Neuromyelitis optica study model based on chronic infusion of autoantibodies in rat cerebrospinal fluid.

BACKGROUND: Devic's neuromyelitis optica (NMO) is an autoimmune astrocytopathy, associated with central nervous system inflammation, demyelination, and neuronal injury. Several studies confirmed that autoantibodies directed against aquaporin-4 (AQP4-IgG) are relevant in the pathogenesis of NMO, mainly through complement-dependent toxicity leading to astrocyte death. However, the effect of the autoantibody per se and the exact role of intrathecal AQP4-IgG are still controversial. METHODS: To explore the intrinsic effect of intrathecal AQP4-IgG, independent from additional inflammatory effector mechanisms, and to evaluate its clinical impact, we developed a new animal model, based on a prolonged infusion of purified immunoglobulins from NMO patient (IgG(AQP4+), NMO-rat) and healthy individual as control (Control-rat) in the cerebrospinal fluid (CSF) of live rats. RESULTS: We showed that CSF infusion of purified immunoglobulins led to diffusion in the brain, spinal cord, and optic nerves, the targeted structures in NMO. This was associated with astrocyte alteration in NMO-rats characterized by loss of aquaporin-4 expression in the spinal cord and the optic nerves compared to the Control-rats (p = 0.001 and p = 0.02, respectively). In addition, glutamate uptake tested on vigil rats was dramatically reduced in NMO-rats (p = 0.001) suggesting that astrocytopathy occurred in response to AQP4-IgG diffusion. In parallel, myelin was altered, as shown by the decrease of myelin basic protein staining by up to 46 and 22 % in the gray and white matter of the NMO-rats spinal cord, respectively (p = 0.03). Loss of neurofilament positive axons in NMO-rats (p = 0.003) revealed alteration of axonal integrity. Then, we investigated the clinical consequences of such alterations on the motor behavior of the NMO-rats. In a rotarod test, NMO-rats performance was lower compared to the controls (p = 0.0182). AQP4 expression, and myelin and axonal integrity were preserved in AQP4-IgG-depleted condition. We did not find a major immune cell infiltration and microglial activation nor complement deposition in the central nervous system, in our model. CONCLUSIONS: We establish a link between motor-deficit, NMO-like lesions and astrocytopathy mediated by intrathecal AQP4-IgG. Our study validates the concept of the intrinsic effect of autoantibody against surface antigens and offers a model for testing antibody and astrocyte-targeted therapies in NMO.

Prospective evaluation of clinically relevant type B hyperlactatemia in dogs with cancer.

BACKGROUND: Cancer is considered a cause of type B hyperlactatemia in dogs. However, studies evaluating cancer as a cause of clinically relevant type B hyperlactatemia (>2.5 mmol/L) are lacking. Cancer cells have a higher lactate production because of increased aerobic glycolysis, known as the "Warburg effect." The mechanisms through which aerobic glycolysis occurs are not well elucidated, but neoplasia may cause type B hyperlactatemia via this process. OBJECTIVES: To determine if malignant tumors of dogs are associated with clinically relevant type B hyperlactatemia (>2.5 mmol/L). ANIMALS: Thirty-seven client-owned dogs with malignant tumors: 22 with hematopoietic and 15 with solid tumors. METHODS: Histology was used to confirm the diagnosis (cytology was considered adequate for diagnosis of lymphoma). Confounding conditions associated with hyperlactatemia were excluded. Lactate measurements were immediately performed on free-flow jugular whole blood samples using the LactatePro analyzer. RESULTS: All dogs had lactate concentrations<2.5 mmol/L. Mean blood lactate concentration was 1.09 mmol/L. Mean blood lactate concentrations for solid and hematopoietic tumors were 0.95 and 1.19 mmol/L, respectively. Dogs with lymphoma (n=18) had a mean blood lactate concentration of 1.15 mmol/L. CONCLUSIONS: Malignant tumors were not considered a cause of clinically relevant type B hyperlactatemia. Therefore, cancer-related type B hyperlactatemia in dogs is uncommon, and hyperlactatemia should prompt careful investigation for causes other than cancer.

The differential response of astrocytes within the vestibular and cochlear nuclei following unilateral labyrinthectomy or vestibular afferent activity blockade by transtympanic tetrodotoxin injection in the rat.

In this study, we investigated whether changes in the vestibular neuronal activity per se influence the pattern of astrocytes morphology, glial fibrillary acidic protein (GFAP) expression and ultimately their activation within the vestibular nuclei after unilateral transtympanic tetrodotoxin (TTX) injections and after unilateral inner ear lesion. The rationale was that, theoretically the noninvasive pharmacological functional blockade of peripheral vestibular inputs with TTX, allowed us to dissociate the signals exclusively related to the shutdown of the resting activity of the first-order vestibular neurons and from neuronal signals associated with trans-ganglionic changes in first order vestibular neurons induced by unilateral labyrinthectomy (UL). Since the cochlea was removed during the surgical procedure, we also studied the astrocytic reaction within the deafferented cochlear nuclei. No significant changes in the distribution or relative levels of GFAP mRNA expression, relative levels of GFAP protein or immunoreactivity for GFAP were found in the ipsilateral vestibular nuclei at any post-TTX injection times studied. In addition, no sign of microglia activation was observed. In contrast, a robust increase of the distribution and relative levels of GFAP mRNA expression, protein levels and immunoreactivity was observed in the deafferented vestibular and cochlear nuclei beginning at 1 day after inner ear lesion. GFAP mRNA expression and immunoreactivity in the cochlear nucleus was qualitatively stronger than in the ipsilateral vestibular nuclei. The results suggest that astrocyte activation in the vestibular nuclei is not related to drastic changes of vestibular nuclei neuronal activity per se. Early trans-ganglionic changes due to vestibular nerve dendrites lesion provoked by the mechanical destruction of vestibular receptors, most probably induced the glial reaction. Its functional role in the vestibular compensation process remains to be elucidated.

5-Hydroxytryptophan uptake and decarboxylating neurons in the cat hypothalamus.

Parachlorophenylalanine, an inhibitor of tryptophan hydroxylase, induced a virtually total disappearance of serotonin-immunoreactivity in the hypothalamus of the cat. After intrahypothalamic injection of 5-hydroxytryptophan, an immediate precursor of serotonin in cats pretreated with parachlorophenylalanine, serotonin-immunoreactivity was detected in many fibers surrounding the injection site. Furthermore, when 5-hydroxytryptophan was injected with inhibitor of monoamine oxidase, a large number of small neurons immunoreactive to serotonin was identified in many discrete regions: the anterior and lateral hypothalamic areas, preoptic area, suprachiasmatic nucleus, dorsal hypothalamic area, dorsomedial nucleus, posterior hypothalamic area and nucleus of the fields of Forel. Serotonin-immunoreactivity was also evident in the thick axon bundles in the lateral hypothalamus. The distribution pattern of these cells was quite similar to that of aromatic L-amino acid decarboxylase, which catalyses the conversion of 5-hydroxytryptophan to serotonin and that of L-3,4-dihydroxyphenylalanine to dopamine. However, we failed to demonstrate serotonin-immunoreactivity in these parvocellular neurons without monoamine oxidase inhibitor. It is possible that 5-hydroxytryptophan is decarboxylated to serotonin by aromatic L-amino acid decarboxylase but rapidly degraded by monoamine oxidase-A, the enzyme which preferentially deaminates serotonin. In contrast, serotonin-immunostaining was always demonstrable after intrahypothalamic injection of 5-hydroxytryptophan without monoamine oxidase inhibitor in magnocellular neurons located in the ventrolateral posterior hypothalamus and which contain exclusively monoamine oxidase-B and histidine decarboxylase. It appears that in these cells and axons, serotonin, possibly formed by histidine decarboxylase, is not rapidly oxidized by monoamine oxidase-B. Possible roles of serotonin as a neurohormone in sleep-waking regulation and of trace amines in the brain are discussed.

Paradoxical sleep deprivation increases the content of glutamate and glutamine in rat cerebral cortex.

We investigated the influence of the sleep/waking cycle, the effects of paradoxical sleep deprivation (PSD) and of the vigilance-promoting drug modafinil on the amino acid contents of rat brain cortex. No significant nycthemeral variations in amino acid levels could be detected. PSD (12-24 hours), using the water tank method, significantly increased the levels of glutamate and glutamine. The increase was still observed after the sleep rebound period. gamma-Aminobutyric acid (GABA) levels did not change significantly during the instrumental sleep deprivation but increased during the rebound period. Control experiments indicate that the increase in glutamate and glutamine levels is due to PSD rather than to the stress associated with the experimental procedure. The increase in glutamate content cannot arise only from transamination reactions, because the levels of other amino acids (such as aspartate) did not decrease. Modafinil treatment did not significantly modify the brain cortex content of any of the amino acids tested.

Glutamine synthetase modulation in the brain of rats subjected to deprivation of paradoxical sleep.

Changes in the level of glutamine synthetase (GS), an enzyme mainly located in astrocytes, were investigated in rat brain after deprivation of paradoxical sleep (PSD) and during recovery. An immunotitration method was used to evaluate the relative level of GS in brain tissue. At the end of a 24 h PSD, a significant increase in GS protein was observed both in the frontoparietal cortex (CX) and in the locus coeruleus area (LC). Four hours later during recovery, the level of GS protein returned to normal level in the CX but fell below control levels in the LC. In contrast, in the CX, the level of glial fibrillary acidic protein, an astroglial marker, did not change after PSD or during recovery. GS mRNA was quantified in the entire cortex by northern blot hybridization using of an oligonucleotidic GS-cDNA probe. We observed an increase in the GS mRNA level in the cortex of PSD rats of the same magnitude as the increase in GS protein. Both GS mRNA and GS protein tended to return to control values 4 h later during recovery. These results are discussed with particular attention to stress effects and possible physiological mechanisms regarding the regulation of amino acid levels by neurotransmitters during prolonged waking or neuronal excitation.

Effects of modafinil-induced wakefulness on glutamine synthetase regulation in the rat brain.

Changes in the level of glutamine synthetase (GS), an enzyme chiefly found in glial cells, were investigated in the brains of rats treated with modafinil, an awakening drug interfering with central catecholamine function. Two hours (waking period) and 7 h (recovery period) after intra-peritoneal injection of 128 mg/kg modafinil, a significant increase in the level of GS protein was observed by immunotitration in both the locus coeruleus (+30%) and in the frontoparietal cortex (+50%). No changes were observed with 64 mg/kg of modafinil. GS mRNA was quantified in the entire cortex by Northern blot hybridization using an oligonucleotidic GS cDNA probe. A significant increase in the GS-mRNA level (+70%) was observed in the CX of rats 2 h after injection of 128 mg/kg modafinil; the level tended to return to control values 7 h later during the recovery period. The level of glial acid fibrillary protein (GFAP), an astroglial marker, was unchanged after modafinil treatment. These changes in GS levels after modafinil treatment are discussed in terms of neuron-glia interactions in the regulation of brain metabolism during pharmacologically induced wakefulness, excluding possible stress effects.

First characterization of 5-hydroxytryptophan in rat brain by using specific antibodies.

DL-5-Hydroxytryptophan (5-HTP) was conjugated to bovine serum albumin and human serum albumin with glutaraldehyde (G). These conjugates made it possible to raise specific antisera in two rabbits. Their specificity and affinity were evaluated using an enzyme-linked immunosorbent assay and immunocytochemistry. For two antisera obtained, the most immunoreactive antigen was 5-HTP-G-protein, indicating that the same immune response was developed. The other conjugated indoleamines (5-methoxytryptophan-G-protein, tryptophan-G-protein) were poorly recognized or not at all (5-methoxytryptamine-G-protein, serotonin-G-protein, tryptamine-G-protein). These 5-HTP antisera enabled us to specifically visualize the precursor of serotonin in the raphe nuclei of G-fixed rat brains.

The locus coeruleus: a quantitative and genetic study in mice.

A fluorescence histochemical study of the pontine catecholaminergic (CA) neurons of two inbred strains of mice, BALB/c (C) and C57BL/6 (B6) and their F1 hybrids revealed that the total number of the fluorescent cells in the C strain was 38% lower than that of the B6 strain. In F1 hybrids, the total number of neurons was not statistically different from that of the B6 parent. During the development from birth to adulthood, the same quantitative differences between strains were observed.

Afferent connections of the nucleus raphe dorsalis in the cat as visualized by the horseradish peroxidase technique.

Using a retrograde tracer technique with protein horseradish peroxidase (HRP), attempts were made to determine afferent projections to the nucleus raphe dorsalis (NRD). As a control, the injection of the HRP was also made into one of the following structures adjacent to the NRD: (1) mesencephalic periaque ductal gray; (2) nucleus linearis intermedius; and (3) third cranial nucleus. The present results indicate that the NRD, particularly is rostral part, receives direct projections arising from: (1) locus coeruleus complex (locus coeruleus, locus coerulus alpha, and locus subcoeruleus); (2) parabrachial nuclei (nucleus parabrachialis lateralis and medialis); (3) nucleus laterodorsalis tegmenti; (4) griseum centrale pontis, particularly the caudal part of the nucleus incertus; (5) substantia nigra; (6) lateral habenular nucleus; (7) hypothalamic areas, particularly dorsal and lateral hypothalamic areas; (8) preoptic areas; (9) anarea dorso-lateral to the inferior olivary complex and medial to the lateral reticular nucleus; and (10) raphe nuclei; particularly nucleus linearis intermedius, centralis superior, pontis and magnus. The present findings thus confirm some previous reports on the afferent projections to the NRD described in the cat and rat, and further indicate the richness of afferent connections of the NRD. Some problems of the peroxidase technique have also been discussed.

Spinal projections from the lower brain stem in the cat as demonstrated by the horseradish peroxidase technique. I. Origins of the reticulospinal tracts and their funicular trajectories.

Using a retrograde tracer technique with horseradish peroxidase (HRP) attempts were made to determine the origins of reticulospinal tracts and their funicular trajectories. Reticulospinal tracts originating from the mesencephalic reticular formation (RF) were composed of: (1) descending projections arising from the cluster of cells located just lateral to the periaqueductal gray that course in the anterior funiculus (AF) and ventral part of the lateral funiculus (LF) with ipsilateral predominance; and (2) projections from the cluster of cells located dorsal to the brachium conjunctivum that course in the ipsilateral LF. Origins of the pontine reticulospinal tracts arising from the n. reticularis pontis oralis (Poo) have been divided qnto three parts: (1) medial one-third; (2) middle; and (3) ventrolateral. The axons from the medial part descend ipsilaterally via the medial part of the AF, while the axons from the ventrolateral part of the Poo give rise to diffuse descending projections in the AF and LF. The middle part of the Poo has been further subdivided into: (1) dorsal part that gives rise to spinal projections ipsilaterally in the ventrolateral funiculus (VLF); and (2) ventral, particularly its upper part, whose axons descend bilaterally via the DLF. Origins of reticulospinal tracts from the n. reticularis pontis caudalis (Poc) could be divided into three parts: (1) medial; (2) dorsolateral; and (3) ventrolateral. The medial part of the Poc is a source of axons via the medial part of the ipsilateral AF, while the ventrolateral part of the nucleus is a source of axons via the contralateral LF. The spinal projections from the dorsolateral part of the Poc appears to course diffusely in the AF and LF, but with DLF predominance. The n. reticularis gigantocellularis (Gc) was found to be a main medullary source of the spinal projections in the ipsilateral AF, while n. reticularis magnocellularis (Mc) is the major source of the fibers coursing ipsilaterally in the VLF. The most medial part of the Mc descends ipsilaterally via the medial part of the AF, while the ventrolateral part of the nucleus together with the n. reticularis lateralis of Meesen and Olszewski descends ipsilaterally via the DLF. It has also been found that the axons from the n. reticularis paramedianus pass via both the AF and LF with ipsilateral predominance, while the n. reticularis dorsalis and ventralis course via the LF with ipsilateral predominance.

Spinal projections from the lower brain stem in the cat as demonstrated by the horseradish peroxidase technique. II. Projections from the dorsolateral pontine tegmentum and raphe nuclei.

The descending projections to the spinal cord arising from the dorsolateral pontine tegmentum and brain stem raphe nuclei have been investigated by means of the horseradish peroxidase (HRP) technique. Particular attention was taken to clarify the cells of origin and the funicular trajectory of these spinal projections. After injections of HRP into the spinal cord, a significant of HRP labeled neurons were observed in the following dorsolateral pontine tegmental structures: (1) an area ventral to the nucleus cuneiformis; (2) principal locus coeruleus; (3) locus coeruleus a; (4) locuse subcoeruleus; (5) Kölliker-Fuse nucleus; and (6) nucleus parabrachialis lateralis. As a rule, the projections are ipsilateral and descendaphe-spinal projections, we have demonstrated that the nucleus raphe dorsalis also sends axons to the cervical segment of the spinal cord. Furthermore, in accord with previous reports, HRP labeled cells were also identified in the nucleus raphe magnus, pallidus and obscurus, but not in the nucleus raphe centralis superior and pontis. On the whole the present study further clarified the organization of spinal projections from the dorsolateral pons and raphe nuclei and provided some additional anatomical data for the physiology of the tegmentospinal and raphe-spinal projections.

Tegmentoreticular projections with special reference to the muscular atonia during paradoxical sleep in the cat: an HRP study.

Using the retrograde tracer technique with horseradish peroxidase (HRP), attempts were made to determine the cells of origin and the descending pathway of the tegmentoreticular projections in order to give an anatomical substrate for the physiological phenomenon of the postural atonia observed during paradoxical sleep (PS) in the cat. The HRP was injected into various parts of the pontomedullary reticular formation (RF) including the caudal raphe nuclei, nucleus (n.) reticularis gigantocellularis (Gc), n. reticularis magnocellularis (Mc), and other pontomedullary structures adjacent to the Mc. The results indicated that the HRP injection into the Mc, particularly its caudal and lateral two-thirds, resulted in specific labeling of cells located in an area just medial to the LCa together with those in the most medial part of the LCa. Bilateral lesions of these pontine structures have been reported to suppress the atonia otherwise observed during PS in the normal cat. In addition to the HRP labeled cells, we have also observed HRP filled fiber bundles directed to labeled cells in the medial part of the LCa and immediately adjacent tegmental RF area. The same course of HRP labeled fiber bundles was also observed together with HRP labeled cells in the Mc after HRP injections into the medial part of the LCa area, indicating the existence of an interconnection between the LCa area and the Mc. The location of the tegmentoreticular pathway corresponded to that of the lesions effective to suppress the muscular atonia during PS. HRP injections into the caudal medullar caudal to the Mc, on the other hand, resulted in no or almost no HRP labeled cells in the area medial to the LCa, in spite of the presence of HRP containing neurons in other parts of the pontomedullary RF areas, showing that the tegmentoreticular projections as described above terminate almost exclusively in the Mc.

Preferential expression of kin, a nuclear protein binding to curved DNA, in the neurons of the adult rat.

The KIN17 gene product has been identified by cross immunoreactivity with anti-RecA antibodies and by DNA recombination techniques, and is probably part of the DNA recombination-repair machinery. Following Western blotting and immunocytochemistry using anti-RecA antibodies, and in situ hybridization with specific KIN17 cDNA probes, we here report the detection of high levels of KIN protein and KIN17 mRNA in the CNS of adult rats. The RecA cross-reacting protein has an apparent molecular weight of 41 kDa and is located in the nucleus of brain cells. Both the KIN17 transcript and the protein were found to be widespread, but they were present in different proportions, depending on the type of brain cells. High levels of KIN protein were seen in neurons of the motor nuclei of the brainstem, the locus coeruleus, hippocampal formation, entorhinal cortex, Purkinje cells, pyramidal cells of the cortex and mitral cells. In contrast, using a combination of KIN17 mRNA in situ hybridization and GFAP immunocytochemistry (a marker of glial cells) showed that the KIN17 messenger is preferentially transcribed in neurons, the post-mitotic and long lived brain cells. We postulate that KIN17 play a role in the illegitimate recombination of DNA sequences and/or the repair of alterations of the genome in neurons.

The raphe nuclei of the cat brain stem: a topographical atlas of their efferent projections as revealed by autoradiography.

Stereotaxic injections of [14C]leucine were made in nulei raphe centralis superior, raphe dorsalis, raphe magnus and raphe pontis of the cat. The organization of the regional connections was outlined in a stereotaxic atlas using the autoradiographic tracing method: the majority of the ascending pathways from the rostral raphe nuclei are directed mainly through a ventrolateral bundle via the ventral tegmental area of Tsai, with some lateral extensions to the substantia nigra, and then through the fields of Forel and the zona incerta. More rostrally the fibers are joined to the medial forebrain bundle through the hypothalamic region up to the preoptic area or the diagonal band of Broca. Multiple divisions leave this tract towards the epithalamic or the intralaminar thalamic nuclei, the stria terminalis, the septum, the capsula interna and the ansa lenticularis. The bulk of the rostral projections terminates in the frontal lobe, while some labeling is scarcely distributed throughout the rest of the neocortex. The projections of nucleus (n.) raphe centralis superior are specifically associated with the n. interpeduncularis, the mammillary bodies and the hippocampal formation while the n. raphe dorsalis innervates selectively the lateral geniculate bodies, striatus, piriform lobes, olfactory bulb and amygdala. The rest of the ascending fibers form the centrolateral or the dorsal ascending tracts radiating either in the reticular mesencephalic formation or in the periventricular gray matter. On the contrary there are heavy descending projections from n. raphe centralis superior which distribute to the main nuclei of the brain stem, the central gray matter and the cerebellum. The ascending projections form the caudal raphe nuclei are much less dense. They disseminate mainly in the colliculus superior, the pretectum, the nucleus of the posterior commissure, the preoculomotor complex and the intralaminar nuclei of the thalamus. From n. raphe pontis, a dense labeling is selectively localized at the n. paraventricularis hypothalami with some rostral extensions to limbic areas. Diffuse caudal and rostral projections from both nuclei are observed in the mesencephalic, pontobulbar reticular formation and the cerebellum. The main differences come from the specific localization of their descending bulbospinal tracts inside the lateroventral funiculus of the spinal cervical cord.

Inhibition of methionine incorporation into brain proteins after the systemic administration of p-chlorophenylalanine and L-5-hydroxytryptophan.

The effects of p-chlorophenylalanine (p-CPA) and L-5-hydroxytryptophan (L-5-HTP) on local rates of plasma methionine incorporation into brain proteins were investigated by a quantitative autoradiographic method. The sequential i.v. administration of p-CPA (280 mg/kg, 42 h before the measurement) and L-5-HTP (60 mg/kg, 40 min before the measurement) resulted in an average 82% decrease of plasma methionine incorporation. The two treatments given separately also reduced the rates of plasma methionine incorporation in all the brain areas examined by 33 and 50%, respectively for p-CPA and L-5-HTP. These results indicate that: (1) p-CPA and L-5-HTP, two drugs which affect brain serotonin production in opposite ways, both produce large and general decreases of brain protein synthesis; (2) the administration of L-5-HTP does not restore the p-CPA-induced inhibition of brain protein synthesis but induces further decreases of protein synthesis. These results suggest that the reduction of brain protein synthesis in p-CPA-treated rats is mainly related to high circulating levels of p-CPA and phenylalanine; and that brain serotonin is not the only factor involved in the widespread metabolic changes observed. Such profound alterations of brain metabolism should be considered when interpreting the behavioral and neurochemical effects of p-CPA and L-5-HTP.

5-Hydroxytryptophan (5-HTP)-immunoreactive neurons in the rat brain tissue.

We demonstrated the presence of 5-hydroxytryptophan (5-HTP), the immediate precursor of serotonin (5-HT), in the rat brain tissue using a glutaraldehyde-coupled immunohistochemical technique. The immunoreactivity of 5-HTP was intensified in the colchicine-pretreated rat. The distribution of labelled cells was the same as for 5-HT-immunoreactive cells, but they were fewer in number.

Awakening properties of modafinil without paradoxical sleep rebound: comparative study with amphetamine in the rat.

We have studied the effect of modafinil and amphetamine, two waking drugs, on the sleep-wake cycle of Sprague-Dawley rat. Both modafinil (64 or 128 mg/kg) and amphetamine (2.5 or 5 mg/kg) cause a dose dependent increase in wakefulness. However, amphetamine wakefulness is followed by a paradoxical sleep rebound on the injection day, whereas modafinil does not produce this effect. In modafinil-treated rats, the sleep pattern on the post-injection day is similar to that of controls, while that of amphetamine-treated rats is modified.

Decreased protein synthesis in hypothalamic nuclei following L-5-hydroxytryptophan in intact and p-chlorophenylalanine-pretreated rats.

The rate of protein synthesis was estimated in individual hypothalamic nuclei by a quantitative autoradiographic technique with L-[35S]methionine. The i.v. administration of 60 mg/kg L-5-hydroxytryptophan (40 min before) resulted in a 45-55% decrease of overall protein synthesis rate in all the hypothalamic nuclei examined. In rats pretreated (42 h before) with a single i.v. injection of 280 mg/kg p-chlorophenylalanine, a drug which is known to deplete brain serotonin concentration, the administration of 60 mg/kg L-5-hydroxytryptophan resulted in a 50-75% decrease of protein synthesis rates in the hypothalamic nuclei. These results suggest that the systemic administration of large doses of L-5-hydroxytryptophan may inhibit protein synthesis in hypothalamic nuclei directly or indirectly after the conversion of this compound to serotonin.