Local corticosterone infusion enhances nocturnal pineal melatonin production in vivo

Melatonin, an important marker of the endogenous rhythmicity in mammals, also plays a role in the body defence against pathogens and injuries. In vitro experiments have shown that either pro- or anti-inflammatory agents, acting directly in the organ, are able to change noradrenaline-induced pineal indoleamine production. Whereas corticosterone potentiates melatonin production, incubation of the gland with tumour necrosis factor-alpha decreases pineal hormonal production. In the present study, we show that nocturnal melatonin production measured by intra-pineal microdialysis is enhanced in pineals perfused with corticosterone at concentrations similar to those measured in inflamed animals. In vitro experiments suggest that this enhancement may be due to an increase in the activity of the two enzymes that convert serotonin to N-acetylserotonin (NAS) and NAS to melatonin. The present results support the hypothesis that the pineal gland is a sensor of inflammation mediators and that it plays a central role in the control of the inflammatory response.

The serotonergic inhibition of slowly bursting cells in the intergeniculate leaflet of the rat

Electrophysiological studies combined with local neurotoxic lesions were conducted on anaesthetized rats in order to determine whether the dorsal raphe nucleus (DRN) inhibits the intergeniculate leaflet (IGL) of the lateral geniculate nucleus by means of innervation by serotonin-containing fibres. In the control animals, electrical stimulation of the DRN induced the long-latency and long-lasting inhibition of the neuronal firing of the IGL cells that are characterized by rhythmic, slow-bursting activity in light conditions. The electrical destruction of the DRN resulted in an increase in the firing rate of the recorded IGL cells, whilst at the same time not affecting the rhythmic, bursting pattern of the activity. In the second group of animals, local neurotoxic lesion of serotonergic fibres was performed by injection of the toxin 5,7-dihydroxytryptamine into the IGL. After 10 days of postoperative recovery, electrophysiological experiments were performed on the toxin-treated rats. In these animals, electrical stimulation as well as electrical lesion of the DRN did not induce any change in the firing of the slowly bursting cells in the 5,7-dihydroxytryptamine-injected IGL. The results obtained provide evidence that inhibition of the IGL slowly bursting cells, by innervation from the dorsal raphe, is mediated by the release of serotonin. Furthermore, the observed serotonergic inhibition of the light-dependent activity of slowly bursting cells can contribute to the neuronal mechanism gating the information that flows through this nucleus to the vestibular, visuomotor, circadian and sleep/arousal systems, with which the IGL is strongly interconnected.

Circadian change in tryptophan hydroxylase protein levels within the rat intergeniculate leaflets and raphe nuclei

Serotonin (5-HT) is involved in the synchronisation of the mammalian circadian clock located in the suprachiasmatic nuclei of the hypothalamus (SCN). This clock is synchronised by light (photic cues) and by non-photic cues. Non-photic cues are notably conveyed to the SCN by a direct 5-HT pathway arising from the mesencephalic median raphe nucleus (MRN). Furthermore, an indirect projection conveys non-photic inputs by 5-HT fibres from the mesencephalic dorsal raphe nucleus (DRN) to the intergeniculate leaflets of the thalamus (IGL) which project to the SCN. In the rat, the quantitative distribution of tryptophan hydroxylase (TpH), used as an index of 5-HT synthesis, was studied by in situ immunoautoradiography in both the serotoninergic cell bodies area of the raphe nuclei and the serotoninergic terminal field of the IGL. Under a 12 h light: 12 h dark (LD 12:12), TpH protein amount exhibited a rhythmic variation within the IGL. The maximum levels were reached at the day/night transition. In both MRN and the lateral groups of the DRN, TpH variations were opposite to those observed in the IGL. Such phase opposition was reported previously in the MRN/SCN pathway and was correlated with a rhythmic release of 5-HT within the SCN [Eur J Neurosci 15 (2002) 833]. Thus, the daily rhythmicity of TpH levels observed in DRN-IGL pathway may be correlated with a rhythmic release of 5-HT in the IGL at the beginning of the night. Under constant darkness, TpH rhythmic variations in the two serotoninergic pathways were maintained and similar to those observed under light/dark cycle. These results demonstrate the existence of a circadian endogenous functioning in the 5-HT neurones projecting to the rat circadian system.

Ca increase in secretory granules of stimulated mast cells

The elemental content of rat peritoneal mast-cell secretory granules has been measured by X-ray micro-analysis. Two distinct categories of granules were analyzed: intact granules, seen in control samples, and spumous granules, corresponding to exocytosed granule matrices. The average Ca content of intact granules was found to be approximately equal to cytosolic concentration, and to increase up to 40-fold in spumous granules. A significant increase was also observed for Na and Cl. These changes were not observed (for Ca) or weaker (for Na and Cl) if the cells had been challenged in the absence of nominal extracellular Ca; in this case, there was also a significant decrease in the sulphur content, suggesting a partial dispersion of the organic matrix components. In exocytosed granule matrices, in the presence but not in the absence of extracellular Ca, a slow and long-lasting increase of intragranular free Ca was monitored by changes in the fluorescence of the Ca-sensitive probes Fluo-3 and Calcium Green-5N, accumulated within rat mast-cell secretory granules. These findings are discussed along two lines: It is proposed that the calcium uptake by the exocytosed mast-cell granule matrices can have a physiological relevance for the surrounding tissue. Mast-cell granules do not disperse after exocytosis. The major uptake of Ca which is seen after opening of the exocytotic pore could be responsible for the exceptional stability of the externalized matrices.

Plasticity of tyrosine hydroxylase gene expression within BALB/C and C57Black/6 mouse locus coeruleus

The plasticity of tyrosine hydroxylase (TH) phenotype in the locus coeruleus (LC) of two pure inbred strains of mice, Balb/C (C) and C57Black/6 (B6), was investigated at the molecular level by radioactive in situ hybridization. The results demonstrated that in basal conditions, C mouse LC contains less TH-mRNA-expressing cells than B6. After RU 24722-treatment, which induces long lasting TH gene expression in the LC, we previously reported an increase in TH-expressing cell number in C mouse LC only, equalizing TH phenotype between the two strains. Here, we demonstrate that strain specific plasticity of TH phenotype detected in spatially organized cells is associated with the regulation of TH-mRNA expression above a detectable level. These results suggest that interstrain differences and pharmacologically-induced phenotypic plasticity in TH phenotype may occur at the transcriptional level.

p-Chlorophenylalanine-induced alteration of somatodendritic levels of tryptophan hydroxylase within the rat mesencephalic raphe nuclei

Tryptophan hydroxylase distribution was examined across the nuclei raphe dorsalis, medianus, and pontis of the adult rat, under basal conditions and 2 days after a single injection of p-chlorophenylalanine, an irreversible tryptophan hydroxylase inhibitor. Tryptophan hydroxylase-expressing cells were numbered in transverse sections processed for immunohistochemistry, and the area of tryptophan hydroxylase distribution was delineated in adjacent sections transferred onto nitrocellulose and processed for immunoautoradiography. Two distinct areas were visualized: an inner zone, corresponding to the area displaying tryptophan hydroxylase-immunoreactive cells (so-called somatic area), and an outer zone, here called perisomatic, devoid of perikarya yet rich in tryptophan hydroxylase-positive neuropil in the histological sections. After treatment with p-chlorophenylalanine, a significant decrease in the number of tryptophan hydroxylase-immunoreactive cells could be observed only in the rostral raphe dorsalis, particularly within its ventromedian and dorsomedian subdivisions. In all raphe nuclei, the topological reconstruction of the somatic area was not modified. Based on the densitometric measurements in the immunoautoradiographs, however, a dramatic decrease in the content, concentration, and volume of expression of tryptophan hydroxylase could be documented in the three raphe nuclei. Detailed analysis of these results led to the conclusion that (a) tryptophan hydroxylase expression is differentially regulated in different serotoninergic cell body subpopulations of the raphe, some of which are more sensitive to p-chlorophenylalanine, and (b) distribution of tryptophan hydroxylase protein is modified also in the somatodendritic area in all raphe nuclei.

Ipsilateral alterations in tryptophan hydroxylase activity in rat brain after hypothalamic 5,7-di-hydroxytryptamine lesion

The in vivo relationship between the amounts of tryptophan hydroxylase (TPH) protein and its intrinsic synthetic activity, measured by quantifying the amounts of alpha-[3H]methyl-5-hydroxytryptamine (alpha-[3H]M5-HT), is reported in cell body and terminal areas of intact and disturbed serotonergic neurons following a unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesion of the dorsolateral hypothalamus. Five days after the lesion, the relationships between TPH and its synthetic product 5-HT were evaluated on adjacent brain sections in serotonergic cells bodies of the dorsal raphe nucleus (DRN) and nerve fibres of the medial forebrain bundle (MFB). On the side contralateral to the lesion, TPH and alpha-[3H]M5-HT levels in the intact hemi-DRN exhibited a caudo-rostral distribution and were positively and significantly correlated (p < or = 0.001); the calculated TPH-specific activity was 0.76 nCi of alpha-[3H]M5-HT formed per U TPH. In the MFB, quantitative measurements of TPH and alpha-[3H]M5-HT showed no correlation between enzyme and product and no specific activity for TPH could be determined. On the side ipsilateral to the lesion, the density of TPH-immunoreactive fibers was drastically decreased in the dorsolateral hypothalamus where a significant reduction in TPH content (45.5% of control side, P < 0.001) was found. In the overall ipsilateral hemi-DRN, TPH and alpha-[3H]M5-HT levels, their correlation as well as TPH-specific activity were unaltered by the lesion but a significant increase in alpha-[3H]M5-HT and TPH contents was observed in the lateral wings of the DRN. The lesion also induced a significant increase in alpha-[3H]M5-HT and TPH levels (136% and 93.8%, P < 0.001, respectively) in the ipsilateral MFB, which resulted in a positive and significant correlation between these two markers and yielded a TPH-specific activity of 1.0 nCi of alpha-[3H]M5-HT formed per U TPH. TPH topological area was also significantly increased in the lateral aspect of the ipsilateral MFB 5 days post lesion. These results show that 5-HT synthesis in the intact DRN is proportional to and dependent on TPH activity while in the MFB, 5-HT accumulation appears unrelated to TPH content which is most likely in an inactive enzymatic form. Moreover, the data show that a local disruption of serotonergic terminals in the dorsolateral hypothalamus does not affect 5-HT synthesis in the overall ipsilateral DRN neurons but results in local activation of TPH within the serotonergic projection neurons and the ipsilateral MFB, as evidenced by active de novo synthesis of 5-HT. Altogether the results point to circumscribed activation of compensatory mechanisms in 5-HT synthesis after selective destruction of serotonergic terminals.

Changes in steady-state levels of tryptophan hydroxylase protein in adult rat brain after neonatal 6-hydroxydopamine lesion

A recently developed technique of immunoautoradiography on nitrocellulose transfers of serial frozen sections was used to determine tryptophan hydroxylase concentration in selected areas of the adult rat brain following neonatal 6-hydroxydopamine destruction of nigrostriatal dopamine neurons. Particular attention was paid to the neostriatum, known to be serotonin-hyperinnervated under these conditions, and to the nucleus raphe dorsalis, containing the cell bodies of origin for these nerve terminals. The hippocampus was also investigated as a territory of structurally intact serotonin innervation arising primarily from the nucleus raphe medianus. Tryptophan hydroxylase protein was measured at successive transverse levels across the entire caudorostral extent of all these regions. Similar measurements of tyrosine hydroxylase protein across the substantia nigra and the neostriatum verified the disappearance of the nigrostriatal dopamine neurons. The average tryptophan hydroxylase tissue concentration in the dorsal third of the serotonin-hyperinnervated neostriatum was up by 36% above control, i.e. significantly less than the number of its serotonin axon terminals or varicosities. This was therefore indicative of a lowering of the tryptophan hydroxylase protein content per serotonin ending. Interestingly, a tight correlation between the respective level-by-level concentrations of tryptophan hydroxylase and tyrosine hydroxylase protein in the control neostriatum allowed the prediction the tryptophan hydroxylase concentration after dopamine denervation with a serotonin hyperinnervation. Tryptophan hydroxylase concentration was also significantly reduced in both the nucleus raphe dorsalis and nucleus raphe medianus, notably at those raphe dorsalis levels known to give rise to the serotonin hyperinnervation of neostriatum. It is hypothesized that the lower steady-state level of tryptophan hydroxylase inside the terminals and cell bodies of hyperinnervating serotonin neurons was the result of a feedback inhibition of the synthesis of the enzyme by its end-product, presumably because of the increased amount of serotonin in these terminals.