Manipulation of familiarity reveals a necessary lexical component of the word-stem completion priming effect

These experiments were motivated by the idea that many types of nondeclarative memory are by-products arising from the plasticity that is inherent in much of the nervous system. We hypothesized that two types of repetition priming, word-stem completion (WSC) priming and perceptual identification (PI) priming, rely on different mechanisms because the WSC task and the PI task engage different cognitive and brain processes. We tested this hypothesis by manipulating word familiarity. The results, impaired WSC priming but intact PI priming with unfamiliar words, indicate that WSC priming relies primarily on a modification mechanism, whereas PI priming relies primarily on an acquisition mechanism. Our conclusions are consistent with component processes theories of nondeclarative memory.

Cultured insect mushroom body neurons express functional receptors for acetylcholine, GABA, glutamate, octopamine, and dopamine

Fluorescence calcium imaging with fura-2 and whole cell, patch-clamp electrophysiology was applied to cultured Kenyon cells (interneurons) isolated from the mushroom bodies of adult crickets (Acheta domesticus) to demonstrate the presence of functional neurotransmitter receptors. In all cells investigated, 5 microM acetylcholine (ACh, n = 52) evoked an increase in intracellular free calcium ([Ca2+]i). Similar effects were observed in response to 10 microM nicotine. The ACh response was insensitive to atropine (50 microM) but was reduced by mecamylamine (50 microM) and alpha-bungarotoxin (alpha-bgt, 10 microM). ACh-induced inward ion currents (n = 28, EACh approximately 0 mV) were also blocked by 1 microM mecamylamine and by 1 microM alpha-bgt. Nicotine-induced inward currents desensitized more rapidly than ACh responses. Thus functional alpha-bgt-sensitive nicotinic ACh receptors are abundant on all Kenyon cells tested, and their activation leads to an increase in [Ca2+]i. gamma-Aminobutyric acid (GABA, 100 microM) triggered a sustained decrease in [Ca2+]i. Similar responses were seen with a GABAA agonist, muscimol (100 microM), and a GABAB agonist, 3-APPA (1 mM), suggesting that more than one type of GABA receptor can affect [Ca2+]i. This action of GABA was not observed when the extracellular KCl concentration was lowered. All cells tested (n = 26) with patch-clamp electrophysiology showed picrotoxinin (PTX)-sensitive, GABA-induced (30-100 microM) currents with a chloride-sensitive reversal potential. Thus, an ionotropic PTX-sensitive GABA receptor was found on all Kenyon cells tested. Most (61%) of the 54 cells studied responded to -glutamate (100 microM) application either with a biphasic increase in [Ca2+]i or with a single, delayed, sustained [Ca2+]i increase. Nearly all cells tested (95%, n = 19) responded to (100 microM) -glutamate with rapidly desensitizing, inward currents that reversed at approximately -30 mV. Dopamine (100 microM) elicited either a rapid or a delayed increase in [Ca2+]i in 63% of the 26 cells tested. The time course of these responses varied greatly among cells. Dopamine failed to elicit currents in patch-clamped cells (n = 4). A brief decrease in [Ca2+]i was induced by octopamine (100 microM) in approximately 54% of the cells tested (n = 35). However, when extracellular CaCl2 was lowered, octopamine triggered a substantial increase in [Ca2+]i in 35% of the cells tested (n = 26). No octopamine-elicited currents were detected in patched-clamped cells (n = 10).

Cyclic AMP induces functional presynaptic boutons in hippocampal CA3-CA1 neuronal cultures

Long-term forms of synaptic plasticity that may underlie learning and memory have been suggested to depend on changes in the number of synapses between presynaptic and postsynaptic neurons. Here we have investigated a form of synaptic plasticity in cultures of hippocampal CA3 and CA1 neurons related to the late phase of long-term potentiation, which depends on cAMP and protein synthesis. Using the fluorescent dye FM 1-43 to label active presynaptic terminals, we find that a membrane permeable analog of cAMP enhances the number of active presynaptic terminals and that this effect requires protein synthesis.

Effect of chronic nimodipine on spatial learning and on long-term potentiation

The present study examined the effect of nimodipine on a reference memory task and on the induction and maintenance of long-term potentiation (LTP) in the hippocampal dentate gyrus. Young rats, subcutaneously implanted with either a 30 mg nimodipine or placebo pellet, were trained on the Barnes circular platform task. Retention was tested 15 days following acquisition. Following behavioural testing, recording and stimulating electrodes were implanted in the granule cell layer of the dentate gyrus and the perforant path, respectively. Pre-pellet baseline evoked potentials were collected. Nimodipine or placebo pellets were again subcutaneously implanted, according to the original groupings, and post-pellet baseline evoked potentials were obtained. LTP was then induced in the granule cell population by perforant path tetanization and the decay of LTP was followed for 15 days. Nimodipine significantly decreased the number of trials to reach both the acquisition and the retention criterion on the circular platform task, but did not alter granule cell excitability, LTP threshold, or the magnitude of LTP. Sustained nimodipine administration, however, increased the decay rate of LTP of the population spike, but did not affect the decay rate for LTP of the EPSP. No significant correlations were obtained between behavioural and electrophysiological measures. These results provide further evidence against a simple direct relationship between LTP and spatial learning.

A vector-integration-to-endpoint model for performance of viapoint movements

Viapoint (VP) movements are movements to a desired point that are constrained to pass through an intermediate point. Studies have shown that VP movements possess properties, such as smooth curvature around the VP, that are not explicable by treating VP movements as strict concatenations of simpler point-to-point (PTP) movements. Such properties have led some theorists to propose whole-trajectory optimization models, which imply that the entire trajectory is precomputed before movement initiation. This paper reports new experiments conducted to systematically compare VP with PTP trajectories. Analyses revealed a statistically significant early directional deviation in VP movements but no associated curvature change. An explanation of this effect is offered by extending the vector-integration-to-endpoint (VITE) model (Bullock, D., & Grossberg, S. (1988a). Neural dynamics of planned arm movements: Emergent invariants and speed-accuracy properties during trajectory formation. Psychological Review, 95, 49-90; Bullock, D., & Grossberg, S. (1988b). The VITE model: A neural command circuit for generating arm and articulator trajectories. In J.A.S. Kelso, A.J. Mandell & M.F. Schlesinger (Eds.), Dynamic patterns in complex systems (pp. 305-326). Singapore: World Scientific Publishers.), which postulates that voluntary movement trajectories emerge as internal gating signals control the integration of continuously computed vector commands based on the evolving, perceptible difference between desired and actual position variables. The model explains the observed trajectories of VP and PTP movements as emergent properties of a dynamical system that does not precompute entire trajectories before movement initiation. The new model includes a working memory and a stage sensitive to time-to-contact information. These cooperate to control serial performance. The structural and functional relationships proposed in the model are consistent with available data on forebrain physiology and anatomy.

Polysialylated neural cell adhesion molecule expression in the dentate gyrus of the human hippocampal formation from infancy to old age

Modulation of neural cell adhesion molecule polysialylation (NCAM PSA) state has been proposed to underlie morphofunctional change associated with consolidation of memory in the rodent, and its age-dependent decline to be related to impaired cognitive function. To establish whether this may be a human correlate of cognitive decline, we determined the age-dependent expression of PSA in the human hippocampal dentate gyrus using postmortem tissue derived from individuals who exhibited no obvious neuropathology. As in the rodent, PSA immunoreactivity in the 5-month human infant was associated mainly with a population of granule-like cells and their mossy fibre axons. Cell numbers were maximal during the first 3 years of life but declined by an order of magnitude between the second and third decades and remained relatively constant thereafter and was restricted to the granule cell layer/hilar border. In contrast to the rodent, diffuse immunostaining was observed in the inner molecular layer; however, as development advanced, this became relocated to the outer molecular layer from 2 years of age onwards. In addition, numerous polysialylated hilar neurons became evident at 2-3 years of age and remained constant until the eighth decade of life. These findings suggest NCAM polysialylation to play a crucial developmental role within a period concluding with adolescence, and that an attenuated NCAM PSA-mediated neuroplasticity continues throughout the human lifespan. The importance of the developmental phase of NCAM PSA expression in the emergence of schizophrenia is discussed.

Neuromodulation of dendritic action potentials

The extent to which regenerative action potentials invade hippocampal CA1 pyramidal dendrites is dependent on both recent activity and distance from the soma. Previously, we have shown that the amplitude of back-propagating dendritic action potentials can be increased by activating either protein kinase A (PKA) or protein kinase C (PKC) and a subsequent depolarizing shift in the activation curve for dendritic K+ channels. Physiologically, an increase in intracellular PKA and PKC would be expected upon activation of beta-adrenergic and muscarinic acetylcholine receptors, respectively. Accordingly, we report here that activation of either of these neurotransmitter systems results in an increase in dendritic action-potential amplitude. Activation of the dopaminergic neurotransmitter system, which is also expected to raise intracellular adenosine 3',5'-cyclic monophosphate (cAMP) and PKA levels, increased action-potential amplitude in only a subpopulation of neurons tested.

Nitric oxide is required for expression of LTP that is induced by stimulation phase-locked with theta rhythm

Long-term potentiation (LTP) can be induced by giving only one burst (five stimuli at 200 Hz) on the positive phase of sensory-induced theta rhythm in awake or anaesthetized rats, a stimulation protocol that mimics naturally occurring neuronal activity. Nitric oxide has been discussed as an important neuronal messenger in the induction of LTP. However, experiments testing inhibitors of nitric oxide synthase (NOS) in vitro produced contradictory results. The non-specific NOS inhibitor Nitro-L-arginine (L-NARG) impaired LTP induced by high-frequency stimulation (HFS) [from 155 +/- 7% to 122 +/- 8%), but completely blocked theta-dependent LTP induction (161 +/- 8% to 102 +/- 5%). NOS inhibitors, e.g. 7-nitro indazole (7-NI) or 1-(2-trifluoromethylphenyl) imidazole (TRIM) that are more selective for neuronal NOS and affect blood pressure less also impaired HFS-induced LTP (186 +/- 11% to 135 +/- 9% for TRIM) but completely blocked theta-dependent LTP (154 +/- 7 to 91 +/- 8). L-Arginine reversed the effects of the NOS inhibitors tested. Therefore, NO appears to be a modulator that is important for synaptic plasticity in this more physiological stimulation technique in vivo. NO is not released in slice preparations in sufficient quantities or at the right timing. Instead, the unphysiologically strong HFS protocol appears to induce an NO-independent type of LTP in some cases.

Mechanisms for generating the autonomous cAMP-dependent protein kinase required for long-term facilitation in Aplysia

The formation of a persistently active cAMP-dependent protein kinase (PKA) is critical for establishing long-term synaptic facilitation (LTF) in Aplysia. The injection of bovine catalytic (C) subunits into sensory neurons is sufficient to produce protein synthesis-dependent LTF. Early in the LTF induced by serotonin (5-HT), an autonomous PKA is generated through the ubiquitin-proteasome-mediated proteolysis of regulatory (R) subunits. The degradation of R occurs during an early time window and appears to be a key function of proteasomes in LTF. Lactacystin, a specific proteasome inhibitor, blocks the facilitation induced by 5-HT, and this block is rescued by injecting C subunits. R is degraded through an allosteric mechanism requiring an elevation of cAMP coincident with the induction of a ubiquitin carboxy-terminal hydrolase.

Chloramphenicol-induced amnesia for passive avoidance training in the day-old chick

The antibiotic chloramphenicol, an inhibitor of mitochondrial protein synthesis, was used to investigate the time-related changes in protein synthesis following passive avoidance training in the day-old chick (white leghorn-black Australorp). Retention of memory for this simple learning task is known to be prevented by an inhibitor of cytosolic protein synthesis, anisomycin, in a biphasic manner, with the first phase of sensitivity occurring up to 90 min post-training and the second phase between 4 and 5 h post-training (Freeman, Rose, & Scholey, 1995). Birds received bilateral intracranial injections of chloramphenicol (10 microl/hemisphere of a 7.4 mM solution) at various times relative to training and were tested 24 h later. This report shows that at the second phase of anisomycin susceptibility there was a chloramphenicol-sensitive period (5 h post-training) which had an onset time less than 1 h after injection. The effect of chloramphenicol appears not to be due to the mitochondria being energetically compromised since intracranial injections of an uncoupler of mitochondrial oxidative phosphorylation, 2,4-dinitrophenol (0.1 mM), did not disrupt memory formation when injected 5 h after training, even though it did cause amnesia when injected at the earlier time point of 20 min post-training. These results are discussed in the context of what is already known about memory formation in the day-old chick.

Temporary amnesia induced by cold anesthesia and hypoxia in Drosophila

Memory consolidation in Drosophila was investigated using cold anesthesia- and hypoxia-induced amnesia. Individual flies were operantly trained to avoid the specific flight orientations with respect to the landmarks surrounding them when paired with heat reinforcement at a flight simulator. Cold anesthesia, introduced immediately after training, exerted a significantly diminishing effect on memory between 15 and 150 min after training. Hypoxia delivered immediately after training had a significantly diminishing effect on memory between 30 and 150 min after training. In addition, cold anesthesia disrupted memory only when introduced within the first 20 min, while hypoxia worked only when delivered within the first 2 min after training. When interpreted in the context of a four-phase model of memory consolidation, the results suggest that 1) cold anesthesia disrupts both short-term memory (STM) and anesthesia-resistant memory (ARM), 2) hypoxia disrupts ARM specifically, 3) both of them leave long-term memory (LTM) intact, and 4) LTM may be independent of availability of STM and ARM in flies.

Temporal sequence compression by an integrate-and-fire model of hippocampal area CA3

Cells in the rat hippocampus fire as a function of the animal's location in space. Thus, a rat moving through the world produces a statistically reproducible sequence of "place cell" firings. With this perspective, spatial navigation can be viewed as a sequence learning problem for the hippocampus. That is, learning entails associating the relationships among a sequence of places that are represented by a sequence of place cell firing. Recent experiments by McNaughton and colleagues suggest the hippocampus can recall a sequence of place cell firings at a faster rate than it was experienced. This speedup, which occurs during slow-wave sleep, is called temporal compression. Here, we show that a simplified model of hippocampal area CA3, based on integrate-and-fire cells and unsupervised Hebbian learning, reproduces this temporal compression. The amount of compression is proportional to the activity level during recall and to the relative timespan of associativity during learning. Compression seems to arise from an alteration of network dynamics between learning and recall. During learning, the dynamics are paced by external input and slowed by a low overall level of activity. During recall, however, external input is absent, and the dynamics are controlled by intrinsic network properties. Raising the activity level by lowering inhibition increases the rate at which the network can transition between previously learned states and thereby produces temporal compression. The tendency for speeding up future activations, however, is limited by the temporal range of associations that were present during learning.

Retaining the equilibrium point hypothesis as an abstract description of the neuromuscular system

The lambda version of the equilibrium point (EP) hypothesis for motor control is examined in light of recent criticisms of its various instantiations. Four important assumptions that have formed the basis for recent criticism are analyzed: First, the assumption that intact muscles possess invariant force-length characteristics (ICs). Second, that these ICs are of the same form in agonist-antagonist pairs. Third, that muscle control is monoparametric and that the control parameter, lambda, can be given a neurophysiological interpretation. Fourth, that reflex loop time delays and the known, asymmetric, nonlinear mechanical properties of muscles can be ignored. Mechanical and neurophysiological investigations of the neuromuscular system suggests that none of these assumptions is likely to be correct. This has been taken to mean that the EP hypothesis is oversimplified and a new approach is needed. It is argued that such an approach can be provided without rejecting the EP hypothesis, rather to regard it as an input-output description of muscle and associated segmental circuits. The operation of the segmental circuitry can be interpreted as having the function, at least in part, of compensating for a variety of nonlinearities and asymmetries such that the overall system implements the lambda-EP model equations.

Selective acquisition of AMPA receptors over postnatal development suggests a molecular basis for silent synapses

Early in postnatal development, glutamatergic synapses transmit primarily through NMDA receptors. As development progresses, synapses acquire AMPA receptor function. The molecular basis of these physiological observations is not known. Here we examined single excitatory synapses with immunogold electron-microscopic analysis of AMPA and NMDA receptors along with electrophysiological measurements. Early in postnatal development, a significant fraction of excitatory synapses had NMDA receptors and lacked AMPA receptors. As development progressed, synapses acquired AMPA receptors with little change in NMDA receptor number. Thus, synapses with NMDA receptors but no AMPA receptors can account for the electrophysiologically observed 'silent synapse'.

Rapid, Labile, and Protein Synthesis– Independent Short-Term Memory in Conditioned Taste Aversion

Short-term memory is a rapid, labile, and protein-synthesis-independent phase of memory. The existence of short-term memory in conditioned taste aversion (CTA) learning has not been demonstrated formally. To determine the earliest time at which a CTA is expressed, we measured intraoral intake of sucrose at 15 min, 1 hr, 6 hr, or 48 h after contingent pairing of an intraoral infusion of 5% sucrose (6.6 ml over 6 min) and toxic lithium chloride injection (76 mg/kg). Rats were implanted with intraoral catheters to allow presentation of taste solutions at arbitrary times. Intraoral intake was measured under conditions of long-delay, single-trial learning typical of CTA. Rats decreased intraoral intake of sucrose at 15 min after contingent pairing of sucrose and LiCl, but not after noncontingent LiCl or sucrose. Thus CTA learning can be expressed rapidly. To determine if short-term CTA memory is labile and decays in the absence of long-term memory, we measured intraoral intake of sucrose after pairing sucrose with low doses of LiCl. Rats received an intraoral infusion of 5% sucrose (6 ml/6 min); 30 min later LiCl was injected at three different doses (19, 38, or 76 mg/kg). A second intraoral infusion of sucrose was administered 15 min, 1 hr, 3 hr, 4.5 hr, 6 hr, or 48 hr later. The formation of long-term CTA memory was dependent on the dose of LiCl paired with sucrose during acquisition. Low doses of LiCl induced a CTA that decayed within 6 hr after pairing. Central administration of the protein synthesis inhibitor cycloheximide prior to LiCl injection blocked long-term CTA expression at 6 and 48 hr, but not short-term CTA expression at 1 hr. Thus, short-term memory for CTA learning exists that is acquired rapidly and independent of protein synthesis, but labile in the absence of long-term memory formation.

On the Respective Roles of Nitric Oxide and Carbon Monoxide in Long-Term Potentiation in the Hippocampus

Perfusion of hippocampal slices with an inhibitor nitric oxide (NO) synthase blocked induction of long-term potentiation (LTP) produced by a one-train tetanus and significantly reduced LTP by a two-train tetanus, but only slightly reduced LTP by a four-train tetanus. Inhibitors of heme oxygenase, the synthetic enzyme for carbon monoxide (CO), significantly reduced LTP by either a two-train or four-train tetanus. These results suggest that NO and CO are both involved in LTP but may play somewhat different roles. One possibility is that NO serves a phasic, signaling role, whereas CO provides tonic, background stimulation. Another possibility is that NO and CO are phasically activated under somewhat different circumstances, perhaps involving different receptors and second messengers. Because NO is known to be activated by stimulation of NMDA receptors during tetanus, we investigated the possibility that CO might be activated by stimulation of metabotropic glutamate receptors (mGluRs). Consistent with this idea, long-lasting potentiation by the mGluR agonist tACPD was blocked by inhibitors of heme oxygenase but not NO synthase. Potentiation by tACPD was also blocked by inhibitors of soluble guanylyl cyclase (a target of both NO and CO) or cGMP-dependent protein kinase, and guanylyl cyclase was activated by tACPD in hippocampal slices. However, biochemical assays indicate that whereas heme oxygenase is constitutively active in hippocampus, it does not appear to be stimulated by either tetanus or tACPD. These results are most consistent with the possibility that constitutive (tonic) rather than stimulated (phasic) heme oxygenase activity is necessary for potentiation by tetanus or tACPD, and suggest that mGluR activation stimulates guanylyl cyclase phasically through some other pathway.

Methods for whole-cell recording from visually preselected neurons of perirhinal cortex in brain slices from young and aging rats

This manuscript describes methods for preparing, visualizing, and recording from healthy perirhinal cortex neurons in brain slices from young and aging rats. We focused on perirhinal cortex because of its role in learning, memory, and aging-related cognitive decline. Detailed accounts of our dissection procedures are reported. Procedures that reliably yielded healthy neurons from juvenile rats were not conducive to obtaining healthy, readily-patchable neurons from aging rats, suggesting a procedure-by-age interaction. Performing an intracardiac perfusion, using a temperature-controlled vibratome, matching osmolarity between the cutting and incubation saline, using a slow cutting speed, and incubating slices at a warm temperature for 30 min were important when working with older tissue. Excellent visualization of neurons at depths of up to 100 microm was achieved in slices from all ages (without tissue clearing) avoiding the need to record from surface neurons, which are more likely to have truncated processes. Whole-cell recordings typically remained stable for several hours in neurons prepared from rats at all ages. These procedures should benefit neuroscientists interested in applying visually-guided whole-cell patch-clamp techniques to brain slice experiments using aged tissue. These methods should also facilitate the application of fluorescent imaging technology to brain slices for studying aging-related changes.

Local calcium signalling by inositol-1,4,5-trisphosphate in Purkinje cell dendrites

The second messenger inositol-1,4,5-trisphosphate (InsP3) releases Ca2+ from intracellular Ca2+ stores by activating specific receptors on the membranes of these stores. In many cells, InsP3 is a global signalling molecule that liberates Ca2+ throughout the cytoplasm. However, in neurons the situation might be different, because synaptic activity may produce InsP3 at discrete locations. Here we characterize InsP3 signalling in postsynaptic cerebellar Purkinje neurons, which have a high level of InsP3 receptors. We find that repetitive activation of the synapse between parallel fibres and Purkinje cells causes InsP3-mediated Ca2+ release in the Purkinje cells. This Ca2+ release is restricted to individual postsynaptic spines, where both metabotropic glutamate receptors and InsP3 receptors are located, or to multiple spines and adjacent dendritic shafts. Focal photolysis of caged InsP3 in Purkinje cell dendrites also produces Ca2+ signals that spread only a few micrometres from the site of InsP3 production. Uncaged InsP3 produces a long-lasting depression of parallel-fibre synaptic transmission that is limited to synapses where the Ca2+ concentration is raised. Thus, in Purkinje cells InP3 acts within a restricted spatial range that allows it to regulate the function of local groups of parallel-fibre synapses.

Adverse psychological impact, glutamatergic dysfunction, and risk factors for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cell loss and pathological changes in neuronal transmission. In particular, malfunction in glutamatergic activity may be associated with the impairment of memory seen in Alzheimer patients. Both hypoactivation and hyperactivation of glutamatergic systems seem to cause impeded cognitive processing in animals. Rats subjected to rearing in isolation display reduced levels of glutamate in temporal regions accompanied by impaired learning and memory. Similar cognitive deficits are also seen in animals exposed to behavioral stress. Stress appears to have deleterious effects on cognition caused by glutamate neurotoxicity leading to attenuated synaptic activity. It is suggested that stress may represent a potential risk factor for AD. The known risk factors for AD (age, heredity, head trauma, low education, depression) may all be related to glutamatergic dysfunction. Some difficulties with pharmacological approaches based on glutamatergic agonists are discussed. It is suggested that optimal glutamate-mediated neurotransmission throughout life may prevent the occurrence of mental decline associated with AD.

A family of cAMP-binding proteins that directly activate Rap1

cAMP (3',5' cyclic adenosine monophosphate) is a second messenger that in eukaryotic cells induces physiological responses ranging from growth, differentiation, and gene expression to secretion and neurotransmission. Most of these effects have been attributed to the binding of cAMP to cAMP-dependent protein kinase A (PKA). Here, a family of cAMP-binding proteins that are differentially distributed in the mammalian brain and body organs and that exhibit both cAMP-binding and guanine nucleotide exchange factor (GEF) domains is reported. These cAMP-regulated GEFs (cAMP-GEFs) bind cAMP and selectively activate the Ras superfamily guanine nucleotide binding protein Rap1A in a cAMP-dependent but PKA-independent manner. Our findings suggest the need to reformulate concepts of cAMP-mediated signaling to include direct coupling to Ras superfamily signaling.

Fear and the brain: where have we been, and where are we going?

In recent years, there has been an explosion of interest in the neural basis of emotion. Much of this enthusiasm has been triggered by studies of the amygdala and its contribution to fear. This work has shown that the amygdala detects and organizes responses to natural dangers (like predators) and learns about novel threats and the stimuli that predict their occurrence. The latter process has been studied extensively using a procedure called classical fear conditioning. This article surveys the progress that has been made in understanding the neural basis of fear and its implications for anxiety disorders, as well as the gaps in our knowledge.

Neurobiology of the Caenorhabditis elegans genome

Neurotransmitter receptors, neurotransmitter synthesis and release pathways, and heterotrimeric GTP-binding protein (G protein)-coupled second messenger pathways are highly conserved between Caenorhabditis elegans and mammals, but gap junctions and chemosensory receptors have independent origins in vertebrates and nematodes. Most ion channels are similar to vertebrate channels but there are no predicted voltage-activated sodium channels. The C. elegans genome encodes at least 80 potassium channels, 90 neurotransmitter-gated ion channels, 50 peptide receptors, and up to 1000 orphan receptors that may be chemoreceptors. For many gene families, C. elegans has both conventional members and divergent outliers with weak homology to known genes; these outliers may provide insights into previously unknown functions of conserved protein families.

Serotonin modulation of sensory inputs to the lateral amygdala: dependency on corticosterone

The lateral nucleus of the amygdala (LA) receives excitatory (glutamatergic) inputs from thalamic and cortical sensory processing areas and is believed to be involved in evaluation of the affective significance of sensory events. We examined whether serotonin (5-HT) affects excitatory transmission in auditory afferents to the LA and, if so, whether this modulation of sensory transmission is regulated by the stress hormone corticosterone (CORT). Neuronal activity in the LA was elicited via iontophoretic ejection of L-glutamate or synaptically via electrical stimulation of auditory afferent pathways. In the intact rat, iontophoretically applied 5-HT inhibited both synaptically and glutamate-evoked action potentials in most neurons examined. However, after adrenalectomy (ADX), which eliminates endogenous CORT, 5-HT no longer inhibited evoked activity in the LA. High-CORT doses given to ADX animals reinstated the inhibition of excitatory transmission of 5-HT, whereas low-CORT doses had little effect. Immunocytochemical labeling of the glucocorticoid receptor in the intact rat demonstrated nuclear staining throughout several amygdala regions, including the LA. However, after ADX, no nuclear labeling was visible. With a high replacement dose of CORT (5 or 10 mg) after ADX, dense nuclear staining returned, but with a low replacement dose (1 mg/kg), there was only light nuclear staining. Thus, the ability of 5-HT to modulate glutamatergic activity in auditory pathways to the amygdala is dependent on the presence of CORT and possibly glucocorticoid activation. Via this mechanism, 5-HT modulates the processing of sensory information within the LA and thus may regulate amygdala-related functions.

Perturbed dentate gyrus function in serotonin 5-HT2C receptor mutant mice

Serotonin systems have been implicated in the regulation of hippocampal function. Serotonin 5-HT2C receptors are widely expressed throughout the hippocampal formation, and these receptors have been proposed to modulate synaptic plasticity in the visual cortex. To assess the contribution of 5-HT2C receptors to the serotonergic regulation of hippocampal function, mice with a targeted 5-HT2C-receptor gene mutation were examined. An examination of long-term potentiation at each of four principal regions of the hippocampal formation revealed a selective impairment restricted to medial perforant path-dentate gyrus synapses of mutant mice. This deficit was accompanied by abnormal performance in behavioral assays associated with dentate gyrus function. 5-HT2C receptor mutants exhibited abnormal performance in the Morris water maze assay of spatial learning and reduced aversion to a novel environment. These deficits were selective and were not associated with a generalized learning deficit or with an impairment in the discrimination of spatial context. These results indicate that a genetic perturbation of serotonin receptor function can modulate dentate gyrus plasticity and that plasticity in this structure may contribute to neural mechanisms underlying hippocampus-dependent behaviors.

Rolipram, a type IV-specific phosphodiesterase inhibitor, facilitates the establishment of long-lasting long-term potentiation and improves memory

In an attempt to improve behavioral memory, we devised a strategy to amplify the signal-to-noise ratio of the cAMP pathway, which plays a central role in hippocampal synaptic plasticity and behavioral memory. Multiple high-frequency trains of electrical stimulation induce long-lasting long-term potentiation, a form of synaptic strengthening in hippocampus that is greater in both magnitude and persistence than the short-lasting long-term potentiation generated by a single tetanic train. Studies using pharmacological inhibitors and genetic manipulations have shown that this difference in response depends on the activity of cAMP-dependent protein kinase A. Genetic studies have also indicated that protein kinase A and one of its target transcription factors, cAMP response element binding protein, are important in memory in vivo. These findings suggested that amplification of signals through the cAMP pathway might lower the threshold for generating long-lasting long-term potentiation and increase behavioral memory. We therefore examined the biochemical, physiological, and behavioral effects in mice of partial inhibition of a hippocampal cAMP phosphodiesterase. Concentrations of a type IV-specific phosphodiesterase inhibitor, rolipram, which had no significant effect on basal cAMP concentration, increased the cAMP response of hippocampal slices to stimulation with forskolin and induced persistent long-term potentiation in CA1 after a single tetanic train. In both young and aged mice, rolipram treatment before training increased long- but not short-term retention in freezing to context, a hippocampus-dependent memory task.

Operant visual learning and memory in Drosophila mutants dunce, amnesiac and radish

Learning and memory of Drosophila mutants dunce, amnesiac and radish which were isolated originally from the classical olfactory learning paradigm are analyzed in an operant visual learning paradigm. Dunce appears to show normal ability to learn during training, but its memory is significantly affected. Though the learning index during the first minute after training is normal, its short-term memory (STM), anesthesia-resistant memory (ARM) and long-term memory (LTM) are all significantly damaged. Amnesiac displays disrupted middle-term memory (MTM), while its STM and LTM remain unchanged. Learning and memory in radish mutants seem to be unaffected. These results lend support to the argument that there are certain common molecular mechanisms underlying learning and memory through different tasks and the previous multi-phase model of visual memory is modified in a genetic way.

Brain-derived neurotrophic factor (BDNF) modulates inhibitory, but not excitatory, transmission in the CA1 region of the hippocampus

Brain-derived neurotrophic factor (BDNF) modulates inhibitory, but not excitatory, transmission in the CA1 region of the hippocampus. J. Neurophysiol. 80: 3383-3386, 1998. Brain-derived neurotrophic factor (BDNF) has been reported to have rapid effects on synaptic transmission in the hippocampus. We report here that bath application of BDNF causes a small but significant decrease in stimulus-evoked inhibitory postsynaptic currents (IPSCs) on CA1 pyramidal cells, which is prevented by the tyrosine kinase inhibitor lavendustin A. BDNF causes a decrease in the 1/CV2 of the IPSC, and also reduces paired-pulse depression of the IPSC, suggesting a presynaptic site of action. In contrast, BDNF did not have a detectable effect on field excitatory postsynaptic potentials measured in stratum radiatum. We conclude that BDNF has a selective depressant action on inhibitory transmission in the hippocampus, due at least in part to a presynaptic mechanism.

Learning in the oculomotor system: from molecules to behavior

A combination of system-level and cellular-molecular approaches is moving studies of oculomotor learning rapidly toward the goal of linking synaptic plasticity at specific sites in oculomotor circuits with changes in the signal-processing functions of those circuits, and, ultimately, with changes in eye movement behavior. Recent studies of saccadic adaptation illustrate how careful behavioral analysis can provide constraints on the neural loci of plasticity. Studies of vestibulo-ocular adaptation are beginning to examine the molecular pathways contributing to this form of cerebellum-dependent learning.

Stimulation of airway sensory nerves by cyclosporin A and FK506 in guinea-pig isolated bronchus

We have investigated the contractile property of cyclosporin A and FK506 in guinea-pig isolated bronchus. Cyclosporin A (10 microM) failed to significantly attenuate the excitatory non-adrenergic non-cholinergic (eNANC) and cholinergic contractile response (per cent methacholine Emax) induced by electrical field stimulation (EFS). In contrast, eNANC responses were significantly attenuated by both the neurokinin (NK)-1 and (NK)-2 receptor antagonists, N-acetyl-L-tryptophan 3,5-bis (trifluoromethyl)-benzyl and SR48968, respectively. Cyclosporin A and FK506 caused a concentration-dependent contraction in guinea-pig isolated bronchus, which was significantly attenuated by NK-1 and NK-2 receptor antagonists. The capsaicin receptor antagonist, capsazepine (10 microM) significantly reduced the contractile response to cyclosporin A and capsaicin, but not to FK506. The N-type calcium channel blocker, omega-Conotoxin (omegaCTX: 10 nM), significantly reduced the contractile response to FK506 and the eNANC response following EFS. In contrast, omega-CTX failed to significantly reduce the contractile potency to capsaicin or cyclosporin A. In bronchial preparations desensitized by repeated application of capsaicin (1 microM), the contractile responses to both cyclosporin A (100 microM) and FK506 (100 microM), were significantly reduced. In contrast, the contractile responses to substance P and neurokinin A (10 microM) were not altered. Furthermore, repeated application of cyclosporin A (100 microM) significantly inhibited the contractile response to capsaicin (1 microM). The findings from this study would indicate that cyclosporin A and FK506 mediate contraction of guinea-pig isolated bronchus secondary to the release of neuropeptides from airway sensory nerves. However, the release of sensory neuropeptides appears to be mediated via different mechanisms for cyclosporin A and FK506, the former by stimulation of the vanilloid receptor and the latter via opening of N-type calcium channels.

Serotonin induces temporally and mechanistically distinct phases of persistent PKA activity in Aplysia sensory neurons

The cAMP signaling cascade has been implicated in several stages of memory formation. We have examined activation of this cascade by serotonin (5-HT) in the sensory neurons of Aplysia. We find that different patterns of 5-HT exposure induce three distinct modes of PKA activation. First, a single 5 min pulse induces transient (5 min) PKA activation that requires neither transcription nor translation. Second, 4-5 pulses induce intermediate-term persistent activation (3 hr duration) that requires translation but not transcription. Third, 5 pulses of 5-HT, as well as continuous (90 min) exposure, induce long-term persistent activation 20 hr later, which requires both transcription and translation. Thus, in the sensory neurons, different patterns of 5-HT give rise to three independent phases of PKA activation that differ in their induction requirements, their temporal profiles, and their molecular mechanisms.

Inositol-1,4,5-trisphosphate receptor-mediated Ca mobilization is not required for cerebellar long-term depression in reduced preparations

Inositol-1,4,5-trisphosphate receptor-mediated Ca mobilization is not required for cerebellar long-term depression in reduced preparations. J. Neurophysiol. 80: 2963-2974, 1998. Cerebellar long-term depression (LTD) is a cellular model system of information storage in which coincident parallel fiber and climbing fiber activation of a Purkinje neuron (PN) gives rise to a sustained attenuation of parallel fiber-PN synaptic strength. Climbing fiber and parallel fiber inputs may be replaced by direct depolarization of the PN and exogenous glutamate pulses, respectively. The parallel fiber-PN synapse has a high-density of mGluR1 receptors that are coupled to phosphoinositide turnover. Several lines of evidence indicated that activation of mGluR1 by parallel fiber stimulation is necessary for the induction of cerebellar LTD. Because phosphoinositide hydrolysis has two initial products, 1, 2-diacylglycerol and inositol-1,4,5-trisphosphate (IP3), we wished to determine whether IP3 signaling via IP3 receptors and consequent Ca mobilization were necessary for the induction of cerebellar LTD. First, ratiometric imaging of free cytosolic Ca was performed on both acutely dissociated and cultured PNs. It was determined that the threshold for glutamate pulses to contribute to LTD induction was below the threshold for producing a Ca transient. Furthermore, the Ca transients produced by depolarization alone and glutamate plus depolarization were not significantly different. Second, the potent and selective IP3 receptor channel blocker xestospongin C was not found to affect the induction of LTD in either acutely dissociated or cultured PNs at a concentration that was sufficient to block mGluR1-evoked Ca mobilization. Third, replacement of mGluR activation by exogenous synthetic diacylglycerol in an LTD induction protocol was successful. Taken together, these results suggest that activation of an IP3 signaling cascade is not required for induction of cerebellar LTD in reduced preparations.

Neural learning rules for the vestibulo-ocular reflex

Mechanisms for the induction of motor learning in the vestibulo-ocular reflex (VOR) were evaluated by recording the patterns of neural activity elicited in the cerebellum by a range of stimuli that induce learning. Patterns of climbing-fiber, vestibular, and Purkinje cell simple-spike signals were examined during sinusoidal head movement paired with visual image movement at stimulus frequencies from 0.5 to 10 Hz. A comparison of simple-spike and vestibular signals contained the information required to guide learning only at low stimulus frequencies, and a comparison of climbing-fiber and simple-spike signals contained the information required to guide learning only at high stimulus frequencies. Learning could be guided by comparison of climbing-fiber and vestibular signals at all stimulus frequencies tested, but only if climbing fiber responses were compared with the vestibular signals present 100 msec earlier. Computational analysis demonstrated that this conclusion is valid even if there is a broad range of vestibular signals at the site of plasticity. Simulations also indicated that the comparison of vestibular and climbing-fiber signals across the 100 msec delay must be implemented by a subcellular "eligibility" trace rather than by neural circuits that delay the vestibular inputs to the site of plasticity. The results suggest two alternative accounts of learning in the VOR. Either there are multiple mechanisms of learning that use different combinations of neural signals to drive plasticity, or there is a single mechanism tuned to climbing-fiber activity that follows activity in vestibular pathways by approximately 100 msec.

The steroid hormone 20-hydroxyecdysone enhances neurite growth of Drosophila mushroom body neurons isolated during metamorphosis

Mushroom bodies (MBs) are symmetrically paired neuropils in the insect brain that are of critical importance for associative olfactory learning and memory. In Drosophila melanogaster, the MB intrinsic neurons (Kenyon cells) undergo extensive reorganization at the onset of metamorphosis. A phase of rapid axonal degeneration without cell death is followed by axonal regeneration. This re-elaboration occurs as levels of the steroid hormone 20-hydroxyecdysone (20E) are rising during the pupal stage. Based on the known role of 20E in directing many features of CNS remodeling during insect metamorphosis, we hypothesized that the outgrowth of MB axonal processes is promoted by 20E. Using a GAL4 enhancer trap line (201Y) that drives MB-restricted reporter gene expression, we identified Kenyon cells in primary cultures dissociated from early pupal CNS. Paired cultures derived from single brains isolated before the 20E pupal peak were incubated in medium with or without 20E for 2-4 d. Morphometric analysis demonstrated that MB neurons exposed to 20E had significantly greater total neurite length and branch number compared with that of MB neurons grown without hormone. The relationship between branch number and total neurite length remained constant regardless of hormone treatment in vitro, suggesting that 20E enhances the rate of outgrowth from pupal MB neurons in a proportionate manner and does not selectively increase neuritic branching. These results implicate 20E in enhancing axonal outgrowth of Kenyon cells to support MB remodeling during metamorphosis.

Olfactory development in invertebrates. On the scent of central developmental issues

Invertebrate olfactory systems offer many advantages for cellular and molecular studies of development and for functional studies of developmental plasticity. For example, nematodes have chemical senses that can be studied using genetic approaches. Arthropods, which include insects and crustacea, have the advantages that certain neurons can be reliably identified from one individual to another, and that olfactory receptor neurons are located on peripheral appendages and thus can be manipulated independently of their brain targets even very early in development. Among the insects, olfactory learning can be displayed and used as a basis for studying olfactory plasticity in bees; genes are especially tractable in flies; individual growth cones can be visualized in the grasshopper embryo; and receptor neurons and glomeruli of known olfactory specificity and behavioral significance can be followed during early development in moths. In addition, many insect nervous systems are amenable to organ culture and dissociated-cell culture, opening the door to experimental studies of cellular interactions that can not be performed in situ. Recent research in the moth Manduca sexta attempts to identify the nature of the interactions between olfactory sensory axons, olfactory neurons of the brain, and glial cells in the creation of the array of glomeruli that underlie olfaction in the adult. Results indicate that timing of the ingrowth of olfactory receptor axons is critical for normal glomerulus development, that a subset of axons expresses a fasciclin II-like molecule that may play a role in guidance of their growth, and that glial cells must surround developing glomeruli in order to stabilize the 'protoglomerular' template made by receptor axon terminals. Moreover, glial cells are dye-coupled to each other early in glomerulus development and gradually become uncoupled. Electrical activity in neurons is not necessary for glomerulus formation; and some intercellular interactions, perhaps involving soluble factors, appear to involve tyrosine phosphorylation. In sum, a detailed picture is emerging of the cellular interactions that lead to the formation of glomeruli.

Synchronous modulation of perirhinal cortex neuronal activity during cholinergically mediated (type II) hippocampal theta

The perirhinal cortex (PRC) plays a major role in memory processes. This role may be influenced by activity in the adjacent entorhinal cortex (EC) and hippocampus (HPC), particularly during the processing of spatial information. In the current experiment we sought to determine whether the cholinergically mediated (type II) theta rhythm, which is a prominent electrophysiological feature of both HPC and EC activity, influenced neuronal firing in the PRC of urethane-anesthetized rats. When the spontaneous firing activity of single units recorded in PRC was related to theta recorded from the hippocampal fissure, it was determined that the firing of 50/163 (31%) PRC neurons exhibited a statistically significant phase relationship (mean phase angle = 188 degrees) to HPC theta. Thirty-three (66%) of these neurons tended to fire near the trough, and 17 near the peak, of this activity. These data indicate that a high proportion of PRC neurons participate in hippocampal-entorhinal theta activity. This activity may support information transmission and storage within and between these structures.

Gene-trapping to identify and analyze genes expressed in the mouse hippocampus

Mice harboring random gene-trap insertions of a lacZ (beta-galactosidase)-neomycin resistance fusion cassette (beta-geo) were analyzed for expression in the hippocampus. In 4 of 15 lines reporter gene activity was observed in the hippocampal formation. In the obn line, enzyme activity was detected in the CA1-3 hippocampal subfields, in hpk expression was restricted to CA1, but in both lines reporter activity was also present in other brain regions. In the third line, kin, reporter activity was robustly expressed throughout the stratum pyrimidale of CA1-3, with only low-level expression elsewhere. The final line (glnC) displayed ubiquitous expression of the reporter and was not analyzed further. Fusion transcripts for the first three lines were characterized; all encode polypeptides with features of membrane-associated signalling proteins. The obn fusion identified a human cDNA (B2-1) encoding a pleckstrin homology (PH) domain, while hpk sequences matched the Epstein-Barr Virus (EBV) inducible G-protein coupled receptor, EBI-1. kin identified an alternative form of the abl-related nonreceptor tyrosine kinase c-arg. Electrophysiological studies on mice homozygous for the insertions revealed normal synaptic transmission, paired pulse facilitation and paired-pulse depression at Schaffer collateral-commissural CA1 synapses, and normal long-term potentiation (LTP) in obn and kin. hpk mice displayed an increase in hippocampal CA1 long-term potentiation (LTP), suggesting a role for this receptor in synaptic plasticity.

Purkinje cell responses to pontine stimulation CS during rabbit eyeblink conditioning

Previous studies have shown that stimuli typically used as CSs in eyeblink conditioning converge with US information in the cerebellum. Extracellular recordings of Purkinje cells have shown learning-related as well as stimulus-related discharge patterns. Stimulation of a portion of the auditory CS pathway, the pontine nucleus, also serves as a highly effective CS. Using a short-latency pontine stimulation CS and air puff US, single Purkinje cell responses were recorded and compared to those elicited with an auditory stimulus in previous work. Purkinje cell recordings in trained and untrained rabbits revealed patterns of responses very similar to those seen in rabbits trained to a tone CS or those given unpaired-tone/air-puff training. Similarities included the proportion of stimulus-related and behavior-related cell responses. However, fewer inhibitory responses were seen than in earlier studies and these differences are considered in light of the differences between an extracellular stimulation CS and a peripherally administered auditory CS.

Developmental changes of cortical and cerebellar motor control: a clinical positron emission tomography study with children and adults

Functional neuroimaging data regarding the development of motor organization in normal children and adolescents are virtually unavailable because of ethical concerns. As an alternative approach, we studied child and adult lesion patients, focusing on movement of the hand ipsilateral to the lesion and on brain activations in the contralesional hemisphere. [15O]-water positron emission tomography was performed during rest and sequential finger-thumb tapping in 10 children (aged 6 to 14 years) and 15 adults (aged 18 to 74 years) with unilateral lesion. We expected more distinct activation/deactivation patterns during movement in adults than in children. While there were no group differences in activation of primary and secondary motor cortices, deactivations in nonmotor cortex were significantly more pronounced in adults than in children. This indirectly supports our hypothesis of developmental focalization of cerebral motor control. Activations in the cerebellum and vermis were significantly stronger in the adults than in the children, possibly reflecting normal developmental patterns.

Olfactory traces and spatial learning in rats

We conducted an experiment to assess the use of olfactory traces for spatial orientation in an open environment in rats, Rattus norvegicus. We trained rats to locate a food source at a fixed location from different starting points, in the presence or absence of visual information. A single food source was hidden in an array of 19 petri dishes regularly arranged in an open-field arena. Rats were trained to locate the food source either in white light (with full access to distant visuospatial information) or in darkness (without any visual information). In both cases, the goal was in a fixed location relative to the spatial frame of reference. The results of this experiment revealed that the presence of noncontrolled olfactory traces coherent with the spatial frame of reference enables rats to locate a unique position as accurately in darkness as with full access to visuospatial information. We hypothesize that the olfactory traces complement the use of other orientation mechanisms, such as path integration or the reliance on visuospatial information. This experiment demonstrates that rats can rely on olfactory traces for accurate orientation, and raises questions about the establishment of such traces in the absence of any other orientation mechanism. Copyright 1998 The Association for the Study of Animal Behaviour.

The MAPK cascade is required for mammalian associative learning

Mitogen-activated protein kinase (MAPK) is an integral component of cellular signaling during mitogenesis and differentiation of mitotic cells. Recently MAPK activation in post-mitotic cells has been implicated in hippocampal long-term potentiation (LTP), a potential cellular mechanism of learning and memory. Here we investigate the involvement of MAPK in learning and memory in behaving animals. MAPK activation increased in the rat hippocampus after an associative learning task, contextual fear conditioning. Two other protein kinases known to be activated during hippocampal LTP, protein kinase C and alpha-calcium/calmodulin protein kinase II, also were activated in the hippocampus after learning. Inhibition of the specific upstream activator of MAPK, MAPK kinase (MEK), blocked fear conditioning. Thus, classical conditioning in mammals activates MAPK, which is necessary for consolidation of the resultant learning.

Interactions between Depression and Facilitation within Neural Networks: Updating the Dual-Process Theory of Plasticity

Repetitive stimulation often results in habituation of the elicited response. However, if the stimulus is sufficiently strong, habituation may be preceded by transient sensitization or even replaced by enduring sensitization. In 1970, Groves and Thompson formulated the dual-process theory of plasticity to explain these characteristic behavioral changes on the basis of competition between decremental plasticity (depression) and incremental plasticity (facilitation) occurring within the neural network. Data from both vertebrate and invertebrate systems are reviewed and indicate that the effects of depression and facilitation are not exclusively additive but, rather, that those processes interact in a complex manner. Serial ordering of induction of learning, in which a depressing locus precedes the modulatory system responsible for inducing facilitation, causes the facilitation to wane. The parallel and/or serial expression of depression and waning facilitation within the stimulus–response pathway culminates in the behavioral changes that characterize dual-process learning. A mathematical model is presented to formally express and extend understanding of the interactions between depression and facilitation.

Neurotrophin-dependent modulation of glutamatergic synaptic transmission in the mammalian CNS

1. The protein family of the neurotrophins, consisting of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and Neurotrophin-3, -4/5, and -6 (NT-3; NT-4/5; NT-6) is well known to enhance the survival and to stabilize the phenotype of different populations of neurons in the central and the peripheral nervous system. These effects are mediated via binding to specific tyrosine kinase receptors (Trks) and to the low-affinity p75 neurotrophin receptor. 2. The neurotrophins NGF, BDNF, and NT-3 and the BDNF and NT-3 selective receptors (TrkB, TrkC) are expressed at high levels in neurons of the basal forebrain, the hippocampus, and the neocortex of the mammalian brain. The expression and secretion of NGF and BDNF in these brain areas is regulated by (physiological levels of) neuronal activity. 3. Exogenous application of the neurotrophins to hippocampal and neocortical neurons can enhance excitatory glutamatergic synaptic transmission via activation of Trk receptors. In addition, long-term potentiation (a potential cellular correlate for learning and memory formation in mammals) in the rodent hippocampus depends on endogenous supply of neurons with BDNF. 4. Judged by the analysis of electrophysiological data, the BDNF- and NT-3-induced enhancement of glutamatergic synapses is mediated by increasing the efficacy of glutamate release from the presynaptic neuron. However, neurotrophin-dependent postsynaptic enhancement of NMDA (but not AMPA) receptor responsiveness has also been shown. 5. On the molecular level, neither the pre- nor the postsynaptic modulation of glutamatergic synapses by neurotrophins is well understood. However, neurotrophins were shown to acutely affect intraneuronal Ca2+ levels and to influence molecular components of the transmitter release machinery, which could underly the presynaptic modifications, whereas BDNF-induced phosphorylation of NMDA-type glutamate receptors could account for the postsynaptic effects. 6. Taken together, these results suggest that the activity-dependent release of neurotrophins at frequently used synapses could modulate the synaptic efficacy at these junctions. Thus, neurotrophins might operate as locally released feedback modulators of synaptic transmission, and this could be a cellular correlate for certain aspects of information processing in the mammalian brain.

Background gamma-oscillations in neuronal networks with interhemisphere connections

Results obtained in studies of the high-frequency components of EEG recordings and in modeling, determining the conditions for the appearance of gamma oscillations in interneuronal interactions, were compared with features of the background gamma oscillations recorded in the activity of interacting neurons located in symmetrical loci of the right and left hemisphere motor areas in anesthetized rats. Similarities in high frequencies extracted from EEG recordings and in the most commonly observed gamma oscillation frequencies suggested that these oscillations may represent one of the mechanisms underlying the high-frequency EEG component. Published modeling data indicating that the formation of these oscillations involves reciprocal inhibitory connections, along with our own data that interhemisphere oscillations are seen 1.5 times more commonly than ipsilateral oscillations, suggested that transcallosal inhibition is more effective than inhibition between neighboring cells. Simultaneously extracted background oscillations in the interacting activity of callosal cells and neighboring cells could be different, as could those characterizing the activity of individual neurons. It is suggested that these differences underlie the functional heterogeneity of local cortical neuronal networks and explain the fact that these networks contain various types of inhibitory neurons.

Involvement of a postsynaptic protein kinase A substrate in the expression of homosynaptic long-term depression

Hippocampal N-methyl-D-aspartate (NMDA) receptor-dependent long-term synaptic depression (LTD) is associated with a persistent dephosphorylation of the GluR1 subunit of AMPA receptors at a site (Ser-845) phosphorylated by cAMP-dependent protein kinase (PKA). In the present study, we show that dephosphorylation of a postsynaptic PKA substrate may be crucial for LTD expression. PKA activators inhibited both AMPA receptor dephosphorylation and LTD. Injection of a cAMP analog into postsynaptic neurons prevented LTD induction and reversed previously established homosynaptic LTD without affecting baseline synaptic transmission. Moreover, infusing a PKA inhibitor into postsynaptic cells produced synaptic depression that occluded homosynaptic LTD. These findings suggest that dephosphorylation of a PKA site on AMPA receptors may be one mechanism for NMDA receptor-dependent homosynaptic LTD expression.

On the Respective Roles of Nitric Oxide and Carbon Monoxide in Long-Term Potentiation in the Hippocampus

Perfusion of hippocampal slices with an inhibitor of nitric oxide (NO) synthase-blocked induction of long-term potentiation (LTP) produced by a one-train tetanus and significantly reduced LTP by a two-train tetanus, but only slightly reduced LTP by a four-train tetanus. Inhibitors of heme oxygenase, the synthetic enzyme for carbon monoxide (CO), significantly reduced LTP by either a two-train or four-train tetanus. These results suggest that NO and CO are both involved in LTP but may play somewhat different roles. One possibility is that NO serves a phasic, signaling role, whereas CO provides tonic, background stimulation. Another possibility is that NO and CO are phasically activated under somewhat different circumstances, perhaps involving different receptors and second messengers. Because NO is known to be activated by stimulation of NMDA receptors during tetanus, we investigated the possibility that CO might be activated by stimulation of metabotropic glutamate receptors (mGluRs). Consistent with this idea, long-lasting potentiation by the mGluR agonist tACPD was blocked by inhibitors of heme oxygenase but not NO synthase. Potentiation by tACPD was also blocked by inhibitors of soluble guanylyl cyclase (a target of both NO and CO) or cGMP-dependent protein kinase, and guanylyl cyclase was activated by tACPD in hippocampal slices. However, biochemical assays indicate that whereas heme oxygenase is constitutively active in hippocampus, it does not appear to be stimulated by either tetanus or tACPD. These results are most consistent with the possibility that constitutive (tonic) rather than stimulated (phasic) heme oxygenase activity is necessary for potentiation by tetanus or tACPD, and suggest that mGluR activation stimulates guanylyl cyclase phasically through some other pathway.

Connectedness and part-relation integration in shape category learning

We investigated the role of connectedness in the use of part-relation conjunctions for object category learning. Participants learned categories of two-part objects defined by the shape of one part and its location relative to the other (part-relation conjunctions). The topological relationship between the parts (connected, separated, or embedded) varied between participants but was invariant for any given participant. In Experiment 1, category learning was faster and more accurate when an object's parts were connected than when they were either separated or embedded. Subsequent experiments showed that this effect is not due to conscious strategies, differences in the salience of the individual attributes, or differences in the integrality/separability of dimensions across stimuli. The results suggest that connectedness affects the integration of parts with their relations in object category learning.

Acute inactivation of the inferior olive blocks associative learning

Can acute inactivation of the inferior olive block associative learning? We anaesthetized the inferior olive with lidocaine while rabbits simultaneously: (i) performed conditioned nictitating membrane responses to a flashing light to which they had already been trained; and (ii) underwent their first experience with classical conditioning of the same response to a tone. Inactivation of the inferior olive immediately and reversibly abolished the performance of conditioned responses and prevented learning during rabbits' initial conditioning with a tone-conditioned stimulus. When olivary function was restored, rabbits showed no signs of having learned under olivary anaesthesia. The experiment demonstrates that an acute disruption in olivary function can block learning, in addition to severely degrading motor control. The results are interpreted to indicate the importance of the inferior olive in optimizing learning, perhaps through a general role in regulating temporal processing.

Conditioning Method Dramatically Alters the Role of Amygdala in Taste Aversion Learning

Although an important role for the amygdala in taste aversion learning has been suggested by work in a number of laboratories, results have been inconsistent and interpretations varied. The present series of studies reevaluated the role of the amygdala in taste aversion learning by examining the extent to which conditioning methods, testing methods and lesioning methods, influence whether amygdala lesions dramatically affect conditioned taste aversion (CTA) learning. Results indicated that when animals are conditioned with an intraoral (I/O) taste presentation, lesions of amygdala eliminate evidence of conditioning whether animals are tested intraorally or with a two-bottle solution presentation. Dramatic effects of amygdala lesions on CTA learning were seen whether lesions were made electrolytically or using an excitotoxin. In contrast, when animals were conditioned using bottle presentation of the taste, electrolytic lesions attenuated CTAs but did not eliminate them, and excitotoxic lesions had no effect. These results are consistent with the hypothesis that neural structures critical for CTA learning may differ depending on the extent to which the method of conditioned stimulus delivery incorporates a response component.