CREB Activation Mediates Plasticity in Cultured Hippocampal Neurons

Activation of cyclic AMP dependent kinase is believed to mediate slow onset, long-term potentiation (LTP) in central neurons. Cyclic- AMP activates a cascade of molecular events leading to phosphorylation of the nuclear cAMP responsive element binding protein (pCREB). Whereas a variety of stimuli lead to activation of CREB, the molecular processes downstream of CREB, which may be relevant to neuronal plasticity, are yet largely unknown. We have recently found that following exposure to estradiol, pCREB mediates the large increase in dendritic spine density in cultured rat hippocampal neurons. We now extend these observations to include other stimuli, such as bicuculline, that cause the formation of new dendritic spines. Such stimuli share with estradiol the same mechanism of action in that both require activity-dependent CREB phosphorylation. Our observations suggest that CREB phosphorylation is a necessary, but perhaps not sufficient, step in the process leading to the generation of new dendritic spines and perhaps to functional plasticity as well

The Hippocampus as a Learning Machine

A series of experiments were conducted with the purposes of describing a functional pathway in the rat hippocampus, characterizing some conditions necessary for activating it, and identifying critical steps in this pathway. In all experiments a classical conditioning paradigm was used and the responses of units in the hippocampus and related forebrain structures to the conditioned stimulus were measured. In the first experiment a few differences between dentate, CA-3, and CA-1, the main fields of the hippocampus, were found. Units in the dentate were first to acquire a conditioned response, CA-3 followed and CA-1 was last. This order fits with the anatomical pathway. However, dentate responses were phasic, that is, did not outlast the CS-US interval, and were not specific to the conditioned stimulus. The responses of CA-3 and CA-1 units, on the other hand, were sustained and specific. The second experiment was devoted to the analysis of conditioned response latencies, in the hippocampus as well as in septum, subiculum, cingulate, entorhinal, and related structures, all known to be input stages to the hippocampus. In this experiment unconditioned short response latencies were found in the medial septum, one of the afferents of the hippocampus. These were not changed in the process of learning. The shortest conditioned response latencies were found in area CA-3 of the hippocampus. Units in area CA-1 followed, but units in dentate did not precede those of CA-3. Units in entorhinal cortex, the other main afferent to the hippocampus did not seem to precede hippocampal units either. The special relations between the hippocampus and the dentate were demonstrated in another part of this experiment, where dentate units lost their conditioned responses, in the process of extinction, before those of CA-3 and CA-1. It was postulated that septal input triggers CA-3 responses and these would be maintained in the presence of reinforcing dentate and entorhinal inputs. The relations between the dentate and the hippocampus were further studied in two experiments in which aversive electric shock served as an unconditioned stimulus. In experiment 3 food and shock served as unconditioned stimuli on alternate days. In experiment 4 food and shock were presented in the same sessions as unconditioned stimuli to two different CS's. Dentate units had an excitatory conditioned response to a food signal and an inhibitory conditioned response to a shock signal in both experiments. Hippocampal units had excitatory responses to both signals. Acquisition of a conditioned response was not demonstrated within the hippocampus when the conditioned stimulus preceded shock and was slow when food or shock were applied following two different signals in the same session. However, when first trained that a signal precedes food, the conditioned response would be maintained in the hippocampus even if shock is now the US. The dentate is probably involved in the initiation of a conditioned response in the hippocampus but not in the maintenance of it. A sensory-sensory paradigm (experiment 5) has demonstrated the presence of unconditioned unhabituated sensory responses in two of the afferents to the hippocampus, that is, the medial septum and the cingulate cortex. It failed to show signs of conditioning in the hippocampus proper. It was proposed that in the absence of an appetitive reward and the activity of the entorhinal-dentate pathway, conditioned responses in hippocampus cannot be established. Conditioned entorhinal responses (experiment 6) had long latency but also long time constant. Their evoked activity was maintained for periods as long as one minute. It was found that hippocampal responses were larger, if the conditioned stimulus was applied within one minute from the previous trial. Hence, a correlation between hippocampal responses and entorhinal firing rate was demonstrated. On the basis of these experiments it was proposed that septal input enters the hippocampus at the CA-3 area, is able to selectively activate these cells only in the presence of facilitation produced by entorhinal and dentate activity. The facilitatory entorhinal activity is triggered mainly by positive reward.</p

Activity Deprivation Induces Neuronal Cell Death: Mediation by Tissue-Type Plasminogen Activator

Spontaneous activity is an essential attribute of neuronal networks and plays a critical role in their development and maintenance. Upon blockade of activity with tetrodotoxin (TTX), neurons degenerate slowly and die in a manner resembling neurodegenerative diseases-induced neuronal cell death. The molecular cascade leading to this type of slow cell death is not entirely clear. Primary post-natal cortical neurons were exposed to TTX for up to two weeks, followed by molecular, biochemical and immunefluorescence analysis. The expression of the neuronal marker, neuron specific enolase (NSE), was down-regulated, as expected, but surprisingly, there was a concomitant and striking elevation in expression of tissue-type plasminogen activator (tPA). Immunofluorescence analysis indicated that tPA was highly elevated inside affected neurons. Transfection of an endogenous tPA inhibitor, plasminogen activator inhibitor-1 (PAI-1), protected the TTX-exposed neurons from dying. These results indicate that tPA is a pivotal player in slowly progressing activity deprivation-induced neurodegeneration

Impaired Functional Connectivity Underlies Fragile X Syndrome

Fragile X syndrome (FXS), the most common form of inherited intellectual disability, is caused by a developmentally regulated silencing of the FMR1 gene, but its effect on human neuronal network development and function is not fully understood. Here, we isolated isogenic human embryonic stem cell (hESC) subclones-one with a full FX mutation and one that is free of the mutation (control) but shares the same genetic background-differentiated them into induced neurons (iNs) by forced expression of NEUROG-1, and compared the functional properties of the derived neuronal networks. High-throughput image analysis demonstrates that FX-iNs have significantly smaller cell bodies and reduced arborizations than the control. Both FX- and control-neurons can discharge repetitive action potentials, and FX neuronal networks are also able to generate spontaneous excitatory synaptic currents with slight differences from the control, demonstrating that iNs generate more mature neuronal networks than the previously used protocols. MEA analysis demonstrated that FX networks are hyperexcitable with significantly higher spontaneous burst-firing activity compared to the control. Most importantly, cross-correlation analysis enabled quantification of network connectivity to demonstrate that the FX neuronal networks are significantly less synchronous than the control, which can explain the origin of the development of intellectual dysfunction associated with FXS

Hippocampal Synaptic Plasticity in Mice Overexpressing an Embryonic Subunit of the NMDA Receptor

The effects of changing NMDA receptor subunit composition on synaptic plasticity in the hippocampus were analyzed by creating transgenic mice overexpressing NR2D, a predominantly embryonic NMDA receptor subunit. NMDA-evoked currents in the transgenic mice had smaller amplitudes and slower kinetics. The transgenics also displayed age-dependent deficits in synaptic plasticity in area CA1 of the hippocampus. Long-term depression was selectively impaired in juvenile mice when NR2D overexpression was moderate. In mature mice, overexpression of NR2D was associated with a reduction of both NR2B and Ca^(2+)-independent activity of Ca^(2+)- and calmodulin-dependent protein kinase II. These biochemical changes were correlated with a marked impairment of NMDA-dependent long-term potentiation, but spatial behavior was normal in these mice. These results show that the developmental regulation of NMDA receptor subunit composition alters the frequency at which modification of synaptic responses occur after afferent stimulation

Lasting Differential Effects on Plasticity Induced by Prenatal Stress in Dorsal and Ventral Hippocampus

Early life adversaries have a profound impact on the developing brain structure and functions that persist long after the original traumatic experience has vanished. One of the extensively studied brain structures in relation to early life stress has been the hippocampus because of its unique association with cognitive processes of the brain. While the entire hippocampus shares the same intrinsic organization, it assumes different functions in its dorsal and ventral sectors (DH and VH, resp.), based on different connectivity with other brain structures. In the present review, we summarize the differences between DH and VH and discuss functional and structural effects of prenatal stress in the two sectors, with the realization that much is yet to be explored in understanding the opposite reactivity of the DH and VH to stressful stimulation

Lasting Differential Effects on Plasticity Induced by Prenatal Stress in Dorsal and Ventral Hippocampus

Early life adversaries have a profound impact on the developing brain structure and functions that persist long after the original traumatic experience has vanished. One of the extensively studied brain structures in relation to early life stress has been the hippocampus because of its unique association with cognitive processes of the brain. While the entire hippocampus shares the same intrinsic organization, it assumes different functions in its dorsal and ventral sectors (DH and VH, resp.), based on different connectivity with other brain structures. In the present review, we summarize the differences between DH and VH and discuss functional and structural effects of prenatal stress in the two sectors, with the realization that much is yet to be explored in understanding the opposite reactivity of the DH and VH to stressful stimulation