Hyperstriatum Ventrale, Pars Caudale Research Articles

Distribution and Abundance of Glucocorticoid and Mineralocorticoid Receptors throughout the Brain of the Great Tit (Parus major).

The glucocorticoid stress response, regulated by the hypothalamic-pituitary-adrenal (HPA) axis, enables individuals to cope with stressors through transcriptional effects in cells expressing the appropriate receptors. The two receptors that bind glucocorticoids—the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR)—are present in a variety of vertebrate tissues, but their expression in the brain is especially important. Neural receptor patterns have the potential to integrate multiple behavioral and physiological traits simultaneously, including self-regulation of glucocorticoid secretion through negative feedback processes. In the present work, we quantified the expression of GR and MR mRNA throughout the brain of a female great tit (Parus major), creating a distribution map encompassing 48 regions. This map, the first of its kind for P. major, demonstrated a widespread but not ubiquitous distribution of both receptor types. In the paraventricular nucleus of the hypothalamus (PVN) and the hippocampus (HP)—the two brain regions that we sampled from a total of 25 birds, we found high GR mRNA expression in the former and, unexpectedly, low MR mRNA in the latter. We examined the covariation of MR and GR levels in these two regions and found a strong, positive relationship between MR in the PVN and MR in the HP and a similar trend for GR across these two regions. This correlation supports the idea that hormone pleiotropy may constrain an individual’s behavioral and physiological phenotype. In the female song system, we found moderate GR in hyperstriatum ventrale, pars caudalis (HVC), and moderate MR in robust nucleus of the arcopallium (RA). Understanding intra- and interspecific patterns of glucocorticoid receptor expression can inform us about the behavioral processes (e.g. song learning) that may be sensitive to stress and stimulate future hypotheses concerning the relationships between receptor expression, circulating hormone concentrations and performance traits under selection, including behavior.

Intracellular Characterization of Song-Specific Neurons in the Zebra Finch Auditory Forebrain

Auditory neurons in the forebrain nucleus HVc (hyperstriatum ventrale pars caudale) are highly sensitive to the temporal structure of the bird's own song. These "song-specific" neurons respond strongly to forward song, weakly to the song with the order of the syllables reversed, and little or not at all to reversed song. To investigate the cellular mechanisms underlying these responses, in vivo intracellular recordings were made from adult HVc neurons. Song-specific cells could be divided into those that responded strongly throughout autogenous song (tonic cells) and those that responded with bursts of action potentials at specific points during the song (phasic cells). Phasic cells were hyperpolarized during autogenous song, even though this stimulus also elicited the strongest response. Less hyperpolarization was seen to the same song with the syllables in reverse order, and none was seen to reversed song. The responses of both types of song-specific cells contained high-frequency bursts of action potentials. The bursts of the phasic cells showed attenuation of the action potential height and lack of full repolarization after each spike. This type of bursting was significantly correlated with the amount of hyperpolarization before each burst in phasic cells and nonauditory cells that generated such bursts spontaneously. These data suggest that song-specific neurons use longlasting hyperpolarization as a mechanism to integrate auditory context, an important component of temporal order selectivity. Hyperpolarization also may increase the precision of spike timing, which could be important for the neural code subserving song learning and production.

Mechanisms underlying the sensitivity of songbird forebrain neurons to temporal order.

Neurons in the songbird forebrain area HVc (hyperstriatum ventrale pars caudale or high vocal center) are sensitive to the temporal structure of the bird's own song and are capable of integrating auditory information over a period of several hundred milliseconds. Extracellular studies have shown that the responses of some HVc neurons depend on the combination and temporal order of syllables from the bird's own song, but little is known about the mechanisms underlying these response properties. To investigate these mechanisms, we recorded intracellular responses to a set of auditory stimuli designed to assess the degree of dependence of the responses on temporal context. This report provides evidence that HVc neurons encode information about temporal structure by using a variety of mechanisms including syllable-specific inhibition, excitatory postsynaptic potentials with a range of different time courses, and burst-firing nonlinearity. The data suggest that the sensitivity of HVc neurons to temporal combinations of syllables results from the interactions of several cells and does not arise in a single step from afferent inputs alone.

Two separate areas of the brain differentially guide the development of a song control nucleus in the zebra finch.

A brain nucleus that is important for the generation of song in the adult male zebra finch (Poephila guttata), the robust nucleus of the archistriatum (RA), receives dual inputs from two other telencephalic song nuclei: the hyperstriatum ventrale pars caudale (HVc) and the lateral magnocellular nucleus of the anterior neostriatum (L-MAN). We lesioned each of these afferent inputs to the RA early in development, either by themselves or both at the same time in the same side of the brain, to determine what influences each of these nuclei exerts on the normal development of the RA into adulthood. We found that lesioning the HVc in a 20-day-old male zebra finch prevents the later increase in RA volume and soma size that would normally occur around 35 days post-hatching. MAN lesions at this same early age, on the other hand, had a large effect on reducing the volume and cell number of RA neurons, without affecting soma size. Lesioning both inputs early in development induced considerable RA neuronal cell death and atrophy of the nucleus. This study shows that the development of the RA is affected differently by each of its two input nuclei.

Computations in the Ascending Auditory Pathway in Songbirds Related to Song Learning

SYNOPSIS. In general, the song of each adult songbird is unique to that individual. Although some of the parameters of song result from maturational processes which are innately specified, the spectral and temporal parameters of song that confer individual specificity are acquired by learning. The specifics of an individual's song are reflected in idiosyncratic physiological properties of auditory neurons in song system nuclei such as the nucleus hyperstriatum ventrale pars caudale (HVc). We are exploring the transformation from classical auditory to song-specific neuronal properties. This transformation is a complex one which to date has not yielded to classical neuroanatomical and neurophysiological methods of analysis. Furthermore, the idiosyncratic nature of song system auditory response properties makes it difficult to collapse data across individuals. To address these issues, we have investigated a connectionist modelling approach. Our initial efforts have been directed at auditory neurons of the thalamus. In the thalamus, simple linear-static models that use average firing rates of responses to tone bursts have proven to be poor predictors of responses to song. In contrast, time-delay neural network (TDNN) architectures that are trained with time-varying responses to tone bursts using the backpropagation algorithm make excellent predictions of the responses to songs. We are currently extending these architectures in an attempt to capture salient features of the responses of song system auditory neurons. INTRODUCTION ticity associated with idiosyncratic

Temporal and harmonic combination-sensitive neurons in the zebra finch's HVc

Song learning shapes the response properties of auditory neurons in the song system to become highly selective for the individual bird's own ("autogenous") song. The auditory representation of autogenous song is achieved in part by neurons that exhibit facilitated responses to combinations of components of song. To understand the circuits that underlie these complex properties, the combination sensitivity of single units in the hyperstriatum ventrale, pars caudale (HVc) of urethane-anesthetized zebra finches was studied. Some neurons exhibited nonlinear temporal summation, spectral summation, or both. The majority of these neurons exhibited low spontaneous rates and phasic responses. Most combination-sensitive neurons required highly accurate copies of sounds derived from the autogenous song and responded weakly to tone bursts, combinations of simple stimuli, or conspecific songs. Temporal combination-sensitive (TCS) neurons required either two or more segments of a single syllable, or two or more syllables of the autogenous song, to elicit a facilitated, excitatory response. TCS neurons integrated auditory input over periods ranging from 80 to 350 msec, although this represents a lower limit. Harmonic combination-sensitive (HCS) neurons required combinations of two harmonics with particular frequency and temporal characteristics that were similar to autogenous song syllables. Both TCS and HCS neurons responded much more weakly when the dynamical spectral features of the autogenous song or syllables were modified than when the dynamical amplitude (waveform) features of the songs were modified. These results suggest that understanding the temporal dynamics of auditory responses in HVc may provide insight into neuronal circuits modified by song learning.

Sodium‐ and calcium‐dependent conductances of neurones in the zebra finch hyperstriatum ventrale pars caudale in vitro.

1. Intracellular recordings were made from zebra finch hyperstriatum ventrale pars caudale (HVc) neurones in in vitro slice preparations. 2. Small depolarizing current pulses elicited tonic firing while relatively large current pulses elicited an initial high-frequency burst followed by tonic firing. Time-dependent and fast-activated inward rectification were observed when a hyperpolarizing current pulse was applied. 3. Fast action potentials were abolished by tetrodotoxin (TTX, 0.5 micrograms/ml). When Co2+ (2.2-2.4 mM) was added to a low-Ca2+ (0-0.2 mM) solution, a plateau potential was elicited by a small depolarizing current pulse. These plateau potentials outlasted the applied current pulse and were abolished by TTX (0.5 micrograms/ml). 4. A long-lasting after-hyperpolarization (AHP), with a duration of tens of seconds, followed long trains of repetitive firing in control solution. Even in a low-Ca2+ (0-0.2 mM) solution containing Co2+ (2.2-2.4 mM) the long-lasting AHPs followed plateau potentials and were associated with an increase in input conductance. The long-lasting AHP, as well as the plateau potential, was blocked by TTX (0.5 micrograms/ml). 5. While TTX abolished fast action potentials, two types of active responses with different thresholds were elicited by depolarizing current in the presence of TTX (0.5 micrograms/ml), particularly when tetraethylammonium (5 mM) was also added to the solution. Both the low-threshold spike (LTS) and the high-threshold spike (HTS) were abolished by Co2+ (2.2-2.3 mM). 6. When Ba2+ (0.5-1 mM) was added to a solution containing TTX (0.5 micrograms/ml), an LTS elicited by a depolarizing current pulse became larger in amplitude. At membrane potentials more positive than -55 mV, an HTS, but not the LTS, was elicited. 7. These results suggest that HVc neurones have three Na(+)-dependent conductances: a fast Na+ conductance responsible for the fast spike, a persistent Na+ conductance responsible for the plateau potential, and a Na(+)-activated K+ conductance responsible for the long-lasting AHP; and that they have two Ca(2+)-dependent conductances: a low-threshold Ca2+ conductance responsible for the LTS, and a high-threshold Ca2+ conductance responsible for the HTS.

The development of afferent projections to the robust archistriatal nucleus in male zebra finches: a quantitative electron microscopic study

Because the projections into the robust nucleus of the archistriatum (RA) are thought to play important roles in song learning and sexual differentiation of the zebra finch (Poephila guttata), quantitative electron microscopic techniques were used to measure the development of synaptic input to the neuropil of RA in this species. Two nuclei [hyperstriatum ventrale pars caudalis (HVc) and lateral magnocellular nucleus of the anterior neostriatum (IMAN)] that send projections to RA were lesioned at each of three different ages: 25 d after hatching, 53 d, and adulthood. In tissue from RA processed for conventional electron microscopy, lesion-induced degeneration was used to identify synapses from either HVc or IMAN. Axosomatic synapses were excluded from analysis. In control (unlesioned) animals, the density of synapses in neuropil increased slightly between days 28 and 56 and remained constant thereafter. Because of a large increase in the volume of RA, the total number of synapses in neuropil of RA tripled between days 28 and 56 and decreased significantly between day 56 and adulthood. The density and total number of synapses in neuropil originating from HVc increased significantly between days 25 and 53, but did not change significantly thereafter. In contrast, the density and total number of synapses from IMAN decreased significantly between days 25 and 53 and did not change thereafter. Presynaptic terminals from IMAN were larger than those from HVc. These data demonstrate that the most rapid phase of song learning is accompanied by a major rearrangement of synaptic contacts into RA that stem from HVc and IMAN.

Testosterone metabolism in the avian hypothalamus

Many central actions of testosterone (T) require the transformation of T into several metabolites including 5α-dihydrotestosterone (5α-DHT) and estradiol (E 2). In birds as in mammals, 5α-DHT and E 2, alone or in combination, mimic most behavioral effects of T. The avian brain is, in addition, able to transform T into 5β-DHT, a metabolite which seems to be devoid of any behavioral or physiological effects, at least in the context of reproduction. By in vitro product-formation assays, we have analyzed the distribution, sex differences and regulation by steroids of the 3 main T metabolizing enzymes (aromatase, 5α- and 5β-reductases) in the brain of the Japanese quail ( Coturnix c. japonica) and the zebra finch ( Taeniopygia guttata castanotis). In the hypothalamus of quail and finches, aromatose activity is higher in males than in females. It is also decreased by castration and increased by T. The activity of the 5α-reductase is not sexually differentiated nor controlled by T. The 5β-reductase activity is often higher in females than in males but this difference disappears in gonadectomized birds and no clear effect of T can be observed at this level. The zebra finch brain also contains a number of steroid-sensitive telencephalic nuclei [e.g. hyperstriatum ventrale, pars caudale (HVc) and robustus archistriatalis (RA)] which play a key role in the control of vocalizations. These nuclei also contain T-metabolizing enzymes but the regulation of their activity is substantially different from what has been observed in the hypothalamus. Aromatase activity is for example higher in females than in males in HVc and RA and the enzyme in these nuclei is not affected by castration nor T treatment. In these nuclei, the 5α-reductase activity is higher in males than in females and the reverse is true for the 5β-reductase. These sex differences in activity are not sensitive to gonadectomy and T treatment and might therefore be organized by neonatal steroids. We have been recently able to localize aromatase-immunoreactive (AR-ir) neurons by ICC in the brain of the quail and zebra finch. Positive cells are found in the preoptic area, ventromedial and tuberal hypothalamus. AR-ir material is found in the perikarya of cells and fills the entire cellular processes including axons. At the electron microscope level, immunoreactive material can clearly be observed in the synaptic boutons. This observation raises questions concerning the mode of action of estrogens produced by central aromatization of T.

Distribution of choline acetyltransferase and acetylcholinesterase in the vocal motor system of zebra finches

The distribution of choline acetyltransferase immunoreactivity (ChAT-IR) was surveyed in the vocal motor system of adult male and female zebra finches and was compared with the pattern of histochemical acetylcholinesterase (AChE-His). In the vocal motor system the most prominent accumulation of ChAT-IR somata was found in lobus parolfactorius (LPO) including Area X. Immunoreactive neuropil was found to be concentrated in pericellular networks of fibers in male's Area X while the corresponding area in females could not be demarcated within the LPO. The density of ChAT-IR fiber networks was much higher in LPO, paleostriatum augmentatum and in a shelf region around nucleus robustus archistriatalis (RA) thanin neostriatal and hyperstriatal parts of the telencephalon. AChE positive neurons and neuropil were observed in all ChAT-IR regions and, in addition, in the vocal motor nucleus hyperstriatum ventrale pars caudalis (HVc), nucleus magnocellularis in the anterior neostriatum (MAN), nucleus interfacialis (NIF) and RA. However, none of the latter nuclei contained ChAT-IR cell bodies. They were characterized by rare ChAT-IR neuropil. MAN and RA exhibited shelf regions with a higher degree of stained fibers. The discrepancy between the localization of AChE-His and ChAT-IR can hardly be explained by different classes of ChAT isoenzymes in neurons within the basal forebrain and the neostriatal, hyperstriatal and archistriatal vocal control nuclei not detected by our antibody. On the other hand, vocal control centers while receiving cholinergic inputs, might — except for Area X — not possess cholinergic efferent projections within the telencephalon.

Genesis and death of vocal control neurons during sexual differentiation in the zebra finch

Several song-related regions in the adult zebra finch brain have substantially more neurons in males than in females. Such differences appear to arise from sex differences in circulating steroids during early posthatch life. In the present study, developmental mechanisms involved in the production of sex differences are explored by examinations of the normal time course of posthatch neurogenesis and cell death in vocal control circuits. As a first step toward determining whether rates of neuron production may be different in males and females, tritiated thymidine, a marker of cell division, was administered to zebra finches at various times during the first month after hatching. Birds were sacrificed at 60 d. The number of cells formed after hatching and present at 60 d was then evaluated in 3 vocal control regions--HVc (hyperstriatum ventralis pars caudalis) and its 2 principal targets, RA (robust nucleus of the archistriatum) and Area X. Cell death was quantified by counts of normal and pyknotic, degenerating cells made in these nuclei in additional, untreated birds of both sexes at 5 d intervals from 5 to 45 d of age. The combined results of these experiments suggest that differential cell death is a major factor in the development of sex differences in the song control system and provide the first direct evidence for sex differences in cell death in the developing telencephalon. Although developmental time tables differ among the 3 brain areas examined, at specific ages significantly higher numbers of pyknotic cells were observed in HVc, RA, and presumptive Area X in females compared to males. Peak levels of cell death in RA occur 4-6 weeks after hatching. This is about 3 weeks after the onset of sex differences in steroid levels that, in turn, lead to differential organization of song system nuclei. This pattern of results suggests that designation for death and actual cell loss are temporally dissociated in this system. Neuron proliferation for HVc and Area X, but not RA, continues throughout the first 30 d after hatching, and a significant sex difference was found in the number of cells present in HVc at 60 d that were formed after hatching. Comparisons of the timing of cell death and cell incorporation suggest that this difference may be best accounted for by differential survival of neurons formed after hatching rather than differential rates of neuron production. Neither differential neurogenesis nor differential neuron death can fully account for the apparent extreme sexual dimorphism in the number of neurons in Area X.(ABSTRACT TRUNCATED AT 400 WORDS)

Estrogen stimulates the incorporation of new neurons into avian song nuclei during adolescence

In zebra finches the song control nuclei hyperstriatum ventralis pars caudalis (HVc) and area X of the lobus parolfactorius (LPO), continue to add new neurons during the juvenile period of song learning. Normally, males add many more of these new cells than do females (who do not sing), leading to pronounced sexual dimorphism within these regions. Exposing females to estradiol (E2) shortly after hatching masculinizes the HVc and area X and such females sing in response to later androgen stimulation. We investigated whether exposing female hatchlings to E2 stimulates the incorporation of HVc and area X neurons born during the juvenile period, and whether later androgen stimulation further influences addition of these late-generated neurons. Females were implanted with E2 on day 3 and received either empty or dihydrotestosterone (DHT)-filled capsules on day 25. These females, and normal males and females received [ 3H]thymidine daily between 20 and 40 days and were killed at 65 days. Autoradiographic analyses of HVc and area X-LPO revealed that neuron number, as well as the incidence and number of thymidine-labeled neurons was increased in E2-treated females to levels approaching those typical of males. DHT did not further influence these measures in females. These data indicate that E2 promotes either the production, migration, or survival of HVc and area X neurons born during the juvenile period.

Morphology of Golgi‐impregnated neurons in hyperstriatum ventralis, pars caudalis in adult male and female canaries

Golgi-impregnated neurons in the song control nucleus hyperstriatum ventralis, pars caudalis (HVc) in male and female canaries (Serinus canarius) have been divided into classes, primarily on the basis of interneuronal variability in spine density and dendritic branching pattern. At least four neuronal classes are found in HVc: aspinous neurons and three classes of spiny neurons. The "furry" dendrite (FD) cell class consists of neurons with long dendrites that are densely packed with spines. Their cell bodies are between 10 and 15 microns in diameter. Neurons of the thick dendrite (TD) cell class also have long dendrites but only about half as many spines along their dendritic branches. Their cell bodies are between 12 and 18 microns in diameter. Neurons of the short dendrite (SD) cell class are characterized by a low spin density, very thin dendrites, and a small dendritic field. Their cell bodies are between 9 and 13 microns in diameter. The TD class can be divided into two subclasses on the basis of the shape of the dendritic field. Principal component factor analysis and cluster analysis provide objective support for this classification scheme. Neurons of subclass TD2 are sexually dimorphic. Neurons from males have dendritic trees that are about 70% larger and have 40% more dendritic endings than do neurons from females. There may also be small sex differences in dendritic morphology in the SD class and in the remainder of the TD class. There are clearly no sex differences in the dendritic morphology of neurons from the FD class. The direct pathway which is believed responsible for dimorphic song production in canaries is from HVc to nucleus robustus archistriatalis (RA) and then to the motor neurons which control the avian vocal organ. It is surprising that the most striking dimorphism in the present data occurs in neurons which, on morphological grounds, are unlikely to project to RA.

Projections of androgen-accumulating neurons in a nucleus controlling avian song

In zebra finches, androgens stimulate sing production and promote growth of the neural regions controlling song. Early exposure to estrogen establishes this sensitivity to androgens and increases the number of androgen-accumulating cells in two song regions, the hyperstriatum ventralis pars caudalis (HVc) and the anterior neostriatum. To examine if these regional changes in androgen accumulation could directly influence androgen responsiveness elsewhere in the song system, we combined autoradiographic and retrograde tracing techniques to determine if androgen-accumulating HVc neurons project to other vocal control nuclei. We report here that both major efferent projections from HVc (to Area X and the nucleus robustus of the archistriatum) include a substantial proportion of androgen-accumulating neurons. These data are consistent with the hypothesis that the extent of androgen accumulation in HVc may in turn regulate the androgenic sensitivity of other song regions.

特集〈聴覚と音声言語〉 発声と聴覚の中枢における相互関係 ヒトのモデルとしてのトリの場合

The mammalian motor cortex sends efferent motor commands and receives two kinds of feedback, internal and external. The avian vocal command center, robustus archistriatalis (RA) which corresponds to the mammalian motor cortex, also receives internal and external feedback which are integrated in the hyperstriatum ventralis pars caudalis (HVc) . The present study clarifies the avian feedback path by analysing neural substrates in the HVc: the HVc contains 3 kinds of neurons in which auditory responses interact and constitute both feedback paths. A speculative theory of the song production mechanism is discussed on the basis of the evidence of synaptic connections between three kinds of the HVc neurons.

Effect of testosterone on input received by an identified neuron type of the canary song system: a Golgi/electron microscopy/degeneration study

Combinations of the Golgi stain, anterograde degeneration, and electron microscopy are used to further characterize the hormone-sensitive "type IV" neuron of the forebrain nucleus robustus archistriatalis (RA) of adult female canaries. Anterograde degeneration was used to "stain," at the electron-microscopic level, the axon terminals of neurons projecting to RA from hyperstriatum ventralis, pars caudalis (HVc) and from the lateral magnocellular nucleus of the anterior neostriatum (L-MAN). The HVc neurons projecting to RA type IV cells form synapses predominantly on the dendritic spines of those cells, while L-MAN neurons that project to RA type IV cells form a 2.5:1 mixture of shaft and spine synapses. There were about 1000 synapses from HVc neurons (about 30% of all spine synapses) on typical type IV cells and about 50 synapses from L-MAN neurons. Earlier work had shown that in female canaries the dendrites of type IV neurons of the avian song control nucleus RA increase in total length after systemic testosterone treatment, and that this increase in dendritic length was accompanied by the development of malelike song. We now show that testosterone treatment also increases the number of dendritic spines present in type IV neurons. Presumably this is accompanied by an increase in the number of synaptic inputs received by type IV cells. Earlier evidence suggested that the testosterone-induced addition of extra dendritic length to type IV cells occurred at existing dendritic tips. We tested the hypothesis that these added peripheral ends received a special subset of inputs, which might then account for the change in behavior, and found it to be false. Mapping and counts of degenerating synapses resulting from lesion of HVc and L-MAN suggest that under the influence of hormone, new synapses are added throughout the dendritic tree, with no special distribution or change in ratio of inputs occurring at the tip of dendrites. Under the influence of testosterone, each type IV cell may receive only "more of the same" inputs it received before onset of treatment. We speculate on how such changes in circuitry may relate to song stability and learning.

Sex and regional differences in the incorporation of neurons born during song learning in zebra finches

In zebra finches only males sing, and several nuclei controlling song contain more neurons in adult males than in females. The ontogeny of sex differences in neuron number differs across song regions and overlaps with song learning in males. We examined the development of neuron number in several song regions in both sexes. We then determined whether neurons are born and incorporated into song nuclei as sex differences in neuron number emerge, and whether sex or regional differences in the insertion of such neurons may account for differences in the development of these areas. Males add neurons to hyperstriatum ventralis pars caudalis (HVc) and Area X between 20 and 55 d of age. In females there is no change in HVc neuron number during this time, and Area X never appears as a distinct nucleus. In both sexes, 3H-thymidine administration between 20 and 30 d results in neuronal labeling at 55 d in HVc and the region of Area X. However, in these areas the incidence of labeled neurons is higher in males than in females. In contrast to HVc and Area X, sex differences in neuron number in the robustus nucleus of the archistriatum (RA) and the magnocellular nucleus of the neostriatum (MAN) emerge because males retain neurons that are lost in females between 20 and 55 d of age. Accordingly, RA and MAN neurons are not labeled following 3H-thymidine administration between 20 and 30 d of age. These data indicate that sex and regional differences in the ontogeny of song nuclei are related to differences in the incorporation of neurons born during song learning.

Projection neurons within a vocal motor pathway are born during song learning in zebra finches.

Many birds learn song during a restricted 'sensitive' period. Juveniles memorize a song model, and then learn the pattern of muscle contractions necessary to reproduce the song. Of the neural changes accompanying avian song learning, perhaps the most remarkable is the production of new neurons which are inserted into the hyperstriatum ventralis pars caudalis (HVc), a region critical for song production. We report here that in young male zebra finches many of the new neurons incorporated into the HVc innervate the robust nucleus of the archistriatum (RA) which projects to motor neurons controlling the vocal musculature. Furthermore, far fewer of these new neurons are incorporated into the HVc of either adult males that are beyond the sensitive learning period, or young females (who do not develop song). Thus, a major portion of the vocal motor pathway is actually created during song learning. This may enable early sensory experience and vocal practice to not only modify existing neuronal circuits, but also shape the insertion and initial synaptic contacts of neurons controlling adult song.

Estrogen accumulation in zebra finch song control nuclei: implications for sexual differentiation and adult activation of song behavior.

In zebra finches the gonadal steroid estradiol (E2) directs the sexual differentiation of neural regions controlling song and synergizes with androgens to stimulate song in adulthood. To identify regions where E2 may act to exert these effects, steroid autoradiographic techniques were used to assess cellular accumulation of 3[H]-E2 or its metabolites within various nuclei of the zebra finch brain. In Experiment 1 we examined brains from juvenile females, still within the critical period for E2's effect on sexual differentiation. In Experiment 2 the pattern and extent of labeling in adult male brains was determined following injection of 3[H]-E2, 3[H]-testosterone, or 3[H]-dihydrotestosterone. The results suggest that, both during development and in adulthood, most song-control nuclei contain few E2-accumulating cells. In contrast, many cells densely labeled by 3[H]-E2 or its metabolites are present in the hypothalamus and in close proximity to one song-control region, the hyperstriatum ventralis pars caudalis (HVc). The distribution of these latter cells overlaps with cells that project to another song-related nucleus, Area X. Thus, in Experiment 3 fluorescent retrograde tracing and steroid autoradiographic techniques were combined to determine if E2-accumulating cells project to Area X in adult males. Although a few retrogradely labeled cells were lightly labeled by 3[H]-E2 or its metabolites, for the most part these appear to be two distinct populations of cells. The sparse accumulation of E2 in the zebra finch song system contrasts with that described in other song birds and has important implications as to the mechanism of E2 action on the developing and mature song system.

Synaptic connections of thalamo-cerebral vocal nuclei of the canary

The canary vocal nuclei include two systems: hyperstriatum ventrale, pars caudale (HVc), nucleus robustus archistriatalis (RA) and nucleus hypoglossus, pars tracheosyringealis (nXIIts) compose a motor driving system for vocalization. The other group of nuclei including HVc, nucleus X of the lobus parolfactorius (Area X), nucleus dorsointermedius posterior thalami (DIP) and nucleus magnocellularis of anterior neostriatum (MAN) is a modulator of the driving system. The HVc neurons receive mono- or polysynaptic innervation from the MAN and send their fibers to Area X. Axons of Area X terminated in the thalamic nucleus, the DIP, from which neurons extended axons back to the cerebral nucleus, the MAN. Accordingly, the HVc, Area X, DIP and MAN are in a closed loop. Auditory inflow may converge on HVc neurons, partly from either the MAN or Area X during feedback control of the song. Thalamic neurons in the DIP responded to MAN stimulation, and to tonal stimuli, with relatively long latency. The interconnections between the HVc and MAN neurons are presumably central in voco-auditory integration during song-learning.