CiteULike: nelmor's Wilson

Neurobiology of a Simple Memory

Donald Wilson — 2008-07-16T09:45:22-00:00

J Neurophysiol, Vol. 100, No. 1. (1 July 2008), pp. 2-7.

Habituation is one of the simplest forms of memory, yet its neurobiological mechanisms remain largely unknown in mammalian systems. This review summarizes recent multidisciplinary analyses of the neurobiology of mammalian odor habituation including in vitro and in vivo synaptic physiology, sensory physiology, behavioral pharmacology, and computational modeling approaches. The findings show that a metabotropic glutamate receptor-mediated depression of afferent synapses to the olfactory cortex is necessary and perhaps sufficient to account for cortical sensory adaptation and short-term behavioral habituation. Furthermore, long-term habituation is an N-methyl-D-aspartate (NMDA) receptor-dependent process within the olfactory bulb. Thus there is both a pharmacological and anatomical distinction between short-term and long-term memory for habituation. The differential locus of change underlying short- and long-term memory leads to predictable differences in their behavioral characteristics, such as specificity. 10.1152/jn.90479.2008

The mechanism of intrinsic amplification of hyperpolarizations and spontaneous bursting in striatal cholinergic interneurons.

CJ Wilson — 2005-09-13T13:19:00-00:00

Neuron, Vol. 45, No. 4. (17 February 2005), pp. 575-585.

Striatal cholinergic interneurons pause their ongoing firing in response to sensory stimuli that have acquired meaning as a signal for learned behavior. In slices, these cells exhibit both spontaneous activity patterns and spontaneous pauses very similar to those seen in vivo. The mechanisms responsible for ongoing firing and spontaneous pauses were studied in striatal slices using perforated patch recordings. All hyperpolarizations, whether spontaneous or generated by current injection, were amplified and shaped by two hyperpolarization-activated currents. Hyperpolarization onsets were regeneratively amplified by a potassium current (KIR) whose activation promoted further hyperpolarization. The termination of hyperpolarizations was controlled by a time-dependent nonspecific cation current (HCN). The duration and even the sizes of spontaneous and driven hyperpolarizations and pauses in spontaneous activity in cholinergic interneurons are largely autonomous properties of the neuron, rather than reflections of characteristics of the input eliciting the response.

Cholinergic interneuron characteristics and nicotinic properties in the striatum.

FM Zhou — 2008-05-21T09:01:07-00:00

Journal of neurobiology, Vol. 53, No. 4. (December 2002), pp. 590-605.

The neostriatum (dorsal striatum) is composed of the caudate and putamen. The ventral striatum is the ventral conjunction of the caudate and putamen that merges into and includes the nucleus accumbens and striatal portions of the olfactory tubercle. About 2% of the striatal neurons are cholinergic. Most cholinergic neurons in the central nervous system make diffuse projections that sparsely innervate relatively broad areas. In the striatum, however, the cholinergic neurons are interneurons that provide very dense local innervation. The cholinergic interneurons provide an ongoing acetylcholine (ACh) signal by firing action potentials tonically at about 5 Hz. A high concentration of acetylcholinesterase in the striatum rapidly terminates the ACh signal, and thereby minimizes desensitization of nicotinic acetylcholine receptors. Among the many muscarinic and nicotinic striatal mechanisms, the ongoing nicotinic activity potently enhances dopamine release. This process is among those in the striatum that link the two extensive and dense local arbors of the cholinergic interneurons and dopaminergic afferent fibers. During a conditioned motor task, cholinergic interneurons respond with a pause in their tonic firing. It is reasonable to hypothesize that this pause in the cholinergic activity alters action potential dependent dopamine release. The correlated response of these two broad and dense neurotransmitter systems helps to coordinate the output of the striatum, and is likely to be an important process in sensorimotor planning and learning.

Firing Rate Dynamics in the Hippocampus Induced by Trajectory Learning

Daoyun Ji — 2008-05-02T09:18:17-00:00

J. Neurosci., Vol. 28, No. 18. (30 April 2008), pp. 4679-4689.

The hippocampus is essential for spatial navigation, which may involve sequential learning. However, how the hippocampus encodes new sequences in familiar environments is unknown. To study the impact of novel spatial sequences on the activity of hippocampal neurons, we monitored hippocampal ensembles while rats learned to switch from two familiar trajectories to a new one in a familiar environment. Here, we show that this novel spatial experience induces two types of changes in firing rates, but not locations of hippocampal place cells. First, place-cell firing rates on the two familiar trajectories start to change before the actual behavioral switch to the new trajectory. Second, repeated exposure on the new trajectory is associated with an increased dependence of place-cell firing rates on immediate past locations. The result suggests that sequence encoding in the hippocampus may involve integration of information about the recent past into current state. 10.1523/JNEUROSCI.4597-07.2008

Lateral presynaptic inhibition mediates gain control in an olfactory circuit

Shawn Olsen — 2008-03-18T04:25:40-00:00

Nature (16 March 2008)

Sensory processing in the Drosophila antennal lobe increases reliability and separability of ensemble odor representations

Vikas Bhandawat — 2007-10-26T17:19:30-00:00

Nature Neuroscience, Vol. 10, No. 11. (07 October 2007), pp. 1474-1482.

Surround inhibition among projection neurons is weak or nonexistent in the rat neostriatum.

D Jaeger — 2007-08-17T08:39:31-00:00

J Neurophysiol, Vol. 72, No. 5. (November 1994), pp. 2555-2558.

1. Antidromic activation of striatal spiny projection neurons by substantia nigra stimulation in vivo did not evoke inhibitory postsynaptic potentials (IPSPs) in the antidromically activated neurons, or in neighboring spiny neurons. 2. More generalized activation of projection cells by stimulation of the efferent pathway in slices did not evoke IPSPs in spiny neurons. Inhibitory mechanisms were operative in these slices, as indicated by the presence of an IPSP component in the orthodromic response to local stimulation. 3. Dual intracellular recordings, obtained from cells located within 50-200 microns of each other in striatal slices also failed to demonstrate any inhibition among striatal spiny cells. Spikes triggered in one spiny neuron by current injection failed to produce any postsynaptic potential in nearby spiny cells at resting or depolarized membrane potentials. Excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the cortex were not affected by spiking of neighboring cells. 4. It is impossible to rule out the presence of inhibition among striatal spiny neurons in all circumstances. However, the absence of demonstrable IPSPs in these experiments argue against the common view that mutual inhibition among spiny neurons is a central organizing principle of striatal function.

Olfactory Cortical Adaptation Facilitates Detection of Odors Against Background

Mikiko Kadohisa — 2006-06-26T23:52:35-00:00

J Neurophysiol, Vol. 95, No. 3. (1 March 2006), pp. 1888-1896.

Detection and discrimination of odors generally, if not always, occurs against an odorous background. On any given inhalation, olfactory receptor neurons will be activated by features of both the target odorant and features of background stimuli. To identify a target odorant against a background therefore, the olfactory system must be capable of grouping a subset of features into an odor object distinct from the background. Our previous work has suggested that rapid homosynaptic depression of afferents to the anterior piriform cortex (aPCX) contributes to both cortical odor adaptation to prolonged stimulation and habituation of simple odor-evoked behaviors. We hypothesize here that this process may also contribute to figure-ground separation of a target odorant from background stimulation. Single-unit recordings were made from both mitral/tufted cells and aPCX neurons in urethan-anesthetized rats and mice. Single-unit responses to odorant stimuli and their binary mixtures were determined. One of the odorants was randomly selected as the background and presented for 50 s. Forty seconds after the onset of the background stimulus, the second target odorant was presented, producing a binary mixture. The results suggest that mitral/tufted cells continue to respond to the background odorant and, when the target odorant is presented, had response magnitudes similar to that evoked by the binary mixture. In contrast, aPCX neurons filter out the background stimulus while maintaining responses to the target stimulus. Thus the aPCX acts as a filter driven most strongly by changing stimuli, providing a potential mechanism for olfactory figure-ground separation and selective reading of olfactory bulb output. 10.1152/jn.00812.2005

Coordinate synaptic mechanisms contributing to olfactory cortical adaptation.

AR Best — 2006-07-27T19:58:01-00:00

J Neurosci, Vol. 24, No. 3. (21 January 2004), pp. 652-660.

Anterior piriform cortex (aPCX) neurons rapidly filter repetitive odor stimuli despite relatively maintained input from mitral cells. This cortical adaptation is correlated with short-term depression of afferent synapses, in vivo. The purpose of this study was to elucidate mechanisms underlying this nonassociative neural plasticity using in vivo and in vitro preparations and to determine its role in cortical odor adaptation. Lateral olfactory tract (LOT)-evoked responses were recorded in rat aPCX coronal slices. Extracellular and intracellular potentials were recorded before and after simulated odor stimulation of the LOT. Results were compared with in vivo intracellular recordings from aPCX layer II/III neurons and field recordings in urethane-anesthetized rats stimulated with odorants. The onset, time course, and extent of LOT synaptic depression during both in vitro electrical and in vivo odorant stimulation methods were similar. Similar to the odor specificity of cortical odor adaptation in vivo, there was no evidence of heterosynaptic depression between independent inputs in vitro. In vitro evidence suggests at least two mechanisms contribute to this activity-dependent synaptic depression: a rapidly recovering presynaptic depression during the initial 10-20 sec of the post-train recovery period and a longer lasting (approximately 120 sec) depression that can be blocked by the metabotropic glutamate receptor (mGluR) II/III antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) and by the beta-adrenergic receptor agonist isoproterenol. Importantly, in line with the in vitro findings, both adaptation of odor responses in the beta (15-35 Hz) spectral range and the associated synaptic depression can also be blocked by intracortical infusion of CPPG in vivo.

Role of experience and oscillations in transforming a rate code into a temporal code.

MR Mehta — 2005-02-17T21:47:07-00:00

Nature, Vol. 417, No. 6890. (13 June 2002), pp. 741-746.

In the vast majority of brain areas, the firing rates of neurons, averaged over several hundred milliseconds to several seconds, can be strongly modulated by, and provide accurate information about, properties of their inputs. This is referred to as the rate code. However, the biophysical laws of synaptic plasticity require precise timing of spikes over short timescales (<10 ms). Hence it is critical to understand the physiological mechanisms that can generate precise spike timing in vivo, and the relationship between such a temporal code and a rate code. Here we propose a mechanism by which a temporal code can be generated through an interaction between an asymmetric rate code and oscillatory inhibition. Consistent with the predictions of our model, the rate and temporal codes of hippocampal pyramidal neurons are highly correlated. Furthermore, the temporal code becomes more robust with experience. The resulting spike timing satisfies the temporal order constraints of hebbian learning. Thus, oscillations and receptive field asymmetry may have a critical role in temporal sequence learning.

The fundamental role of memory in olfactory perception.

DA Wilson — 2005-12-06T16:23:22-00:00

Trends Neurosci, Vol. 26, No. 5. (May 2003), pp. 243-247.

Current emphasis on odorant physiochemical features as the basis for perception largely ignores the synthetic and experience-dependent nature of olfaction. Olfaction is synthetic, as mammals have only limited ability to identify elements within even simple odor mixtures. Furthermore, olfaction is experience-bound, as exposure alone can significantly affect the extent to which stimuli can be discriminated. We propose that early analytical processing of odors is inaccessible at the behavioral level and that all odors are initially encoded as 'objects' in the piriform cortex. Moreover, we suggest that odor perception is wholly dependent on the integrity of this memory system and that its loss severely impairs normal perception.

Dentate Gyrus NMDA Receptors Mediate Rapid Pattern Separation in the Hippocampal Network

Thomas Mchugh — 2007-06-08T17:10:46-00:00

Science (7 June 2007), 1140263.

Forming distinct representations of multiple contexts, places and episodes is a crucial function of the hippocampus. The dentate gyrus subregion has been suggested to fulfill this role. We have tested this hypothesis by generating and analyzing a mouse strain that lacks the gene encoding the essential subunit of the N-methyl-D-aspartate (NMDA) receptor, NR1, specifically in dentate gyrus granule cells. The mutant mice performed normally in contextual fear conditioning, but were impaired in the ability to distinguish two similar contexts. A significant reduction in the context-specific modulation of firing rate was observed in the CA3 pyramidal cells when the mutant mice were transferred from one context to another. These results provide evidence that NMDA receptors in the granule cells of the dentate gyrus play a crucial role in the process of pattern separation. 10.1126/science.1140263

Cortical oscillations and temporal interactions in a computer simulation of piriform cortex

M Wilson — 2007-04-02T08:51:22-00:00

J Neurophysiol, Vol. 67, No. 4. (1 April 1992), pp. 981-995.

Spatial and Temporal Distribution of Odorant-Evoked Activity in the Piriform Cortex

Robert Rennaker — 2007-02-15T14:29:16-00:00

J. Neurosci., Vol. 27, No. 7. (14 February 2007), pp. 1534-1542.

Despite a remarkably precise spatial representation of odorant stimuli in the early stages of olfactory processing, the projections to the olfactory (piriform) cortex are more diffuse and show characteristics of a combinatorial array, with extensive overlap of afferent inputs and widespread intracortical association connections. Furthermore, although there is increasing evidence for the importance of temporal structure in olfactory bulb odorant-evoked output, little is known about how this temporal patterning is translated within cortical neural ensembles. The present study used multichannel electrode arrays and paired single-unit recordings in rat anterior piriform cortex to test several predictions regarding ensemble coding in this system. The results indicate that odorants evoke activity in a spatially scattered ensemble of anterior piriform cortex neurons, and the ensemble activity includes a rich temporal structure. The most pronounced discrimination between different odorants by cortical ensembles occurs during the first inhalation of a 2 s stimulus. The distributed spatial and temporal structure of cortical activity is present at both global and local scales, with neighboring single units contributing to coding of different odorants and active at different phases of the respiratory cycle. Finally, cross-correlogram analyses suggest that cortical unit activity reflects not only afferent input from the olfactory bulb but also intrinsic activity within the intracortical association fiber system. These results provide direct evidence for predictions stemming from anatomical- and theoretical-based models of piriform cortex. 10.1523/JNEUROSCI.4072-06.2007

Coordinated interactions between hippocampal ripples and cortical spindles during slow-wave sleep.

AG Siapas — 2005-02-18T16:58:59-00:00

Neuron, Vol. 21, No. 5. (November 1998), pp. 1123-1128.

Sleep is characterized by a structured combination of neuronal oscillations. In the hippocampus, slow-wave sleep (SWS) is marked by high-frequency network oscillations (approximately 200 Hz "ripples"), whereas neocortical SWS activity is organized into low-frequency delta (1-4 Hz) and spindle (7-14 Hz) oscillations. While these types of hippocampal and cortical oscillations have been studied extensively in isolation, the relationships between them remain unknown. Here, we demonstrate the existence of temporal correlations between hippocampal ripples and cortical spindles that are also reflected in the correlated activity of single neurons within these brain structures. Spindle-ripple episodes may thus constitute an important mechanism of cortico-hippocampal communication during sleep. This coactivation of hippocampal and neocortical pathways may be important for the process of memory consolidation, during which memories are gradually translated from short-term hippocampal to longer-term neocortical stores.