Bilateral multielectrode neurophysiological recordings coupled to local pharmacology in awake songbirds

Thalamic Kv 7 channels: pharmacological properties and activity control during noxious signal processing

BACKGROUND AND PURPOSE

The existence of functional K(v)7 channels in thalamocortical (TC) relay neurons and the effects of the K(+)-current termed M-current (I(M)) on thalamic signal processing have long been debated. Immunocytochemical evidence suggests their presence in this brain region. Therefore, we aimed to verify their existence, pharmacological properties and function in regulating activity in neurons of the ventrobasal thalamus (VB).

EXPERIMENTAL APPROACH

Characterization of K(v)7 channels was performed by combining in vitro, in vivo and in silico techniques with a pharmacological approach. Retigabine (30 μM) and XE991 (20 μM), a specific K(v)7 channel enhancer and blocker, respectively, were applied in acute brain slices during electrophysiological recordings. The effects of intrathalamic injection of retigabine (3 mM, 300 nL) and/or XE991 (2 mM, 300 nL) were investigated in freely moving animals during hot-plate tests by recording behaviour and neuronal activity.

KEY RESULTS

K(v)7.2 and K(v)7.3 subunits were found to be abundantly expressed in TC neurons of mouse VB. A slow K(+)-current with properties of IM was activated by retigabine and inhibited by XE991. K(v)7 channel activation evoked membrane hyperpolarization, a reduction in tonic action potential firing, and increased burst firing in vitro and in computational models. Single-unit recordings and pharmacological intervention demonstrated a specific burst-firing increase upon I(M) activation in vivo. A K(v)7 channel-mediated increase in pain threshold was associated with fewer VB units responding to noxious stimuli, and increased burst firing in responsive neurons.

CONCLUSIONS AND IMPLICATIONS

K(v)7 channel enhancement alters somatosensory activity and may reflect an anti-nociceptive mechanism during acute pain processing.

Mechanistic basis and functional roles of long-term plasticity in auditory neurons induced by a brain-generated estrogen

The classic estrogen 17β-estradiol (E2) was recently identified as a novel modulator of hearing function. It is produced rapidly, in an experience-dependent fashion, by auditory cortical neurons of both males and females. This brain-generated E2 enhances the efficiency of auditory coding and improves the neural and behavioral discrimination of auditory cues. Remarkably, the effects of E2 are long-lasting and persist for hours after local rises in hormone levels have subsided. The mechanisms and functional consequences of this E2-induced plasticity of auditory responses are unknown. Here, we addressed these issues in the zebra finch model by combining intracerebral pharmacology, biochemical assays, in vivo neurophysiology in awake animals, and computational and information theoretical approaches. We show that auditory experience activates the MAPK pathway in an E2-dependent manner. This effect is mediated by estrogen receptor β (ERβ), which directly associates with MEKK1 to sequentially modulate MEK and ERK activation, where the latter is required for the engagement of downstream molecular targets. We further show that E2-mediated activation of the MAPK cascade is required for the long-lasting enhancement of auditory-evoked responses in the awake brain. Moreover, a functional consequence of this E2/MAPK activation is to sustain enhanced information handling and neural discrimination by auditory neurons for several hours following hormonal challenge. Our results demonstrate that brain-generated E2 engages, via a nongenomic interaction between an estrogen receptor and a kinase, a persistent form of experience-dependent plasticity that enhances the neural coding and discrimination of behaviorally relevant sensory signals in the adult vertebrate brain.

Changing neuroestrogens within the auditory forebrain rapidly transform stimulus selectivity in a downstream sensorimotor nucleus

The activity of sensory circuits is shaped by neuromodulators, which can have downstream consequences for both sensorimotor integration and behavioral output. Recent evidence indicates that brain-derived estrogens ("neuroestrogens") can act as local circuit modulators in the songbird auditory forebrain. Specifically, neuroestrogens fluctuate in the auditory caudomedial nidopallium (NCM) during social interactions and in response to song stimuli. Within minutes of elevation, neuroestrogens also enhance auditory response properties of NCM neurons, and acute blockade of estrogen production in NCM disrupts behavioral song preferences. Here, we test the hypothesis that fluctuating neuroestrogens within NCM influence stimulus selectivity in a downstream sensorimotor nucleus (HVC, used as a proper name) that receives indirect auditory input from NCM. Dual extracellular recordings coupled with retrodialysis delivery show that song selectivity in HVC is rapidly enhanced by increasing neuroestrogens in NCM in adult males. Conversely, inhibiting neuroestrogen production in NCM causes a rapid decline in song selectivity in HVC, demonstrating the endogenous nature of this modulatory network. In contrast, HVC selectivity is unaffected by neuroestrogen delivery to either nearby caudomedial mesopallium or into HVC itself, indicating that neuroestrogen actions are restricted to NCM. In juvenile males, identical neuroestrogen treatment in NCM also does not alter HVC selectivity, consistent with a developmental maturation of the auditory network. Lastly, the rapid actions of estrogens leading to enhanced HVC selectivity appear to be mediated by membrane-bound receptors in NCM. These findings indicate that steroid-dependent modulation of sensory processing is not locally restricted and can be transmitted transynaptically to influence downstream sensorimotor and premotor targets.

Brain-generated estradiol drives long-term optimization of auditory coding to enhance the discrimination of communication signals

Auditory processing and hearing-related pathologies are heavily influenced by steroid hormones in a variety of vertebrate species, including humans. The hormone estradiol has been recently shown to directly modulate the gain of central auditory neurons, in real time, by controlling the strength of inhibitory transmission via a nongenomic mechanism. The functional relevance of this modulation, however, remains unknown. Here we show that estradiol generated in the songbird homolog of the mammalian auditory association cortex, rapidly enhances the effectiveness of the neural coding of complex, learned acoustic signals in awake zebra finches. Specifically, estradiol increases mutual information rates, coding efficiency, and the neural discrimination of songs. These effects are mediated by estradiol's modulation of both the rate and temporal coding of auditory signals. Interference with the local action or production of estradiol in the auditory forebrain of freely behaving animals disrupts behavioral responses to songs, but not to other behaviorally relevant communication signals. Our findings directly show that estradiol is a key regulator of auditory function in the adult vertebrate brain.