Stimulus-specific adaptation at the synapse level in vitro

Predictive encoding of pure tones and FM-sweeps in the human auditory cortex

Expectations substantially influence perception, but the neural mechanisms underlying this influence are not fully understood. A prominent view is that sensory neurons encode prediction error with respect to expectations on upcoming sensory input. Although the encoding of prediction error has been previously demonstrated in the human auditory cortex (AC), previous studies often induced expectations using stimulus repetition, potentially confounding prediction error with neural habituation. These studies also measured AC as a single population, failing to consider possible predictive specializations of different AC fields. Moreover, the few studies that considered prediction error to stimuli other than pure tones yielded conflicting results. Here, we used functional magnetic resonance imaging (fMRI) to systematically investigate prediction error to subjective expectations in auditory cortical fields Te1.0, Te1.1, Te1.2, and Te3, and two types of stimuli: pure tones and frequency modulated (FM) sweeps. Our results show that prediction error is elicited with respect to the participants' expectations independently of stimulus repetition and similarly expressed across auditory fields. Moreover, despite the radically different strategies underlying the decoding of pure tones and FM-sweeps, both stimulus modalities were encoded as prediction error in most fields of AC. Altogether, our results provide unequivocal evidence that predictive coding is the general encoding mechanism in AC.

Adjudicating Between Local and Global Architectures of Predictive Processing in the Subcortical Auditory Pathway

Predictive processing, a leading theoretical framework for sensory processing, suggests that the brain constantly generates predictions on the sensory world and that perception emerges from the comparison between these predictions and the actual sensory input. This requires two distinct neural elements: generative units, which encode the model of the sensory world; and prediction error units, which compare these predictions against the sensory input. Although predictive processing is generally portrayed as a theory of cerebral cortex function, animal and human studies over the last decade have robustly shown the ubiquitous presence of prediction error responses in several nuclei of the auditory, somatosensory, and visual subcortical pathways. In the auditory modality, prediction error is typically elicited using so-called oddball paradigms, where sequences of repeated pure tones with the same pitch are at unpredictable intervals substituted by a tone of deviant frequency. Repeated sounds become predictable promptly and elicit decreasing prediction error; deviant tones break these predictions and elicit large prediction errors. The simplicity of the rules inducing predictability make oddball paradigms agnostic about the origin of the predictions. Here, we introduce two possible models of the organizational topology of the predictive processing auditory network: (1) the global view, that assumes that predictions on the sensory input are generated at high-order levels of the cerebral cortex and transmitted in a cascade of generative models to the subcortical sensory pathways; and (2) the local view, that assumes that independent local models, computed using local information, are used to perform predictions at each processing stage. In the global view information encoding is optimized globally but biases sensory representations along the entire brain according to the subjective views of the observer. The local view results in a diminished coding efficiency, but guarantees in return a robust encoding of the features of sensory input at each processing stage. Although most experimental results to-date are ambiguous in this respect, recent evidence favors the global model.

Abstract rules drive adaptation in the subcortical sensory pathway

The subcortical sensory pathways are the fundamental channels for mapping the outside world to our minds. Sensory pathways efficiently transmit information by adapting neural responses to the local statistics of the sensory input. The long-standing mechanistic explanation for this adaptive behaviour is that neural activity decreases with increasing regularities in the local statistics of the stimuli. An alternative account is that neural coding is directly driven by expectations of the sensory input. Here, we used abstract rules to manipulate expectations independently of local stimulus statistics. The ultra-high-field functional-MRI data show that abstract expectations can drive the response amplitude to tones in the human auditory pathway. These results provide first unambiguous evidence of abstract processing in a subcortical sensory pathway. They indicate that the neural representation of the outside world is altered by our prior beliefs even at initial points of the processing hierarchy.

The effect of inhibition on stimulus-specific adaptation in the inferior colliculus

The inferior colliculus is a center of convergence for inhibitory and excitatory synaptic inputs that may be activated simultaneously by sound stimulation. Stimulus repetition may generate response habituation by changing the efficacy of neuron's synaptic inputs. Specialized IC neurons reduce their response to repetitive tones, but restore their firing when a different and infrequent tone occurs, a phenomenon known as stimulus specific adaptation. Here, using the microiontophoresis technique, we determined the role of GABA_A-, GABA_B-, and glycinergic receptors in stimulus-specific adaptation (SSA). We found that blockade of postsynaptic GABA_B receptors selectively modulated response adaptation to repetitive sounds, whereas blockade of presynaptic GABA_B receptors exerted a gain control effect on neuron excitability. Adaptation decreased when postsynaptic GABA_B receptors were blocked, but increased if the blockade affected the presynaptic GABA_B receptors. A dual, paradoxical effect was elicited by blockade of glycinergic receptors, i.e., both increase and decrease in adaptation. Moreover, simultaneous co-application of GABA_A, GABA_B, and glycinergic antagonists demonstrated that local GABA- and glycine-mediated inhibition contributes to only about 50% of SSA. Therefore, inhibition via chemical synapses dynamically modulate the strength and dynamics of stimulus-specific adaptation, but does not generate it.

5-HT2 receptors mediate functional modulation of GABAa receptors and inhibitory synaptic transmissions in human iPS-derived neurons

Neural progenitors differentiated from induced pluripotent stem cells (iPS) hold potentials for treating neurological diseases. Serotonin has potent effects on neuronal functions through multiple receptors, underlying a variety of neural disorders. Glutamate and GABA receptors have been proven functional in neurons differentiated from iPS, however, little is known about 5-HT receptor-mediated modulation in such neuronal networks. In the present study, human iPS were differentiated into cells possessing featured physiological properties of cortical neurons. Whole-cell patch-clamp recording was used to examine the involvement of 5-HT₂ receptors in functional modulation of GABAergic synaptic transmission. We found that serotonin and DOI (a selective agonist of 5-HT_2A/C receptor) reversibly reduced GABA-activated currents, and this 5-HT_2A/C receptor mediated inhibition required G protein, PLC, PKC, and Ca²⁺ signaling. Serotonin increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), which could be mimicked by α-methylserotonin, a 5-HT₂ receptor agonist. In contrast, DOI reduced both frequency and amplitude of mIPSCs. These findings suggested that in iPS-derived human neurons serotonin postsynaptically reduced GABAa receptor function through 5-HT_2A/C receptors, but presynaptically other 5-HT₂ receptors counteracted the action of 5-HT_2A/C receptors. Functional expression of serotonin receptors in human iPS-derived neurons provides a pre-requisite for their normal behaviors after grafting.

An economic method to build a puffing instrument for drug application in vitro

BACKGROUND

In in vitro electrophysiological studies, a quick application of picoliters of drug within milliseconds is required to avoid the desensitization of membrane receptors. However, conventional gravity-fed drug delivery devices sometime fail to achieve this. Moreover, the high financial cost of the advanced drug delivery system often limits the application of commercial instruments in academic research.

NEW METHOD

Taking advantage of the availability of data acquisition system and software in almost every electrophysiology laboratory, a simple puffing device was designed and assembled using low-cost commercially off-the-shelf components to inject picoliter amounts of drugs.

RESULTS

An optimal drug delivery with precise timing and volume was achieved using the custom made puffing device. The glutamate-evoked currents of cortical neurons recorded with patch-clamp technique were maintained for a prolonged period of time. Similarly, puffed inhibitory transmitters including GABA and glycine also produced satisfactory currents.

COMPARISON WITH EXISTING METHOD(S)

Our custom-made puffing system holds the advantage over conventional gravity-fed systems in operating within milliseconds of time. The channel number of the new device can easily be increased by simply adding more identical modules in parallel, and thus offering more flexibility than commercial puffing devices.

CONCLUSIONS

This custom-made puffing device can be characterized as reliable, modular and inexpensive system for modern drug delivery research and application.

Activation of 5-HT2A/C receptor reduces glycine receptor-mediated currents in cultured auditory cortical neurons

Glycine receptors (GlyRs) permeable to chloride only mediate tonic inhibition in the cerebral cortex where glycinergic projection is completely absent. The functional modulation of GlyRs was largely studied in subcortical brain regions with glycinergic transmissions, but the function of cortical GlyRs was rarely addressed. Serotonin could broadly modulate many ion channels through activating 5-HT2 receptor, but whether cortical GlyRs are subjected to serotonergic modulation remains unexplored. The present study adopted patch clamp recordings to examine functional regulation of strychnine-sensitive GlyRs currents in cultured cortical neurons by DOI (2,5-Dimethoxy-4-iodoamphetamine), a 5-HT2A/C receptor agonist. DOI caused a concentration-dependent reduction of GlyR currents with unchanged reversal potential. This reduction was blocked by the selective receptor antagonists (ritanserin and risperidone) and G protein inhibitor (GDP-β-s) demonstrated that the reducing effect of DOI on GlyR current required the activation of 5-HT2A/C receptors. Strychnine-sensitive tonic currents revealed the inhibitory tone mediated by nonsynaptic GlyRs, and DOI similarly reduced the tonic inhibition. The impaired microtube-dependent trafficking or clustering of GlyRs was thought to be involved in that nocodazole as a microtube depolymerizing drug largely blocked the inhibition mediated by 5-HT2A/C receptors. Our results suggested that activation of 5-HT2A/C receptors might suppress cortical tonic inhibition mediated by GlyRs, and the findings would provide important insight into serotonergic modulation of tonic inhibition mediated by GlyRs, and possibly facilitate to develop the therapeutic treatment of neurological diseases such as tinnitus through regulating cortical GlyRs.