Biomedical subjects

Robert D Hawkins

Publications and source records attributed to Robert D Hawkins.

12 recordsLinked to original sources

PubMed2006

Dishabituation in Aplysia can involve either reversal of habituation or superimposed sensitization.

Dishabituation has been thought to be due either to reversal of the process of habituation or to a second process equivalent to sensitization superimposed on habituation. One way to address this question is by testing whether dishabituation and sensitization can be dissociated. Previous studies using this approach in Aplysia have come to different conclusions about the nature of dishabituation, perhaps because those studies differed in many respects, including (1) whether they also observed transient behavioral inhibition, and (2) whether they used test stimuli that activated the LE siphon sensory neurons or as yet unidentified sensory neurons. To attempt to resolve the apparent contradictions between the previous studies, we have explored the importance of these two factors by performing a parametric study of dishabituation and sensitization of gill withdrawal in a simplified preparation that does not exhibit transient behavioral inhibition, using two different test stimuli that are known to activate the LE (Touch) or unidentified (Not Touch) sensory neurons. We find that dishabituation and sensitization in this preparation have similar time courses and generally similar functions of shock intensity. However, under one condition, with the Not Touch stimulus 2.5 min after the shock, dishabituation has a reverse effect of shock intensity. Additional analyses suggest that dishabituation with the Not Touch stimulus 2.5 min after the shock is due to reversal of habituation, whereas 12.5 min after the shock, dishabituation is due to superimposed sensitization. These results thus suggest that dishabituation may involve either process in the same preparation, and begin to define the conditions that favor one or the other.

Animals↗

PubMed2006

Presynaptic and postsynaptic Ca(2+) and CamKII contribute to long-term potentiation at synapses between individual CA3 neurons.

Long-term potentiation (LTP) in the Schaffer collateral pathway from the CA3 to the CA1 region of the hippocampus is thought to involve postsynaptic mechanisms including Ca(2+)- and CamKII-dependent alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor insertion. However, very little is known about possible presynaptic mechanisms. It is easier to address that question at synapses between individual neurons in the CA3 region, where both sides of the synapses are accessible to substances injected into the cell bodies. Previous studies using that method showed that CA3-CA3 LTP involves presynaptic protein kinases as well as postsynaptic receptor insertion. We have extended those findings by exploring the pre- and postsynaptic roles of Ca(2+) and CamKII, and we have also compared results with two induction protocols, 1-Hz-paired and -burst-paired, which may involve pre- and/or postsynaptic mechanisms in addition to receptor insertion in CA1. Similar to results in CA1, we find that CA3-CA3 LTP completely depends on postsynaptic Ca(2+) with the 1-Hz-paired protocol but depends only partially on postsynaptic Ca(2+) or CamKII with the -burst-paired protocol. Potentiation with that protocol also partially depends on presynaptic Ca(2+) or CamKII, suggesting that the additional mechanisms of potentiation, at least in part, are presynaptic. Furthermore, the pre- and postsynaptic mechanisms seem to act in series, suggesting coordinate regulation of the two sides of the synapses. CA3-CA3 LTP with the 1-Hz-paired protocol also partially depends on presynaptic Ca(2+), suggesting that it may involve presynaptic mechanisms as well.

Animals↗

PubMed2006

Operant conditioning of gill withdrawal in Aplysia.

A basic question in neuroscience is how different forms of learning are related. To further address that question, we examined whether gill withdrawal in Aplysia, which has already been studied extensively for neuronal mechanisms contributing to habituation, sensitization, and classical conditioning, also undergoes operant conditioning. Animals were run in pairs. During the initial training period, the contingent (experimental) animal received a siphon shock each time its gill relaxed below a criterion level, and the yoked control animal received a shock whenever the experimental animal did, regardless of its own gill position. This was followed by an extinction period when there was no shock, a retraining period when both animals were contingent, and another extinction period. The experimental animals spent more time with their gills contracted above the criterion level than did the control animals during each period, demonstrating operant conditioning. The type of gill behavior modified by learning shifted over time: the experimental animals had a larger increase in the frequency and duration of spontaneous contractions than did the control animals during the first but not the last extinction period and a larger increase in the level of tonic contraction during the last but not the first extinction period. Because many of the neurons controlling spontaneous and tonic gill withdrawal have already been identified, it should now be possible to examine the cellular locus and mechanism of operant conditioning and compare them with those for other forms of learning of the same behavior.

Animals↗

PubMed2006

Molecular mechanisms of memory storage in Aplysia.

Cellular studies of implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded, processed, and stored within the brain. In this review, we focus on recent advances in our understanding of the molecular mechanisms that underlie short-term, intermediate-term, and long-term forms of implicit memory in the marine invertebrate Aplysia californica, and consider how the conservation of common elements in each form may contribute to the different temporal phases of memory storage.

Animals↗

PubMed2005

Serotonin-induced regulation of the actin network for learning-related synaptic growth requires Cdc42, N-WASP, and PAK in Aplysia sensory neurons.

Application of Clostridium difficile toxin B, an inhibitor of the Rho family of GTPases, at the Aplysia sensory to motor neuron synapse blocks long-term facilitation and the associated growth of new sensory neuron varicosities induced by repeated pulses of serotonin (5-HT). We have isolated cDNAs encoding Aplysia Rho, Rac, and Cdc42 and found that Rho and Rac had no effect but that overexpression in sensory neurons of a dominant-negative mutant of ApCdc42 or the CRIB domains of its downstream effectors PAK and N-WASP selectively reduces the long-term changes in synaptic strength and structure. FRET analysis indicates that 5-HT activates ApCdc42 in a subset of varicosities contacting the postsynaptic motor neuron and that this activation is dependent on the PI3K and PLC signaling pathways. The 5-HT-induced activation of ApCdc42 initiates reorganization of the presynaptic actin network leading to the outgrowth of filopodia, some of which are morphological precursors for the learning-related formation of new sensory neuron varicosities.

Actin Cytoskeleton↗

PubMed2005

Presynaptic and postsynaptic roles of NO, cGK, and RhoA in long-lasting potentiation and aggregation of synaptic proteins.

Recent results suggest that long-lasting potentiation at hippocampal synapses involves the rapid formation of clusters or puncta of presynaptic as well as postsynaptic proteins, both of which are blocked by antagonists of NMDA receptors and an inhibitor of actin polymerization. We have investigated whether the increase in puncta involves retrograde signaling through the NO-cGMP-cGK pathway and also examined the possible roles of two classes of molecules that regulate the actin cytoskeleton: Ena/VASP proteins and Rho GTPases. Our results suggest that NO, cGMP, cGK, actin, and Rho GTPases including RhoA play important roles in the potentiation and act directly in both the presynaptic and postsynaptic neurons, where they contribute to the increase in puncta of synaptic proteins. cGK phosphorylates synaptic VASP during the potentiation, whereas Rho GTPases act both in parallel and upstream of cGMP, in part by maintaining the synaptic localization of soluble guanylyl cyclase.

Actins↗

PubMed2004

alpha-Synuclein produces a long-lasting increase in neurotransmitter release.

Wild-type alpha-synuclein, a protein of unknown function, has received much attention because of its involvement in a series of diseases that are known as synucleinopathies. We find that long-lasting potentiation of synaptic transmission between cultured hippocampal neurons is accompanied by an increase in the number of alpha-synuclein clusters. Conversely, suppression of alpha-synuclein expression through antisense nucleotide and knockout techniques blocks the potentiation, as well as the glutamate-induced increase in presynaptic functional bouton number. Consistent with these findings, alpha-synuclein introduction into the presynaptic neuron of a pair of monosynaptically connected cells causes a rapid and long-lasting enhancement of synaptic transmission, and rescues the block of potentiation in alpha-synuclein null mouse cultures. Also, we report that the application of nitric oxide (NO) increases the number of alpha-synuclein clusters, and inhibitors of NO-synthase block this increase, supporting the hypothesis that NO is involved in the enhancement of the number of alpha-synuclein clusters. Thus, alpha-synuclein is involved in synaptic plasticity by augmenting transmitter release from the presynaptic terminal.

Animals↗

PubMed2004

Increased attention to spatial context increases both place field stability and spatial memory.

The hippocampal formation is critical for the acquisition and consolidation of memories. When recorded in freely moving animals, hippocampal pyramidal neurons fire in a location-specific manner: they are "place" cells, comprising a hippocampal representation of the animal's environment. To explore the relationship between place cells and spatial memory, we recorded from mice in several behavioral contexts. We found that long-term stability of place cell firing fields correlates with the degree of attentional demands and that successful spatial task performance was associated with stable place fields. Furthermore, conditions that maximize place field stability greatly increase orientation to novel cues. This suggests that storage and retrieval of place cells is modulated by a top-down cognitive process resembling attention and that place cells are neural correlates of spatial memory. We propose a model whereby attention provides the requisite neuromodulatation to switch short-term homosynaptic plasticity to long-term heterosynaptic plasticity, and we implicate dopamine in this process.

Animals↗

PubMed2004

The long-term stability of new hippocampal place fields requires new protein synthesis.

The hippocampus is critical for formation of spatial memories. Hippocampal pyramidal neurons in freely behaving animals exhibit spatially selective firing patterns, which taken together form an internal representation of the environment. This representation is thought to contribute to the hippocampal spatial memory system. Behavioral long-term memories differ from short-term memories in requiring the synthesis of new proteins. Does the development of the internal hippocampal representation also require the synthesis of new proteins? We found that blocking protein synthesis in the brain of mice by 95% does not affect short-term stability of newly formed hippocampal place fields but abolishes stability in the long term. By contrast, inhibiting protein synthesis does not affect the retention and recall of previously established fields in a familiar environment, indicating that protein synthesis-dependent reconsolidation is not required for recall. Our results indicate that place fields parallel both behavioral memories and the late phase of long-term potentiation in requiring the synthesis of new proteins for consolidation.

Action Potentials↗

PubMed2003

Presynaptic and postsynaptic mechanisms of a novel form of homosynaptic potentiation at aplysia sensory-motor neuron synapses.

Previous studies have shown that homosynaptic potentiation produced by rather mild tetanic stimulation (20 Hz, 2 sec) at Aplysia sensory-motor neuron synapses in isolated cell culture involves both presynaptic and postsynaptic Ca2+ (Bao et al., 1997). We have now investigated the sources of Ca2+ and some of its downstream targets. Although the potentiation lasts >30 min, it does not require Ca2+ influx through either NMDA receptor channels or L-type Ca2+ channels. Rather, the potentiation involves metabotropic receptors and intracellular Ca2+ release from both postsynaptic IP3-sensitive and presynaptic ryanodine-sensitive stores. In addition, it involves protein kinases, including both presynaptic and postsynaptic CamKII and probably MAP kinase. Finally, it does not require transsynaptic signaling by nitric oxide but it may involve AMPA receptor insertion. The potentiation, thus, shares components of the mechanisms of post-tetanic potentiation, NMDA- and mGluR-dependent long-term potentiation, and even long-term depression, but is not identical to any of them. These results are consistent with the more general idea that there is a molecular alphabet of basic components that can be combined in various ways to create novel as well as known types of plasticity.

Animals↗

PubMed2003

Activity-dependent presynaptic facilitation and hebbian LTP are both required and interact during classical conditioning in Aplysia.

Using a simplified preparation of the Aplysia siphon-withdrawal reflex, we previously found that associative plasticity at synapses between sensory neurons and motor neurons contributes importantly to classical conditioning of the reflex. We have now tested the roles in that plasticity of two associative cellular mechanisms: activity-dependent enhancement of presynaptic facilitation and postsynaptically induced long-term potentiation. By perturbing molecular signaling pathways in individual neurons, we have provided the most direct evidence to date that each of these mechanisms contributes to behavioral learning. In addition, our results suggest that the two mechanisms are not independent but rather interact through retrograde signaling.

Action Potentials↗

PubMed2002

Ryanodine receptors contribute to cGMP-induced late-phase LTP and CREB phosphorylation in the hippocampus.

We previously found that the nitric oxide (NO)-cGMP-cGMP-dependent protein kinase (PKG) signaling pathway acts in parallel with the cAMP-cAMP-dependent protein kinase (PKA) pathway to produce protein and RNA synthesis-dependent late-phase long-term potentiation (L-LTP) and cAMP response element-binding protein (CREB) phosphorylation in the CA1 region of mouse hippocampus. We have now investigated the possible involvement of a downstream target of PKG, ryanodine receptors. L-LTP can be induced by either multiple-train tetanization, NO or 8-Br-cGMP paired with one-train tetanization, or the cAMP activator forskolin, and all three types of potentiation are accompanied by an increase in phospho-CREB immunofluorescence in the CA1 cell body area. Both the potentiation and the increase in phospho-CREB immunofluorescence induced by multiple-train tetanization or 8-Br-cGMP paired with one-train tetanization are reduced by prolonged perfusion with ryanodine, which blocks Ca(2+) release from ryanodine-sensitive Ca(2+) stores. By contrast, neither the potentiation nor the increase in immunofluorescence induced by forskolin are reduced by depletion of ryanodine and inositol-1,4,5-triphosphate (IP3)-sensitive Ca(2+) stores. These results suggest that NO, cGMP, and PKG cause release of Ca(2+) from ryanodine-sensitive stores, which in turn causes phosphorylation of CREB in parallel with PKA during the induction of L-LTP.

Animals↗