The role of acetylcholine in cortical synaptic plasticity

The nucleus basalis and memory codes: auditory cortical plasticity and the induction of specific, associative behavioral memory

Receptive field (RF) plasticity develops in the primary auditory cortex (ACx) when a tone conditioned stimulus (CS) becomes associated with an appetitive or aversive unconditioned stimulus (US). This prototypical stimulus-stimulus (S-S) association is accompanied by shifts of frequency tuning of neurons toward or to the frequency of the CS such that the area of best tuning of the CS frequency is increased in the tonotopic representation of the ACx. RF plasticity has all of the major characteristics of behavioral associative memory: it is highly specific, discriminative, rapidly induced, consolidates (becomes stronger and more specific over hours to days) and can be retained indefinitely (tested to two months). Substitution of nucleus basalis (NB) stimulation for a US induces the same associative RF plasticity, and this requires the engagement of muscarinic receptors in the ACx. Pairing a tone with NB stimulation actually induces specific, associative behavioral memory, as indexed by post-training frequency generalization gradients. The degree of acquired behavioral significance of sounds appears to be encoded by the number of neurons that become retuned in the ACx to that acoustic stimulus, the greater the importance, the greater the number of re-tuned cells. This memory code has recently been supported by direct neurobehavioral tests. In toto, these findings support the view that specific, learned auditory memory content is stored in the ACx, and further that this storage of information during learning and the instantiation of the memory code involves the engagement of the nucleus basalis and its release of acetylcholine into target structures, particularly the cerebral cortex.

Basal forebrain cholinergic modulation of auditory activity in the zebra finch song system

The cholinergic basis of auditory "gating" in the sensorimotor nucleus HVc and its efferent target robustus archistriatalis (RA) was investigated in anesthetized zebra finches. Injections of cholinergic agonists carbachol or muscarine into HVc strongly affected discharge rates and diminished auditory responsiveness in both HVc and its target RA, changes toward an awake-like condition. HVc nicotine injections produced similar strong effects in HVc, but weaker and inconsistent effects in RA. Stimulation of basal forebrain (BF) produced an initial transient network shutdown followed by diminished auditory responsiveness in HVc and RA. All stimulation effects were blocked when preceded by HVc injections of nicotinic or muscarinic antagonists. Thus, BF cholinergic modulation of song system auditory activity acting via functionally distinct HVc circuits can contribute to auditory gating. We hypothesize that wakeful BF activity levels block sensory input to motor systems and adaptively change during behavior to allow sensorimotor feedback such as auditory feedback during singing.

Selective cholinergic denervation of the cingulate cortex impairs the acquisition and performance of a conditional visual discrimination in rats

Results from excitotoxic lesion studies have implicated the cingulate cortex and its basal forebrain afferents in the acquisition and performance of conditional discrimination tasks. In the present work, we sought to clarify the role of specifically cholinergic projections from the vertical limb nucleus of the diagonal band (VDB) to the cingulate cortex in conditional visual discrimination (CVD) learning and performance in rats. We injected the cholinergic immunotoxin 192 IgG-saporin into the cingulate cortex to produce selective retrograde lesions of the cholinergic neurons projecting from the VDB to the cingulate cortex with the aim of sparing afferents of non-cingulate regions that can be disrupted by excitotoxic or immunotoxic VDB injections and non-cholinergic VDB projections that can also be damaged by excitotoxic lesions. Rats sustaining selective cholinergic denervation in this manner were significantly impaired relative to sham-operated animals in the acquisition and performance of a CVD rule of the type 'If lights are flashing FAST, press the left lever; if SLOW, press right'. Asymptotic performance of the lesion group was substantially lower than for control rats, indicating an enduring performance deficit. This impairment was associated with a selective disruption on trials with the FAST flashing stimulus. The results confirm the involvement of cholinergic innervation of the cingulate cortex in CVD performance; however, the nature of the deficit suggests a role for cholinergic modulation in task-relevant stimulus processing rather than stimulus-response learning per se.

Cortical acetylcholine release and electroencephalogram activation evoked by ionotropic glutamate receptor agonists in the rat basal forebrain

To determine the sensitivity of basal forebrain cholinergic neurons to ionotropic glutamate receptor activation, acetylcholine was collected from the cerebral cortex of urethane-anesthetized rats using microdialysis while monitoring cortical electroencephalographic (EEG) activity. alpha-Amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA; 1, 10, or 100 microM), N-methyl-D-aspartate (NMDA; 100 or 1000 microM) or a combination of AMPA (10 microM) and NMDA (100 microM) was administered to the basal forebrain using reverse microdialysis. Both glutamate receptor agonists produced concentration-dependent, several-fold increases in acetylcholine release indicating that they activated basal forebrain cholinergic neurons; AMPA was more potent, increasing acetylcholine release at a lower concentration than NMDA. The combination of AMPA and NMDA did not produce any greater release than each drug alone, indicating that the effects of these two drugs on cholinergic neurons are not additive. EEG was analyzed by fast Fourier transforms to determine the extent of physiological activation of the cortex. The highest concentrations of AMPA and NMDA tested produced small (25%) but significant increases in high frequency activity. There was a positive correlation across animals between the increases in power in the beta (14-30 Hz) and gamma (30-58 Hz) ranges and increases in acetylcholine release. These results indicate that glutamate can activate cholinergic basal forebrain neurons via both AMPA and NMDA ionotropic receptors but has a more modest effect on EEG activation.

A 14-day period of hindpaw sensory deprivation enhances the responsiveness of rat cortical neurons

Hypodynamia-hypokinesia (HH) is a model of hindpaw sensory deprivation. It is obtained by unloading of the hindquarters during 14 days. In this situation, the feet are not in contact with the ground and as a consequence, the cutaneous receptors are not activated; the sensory input to the primary somatosensory cortex (SmI) is thus reduced. In a previous study, we have shown that HH induced a cortical reorganisation of the hindlimb representation. The understanding of the mechanisms involved in cortical map plasticity requires a close examination of the changes in response properties of cortical neurons during HH. The aim of the present study was thus to study the characteristics of neurons recorded from granular and infragranular layers in hindlimb representation of SmI. A total of 289 cortical neurons were recorded (158 from control rats and 131 from HH rats) in pentobarbital-anaesthetized rats. Cutaneous threshold, cutaneous receptive fields, spontaneous activity (discharge rate and instantaneous frequency) and activity evoked by air-jet stimulation (response latency and duration, amplitude) were analysed. The present study suggests that activity-dependent changes occur in the cortex. The duration of the spike waveform presented two populations of spikes: thin-spike cells (<1 ms, supposed to be inhibitory interneurons) and regular cells (>1 ms). Thin-spike cells were less frequently encountered in HH than in control rats. The analysis of regular cells revealed that after HH (1) spontaneous activity was unchanged and (2) cortical somatosensory neurons were more responsive: the cutaneous threshold was reduced and the response magnitude increased. Taken together, these results suggest a down-regulation of GABAergic function.

Parafascicular electrical stimulation attenuates nucleus basalis magnocellularis lesion-induced active avoidance retention deficit

Previous experiments from our laboratory showed that retention of two-way active avoidance learning is improved by post-training intracranial electrical stimulation (ICS) of the parafascicular nucleus (PF) and impaired by pre-training electrolytic lesions of the nucleus basalis magnocellularis (NBM). The question investigated here was whether post-training PF ICS is able to attenuate the active avoidance retention deficit observed in rats lesioned pre-training in the NBM. To this goal, the following experimental design was used: rats bilaterally lesioned in the NBM and stimulated in the PF, rats lesioned in the NBM, rats stimulated in the PF, control rats implanted in the PF, and sham-operated rats were first trained in a shuttle-box for a single 30-trial session and tested again following two successive retention intervals (24 h and 11 days). The results showed that: (1) NBM lesions impaired the 11-day performance without affecting either the acquisition or the 24-h retention of the avoidance learning; (2) PF ICS treatment in unlesioned rats improved performance in both retention sessions only when the stimulation was applied in the posterior region of the nucleus; and (3) stimulation of the posterior PF compensated the 11-day retention impairment induced by NBM lesions. These results are discussed in relation to the interaction of arousal systems in the modulation of cognitive processes.

A morphogenetic role for acetylcholine in mouse cerebral neocortex

Recently, cholinergic afferents to cerebral cortex have met renewed attention regarding the regulation of plasticity as well as cognitive processing. My laboratory has developed a mouse neonatal basal forebrain lesion paradigm that has contributed considerably to the understanding of cholinergic mechanisms in cortical development. We have shown that transient cholinergic deafferentation, beginning at birth, precipitates alterations in neuronal differentiation and synaptic connectivity that persist into maturity, and contribute to altered cognitive behavior. These data are in general agreement with studies in rats in which the cholinergic basal forebrain is lesioned very early in development but contrast with effects of later developmental lesions. Moreover, in mouse, both morphological and behavioral consequences of the lesion are sex dependent. Studies of receptors and secondary messengers that are instrumental in morphogenesis and plasticity suggest that sex dependent molecular alterations occur within days if not hours following cortical cholinergic deafferentation.

Development of reorganization of the auditory cortex caused by fear conditioning: effect of atropine

Reorganization of the frequency map in the central auditory system is based on shifts in the best frequencies (BFs; hereafter, BF shifts), together with the frequency-response curves, of auditory neurons. In the big brown bat, conditioning with acoustic stimulation followed by electric leg-stimulation causes BF shifts of collicular and cortical neurons. The collicular BF shift develops quickly and is short term, whereas the cortical BF shift develops slowly and is long term. The acetycholine level in the auditory cortex must be high during conditioning to develop these BF shifts. We studied the effect of atropine (an antagonist of muscarinic acetylcholine receptors) applied to the auditory cortex on the development of the long-term cortical BF shift in the awake bat caused by a 30-min conditioning session. We found 1) the cortical BF shift starts to develop approximately 15 min after the onset of the conditioning, gradually increases over 60 min, and reaches a plateau, 2) the cortical BF shift changes from short to long term approximately 45 min after the onset of the conditioning, 3) the cortical BF shift can plateau at different frequencies between the BF of a given neuron in the control condition and the frequency of the conditioning tone, 4) the maximum BF shift is determined approximately 70 min after the onset of the conditioning, and 5) acetylcholine plays an important role in the development of the cortical BF shift. Its role ends approximately 180 min after the onset of the conditioning.

Effects of attention and emotion on repetition priming and their modulation by cholinergic enhancement

We examined whether behavioral and neural effects of repeating faces are modulated by independent factors of selective attention, emotion, and cholinergic enhancement, during functional MRI. Face repetition occurred either between task-relevant (spatially attended) or task-irrelevant (unattended) stimuli; faces could be fearful or neutral; subjects received either placebo or physostigmine. Under placebo, a reaction time advantage occurred with repetition (i.e., priming) that did not differ between levels of attention, but was attenuated with emotion. Inferior temporo-occipital cortex demonstrated repetition decreases to both attended and unattended faces, and showed either equivalent or greater repetition decreases with emotional compared with neutral faces. By contrast, repetition decreases were attenuated for emotional relative to neutral faces in lateral orbitofrontal cortex. These results distinguish automatic repetition effects in sensory cortical regions from repetition effects modulated by emotion in orbitofrontal cortex, which parallel behavioral effects. Under physostigmine, unlike placebo, behavioral repetition effects were seen selectively for attended faces only, whereas emotional faces no longer impaired priming. Physostigmine enhanced repetition decreases in inferior occipital cortex selectively for attended faces, and reversed the emotional interaction with repetition in lateral orbitofrontal cortex. Thus we show that cholinergic enhancement both augments a neural signature of priming and modulates the effects of attention and emotion on behavioral and neural consequences of repetition.

Electrophysiological evidence for the existence of a posterior cortical-prefrontal-basal forebrain circuitry in modulating sensory responses in visual and somatosensory rat cortical areas

The prefrontal cortex (PFC) receives input from sensory neocortical regions and sends projections to the basal forebrain (BF). The present study tested the possibility that pathways from sensory cortical regions via the PFC-BF and from the BF back to specific sensory cortical areas could modulate sensory responses. Two prefrontal areas that responded to stimulation of the primary somatosensory and visual cortices were delineated: an area encompassing the rostral part of the cingulate cortex that responded to visual cortex stimulation, and a region dorso-lateral to the first in the precentral-motor association area that reacted to somatosensory cortex stimulation. Moreover, BF neurons responded to PFC electrical stimulation. They were located in the ventral pallidum, substantia innominata and the horizontal limb of the diagonal-band areas. Of the responsive BF neurons 42% reacted only to stimulation of 'visually-responsive,' 33% responded only to the 'somatosensory-responsive' prefrontal sites and the remaining neurons reacted to both prefrontal cortical areas. The effect of BF and PFC stimulations on somatosensory and visual-evoked potentials was tested. BF stimulation increased the amplitude of both sensory-evoked potentials. However, stimulation of the 'somatosensory-responsive' prefrontal area increased only somatosensory-evoked potentials while 'visually-responsive' prefrontal-area stimulation increased only visual-evoked potentials. Atropine blocked both facilitatory effects. The proposed cortico-prefronto-basalo-cortical circuitry may have an important role in cortical plasticity and selective attention.

Cholinergic neurotransmission is essential for perirhinal cortical plasticity and recognition memory

We establish the importance of cholinergic neurotransmission to both recognition memory and plasticity within the perirhinal cortex of the temporal lobe. The muscarinic receptor antagonist scopolamine impaired the preferential exploration of novel over familiar objects, disrupted the normal reduced activation of perirhinal neurones to familiar compared to novel pictures, and blocked production of long-term depression (LTD) but not long-term potentiation (LTP) of synaptic transmission in perirhinal slices. The consistency of these effects across the behavioral, systems, and cellular levels of analysis provides strong evidence for the involvement of cholinergic mechanisms in synaptic plastic processes within perirhinal cortex that are necessary for recognition memory.

Rabbit forebrain cholinergic system: morphological characterization of nuclei and distribution of cholinergic terminals in the cerebral cortex and hippocampus

Although the rabbit brain, in particular the basal forebrain cholinergic system, has become a common model for neuropathological changes associated with Alzheimer's disease, detailed neuroanatomical studies on the morphological organization of basal forebrain cholinergic nuclei and on their output pathways are still awaited. Therefore, we performed quantitative choline acetyltransferase (ChAT) immunocytochemistry to localize major cholinergic nuclei and to determine the number of respective cholinergic neurons in the rabbit forebrain. The density of ChAT-immunoreactive terminals in layer V of distinct neocortical territories and in hippocampal subfields was also measured. Another cholinergic marker, the low-affinity neurotrophin receptor (p75(NTR)), was also employed to identify subsets of cholinergic neurons. Double-immunofluorescence labeling of ChAT and p75(NTR), calbindin D-28k (CB), parvalbumin, calretinin, neuronal nitric oxide synthase (nNOS), tyrosine hydroxylase, or substance P was used to elucidate the neuroanatomical borders of cholinergic nuclei and to analyze the neurochemical complexity of cholinergic cell populations. Cholinergic projection neurons with heterogeneous densities were found in the medial septum, vertical and horizontal diagonal bands of Broca, ventral pallidum, and magnocellular nucleus basalis (MBN)/substantia innominata (SI) complex; cholinergic interneurons were observed in the caudate nucleus, putamen, accumbens nucleus, and olfactory tubercule, whereas the globus pallidus was devoid of cholinergic nerve cells. Cholinergic interneurons were frequently present in the hippocampus and to a lesser extent in cerebral cortex. Cholinergic projection neurons, except those localized in SI, abundantly expressed p75(NTR), and a subset of cholinergic neurons in posterior MBN was immunoreactive for CB and nNOS. A strict laminar distribution pattern of cholinergic terminals was recorded both in the cerebral cortex and in CA1-CA3 and dentate gyrus of the hippocampus. In summary, the structural organization and chemoarchitecture of rabbit basal forebrain may be considered as a transition between that of rodents and that of primates.

CS-specific gamma, theta, and alpha EEG activity detected in stimulus generalization following induction of behavioral memory by stimulation of the nucleus basalis

Tone paired with stimulation of the nucleus basalis (NB) induces behavioral memory that is specific to the frequency of the conditioned stimulus (CS), assessed by cardiac and respiration behavior during post-training stimulus generalization testing. This paper focuses on CS-specific spectral and temporal features of conditioned EEG activation. Adult male Sprague-Dawley rats, chronically implanted with a stimulating electrode in the NB and a recording electrode in the ipsilateral auditory cortex, received either tone (6kHz, 70dB, 2s) paired with co-terminating stimulation of the nucleus basalis (0.2s, 100Hz, 80-105 microA, ITI approximately 45s) or unpaired presentation of the stimuli (approximately 200 trials/day for approximately 14 days). CS-specificity was tested 24h post-training by presenting test tones to obtain generalization gradients for the EEG, heart rate, and respiration. Behavioral memory was evident in cardiac and respiratory responses that were maximal to the CS frequency of 6kHz. FFT analyses of tone-elicited changes of power in the delta, theta, alpha, beta1, beta2, and gamma bands in the paired group revealed that conditioned EEG activation (shift from lower to higher frequencies) was differentially spectrally and temporally specific: theta, and alpha to a lesser extent, decreased selectively to 6kHz during and for several seconds following tone presentation while gamma power increased transiently during and after 6kHz. Delta exhibited no CS-specificity and the beta bands showed transient specificity only after several seconds. The unpaired group exhibited neither CS-specific behavioral nor EEG effects. Thus, stimulus generalization tests reveal that conditioned EEG activation is not unitary but rather reflects CS-specificity, with band-selective markers for specific, associative neural processes in learning and memory.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Augmentation of plasticity of the central auditory system by the basal forebrain and/or somatosensory cortex

Auditory conditioning (associative learning) or focal electric stimulation of the primary auditory cortex (AC) evokes reorganization (plasticity) of the cochleotopic (frequency) map of the inferior colliculus (IC) as well as that of the AC. The reorganization results from shifts in the best frequencies (BFs) and frequency-tuning curves of single neurons. Since the importance of the cholinergic basal forebrain for cortical plasticity and the importance of the somatosensory cortex and the corticofugal auditory system for collicular and cortical plasticity have been demonstrated, Gao and Suga proposed a hypothesis that states that the AC and corticofugal system play an important role in evoking auditory collicular and cortical plasticity and that auditory and somatosensory signals from the cerebral cortex to the basal forebrain play an important role in augmenting collicular and cortical plasticity. To test their hypothesis, we studied whether the amount and the duration of plasticity of both collicular and cortical neurons evoked by electric stimulation of the AC or by acoustic stimulation were increased by electric stimulation of the basal forebrain and/or the somatosensory cortex. In adult big brown bats (Eptesicus fuscus), we made the following major findings. 1) Collicular and cortical plasticity evoked by electric stimulation of the AC is augmented by electric stimulation of the basal forebrain. The amount of augmentation is larger for cortical plasticity than for collicular plasticity. 2) Collicular and cortical plasticity evoked by AC stimulation is augmented by somatosensory cortical stimulation mimicking fear conditioning. The amount of augmentation is larger for cortical plasticity than for collicular plasticity. 3) Collicular and cortical plasticity evoked by both AC and basal forebrain stimulations is further augmented by somatosensory cortical stimulation. 4) A lesion of the basal forebrain tends to reduce collicular and cortical plasticity evoked by AC stimulation. The reduction is small and statistically insignificant for collicular plasticity but significant for cortical plasticity. 5) The lesion of the basal forebrain eliminates the augmentation of collicular and cortical plasticity that otherwise would be evoked by somatosensory cortical stimulation. 6) Collicular and cortical plasticity evoked by repetitive acoustic stimuli is augmented by basal forebrain and/or somatosensory cortical stimulation. However, the lesion of the basal forebrain eliminates the augmentation of collicular and cortical plasticity that otherwise would be evoked by somatosensory cortical stimulation. These findings support the hypothesis proposed by Gao and Suga.

Effects of cholinergic enhancement on conditioning-related responses in human auditory cortex

It has previously been shown that cholinergic blockade attenuates conditioning-related neuronal responses in human auditory cortex. The present study was conducted to investigate the effect of cholinergic enhancement on such experience-dependent cortical responses. The cholinesterase inhibitor physostigmine, or a placebo control, were continuously infused into healthy young volunteers, during differential aversive conditioning whilst brain activity was measured using event-related functional magnetic resonance imaging (fMRI). Volunteers were presented with two tones, one of which (CS+) was conditioned by pairing with an electrical shock whereas the other was always presented without the shock (CS-). Conditioning-related activations, expressed as an enhanced blood oxygenation level dependent (BOLD) response to the salient CS+, were evident in left auditory cortex under placebo but not under physostigmine. This absence of conditioning-related activations under physostigmine was due to enhanced responses to the CS- under physostigmine as compared to placebo. We suggest that an overactive cholinergic system leads to increased processing of behaviourally irrelevant stimuli and thus attenuates differential conditioning-related cortical activations.

Reorganization of the cochleotopic map in the bat's auditory system by inhibition

The central auditory system of the mustached bat shows two types of reorganization of cochleotopic (frequency) maps: expanded reorganization resulting from shifts in the best frequencies (BFs) of neurons toward the BF of repetitively stimulated cortical neurons (hereafter centripetal BF shifts) and compressed reorganization resulting from the BF shifts of neurons away from the BF of the stimulated cortical neurons (hereafter centrifugal BF shifts). Facilitation and inhibition evoked by the corticofugal system have been hypothesized to be respectively related to centripetal and centrifugal BF shifts. If this hypothesis is correct, bicuculline (an antagonist of inhibitory GABA-A receptors) applied to cortical neurons would change centrifugal BF shifts into centripetal BF shifts. In the mustached bat, electric stimulation of cortical Doppler-shifted constant-frequency neurons, which are highly specialized for frequency analysis, evokes the centrifugal BF shifts of ipsilateral collicular and cortical Doppler-shifted constant-frequency neurons and contralateral cochlear hair cells. Bicuculline applied to the stimulation site changed the centrifugal BF shifts into centripetal BF shifts. On the other hand, electric stimulation of neurons in the posterior division of the auditory cortex, which are not particularly specialized for frequency analysis, evokes centripetal BF shifts of cortical neurons located near the stimulated cortical neurons. Bicuculline applied to the stimulation site augmented centripetal BF shifts but did not change the direction of the shifts. These observations support the hypothesis and indicate that centripetal and centrifugal BF shifts are both based on a single mechanism consisting of two components: facilitation and inhibition.

Neonatal electrolytic lesions of the basal forebrain stunt plasticity in mouse barrel field cortex

Previous studies have shown that neonatal electrolytic lesions of basal forebrain cholinergic projections in mice lead to a transient cholinergic depletion of neocortex and to permanent alterations in cortical cytoarchitecture and in cognitive performance. The present study examines whether neonatal electrolytic lesions of the basal forebrain modify neocortical plasticity. Using cytochrome oxidase histochemistry, we compared cross-sectional areas of individual barrels in the barrel field of four groups of postnatal day 8 (P8) old mice that on P1 received either (1) right electrolytic lesions of the basal forebrain, (2) left C row 1-4 whisker follicle ablations, (3) combined lesion treatments or (4) ice anesthesia only. The size of barrels in basal forebrain lesioned animals was not significantly different from controls. However, the plastic response to whisker removal was compromised in basal forebrain lesioned animals. An index of plasticity, the ratio of row D/row C areas, was reduced significantly in the combined nBM lesioned/follicle ablation group. Compared to whisker-lesioned mice, the expansion in rows B and D and the shrinkage in the lesioned row C area were diminished in the combined treatment group. The present findings correspond to those from a study of rats injected with a cholinergic immunotoxin [Cereb. Cortex 8 (1998) 63]. These results suggest that cholinergic inputs play a role in regulating plasticity as well as in the morphogenesis of mouse sensory-motor cortex.

Plasticity and corticofugal modulation for hearing in adult animals

The descending (corticofugal) auditory system adjusts and improves auditory signal processing in the subcortical auditory nuclei. The auditory cortex and corticofugal system evoke small, short-term changes of the subcortical auditory nuclei in response to a sound repetitively delivered to an animal. These changes are specific to the parameters characterizing the sound. When the sound becomes significant to the animal through conditioning (associative learning), the changes are augmented and the cortical changes become long-term. There are two types of reorganizations: expanded reorganization resulting from centripetal shifts in tuning curves of neurons toward the values of the parameters characterizing a sound and compressed reorganization resulting from centrifugal shifts in tuning curves of neurons away from these values. The two types of reorganizations are based on a single mechanism consisting of two components: facilitation and inhibition.

The modular organization of brain systems. Basal forebrain: the last frontier

Computational anatomical studies suggest that specific clusters of projection neurons in the basal forebrain together with specific prefrontal and posterior cortical associational regions constitute distributed parts of functional parallel circuits. The predictable sequence of cell clusters consisting of various types of noncholinergic cell populations in the basal forebrain suggests further subdivisions within these circuits. It is possible that similar to the parallel basal ganglia circuits (Alexander and Crutcher, 1990), large number of specialized channels and sub-channels exist within this triangular circuitry that permit parallel, multilevel processing concurrently. The location and size of the active modules may temporarily vary according to the prevalence of state-related diffuse ascending brain stem and specific telencephalic inputs. From this latter group of afferents, the prefrontal input may function as an external threshold control which allocates attentional resources via the basal forebrain to distributed cortical processes in a selective, self-regulatory fashion.

Cholinergic modulation of experience-dependent plasticity in human auditory cortex

The factors that influence experience-dependent plasticity in the human brain are unknown. We used event-related functional magnetic resonance imaging (fMRI) and a pharmacological manipulation to measure cholinergic modulation of experience-dependent plasticity in human auditory cortex. In a differential aversive conditioning paradigm, subjects were presented with high (1600 Hz) and low tones (400 Hz), one of which was conditioned by pairing with an electrical shock. Prior to presentation, subjects were given either a placebo or an anticholinergic drug (0.4 mg iv scopolamine). Experience-dependent plasticity, expressed as a conditioning-specific enhanced BOLD response, was evident in auditory cortex in the placebo group, but not with scopolamine. This study provides in vivo evidence that experience-dependent plasticity, evident in hemodynamic changes in human auditory cortex, is modulated by acetylcholine.

Alpha7 nicotinic acetylcholine receptors occur at postsynaptic densities of AMPA receptor-positive and -negative excitatory synapses in rat sensory cortex

NMDA receptor (NMDAR) activation requires concurrent membrane depolarization, and glutamatergic synapses lacking AMPA receptors (AMPARs) are often considered "silent" in the absence of another source of membrane depolarization. During the second postnatal week, NMDA currents can be enhanced in rat auditory cortex through activation of the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). Electrophysiological results support a mainly presynaptic role for alpha7nAChR at these synapses. However, immunocytochemical evidence that alpha7nAChR is prevalent at postsynaptic sites of glutamatergic synapses in hippocampus and neocortex, along with emerging electrophysiological evidence for postsynaptic nicotinic currents in neocortex and hippocampus, has prompted speculation that alpha7nAChR allows for activation of NMDAR postsynaptically at synapses lacking AMPAR. Here we used dual immunolabeling and electron microscopy to examine the distribution of alpha7nAChR relative to AMPAR (GluR1, GluR2, and GluR3 subunits combined) at excitatory synapses in somatosensory cortex of adult and 1-week-old rats. alpha7nAChR occurred discretely over most of the thick postsynaptic densities in all cortical layers of both age groups. AMPAR immunoreactivity was also detectable at most synapses; its distribution was independent of that of alpha7nAChR. In both age groups, approximately one-quarter of asymmetrical synapses were alpha7nAChR positive and AMPAR negative. The variability of postsynaptic alpha7nAChR labeling density was greater at postnatal day (PD) 7 than in adulthood, and PD 7 neuropil contained a subset of small AMPA receptor-negative synapses with a high density of alpha7nAChR immunoreactivity. These observations support the idea that acetylcholine receptors can aid in activating glutamatergic synapses and work together with AMPA receptors to mediate postsynaptic excitation throughout life.

Metalearning and neuromodulation

This paper presents a computational theory on the roles of the ascending neuromodulatory systems from the viewpoint that they mediate the global signals that regulate the distributed learning mechanisms in the brain. Based on the review of experimental data and theoretical models, it is proposed that dopamine signals the error in reward prediction, serotonin controls the time scale of reward prediction, noradrenaline controls the randomness in action selection, and acetylcholine controls the speed of memory update. The possible interactions between those neuromodulators and the environment are predicted on the basis of computational theory of metalearning.

Serotonin release evoked by tail nerve stimulation in the CNS of aplysia: characterization and relationship to heterosynaptic plasticity

Considerable experimental evidence suggests that serotonin (5-HT) at sensory neuron-->motor neuron (SN-->MN) synapses, as well as other neuronal sites, contributes importantly to simple forms of learning such as sensitization and classical conditioning in Aplysia. However, the actual release of 5-HT in the CNS induced by sensitizing stimuli such as tail shock has not been directly demonstrated. In this study, we addressed this question by (1) immunohistochemically labeling central 5-HT processes and (2) directly measuring with chronoamperometry the release of 5-HT induced by pedal tail nerve (P9) shock onto tail SNs in the pleural ganglion and their synapses onto tail MNs in the pedal ganglion. We found that numerous 5-HT-immunoreactive fibers surround both the SN cell bodies in the pleural ganglion and SN axons in the pedal ganglion. Chronoamperometric detection of 5-HT performed with carbon fiber electrodes implanted in the vicinity of tail SN somata and synapses revealed an electrochemical 5-HT signal lasting approximately 40 sec after a brief shock of P9. 5-HT release was restricted to discrete subregions (modulatory fields) of the CNS, including the vicinity of tail SN soma and synapses ipsilateral to the stimulation. Increasing P9 shock frequency augmented the amplitude of the 5-HT signal and, in parallel, increased SN excitability and SN synaptic transmission onto tail MNs. However, the relationship between the amount of 5-HT release and the two forms of SN plasticity was not uniform: SN excitability increased in a graded manner with increased 5-HT release, whereas synaptic facilitation exhibited a highly nonlinear relationship. The development of chronoamperometric techniques in Aplysia now paves the way for a more complete understanding of the contribution of the serotonergic modulatory pathway to memory processing in this system.

Induction of behavioral associative memory by stimulation of the nucleus basalis

The nucleus basalis (NB) has been implicated in memory formation indirectly, by lesions, pharmacological manipulations, and neural correlates of learning. Prior findings imply that engagement of the NB during learning promotes memory storage. We directly tested this NB-memory hypothesis by determining whether stimulation of the NB induces behavioral associative memory. Rats were trained either with paired tone (6 kHz) and NB stimulation or with the two stimuli unpaired. We later determined the specificity of cardiac and respiratory behavioral responses to the training tone and several other acoustic frequencies. Paired subjects exhibited frequency generalization gradients with a peak of 6 kHz for both cardiac and respiratory behavior. Unpaired subjects exhibited no generalization gradient. The development of such specific, associative behavioral responses indicates that tone paired with NB stimulation induced behavioral associative memory. The discovery of memory induction by direct activation of the NB supports the NB-memory hypothesis and provides a potentially powerful way to control and investigate neural mechanisms of memory.

Atropine prevents the changes in the hindlimb cortical area induced by hypodynamia-hypokinesia

It has been demonstrated that hypodynamia-hypokinesia (HH), a model of sensory disruption, induced a decrease in the cortical hindpaw representation and an enlargement of the cutaneous receptive fields (RFs). The present study was carried out to determine whether chronic application of atropine could prevent this reorganisation. The extent of the hindlimb representation on the somatosensory cortex was determined in control rats (C), rats submitted to HH (HH), and rats submitted to HH with a chronic cortical infusion of atropine (70 mM, HH-ATR). Our results show that the hindpaw cortical area was similar for the HH-ATR and C rats, and was smaller for the HH rats. The distribution of RFs was comparable for the C and HH-ATR groups with a high percentage of small RFs. In contrast, for the HH rats, the percentage of large RFs was higher. Atropine can thus prevent the reduction in the hindlimb cortical area induced by HH. These results suggest that cholinergic mechanisms contribute to cortical plasticity.

Corticofugal modulation of duration-tuned neurons in the midbrain auditory nucleus in bats

Animal sounds, as well as human speech sounds, are characterized by multiple parameters such as frequency, intensity, duration, etc. The central auditory system produces neurons tuned to particular durations and frequencies of sounds emitted by a species. In bats, "duration-tuned" neurons are mostly sensitive to short durations and high frequencies of sounds used for echolocation. They are scattered in the frequency maps of the inferior colliculus and auditory cortex. We found that electric stimulation of cortical duration-tuned neurons modulates collicular duration-tuned neurons in both duration and frequency tuning only when collicular and cortical neurons paired for studies are within +/-4 ms in best duration and within +/-6 kHz in best frequency. There are four types of modulations: sharpening or broadening of duration tuning, and lengthening or shortening of best duration. Sharpening is observed in "matched" collicular neurons whose best durations are the same as those of stimulated cortical neurons, and it is accompanied by augmentation of the auditory responses at their best durations. The other three types of modulations are observed in "unmatched" collicular neurons whose best durations are different from those of stimulated cortical neurons. Lengthening or shortening of best duration is linearly related to the amount of the difference in best duration between collicular and cortical neurons. Corticofugal modulation is specific and systematic according to relationships in both duration and frequency between stimulated cortical and recorded collicular neurons.

Cholinergic modulation of synaptic physiology in deep layer entorhinal cortex of the rat

We have recently shown that cholinergic effects on synaptic transmission and plasticity in the superficial (II/III) layers of the rat medial entorhinal cortex (EC) are similar, but not identical, to those in the hippocampus (Yun et al. [2000] Neuroscience 97:671-676). Because the superficial and deep layers of the EC preferentially convey afferent and efferent hippocampal projections, respectively, it is of interest to compare cholinergic effects between the two regions. We therefore investigated the physiological effects of cholinergic agents in the layer V of medial EC slices under experimental conditions identical to those in the previous study. Bath application of carbachol (0.5 microM) induced transient depression of field potential responses in all cases tested (30 of 30; 18.5% +/- 2.3%) and rarely induced long-lasting potentiation (only 3 of 30; 20.4% +/- 3.2% in successful cases). At 5 microM, carbachol induced transient depression only (20 of 20, 48.9% +/- 2.8%), which was blocked by atropine (10 microM). Paired-pulse facilitation was enhanced during carbachol-induced depression, suggesting presynaptic action of carbachol. Long-term potentiation (LTP) could be induced in the presence of 10 microM atropine by theta burst stimulation, but its magnitude was significantly lower (9.1% +/- 4.7%, n = 15) compared to LTP in control slices (22.4% +/- 3.9%, n = 20). These results, combined with our previous findings, demonstrate remarkably similar cholinergic modulation of synaptic transmission and plasticity across the superficial and deep layers of EC.

Pharmacological modulation of behavioral and neuronal correlates of repetition priming

In this experiment we address the pharmacological modulation of repetition priming, a basic form of learning, using event-related functional magnetic resonance imaging. We measured brain activity in a word-stem completion paradigm in which, before study, volunteers were given either placebo, lorazepam (2 mg orally), or scopolamine (0.4 mg, i.v.). Relative to placebo, both drugs attenuated the behavioral expression of priming. Repetition was associated with a decreased neuronal response in left extrastriate, left middle frontal, and left inferior frontal cortices in the placebo group. Both drugs abolished these "repetition suppression" effects. By showing a concurrence of behavioral and neuronal modulations, the results suggest that GABAergic and cholinergic systems influence the neuronal plasticity necessary for repetition priming.

Enhanced frontal cortex activation in rats by convergent amygdaloid and noxious sensory signals

The modulation of frontal cortical EEG activation to noxious somatosensory (tail pressure) and olfactory (acetone) stimulation by the basal amygdala was examined in urethane-anesthetized rats. Mild tail pressure produced no EEG activation, while acetone (sniffed by freely breathing rats or drawn across the olfactory epithelium in tracheotomized rats) produced a moderate suppression of large-amplitude synchronized EEG patterns. Concurrent, low-intensity 100 Hz stimulation of the basal amygdala permitted EEG activation to tail pressure to occur, and strongly enhanced olfactory-induced cortical activation. These results indicate that excitation of the basal amygdala potentiates frontal cortical responsiveness to aversive sensory events. This may provide a mechanism to facilitate cortical excitability and processing by amygdaloid neuronal activity.

Effects of acetylcholine and atropine on plasticity of central auditory neurons caused by conditioning in bats

In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning curves and best frequencies (hereafter BF shifts) of collicular and cortical neurons, i.e., reorganization of the cochleotopic (frequency) maps in the inferior colliculus (IC) and auditory cortex (AC). The collicular BF shift recovers 180 min after the conditioning, but the cortical BF shift lasts longer than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded that the collicular BF shift is caused by the corticofugal auditory system. The collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it has been hypothesized that the cortical BF shift is mostly caused by both the subcortical (e.g., collicular) BF shift and the activity of nonauditory systems such as the somatosensory cortex excited by an unconditioned leg-stimulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augments the collicular and cortical BF shifts caused by the conditioning and whether atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awake bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the corticofugal system. Atropine applied to the AC reduces the collicular BF shift and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affects the cortical BF shift only slightly. ACh makes the cortical BF shift long-lasting beyond 4 h, but it cannot make the collicular BF shift long-lasting beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings favor the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both necessary to evoke a long-lasting cortical BF shift.

Nucleus basalis magnocellularis electrical stimulation facilitates two-way active avoidance retention, in rats

We studied the effects of post-training intracranial electrical stimulation of the nucleus basalis magnocellularis on two-way active avoidance retention. After the acquisition, rats were stimulated for 20 min, and they were tested again after 24 h or 11 days. The treatment improved memory consolidation, especially in animals with a low initial learning ability. These facilitative effects could be attributed to an enhancement of cortical and/or amygdala activation, leading to an improvement in associative processes and/or cortical plasticity.

Bidirectional modulation of basal forebrain N-methyl-d-aspartate receptor function differentially affects visual attention but not visual discrimination performance

Basal forebrain neuronal circuits, specifically the corticopetal cholinergic system, mediate attentional abilities. The effects of infusions of N-methyl-D-aspartate (NMDA) and the competitive NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) into the basal forebrain were assessed in rats trained in an operant task designed to generate measures of sustained attention performance. Control animals were trained in a cued visual discrimination task devoid of explicit demands on attentional performance, but involving similar basic operant components as the sustained attention task. The effects of intrabasalis infusions of NMDA (1, 3 and 6nmol) and APV (3, 10 and 20nmol) were tested in separate groups of animals. Infusion of neither drug affected the animals' response accuracy in the cued visual discrimination task, indicating that performance in this task remains insufficient to activate basal forebrain NMDA receptors. Infusions of APV in sustained attention task-performing animals selectively decreased the animals' ability to detect visual signals, but spared their ability to reject non-signal events. Conversely, infusions of NMDA into the basal forebrain did not affect the animals' hit rate but increased their number of false alarms, i.e. "claims" for signals in non-signal trials. The concentrations of NMDA infused into the basal forebrain did not result in neurotoxic effects as demonstrated by a separate experiment, which indicated neurodegeneration following the infusion of 30 nmol NMDA as visualized by the Fluoro-Jade method.The effects of APV correspond with the attentional consequences of other manipulations known to impair the functions of cortical cholinergic input. Conversely, the effects of NMDA infusions agree with the hypothesis that overactivity of cortical cholinergic inputs mediates an abnormal overprocessing of the stimulus situation. Basal forebrain NMDA receptor manipulations assist in determining the role of this neuronal system in cognitive processes.