Distribution of GABA-like immunoreactivity in the song system of the zebra finch

GABA-like immunoreactivity (GABA-LIR) was mapped in the male and female zebra finch song system using a polyclonal antibody to GABA. GABA-LIR was found throughout the song system in neurons and neuropil of the robust nucleus of the archistriatum (RA), the higher vocal center (HVC), Area X, the magnocellular nucleus of the neostriatum (MAN), and the dorsomedial portion of the nucleus intercollicularis (DM of ICo). Puncta present in the lateral division of MAN (lMAN) may be local interneurons since the only known afferents of lMAN are from the dorsolateral nucleus of the anterior thalamus (DLM), which did not appear to have any cell bodies with GABA-LIR. Distinct and dense puncta with GABA-LIR were present in DLM, and may be projections from Area X/lobus parolfactorius (LPO). Dramatic sex differences in GABA-LIR distribution were found. Females did not appear to have any GABA-LIR above background in either RA or HVC. Females also did not appear to have a distinct Area X, although they did have many small, lightly staining cell bodies in the corresponding LPO. The distribution of GABA-LIR and sex differences in its distribution suggests that GABAergic neurons may play a role in the acquisition and/or production of song in the zebra finch.     

Axonal connections of the high vocal center and surrounding cortical regions in juvenile and adult male zebra finches

Neuronal connections of the High Vocal Center (HVC), a cortical nucleus of songbirds necessary for learned vocal behavior, and the region adjacent to HVC called paraHVC (pHVC), were studied in adult and juvenile male zebra finches. Extremely small injections of fluorescent dextran amines or biocytin were made within subregions of HVC and pHVC to define the precise nature and development of these pathways. In adults, all HVC injections produced an even, nontopographic distribution of retrograde label throughout the medial magnocellular nucleus of the anterior neostriatum (mMAN), the interfacial nucleus (NIf), and the uvaeform nucleus of the thalamus (Uva) and an even distribution of anterograde label within area X of the striatum and the robust nucleus of the archistriatum (RA). These same patterns of projections were present in juvenile birds 20-23 days of age, including the projection from HVC to RA, which has previously been reported to develop only after 25-30 days of age. Results also establish a novel efferent projection from HVC to pHVC in both juvenile and adult birds. Injections into pHVC indicate that this region receives afferent input from song control areas HVC, mMAN, medial regions of the parvicellular shell of lateral MAN, NIf, and Uva and projects to Area X, caudomedial regions of striatum, and regions of the caudomedial neostriatum (NCM). Thus, neuronal connections of pHVC are highly integrated with circuitry important for vocal behavior and are distinct from those of HVC. Such differences establish HVC and pHVC as separate brain areas and suggest that each may serve a different function in vocal behavior. Control injections in both juveniles and adults produced specific patterns of projections from areas outside of HVC to areas outside of RA, illustrating an overall spatial organization of projections from HVC and neighboring cortical areas. Further, although neuronal connections of HVC are not topographic, projections of HVC, pHVC, and surrounding areas demonstrate a broad spatial organization of efferents to striatum and regions surrounding RA, thus defining a level of organization beyond that of individual song control nuclei.     

Noradrenergic system of the zebra finch brain: immunocytochemical study of dopamine-beta-hydroxylase

Oscine birds are among the few animal groups that have vocal learning, and their brains contain a specialized system for song learning and production. We describe here the immunocytochemical distribution of dopamine-beta-hydroxylase (DBH), a noradrenergic marker, in the brain of an oscine, the zebra finch (Taeniopygia guttata). DBH-positive cells were seen in the locus coeruleus, the nucleus subcoeruleus ventralis, the nucleus of the solitary tract, and the caudolateral medulla. Immunoreactive fibers and varicosities had a much wider brain distribution. They were particularly abundant in the hippocampus, septum, hypothalamus, area ventralis of Tsai, and substantia nigra, where they formed dense pericellular arrangements. Significant immunoreactivity was observed in auditory nuclei, including the nucleus mesencephalicus lateralis pars dorsalis, the thalamic nucleus ovoidalis, field L, the shelf of the high vocal center (HVC), and the cup of the nucleus robustus archistriatalis (RA), as well as in song control nuclei, including the HVC, RA, the lateral magnocellular nucleus of the anterior neostriatum, and the dorsomedial nucleus (DM) of the intercollicular complex. Except for the DM, DBH immunoreactivity within song nuclei was comparable to that of surrounding tissues. Conspicuously negative were the lobus paraolfactorius, including song nucleus area X, and the paleostriatum. Our results are in agreement with previous studies of the noradrenergic system performed in nonoscines. More importantly, they provide direct evidence for a noradrenergic innervation of auditory and song control nuclei involved in song perception and production, supporting the notion that noradrenaline is involved in vocal communication and learning in oscines.     

Lesions of the melatonin- and androgen-responsive tissue of the dorsomedial nucleus of the hypothalamus block the gonadal response of male Syrian hamsters to programmed infusions of melatonin

The objective of this study was to characterize a site at which it is likely that melatonin mediates photoperiodic control of reproduction in the male Syrian hamster. The first experiment was a comparison of the distributions of iodomelatonin (IMEL)-binding sites and cells immunoreactive to androgen receptors (AR-ir) in the medio-basal hypothalamus (MBH). AR-ir cells extended throughout the MBH, whereas IMEL binding was restricted to the dorsomedial nucleus (DMN). Comparisons between IMEL binding and AR-ir on adjacent cryostat sections revealed a clear overlap between the IMEL-binding sites and a distinct subpopulation of AR-ir cells within the DMN. The second experiment examined whether lesions of these IMEL- and androgen-responsive cells affected the response of the hamsters to short-day (SD)-like infusions of melatonin. Animals received sham or bilateral electrolytic lesions of the IMEL-binding sites within the DMN of the hypothalamus (MBH-X). Animals were pinealectomized and 4 wk later fitted with an s.c. cannula for the daily infusion of either melatonin (50 ng/h) or saline (500 microliters/10 h). After 6 wk the animals with sham lesions showed gonadal atrophy and lower serum concentrations of LH and prolactin (PRL) after infusions with melatonin. In contrast, MBH-X animals given melatonin had large testes and long-day (LD)-like serum LH concentrations. Infusions of melatonin did, however, cause a significant decline in serum PRL level. This study shows that an intact MBH is essential for the expression of gonadotrophic but not lactotrophic responses to melatonin and/or photoperiod. It also suggests that cells responsive to both gondal steroids and melatonin may be involved in the seasonal variation in GnRH release, and indicates a site at which melatonin might influence sensitivity to steroid feedback, a hypothalamic function known to be regulated by photoperiod.     

Changes in catecholamine levels and turnover rates in hypothalamic, vocal control, and auditory nuclei in male zebra finches during development

The catecholamines norepinephrine (NE) and dopamine (DA) have been implicated in the sexual differentiation of brain and behavior and in species-specific learning in several species. To determine if these neurotransmitters might be involved in sexual differentiation of the vocal control system and song learning in male zebra finches, NE and DA levels and turnover rates were quantified in 10 behaviorally relevant brain nuclei [6 vocal control (VCN), 2 auditory (AN), and 2 hypothalamic (HN)] at four critical points during sexual differentiation of the VCN and the period of song learning, 25, 35, 55, and 90 days of age. Some birds were pretreated with alpha-methyl-para-tyrosine (alphaMPT) to allow estimation of NE and DA turnover rates. NE and DA levels in microdissected nuclei were quantified using high-performance liquid chromatography with electrochemical detection. AlphaMPT treatment suppressed catecholamine synthesis just as effectively in juveniles as it does in adults and proved an effective method for estimating NE and DA turnover rates. Patterns of NE and DA function in most VCN and AN over development were quite different from those in HN in which NE and DA function changed gradually and showed no striking peaks. NE turnover rates changed significantly over development in all six VCN [nucleus interfacialis (Nlf), high vocal center (HVC), nucleus robustus of the archistriatum (RA), dorsomedial portion of the intercollicular nucleus (DM), Area X of the parolfactory lobe, and lateral portion of the magnocellular nucleus of the anterior neostriatum (IMAN)]; one AN [nucleus mesencephalicus lateralis pars dorsalis (MLd)], and one HN [preopticus anterior (POA)]. NE levels changed significantly in two VCN (Nlf and Area X). In Nlf, RA, Area X, IMAN, and MLd, NE levels and/or turnover rates showed a striking peak at day 25, which was not seen in HN. Both DA levels and turnover rates changed profoundly over development in 5 of 6 VCN (Nlf, RA, DM, Area X, and IMAN) and both AN (MLd and Field L). These nuclei showed striking peaks in DA levels and turnover rates, primarily on day 35 and/or 55, which then declined profoundly by day 90. This contrasted with the minimal change in DA turnover rates seen in one HN (POA) and the sixth VCN, HVC. In several VCN and AN, NE and DA levels and turnover rates during development reached levels never seen in adult males. Previous research has shown that catecholamine function is heightened in VCN during development compared to surrounding tissues. Our data demonstrate that NE and DA function during development shows pronounced peaks in most VCN not seen in HN. This is interesting because both VCN and HN are hormone sensitive, and both show hormone-modulated NE and DA function in adult males. The timing of these peaks suggests that increased catecholaminergic function may be involved in sexual differentiation of the VCN and song learning in finches.     

Sex difference in the size of the neural song control regions in a dueting songbird with similar song repertoire size of males and females

Previous studies have suggested a causal relation between sex differences in behavior such as singing and sex differences in the size of brain areas such as the forebrain song control areas of songbirds. In the present study we show that the size of the forebrain vocal control areas nucleus hyperstriatalis ventrale pars caudale (HVC) and nucleus robustus archistriatalis (RA) and its neuron numbers are about twice as large in males as in females of the African dueting bush shrike Laniarius funebris. However, song types are of similar complexity (number of elements per song type, physical properties of elements) in both sexes, and repertoire size does not differ between males and females. Furthermore, in captivity male and female shrikes are able to learn the same song types. This demonstrates for the shrike that sex differences in the size of vocal control areas and in its neuron numbers do not predict the type of sex-typical vocal behavior. This result is supported by a statistical comparison of the sex differences in HVC size, RA size, and song repertoire size of all songbird species studied to date. Sex differences in species in which only the males sing are indeed larger than in species in which the females also sing; in songbird species with singing females, however, the sex differences in HVC and RA volume appear to be independent of the vocal repertoire size of females. The songbird model therefore does not support the notion that sex differences in area size and neuron number explain sex differences in a behavior that occurs in both sexes. Furthermore, in the shrike, neuron soma size is similar in males and females in the song motonucleus hypoglossus pars tracheosyringealis (nXIIts) and in the premotor nucleus RA, but is sexually dimorphic in the higher vocal center HVC. Thus, male and female shrikes produce songs of similar complexity with different neuron phenotypes.     

Selective expression of insulin-like growth factor II in the songbird brain

Neuronal replacement occurs in the forebrain of juvenile and adult songbirds. To address the molecular processes that govern this replacement, we cloned the zebra finch insulin-like growth factor II (IGF-II) cDNA, a factor known to regulate neuronal development and survival in other systems, and examined its expression pattern by in situ hybridization and immunocytochemistry in juvenile and adult songbird brains. The highest levels of IGF-II mRNA expression occurred in three nuclei of the song system: in the high vocal center (HVC), in the medial magnocellular nucleus of the neostriatum (mMAN), which projects to HVC, and to a lesser extent in the robust nucleus of the archistriatum (RA), which receives projections from HVC. IGF-II mRNA expression was developmentally regulated in zebra finches. In canary HVC, monthly changes in IGF-II mRNA expression covaried with previously reported monthly differences in neuron incorporation. Combining retrograde tracers with in situ hybridization and immunocytochemistry, we determined that the HVC neurons that project to area X synthesize the IGF-II mRNA, whereas the adjacent RA-projecting neurons accumulate the IGF-II peptide. Our findings raise the possibility that within HVC IGF-II acts as a paracrine signal between nonreplaceable area X-projecting neurons and replaceable RA-projecting neurons, a mode of action that is compatible with the involvement of IGF-II with the replacement of neurons. Additional roles for IGF-II expression in songbird brain are likely, because expression also occurs in some brain areas outside the song system, among them the cerebellar Purkinje cells in which neurogenesis is not known to occur.     

Kinetics of humoral response in children with acute lymphoblastic leukemia immunized with influenza vaccine in 1993 in Poland

Songbirds exhibit seasonal changes in the volumes of song control nuclei. Birds on long, spring-like days have larger nuclei than do birds on short, winter-like days. The mechanisms mediating volumetric changes have not been determined unequivocally, but testosterone (T) is probably involved. This study examined whether testicular factors are uniquely responsible for seasonal changes in the song system, or whether photoperiod has testis-independent effects. Male American tree sparrows were exposed to one of three photoperiodic conditions: (1) Photosensitive birds were retained on short days (8L:16D). Plasma T is rarely detected in such birds. (2) Photosensitive birds were moved from short days to long days (20L:4D) and photostimulated for three weeks. Photostimulation elevates circulating T in photosensitive birds. (3) Photorefractory birds were held at least four months on 20L:4D. Such birds seldom have detectable levels of T, even though they are on long days. In each condition, there were both intact and castrated birds. Castration typically removes circulating T in tree sparrows. The volumes of the high vocal center (HVC), nucleus robustus archistriatalis (RA), and area X were measured. Song nuclei were largest in intact photostimulated birds. Other long-day birds (i.e. castrated photostimulated, and intact and castrated photorefractory groups) had larger song nuclei than did short-day intact or castrated photosensitive birds and did not differ from each other. These data indicate that photoperiod has both testis-dependent and -independent effects on the volumes of song control nuclei.     

Axonal connections of the medial magnocellular nucleus of the anterior neostriatum in zebra finches

The medial magnocellular nucleus of the anterior neostriatum (mMAN) is a small cortical nucleus which was previously identified as a component of the neural circuitry controlling vocal behavior in songbirds based on its efferent connection to the High Vocal Center (HVC), a major song control nucleus (Nottebohm et al. [1982] J. Comp. Neurol. 207:344-357; Bottjer et al. [1989] J. Comp. Neurol. 279:312-326). We have conducted tract tracing experiments (using wheat-germ agglutinin-horseradish peroxidase (WGA-HRP), the carbocyanine dye DiI, and biocytin) to determine the complete pattern of afferent and efferent connections of mMAN in adult male zebra finches. We confirmed the existence of an efferent projection from mMAN to HVC and discovered a novel projection to the region medial to caudal HVC called paraHVC (pHVC). Injections of retrograde tracers into mMAN showed that afferent input to mMAN originates from the dorsomedial nucleus of the posterior thalamus (DMP). Injections of DiI into DMP produced anterograde label over mMAN, thus confirming the DMP-to-mMAN projection. Interestingly, this anterograde label extended beyond the region of mMAN defined by HVC-projecting neurons into the immediately surrounding cortex. This extended terminal field of DMP efferents indicates that mMAN encompasses a core population of projection neurons surrounded by a shell of non-HVC-projecting neurons, both of which receive input from the dorsal thalamus. Analysis of retrograde DiI label resulting from DMP injections revealed two major sources of afferent input to DMP originating in regions of the archistriatum and hypothalamus. Inputs to DMP were distributed throughout the dorsal archistriatum and included the area that receives a projection from the parvicellular shell region of the lateral magnocellular nucleus of the anterior neostriatum, a song control nucleus, as well as the dorsal portion of the robust nucleus of the archistriatum, the motor-cortical output of the song control system. The projections from song control regions of the archistriatum to DMP may feed information back into telencephalic song control circuitry via the DMP-->mMAN-->HVC/pHVC pathway. The other source of afferent input to DMP is located in the external cellular stratum of the lateral hypothalamus (SCE). This newly delineated SCE-->DMP-->mMAN-->HVC/pHVC pathway is the first report of a hypothalamic brain region neuroanatomically integrated with song control circuitry. Because hypothalamic brain regions are important for homeostasis and regulating behavior, the trans-synaptic circuitry of mMAN may help to integrate information about the bird's internal state, such as sexual maturation, with song learning and production.     

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.     

Soldiers, surgeons and the campaigns to combat sexually transmitted diseases in colonial India, 1805-1860

Here, we examine the connectivity of two previously identified telencephalic stations of the auditory system of adult zebra finches, the neostriatal "shelf" that underlies the high vocal center (HVC) and the archistriatal "cup" adjacent to the robust nucleus of the archistriatum (RA). We used different kinds of neuroanatomical tracers to visualize the projections from the shelf to the HVC. In addition, we show that the shelf projects to the cup and that the cup projects to thalamic, midbrain, and pontine nuclei of the ascending auditory pathway. Our observations extend to songbirds anatomical features that are found in the auditory pathways of a nonoscine bird, the pigeon (Wild et al. [1993] J. Comp. Neurol. 337:32-62), and we suggest that the descending auditory projections found in mammals may also be a general property of the avian brain. Finally, we show that the oscine song control system is closely apposed to auditory pathways at many levels. Our observations may help in understanding the evolution and organization of networks for vocal communication and vocal learning in songbirds.     

Activity Dependent Brain Plasticity: Does Singing Increase the Volume of a Song System Nucleus? Theoretical Comment on Sartor and Ball (2005).

The avian telencephalic song system, including nucleus high vocal center (HVC), is an important model for the study of brain plasticity. J. J. Sartor and G. F. Ball (2004) asked whether song activity itself could increase HVC volume (see record 2005-01705-022). They report that male starlings that sang more, motivated by gaining possession of a nest box in competition with another male, had larger HVCs than males that sang less. These results are suggestive but correlational. A more direct experimental approach will be required to establish that HVC size is activity dependent. Their experiment is an important first step, however. The song activity hypothesis merits continued investigation because of its potential to produce unique insights into naturally occurring brain plasticity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of melatonin in the human fetus (review)

Melatonin, an indole amine, primarily derived from the pineal gland is secreted during the hours of darkness. Melatonin acts as a hormonal transduction of photoperiod influencing the timing of seasonal and daily (circadian) physiological rhythms. Maternal melatonin crosses the placenta and enters the fetal circulation providing photoperiodic information to the fetus influencing the subsequent circadian and seasonal rhythms of the offspring. The function of melatonin in humans is more obscure. However, melatonin has attained prominence as a treatment for disturbed circadian rhythms and sleep patterns which occur as a result of transmeridian travel, shift work or blindness. The biological clock, the hypothalamic suprachiasmatic nuclei (SCN), possesses melatonin receptors, in both the adult and fetal human. This concurs with the reported influence of melatonin on human circadian rhythmicity and indicates that this influence may begin in utero. Melatonin receptors are widespread in the human fetus and occur in both central and peripheral tissue from early in fetal development. Thus, the influence of melatonin on the developing human fetus may not be limited to entraining circadian rhythmicity. Considering the transplacental availability of melatonin to the fetus the ingestion of melatonin by pregnant women may be inadvisable.     

Behavioral state modulation of auditory activity in a vocal motor system

Neurons of the song motor control nucleus robustus archistriatalis (RA) exhibited far weaker auditory responses in awake than in anesthetized zebra finches. Remarkably, sleep induced complex patterns of bursts in ongoing activity and uncovered vigorous auditory responses of RA neurons. Local injections of norepinephrine suggested that the changes in response strength occur through neuromodulatory control of the sensorimotor nucleus HVc, which projects to RA. Thus, motor access to auditory feedback, which zebra finches require for song learning and maintenance, may be regulated through neuromodulation. During sleep, the descending motor system may gain access to sensorimotor song memories represented as bursting patterns of activity.     

Melatonin, its receptors, and relationships with biological rhythm disorders

Melatonin is a neurohormone produced during the night by the pineal gland. Its secretion is regulated by circadian and seasonal variations in daylength, transmitted via visual projections to the suprachiasmatic nucleus which functions as a circadian clock in mammals. Melatonin has been proposed to act as an internal synchronizer of circadian rhythms generated at different levels of the organism. The chronobiotic effects of melatonin in humans have been mainly studied in circadian rhythm sleep disorders related to jet lag, shift work, blindness or aging. Alterations of the melatonin profiles have also been reported in other biological rhythm disorders.     

Song syllable discrimination by song sparrows (Melospiza melodia).

A habituation test paradigm was used to examine the responses of free-living territorial adult male song sparrows (Melospiza melodia) to a range synthetic songs. The three-phrased test songs differed from one another in having either conspecific or heterospecific (swamp sparrow, M. georgiana) syllables, or silence, in the second phrase. Subjects were exposed to repeated presentations of one song type until their approach distance to a loudspeaker increased. In one experiment, birds were habituated with a song consisting of three phrases of song sparrow syllables and then tested for generalization to either novel song sparrow syllables in the second phrase, swamp sparrow syllables, or silence. Birds discriminated between song sparrow syllables on two response measures, and between song sparrow and swamp sparrow syllables on one measure. In a second experiment, after habituation to a song with swamp sparrow syllables in the second phrase, birds did not generalize to novel song sparrow syllables, but they did generalize to novel swamp sparrow syllables. The results further suggest that subtle species-specific differences in note structure within syllables are discriminated by song sparrows and potentially provide an adequate basis for individual recognition by song. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Brain space for a learned task

Forty-six adult male and female canaries were sacrificed, their brains were weighed and the volume of several brain nuclei reconstructed from the cresyl violet-stained material. Two forebrain vocal control nuclei, hyperstriatum ventrale, pars caudale (HVc) and nucleus robustus archistriatalis (RA), were approximately 4 and 3 times larger, respectively, in males than in females, confirming previous findings. There was no consistent right-left asymmetry in the volume of these nuclei in males and females. Twenty-five male birds in this study had their song repertoire recorded during the peak of the singing season. They were sacrificed 3 to 4 months later. The size of the song repertoire, measured as number of different syllable types, showed a positive and significant correlation with the size of HVc and RA. There was no significant correlation between size of the syllable repertoire and age, brain weight or the volume of two brain nuclei not involved in song control. This is the first time that the amount of brain allotted to a specific learned skill has been shown to correlate positively with the amount of that skill that is learned. Interestingly, too, there was a positive and significant correlation between testis weight at the end of the breeding season and the volume of RA at that time, suggesting a hormone-mediated seasonal modulation of part of the brain space occupied by song control pathways. This material seems well suited for studying the relation between space and learning, and the manner in which this relation is influenced by gonadal hormones.     

Melatonin's role in vertebrate circadian rhythms

The circadian secretion of melatonin by the pineal gland and retinae is a direct output of circadian oscillators and of the circadian system in many species of vertebrates. This signal affects a broad array of physiological and behavioral processes, making a generalized hypothesis for melatonin function an elusive objective. Still, there are some common features of melatonin function. First, melatonin biosynthesis is always associated with photoreceptors and/or cells that are embryonically derived from photoreceptors. Second, melatonin frequently affects the perception of the photic environment and has as its site of action structures involved in vision. Finally, melatonin affects overt circadian function at least partially via regulation of the hypothalamic suprachiasmatic nucleus (SCN) or its homologues. The mechanisms by which melatonin affects circadian rhythms and other downstream processes are unknown, but they include interaction with a class of membrane-bound receptors that affect intracellular processes through guanosine triphosphate (GTP)-binding protein second messenger systems. Investigation of mechanisms by which melatonin affects its target tissues may unveil basic concepts of neuromodulation, visual system function, and the circadian clock.     

Melatonin in light treatment of patients with seasonal and nonseasonal depression

Melatonin as a marker of circadian rhythm and the effect of bright light on melatonin were studied in 63 depressed patients, 42 with a seasonal pattern and 21 with a nonseasonal pattern. The patients were matched for age, time of treatment and severity of depression. Before light treatment, blood was sampled for melatonin and depression was clinically rated with the Comprehensive Psychopathological Rating Scale and Hamilton Depression Rating Scale. Two hours of light treatment, 350 cd/m2, was given daily for 10 days 0600 to 0800 or 1800 to 2000. Of the 42 patients with seasonal depression, 26 were treated with morning light and, 16 with evening light. The melatonin amplitude was significantly decreased by light, and the melatonin phase position was advanced by morning light and delayed by evening light. All patients except for 3 in each group changed in the expected direction. Although the patients with seasonal pattern had a more favorable outcome than patients with nonseasonal pattern, there was no difference in therapeutic outcome related to the baseline melatonin phase position. The hypothesis that the short term clinical effects of light therapy either in the morning or evening are related to pretreatment melatonin levels or alteration of melatonin amplitude or phase position was not supported in the study. There was also no significant difference between the seasonal and nonseasonal patients related to the degree of light suppression of melatonin and the rebound effect of serum melatonin levels following bright level exposure between 2200 and 2300 before regular light treatment.