Duration of NMDA-dependent synaptic potentiation in piriform cortex in vivo is increased after epileptiform bursting

Aberrant excitatory rewiring of layer V pyramidal neurons early after neocortical trauma

Lesioned neuronal circuits form new functional connections after a traumatic brain injury (TBI). In humans and animal models, aberrant excitatory connections that form after TBI may contribute to the pathogenesis of post-traumatic epilepsy. Partial neocortical isolation ("undercut" or "UC") leads to altered neuronal circuitry and network hyperexcitability recorded in vivo and in brain slices from chronically lesioned neocortex. Recent data suggest a critical period for maladaptive excitatory circuit formation within the first 3days post UC injury (Graber and Prince 1999, 2004; Li et al. 2011, 2012b). The present study focuses on alterations in excitatory connectivity within this critical period. Immunoreactivity (IR) for growth-associated protein (GAP)-43 was increased in the UC cortex 3days after injury. Some GAP-43-expressing excitatory terminals targeted the somata of layer V pyramidal (Pyr) neurons, a domain usually innervated predominantly by inhibitory terminals. Immunocytochemical analysis of pre- and postsynaptic markers showed that putative excitatory synapses were present on somata of these neurons in UC neocortex. Excitatory postsynaptic currents from UC layer V Pyr cells displayed properties consistent with perisomatic inputs and also reflected an increase in the number of synaptic contacts. Laser scanning photostimulation (LSPS) experiments demonstrated reorganized excitatory connectivity after injury within the UC. Concurrent with these changes, spontaneous epileptiform bursts developed in UC slices. Results suggest that aberrant reorganization of excitatory connectivity contributes to early neocortical hyperexcitability in this model. The findings are relevant for understanding the pathophysiology of neocortical post-traumatic epileptogenesis and are important in terms of the timing of potential prophylactic treatments.

In vitro studies of neuronal networks and synaptic plasticity in invertebrates and in mammals using multielectrode arrays

Brain functions are strictly dependent on neural connections formed during development and modified during life. The cellular and molecular mechanisms underlying synaptogenesis and plastic changes involved in learning and memory have been analyzed in detail in simple animals such as invertebrates and in circuits of mammalian brains mainly by intracellular recordings of neuronal activity. In the last decades, the evolution of techniques such as microelectrode arrays (MEAs) that allow simultaneous, long-lasting, noninvasive, extracellular recordings from a large number of neurons has proven very useful to study long-term processes in neuronal networks in vivo and in vitro. In this work, we start off by briefly reviewing the microelectrode array technology and the optimization of the coupling between neurons and microtransducers to detect subthreshold synaptic signals. Then, we report MEA studies of circuit formation and activity in invertebrate models such as Lymnaea, Aplysia, and Helix. In the following sections, we analyze plasticity and connectivity in cultures of mammalian dissociated neurons, focusing on spontaneous activity and electrical stimulation. We conclude by discussing plasticity in closed-loop experiments.

Slow-wave sleep-imposed replay modulates both strength and precision of memory

Odor perception is hypothesized to be an experience-dependent process involving the encoding of odor objects by distributed olfactory cortical ensembles. Olfactory cortical neurons coactivated by a specific pattern of odorant evoked input become linked through association fiber synaptic plasticity, creating a template of the familiar odor. In this way, experience and memory play an important role in odor perception and discrimination. In other systems, memory consolidation occurs partially via slow-wave sleep (SWS)-dependent replay of activity patterns originally evoked during waking. SWS is ideal for replay given hyporesponsive sensory systems, and thus reduced interference. Here, using artificial patterns of olfactory bulb stimulation in a fear conditioning procedure in the rat, we tested the effects of imposed post-training replay during SWS and waking on strength and precision of pattern memory. The results show that imposed replay during post-training SWS enhanced the subsequent strength of memory, whereas the identical replay during waking induced extinction. The magnitude of this enhancement was dependent on the timing of imposed replay relative to cortical sharp-waves. Imposed SWS replay of stimuli, which differed from the conditioned stimulus, did not affect conditioned stimulus memory strength but induced generalization of the fear memory to novel artificial patterns. Finally, post-training disruption of piriform cortex intracortical association fiber synapses, hypothesized to be critical for experience-dependent odor coding, also impaired subsequent memory precision but not strength. These results suggest that SWS replay in the olfactory cortex enhances memory consolidation, and that memory precision is dependent on the fidelity of that replay.

Odor-specific habituation arises from interaction of afferent synaptic adaptation and intrinsic synaptic potentiation in olfactory cortex

Segmentation of target odorants from background odorants is a fundamental computational requirement for the olfactory system and is thought to be behaviorally mediated by olfactory habituation memory. Data from our laboratory have shown that odor-specific adaptation in piriform neurons, mediated at least partially by synaptic adaptation between the olfactory bulb outputs and piriform cortex pyramidal cells, is highly odor specific, while that observed at the synaptic level is specific only to certain odor features. Behavioral data show that odor habituation memory at short time constants corresponding to synaptic adaptation is also highly odor specific and is blocked by the same pharmacological agents as synaptic adaptation. Using previously developed computational models of the olfactory system we show here how synaptic adaptation and potentiation interact to create the observed specificity of response adaptation. The model analyzes the mechanisms underlying the odor specificity of habituation, the dependence on functioning cholinergic modulation, and makes predictions about connectivity to and within the piriform neural network. Predictions made by the model for the role of cholinergic modulation are supported by behavioral results.

Odor-induced persistent discharge of mitral cells in the mouse olfactory bulb

Short-term retention of sensory information in the form of persistent activity of central neurons plays a key role in transforming a brief sensory stimulation into longer-lasting brain responses. The olfactory system uses this transformation for various functional purposes, but the underlying neuronal mechanisms remain elusive. Here, we recorded odor-evoked, single-unit spike responses of mitral and tufted (M/T) cells in the mouse olfactory bulb (OB) under urethane anesthesia and examined the neuronal mechanisms of the persistent discharge (PD) of M/T cells that outlasts the odor stimulus for tens of seconds. The properties of the persistent afterdischarge that occurred after odor stimulation were distinct from those of odor-induced immediate spike responses in terms of the magnitude, odorant specificity, and odorant concentration-response relationship. This suggests that neuronal mechanisms other than prolonged input from olfactory sensory neurons are involved in generating these afterdischarges. Metabotropic glutamate receptor 1 (mGluR1) is expressed in the dendrites of M/T cells and is thought to participate in intraglomerular interactions among M/T cells. In OBs lacking mGluR1, or treated locally with an mGluR1-selective antagonist, the duration of the odor-induced spike responses was significantly lower than that in control OBs, indicating that mGluR1 within the bulbar neuronal circuits participates in the PD generation. These results suggest that neuronal circuits in the OB can actively prolong the odor-induced spike activity of bulbar output neurons and thus transform a brief odor input into longer-lasting activity in the central olfactory system.

Changes in neuromodulatory effect of adenosine A1 receptors on piriform cortex field potentials in amygdala kindled rats

Adenosine exerts its anticonvulsants effect through different brain regions including piriform cortex. In this study, the effect of amygdala kindled seizures on adenosine A1 receptor-mediated neuromodulation in piriform cortex pyramidal neurons was tested at 24 h and 1 month after kindling. Animals were kindled by daily electrical stimulation of amygdala. Field potentials were recorded from layer II of piriform cortex pyramidal cells following stimulation of the lateral olfactory tract. Obtained results showed that N6-cyclohexyladenosine (CHA), a selective adenosine A1 receptor agonist (1, 10 and 100 microM; i.c.v.), reduced A1 slope and B1 amplitude of field potentials in both kindled and non-kindled (control) rats. However, its effects on kindled animals were more potent at 24 h, but not 1 month post-kindling. 8 cyclopenthyl-1,3-dimethylxanthine (CPT), a selective adenosine A1 receptor antagonist (50 microM, i.c.v.), had no significant effect on the field potential parameters. However, CPT (50 microM, i.c.v.) pretreatment eliminated effects of CHA (10 microM; i.c.v.) on the field potentials. These results indicate that activation of adenosine A1 receptors has an inhibitory effect on the field potentials of piriform cortex pyramidal neurons and the efficiency of adenosine A1 receptor neuromodulation in piriform cortex is increased at short-term (24 h) but return to normal at long-term (1 month) after kindling implementation.

Hyperexcitability induced by GABA withdrawal facilitates hippocampal long-term potentiation

In some mammals, epileptic seizures have been induced in the cerebral cortex, hippocampus and other limbic structures after the sudden suppression of chronically infused GABA. This hyperexcitability state induced by the endogenous neurotransmitter resembles the withdrawal seizure-responses to other GABA(A) receptor agonists such as benzodiazepines, barbiturates and alcohol. Hyperexcitability induced by GABA withdrawal also persists in in vitro preparation. Hippocampal slices, obtained from rats with seizures induced by GABA-withdrawal showed field potential oscillations and paroxysmal activity in the Ammon's horn region 1. During GABA-withdrawal hyperexcitability the threshold of hippocampal long-term potentiation (LTP) decreased to a point in which a brief frequency stimulation that normally failed to produce long lasting changes in synaptic strength, was now able to induce LTP. Facilitation of the LTP induction was associated with a decreased GABA(A)-mediated inhibitory activity, because the effect of the GABA(A) receptor antagonist, bicuculline, was occluded during hyperexcitability and the dose-response curve for bicuculline showed a 50% efficacy reduction with a shift in the effective concentration required for half-maximal activation from 4.5-1.1 microM relative to controls. Nevertheless, the dissociation constant of the antagonist did not change significantly. Our results support the idea that changes in hippocampal plasticity under altered inhibitory neurotransmission states, like those induced by withdrawal syndromes to anxiolytic, sedative or anticonvulsant drugs may be engaged during seizures.

A postnatal sensitive period for plasticity of cortical afferents but not cortical association fibers in rat piriform cortex

Male and female rats underwent unilateral naris occlusion or sham surgery on either post-natal day (PN) 1 or after PN30. Following at least 30 days of unilateral olfactory deprivation, rats were urethane anesthetized and recordings were made from anterior piriform cortex (aPCX). Shock stimulation of afferent fibers (lateral olfactory tract) and association/commissural fibers evoked field potentials in aPCX that were analyzed across groups and between ages. The results demonstrate that early-onset unilateral olfactory deprivation depresses field potentials evoked by stimulation of the deprived cortical afferent, while late-onset deprivation did not. In contrast, intracortical association fiber mediated field potentials in the deprived cortex were enhanced after both early-onset and late-onset deprivation. These results suggest differential developmental plasticity of afferent and association fiber pathways in paleocortex that mirrors that previously described in neocortical sensory systems.

Olfactory learning induces differential long-lasting changes in rat central olfactory pathways

In the present work, we investigated lasting changes induced by olfactory learning at different levels of the olfactory pathways. For this, evoked field potentials induced by electrical stimulation of the olfactory bulb were recorded simultaneously in the anterior piriform cortex, the posterior piriform cortex, the lateral entorhinal cortex and the dentate gyrus. The amplitude of the evoked field potential's main component was measured in each site before, immediately after, and 20 days after completion of associative learning. Evoked field potential recordings were carried out under two experimental conditions in the same animals: awake and anesthetized. In the learning task, rats were trained to associate electrical stimulation of one olfactory bulb electrode with the delivery of sucrose (positive reward), and stimulation of a second olfactory bulb electrode with the delivery of quinine (negative reward). In this way, stimulation of the same olfactory bulb electrodes used for inducing field potentials served as a discriminative cue in the learning paradigm. The data showed that positively reinforced learning resulted in a lasting increase in evoked field potential amplitude restricted to posterior piriform cortex and lateral entorhinal cortex. In contrast, negatively reinforced learning was mainly accompanied by a decrease in evoked field potential amplitude in the dentate gyrus. Moreover, the expression of these learning-related changes occurred to be modulated by the animals arousal state. Indeed, the comparison between anesthetized versus awake animals showed that although globally similar, the changes were expressed earlier with respect to learning, under anesthesia than in the awake state. From these data we suggest that associative olfactory learning involves different neural circuits depending on the acquired value of the stimulus. Furthermore, they show the existence of a functional dissociation between anterior and posterior piriform cortex in mnesic processes, and stress the importance of the animal's arousal state on the expression of learning-induced plasticity.