Telencephalic Song Control Nuclei Research Articles

Inflammation Induced by Natural Neuronal Death and LPS Regulates Neural Progenitor Cell Proliferation in the Healthy Adult Brain.

Inflammation is typically considered a negative response to injury or insult; however, recent advances demonstrate that inflammatory cells regulate development, plasticity, and homeostasis through anticytotoxic, progenerative responses. Here, we extend analyses of neuroinflammation to natural neurodegenerative and homeostatic states by exploiting seasonal plasticity in cytoarchitecture of the avian telencephalic song control nucleus, high vocal center [HVC (proper name)], in the songbird Gambel’s white-crowned sparrow (Zonotrichia leucophrys gambelii). We report that local injection of the endotoxin lipopolysaccharide into HVC of birds in both breeding (high circulating testosterone level) and nonbreeding (low circulating testosterone level) conditions increased neural progenitor cell proliferation in the nearby but distinct ventricular zone. Additionally, we found that oral administration of the anti-inflammatory drug minocycline during seasonal regression of HVC reduced microglia activation in HVC and prevented the normal proliferative response in the ventricular zone to apoptosis in HVC. Our results suggest that local neuroinflammation positively regulates neural progenitor cell proliferation and, in turn, contributes to the previously described repatterning of HVC cytoarchitecture following seasonally induced neuronal loss.

Testosterone stimulates perineuronal nets development around parvalbumin cells in the adult canary brain in parallel with song crystallization

Perineuronal nets (PNN) of the extracellular matrix are dense aggregations of chondroitin-sulfate proteoglycans that usually surround fast-spiking parvalbumin-expressing inhibitory interneurons (PV). The development of PNN around PV appears specifically at the end of sensitive periods of visual learning and limits the synaptic plasticity in the visual cortex of mammals. Seasonal songbirds display a high level of adult neuroplasticity associated with vocal learning, which is regulated by fluctuations of circulating testosterone concentrations. Seasonal changes in testosterone concentrations and in neuroplasticity are associated with vocal changes between the non-breeding and breeding seasons. Increases in blood testosterone concentrations in the spring lead to the annual crystallization of song so that song becomes more stereotyped. Here we explore whether testosterone also regulates PNN expression in the song control system of male and female canaries. We show that, in both males and females, testosterone increases the number of PNN and of PV neurons in the three main telencephalic song control nuclei HVC, RA (nucleus robustus arcopallialis) and Area X and increases the PNN localization around PV interneurons. Singing activity was recorded in males and quantitative analyses demonstrated that testosterone also increased male singing rate, song duration and song energy while decreasing song entropy. Together, these data suggest that the development of PNN could provide the synaptic stability required to maintain the stability of the testosterone-induced crystallized song. This provides the new evidence for a role of PNN in the regulation of adult seasonal plasticity in seasonal songbirds.

Timing of perineuronal net development in the zebra finch song control system correlates with developmental song learning.

The appearance of perineuronal nets (PNNs) represents one of the mechanisms that contribute to the closing of sensitive periods for neural plasticity. This relationship has mostly been studied in the ocular dominance model in rodents. Previous studies also indicated that PNN might control neural plasticity in the song control system of songbirds. To further elucidate this relationship, we quantified PNN expression and their localization around parvalbumin interneurons at key time-points during ontogeny in both male and female zebra finches, and correlated these data with the well-described development of song in this species. We also extended these analyses to the auditory system. The development of PNN during ontogeny correlated with song crystallization although the timing of PNN appearance in the four main telencephalic song control nuclei slightly varied between nuclei in agreement with the established role these nuclei play during song learning. Our data also indicate that very few PNN develop in the secondary auditory forebrain areas even in adult birds, which may allow constant adaptation to a changing acoustic environment by allowing synaptic reorganization during adulthood.

Seasonal Changes in Patterns of Gene Expression in Avian Song Control Brain Regions

Photoperiod and hormonal cues drive dramatic seasonal changes in structure and function of the avian song control system. Little is known, however, about the patterns of gene expression associated with seasonal changes. Here we address this issue by altering the hormonal and photoperiodic conditions in seasonally-breeding Gambel's white-crowned sparrows and extracting RNA from the telencephalic song control nuclei HVC and RA across multiple time points that capture different stages of growth and regression. We chose HVC and RA because while both nuclei change in volume across seasons, the cellular mechanisms underlying these changes differ. We thus hypothesized that different genes would be expressed between HVC and RA. We tested this by using the extracted RNA to perform a cDNA microarray hybridization developed by the SoNG initiative. We then validated these results using qRT-PCR. We found that 363 genes varied by more than 1.5 fold (>log2 0.585) in expression in HVC and/or RA. Supporting our hypothesis, only 59 of these 363 genes were found to vary in both nuclei, while 132 gene expression changes were HVC specific and 172 were RA specific. We then assigned many of these genes to functional categories relevant to the different mechanisms underlying seasonal change in HVC and RA, including neurogenesis, apoptosis, cell growth, dendrite arborization and axonal growth, angiogenesis, endocrinology, growth factors, and electrophysiology. This revealed categorical differences in the kinds of genes regulated in HVC and RA. These results show that different molecular programs underlie seasonal changes in HVC and RA, and that gene expression is time specific across different reproductive conditions. Our results provide insights into the complex molecular pathways that underlie adult neural plasticity.

Thalamic Gating of Auditory Responses in Telencephalic Song Control Nuclei

In songbirds, nucleus Uvaeformis (Uva) is the sole thalamic input to the telencephalic nucleus HVC (used as a proper name), a sensorimotor structure essential to learned song production that also exhibits state-dependent responses to auditory presentation of the bird's own song (BOS). The role of Uva in influencing HVC auditory activity is unknown. Using in vivo extracellular and intracellular recordings in urethane-anesthetized zebra finches, we characterized the auditory properties of Uva and examined its influence on auditory activity in HVC and in the telencephalic nucleus interface (NIf), the main auditory afferent of HVC and a corecipient of Uva input. We found robust auditory activity in Uva and determined that Uva is innervated by the ventral nucleus of lateral lemniscus, an auditory brainstem component. Thus, Uva provides a direct linkage between the auditory brainstem and HVC. Although low-frequency electrical stimulation in Uva elicited short-latency depolarizing postsynaptic potentials in HVC neurons, reversibly silencing Uva exerted little effect on BOS-evoked activity in HVC neurons. However, high-frequency stimulation in Uva suppressed auditory-evoked synaptic and suprathreshold activity in all HVC neuron types, a process accompanied by decreased input resistance of individual HVC neurons. Furthermore, high-frequency stimulation in Uva simultaneously suppressed auditory activity in HVC and NIf. These results suggest that Uva can gate auditory responses in HVC through a mechanism that involves inhibition local to HVC as well as withdrawal of auditory-evoked excitatory drive from NIf. Thus, Uva could play an important role in state-dependent gating of auditory activity in telencephalic sensorimotor structures important to learned vocal control.

Effects of visual stimulation on the auditory responses of the HVC song control nucleus in anesthetized Bengalese Finches

ABSTRACT In songbirds, the auditory neurons of the telencephalic song control nuclei, especially those in the high vocal center (HVC), respond to the bird's own song (BOS) selectively. Since songs are elicited by the sight of conspecific females and interrupted by intense visual stimulation, such as strobe lights, visual input might modulate this auditory selectivity. This study used acute electrophysiological experiments using Bengalese Finches (Lonchura striata var. domestica) to examine whether strobe lights affect this auditory response. The results showed that visual inputs did not affect the neural activities in response to the BOS. When the visual stimulus was presented alone, we did not record comparable neural activities to auditory stimuli, although vague, weak electrical potential fluctuations were observed. This means that direct visual inputs do not reach all HVC neurons that have BOS selectivity, and the effects of visual information might be very limited in the song control system. Although...

Estrogen contributes to seasonal plasticity of the adult avian song control system.

Songbirds show dramatic neural plasticity as adults, including large-scale anatomical changes in discrete brain regions ("song control nuclei") controlling the production of singing behavior. The volumes of several song control nuclei are much larger in the breeding season than in the nonbreeding season, and these seasonal neural changes are regulated by plasma testosterone (T) levels. In many cases, the effects of T on the central nervous system are mediated by neural conversion to estradiol (E(2)) by the enzyme aromatase. The forebrain of male songbirds expresses very high levels of aromatase, in some cases adjacent to song control nuclei. We examined the effects of aromatase inhibition and estrogen treatment on song nuclei size using wild male songbirds in both the breeding and nonbreeding seasons. In breeding males, aromatase inhibition caused the volume of a telencephalic song control nucleus (HVC) to decrease, and this effect was partially rescued by concurrent estrogen replacement. In nonbreeding males, estradiol treatment caused HVC to grow to maximal spring size within 2 weeks. Overall, these data suggest that aromatization of T is an important mediator of song control system plasticity, and that estradiol has neurotrophic effects in adult male songbirds. This study demonstrates that estrogen can affect adult neural plasticity on a gross anatomical scale and is the first examination of estrogen effects on the brain of a wild animal.

Song activation by testosterone is associated with an increased catecholaminergic innervation of the song control system in female canaries

In canaries, singing and a large number of morphological features of the neural system that mediates the learning, perception and production of song exhibit marked sex differences. Although these differences have been mainly attributed to sex-specific patterns of the action of testosterone and its metabolites, the mechanisms by which sex steroids regulate brain and behavior are far from being completely understood. Given that the density of immunoreactive catecholaminergic fibers that innervate telencephalic song nuclei in canaries is higher in males, which sing, than in females, which usually do not sing, we hypothesized that some of the effects induced by testosterone on song behavior are mediated through the action of the steroid on the catecholaminergic neurons which innervate the song control nuclei. Therefore, we investigated in female canaries the effects of a treatment with exogenous testosterone on song production, on the volume of song control nuclei, and on the catecholaminergic innervation of these nuclei as assessed by immunocytochemical visualization of tyrosine hydroxylase. Testosterone induced male-like singing in all females and increased by about 80% the volume of two telencephalic song control nuclei, the high vocal center (HVC) and the nucleus robustus archistriatalis (RA). Testosterone also significantly increased the fractional area covered by tyrosine hydroxylase-immunoreactive structures (fibers and varicosities) in most telencephalic song control nuclei (HVC, the lateral and medial parts of the magnocellular nucleus of the anterior neostriatum, the nucleus interfacialis, and to a lesser extent RA). By contrast, testosterone did not affect the catecholaminergic innervation of the telencephalic areas adjacent to HVC and RA. Together these data demonstrate that, in parallel to its effects on song behavior and on the morphology of the song control system, testosterone also regulates the catecholaminergic innervation of most telencephalic song control nuclei in canaries. The endocrine regulation of singing may thus involve the neuromodulatory action of specialized dopaminergic and/or noradrenergic projections onto several key parts of the song control system.

Sex differences in the distribution of the steroid receptor coactivator SRC-1 in the song control nuclei of male and female canaries

The steroid receptor coactivator SRC-1 modulates ligand-dependent transactivation of several nuclear receptors, including the receptors for sex steroid hormones. The distribution of SRC-1 transcripts was analyzed here by in situ hybridization in coronal sections through the brain of male and female canaries. A broad but heterogeneous distribution of SRC-1 transcripts was observed with high numbers of densely labeled cells being present in many steroid-sensitive areas including the medial preoptic nucleus, several hypothalamic nuclei, five song control nuclei (HVc, the lateral and medial portion of the magnocellular nucleus of the anterior neostriatum, area X and the nucleus uvaeformis) and several catecholaminergic areas (area ventralis of Tsai, substantia nigra, locus coeruleus). The volume of two song control nuclei, HVc and area X were reconstructed based on the boundaries of the cell groups exhibiting a denser SRC-1 expression as compared to the surrounding areas. Sex differences in the expression of SRC-1 were also detected in several song control nuclei. In particular, the volume of HVc based on the high density of SRC-1 expression was significantly larger in males than in females. The effect of steroids on the song control system could be, at least in part, indirect and result from a modulation by steroids of the catecholaminergic inputs to the song control nuclei. The presence of the steroid receptor coactivator SRC-1 in the telencephalic song control nuclei and in the catecholaminergic cell groups that innervate the song system supports the idea that SRC-1 expression could play an active role in the control of singing behavior by modulating estrogen and androgen receptor action at both locations.

Lateralization and effects of adult androgen in a sexually dimorphic neuromuscular system controlling song in zebra finches

Unlike many other sexually dimorphic systems, gonadal secretions do not explain sex differences in the morphology of the telencephalic song control nuclei of zebra finches. It is important to understand whether a novel mechanism for controlling structure is restricted to the forebrain regions specialized for song, and whether other areas respond more typically to gonadal steroids. Therefore, the effects of sex and adult androgen manipulation on the neuromuscular end of the song control system (tracheosyringeal portion of the hypoglossal nucleus, nXIIts, and the syrinx, or vocal organ) were investigated. Because lesion and axotomy experiments indicate a right-side bias in the functional control of song, asymmetry in the motor nucleus and in the ventralis and dorsalis syrinx muscles was also tested. Male-biased dimorphisms existed in the volume of nXIIts, and in syrinx mass and size of muscle fibers, but not in motoneuron number or size. Asymmetries favoring the right side were detected in nXIIts volume and motoneuron number in males, as well as in ventralis and dorsalis fiber size in both sexes. Hormone manipulations had no effect on nXIIts size, neuron size, or number. Testosterone treatment of adult females increased all of the syringeal measures, but the only effect of flutamide in males was to decrease syrinx weight. Thus, male-biased sexual dimorphisms and right side dominance in both nXIIts and the syrinx may facilitate singing behavior. Adult androgen exposure can induce partial masculinization of the syrinx, but other factors must be important in mediating the sex differences in both that structure and the volume of nXIIts. J. Comp. Neurol. 426:154–164, 2000. © 2000 Wiley-Liss, Inc.

Greater song complexity is associated with augmented song system anatomy in zebra finches.

We revisited the relationship between brain anatomy and song behavior in zebra finches. Consistent with previous studies in other songbirds, we find that differences in volume of the telencephalic song control nucleus HVc is predictive of differences in repertoire size and phrase duration in zebra finches. We extend the study of brain and behavior correlations in song birds by showing that repertoire size in zebra finches can be predicted by variance in several brain nuclei, providing the first demonstration that volumetric differences across multiple components of a neural network are predictive of song behavior.

Greater song complexity is associated with augmented song system anatomy in zebra finches.

We revisited the relationship between brain anatomy and song behavior in zebra finches. Consistent with previous studies in other songbirds, we find that differences in volume of the telencephalic song control nucleus HVc is predictive of differences in repertoire size and phrase duration in zebra finches. We extend the study of brain and behavior correlations in song birds by showing that repertoire size in zebra finches can be predicted by variance in several brain nuclei, providing the first demonstration that volumetric differences across multiple components of a neural network are predictive of song behavior.

Lateralization and effects of adult androgen in a sexually dimorphic neuromuscular system controlling song in zebra finches

Unlike many other sexually dimorphic systems, gonadal secretions do not explain sex differences in the morphology of the telencephalic song control nuclei of zebra finches. It is important to understand whether a novel mechanism for controlling structure is restricted to the forebrain regions specialized for song, and whether other areas respond more typically to gonadal steroids. Therefore, the effects of sex and adult androgen manipulation on the neuromuscular end of the song control system (tracheosyringeal portion of the hypoglossal nucleus, nXIIts, and the syrinx, or vocal organ) were investigated. Because lesion and axotomy experiments indicate a right-side bias in the functional control of song, asymmetry in the motor nucleus and in the ventralis and dorsalis syrinx muscles was also tested. Male-biased dimorphisms existed in the volume of nXIIts, and in syrinx mass and size of muscle fibers, but not in motoneuron number or size. Asymmetries favoring the right side were detected in nXIIts volume and motoneuron number in males, as well as in ventralis and dorsalis fiber size in both sexes. Hormone manipulations had no effect on nXIIts size, neuron size, or number. Testosterone treatment of adult females increased all of the syringeal measures, but the only effect of flutamide in males was to decrease syrinx weight. Thus, male-biased sexual dimorphisms and right side dominance in both nXIIts and the syrinx may facilitate singing behavior. Adult androgen exposure can induce partial masculinization of the syrinx, but other factors must be important in mediating the sex differences in both that structure and the volume of nXIIts.

A field study of seasonal neuronal incorporation into the song control system of a songbird that lacks adult song learning.

Adult songbirds can incorporate new neurons into HVc, a telencephalic song control nucleus. Neuronal incorporation into HVc is greater in the fall than in the spring in adult canaries (open-ended song learners) and is temporally related to seasonal song modification. We used the western song sparrow, a species that does not modify its adult song, to test the hypothesis that neuronal incorporation into adult HVc is not seasonally variable in age-limited song learners. Wild song sparrows were captured during the fall and the spring, implanted with osmotic pumps containing [3H]thymidine, released onto their territories, and recaptured after 30 days. The density, proportion, and number of new HVc neurons were all significantly greater in the fall than in the spring. There was also a seasonal change in the incorporation of new neurons into the adjacent neostriatum that was less pronounced than the change in HVc. This is the first study of neuronal recruitment into the song control system of freely ranging wild songbirds. These results indicate that seasonal changes in HVc neuronal incorporation are not restricted to open-ended song learners. The functional significance of neuronal recruitment into HVc therefore remains elusive.

Effects of embryonic estrogen on differentiation of the gonads and secondary sexual characteristics of male zebra finches.

Male zebra finches sing to court females, whereas females do not normally sing. In parallel, the telencephalic brain regions that control song are larger in volume and contain larger cells in males than in females. The vocal control organ (syrinx) is also larger in males. Some evidence suggests that the sexual differentiation of both anatomy and behavior is under the regulation of gonadal hormones during early development, yet recent data conflict with the idea that the sole source of masculinization of the neural song system is the testes. In the present experiment, we treated genetic males with estradiol benzoate on embryonic day 5 and measured the volume of and neuron soma size in robust nucleus of the archistriatum (RA) and the high vocal center (HVC), two telencephalic song control nuclei. We also weighed the syrinx, the muscles of which are the target of the motor pathway containing the two brain regions. The estrogen treatment disrupted testicular morphology, and induced an oviduct in six of seven animals, but it had no effect on any of four measures of masculinization of the neural song system. These results suggest that normal testicular tissue is not required for masculine development of the neural song system.

Sex differences in neuropeptide staining of song-control nuclei in zebra finch brains.

This study examined the distribution of the neuropeptides somatostatin (SS) and calcitonin gene-related peptide (CGRP) in forebrain and midbrain song-control nuclei of male versus female brains in adult zebra finches (Poephila guttata) using immunohistochemical techniques. Vocal learning in songbirds is controlled by an interconnected, highly-localized system of brain nuclei. Male zebra finches produce learned vocalizations, and females do not. This behavioral sexual dimorphism is reflected in a substantial difference in the size of these song-control nuclei: males have larger nuclei containing a greater number of neurons than do females. Interestingly, previous studies describing neurochemical aspects of the song-control system in other songbird species have not reported any obvious sex differences. However, the results of this study showed that the level of neuropeptide staining was substantially greater in telencephalic song-control nuclei of male brains compared to female brains in zebra finches. In some brain regions females lacked any apparent staining, whereas male brains were intensely labeled. In other cases the number of neurons and/or intensity of labeling were diminished in females relative to males. The telencephalic pattern of somatostatin labeling revealed a large number of SS-labeled somata in the magnocellular nucleus of the anterior neostriatum species that occupy habitats with different hydrodynamic conditions. (MAN), the high vocal center (HVC) and the robust nucleus of the archistriatum (RA) of males, whereas female song-control nuclei contained many fewer or no labeled cells in these regions. Area X of the striatum appeared to contain a slightly higher level of somatostatin immunoreactivity than surrounding striatum in males, but the region corresponding to Area X in females could not be distinguished from the rest of striatum. The telencephalic pattern of CGRP staining was less extensive than that seen for SS. In male brains the magnocellular core region of lateral MAN contained many darkly labeled neurons, and RA was stained by a dense field of anterograde terminal label. Well labeled somata were also seen in medial MAN of males, but HVC was devoid of immunoreactivity. Area X in male brains contained a light field of terminal immunoreactivity. The pattern of labeling seen in males indicates that CGRP acts selectively as a neuromodulator along the efferent projection from lateral MAN to RA and Area X but not in the HVC-to-RA pathway. No telencephalic song-control nuclei in female brains contained CGRP staining. Although some diencephalic and midbrain nuclei contained well-labeled somata or fibers with either or both neuropeptides, there was little or no evidence of a sex difference in neuropeptide expression in these regions. This latter finding suggests that the greater anatomical specialization seen in cortical regions of songbirds compared to those regions in non-oscine species is accompanied by a greater neurochemical differentiation, whereas thalamic and midbrain regions may be more conserved across sex as well as species. These findings indicate that male neurons produce high levels of somatostatin and calcitonin gene-related peptide in major telencephalic vocal-control regions in zebra finches, whereas female neurons produce less or none at all. These dramatic neurochemical sex differences may be directly related to production of learned vocalizations in males, as well as to other aspects of song behavior and courtship.

Localization of met-enkephalin and vasoactive intestinal polypeptide in the brains of male zebra finches.

An interconnected series of brain nuclei controls song learning and behavior in male zebra finches (Poephila guttata). This study examined the distribution of fibers, terminals, and somata immunoreactive for two neuropeptides, methionine-enkephalin (ENK) and vasoactive intestinal polypeptide (VIP), in song-control nuclei of adult males. In addition, the broad pattern of major regions of labeling throughout the forebrain and midbrain was determined. The telencephalic song-control nuclei MAN (magnocellular nucleus of the anterior neostriatum), Area X of the striatum, HVC (higher vocal center), and RA (robust nucleus of the archistriatum) contained abundant ENK immunoreactivity, including labeled fibers and somata. In addition, intensely labeled fibers and terminals were seen in the thalamic nucleus DLM (medial portion of the dorsolateral nucleus of the anterior thalamus). High levels of VIP immunoreactivity were also seen in MAN, HVC, and RA, but this label consisted of fiber and terminals only. Area X and surrounding striatum contained extremely sparsely distributed VIP-labeled processes. Somata positive for VIP were not seen throughout cortical regions such as the neostriatum and hyperstriatum but were abundant in the lateral striatum (paleostriatum augmentatum, PA) and may contribute to a dense field of terminal labeling seen in the globus pallidus. The apparent presence of a robust VIP-positive striato-pallidal projection is not typical of major basal ganglia pathways in vertebrates, raising the possibility that passerine birds have diverged from the typical amniote pattern.

Localization of Met-Enkephalin and Vasoactive Intestinal Polypeptide in the Brains of Male Zebra Finches (Part 2 of 2)

An interconnected series of brain nuclei controls song learning and behavior in male zebra finches (Poephila guttata). This study examined the distribution of fibers, terminals, and somata immunoreactive for two neuropeptides, methionine-enkephalin (ENK) and vasoactive intestinal polypeptide (VIP), in song-control nuclei of adult males. In addition, the broad pattern of major regions of labeling throughout the forebrain and midbrain was determined. The telencephalic song-control nuclei MAN (magnocellular nucleus of the anterior neostriatum), Area X of the striatum, HVC (higher vocal center), and RA (robust nucleus of the archistriatum) contained abundant ENK immunoreactivity, including labeled fibers and somata. In addition, intensely labeled fibers and terminals were seen in the thalamic nucleus DLM (medial portion of the dorsolateral nucleus of the anterior thalamus). High levels of VIP immunoreactivity were also seen in MAN, HVC, and RA, but this label consisted of fiber and terminals only. Area X and surrounding striatum contained extremely sparsely distributed VIP-Iabeled processes. Somata positive for VIP were not seen throughout cortical regions such as the neostriatum and hyperstriatum but were abundant in the lateral striatum (paleostriatum augmentatum, PA) and may contribute to a dense field of terminal labeling seen in the globus pallidus. The apparent presence of a robust VIP-positive striato-pallidal projection is not typical of major basal ganglia pathways in vertebrates, raising the possibility that passerine birds have diverged from the typical amniote pattern.

A brain for all seasons: cyclical anatomical changes in song control nuclei of the canary brain.

Male canaries that have reached sexual maturity can, in subsequent years, learn new song repertoires. Two telencephalic song control nuclei, the hyperstriatum ventrale, pars caudale, and nucleus robustus archistriatalis are, respectively, 99 and 76 percent larger in the spring, when male canaries are producing stable adult song, than in the fall, at the end of the molt and after several months of not singing. It is hypothesized that such fluctuations reflect an increase and then reduction in numbers of synapses and are related to the yearly ability to acquire new motor coordinations.

Sex differences in dendritic morphology of a song control nucleus in the canary: a quantitative Golgi study.

Singing in the canary is a learned male behavior controlled predominantly by nuclei in the left hemisphere (Nottebohm and Nottebohm, '76; Nottebohm et al., '76; Nottebohm, '77). These nuclei are several times larger in males than in females (Nottebohm and Arnold, '76). One of the telencephalic song control nuclei, robustus archistriatalis (RA), was examined in Golgi-stained tissue sections from the left and right hemispheres of male and female canaries. At least four cell classes were present in each sex. One of these cell classes was further studied with a variety of quantitative techniques. No hemispheric differences were seen in either sex. However, dendrites from male cells tend to branch and end further from the cell body than do dendrites from female cells. This difference is seen most clearly when serial sections are used to reconstruct the entire dendritic tree.