Tracking the flow of learned information from membrane receptors to genome

Transition to endogenous bursting after long-term decentralization requires De novo transcription in a critical time window

Rhythmic motor pattern generation by the pyloric network in the lobster stomatogastric ganglion (STG) requires neuromodulatory inputs from adjacent ganglia. However, although suppression of these inputs by cutting the stomatogastric nerve (stn) causes the pyloric network to fall silent, network output similar to that expressed when the stn is intact returns after 3-4 days in organ culture. Intracellular recordings from identified pyloric dilator (PD) neurons indicate that the fundamental change underlying rhythm recovery resides with the intrinsic excitability of pyloric neurons themselves, since the prolonged absence of extrinsic modulatory inputs allows the expression of an endogenous oscillatory capability that is maintained in a strictly conditional state when these inputs are present. To examine whether gene transcription was involved in this change in neuronal behavior, we performed in vitro experiments in which the STG was exposed to the RNA-synthesis inhibitor actinomycin D (ACD). ACD (50 microM) incubation at the time of decentralization prevented subsequent reacquisition of PD neuron bursting, but the inhibitor was much less effective when applied at later postdecentralization times, suggesting that the recovery process arises from new protein synthesis triggered when modulatory inputs are first removed. Moreover, in the nondecentralized STG, trans-synaptic modulatory instruction may sustain the conditional pyloric network phenotype by continuously regulating expression of genes responsible for intrinsic neuronal rhythmogenesis.

Involvement of the genetic apparatus in memory formation mechanisms: the role of the neuronal calcium-regulatory system in rats

Stimulators of long-term memory (ethylnorantiphein and its analogs M1 and M2) were used to study the dynamics of several components of the neuronal calcium-regulatory system in the rat cortex and hippocampus. There were no changes in the activity of the Mg, Ca-ATPase transporter and actomyosin-like Ca-ATPase in synaptosomes 5, 15, 60, and 180 min after dosage with these agents. On exposure to ethylnorantiphein, M1, and M2, activation of RNA transcription at 60 min was accompanied by notable increases in chromatin Ca-ATPase activity, along with an increase in the synthesis of synaptosomal proteins at 180 min, with an increase in synaptic membrane protein kinase C activity. An increase in chromatin Ca-ATPase activity was also seen during fixation of a conditioned active escape reflex. It is suggested that the increase in protein kinase C activity is associated with secondary rearrangements of the synaptic membranes. The question of the role of direct activation of the genetic apparatus by neuroactive substances in the molecular mechanisms of memory formation is discussed.

Putative roles for the inducible transcription factor c-fos in the central nervous system: studies with antisense oligonucleotides

Although immediate-early genes such as c-fos are widely believed to play an important role in neuroplasticity, there is limited evidence to support involvement in the initiation of molecular events leading to medium- and long-term changes in brain function following a stimulus. Results using techniques such as transgenic knockout of the gene are often difficult to interpret. Antisense oligonucleotide technology offers an alternative. Infusion of antisense oligonucleotide to modify the expression of c-fos in the brain results in dramatic changes in rotation behaviour in animals challenged with psychostimulant drugs such as amphetamine. Similarly, the knockdown of c-fos expression using antisense oligonucleotides can also alter the rate of amygdala kindling in response to electrical stimulation of the brain. While studies using antisense oligonucleotides to knockdown c-fos expression provide evidence that the expression of c-fos plays an important role in regulating neuronal function, the use of antisense nucleotides has limitations and experiments must be very carefully controlled. Many details of antisense oligonucleotide actions remain unknown.

CREB as a memory modulator: induced expression of a dCREB2 activator isoform enhances long-term memory in Drosophila

Genetic studies of memory formation in Drosophila have revealed that the formation of a protein synthesis-dependent long-term memory (LTM) requires multiple training sessions. LTM is blocked specifically by induced expression of a repressor isoform of the cAMP-responsive element-binding protein (CREB). Here, we report an enhancement of LTM formation after induced expression of an activator isoform of dCREB2. Maximum LTM is achieved after one training session, and its formation depends on phosphorylation of the activator transgene. A model of LTM formation based on differential regulation of CREB isoforms is proposed.

Biology of Memory and Childhood Trauma

Examples have been presented of children's behaviors that demonstrate the trauma-learning pattern of re-enactment, repetition, and displacement. They become persistent parts of the symptom complex of PTSD. The encapsulation phase occurs when the trauma event occurs and symptoms present themselves, but the events as yet are undisclosed. The trauma-specific behavior patterns, the general hyper-arousal symptoms, and the avoidant, numbing symptoms persist; the emerging disruptive behaviors are not linked to the traumatic event and reactions to the trauma. The response of the child's social and interpersonal context to the internalizing or externalizing behaviors post-trauma, continue shaping the internal cognitive schema of the child. When the child is unable to link ongoing, self-defeating, disruptive behavior to trauma experience, the underlying fear persists. This interferes with the child's ability to modulate emotions either through altering the persistence of refractory, self-limiting cognitive schema or the inability to use new experience to develop and grow. The flexibility of children to discriminate new information may be lost; the children are either numb to new information or hyperalert and perceive danger. Issues for treatment include children's distress over memories of the trauma and the lack of capacity to learn and develop from new interpersonal experiences. It has been our experience that nurses first must help the child relearn flexibility through self-observation, the element of self-soothing and calming behaviors, processing of new information, and strengthening of social relationships. With new and strengthened personal resources, the child then is able to begin to process the traumatic memories.

Biochemical correlates of long-term potentiation in hippocampal synapses

Figure 2 summarizes biochemical events which are currently known or hypothesized to participate in LTP induction/maintenance. Current evidence strongly suggests that postsynaptic Ca2+, both entered from the outside of cells and released from intracellular stores, is the initial key substance for the induction of LTP. A rise of [Ca2+]i triggers a variety of enzymatic reactions and initiates the enhancement of synaptic transmission. This first step may be achieved by direct/indirect phosphorylations of protein molecules in postsynaptic receptors/ion channels. This would result in an increase in receptor sensitivity. An immediate increase in the number of available postsynaptic receptors by modifications of spine morphology is another candidate. Such modifications may be accomplished by cytoskeleton rearrangements or changes in extracellular environments. A change in spine structure may also cause an increase in spine neck conductance. Although it is unknown to what extent the increase in [Ca2+]i affects cellular chemistry, Ca2+ probably also directly/indirectly stimulates cascades which exert effects more slowly. A delayed increase in metabotropic receptor sensitivity may occur. New synthesis of protein molecules may be involved in late periods of LTP by replacing turnovered molecules and/or by supplying new materials. Some of these chains of biochemical events may also apply to presynaptic terminals, although the existence of retrograde messenger substances must still be confirmed. In addition, interactions between different protein kinases and second messengers appear to occur to bring about final effects.

Maintenance of long-term potentiation in rat dentate gyrus requires protein synthesis but not messenger RNA synthesis immediately post-tetanization

The involvement of new protein and messenger ribonucleic acid synthesis in long-term potentiation was studied in the anaesthetized rat dentate gyrus using several inhibitors of protein synthesis (anisomycin, emetine, cycloheximide and puromycin) and an inhibitor of messenger ribonucleic acid synthesis (actinomycin D). When injected for 1 h just prior to tetanization, the four inhibitors of protein synthesis produced a mild reduction of long-term potentiation of the excitatory postsynaptic potential measured 10 min after tetanization. Anisomycin produced a significantly faster decay of long-term potentiation, while the other inhibitors had more moderate effects. Actinomycin D failed to affect long-term potentiation. In a second experiment, the time-dependency of the anisomycin effect was examined. Anisomycin injected immediately after tetanization promoted decay of long-term potentiation, but when injected after a 15-min delay, the drug had no effect. Inhibition of protein synthesis for 4 h prior to tetanization did not have any more effect on long-term potentiation than inhibition for 1 h. In no experiment was long-term potentiation of the population spike affected by drug manipulation. These results suggest that for long-term potentiation of the excitatory postsynaptic potential to be maintained for at least 3 h proteins must be synthesized from already existing messenger ribonucleic acid, and that this synthesis is mostly completed within 15 min after tetanization.

Discriminative conditioning alters food preferences in the leech, Haemopis marmorata

The feeding behavior of the carnivorous leech, Haemopis marmorata, was aversively trained in a discriminative classical conditioning task. Two conditioned stimuli were used: One consisted of a food (chicken or liver) paired with an unconditioned stimulus of quinidine (bitter chemical); the other consisted of the alternate food presented in an unpaired relationship with the quinidine. Training consisted of alternating exposures to the two conditioned stimuli. Testing consisted of the simultaneous presentation of a conditioned stimulus food and a neutral food, beef. The percentages of responding to the conditioned stimuli were tabulated. Haemopis could discriminate between the conditioned stimuli. As a result of pairing a food with quinidine, the leeches selectively reduced their preference for that paired food, while they did not alter their preference for the unpaired food.

A quantitative analysis of 2-D gels identifies proteins in which labeling is increased following long-term sensitization in Aplysia

Long-term memory for sensitization of the gill- and siphon-withdrawal reflex in Aplysia, produced by 4 days of training, is associated with increased synaptic efficacy of the connection between the sensory and motor neurons. This training is also accompanied by neuronal growth; there is an increase in the number of synaptic varicosities per sensory neuron and in the number of active zones. Such structural changes may be due to changes in the rates of synthesis of certain proteins. We have searched for proteins in which the rates of [35S]methionine labeling are altered during the maintenance phase of long-term memory for sensitization by using computer-assisted quantitative 2-D gel analysis. This method has allowed us to detect 4 proteins in which labeling is altered after 4 days of sensitization training.