Reticular facilitation of cat visual cortical responses is mediated by nicotinic and muscarinic cholinergic mechanisms

Distribution and effects of the muscarinic receptor subtypes in the primary visual cortex

Muscarinic cholinergic receptors modulate the activity and plasticity of the visual cortex. Muscarinic receptors are divided into five subtypes that are not homogeneously distributed throughout the cortical layers and cells types. This distribution results in complex action of the muscarinic receptors in the integration of visual stimuli. Selective activation of the different subtypes can either strengthen or weaken cortical connectivity (e.g., thalamocortical vs. corticocortical), i.e., it can influence the processing of certain stimuli over others. Moreover, muscarinic receptors differentially modulate some functional properties of neurons during experience-dependent activity and cognitive processes and they contribute to the fine-tuning of visual processing. These functions are involved in the mechanisms of attention, maturation and learning in the visual cortex. This minireview describes the anatomo-functional aspects of muscarinic modulation of the primary visual cortex's (V1) microcircuitry.

Nicotinic filtering of sensory processing in auditory cortex

Although it has been known for decades that the drug nicotine can improve cognitive function, the nature of its effects and the underlying mechanisms are not well understood. Nicotine activates nicotinic acetylcholine (ACh) receptors (nAChRs) that normally are activated by endogenous ACh, presumably "hijacking" the cholinergic contribution to multiple cognitive functions, notably attention. Thus, studying nicotine's effects helps to better understand a commonly used drug as well as functions of nAChRs. Moreover, nicotinic agonists are being developed to treat a variety of disorders that involve attention-related or age-related cognitive dysfunction. Studies have shown that nicotine can enhance processing of attended stimuli and/or reduce processing of distracters; that is, nicotine enhances attentional filtering. To examine potential mechanisms within sensory cortex that may contribute to cognitive functions, here we describe nicotinic actions in primary auditory cortex, where well-characterized neural "filters"-frequency receptive fields-can be exploited to examine nicotinic regulation of cortical processing. Using tone-evoked current-source density (CSD) profiles, we show that nicotine produces complex, layer-dependent effects on spectral and temporal processing that, broadly speaking, enhance responses to characteristic frequency (optimal) stimuli while simultaneously suppressing responses to spectrally distant stimuli. That is, nicotine appears to narrow receptive fields and enhances processing within the narrowed receptive field. Since basic cortical circuitry and nAChR distributions are similar across neocortex, these findings may generalize to neural processing in other sensory regions, and to non-sensory regions where afferent inputs are more difficult to manipulate experimentally. Similar effects across sensory and non-sensory cortical circuits could contribute to nicotinic enhancement of cognitive functions.

Cholinergic modulation of cognition: insights from human pharmacological functional neuroimaging

Evidence from lesion and cortical-slice studies implicate the neocortical cholinergic system in the modulation of sensory, attentional and memory processing. In this review we consider findings from sixty-three healthy human cholinergic functional neuroimaging studies that probe interactions of cholinergic drugs with brain activation profiles, and relate these to contemporary neurobiological models. Consistent patterns that emerge are: (1) the direction of cholinergic modulation of sensory cortex activations depends upon top-down influences; (2) cholinergic hyperstimulation reduces top-down selective modulation of sensory cortices; (3) cholinergic hyperstimulation interacts with task-specific frontoparietal activations according to one of several patterns, including: suppression of parietal-mediated reorienting; decreasing ‘effort’-associated activations in prefrontal regions; and deactivation of a ‘resting-state network’ in medial cortex, with reciprocal recruitment of dorsolateral frontoparietal regions during performance-challenging conditions; (4) encoding-related activations in both neocortical and hippocampal regions are disrupted by cholinergic blockade, or enhanced with cholinergic stimulation, while the opposite profile is observed during retrieval; (5) many examples exist of an ‘inverted-U shaped’ pattern of cholinergic influences by which the direction of functional neural activation (and performance) depends upon both task (e.g. relative difficulty) and subject (e.g. age) factors. Overall, human cholinergic functional neuroimaging studies both corroborate and extend physiological accounts of cholinergic function arising from other experimental contexts, while providing mechanistic insights into cholinergic-acting drugs and their potential clinical applications.

Input-selective potentiation and rebalancing of primary sensory cortex afferents by endogenous acetylcholine

Acetylcholine (ACh) plays important roles in the modulation of activity and plasticity of primary sensory cortices, thus influencing sensory detection and integration. We examined this in urethane-anesthetized rats, comparing cholinergic modulation of short latency, large amplitude field postsynaptic potentials (fPSPs) in the visual cortex (V1) evoked by stimulation of the ipsilateral lateral geniculate nucleus (LGN), reflecting direct thalamocortical inputs, with longer latency, smaller amplitude fPSPs elicited by contralateral LGN stimulation, reflecting indirect, polysynaptic inputs. Basal forebrain (BF) stimulation (100 Hz) produced a significant (approximately 45%), gradually developing potentiation of the smaller, contralateral fPSPs, while ipsilateral fPSPs showed less enhancement (approximately 15%), shifting the relative strength of dominant/ipsi- and weaker/contralateral inputs to V1. Systemic or local, cortical blockade of muscarinic receptors (scopolamine) reduced potentiation of contralateral fPSP without affecting ipsilateral enhancement, thus preventing the relative amplification of contralateral inputs following BF stimulation. Systemic nicotinic receptor blockade (mecamylamine) resulted in depression of ipsilateral, and reduced enhancement of contralateral fPSPs after BF stimulation. N-methyl-D-aspartate receptor blockade (systemic MK-801) abolished ipsilateral fPSP enhancement without affecting contralateral potentiation. Neither drug reduced the amplification of contralateral relative to ipsilateral signals in V1. In a second experiment in the barrel cortex, BF stimulation enhanced multiunit activity elicited by whisker deflection in a muscarinic-sensitive manner. Similar to the observations in V1, this effect was more pronounced for weaker multiunit activity driven by a surround whisker than activity following principal whisker deflection. These experiments demonstrate that ACh release following BF stimulation exerts surprisingly selective effects to amplify non-dominant inputs to sensory cortices. We suggest that, by diminishing the imbalance between different afferent signals, ACh release during states of behavioral activation acts to induce a long-lasting facilitation of the detection and/or integration of signals in primary sensory fields of the cortical mantle.

(Micro)Saccades, corollary activity and cortical oscillations

In natural vision, attention and eye movements are linked. Furthermore, eye movements structure the inflow of information into the visual system. Saccades, where little vision occurs, alternate with fixations, when most vision occurs. A mechanism must be in place to maximize information intake during fixations. Oscillatory synchrony has been proposed as a mechanism for rapid and reliable communication of signals, subserving cognitive functions such as attention and object identification. We propose that saccade-related corollary activity has a crucial role in anticipatory preparation of visual centers, which interacts with ongoing oscillation, favoring the processing of postfixational signals. During prolonged fixations, microsaccades could be generated to exploit this mechanism. Studying this interplay between the sensory and the motor system will provide novel insight into the dynamics of natural vision.

Layout of transcallosal activity in cat visual cortex revealed by optical imaging

The contribution of interhemispheric connections to functional maps in cat visual cortex was investigated by using optical imaging of intrinsic signals. In order to isolate the functional inputs arriving via the corpus callosum (CC) from other inputs, we used the split-chiasm preparation. The regions activated through the CC in visual areas 17 (A17) and 18 (A18) were localized and characterized by stimulating monocularly split-chiasm cats with moving, high contrast oriented gratings. We found that the CC mediates the activation of orientation selective domains in the transition zone (TZ) between A17 and A18 and occasionally within portions of both of these areas. We observed transcallosally activated orientation domains all along the TZ without any obvious interruption, and these domains were arranged around "pinwheel" centers. Interestingly, the TZ was divided in two parallel regions, which resemble A17 and A18 in their preferred temporal and spatial frequencies. Finally, we demonstrated that orientation maps evoked through the transcallosal and geniculo-cortical pathways were similar within the TZ, indicating a convergence of inputs of matching orientations in this region. These results contribute to a better understanding of the role of the CC in visual perception of orientations and shapes, at the level of the visual cortex.

Stabilization of thalamo-cortical long-term potentiation by the amygdala: cholinergic and transcription-dependent mechanisms

Synaptic potentiation allows neurons to enhance excitability and store information for extended time periods. We examined the role of the amygdaloid complex, known to facilitate long-term memory encoding, to influence synaptic strength at thalamo-cortical synapses. In urethane-anaesthetized rats, theta-burst stimulation of the dorsal lateral geniculate nucleus of the thalamus induced early phase (1-2 h) long-term potentiation (LTP) of the field postsynaptic potential (fPSP) recorded in the ipsilateral primary visual cortex. Electrical stimulation (100 Hz) of the amygdala 5 min after thalamic stimulation converted early phase LTP to stable late-phase (> 4 h) LTP. This effect was not correlated with the degree of electrocorticographic activation of V1 induced by amygdala stimulation. Amygdala stimulation without thalamic theta-burst stimulation did not change thalamo-cortical fPSPs. The centrally acting cholinergic-muscarinic receptor antagonist scopolamine (1 mg/kg, i.p.), but not peripherally acting methyl-scopolamine, completely blocked the amygdala-induced conversion of early to late-phase thalamo-cortical LTP. Further, ventricular application of the transcription inhibitor anisomycin (250 micro g) reduced amygdala-induced late-phase LTP induction. These results demonstrate that the amygdaloid complex transforms time-limited synaptic enhancement of thalamo-cortical transmission into long lasting increases in synaptic strength. These processes are mediated, at least in part, by cholinergic and transcription-dependent mechanisms. These amygdaloid-induced effects provide a potential mechanism underlying long-term enhancement of sensory transmission and information encoding in thalamo-cortical networks.

Contribution of acetylcholine to visual cortex plasticity

Acetylcholine is involved in a variety of brain functions. In the visual cortex, the pattern of cholinergic innervation varies considerably across different mammalian species and across different cortical layers within the same species. The physiological effects of acetylcholine in the visual cortex display complex responses, which are likely due to cholinergic receptor subtype composition in cytoplasm membrane as well as interaction with other transmitter systems within the local neural circuitry. The functional role of acetylcholine in visual cortex is believed to improve the signal-to-noise ratio of cortical neurons during visual information processing. Available evidence suggests that acetylcholine is also involved in experience-dependent visual cortex plasticity. At the level of synaptic transmission, activation of muscarinic receptors has been shown to play a permissive role in visual cortex plasticity. Among the muscarinic receptor subtypes, the M(1) receptor seems to make a predominant contribution towards modifications of neural circuitry. The signal transduction cascade of the cholinergic pathway may act synergistically with that of the NMDA receptor pathway, whose activation is a prerequisite for cortical plasticity.

The role of acetylcholine in cortical synaptic plasticity

This review examines the role of acetylcholine in synaptic plasticity in archi-, paleo- and neocortex. Studies using microiontophoretic application of acetylcholine in vivo and in vitro and electrical stimulation of the basal forebrain have demonstrated that ACh can produce long-lasting increases in neural responsiveness. This evidence comes mainly from models of heterosynaptic facilitation in which acetylcholine produces a strengthening of a second, noncholinergic synaptic input onto the same neuron. The argument that the basal forebrain cholinergic system is essential in some models of plasticity is supported by studies that have selectively lesioned the cholinergic basal forebrain. This review will examine the mechanisms whereby acetylcholine might induce synaptic plasticity. It will also consider the neural circuitry implicated in these studies, namely the pathways that are susceptible to cholinergic plasticity and the neural regulation of the cholinergic system.

Cellular and network mechanisms of rhythmic recurrent activity in neocortex

The neocortex generates periods of recurrent activity, such as the slow (0.1-0.5 Hz) oscillation during slow-wave sleep. Here we demonstrate that slices of ferret neocortex maintained in vitro generate this slow (< 1 Hz) rhythm when placed in a bathing medium that mimics the extracellular ionic composition in situ. This slow oscillation seems to be initiated in layer 5 as an excitatory interaction between pyramidal neurons and propagates through the neocortex. Our results demonstrate that the cerebral cortex generates an 'up' or depolarized state through recurrent excitation that is regulated by inhibitory networks, thereby allowing local cortical circuits to enter into temporarily activated and self-maintained excitatory states. The spontaneous generation and failure of this self-excited state may account for the generation of a subset of cortical rhythms during sleep.

The effects of nicotine on the 13 Hz steady-state visually evoked potential

OBJECTIVES

The high alpha/low beta range of the spontaneous EEG appears to be particularly sensitive to the effects of nicotine. The present study examined the acute effects of nicotine on the topography of the 13 Hz steady-state visually evoked potential (SSVEP).

METHODS

Thirteen moderate smokers participated in a repeated-measures design. The amplitude and latency of the SSVEP elicited by an unstructured sinusoidal 13 Hz flicker following a <0. 05 mg nicotine cigarette were compared to those following a 0.8 mg nicotine cigarette.

RESULTS

The nicotine condition was associated with an increase in the amplitude of the SSVEP, when compared to the placebo condition, and this increase was greatest in central and right parietal regions. The latency of the SSVEP was reduced in the nicotine condition in bilateral frontal and right parietal regions.

CONCLUSIONS

These results are similar to the effects of nicotine seen in studies examining spontaneous EEG, and are consistent with other studies indicating that the 13 Hz SSVEP indexes brain electrical activity in the high alpha/low beta range. The findings are discussed in terms of possible functional significance of nicotine-induced cortical activation in this frequency range.

The basal forebrain corticopetal system revisited

The medial septum, diagonal bands, ventral pallidum, substantia innominata, globus pallidus, and internal capsule contain a heterogeneous population of neurons, including cholinergic and noncholinergic (mostly GABA containing), corticopetal projection neurons, and interneurons. This highly complex brain region, which constitutes a significant part of the basal forebrain has been implicated in attention, motivation, learning, as well as in a number of neuropsychiatric disorders, such as Alzheimer's disease, Parkinson's disease, and schizophrenia. Part of the difficulty in understanding the functions of the basal forebrain, as well as the aberrant information-processing characteristics of these disease states lies in the fact that the organizational principles of this brain area remained largely elusive. On the basis of new anatomical data, it is proposed that a large part of the basal forebrain corticopetal system be organized into longitudinal bands. Considering the topographic organization of cortical afferents to different divisions of the prefrontal cortex and a similar topographic projection of these prefrontal areas to basal forebrain regions, it is suggested that several functionally segregated cortico-prefronto-basal forebrain-cortical circuits exist. It is envisaged that such specific "triangular" circuits could amplify selective attentional processing in posterior sensory cortical areas.

The effect of brainstem stimulation on the evoked potentials in the intergeniculate leaflet

We have investigated the effect of the laterodorsal tegmental (LDTg) stimulation on evoked potentials in the intergeniculate leaflet (IGL) of the rat, in order to characterize how non-specific systems of the brain, whose activity indicates the influence of non-photic information, impact the activity of the IGL. IGL responses were evoked by electrical stimulation of contralateral suprachiasmatic nuclei (SCN). The amplitude of the evoked potentials was, in all experiments, significantly reduced after the LDTg stimulation. This effect indicated strong neuronal integration between the brainstem reticular formation and the IGL. These results are discussed in relation to the putative role of GABAergic projection.

Differential regulation of neocortical synapses by neuromodulators and activity

Synapses are continually regulated by chemical modulators and by their own activity. We tested the specificity of regulation in two excitatory pathways of the neocortex: thalamocortical (TC) synapses, which mediate specific inputs, and intracortical (IC) synapses, which mediate the recombination of cortical information. Frequency-sensitive depression was much stronger in TC synapses than in IC synapses. The two synapse types were differentially sensitive to presynaptic neuromodulators: only IC synapses were suppressed by activation of GABA(B) receptors, only TC synapses were enhanced by nicotinic acetylcholine receptors, and muscarinic acetylcholine receptors suppressed both synapse types. Modulators also differentially altered the frequency sensitivity of the synapses. Our results suggest a mechanism by which the relative strength and dynamics of input and associational pathways of neocortex are regulated during changes in behavioral state.

Different effects of cholinergic agents on responses recorded from the cat visual cortex and lateral geniculate nucleus dorsalis

We investigated the effect of cholinergic agents on the cat visual evoked potentials (VEPs) recorded from the primary visual cortex (V1) and lateral geniculate nucleus dorsalis (LGNd) to determine on which level of the visual pathway the cholinergic system acts. VEPs to the alternation of 0.1 cycles per degree sinusoidal gratings at 1 and 4 Hz were recorded from N2O-anesthetized cats directly from the surface of V1 and LGNd. The depth of recording in LGNd was determined by the site where the maximal response was obtained by 1 Hz stimulation. VEPs to 4 Hz stimulation, which showed sinusoidal waveforms and were analyzed by fast Fourier transforms, were used as indicators for modulation by cholinergic agents. Physostigmine, an acetylcholinesterase inhibitor, 0.7 mg/kg i.v., suppressed the amplitude of the responses more at V1 (suppression ratio: mean +/- SD, 85.4 +/- 9.3%) than at LGNd (32.4 +/- 30.7%) (P < 0.05). Conversely, scopolamine, a muscarinic receptor blocker, 0.7 mg/kg i.v., increased the amplitude of the responses more at V1 (enhancement ratio: mean +/- SD, 60.3 +/- 22.3%) than at LGNd (-22.2 +/- 22.5%) (P < 0.05). These results indicate that the V1 changes reflect a direct cortical cholinergic effect, probably by modulating the cholinergic projection from the nucleus basalis of Meynert to V1.

A population of nicotinic receptors is associated with thalamocortical afferents in the adult rat: laminal and areal analysis

In the adult rat brain, a prominent population of nicotinic cholinoceptors binds 3H-nicotine with nanomolar affinity. These receptors are abundant in most thalamic nuclei and in neocortical layers 3/4, which receive a major thalamic input. To test whether cortical nicotinic receptors are associated with thalamocortical afferents, unilateral excitotoxic (N-methyl-D-aspartate) lesions were made in one of four thalamic nuclear groups (anterior, ventral, medial geniculate, or dorsal lateral geniculate) or in temporal cortex. After 1 or 4 weeks of survival, cortical 3H-nicotine binding was quantified via autoradiography. Thalamic lesions resulted in a partial loss of 3H-nicotine binding in ipsilateral cerebral cortex. In each thalamic lesion group, the greatest decrease (35-45%) occurred within the cortical layers and area (i.e., cingulate, parietal, temporal, or occipital cortex) receiving the densest thalamocortical innervation. Binding of 3H-nicotine was also reduced within the thalamus local to the lesion, particularly at the longer survival time. Saturation analysis, performed in frontoparietal cortical tissue homogenates following ventral thalamic lesions, revealed a significant (34%) reduction in receptor density but not affinity. Direct excitotoxic lesions of the neocortex (temporal cortex) tended to preserve 3H-nicotine binding in layers 3/4, despite local neuronal loss. These results, taken with other published findings, suggest that some nicotinic cholinoceptors in adult rat cerebral cortex are located on thalamocortical terminals. This organizing principle appears to apply not only to sensory and motor relay projections but also to association nuclei that project to allocortical areas. These receptors may provide a local mechanism for nicotinic cholinergic modulation of thalamocortical input.

Contrast discrimination: a model and a hypothesis concerning the role of cholinergic modulation in contrast perception

A model of contrast discrimination performance in human observers is developed and then extended to cover effects on performance of anticholinergic drugs. It is shown that it is necessary to assume that neural noise increases at high spatial frequencies in order to provide a satisfactory model of variations in discrimination performance with spatial frequency. The model results are compared with the results of empirical studies in which the effects of the muscarinic antagonist scopolamine (hyoscine) on contrast discrimination performance in human observers are examined. The purpose of the pharmacological work is to test the hypothesis that the differential contrast gain found psychophysically at different spatial frequencies might reflect differential facilitation by extrinsic cholinergic neurons. Contrast discrimination and contrast increment detection are found to be impaired by scopolamine in a manner that depends on both spatial frequency and base contrast. By comparing the empirical data with the predictions of the model, it is concluded that contrast constancy may reflect differential cholinergic modulation.

The effect of muscarinic and nicotinic ACh antagonist on the facilitation of rat visual cortical responses

Combined blockade of both muscarinic and nicotinic receptors fully eliminated the effect of reticular facilitation in rat visual cortex. However, this effect lasted only 2 h, what can suggest that there may be other excitatory input to the cholinergic neurones in rat visual cortex which is activated after blockade of cholinergic transmission.

Structures mediating cholinergic reticular facilitation of cortical responses in the cat: effects of lesions in immunocytochemically characterized projections

Cholinergic afferents to the neocortex controlled by the mesencephalic reticular formation (MRF) are known to transiently facilitate cortical excitability. In an attempt to identify the pathway mediating this effect in the cat visual cortex we combined retrograde tracing techniques with immunocytochemical methods to visualize the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT). In addition we examined, in acute electrophysiological experiments, whether local neurotoxin injections into nuclei of the basal forebrain interfered with the reticular facilitation of cortical evoked potentials. Cholinergic projections to area 17 originate from different centers in the homolateral substantia innominata/internal capsule, the septal nuclei, and the nuclei of the diagonal band of Broca. No direct cholinergic projection from the MRF to the visual cortex was observed. Retrogradely labelled cells intermingled with ChAT-positive neurons in the brainstem generally revealed immunopositivity for catecholaminergic markers. Local injections of neurotoxins in the substantia innominata blocked reticular facilitation, whereas local lesions of the septal nuclei and the nuclei of the diagonal band had no effect on MRF-induced facilitation. The blockage of the reticular facilitation of cortical evoked responses after unilateral lesions of the substantia innominata was bilateral, suggesting a cooperative interaction between basal forebrain structures of the two hemispheres. The anatomical and physiological data are discussed with respect to possible mechanisms of transient brainstem influences on cortical excitability.