Heightened nicotinic regulation of auditory cortex during adolescence

Task-dependent effects of nicotine treatment on auditory performance in young-adult and elderly human nonsmokers

Electrophysiological studies show that nicotine enhances neural responses to characteristic frequency stimuli. Previous behavioral studies partially corroborate these findings in young adults, showing that nicotine selectively enhances auditory processing in difficult listening conditions. The present work extended previous work to include both young and older adults and assessed the nicotine effect on sound frequency and intensity discrimination. Hypotheses were that nicotine improves auditory performance and that the degree of improvement is inversely proportional to baseline performance. Young (19-23 years old) normal-hearing nonsmokers and elderly (61-80) nonsmokers with normal hearing between 500 and 2000 Hz received nicotine gum (6 mg) or placebo gum in a single-blind, randomized crossover design. Participants performed three experiments (frequency discrimination, frequency modulation identification, and intensity discrimination) before and after treatment. The perceptual differences were analyzed between pre- and post-treatment, as well as between post-treatment nicotine and placebo conditions as a function of pre-treatment baseline performance. Compared to pre-treatment performance, nicotine significantly improved frequency discrimination. Compared to placebo, nicotine significantly improved performance for intensity discrimination, and the improvement was more pronounced in the elderly with lower baseline performance. Nicotine had no effect on frequency modulation identification. Nicotine effects are task-dependent, reflecting possible interplays of subjects, tasks and neural mechanisms.

Sex differences in the auditory functions of rodents

BACKGROUND

In humans, it is well known that females have better hearing than males. The mechanism of this influence of sex on auditory function in humans is not well understood. Testing the hypothesis of underlying mechanisms often relies on preclinical research, a field in which sex bias still exists unconsciously. Rodents are popular research models in hearing, thus it is crucial to understand the sex differences in these rodent models when studying health and disease in humans.

OBJECTIVES

This review aims to summarize the existing sex differences in the auditory functions of rodent species including mouse, rat, Guinea pig, Mongolian gerbil, and chinchilla. In addition, a concise summary of the hearing characteristics and the advantages and the drawbacks of conducting auditory experiments in each rodent species is provided.

DESIGNS

Manuscripts were identified in PubMed and Ovid Medline for the queries "Rodent", "Sex Characteristics", and "Hearing or Auditory Function". Manuscripts were included if they were original research, written in English, and use rodents. The content of each manuscript was screened for the sex of the rodents and the discussion of sex-based results.

CONCLUSIONS

The sex differences in auditory function of rodents are prevalent and influenced by multiple factors including physiological mechanisms, sex-based anatomical variations, and stimuli from the external environment. Such differences may play a role in understanding and explaining sex differences in hearing of humans and need to be taken into consideration for developing clinical therapies aim to improve auditory performances.

Nicotine enhances auditory processing in healthy and normal-hearing young adult nonsmokers

RATIONALE

Electrophysiological studies show that systemic nicotine narrows frequency receptive fields and increases gain in neural responses to characteristic frequency stimuli. We postulated that nicotine enhances related auditory processing in humans.

OBJECTIVES

The main hypothesis was that nicotine improves auditory performance. A secondary hypothesis was that the degree of nicotine-induced improvement depends on the individual's baseline performance.

METHODS

Young (18-27 years old), normal-hearing nonsmokers received nicotine (Nicorette gum, 6mg) or placebo gum in a single-blind, randomized, crossover design. Subjects performed four experiments involving tone-in-noise detection, temporal gap detection, spectral ripple discrimination, and selective auditory attention before and after treatment. The perceptual differences between posttreatment nicotine and placebo conditions were measured and analyzed as a function of the pre-treatment baseline performance.

RESULTS

Nicotine significantly improved performance in the more difficult tasks of tone-in-noise detection and selective attention (effect size = - 0.3) but had no effect on relatively easier tasks of temporal gap detection and spectral ripple discrimination. The two tasks showing significant nicotine effects further showed no baseline-dependent improvement.

CONCLUSIONS

Nicotine improves auditory performance in difficult listening situations. The present results support future investigation of nicotine effects in clinical populations with auditory processing deficits or reduced cholinergic activation.

Enhanced Sensory-Cognitive Processing by Activation of Nicotinic Acetylcholine Receptors

Activation of nicotinic acetylcholine receptors (nAChRs) enhances sensory-cognitive function in human subjects and animal models, yet the neural mechanisms are not fully understood. This review summarizes recent studies on nicotinic regulation of neural processing in the cerebral cortex that point to potential mechanisms underlying enhanced cognitive function. Studies from our laboratory focus on nicotinic regulation of auditory cortex and implications for auditory-cognitive processing, but relevant emerging insights from multiple brain regions are discussed. Although the major contributions of the predominant nAChRs containing α7 (homomeric receptors) or α4 and β2 (heteromeric) subunits are well recognized, recent results point to additional, potentially critical contributions from α2 subunits that are relatively sparse in cortex. Ongoing studies aim to elucidate the specific contributions to cognitive and cortical function of diverse nAChRs.

IMPLICATIONS

This review highlights the therapeutic potential of activating nAChRs in the cerebral cortex to enhance cognitive function. Future work also must determine the contributions of relatively rare but important nAChR subtypes, potentially to develop more selective treatments for cognitive deficits.

Nicotine excites VIP interneurons to disinhibit pyramidal neurons in auditory cortex

Nicotine activates nicotinic acetylcholine receptors and improves cognitive and sensory function, in part by its actions in cortical regions. Physiological studies show that nicotine amplifies stimulus-evoked responses in sensory cortex, potentially contributing to enhancement of sensory processing. However, the role of specific cell types and circuits in the nicotinic modulation of sensory cortex remains unclear. Here, we performed whole-cell recordings from pyramidal (Pyr) neurons and inhibitory interneurons expressing parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP) in mouse auditory cortex, in vitro. Bath application of nicotine strongly depolarized and excited VIP neurons, weakly depolarized Pyr neurons, and had no effect on the membrane potential of SOM or PV neurons. The use of receptor antagonists showed that nicotine's effects on VIP and Pyr neurons were direct and indirect, respectively. Nicotine also enhanced the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in Pyr, VIP, and SOM, but not PV, cells. Using Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), we show that chemogenetic inhibition of VIP neurons prevents nicotine's effects on Pyr neurons. Since VIP cells preferentially contact other inhibitory interneurons, we suggest that nicotine drives VIP cell firing to disinhibit Pyr cell somata, potentially making Pyr cells more responsive to auditory stimuli. In parallel, activation of VIP cells also directly inhibits Pyr neurons, likely altering integration of other synaptic inputs. These cellular and synaptic mechanisms likely contribute to nicotine's beneficial effects on cognitive and sensory function.

Nicotine Gateway Effects on Adolescent Substance Use

Given the rise in teenage use of electronic nicotine delivery systems ("vaping") in congruence with the increasing numbers of drug-related emergencies, it is critical to expand the knowledge of the physical and behavioral risks associated with developmental nicotine exposure. A further understanding of the molecular and neurochemical underpinnings of nicotine's gateway effects allows emergency clinicians to advise patients and families and adjust treatment accordingly, which may minimize the use of tobacco, nicotine, and future substances. Currently, the growing use of tobacco products and electronic cigarettes among teenagers represents a major public health concern. Adolescent exposure to tobacco or nicotine can lead to subsequent abuse of nicotine and other substances, which is known as the gateway hypothesis. Adolescence is a developmentally sensitive time period when risk-taking behaviors, such as sensation seeking and drug experimentation, often begin. These hallmark behaviors of adolescence are largely due to maturational changes in the brain. The developing brain is particularly vulnerable to the harmful effects of drugs of abuse, including tobacco and nicotine products, which activate nicotinic acetylcholine receptors (nAChRs). Disruption of nAChR development with early nicotine use may influence the function and pharmacology of the receptor subunits and alter the release of reward-related neurotransmitters, including acetylcholine, dopamine, GABA, serotonin, and glutamate. In this review, we emphasize that the effects of nicotine are highly dependent on timing of exposure, with a dynamic interaction of nAChRs with dopaminergic, endocannabinoid, and opioidergic systems to enhance general drug reward and reinforcement. We analyzed available literature regarding adolescent substance use and nicotine's impact on the developing brain and behavior using the electronic databases of PubMed and Google Scholar for articles published in English between January 1968 and November 2018. We present a large collection of clinical and preclinical evidence that adolescent nicotine exposure influences long-term molecular, biochemical, and functional changes in the brain that encourage subsequent drug abuse.

Systemic Nicotine Increases Gain and Narrows Receptive Fields in A1 via Integrated Cortical and Subcortical Actions

Nicotine enhances sensory and cognitive processing via actions at nicotinic acetylcholine receptors (nAChRs), yet the precise circuit- and systems-level mechanisms remain unclear. In sensory cortex, nicotinic modulation of receptive fields (RFs) provides a model to probe mechanisms by which nAChRs regulate cortical circuits. Here, we examine RF modulation in mouse primary auditory cortex (A1) using a novel electrophysiological approach: current-source density (CSD) analysis of responses to tone-in-notched-noise (TINN) acoustic stimuli. TINN stimuli consist of a tone at the characteristic frequency (CF) of the recording site embedded within a white noise stimulus filtered to create a spectral “notch” of variable width centered on CF. Systemic nicotine (2.1 mg/kg) enhanced responses to the CF tone and to narrow-notch stimuli, yet reduced the response to wider-notch stimuli, indicating increased response gain within a narrowed RF. Subsequent manipulations showed that modulation of cortical RFs by systemic nicotine reflected effects at several levels in the auditory pathway: nicotine suppressed responses in the auditory midbrain and thalamus, with suppression increasing with spectral distance from CF so that RFs became narrower, and facilitated responses in the thalamocortical pathway, while nicotinic actions within A1 further contributed to both suppression and facilitation. Thus, multiple effects of systemic nicotine integrate along the ascending auditory pathway. These actions at nAChRs in cortical and subcortical circuits, which mimic effects of auditory attention, likely contribute to nicotinic enhancement of sensory and cognitive processing.

Nicotinic regulation of experience-dependent plasticity in visual cortex

While the cholinergic neuromodulatory system and muscarinic acetylcholine receptors (AChRs) have been appreciated as permissive factors for developmental critical period plasticity in visual cortex, it was unknown why plasticity becomes limited after the critical period even in the presence of massive cholinergic projections to visual cortex. In this review we highlighted the recent progresses that started to shed light on the role of the nicotinic cholinergic neuromodulatory signaling on limiting juvenile form of plasticity in the adult brain. We introduce the Lynx family of proteins and Lynx1 as its representative, as endogenous proteins structurally similar to α-bungarotoxin with the ability to bind and modulate nAChRs to effectively regulate functional and structural plasticity. Remarkably, Lynx family members are expressed in distinct subpopulations of GABAergic interneurons, placing them in unique positions to potentially regulate the convergence of GABAergic and nicotinic neuromodulatory systems to regulate plasticity. Continuing studies of the potentially differential roles of Lynx family of proteins may further our understanding of the fundamentals of molecular and cell type-specific mechanisms of plasticity that we may be able to harness through nicotinic cholinergic signaling.

Spectral breadth and laminar distribution of thalamocortical inputs to A1

The GABAergic agonist muscimol is used to inactivate brain regions in order to reveal afferent inputs in isolation. However, muscimol's use in primary auditory cortex (A1) has been questioned on the grounds that it may unintentionally suppress thalamocortical inputs. We tested whether muscimol can preferentially suppress cortical, but not thalamocortical, circuits in urethane-anesthetized mice. We recorded tone-evoked current source density profiles to determine frequency receptive fields (RFs) for three current sinks: the "layer 4" sink (fastest onset, middle-layer sink) and current sinks 100 μm above ("layer 2/3") and 300 μm below ("layer 5/6") the main input. We first determined effects of muscimol dose (0.01-1 mM) on the characteristic frequency (CF) tone-evoked layer 4 sink. An "ideal" dose (100 μM) had no effect on CF-evoked sink onset latency or initial response but reduced peak amplitude by >80%, implying inhibition of intracortical, but not thalamocortical, activity. We extended the analysis to current sinks in layers 2/3 and 5/6 and for all three sinks determined RF breadth (quarter-octave steps, 20 dB above CF threshold). Muscimol reduced RF breadth 42% in layer 2/3 (from 2.4 ± 0.14 to 1.4 ± 0.11 octaves), 14% in layer 4 (2.2 ± 0.12 to 1.9 ± 0.10 octaves), and not at all in layer 5/6 (1.8 ± 0.10 to 1.7 ± 0.12 octaves). The results provide an estimate of the laminar and spectral extent of thalamocortical projections and support the hypothesis that intracortical pathways contribute to spectral integration in A1.

Nicotine and the adolescent brain

Adolescence encompasses a sensitive developmental period of enhanced clinical vulnerability to nicotine, tobacco, and e-cigarettes. While there are sociocultural influences, data at preclinical and clinical levels indicate that this adolescent sensitivity has strong neurobiological underpinnings. Although definitions of adolescence vary, the hallmark of this period is a profound reorganization of brain regions necessary for mature cognitive and executive function, working memory, reward processing, emotional regulation, and motivated behavior. Regulating critical facets of brain maturation are nicotinic acetylcholine receptors (nAChRs). However, perturbations of cholinergic systems during this time with nicotine, via tobacco or e-cigarettes, have unique consequences on adolescent development. In this review, we highlight recent clinical and preclinical data examining the adolescent brain's distinct neurobiology and unique sensitivity to nicotine. First, we discuss what defines adolescence before reviewing normative structural and neurochemical alterations that persist until early adulthood, with an emphasis on dopaminergic systems. We review how acute exposure to nicotine impacts brain development and how drug responses differ from those seen in adults. Finally, we discuss the persistent alterations in neuronal signaling and cognitive function that result from chronic nicotine exposure, while highlighting a low dose, semi-chronic exposure paradigm that may better model adolescent tobacco use. We argue that nicotine exposure, increasingly occurring as a result of e-cigarette use, may induce epigenetic changes that sensitize the brain to other drugs and prime it for future substance abuse.

Cholinergic modulation of dopamine pathways through nicotinic acetylcholine receptors

Nicotine addiction is highly prevalent in current society and is often comorbid with other diseases. In the central nervous system, nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs) and its effects depend on location and receptor composition. Although nicotinic receptors are found in most brain regions, many studies on addiction have focused on the mesolimbic system and its reported behavioral correlates such as reward processing and reinforcement learning. Profound modulatory cholinergic input from the pedunculopontine and laterodorsal tegmentum to dopaminergic midbrain nuclei as well as local cholinergic interneuron projections to dopamine neuron axons in the striatum may play a major role in the effects of nicotine. Moreover, an indirect mesocorticolimbic feedback loop involving the medial prefrontal cortex may be involved in behavioral characteristics of nicotine addiction. Therefore, this review will highlight current understanding of the effects of nicotine on the function of mesolimbic and mesocortical dopamine projections in the mesocorticolimbic circuit.

Neuropharmacological modulation of the P3-like event-related potential in a rat two-tone auditory discrimination task with modafinil and NS9283, a positive allosteric modulator of α4β2 nAChRs

The P300 (P3) event-related potential (ERP) is a neurophysiological signal believed to reflect cognitive processing of salient cues, and is thus used as a measure of attention and working memory. Additionally, P3 amplitude and latency is altered in neurological diseases and can be pharmacologically modulated. As P3-like ERPs can be recorded in rodents, it may serve as a potential translational biomarker of value for drug discovery. Here we investigated whether a positive allosteric modulator of α4β2 nicotinic acetylcholine receptors, NS9283, and the psychostimulant modafinil could modulate P3-like ERPs in healthy adult rats performing an auditory oddball discrimination task. ERPs were recorded with electroencephalography electrodes implanted into mediodorsal (MD) thalamus, medial prefrontal cortex, hippocampus and auditory cortex (AC). P3-like ERPs were detected in all brain regions, displaying larger amplitudes in target trials compared to non-target trials. Administration of modafinil (64 mg/kg) decreased P3-like ERP latency in MD thalamus and AC, whereas NS9283 augmented P3-like ERP amplitude in MD thalamus at 0.3 mg/kg and in AC at 3.0 mg/kg. Additionally, N1 pre-attention peak amplitude in MD thalamus was increased with 0.3 mg/kg NS9283. Neither of the compounds enhanced task performance. Rather, modafinil lowered correct rejections in non-target trials. In summary, our findings reveal pharmacological modulation of the rat P3-like ERP in cortical and subcortical regions by modafinil and NS9283. These findings encourage further exploration of the rat P3-like ERP in order to promote the understanding of its meaning within cognition, as well as its applicability as a translatable biomarker in drug development.

Regulation of glutamate release by heteromeric nicotinic receptors in layer V of the secondary motor region (Fr2) in the dorsomedial shoulder of prefrontal cortex in mouse

We studied how nicotinic acetylcholine receptors (nAChRs) regulate glutamate release in the secondary motor area (Fr2) of the dorsomedial murine prefrontal cortex, in the presence of steady agonist levels. Fr2 mediates response to behavioral situations that require immediate attention and is a candidate for generating seizures in the frontal epilepsies caused by mutant nAChRs. Morphological analysis showed a peculiar chemoarchitecture and laminar distribution of pyramidal cells and interneurons. Tonic application of 5 µM nicotine on Layer V pyramidal neurons strongly increased the frequency of spontaneous glutamatergic excitatory postsynaptic currents. The effect was inhibited by 1 µM dihydro-β-erythroidine (which blocks α4-containing nAChRs) but not by 10 nM methyllicaconitine (which blocks α7-containing receptors). Excitatory postsynaptic currents s were also stimulated by 5-iodo-3-[2(S)-azetidinylmethoxy]pyridine, selective for β2-containing receptors, in a dihydro-β-erythroidine -sensitive way. We next studied the association of α4 with different populations of glutamatergic terminals, by using as markers the vesicular glutamate transporter type (VGLUT) 1 for corticocortical synapses and VGLUT2 for thalamocortical projecting fibers. Immunoblots showed higher expression of α4 in Fr2, as compared with the somatosensory cortex. Immunofluorescence showed intense VGLUT1 staining throughout the cortical layers, whereas VGLUT2 immunoreactivity displayed a more distinct laminar distribution. In Layer V, colocalization of α4 nAChR subunit with both VGLUT1 and VGLUT2 was considerably stronger in Fr2 than in somatosensory cortex. Thus, in Fr2, α4β2 nAChRs are expressed in both intrinsic and extrinsic glutamatergic terminals and give a major contribution to control glutamate release in Layer V, in the presence of tonic agonist levels.

Convergence of nicotine-induced and auditory-evoked neural activity activates ERK in auditory cortex

Enhancement of sound-evoked responses in auditory cortex (ACx) following administration of systemic nicotine is known to depend on activation of extracellular-signaling regulated kinase (ERK), but the nature of this enhancement is not clear. Here, we show that systemic nicotine increases the density of cells immunolabeled for phosphorylated (activated) ERK (P-ERK) in mouse primary ACx (A1). Cortical injection of dihydro-β-erythroidine reduced nicotine-induced P-ERK immunolabel, suggesting a role for nicotinic acetylcholine receptors located in A1 and containing α4 and β2 subunits. P-ERK expressing cells were distributed mainly in layers 2/3 and more sparsely in lower layers, with many cells exhibiting immunolabel within pyramidal-shaped somata and proximal apical dendrites. About one-third of P-ERK positive cells also expressed calbindin. In the thalamus, P-ERK immunopositive cells were found in the nonlemniscal medial geniculate (MG) and adjacent nuclei, but were absent in the lemniscal MG. Pairing broad spectrum acoustic stimulation (white noise) with systemic nicotine increased P-ERK immunopositive cell density in ACx as well as the total amount of P-ERK protein, particularly the phosphorylated form of ERK2. However, narrow spectrum (tone) stimulation paired with nicotine increased P-ERK immunolabel preferentially at a site within A1 where the paired frequency was characteristic frequency (CF), relative to a second site with a spectrally distant CF (two octaves above or below the paired frequency). Together, these results suggest that ERK is activated optimally where nicotinic signaling and sound-evoked neural activity converge.

Tone-detection training enhances spectral integration mediated by intracortical pathways in primary auditory cortex

Auditory-cued behavioral training can alter neural circuits in primary auditory cortex (A1), but the mechanisms and consequences of experience-dependent cortical plasticity are not fully understood. To address this issue, we trained adult rats to detect a 5 kHz target in order to receive a food reward. After 14 days training we identified three locations within A1: (i) the region representing the characteristic frequency (CF) 5 kHz, (ii) a nearby region with CF ∼10 kHz, and (iii) a more distant region with CF ∼20 kHz. In order to compare functional connectivity in A1 near to, vs. far from, the representation of the target frequency, we placed a 16-channel multiprobe in middle- (∼10 kHz) and high- (∼20 kHz) CF regions and obtained current-source density (CSD) profiles evoked by a range of tone stimuli (CF±1-3 octaves in quarter-octave steps). Our aim was to construct "CSD receptive fields" (CSD RFs) in order to determine the laminar and spectral profile of tone-evoked current sinks, and infer changes to thalamocortical and intracortical inputs. Behavioral training altered CSD RFs at the 10 kHz, but not 20 kHz, site relative to CSD RFs in untrained control animals. At the 10 kHz site, current sinks evoked by the target frequency were enhanced in layer 2/3, but the initial current sink in layer 4 was not altered. The results imply training-induced plasticity along intracortical pathways connecting the target representation with nearby cortical regions. Finally, we related behavioral performance (sensitivity index, d') to CSD responses in individual animals, and found a significant correlation between the development of d' over training and the amplitude of the target-evoked current sink in layer 2/3. The results suggest that plasticity along intracortical pathways is important for auditory learning.

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.

Nicotinic neuromodulation in auditory cortex requires MAPK activation in thalamocortical and intracortical circuits

Activation of nicotinic acetylcholine receptors (nAChRs) by systemic nicotine enhances sensory-cognitive function and sensory-evoked cortical responses. Although nAChRs mediate fast neurotransmission at many synapses in the nervous system, nicotinic regulation of cortical processing is neuromodulatory. To explore potential mechanisms of nicotinic neuromodulation, we examined whether intracellular signal transduction involving mitogen-activated protein kinase (MAPK) contributes to regulation of tone-evoked responses in primary auditory cortex (A1) in the mouse. Systemic nicotine enhanced characteristic frequency (CF) tone-evoked current-source density (CSD) profiles in A1, including the shortest-latency (presumed thalamocortical) current sink in layer 4 and longer-latency (presumed intracortical) sinks in layers 2-4, by increasing response amplitudes and decreasing response latencies. Microinjection of the MAPK kinase (MEK) inhibitor U0126 into the thalamus, targeting the auditory thalamocortical pathway, blocked the effect of nicotine on the initial (thalamocortical) CSD component but did not block enhancement of longer-latency (intracortical) responses. Conversely, microinjection of U0126 into supragranular layers of A1 blocked nicotine's effect on intracortical, but not thalamocortical, CSD components. Simultaneously with enhancement of CF-evoked responses, responses to spectrally distant (nonCF) stimuli were reduced, implying nicotinic "sharpening" of frequency receptive fields, an effect also blocked by MEK inhibition. Consistent with these physiological results, acoustic stimulation with nicotine produced immunolabel for activated MAPK in A1, primarily in layer 2/3 cell bodies. Immunolabel was blocked by intracortical microinjection of the nAChR antagonist dihydro-β-erythroidine, but not methyllycaconitine, implicating α4β2*, but not α7, nAChRs. Thus activation of MAPK in functionally distinct forebrain circuits--thalamocortical, local intracortical, and long-range intracortical--underlies nicotinic neuromodulation of A1.

Null mutations in EphB receptors decrease sharpness of frequency tuning in primary auditory cortex

Primary auditory cortex (A1) exhibits a tonotopic representation of characteristic frequency (CF). The receptive field properties of A1 neurons emerge from a combination of thalamic inputs and intracortical connections. However, the mechanisms that guide growth of these inputs during development and shape receptive field properties remain largely unknown. We previously showed that Eph family proteins help establish tonotopy in the auditory brainstem. Moreover, other studies have shown that these proteins shape topography in visual and somatosensory cortices. Here, we examined the contribution of Eph proteins to cortical organization of CF, response thresholds and sharpness of frequency tuning. We examined mice with null mutations in EphB2 and EphB3, as these mice show significant changes in auditory brainstem connectivity. We mapped A1 using local field potential recordings in adult EphB2^−/−;EphB3^−/− and EphB3^−/− mice, and in a central A1 location inserted a 16-channel probe to measure tone-evoked current-source density (CSD) profiles. Based on the shortest-latency current sink in the middle layers, which reflects putative thalamocortical input, we determined frequency receptive fields and sharpness of tuning (Q₂₀) for each recording site. While both mutant mouse lines demonstrated increasing CF values from posterior to anterior A1 similar to wild type mice, we found that the double mutant mice had significantly lower Q₂₀ values than either EphB3^−/− mice or wild type mice, indicating broader tuning. In addition, we found that the double mutants had significantly higher CF thresholds and longer onset latency at threshold than mice with wild type EphB2. These results demonstrate that EphB receptors influence auditory cortical responses, and suggest that EphB signaling has multiple functions in auditory system development.