Intracellular Stores, Postsynaptic Exocytosis, and Modulation of Postsynaptic AMPA Receptor Efficacy

Sensitization and dishabituation of the defensive withdrawal reflex in Aplysia have been ascribed to presynaptic mechanisms, particularly presynaptic facilitation of transmission at sensorimotor synapses in the CNS of Aplysia. Here, we show that facilitation of sensorimotor synapses in cell culture during and after serotonin (5-HT) exposure depends on a rise in postsynaptic intracellular Ca2+ and release of Ca2+ from postsynaptic stores. We also provide support for the idea that postsynaptic AMPA receptor insertion mediates a component of synaptic facilitation by showing that facilitation after 5-HT offset is blocked by injecting botulinum toxin, an exocytotic inhibitor, into motor neurons before application of 5-HT. Using a reduced preparation, we extend our results to synaptic facilitation in the abdominal ganglion. We show that tail nerve shock-induced facilitation of siphon sensorimotor synapses also depends on elevated postsynaptic Ca2+ and release of Ca2+ from postsynaptic stores and recruits a late phase of facilitation that involves selective enhancement of the AMPA receptor-mediated synaptic response. To examine the potential role of postsynaptic exocytosis of AMPA receptors in learning in Aplysia, we test the effect of injecting botulinum toxin into siphon motor neurons on dishabituation of the siphon-withdrawal reflex. We find that postsynaptic injections of the toxin block dishabituation resulting from tail shock. Our results indicate that postsynaptic mechanisms, particularly Ca2+-dependent modulation of AMPA receptor trafficking, play a critical role in synaptic facilitation as well as in dishabituation and sensitization in Aplysia.

Long-Term Potentiation of Aplysia Sensorimotor Synapses in Cell Culture: Regulation by Postsynaptic Voltage

Long-term potentiation (LTP) has been proposed as a cellular mechanism for associative learning in vertebrates. Induction of one type of LTP — observed at synapses in the CA1 region of the mammalian hippocampus - is regulated by the voltage of the postsynaptic cell. To date, a similar form of LTP has not been demonstrated for any invertebrate synapse. We now report that high-frequency stimulation can induce LTP of sensorimotor synapses of the marine mollusc Aplysia in cell culture. Moreover, induction of this form of LTP appears to involve a voltage-dependent postsynaptic mechanism because pairing tetanic stimulation of the presynaptic cell with strong hyperpolarization of the postsynaptic cell blocks the induction of LTP.

Common Mechanisms of Synaptic Plasticity in Vertebrates and Invertebrates

Until recently, the literature on learning-related synaptic plasticity in invertebrates has been dominated by models assuming plasticity is mediated by presynaptic changes, whereas the vertebrate literature has been dominated by models assuming it is mediated by postsynaptic changes. Here I will argue that this situation does not reflect a biological reality and that, in fact, invertebrate and vertebrate nervous systems share a common set of mechanisms of synaptic plasticity.

Protein Kinase M Maintains Long-Term Sensitization and Long-Term Facilitation in Aplysia

How the brain maintains long-term memories is one of the major outstanding questions in modern neuroscience. Evidence from mammalian studies indicates that activity of a protein kinase C (PKC) isoform, protein kinase Mζ (PKMζ), plays a critical role in the maintenance of long-term memory. But the range of memories whose persistence depends on PKMζ, and the mechanisms that underlie the effect of PKMζ on long-term memory, remain obscure. Recently, a PKM isoform, known as PKM Apl III, was cloned from the nervous system of Aplysia. Here, we tested whether PKM Apl III plays a critical role in long-term memory maintenance in Aplysia. Intrahemocoel injections of the pseudosubstrate inhibitory peptide ZIP (ζ inhibitory peptide) or the PKC inhibitor chelerythrine erased the memory for long-term sensitization (LTS) of the siphon-withdrawal reflex (SWR) as late as 7 d after training. In addition, both PKM inhibitors disrupted the maintenance of long-term (≥24 h) facilitation (LTF) of the sensorimotor synapse, a form of synaptic plasticity previously shown to mediate LTS of the SWR. Together with previous results (Bougie et al., 2009), our results support the idea that long-term memory in Aplysia is maintained via a positive-feedback loop involving PKM Apl III-dependent protein phosphorylation. The present data extend the known role of PKM in memory maintenance to a simple and well studied type of long-term learning. Furthermore, the demonstration that PKM activity underlies the persistence of LTF of the Aplysia sensorimotor synapse, a form of synaptic plasticity amenable to rigorous cellular and molecular analyses, should facilitate efforts to understand how PKM activity maintains memory.

Postsynaptic Regulation of Long-Term Facilitation in Aplysia

Repeated exposure to serotonin (5-HT), an endogenous neurotransmitter that mediates behavioral sensitization in Aplysia[1-3], induces long-term facilitation (LTF) of the Aplysia sensorimotor synapse [4]. LTF, a prominent form of invertebrate synaptic plasticity, is believed to play a major role in long-term learning in Aplysia[5]. Until now, LTF has been thought to be due predominantly to cellular processes activated by 5-HT within the presynaptic sensory neuron [6]. Recent work indicates that LTF depends on the increased expression and release of a sensory neuron-specific neuropeptide, sensorin [7]. Sensorin released during LTF appears to bind to autoreceptors on the sensory neuron, thereby activating critical presynaptic signals, including mitogen-activated protein kinase (MAPK) [8, 9]. Here, we show that LTF depends on elevated postsynaptic Ca2+ and postsynaptic protein synthesis. Furthermore, we find that the increased expression of presynaptic sensorin resulting from 5-HT stimulation requires elevation of postsynaptic intracellular Ca2+. Our results represent perhaps the strongest evidence to date that the increased expression of a specific presynaptic neuropeptide during LTF is regulated by retrograde signals.