Development of Perineuronal Nets during Ontogeny Correlates with Sensorimotor Vocal Learning in Canaries

Adolescent Alcohol Exposure Promotes Mechanical Allodynia and Alters Synaptic Function at Inputs from the Basolateral Amgydala to the Prelimbic Cortex

Binge drinking is common among adolescents despite mounting evidence linking it to various adverse health outcomes that includes heightened pain perception. The prelimbic (PrL) cortex is vulnerable to insults from adolescent alcohol exposure and receives input from the basolateral amygdala (BLA) while sending projections to the ventrolateral periaqueductal gray (vlPAG) - two brain regions implicated in nociception. In this study, adolescent intermittent ethanol (AIE) exposure was carried out in male and female rats using a vapor inhalation procedure. Mechanical and thermal sensitivity, assessed throughout adolescence and into adulthood, revealed that AIE exposure induced protracted mechanical allodynia in both male and female rats. However, a carrageenan inflammatory paw pain challenge in adult rats revealed that AIE did not further augment carrageenan-induced hyperalgesia. To investigate synaptic function at BLA inputs onto defined populations of PrL neurons, retrobeads and viral labelling were combined with optogenetics and slice electrophysiology. Recordings from retrobead labelled cells in the PrL revealed AIE reduced BLA driven feedforward inhibition of neurons projecting from the PrL to the vlPAG (PrLPAG neurons), resulting in augmented excitation/inhibition (E/I) balance and increased intrinsic excitability. Consistent with this finding, recordings from virally tagged PrL parvalbumin interneurons (PVINs) demonstrated that AIE exposure reduced both E/I balance at BLA inputs onto PVINs and PVIN intrinsic excitability when assessed in adulthood. These findings provide compelling evidence that AIE and acute pain alter synaptic function and intrinsic excitability within a prefrontal nociceptive circuit.

Cross-species conservation in the regulation of parvalbumin by perineuronal nets

Parvalbumin (PV) neurons play an integral role in regulating neural dynamics and plasticity. Therefore, understanding the factors that regulate PV expression is important for revealing modulators of brain function. While the contribution of PV neurons to neural processes has been studied in mammals, relatively little is known about PV function in non-mammalian species, and discerning similarities in the regulation of PV across species can provide insight into evolutionary conservation in the role of PV neurons. Here we investigated factors that affect the abundance of PV in PV neurons in sensory and motor circuits of songbirds and rodents. In particular, we examined the degree to which perineuronal nets (PNNs), extracellular matrices that preferentially surround PV neurons, modulate PV abundance as well as how the relationship between PV and PNN expression differs across brain areas and species and changes over development. We generally found that cortical PV neurons that are surrounded by PNNs (PV+PNN neurons) are more enriched with PV than PV neurons without PNNs (PV-PNN neurons) across both rodents and songbirds. Interestingly, the relationship between PV and PNN expression in the vocal portion of the basal ganglia of songbirds (Area X) differed from that in other areas, with PV+PNN neurons having lower PV expression compared to PV-PNN neurons. These relationships remained consistent across development in vocal motor circuits of the songbird brain. Finally, we discovered a causal contribution of PNNs to PV expression in songbirds because degradation of PNNs led to a diminution of PV expression in PV neurons. These findings reveal a conserved relationship between PV and PNN expression in sensory and motor cortices and across songbirds and rodents and suggest that PV neurons could modulate plasticity and neural dynamics in similar ways across songbirds and rodents.

Perineuronal Nets: Subtle Structures with Large Implications

Perineuronal nets (PNNs) are specialized structures of the extracellular matrix that surround the soma and proximal dendrites of certain neurons in the central nervous system, particularly parvalbumin-expressing interneurons. Their appearance overlaps the maturation of neuronal circuits and the closure of critical periods in different regions of the brain, setting their connectivity and abruptly reducing their plasticity. As a consequence, the digestion of PNNs, as well as the removal or manipulation of their components, leads to a boost in this plasticity and can play a key role in the functional recovery from different insults and in the etiopathology of certain neurologic and psychiatric disorders. Here we review the structure, composition, and distribution of PNNs and their variation throughout the evolutive scale. We also discuss methodological approaches to study these structures. The function of PNNs during neurodevelopment and adulthood is discussed, as well as the influence of intrinsic and extrinsic factors on these specialized regions of the extracellular matrix. Finally, we review current data on alterations in PNNs described in diseases of the central nervous system (CNS), focusing on psychiatric disorders. Together, all the data available point to the PNNs as a promising target to understand the physiology and pathologic conditions of the CNS.

Steroid-dependent plasticity in the song control system: Perineuronal nets and HVC neurogenesis

The vocal control nucleus HVC in songbirds has emerged as a widespread model system to study adult brain plasticity in response to changes in the hormonal and social environment. I review here studies completed in my laboratory during the last decade that concern two aspects of this plasticity: changes in aggregations of extracellular matrix components surrounding the soma of inhibitory parvalbumin-positive neurons called perineuronal nets (PNN) and the production/incorporation of new neurons. Both features are modulated by the season, age, sex and endocrine status of the birds in correlation with changes in song structure and stability. Causal studies have also investigated the role of PNN and of new neurons in the control of song. Dissolving PNN with chondroitinase sulfate, a specific enzyme applied directly on HVC or depletion of new neurons by focalized X-ray irradiation both affected song structure but the amplitude of changes was limited and deserves further investigations.

Cross-species conservation in the regulation of parvalbumin by perineuronal nets

Parvalbumin (PV) neurons play an integral role in regulating neural dynamics and plasticity. Therefore, understanding the factors that regulate PV expression is important for revealing modulators of brain function. While the contribution of PV neurons to neural processes has been studied in mammals, relatively little is known about PV function in non-mammalian species, and discerning similarities in the regulation of PV across species can provide insight into evolutionary conservation in the role of PV neurons. Here we investigated factors that affect the abundance of PV in PV neurons in sensory and motor circuits of songbirds and rodents. In particular, we examined the degree to which perineuronal nets (PNNs), extracellular matrices that preferentially surround PV neurons, modulate PV abundance as well as how the relationship between PV and PNN expression differs across brain areas and species and changes over development. We generally found that cortical PV neurons that are surrounded by PNNs (PV+PNN neurons) are more enriched with PV than PV neurons without PNNs (PV-PNN neurons) across both rodents and songbirds. Interestingly, the relationship between PV and PNN expression in the vocal portion of the basal ganglia of songbirds (Area X) differed from that in other areas, with PV+PNN neurons having lower PV expression compared to PV-PNN neurons. These relationships remained consistent across development in vocal motor circuits of the songbird brain. Finally, we discovered a causal contribution of PNNs to PV expression in songbirds because degradation of PNNs led to a diminution of PV expression in PV neurons. These findings in reveal a conserved relationship between PV and PNN expression in sensory and motor cortices and across songbirds and rodents and suggest that PV neurons could modulate plasticity and neural dynamics in similar ways across songbirds and rodents.

Sex differences in song syntax and syllable diversity in testosterone-induced songs of adult male and female canaries

BACKGROUND

Behavioral sex differences are widespread in the animal world. These differences can be qualitative (i.e., behavior present in one sex but not the other, a true sex dimorphism) or quantitative (behavior is present at a higher rate or quality in one sex compared to the other). Singing in oscine songbirds is associated with both types of differences. In canaries, female rarely sing spontaneously but they can be induced to do so by treatments with steroids. Song in these females is, however, not fully masculinized and exhibits relatively subtle differences in quality as compared with male song. We analyzed here sex differences in syllable content and syllable use between singing male and female canaries.

METHODS

Songs were recorded from three groups of castrated male and three groups of photoregressed female canaries that had received Silastic™ implants filled with testosterone (T), with T plus estradiol (E2), or left empty (control). After 6 weeks of hormone treatment, 30 songs were recorded from each of the 47 subjects. Songs were segmented and each syllable was annotated. Various metrics of syllable diversity were extracted and network analysis was employed to characterize syllable sequences.

RESULTS

Male and female songs were characterized by marked sex differences related to syllable use. Compared to females, males had a larger syllable-type repertoire and their songs contained more syllable types. Network analysis of syllable sequences showed that males follow more fixed patterns of syllable transitions than females. Both sexes, however, produced song of the same duration containing the same number of syllables produced at similar rates (numbers per second).

CONCLUSIONS

Under the influence of T, canaries of both sexes are able to produce generally similar vocalizations that nevertheless differ in specific ways. The development of song during ontogeny appears to be a very sophisticated process that is presumably based on genetic and endocrine mechanisms but also on specific learning processes. These data highlight the importance of detailed behavioral analyses to identify the many dimensions of a behavior that can differ between males and females.

Does the syrinx, a peripheral structure, constrain effects of sex steroids on behavioral sex reversal in adult canaries?

We previously confirmed that effects of testosterone (T) on singing activity and on the volume of brain song control nuclei are sexually differentiated in adult canaries: females are limited in their ability to respond to T as males do. Here we expand on these results by focusing on sex differences in the production and performance of trills, i.e., rapid repetitions of song elements. We analyzed >42,000 trills recorded over a period of 6 weeks from 3 groups of castrated males and 3 groups of photoregressed females that received Silastic™ implants filled with T, T plus estradiol or left empty as control. Effects of T on the number of trills, trill duration and percent of time spent trilling were all stronger in males than females. Irrespective of endocrine treatment, trill performance assessed by vocal deviations from the trill rate versus trill bandwidth trade-off was also higher in males than in females. Finally, inter-individual differences in syrinx mass were positively correlated with specific features of trills in males but not in females. Given that T increases syrinx mass and syrinx fiber diameter in males but not in females, these data indicate that sex differences in trilling behavior are related to sex differences in syrinx mass and syrinx muscle fiber diameter that cannot be fully suppressed by sex steroids in adulthood. Sexual differentiation of behavior thus reflects organization not only of the brain but also of peripheral structures.

Effects of the depletion of neural progenitors by focal X-ray irradiation on song production and perception in canaries

The song control nucleus HVC of songbirds has emerged as a widespread model system to study adult neurogenesis and the factors that modulate the incorporation of new neurons, including seasonal state, sex differences or sex steroid hormone concentrations. However, the specific function of these new neurons born in adulthood remains poorly understood. We implemented a new procedure based on focal X-ray irradiation to deplete neural progenitors in the ventricular zone adjacent to HVC and study the functional consequences. A 23 Gy dose depleted by more than 50 percent the incorporation of BrdU in neural progenitors, a depletion that was confirmed by a significant decrease in doublecortin positive neurons. This depletion of neurogenesis significantly increased the variability of testosterone-induced songs in females and decreased their bandwidth. Expression of the immediate early gene ZENK in secondary auditory areas of the telencephalon that respond to song was also inhibited. These data provide evidence that new neurons in HVC play a role in both song production and perception and that X-ray focal irradiation represents an excellent tool to advance our understanding of adult neurogenesis.

Does the syrinx, a peripheral structure, constrain effects of sex steroids on behavioral sex reversal in adult canaries?

We previously confirmed that effects of testosterone (T) on singing activity and on the volume of brain song control nuclei are sexually differentiated in adult canaries: females are limited in their ability to respond to T as males do. Here we expand on these results by focusing on sex differences in the production and performance of trills, i.e., rapid repetitions of song elements. We analyzed more than 42,000 trills recorded over a period of 6 weeks from 3 groups of castrated males and 3 groups of photoregressed females that received Silasticâ"¢ implants filled with T, T plus estradiol or left empty as control. Effects of T on the number of trills, trill duration and percent of time spent trilling were all stronger in males than females. Irrespective of endocrine treatment, trill performance assessed by vocal deviations from the trill rate versus trill bandwidth trade-off was also higher in males than in females. Finally, inter-individual differences in syrinx mass were positively correlated with trill production in males but not in females. Given that T increases syrinx mass and syrinx fiber diameter in males but not in females, these data indicate that sex differences in trilling behavior are related to sex differences in syrinx mass and syrinx muscle fiber diameter that cannot be fully reversed by sex steroids in adulthood. Sexual differentiation of behavior thus reflects organization not only of the brain but also of peripheral structures.

Testosterone treatment reveals marked sex differences in song diversity and syllable syntax in adult canaries

Background. Behavioral sex differences are widespread in the animal world. These differences can be qualitative (i.e., behavior present in one sex but not the other, a true sex dimorphism) or quantitative (behavior is present at a higher rate or quality in one sex compared to the other). Singing in oscine songbirds is associated with both types of differences. In canaries, female rarely sing spontaneously but they can be induced to do so by treatments with steroids. Song in these females is however not fully masculinized and exhibits relatively subtle differences in quality as compared with male song. We analyzed here sex differences in syllable content and syllable use between singing male and female canaries. Methods. Songs were recorded from 3 groups of castrated male and 3 groups of photoregressed female canaries that had received Silasticâ"¢ implants filled with testosterone (T), with T plus estradiol (E2), or left empty (control). After 6 weeks of hormone treatment, 30 songs were recorded from each of the 47 subjects. Songs were segmented and each syllable was annotated. Various metrics of syllable diversity were extracted and network analysis was employed to characterize syllable sequences. Results. Male and female songs were characterized by marked sex differences related to syllable use. Compared to females, males had a larger syllable type repertoire and their songs contained more syllable types. Network analysis of syllable sequences showed that males follow more fixed patterns of syllable transitions than females. Both sexes however produced song of the same duration containing the same number of syllables produced at similar rates (numbers per second). Conclusions. Under the influence of T canaries of both sexes are able to produce generally similar vocalizations that nevertheless differ in specific ways. The development of song during ontogeny appears to be a very sophisticated process that is presumably based on genetic and endocrine mechanisms but also on specific learning processes. These data highlight the importance of detailed behavioral analyses in order to identify the many dimensions of a behavior that can differ between males and females.

Treatment with androgens plus estrogens cannot reverse sex differences in song and the song control nuclei in adult canaries

Adult treatments with testosterone (T) do not activate singing behavior nor promote growth of song control nuclei to the same extent in male and female canaries (Serinus canaria). Because T acts in part via aromatization into an estrogen and brain aromatase activity is lower in females than in males in many vertebrates, we hypothesized that this enzymatic difference might explain the sex differences seen even after exposure to the same amount of T. Three groups of castrated males and 3 groups of photoregressed females (i.e., with quiescent ovaries following exposure to short days) received either 2 empty 10 mm silastic implants, one empty implant and one implant filled with T or one implant filled with T plus one with estradiol (E2). Songs were recorded for 3 h each week for 6 weeks before brains were collected and song control nuclei volumes were measured in Nissl-stained sections. Multiple measures of song were still different in males and females following treatment with T. Co-administration of E2 did not improve these measures and even tended to inhibit some measures such as song rate and song duration. The volume of forebrain song control nuclei (HVC, RA, Area X) and the rate of neurogenesis in HVC was increased by the two steroid treatments, but remained significantly smaller in females than in males irrespective of the endocrine condition. These sex differences are thus not caused by a lower aromatization of the steroid; sex differences in canaries are probably organized either by early steroid action or by sex-specific gene regulation directly in the brain.

Striatal Injury Induces Overall Brain Alteration at the Pallial, Thalamic, and Cerebellar Levels

Simple Summary

Magnetic resonance imaging showed that striatal injury leads to structural changes within several brain areas. Here, we specify these changes via gene expression of synaptic plasticity markers, neuronal markers, assessing the number of newborn cells as well as cell densities. We found that the injury resulted in long-lasting modifications involving plasticity and neural protection mechanisms in areas directly as well as indirectly connected with the damaged striatum, including the cerebellum.

Abstract

The striatal region Area X plays an important role during song learning, sequencing, and variability in songbirds. A previous study revealed that neurotoxic damage within Area X results in micro and macrostructural changes across the entire brain, including the downstream dorsal thalamus and both the upstream pallial nucleus HVC (proper name) and the deep cerebellar nuclei (DCN). Here, we specify these changes on cellular and gene expression levels. We found decreased cell density in the thalamic and cerebellar areas and HVC, but it was not related to neuronal loss. On the contrary, perineuronal nets (PNNs) in HVC increased for up to 2 months post-lesion, suggesting their protecting role. The synaptic plasticity marker Forkhead box protein P2 (FoxP2) showed a bi-phasic increase at 8 days and 3 months post-lesion, indicating a massive synaptic rebuilding. The later increase in HVC was associated with the increased number of new neurons. These data suggest that the damage in the striatal vocal nucleus induces cellular and gene expression alterations in both the efferent and afferent destinations. These changes may be long-lasting and involve plasticity and neural protection mechanisms in the areas directly connected to the injury site and also to distant areas, such as the cerebellum.

Perineuronal nets in HVC and plasticity in male canary song

Songbirds learn their vocalizations during developmental sensitive periods of song memorization and sensorimotor learning. Some seasonal songbirds, called open-ended learners, recapitulate transitions from sensorimotor learning and song crystallization on a seasonal basis during adulthood. In adult male canaries, sensorimotor learning occurs each year in autumn and leads to modifications of the syllable repertoire during successive breeding seasons. We previously showed that perineuronal nets (PNN) expression in song control nuclei decreases during this sensorimotor learning period. Here we explored the causal link between PNN expression in adult canaries and song modification by enzymatically degrading PNN in HVC, a key song control system nucleus. Three independent experiments identified limited effects of the PNN degradation in HVC on the song structure of male canaries. They clearly establish that presence of PNN in HVC is not required to maintain general features of crystallized song. Some suggestion was collected that PNN are implicated in the stability of song repertoires but this evidence is too preliminary to draw firm conclusions and additional investigations should consider producing PNN degradations at specified time points of the seasonal cycle. It also remains possible that once song has been crystallized at the beginning of the first breeding season, PNN no longer play a key role in determining song structure; this could be tested by treatments with chondroitinase ABC at key steps in ontogeny. It would in this context be important to develop multiple stereotaxic procedures allowing the simultaneous bilateral degradation of PNN in several song control nuclei for extended periods.

Plasticity in the hippocampal formation of shorebirds during the wintering period: Stereological analysis of parvalbumin neurons in Actitis macularius

The number of parvalbumin neurons can be modified by social, multisensory, and cognitive stimuli in both mammals and birds, but nothing is known about their plasticity in long-distance migratory shorebirds. Here, in the spotted sandpiper (Actitis macularius), we investigated the plasticity of parvalbumin neurons of two brain areas during this species' wintering period at a lower latitude. We compared individuals in a nonmigratory rest period (November-January) and premigration (May-July) period. We used parvalbumin as a marker for counting a subpopulation of inhibitory neurons in the hippocampal formation (HF), with the magnocellular nucleus of the tectal isthmus (IMC) as a control area. Because the HF is involved in learning and memory and social interaction and the IMC is essential for control of head, neck, and eye movements, we hypothesized that parvalbumin neurons would increase in the HF and remain unchanged in the IMC. We used an optical fractionator to estimate cell numbers. Compared with the nonmigratory rest birds, parvalbumin neuron count estimates in the premigration birds increased significantly in the HF but remained unchanged in IMC. We suggest that the greater number of parvalbuminergic neurons in the HF of A. macularius in the premigration period represents adaptive circuitry changes involved in the migration back to reproductive niches in the northern hemisphere.

An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity

During restricted time windows of postnatal life, called critical periods, neural circuits are highly plastic and are shaped by environmental stimuli. In several mammalian brain areas, from the cerebral cortex to the hippocampus and amygdala, the closure of the critical period is dependent on the formation of perineuronal nets. Perineuronal nets are a condensed form of an extracellular matrix, which surrounds the soma and proximal dendrites of subsets of neurons, enwrapping synaptic terminals. Experimentally disrupting perineuronal nets in adult animals induces the reactivation of critical period plasticity, pointing to a role of the perineuronal net as a molecular brake on plasticity as the critical period closes. Interestingly, in the adult brain, the expression of perineuronal nets is remarkably dynamic, changing its plasticity-associated conditions, including memory processes. In this review, we aimed to address how perineuronal nets contribute to the maturation of brain circuits and the regulation of adult brain plasticity and memory processes in physiological and pathological conditions.