In vivo Hebbian and basal forebrain stimulation treatment in morphologically identified auditory cortical cells

Alternation of Neuronal Feature Selectivity Induced by Paired Optogenetic-Mechanical Stimulation in the Barrel Cortex

Paired stimulation has been applied to modulate neuronal functions in the primary somatosensory cortex but its utility in the alternation of tuning function, such as direction tuning for whisker stimuli, remains unclear. In the present study, we attempted to manipulate feature preferences in barrel cortical neurons using repetitive paired whisker deflection combined with optogenetic stimulation and to obtain optimal parameters that can induce neuroplasticity. We found no significant response changes across stimulus parameters, such as onset asynchronies and paired directions. Only when paired stimulation was applied in the nonpreferred direction of the principal whisker of a neuron, were the neuron’s responses enhanced in that direction. Importantly, this effect was only observed when the optogenetic stimulus preceded the mechanical stimulus. Our findings indicate that repetitive paired optogenetic-mechanical stimulation can induce in vivo neuroplasticity of feature selectivity in limited situations.

Beyond traditional approaches to understanding the functional role of neuromodulators in sensory cortices

Over the last two decades, a vast literature has described the influence of neuromodulatory systems on the responses of sensory cortex neurons (review in Gu, 2002; Edeline, 2003; Weinberger, 2003; Metherate, 2004, 2011). At the single cell level, facilitation of evoked responses, increases in signal-to-noise ratio, and improved functional properties of sensory cortex neurons have been reported in the visual, auditory, and somatosensory modality. At the map level, massive cortical reorganizations have been described when repeated activation of a neuromodulatory system are associated with a particular sensory stimulus. In reviewing our knowledge concerning the way the noradrenergic and cholinergic system control sensory cortices, I will point out that the differences between the protocols used to reveal these effects most likely reflect different assumptions concerning the role of the neuromodulators. More importantly, a gap still exists between the descriptions of neuromodulatory effects and the concepts that are currently applied to decipher the neural code operating in sensory cortices. Key examples that bring this gap into focus are the concept of cell assemblies and the role played by the spike timing precision (i.e., by the temporal organization of spike trains at the millisecond time-scale) which are now recognized as essential in sensory physiology but are rarely considered in experiments describing the role of neuromodulators in sensory cortices. Thus, I will suggest that several lines of research, particularly in the field of computational neurosciences, should help us to go beyond traditional approaches and, ultimately, to understand how neuromodulators impact on the cortical mechanisms underlying our perceptual abilities.

Brain state-dependence of electrically evoked potentials monitored with head-mounted electronics

Inferring changes in brain connectivity is critical to studies of learning-related plasticity and stimulus-induced conditioning of neural circuits. In addition, monitoring spontaneous fluctuations in connectivity can provide insight into information processing during different brain states. Here, we quantified state-dependent connectivity changes throughout the 24-h sleep-wake cycle in freely behaving monkeys. A novel, head-mounted electronic device was used to electrically stimulate at one site and record evoked potentials at other sites. Electrically evoked potentials (EEPs) revealed the connectivity pattern between several cortical sites and the basal forebrain. We quantified state-dependent changes in the EEPs. Cortico-cortical EEP amplitude increased during slow-wave sleep, compared to wakefulness, while basal-cortical EEP amplitude decreased. The results demonstrate the utility of using portable electronics to document state-dependent connectivity changes in freely behaving primates.

Induction of selective plasticity in the frequency tuning of auditory cortex and auditory thalamus neurons by locus coeruleus stimulation

Neurons in primary sensory cortices display selective receptive field plasticity in behavioral situations ranging from classical conditioning to attentional tasks, and it is generally assumed that neuromodulators promote this plasticity. Studies have shown that pairing a pure-tone and a stimulation of the nucleus basalis magnocellularis mimics the selective receptive field facilitations described after classical conditioning. Here, we evaluated the consequences of repeated pairings between a particular sound frequency and a phasic stimulation of locus coeruleus (LC) on the frequency tuning of auditory thalamus and auditory cortex neurons. Selective alterations for the paired frequency were observed for more than 30% of the cells recorded both in cortex and in thalamus. There were as much selective increases as selective decreases at the cortical level, whereas selective increases were prevailing at the thalamic level. Selective changes usually persisted 15 min after pairing in cortex; they dissipated in thalamus, and so did the general increases in both structures. In animals with stimulation sites outside the LC, pairing induced either general changes or no effect. These results indicate that the selective plasticity induced in the frequency tuning of auditory cortex neurons by LC stimulation is bidirectional, thereby suggesting that noradrenergic activation can contribute to the different forms of plasticity observed after distinct behavioral paradigms.

Sound-guided shaping of the receptive field in the mouse auditory cortex by basal forebrain activation

The mammalian auditory cortex undergoes continuous plasticity following auditory experience. This study demonstrates the instructive roles of sound frequency and amplitude in representational plasticity in the primary auditory cortex of the mouse. Electrical stimulation of the basal forebrain paired with a tone led to a pronounced shift in the receptive field of the cortical neurons in both frequency and amplitude domains, the shift being towards the frequency and amplitude of the tone. Importantly, the plasticity in the frequency tuning of cortical neurons appeared to be largely dependent upon frequency-specific decreases in the response threshold. The minimum threshold of cortical neurons could be reduced only if the amplitude of the presented tone was lower than the minimum threshold. This finding suggests that training with low-intensity sound can increase the sensitivity of cortical neurons. Furthermore, all of these effects evoked by basal forebrain activation could be eliminated by cortical application of atropine, the acetylcholine muscarinic receptor antagonist. The data suggest that cortical plasticity is guided by both sound frequency and amplitude. The basal forebrain promotes sound-guided cortical plasticity by facilitating neural mechanisms intrinsic to the auditory system.

Noradrenergic Induction of Selective Plasticity in the Frequency Tuning of Auditory Cortex Neurons

Neuromodulators have long been viewed as permissive factors in experience-induced cortical plasticity, both during development and in adulthood. Experiments performed over the last two decades have reported the potency of acetylcholine to promote changes in functional properties of cortical cells in the auditory, visual, and somatosensory modality. In contrast, very few attempts were made with the monoaminergic systems. The present study evaluates how repeated presentation of brief pulses of noradrenaline (NA) concomitant with presentation of a particular tone frequency changes the frequency tuning curves of auditory cortex neurons determined at 20 dB above threshold. After 100 trials of NA-tone pairing, 28% of the cells (19/67) exhibited selective tuning modifications for the frequency paired with NA. All the selective effects were obtained when the paired frequency was within 1/4 of an octave from the initial best frequency. For these cells, selective decreases were prominent (15/19 cases), and these effects lasted ≥15 min after pairing. No selective effects were observed under various control conditions: tone alone ( n = 10 cells), NA alone ( n = 11 cells), pairing with ascorbic acid ( n = 6 cells), or with GABA ( n = 20 cells). Selective effects were observed when the NA-tone pairing was performed in the presence of propranolol (4/10 cells) but not when it was performed in the presence phentolamine (0/13 cells), suggesting that the effects were mediated by alpha receptors. These results indicate that brief increases in noradrenaline concentration can trigger selective modifications in the tuning curves of cortical neurons that, in most of the cases, go in opposite direction compared with those usually reported with acetylcholine.

Acetylcholine-dependent potentiation of temporal frequency representation in the barrel cortex does not depend on response magnitude during conditioning

The response properties of neurons of the postero-medial barrel sub-field of the somatosensory cortex (the cortical structure receiving information from the mystacial vibrissae can be modified as a consequence of peripheral manipulations of the afferent activity. This plasticity depends on the integrity of the cortical cholinergic innervation, which originates at the nucleus basalis magnocellularis (NBM). The activity of the NBM is related to the behavioral state of the animal and the putative cholinergic neurons are activated by specific events, such as reward-related signals, during behavioral learning. Experimental studies on acetylcholine (ACh)-dependent cortical plasticity have shown that ACh is needed for both the induction and the expression of plastic modifications induced by sensory-cholinergic pairings. Here we review and discuss ACh-dependent plasticity and activity-dependent plasticity and ask whether these two mechanisms are linked. To address this question, we analyzed our data and tested whether changes mediated by ACh were activity-dependent. We show that ACh-dependent potentiation of response in the barrel cortex of rats observed after sensory-cholinergic pairing was not correlated to the changes in activity induced during pairing. Since these results suggest that the effect of ACh during pairing is not exerted through a direct control of the post-synaptic activity, we propose that ACh might induce its effect either pre- or post-synaptically through activation of second messenger cascades.

The thalamo-cortical auditory receptive fields: regulation by the states of vigilance, learning and the neuromodulatory systems

The goal of this review is twofold. First, it aims to describe the dynamic regulation that constantly shapes the receptive fields (RFs) and maps in the thalamo-cortical sensory systems of undrugged animals. Second, it aims to discuss several important issues that remain unresolved at the intersection between behavioral neurosciences and sensory physiology. A first section presents the RF modulations observed when an undrugged animal spontaneously shifts from waking to slow-wave sleep or to paradoxical sleep (also called REM sleep). A second section shows that, in contrast with the general changes described in the first section, behavioral training can induce selective effects which favor the stimulus that has acquired significance during learning. A third section reviews the effects triggered by two major neuromodulators of the thalamo-cortical system--acetylcholine and noradrenaline--which are traditionally involved both in the switch of vigilance states and in learning experiences. The conclusion argues that because the receptive fields and maps of an awake animal are continuously modulated from minute to minute, learning-induced sensory plasticity can be viewed as a "crystallization" of the receptive fields and maps in one of the multiple possible states. Studying the interplays between neuromodulators can help understanding the neurobiological foundations of this dynamic regulation.