Demonstration of direct olfactory projections in the hypothalamus, the mesencephalon and the metencephalon of Microcebus murinus (Primate, Lemurian)

The efferent projections of the olfactory bulb in Microcebus murinus were identified after transection of the olfactory peduncle and after the revelation of the degenerating fibers by different silver staining methods. Total and partial sections have allowed demonstrating the importance of the two olfactory tracts in the olfactory projection areas. Degenerated fibers and endings were evidence not only in the different telencephalic regions, as classically known, but also in various hypothalamic nuclei (lateral hypothalamus, the suprachiasmatic, posterior supraoptic and mammillary nuclei and in the median eminence) and in several mesencephalic and metencephalic nuclei (ventral tegmental area, interpeduncular and raphe nuclei, and locus coeruleus). In all these structures the degenerate fibers were seen on both sides. The olfactory projections appeared not to be limited to the telencephalic areas. Moreover, the olfactory bulbs seem to be directly connected especially with the vegetative and integrative areas localized in the hypothalamus and the brainstem and particularly with the major aminergic nuclei that play an essential role in the neurovegetative, neuroendocrine and behavioral regulations.     

Central connections of the olfactory bulb in the bichir,Polypterus palmas,reexamined

Summary Central connections of the olfactory bulb of Polypterus palmas were studied with the use of horseradish peroxidase and cobalt-tracing techniques. The olfactory bulb projects to subpallial and palliai areas in the ipsilateral telencephalon; a projection to the contralateral subpallium is noted via the habenular commissure. A further target of secondary olfactory fibers is a caudal olfactory projection area in the ipsilateral hypothalamus. No labeling was seen in the anterior commissure and in the contralateral olfactory bulb. The medial and the lateral pallium receive secondary olfactory fibers in distinct areas. Neurons projecting to the bulb are found in the ipsilateral subpallium, mainly in one dorsal longitudinal nucleus. The main connection with the tel- and diencephalon is mediated via the medial olfactory tract. This tract also contains fibers to the contralateral telencephalon, and to the hypothalamus. The smaller lateral olfactory tract mediates fibers to the lateral pallium. The organization of pathways of secondary olfactory fibers in the telencephalon is described. The present findings are compared to those obtained in species possessing an inverted forebrain.This investigation was supported by grants from the Deutsche Forschungsgemeinschaft to DLM     

Desipramine administered chronically inhibits lipopolysaccharide-stimulated production of IL-1β in the brain and plasma of rats

Nowadays, it is assumed that therapeutic efficacy of antidepressants depends, at least partly, on their anti-inflammatory properties. The present study investigated for the first time the effect of 21-day oral administration of desipramine on the lipopolysaccharide (LPS)-stimulated IL-1β concentration in the olfactory bulb, hypothalamus, frontal cortex, hippocampus and plasma of rats, and on the LPS-induced IL-1β mRNA level in the olfactory bulb. Desipramine (15 mg/kg/day) reduced significantly the LPS (250 μg/kg i.p.)-induced IL-1β concentration in the olfactory bulb, hypothalamus and in plasma, and diminished the LPS effect on IL-1β mRNA in the olfactory bulb. Plasma concentration of desipramine was comparable to its therapeutic range. By using the α1/α2-adrenoceptor antagonist prazosin and the unspecific β-adrenoceptor antagonist propranolol given prior to LPS, we found that the effect of desipramine on LPS-induced IL-1β production was partially mediated by both adrenoceptors in the olfactory bulb and plasma, and that β-adrenoceptors contributed also to its effect on the stimulated IL-1β concentration in the hypothalamus. The effect of LPS on the cerebral IL-1β levels was, in part, mediated by β-adrenoceptors and, in a region-specific manner, by α1/α2-adrenoceptors. The findings provide evidence for central and peripheral anti-inflammatory activity of desipramine and confirm the impact of the noradrenergic system on IL-1β production induced by an immunostimulatory challenge.     

Smelling of odorous sex hormone-like compounds causes sex-differentiated hypothalamic activations in humans.

The anatomical pathways for processing of odorous stimuli include the olfactory nerve projection to the olfactory bulb, the trigeminal nerve projection to somatosensory and insular cortex, and the projection from the accessory olfactory bulb to the hypothalamus. In the majority of tetrapods, the sex-specific effects of pheromones on reproductive behavior is mediated via the hypothalamic projection. However, the existence of this projection in humans has been regarded as improbable because humans lack a discernable accessory olfactory bulb. Here, we show that women smelling an androgen-like compound activate the hypothalamus, with the center of gravity in the preoptic and ventromedial nuclei. Men, in contrast, activate the hypothalamus (center of gravity in paraventricular and dorsomedial nuclei) when smelling an estrogen-like substance. This sex-dissociated hypothalamic activation suggests a potential physiological substrate for a sex-differentiated behavioral response in humans.     

In vitro TRH release from hypothalamus slices varies during the diurnal cycle

We have previously described a daily rhythm in thyrotropin releasing hormone (TRH) and TRH mRNA in the rat hypothalamus. To determine whether TRH release fluctuates in a diurnal manner, we have measured basal and potassium stimulated release from hypothalamic slices, and compared it to release from olfactory bulb slices, during the diurnal cycle. Basal TRH release was higher at 7:00 h than at any other time (1:00, 13:00 or 19:00 h) in either hypothalamus or olfactory bulb. The ratio of stimulated over basal release was higher in the hypothalamus at 19:00 h, when TRH content was highest. Potassium stimulated TRH release from olfactory bulb was not different from basal release at any time. TRH release fluctuations were not due to a rhythm of extracellular inactivation: the activity of pyroglutamyl aminopeptidase II, an ectoenzyme responsible for TRH inactivation, was constant throughout the cycle. Our data demonstrate that diurnal variations of TRH release occur in vitro and that the enhanced responsiveness to potassium stimulation in hypothalamus is correlated with increased levels of peptide.     

Vascularization of the brains of the Atlantic and Pacific hagfishes, Myxine glutinosa and Eptatretus stouti: a scanning electron microscope study of vascular corrosion casts

The microvascularization of the brains of the hagfishes, Myxine glutinosa L. and Eptatretus stouti, were studied by scanning electron microscopy (SEM) of microvascular corrosion casts. Sections of these casts were used to determine the vascular territories of defined brain areas. Histological serial sections (10 microm) of the brains served for correlation of findings. Analysis of the microvascular casts of both species revealed that the blood supply to and from these brains arose ventrally and dorsally, respectively. Neither species possesses an arterial circle (Circulus Willisi) and both have similar microvascular patterns. The only difference between Myxine and Eptatretus was that the posterior cerebral artery in Myxine divides into mesencephalic and rhombencephalic branches, and in Eptatretus a third branch, termed telencephalic branch, arises from the posterior cerebral artery. 3D-morphometry revealed that luminal diameters of: 1) intracerebral arteries and arterioles range from 35.11 +/- 5.66 microm (mean +/- SEM) in the hypothalamus to 92.69 +/- 14.48 microm in the thalamus; 2) capillaries range from 17.8 +/- 0.44 microm in the olfactory bulb to 21.70 +/- 0.87 microm in the basal ganglia; and 3) intracerebral venules and veins range from 49.38 +/- 4.17 microm in the hypothalamus to 75.58 +/- 6.59 microm in the rhombencephalon. Interbranching distances of arteries and arterioles range from 179.19 +/- 11.32 microm in the optic tectum to 235.19 +/- 94.64 microm in the hypothalamus. Capillaries range from 91.07 +/- 6.22 microm in the hypothalamus to 116.15 +/- 9.45 microm in the thalamus, and venules and veins range from 137.30 +/- 18.11 microm in the hypothalamus to 189.83 +/- 17.47 microm in the optic tectum. Intervascular distances range from 70.58 +/- 3.58 microm in the olfactory bulb to 89.52 +/- 5.74 microm in the optic tectum. Branching angles of arteries and arterioles range from 38.39 +/- 10.9 degrees in the olfactory bulb to 100.73 +/- 9.4 degrees in the optic tectum, and the branching angles of capillaries range from 74.40 +/- 5.42 degrees in the optic tectum to 90.24 +/- 4.66 degrees in the olfactory bulb. Finally, the branching angles of the venules and veins range from 67.84 +/- 6.83 degrees in the tegmentum of the mesencephalon to 92.30 +/- 6.35 degrees in the optic tectum.     

Levels of specifically bound [3H]ketanserin compared with levels of serotonin (5HT) in the brain regions of juvenile and sexually recrudescing female rainbow trout, Oncorhynchus mykiss

This study compared the distribution of specifically bound [3H]ketanserin (Bsp) with serotonin (5HT) in brain regions of juvenile and sexually recrudescing female trout. Amounts of Bsp varied widely among brain regions and consistently differed between juvenile and sexually recrudescing females. Levels of Bsp were significantly greater in the hypothalamus than the olfactory lobe, which were at least threefold greater than in all other tissues examined (Kruskal-Wallis test, p < 0.05). Bsp densities in the hypothalamus, preoptic area, and optic lobe were significantly greater in juveniles compared with corresponding tissues from sexually recrudescing females (Mann-Whitney U test, p < 0.05); in contrast, Bsp in olfactory lobe and spinal cord did not differ significantly between the two classes of fish. 5HT concentration was determined by high performance liquid chromatography - electrochemical detection (HPLC-EC) analysis. Biogenic amine standards eluted in a stereotypic pattern, with peaks consistently separable in time. 5HT concentration was significantly greater in hypothalamus than in olfactory lobe and undetectable in the pituitary (Kruskal-Wallis test, p < 0.05). Trends in distribution of Bsp and 5HT were comparable in the hypothalamus and preoptic area in juvenile and sexually recrudescing females. In general, density of specific [3H]ketanserin binding sites was directly related to 5HT content of brain regions in juvenile and sexually recrudescing females. 5HT concentrations (pmol/g tissue) were approximately 900-fold greater than Bsp (fmol/g tissue) in all brain regions, and approximately 300-fold greater than Bsp in the olfactory lobe. These results suggest important regulatory role(s) for 5HT in the trout preoptic-hypothalamo-hypophysial axis, which may differ from 5HT role(s) in trout olfactory lobe.     

Electrical responses of olfactory structures and amygdala of dogs in the paradoxical phase of sleep

Electrical activity of the olfactory bulb, olfactory tubercle, amygdala, hippocampus, hypothalamus, and neocortex in the various phases of natural sleep was studied in chronic experiments on dogs under conditions close to those of free behavior. During paradoxical sleep it was found that a high-frequency synchronized rhythm of sinusoidal waves with a frequency of 36–42 Hz arises in the olfactory structures and amygdala. Generation of this activity during paradoxical sleep, by contrast with wakefulness, was unconnected with stimulation of the olfactory receptors and was probably purely central in origin. A study of the dynamics of the olfacto-amygdaloid rhythm during the paradoxical phase, and its comparison with somatic, autonomic, and EEG correlates of sleep, led to the conclusion that this rhythm is a specific EEG correlate of the paradoxical phase of sleep in dogs.     

Presence of specific prolactin binding sites in the rabbit hypothalamus

The presence of specific prolactin binding sites in membranes obtained from rabbit brain was investigated. Among the different brain areas studied (hypothalamus, cerebral cortex, cerebellum, olfactory bulbi and ponsmedulla) only the hypothalamus showed an evident specific binding (5.34% in male and 6.64% in female rabbits), whereas in the other brain regions the binding was lower or negligible. Scatchard analysis of the binding revealed the presence of high-affinity saturable binding sites. The study on hormonal specificity showed that the binding in the hypothalamus was inhibited by ovine and rat prolactin and by human GH, but not by many other polypeptide hormones.     

Distribution of FMRFamide-like immunoreactivity in the brain,retina and nervus terminalis of the sockeye salmon parr,Oncorhynchus nerka

Summary Neurons displaying FMRFamide(Phe-Met-Arg-Phe-NH₂)-like immunoreactivity have recently been implicated in neural plasticity in salmon. We now extend these findings by describing the extent of the FMRF-like immunoreactive (FMRF-IR) system in the brain, retina and olfactory system of sockeye salmon parr using the indirect peroxidase anti-peroxidase technique. FMRF-IR perikarya were found in the periventricular hypothalamus, mesencephalic laminar nucleus, nucleus nervi terminalis and retina (presumed amacrine cells), and along the olfactory nerves. FMRF-IR fibers were distributed throughout the brain with highest densities in the ventral area of the telencephalon, in the medial forebrain bundle, and at the borders between layers III/IV and IV/V in the optic tectum. High densities of immunoreactive fibers were also observed in the area around the torus semicircularis, in the medial hypothalamus, median raphe, ventromedial tegmentum, and central gray. In the retina, immunopositive fibers were localized to the inner plexiform layer, but several fiber elements were also found in the outer plexiform layer. The olfactory system displayed FMRF-IR fibers in the epithelium and along the olfactory nerves. These findings differ from those reported in other species as follows: (i) FMRF-IR cells in the retina have not previously been reported in teleosts; (ii) the presence of FMRF-IR fibers in the outer plexiform layer of the retina is a new finding for any species; (iii) the occurrence of immunopositive cells in the mesencephalic laminar nucleus has to our knowledge not been demonstrated previously.     

Preprotachykinin gene expression in goldfish brain: sexual, seasonal, and postprandial variations

Recently, we described the complete nucleotide sequence of gamma-preprotachykinin (gamma-PPT) mRNA and the deduced amino acid sequence of the precursor on the basis of molecular cloning and sequence analysis of cDNA from goldfish brain. In the present study, gamma-PPT gene expression in the brain of goldfish was examined using quantitative Northern blot analysis. The results showed that the gamma-PPT gene is highly but differentially expressed in the olfactory bulbs, hypothalamus, and posterior brain regions. There are sexual dimorphism and seasonal variations in gamma-PPT gene expression. In addition, the postprandial changes in gamma-PPT gene expression in the olfactory bulbs and hypothalamus suggest that tachykinin peptides are involved in regulation of feeding behavior in goldfish.     

LHRH-systems in the brain of the golden hamster

Summary Vibra tome sections of male hamster brains were treated immunohistochemically with LHRH antiserum, and the anatomical distribution of LHRH immunoreactive cells and nerve fibers was assessed. LHRH-cell bodies are found in the ventral hypothalamus that includes its preoptic, anterior and central parts, in the septum, the olfactory tubercle, the main and accessory olfactory bulb, and the prepiriform cortex. In addition, extracerebral LHRH-neurons and ganglia exist in LHRH-positive nerves at the ventromedial surface of the olfactory tubercle and bulb as well as in olfactory nerves. Dense networks of LHRH-immunoreactive fibers are found in all regions where LHRH-cell bodies exist. Intraseptal connections reach the organum vasculosum of the lamina terminalis, the subfornical organ, and the lateral ventricle. Dorsolateral projections from the septum can be traced via the fimbria hippocampi and alveus to the ventral hippocampus, via the stria terminalis to the amygdala and piriform cortex. Ventrolateral projections extend from the level of the olfactory tubercle and preoptic-anterior hypothalamic area via the ventral amygdalofugal pathway to the prepiriform and piriform cortex as well as the amygdala. Dorsal supracallosal projections via the stria longitudinalis are seen in the induseum griseum and the cingulate cortex. Caudal efferents reach the habenula, interpeduncular nucleus, midbrain raphe, and central gray of the rostral fourth ventricle via the stria medullaris and fasciculus retroflexus and by a ventral projection via the periventricular and subventricular hypothalamus. A major portion of this ventrocaudal projection gives rise to a dense network in the median eminence. Anatomical relationships of LHRH-fibers to certain regions of the inner ventricular and outer brain surface are noted.Postdoctoral fellow of the Deutsche ForschungsgemeinschaftSupported by US PHS grant NS09914 and NRCHD grant HD03110     

Oxidative stress is induced in the rat brain following repeated inhalation exposure to manganese sulfate

Eight-week-old rats inhaled manganese (Mn) in the form of MnSO₄ at 0, 0.03, 0.3, or 3.0 mg Mn/m³ for 6 h/d for 7 d/wk (14 consecutive exposures). Brain manganese concentrations in these animals were reported by Dorman et al. in 2001, noting the following rank order: olfactory bulb>striatum>cerebellum. We assessed biochemical end points indicative of oxidative stress in these three brain regions, as well as the hypothalamus and hippocampus. Glutamine synthetase (GS) protein levels and total glutathione (GSH) levels were determined for all five regions. GS mRNA and metallothionein (MT) mRNA levels were also evaluated for the cerebellum, hypothalamus, and hippocampus. Statistically significant increases (p<0.05) in GS protein were observed in the olfactory bulb upon exposure to the medium and high manganese doses. In the hypothalamus, statistically significant (p<0.05) but more modest increases were also noted in the medium and high manganese dose. Total GSH levels significantly (p<0.05) decreased only in the hypothalamus (high manganese dose), and MT mRNA significantly increased in the hypothalamus (medium manganese dose). No significant changes were noted in any of the measured parameters in the striatum, although manganese concentrations in this region were also increased. These results demonstrate that the olfactory bulb and hypothalamus represent potentially sensitive areas to oxidative stress induced by exceedingly high levels of inhaled manganese sulfate and that other regions, and especially the striatum, are resistant to manganese-induced oxidative stress despite significant accumulation of this metal.     

Circadian rhythms in catecholamine metabolites and cyclic nucleotide production

Circadian rhythms in noradrenergic (NE) and dopaminergic (DA) metabolites and in cyclic nucleotide production were measured in discrete regions of rat brain. A circadian rhythm was found in the concentration of the NE metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), in the hippocampus. No MHPG rhythm was found in frontal, cingulate, parietal, piriform, insular or temporal cortex, or in hypothalamus. Circadian rhythms in the concentration of the NE metabolite, 3,4-dihydroxyphenylglycol (DHPG), occurred in occipital and parietal cortex and hypothalamus, with no rhythm observable in temporal or insular cortex, hippocampus, pons-medulla or cerebellum. The 24-hr mean concentration of MHPG varied 3.5-fold, highest in cingulate and lowest in parietal, temporal and occipital cortex. The 24-hr mean concentration of DHPG varied 6-fold, highest in hypothalamus and lowest in parietal cortex. Circadian rhythms in the concentration of the DA metabolite, homovanillic acid (HVA), were found in olfactory tubercle, amygdala and caudate-putamen, but not in nucleus accumbens. A circadian rhythm in the concentration of the DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), occurred in nucleus accumbens, but not in olfactory tubercle or caudate-putamen. The mean 24-hr concentration of HVA was highest in caudate-putamen, intermediate in nucleus accumbens, and lowest in olfactory tubercle and amygdala. The mean 24-hr concentration of DOPAC was highest in nucleus accumbens and lower in olfactory tubercle and caudate-putamen. Circadian rhythms were found in the concentration of cyclic GMP (cGMP) in all regions measured except parietal cortex. The mean 24-hr concentration varied 128-fold, highest in nucleus accumbens, frontal poles, and hypothalamus and lowest in cingulate cortex. Circadian rhythms in cyclic AMP (cAMP) concentration were found in piriform, temporal, occipital, cingulate, and parietal cortex, amygdala and nucleus accumbens. No rhythms were found in frontal or insular cortex, hypothalamus, hippocampus, caudate-putamen or olfactory tubercle. The 24-hr mean cAMP concentration varied 4-fold, highest in parietal cortex and lowest in caudate-putamen and amygdala. Norepinephrine metabolites and dopamine metabolites were rhythmic in few regions. It is, therefore, unlikely that the rhythmicity measured in adrenergic receptors is, in general, a response to rhythmic changes in adrenergic transmitter release. The putative second messenger response systems, especially cGMP, were more often rhythmic. The rhythms in cGMP are parallel in form and region to those in the alpha 1-adrenergic receptor and may act as 2nd messenger for that receptor.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cocaine

Cocaine HCl (0, 10, or 50 mg/kg) was injected into adult male ICR mice ip. Thirty minutes later, the brains were removed, and nine regions were isolated: olfactory bulbs, olfactory tubercles, prefrontal cortex, septum, striatum, amygdala, hypothalamus, hippocampus, and thalamus. Using high-performance liquid chromatography, concentrations of norepinephrine, dopamine, serotonin, and their major metabolites and the metabolite/neurotransmitter ratios were determined as an indicator of utilization. Serotonergic systems responded most dramatically. 5HIAA/5-HT decreases were seen in all the brain regions, except the septum, hippocampus, and olfactory bulbs. In most instances, the alterations were dose-dependent. The most profound changes were seen in the amygdala, prefrontal cortex, hypothalamus, and thalamus. For noradrenergic systems, significant responses were seen only in the amygdala, prefrontal cortex, and hypothalamus, but then only at the lower dose. The dopaminergic responses were more complex and not always dose-dependent. The DOPAC/DA ratio was decreased only in the amygdala and striatum at the lower dose, and the olfactory tubercles at the higher dose. It was increased in the septum. The HVA/DA ratios were decreased in the amygdala, prefrontal cortex, and hypothalamus, but only at the lower dose (like MHPG/NE). The 3MT/DA ratio was decreased in the thalamus at the lower dose and in the olfactory tubercles at the higher dose, whereas it was increased in the prefrontal cortex at the lower dose. The HVA and DOPAC routes of degradation were both utilized only by the amygdala. Thus, cocaine produced its most comprehensive effects in this nucleus, as well as the greatest absolute percentage changes for all three of the monoamine systems studied.     

Elevated Fos-like immunoreactivity in the brains of postpartum female prairie voles, Microtus ochrogaster

This study investigated the expression of the protein product of the immediate early gene c-fos in the brains of female prairie voles (Microtus ochrogaster) in association with pregnancy and postparturient activities including maternal behavior, lactation and postpartum estrus. Fos expression was assessed in female voles that were late in pregnancy, nonpregnant or at one of three different times postpartum (0-8, 12-24, and 24-48 h, respectively). A significant increase in the number of cells displaying Fos immunoreactivity (Fos-ir) was observed during the 0-8 h and 12-24 h postpartum time periods in the accessory olfactory bulbs, medial preoptic area, hypothalamus (specifically, the supraoptic nucleus, ventro-medial hypothalamus, and paraventricular nucleus), lateral septum, bed nucleus of the stria terminalis, and primary somatosensory area of the brain. The number of Fos-ir cells decreased after 24 h postpartum. There were no significant changes in Fos-ir cell numbers in the primary olfactory bulbs, hippocampus, or caudate putamen. The neural activation of the medial preoptic area, accessory olfactory bulbs, hypothalamus, and bed nucleus is consistent with reports in rats of Fos induction associated with the onset of maternal behavior. In voles postpartum estrous behavior begins and ends 0-12 h after parturition. Maternal behavior, including lactation, is initiated at the same time but persists for several weeks. The highest Fos-ir cell numbers reported here coincide with the timing of postpartum estrous behavior in this species.     

Regional distribution of metallothionein, zinc, and copper in the brain of different strains of rats

The regional brain distribution of metallothionein (MT), zinc, and copper in the brain was determined in nine anatomical regions (olfactory bulb, cortex, corpus striatum, hippocampus, thalamus plus hypothalamus, pons plus medulla oblongata, cerebellum, midbrain, and white matter) and was compared between two different strains of rat (Sprague-Dawley [SD] and Lewis). No significant difference was observed in the whole-brain MT level between the two strains (17.8 ± 3.4 μg/g in SD rats and 20.3 ± 2.3 μg/g in Lewis rats). In SD rats, however, MT was more highly expressed in the white matter than in the other regions studied. In contrast, MT concentration was highest in the cortex and lowest in the olfactory bulb in Lewis rats. The MT levels in the cortex, corpus striatum, hippocampus, and thalamus plus hypothalamus were significantly lower in SD rats than in Lewis rats. In both strains, the olfactory bulb contained markedly higher levels of both zinc and copper than the other regions (27.9 ±6.8 μg/g zinc in SD rats and 27.6 ± 6.9 μg/g zinc in Lewis rats, and 5.2 ± 1.5 μg/g copper in SD rats and 11.1 ± 4.8 μg/g copper in Lewis rats). The next high-est zinc levels were seen in the hippocampus, whereas the next highest copper levels were in the corpus striatum in both SD and Lewis rats. The high levels of zinc and copper in the olfactory bulb were not accompanied by concomitant high MT concentrations. These results indicate that the strain of rat as well as the anatomical brain region should be taken into account in MT and metal distribution studies. However, the highest concentrations of zinc and copper in olfactory bulb were common to both SD and Lewis rats. The discrepancy between MT and the metal levels in olfactory bulb suggests a role for other proteins in addition to MT in the homeostatic control of zinc and copper.     

A reinvestigation of the Gn-RH (gonadotrophin-releasing hormone) systems in the goldfish brain using antibodies to salmon Gn-RH

Summary The organization of Gn-RH systems in the brain of teleosts has been investigated previously by immunohistochemistry using antibodies against the mammalian decapeptide which differs from the teleostean factor. Here, we report the distribution of immunoreactive Gn-RH in the brain of goldfish using antibodies against synthetic teleost peptide.Immunoreactive structures are found along a column extending from the rostral olfactory bulbs to the pituitary stalk. Cell bodies are observed within the olfactory nerves and bulbs, along the ventromedial telencephalon, the ventrolateral preoptic area and the latero-basal hypothalamus. Large perikarya are detected in the dorsal midbrain tegmentum, immediately caudal to the posterior commissure. A prominent pathway was traced from the cells located in the olfactory nerves through the medial olfactory tract and along all the perikarya described above to the pituitary stalk. In the pituitary, projections are restricted to the proximal pars distalis. A second immunoreactive pathway ascends more dorsally in the telencephalon and arches to the periventricular regions of the diencephalon. Part of this pathway forms a periventricular network in the dorsal and posterior hypothalamus, whereas other projections continue caudally to the medulla oblongata and the spinal cord. Lesions of the ventral preoptic area demonstrate that most of the fibers detected in the pituitary originate from the preoptic region.