Serotonin receptors involved in vasopressin and oxytocin secretion

Serotonin (5-HT), 5-HT agonists, the 5-HT precursor 5-hydroxytryptophan, 5-HT-releasers and -reuptake inhibitors stimulate the release of vasopressin and oxytocin. We investigated the involvement of 5-HT receptors in the serotonergic regulation of vasopressin and oxytocin secretion. Vasopressin and oxytocin secretion was stimulated by 5-HT, the 5-HT(1A+1B+5A+7) agonist 5-carboxamidotryptamine (5-CT), the 5-HT(2A+2C) agonist DOI, the 5-HT(2C+2A) agonist mCPP, the 5-HT(2C) agonist MK-212, the 5-HT(3) agonist SR 57277 and the 5-HT(4) agonist RS 67506. The 5-HT(1A) agonist 8-OH-DPAT, which had no effect on vasopressin secretion, stimulated oxytocin secretion. The 5-HT-induced release of vasopressin and oxytocin was inhibited by central infusion of the 5-HT antagonists WAY 100635 (5-HT(1A)), LY 53857 (5-HT(2A+2C)), ICS 205-930 (5-HT(3+4)) and RS 23597 (5-HT(4)). The 5-HT2+6+7 antagonist metergoline in combination with the 5-HT1A+2+7 antagonist methysergide inhibited the stimulatory effect of 5-CT on both hormones, whereas the 5-HT1A+1B antagonist cyanopindolol only inhibited the oxytocin response. The 5-HT(2A) antagonist 4-(4-flourobenzoyl)-1-(4-phenylbutyl)-piperidine oxalate had no effect on DOI-induced hormone response. The 5-HT(2C) antagonist Y 25130 partly inhibited the stimulating effect of MK-212. ICS 205-930 and RS 23597 inhibited vasopressin and oxytocin secretion induced by RS 67506. WAY 100635 inhibited 8-OH-DPAT-induced oxytocin secretion. We conclude that 5-HT-induced vasopressin secretion primarily is mediated via 5-HT(2C), 5-HT(4) and 5-HT(7) receptors, whereas 5-HT(2A), 5-HT(3) and 5-HT(5A) receptors seem to be of minor importance. 5-HT-induced oxytocin secretion involves 5-HT(1A), 5-HT(2C) and 5-HT(4) receptors; in addition an involvement of 5-HT(1B), 5-HT(5A) and 5-HT(7) receptors seems likely, whereas 5-HT(2A) and 5-HT(3) receptors seem to be less important.     

Effect of forskolin and exogenously administered oxytocin mRNA on oxytocin release by dispersed hypothalamic cultures

Differential vasopressin (VP) gene expression and oxytocin (OT) gene expression were observed in hypothalamic cultures derived from 14-day-old rat fetuses, with VP but not OT being induced by treatment with forskolin and 3-isobutyl-1-methylxanthine. These cultures were used to demonstrate that exogenous VP mRNA could be taken up and translated into releasable VP. In the current studies a similar culture preparation was used to test the hypothesis that, due to the similarity in the mRNA and prohormone structures of VP and OT, the VP-expressing neurons in the cultures would be capable of utilizing exogenous OT mRNA for synthesis of releasable OT. Although OT release was increased by the administration of exogenous OT mRNA, endogenous OT gene expression was also observed. To determine what had induced OT gene expression in the current cultures, the undefined components of the culture preparation, e.g., the glial feeder layer and the serum component of the culture medium, were evaluated. Restraining growth of the glial carpet with cytosine-arabinoside did not alter OT gene expression. Use of a defined medium supplemented with B-27 induced optimal OT gene expression. From this, it is possible to conclude that the components included in B-27 are sufficient for OT gene expression.Factors included in earlier lots of sera may have been responsible for suppression of OT gene expression. Cultures maintained in serum-free, B-27-supplemented medium may provide a useful model system for studying OT gene regulation.     

Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parurient rats

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were giben naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximatelyy doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increase and pituitary stores of this peptide are decreased by day 20 gestation, when labor has not yet begun. In contrast, OT secretion becomes elevated only during delivery. Inhibition of OT release by opioid peptides may: (1) allow preferential release of AVP during pregnancy; and (2) prevent depletion of pituitary stores of OT and neuronal fatigue during the 1–2 h period of parturition in the rat.     

Serotonin stimulates vasoactive intestinal polypeptide release from rat hypothalamus in vitro

The effect of serotonin (5-HT) on the release of immunoreactive vasoactive intestinal polypeptide (VIP) from rat hypothalamus was examined in vitro with a perifusion system. 5-HT (10⁻⁶ M) and high potassium (56 mM) stimulated VIP release in a calcium-dependent manner. VIP release induced by 5-HT was blunted by cyproheptadine (10⁻⁵ M). These findings suggest that 5-HT has a stimulating effect on VIP release from the hypothalamus.     

Serotonin neuronal release from dorsal hippocampus following electrical stimulation of the dorsal and median raphé nuclei in conscious rats

We have studied 5-hydroxytryptamine (5-HT) release in the hippocampal formation following electrical stimulation of the dorsal and median raphé nuclei in the behaving rat. The primary finding in this study is a decrease in neuronal release of serotonin in the dorsal hippocampal formation following electrical stimulation of either the dorsal or median raphé nucleus in conscious rats. At no time did electrical stimulation of either raphé nucleus result in behavioral, including vigilance state, changes. The amount of 5-HT released was found to be frequency dependent with higher frequencies (20 Hz) producing larger decreases in release of 5-HT. However, the pattern of release differs between the two raphé nuclei. Extracellular levels of 5-HT decrease during stimulation of the dorsal raphé, whereas levels decrease only following cessation of stimulation of the median raphé nucleus. This may relate to the patterns of innervation of the dorsal hippocampal formation by these two midbrain raphé nuclei and also may reflect an inhibition of median raphé cell firing during stimulation of the dorsal raphé. Electrical stimulation of the dorsal raphé in anesthetized animals resulted in an enhanced release of 5-HT. The suppression of 5-HT release in the dorsal hippocampal formation in behaving animals was long-lasting (over 2 h), suggesting that the control mechanisms that regulate 5-HT release operate over a long time-course. This difference in release between non-anesthetized and anesthetized animals may relate to anesthesia blocking long- and/or short-loop serotonin recurrent axonal collaterals negatively feeding back onto 5-HT_1A and 5-HT_1D somatodendritic autoreceptors on raphé neurons. Further, the anesthetized animal has diminished monoaminergic “gating” influences on the hippocampal formation, whereas the behaving animal is more complex with behavioral (vigilance) states associated with different patterns of gating of information flow through the hippocampal formation. Hippocampus 1998;8:262–273. © 1998 Wiley-Liss, Inc.     

The effects of ionotropic agonists of excitatory amino acids on the release of arginine vasopressin in rat hypothalamic slices

The effects of ionotropic excitatory amino acids agonists on the release of vasopressin from rat hypothalamic slices were studied. Incubation with increasing doses of NMDA, kainate or AMPA decreased the release of vasopressin in a dose-dependent manner. The values of the IC50 were 1.0, 9.6, or 3.7 x 10-8 M, respectively. The inhibitory effect of the various excitatory amino acids tested was blocked by coincubation with their respective antagonists. Vasopressin secretion was stimulated to 140.3 +/- 7.6% of controls when the slices were obtained from chronically (7 days) salt-loaded rats. Addition of 1 x 10-7 M NMDA or 1 x 10-6 M kainate to the incubation medium antagonized the salt loading-induced increase in vasopressin release. Incubation with 1 x 10-4 M tetrodotoxin did not change basal vasopressin release, but it blocked the decrease in vasopressin secretion induced by 1 x 10-7 M NMDA or 1 x 10-6 M kainate or 1 x 10-6 M AMPA. Incubation with 1 x 10-5 M phaclophen (a GABAB antagonist) and 1 x 10-5 M bicuculline (a GABAA antagonist) was without effect on basal vasopressin secretion while it reversed the inhibition of vasopressin release induced by 1 x 10-7 M NMDA. Incubation with 1 x 10-6 M GABA alone decreased vasopressin secretion to 64.6 +/- 6.9% of control values. The inhibitory effect of GABA did not change when 1 x 10-7 M NMDA was added to the incubation medium. These findings demonstrate that ionotropic excitatory amino acids agonists inhibit vasopressin secretion from hypothalamic slices. They strongly suggest that this inhibitory effect is mediated through local GABAergic interneurones.     

Contrasting effects of neurohypophysial peptides on pyramidal and non-pyramidal neurones in the rat hippocampus

Oxytocin and vasopressin increased the rate of firing of a class of presumed non-pyramidal neurones located in the CA1 area of rat hippocampal slices. This excitatory effect persisted in conditions of synaptic uncoupling. In contrast, pyramidal neurones were either unaffected by neurohypophysial peptides or showed one or several of the following effects: (a) a decrease in firing rate in cells which were spontaneously active; (b) a slight membrane hyperpolarization; and (c) an increase in the rate of occurrence os spontaneous inhibitory postsynaptic potentials. We therefore propose that oxytocin and vasopressin excite directly a class of non-pyramidal inhibitory interneurones, whereas their observed effect on pyramidal neurones is indirect and inhibitory.     

精神分裂症急性期患者5-羟色胺转运体mRNA表达相关因素分析

目的:探讨精神分裂症急性期患者5-羟色胺转运体(SERT)mRNA表达水平及其与临床因素的关系。方法:收集44例未经治疗的精神分裂症急性期患者的临床资料,给予阳性与阴性症状量表(PANSS)评估;采用逆转录聚合酶链反应(RT-PCR)方法测定外周血SERT mRNA表达。结果:有攻击行为患者(26例)SERT mRNA表达明显高于无攻击行为患者(18例)(F=9.75,P0.01)。SERT mRNA表达与患者发病年龄无相关(r=0.056,P=0.722);与病程呈正相关(r=0.418,P0.01);与PANSS的阴性症状分呈负相关(r=-0.335,P0.05);与PANSS总分、阳性症状分及一般精神病理分无相关性(r=-0.156,P=0.317;r=0.115,P=0.465;r=-0.097,P=0.538)。结论:精神分裂症急性期患者SERT mRNA高表达与其病程长、伴有攻击行为以及阴性症状量表评分低有关。     

Endotoxin administration stimulates cerebral catecholamine release in freely moving rats as assessed by microdialysis

In vivo microdialysis was used to measure changes in extracellular concentrations of catecholamines and indolamines in freely moving rats in response to administration of endotoxin (lipopolysaccharide, LPS). Dialysis probes were placed stereotaxically in either the medial hypothalamus or the medial prefrontal cortex. We used a repeated-measures design in which each rat received LPS or saline, and each subject was retested with the other treatment one week later. With the dialysis probes in the medial hypothalamus, intraperitoneal (ip) administration of LPS (5 μg) increased dialysate concentrations of norepinephrine (NE, 187%), dopamine (DA, 119%), and all their measured catabolites, except normetanephrine. Dialysate concentrations of NE and DA were elevated significantly in the fourth or fifth (20 min) collection period with a peak response at around 2 hr. They returned to baseline by about 4 hr. When the dialysis probes were placed in the medial prefrontal cortex, the same dose of LPS also elevated dialysate concentrations of NE and DA, but the increases were much smaller (ca. 20%). However, a dose of 100 μg LPS increased dialysate concentrations of NE and DA from the medial prefrontal cortex to an extent comparable to that of the 5 μg dose in the hypothalamus, and the response was more prolonged. Dialysate concentrations of serotonin could not be measured reliably, but those of its catabolite, 5-hydroxyindoleacetic acid (5-HIAA), were also elevated in both regions. The peak of 5-HIAA occurred at around 4 hr. Pretreatment of the rats with indomethacin (10 mg/kg ip) completely prevented the changes due to 100 μg LPS in the medial prefrontal cortex. These results support earlier neurochemical data suggesting that LPS stimulates the release of both DA and NE in the brain, and probably also release of serotonin. © 1995 Wiley-Liss, Inc.     

Serotonin increases the excitability of the hypothalamic paraventricular nucleus magnocellular neurons

Recent evidence that 5-hydroxytryptamine (5-HT or serotonin) enhances the release and the gene expression of vasopressin and oxytocin in the hypothalamic paraventricular nucleus (PVN) suggests that 5-HT can excite the PVN magnocellular neurons. The objective of this study was to examine the underlying mechanisms for such excitatory action in the electrophysiologically identified hypothalamic PVN magnocellular neurons in rats using whole-cell patch-clamp. We found that 5-HT weakly depolarizes 33.3% of PVN magnocellular neurons in the presence of tetrodotoxin. A minuscule inward current was produced by 5-HT in 48% of the cells, which was attenuated when the 5-HT(4) antagonist GR113808 or the 5-HT(7) antagonist SB269970 was added. In addition, 5-HT reduced the frequency of miniature inhibitory postsynaptic currents in a dose-dependent manner. This inhibition was mimicked by the 5-HT(1B) agonist CP93129, and reversed in the presence of 5-HT(1B) antagonists cyanopindolol and SB224289. Besides, 5-HT induced a biphasic effect on the frequency of miniature excitatory postsynaptic currents, comprising a transient inhibition and a delayed concentration-dependent excitation (onset latency approximately 5 min). The facilitation was mimicked by the 5-HT(2A/2C) agonist DOI and abolished in the presence of the 5-HT(2C) antagonist RS102221. Our findings reveal that 5-HT directly increases the excitability of the PVN magnocellular neurons via multiple receptor subtypes and mechanisms. This may help understanding the regulation of 5-HT-induced hormone release and feeding behavior in the PVN.     

Serotonin release in human cerebral cortex and its modulation via serotonin receptors

Human cerebral cortex slices were prepared from brain tissue which had to be removed in order to gain access to deep-seated tumours. Subsequent to incubation with [3H]serotonin, the slices were superfused with physiological salt solution containing paroxetine, and 3H overflow was evoked by electrical field stimulation. The evoked tritium overflow (86% of which was accounted for by unmetabolized [3H]serotonin) was abolished by tetrodotoxin or omission of calcium from the superfusion fluid. Unlabelled serotonin decreased, and the serotonin receptor antagonist metitepin increased, the evoked overflow. The inhibition produced by serotonin was antagonized by metitepin. It is concluded that serotonin release in human cerebral cortex is modulated by inhibitory serotonin receptors, which may be localized presynaptically on the serotoninergic nerve fibers themselves. There are marked similarities between human and rat brain cortex with respect to action potential-induced, Ca2+-dependent serotonin release and its modulation via serotonin receptors.     

Increased levels of 5HT2A receptor mRNA expression in peripheral blood mononuclear cells of patients with major depression: correlations with severity and duration of illness

Background: Neuroimaging, immunologic, and pharmacologic studies have emphasized the role of 5-HT2A and 5-HT3A serotonin receptors in the pathophysiology of major depression.

Aim: The aim of this study was to measure the relative expression of 5-HT2A and 5-HT3A receptor mRNA in peripheral blood mononuclear cells (PBMCs) of patients with major depressive disorder (MDD).

Method: 5-HT2A and 5-HT3A receptor mRNA expressions were examined in PBMCs of 25 medication-naïve-patients with MDD, 25 medication-free MDD patients, and 25 healthy controls. 5-HT2A and 5-HT3A receptor mRNA expressions were measured using real-time quantitative PCR. This study evaluated patients’ clinical symptoms using the Hamilton Depression Rating Scale-17 items (HDRS) and the Beck Depression Inventory (BDI).

Results: Relative 5-HTR2A mRNA expression was significantly higher in PBMCs of all MDD patients when compared with healthy controls (Z = ?3.875, p?Z = ?1.328, p?>?0.05). MDD patients showed significant correlations between 5-HTR2A mRNA expression and HDRS scores (rs?=?0.902, p?rs?=?0.878, p?Conclusion: This study showed that depressed patients, irrespective of treatment, have higher 5-HTR2A mRNA levels in PBMCs than healthy subjects. It also provided evidence that 5-HTR2A mRNA levels in PBMCs of MDD patients could be associated with the severity of depression and the duration of the illness.     

Demonstration of glucocorticoid receptor-like immunoreactivity in glucocorticoid-sensitive vasopressin and corticotropin-releasing factor neurons in the hypothalamic paraventricular nucleus

Many parvocellular neurons in the paraventricular nucleus of the hypothalamus express high levels of corticotropin releasing factor (CRF) or vasopressin following adrenalectomy. To determine whether glucocorticoids feed back directly on these neurons, a mouse monoclonal antibody directed against the rat liver glucocorticoid receptor was used in combination with polyclonal antisera directed against either vasopressin or CRF to permit simultaneous visualization of either peptide with glucocorticoid receptor-like immunoreactivity (IR). Rats were adrenalectomized (ADX) for 2 weeks to optimize numbers of vasopressin - and CRF-IR neurons. Six hours prior to sacrifice, a separate group of adrenalectomized rats was treated with corticosterone (40 mg/kg). This short-term replacement resulted in nuclear localization of glucocorticoid receptor-like-IR but did not attenuate the increased numbers of CRF- and vasopressin-IR neurons observed after adrenalectomy. It was therefore possible to visualize vasopressin- or CFR-IR and nuclear glucocorticoid receptor-like-IR simultaneously. Cell counts of double-labeled neurons in the paraventricular nucleus of the hypothalamus (PVH) demonstrated that glucocorticoid receptor-like-IR is colocalized in virtually all the CRF and vasopressin immunoreactive parvocellular neurons studied, which respond to adrenalectomy by increased peptide expression. These data suggest that a major feedback effect of glucocorticoids on the hypothalamic-pituitary-adrenal axis is exerted directly within nuclei of CRF and vasopressin neurons.     

Neuropeptide expression in rat paraventricular hypothalamic neurons that project to the spinal cord

The paraventricular hypothalamic nucleus (PVH) exerts many of its regulatory functions through projections to spinal cord neurons that control autonomic and sensory functions. By using in situ hybridization histochemistry in combination with retrograde tract tracing, we analyzed the peptide expression among neurons in the rat PVH that send axons to the spinal cord. Projection neurons were labeled by immunohistochemical detection of retrogradely transported cholera toxin subunit B, and radiolabeled long riboprobes were used to identify neurons containing dynorphin, enkephalin, or oxytocin mRNA. Of the spinally projecting neurons in the PVH, approximately 40% expressed dynorphin mRNA, 40% expressed oxytocin mRNA, and 20% expressed enkephalin mRNA. Taken together with our previous findings on the distribution of vasopressin-expressing neurons in the PVH (Hallbeck and Blomqvist [1999] J. Comp. Neurol. 411:201-211), the results demonstrated that the different PVH subdivisions display distinct peptide expression patterns among the spinal cord-projecting neurons. Thus, the lateral parvocellular subdivision contained large numbers of spinal cord-projecting neurons that express any of the four investigated peptides, whereas the ventral part of the medial parvocellular subdivision displayed a strong preponderance for dynorphin- and vasopressin-expressing cells. The dorsal parvocellular subdivision almost exclusively contained dynorphin- and oxytocin-expressing spinal cord-projecting neurons. This parcellation of the peptide-expressing neurons suggested a functional diversity among the spinal cord-projecting subdivisions of the PVH that provide an anatomic basis for its various and distinct influences on autonomic and sensory processing at the spinal level.     

Chronic ethanol ingestion decreases vasopressin mRNA in hypothalamic and extrahypothalamic nuclei of mouse brain

Endogenous arginine vasopressin was previously shown to modulate the rate of loss of functional (CNS) tolerance to ethanol, suggesting that chronic ethanol ingestion might alter vasopressin synthesis and/or release. Since extrahypothalamic vasopressin is believed to be involved in the CNS effects of the peptide, we determined the effect of ethanol on vasopressin mRNA in the bed nucleus of the stria terminalis (BST), as well as in several hypothalamic nuclei. Chronic ethanol ingestion, that produced functional tolerance and physical dependence in mice, resulted in decreased vasopressin mRNA levels in all areas examined. In contrast, as expected, dehydration resulted in increases in vasopressin mRNA in the BST and in all hypothalamic nuclei except the suprachiasmatic nucleus. In the BST, both ethanol ingestion and dehydration affected cells in the central region of the nucleus, while cells in the caudal portion were only affected by ethanol treatment. The results indicate that chronic ethanol ingestion generally reduces the synthesis of vasopressin, and that increased vasopressin synthesis is not necessary in order for the peptide to affect ethanol tolerance.     

Plasticity of neurohypophysial terminals with increased hormonal release during dehydration: ultrastructural and biochemical analyses

Arginine vasopressin- (AVP) and oxytocin- (OXT) secreting magnocellular neurons undergo gross structural changes with chronic physiological stimulation. Here, we investigated subcellular aspects of plasticity in rat neurohypophysial terminals during dehydration. Ultrastructural analyses demonstrated that chronic dehydration by 2% NaCl drinking for 7 days significantly decreased the numbers of neurosecretory granules and microvesicles but not the numbers of mitochondria. Moreover, in dehydrated rats, terminals making neurovascular contacts enlarged, whereas terminals in apposition to astrocytes, i.e., neuroglial contacts, became smaller. Western blot analyses demonstrated significant decreases in the levels of F3 and Thy-1 together with those of AVP- and OXT-neurophysin, but the levels of synaptophysin, SNAP-25, and GAP-43 were unchanged. Both F3 and Thy-1 were recovered in the buffer-insoluble pellet, and phosphatidyl inositol-specific phospholipase C treatment released both molecules from the crude membrane fraction, indicating that they are attached to terminal membranes by glycosylphosphatidyl inositol anchors. Confocal microscopic observations demonstrated that F3 colocalized with Thy-1 in the same terminals of magnocellular neurons. In contrast, the level of calretinin, a Ca(2+) binding protein was significantly increased with chronic dehydration. Thus, the present results suggest that enhancement of neurovascular contacts results from rearrangement of terminal-astrocyte and terminal-vessel contacts rather than enlargement or sprouting of magnocellular terminals themselves. The down-regulation of F3 and Thy-1 may contribute to enhancement of neurovascular contacts that accompany increased peptide release during dehydration.     

Gene structure and expression of the mouse 5-HT2 receptor.

Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter which mediates numerous physiological functions. Using the SacI-EcoRI restriction fragment of the rat brain 5-HT2 receptor cDNA as a probe, we have screened a mouse brain cDNA library, created by random priming and constructed in SWAJ vectors, and have isolated a cDNA encoding a 1.4 kb open reading frame which codes for a functional mouse 5-HT2 receptor identified from pharmacological binding profiles and coupling of phosphoinositide formation in a stably transfected fibroblast cell line. The deduced amino acid sequence is 97.4% identical to the rat 5-HT2 receptor. Using the same 5-HT2 receptor cDNA probe, ten positive genomic clones were isolated from two mouse genomic libraries constructed in the pWE15 cosmid vector and the EMBL-3 phage vector. Extensive mapping and sequencing of these genomic clones indicate the mouse 5-HT2 receptor coding region spans over 20 kb and is composed of three exons split by two introns. Northern blot analysis shows one band of 5-6 kb in the mouse brain, but not in the heart, lung, liver, or kidney total RNA. Southern analysis of mouse liver genomic DNA shows a simple pattern of digestion by several restriction enzymes, which suggests that one copy of the 5-HT2 receptor gene may exist in the mouse genome.     

Inducible Expression of Tryptophan Hydroxylase without Serotonin Synthesis in Hypothalamic Dopaminergic Neurons

In the present study we have further studied the previous findings that rat hypothalamic dopaminergic neuronal cell groups may express tryptophan hydroxylase (TpH), the serotonin synthesizing enzyme, without a detectable serotonin synthesis. Chemical and mechanical neuronal injuries, namely colchicine treatment and axonal transection, respectively, were performed, and distributions of neurons exhibiting immunoreactivity for TpH and/or tyrosine hydroxylase (TH), the dopamine synthesizing enzyme, were analyzed throughout the hypothalamic periventricular and arcuate nuclei. After colchicine treatment there was a statistically significant 87% (P = 0,01) increase in the number of TpH expressing neurons, while TH expression remained essentially similar. Axonal transection resulted also in a statistically significant 131% (P < 0,01) increase in the number of TpH expressing neurons, while TH expression was not significantly altered. All TpH expression coexisted with TH expression, and the induction of TpH expression by neuronal injuries occurred evenly throughout the rostrocaudal length of the territory studied. A possible serotonin synthesis by TpH was examined by giving drugs that increase brain serotonin synthesis, but no immunohistochemically detectable serotonin synthesis could be found in any of the TpH expressing neurons. Finally the possibility was studied that the relative shortage of the cofactor tetrahydrobiopterin would limit serotonin synthesis. However, an administration of tetrahydrobiopterin did not result in detectable serotonin synthesis in these neurons. Taken together these results suggest that dopaminergic neurons in the hypothalamic periventricular and arcuate nuclei are able to express TpH, this expression is induced after neuronal injury, and this induction occurs similarly throughout the territories studied. TpH expression occurs independently of TH expression, and the newly expressed TpH appears not to synthesize serotonin, regardless of pharmacological pretreatments. Thus, our findings (i) support the idea that neurons may possess inducible expression of nonfunctional transmitter-synthesizing enzymes, in this case TpH, and (ii) suggest that expression of an enzyme synthesizing a certain transmitter may not necessarily imply the corresponding transmitter phenotype.