Serotonin depletion by p-chlorophenylalanine decreases VIP mRNA in the suprachiasmatic nucleus

The effects of aging and chronic fluoxetine treatment on circadian rhythms and suprachiasmatic nucleus expression of neuropeptide genes and 5-HT1B receptors

Age-related changes in circadian rhythms, including attenuation of photic phase shifts, are associated with changes in the central pacemaker in the suprachiasmatic nucleus (SCN). Aging decreases expression of mRNA for vasoactive intestinal peptide (VIP), a key neuropeptide for rhythm generation and photic phase shifts, and increases expression of serotonin transporters and 5-HT(1B) receptors, whose activation inhibits these phase shifts. Here we describe studies in hamsters showing that aging decreases SCN expression of mRNA for gastrin-releasing peptide, which also modulates photic phase resetting. Because serotonin innervation trophically supports SCN VIP mRNA expression, and serotonin transporters decrease extracellular serotonin, we predicted that chronic administration of the serotonin-selective reuptake inhibitor, fluoxetine, would attenuate the age-related changes in SCN VIP mRNA expression and 5-HT(1B) receptors. In situ hybridization studies showed that fluoxetine treatment does not alter SCN VIP mRNA expression, in either age group, at zeitgeber time (ZT)6 or 13 (ZT12 corresponds to lights off). However, receptor autoradiographic studies showed that fluoxetine prevents the age-related increase in SCN 5-HT(1B) receptors at ZT6, and decreases SCN 5-HT(1B) receptors in both ages at ZT13. Therefore, aging effects on SCN VIP mRNA and SCN 5-HT(1B) receptors are differentially regulated; the age-related increase in serotonin transporter sites mediates the latter but not the former. The studies also showed that aging and chronic fluoxetine treatment decrease total daily wheel running without altering the phase of the circadian wheel running rhythm, in contrast to previous reports of phase resetting by acute fluoxetine treatment.

Long‐lasting effects of serotonin deficiency on differentiating peptidergic neurons in the rat suprachiasmatic nucleus

Serotonin (5-HT, 5-hydroxytryptamine) is known to be an inductor of the brain development [Whitaker-Azmitia, P.M., Druse, M., Walker, P., Lauder, J.M., 1996. Serotonin as a developmental signal. Behav. Brain Res. 73, 19-29; Ugrumov, M.V., 1997. Hypothalamic monoaminergic systems in ontogenesis: development and functional significance. Int. J. Dev. Biol. 41, 809-816]. This study was aimed to test whether it provides long-lasting effects on the differentiating vasoactive intestinal polypeptide (VIP) and vasopressin (VP) neurons of the suprachiasmatic nucleus (SCN) in rats. To this aim, 5-HT was depleted in fetal brain by daily injections of p-chlorophenylalanine (pCPA), an inhibitor of 5-HT synthesis, to pregnant rats from the 13th to the 21st day of gestation. Pregnant rats injected with saline served as controls. The offsprings (males) of pCPA-treated and control pregnant rats were maintained after birth for two months under normal laboratory conditions. Then, the SCN was processed for immunocytochemistry of VIP and VP and in situ hybridization of appropriate mRNAs. There were no differences in concentrations of VIP and VP mRNAs in the SCN in adult offsprings of the 5-HT-depleted pregnant rats compared to the controls. Moreover, 5-HT deficiency did not induce any change in size of VIP-immunoreactive (IR) and VP-IR neurons. Conversely, both the numbers of VIP- and VP-immunoreactive neurons and concentrations of the peptides in cell bodies increased significantly. It is concluded that 5-HT provides long-lasting effects on differentiating VIP and VP neurons in the SCN resulting in attenuated release rather than elevated synthesis of both peptides in adulthood.

Postnatal regulation by monoamines of vasopressin expression in the neuroendocrine hypothalamus of MAO-A-deficient mice

We studied the influence of noradrenaline (NA) and serotonin (5-HT) on arginine-vasopressin (AVP) expression in the mouse neuroendocrine hypothalamus during the postnatal period. We used 11-day-old transgenic Tg8 mice knock-out for the monoamine oxidase A gene, which are characterized by increased amounts of NA (two-fold) and 5-HT (nine-fold) in the brain compared with wild-type littermates. AVP expression, determined by enzyme immunoassay and in situ hybridization, was increased in the suprachiasmatic nucleus (SCN), decreased in the supraoptic nucleus (SON), and unchanged in the paraventricular nucleus of Tg8 mice compared with wild-types. Inhibiting NA synthesis by injecting alpha-methylparatyrosine to Tg8 mice, AVP levels were decreased in the SCN but increased in the SON. Moreover, the administration of parachlorophenylalanine, a 5-HT synthesis inhibitor, was associated with increased AVP contents in the SCN only. Together, these data show a marked region-specific sensitivity of AVP expression to NA and 5-HT during the postnatal period in the mouse hypothalamus.

Monoaminergic control of vasopressin and VIP expression in the mouse suprachiasmatic nucleus

We studied the effects of serotonin and noradrenaline on the expression of arginine-vasopressin (AVP) and vasoactive intestinal peptide (VIP) in the suprachiasmatic nucleus (SCN). We used transgenic Tg8 mice knockout for the MAO-A (monoamine oxidase A) gene, which are characterized by increased amounts of serotonin and noradrenaline in brain compared to wild-type mice (C3H). The MAO-A deficiency caused an increase in AVP and VIP expression (determined by immunohistochemistry, enzyme immunoassay, and in situ hybridization) compared to C3H mice. The number of peptidergic neurons was also increased. Inhibiting serotonin or noradrenaline synthesis in Tg8 mice by the administration of parachlorophenylalanine or alpha-methylparatyrosine, respectively, the amounts of AVP, VIP and their mRNAs were decreased, but not the number of peptidergic neurons. This study indicates that serotonin and noradrenaline stimulate AVP and VIP expression, and could participate in the differentiation of the neurochemical phenotype in the mouse SCN.

Serotonin modulates expression of VIP and GRP mRNA via the 5-HT(1B) receptor in the suprachiasmatic nucleus of the rat

The expression of vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) in the suprachiasmatic nucleus (SCN) changes depending on light. VIP mRNA increases and GRP mRNA decreases in the light phase, while they do not show change without light. In the present study we investigated the involvement of serotonin (5-HT) in the expression of VIP and GRP messenger RNA in the SCN of the rat. The decrease in VIP mRNA and the increase in GRP mRNA in the light phase were amplified by 5-HT depletion using 5,6-dihydroxytryptamine injected into the lateral ventricle. These enhancements due to 5-HT depletion were reversed to control levels by applying 5-HT(1B) agonists TFMPP and CGS12066A, but not a 5-HT(1A)/5-HT(7) agonist, 8-OH-DPAT. The 5-HT(1B) receptor is known to exist on the terminals of the retinohypothalamic tract (RHT). Therefore, next we investigated the morphological relationship of RHT and 5-HT terminals by double-labeling immunocytochemistry and demonstrated that 5-HT-immunoreactive fibers and cholera toxin B subunit-labeled RHT terminals were intermingled in the ventrolateral SCN, and 5-HT axon processes had close contact with RHT terminals. Collectively, these pharmacological and morphological results suggest that 5-HT afferents from raphe nuclei modulate VIP and GRP expression in neurons of the ventrolateral SCN by activating the 5-HT(1B) receptor in the RHT.

Aging selectively suppresses vasoactive intestinal peptide messenger RNA expression in the suprachiasmatic nucleus of the Syrian hamster

Aging leads to many changes in the expression of circadian rhythms, including reduced amplitude, altered relationship to the environmental illumination cycle, and reduced sensitivity to phase resetting signals. Neuropeptide synthesizing neurons in the suprachiasmatic nucleus (SCN), the principal circadian pacemaker in mammals, play a role in regulating pacemaker function and in coupling the pacemaker to overt circadian rhythms. Aging may alter the activity of neuropeptide neurons in the SCN, which could be reflected in changes in mRNA expression. Therefore, this study investigated whether aging alters the level or rhythm of expression of neuropeptide mRNAs in the SCN of male Syrian hamsters, a well established model for the study of age-related changes in circadian rhythms. Three age groups of hamsters (young [3--5 months old], middle-aged [12--15 months old] and old [19--22 months old] were sacrificed at five times of day. Their brains were dissected and sections through the suprachiasmatic nucleus were prepared and used for in situ hybridization for mRNAs for vasoactive intestinal peptide (VIP), arginine vasopressin (AVP) and somatostatin (SS). Aging selectively decreased the SCN expression of VIP mRNA without affecting AVP mRNA or SS mRNA. Also, only AVP mRNA expression exhibited a robust 24-h rhythm, in contrast to previous findings in other species that VIP mRNA and SS mRNA, as well as AVP mRNA, exhibit 24-h rhythms in the SCN. The present findings suggest that age-related reductions in VIP mRNA expression may contribute to the alterations in entrainment and attenuated sensitivity to phase resetting signals that are characteristic of aging. Furthermore, the results demonstrate that neuropeptide gene expression in the SCN is differentially regulated by aging and varies among species.

5-hydroxytryptamine1A receptor-mediated effects on adenylate cyclase and nitric oxide synthase activities in rat ventral prostate

The rat ventral prostate possesses specific 5-hydroxytryptamine (5-HT1A) receptors coupled to adenylate cyclase. In vivo treatment of rats or in vitro preincubation of minced prostatic tissue with the 5-HT1A receptor agonist 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT) in different experimental conditions shows the possibility of desensitisation mechanisms with switching from inhibitory to stimulatory pattern on adenylate cyclase activity. As in the majority of systems, we observed the inhibition of forskolin-stimulated adenylate cyclase activity as a functional correlate of 5-HT1A receptor activation. A similar feature occurred when the direct stimulation of the enzyme by the diterpene was replaced by a receptor-mediated activation with the neuropeptide vasoactive intestinal peptide. Furthermore, 8-OH-DPAT stimulated nitric oxide synthase (NOS) activity in a dose-dependent manner. Thus, serotonin appears to be able to act in the rat prostate gland through specific 5-HT1A receptors coupled to a complex system of signal transduction involving an inhibitory response of adenylate cyclase that can become stimulatory, as well as an enhancement of NOS activity.

Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats

Our laboratory has shown that the ability of the suprachiasmatic nuclei (SCN) to regulate a number of rhythmic processes may be compromised by the time females reach middle age. Therefore, we examined the effects of aging on the rhythmic expression of two neuropeptides synthesized in the SCN, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP), using in situ hybridization. Because both VIP and AVP are outputs of the SCN, we hypothesized that age-related changes in rhythmicity are associated with alterations in the patterns of expression of these peptides. We found that VIP mRNA levels exhibited a 24 hr rhythm in young females, but by the time animals were middle-aged, this rhythm was gone. The attenuation of rhythmicity was associated with a decline in the level of mRNA per cell and in the number of cells in the SCN producing detectable VIP mRNA. AVP mRNA also showed a robust 24 hr rhythm in young females. However, in contrast to VIP, the AVP rhythm was not altered in the aging animals. The amount of mRNA per cell and the number of cells expressing AVP mRNA also was not affected with age. Based on these results we conclude that (1) various components of the SCN are differentially affected by aging; and (2) age-related changes in various rhythms may be attributable to changes in the ability of the SCN to transmit timing information to target sites. This may explain why the deterioration of various rhythmic processes occurs at different rates and at different times during the aging process.

Extracellular serotonin in the striatum increased after immobilization stress only in the nighttime

The release of serotonin (5-HT) in the striatum and the motor activity of rats given the immobilization stress were measured using in vivo microdialysis techniques and locomotion counts. Serotonin release in the striatum did not change in the daytime and nighttime, but motor activity in the nighttime was significantly higher than in the daytime. Serotonin release was not significantly increased during immobilization stress in the daytime or nighttime. In the nighttime, however, after the end of stress, 5-HT release was significantly increased from 0.002 ng/dialysate to 0.47 ng/dialysate. The motor activities were also significantly increased after the stress in the nighttime. These results suggest that the effects of immobilization stress on serotonin release in the striatum were different in the daytime and nighttime. In the nighttime, serotonin release in the striatum and the motor activities increased not during but after immobilization stress.

Restricted daytime feeding modifies suprachiasmatic nucleus vasopressin release in rats

The authors have shown previously that vasopressin (VP) release from suprachiasmatic nucleus (SCN) efferents in rats is important for the timing of the circadian activity of the hypothalamo-pituitary-adrenal (HPA) axis, resulting in a circadian rise in corticosterone at dusk. When meals are supplied at a fixed time during the light period, however, this normal circadian activity of the HPA axis is strongly modified. Under such a restricted feeding regimen, a corticosterone peak appears just before the daily meal in addition to the circadian corticosterone peak at dusk. This feeding-associated rise in corticosterone is regarded as an SCN-independent circadian rhythm because it is sustained in SCN-lesioned animals. Despite these previous results, the authors investigated a putative involvement of SCN-derived VP in the control of the prefeeding corticosterone peak by measuring the intranuclear release of VP in the SCN and plasma corticosterone levels in rats in ad libitum feeding conditions as well as in animals that were obliged to feed during a 2-h period in the middle of the light period. Restricted daytime feeding caused clear changes in the daily release pattern of VP from SCN terminals. Both a delayed onset of the diurnal rise and a premature decline of the elevated daytime levels were observed, but the acrophase of the VP rhythm was not phase shifted. Concerning the circadian corticosterone peak, no phase shift of its acrophase was observed either. It is concluded that (1) restricted daytime feeding does affect SCN activity, (2) intranuclear release of VP within the SCN is an important mechanism to amplify and synchronize the circadian rhythms as dictated by the light/dark-entrained circadian pacemaker, and (3) VP release observed in animals on restricted feeding is completely compatible with the previously proposed inhibitory action of SCN-derived VP on the HPA axis.

Increase of 5HT and VIP immunoreactivity within the hamster (Mesocricetus auratus) SCN during chronic MAOI treatment

The effects of chronic treatment with the monoamine oxidase inhibitor (MAOI), clorgyline (CLG; 2 mg/kg per day) on serotonin (5HT) and vasoactive intestinal peptide (VIP) immunoreactivity (IR) within the hamster suprachiasmatic nucleus (SCN) were examined. Optical densities of 5HT IR and VIP IR were increased by MAOI treatment. VIP IR was increased in both the ventrolateral and dorsal regions of the SCN, suggesting that VIP content was increased within both perikarya and terminals of VIP neurons. The results suggest that previously described effects of MAOIs on the mammalian circadian system may be mediated in part, by their effects on serotonergic input to VIP neurons within the SCN.

Chronic alcohol consumption and withdrawal do not induce cell death in the suprachiasmatic nucleus, but lead to irreversible depression of peptide immunoreactivity and mRNA levels

There is evidence that chronic ethanol treatment (CET) disrupts the biological rhythms of various brain functions and behaviors. Because the suprachiasmatic nucleus (SCN) is widely recognized as the dominant pacemaker of the circadian system, we have examined the effects of CET and withdrawal on the main morphological features and chemoarchitecture of this hypothalamic nucleus. Groups of rats ethanol-treated for 6 and 12 months were compared with withdrawn rats (ethanol-treated for 6 months and then switched to a normal diet for an additional 6 months) and with groups of age-matched control and pair-fed control rats. The volume and the total number of neurons of the SCN were estimated from conventionally stained material, whereas the total number of astrocytes and of neurons containing vasopressin (AVP), vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), and somatostatin (SS) were estimated from immunostained sections. The estimates were obtained using unbiased stereological methods, based on Cavalieri's principle and the optical fractionator. The volume of the SCN and the total number of SCN neurons and astrocytes did not vary among groups. We found, however, that CET induced a significant reduction in the total number of AVP-, VIP-, GRP-, and SS-containing neurons. Withdrawal from alcohol did not reduce but rather augmented the loss of VIP- and GRP-immunoreactive neurons. The CET-induced neurochemical alterations seem to result from a decrease in neuropeptide synthesis, as revealed by the reduction in AVP and VIP mRNA levels demonstrated by in situ hybridization with radioactively labeled 48-mer AVP and 30-mer VIP probes. It is thus possible to conclude that the irreversible CET-induced changes in the neurochemistry of the SCN might underpin the disturbances in circadian rhythms observed after long-term alcohol consumption.

Loss of day-night differences in VIP mRNA levels in the suprachiasmatic nucleus of aged rats

Age-related decreases in circadian oscillating activity are speculated to be one of the causes of psychiatric symptoms. To explore the effects of aging on vasoactive intestinal peptide (VIP) synthesis in the suprachiasmatic nucleus (SCN), we investigated the changes in VIP mRNA levels in aged rats compared with young-adult rats under a light/dark cycle using in situ hybridization combined with microcomputer-based imaging analysis. In the young-adult rats, total signals of VIP mRNA in the light-phase showed a significant decrease compared with those on the dark-phase. The VIP signal level in the aged rats was markedly lower than that in young-adults in both light and dark phases. Moreover, in the aged rats, there were no significant differences in VIP mRNA level between the light and dark phases. These results suggest that gene expression of VIP neurons, a main component of the circadian oscillating system, becomes disturbed in the aged rat brain.

Methamphetamine modifies the photic entraining responses in the rodent suprachiasmatic nucleus via serotonin release

We examined whether methamphetamine modifies the photic entraining responses in the rat suprachiasmatic nucleus. Optic nerve stimulation increased vasoactive intestinal polypeptide release from rat suprachiasmatic nucleus slices, and methamphetamine inhibited this increase in a concentration-dependent manner. Optic nerve stimulation has been reported to evoke field potentials in rat suprachiasmatic nucleus slices. Methamphetamine attenuated this field potential, and maximal inhibition (75.5%) was achieved at a concentration of 100 microM. Systemic administration of methamphetamine (1-5 mg/kg) inhibited light (300 lux, 1h)-induced Fos expression in the suprachiasmatic nucleus; methamphetamine at a dose of 5 mg/kg, i.p. caused 40% inhibition of light-induced Fos expression. We examined whether the inhibitory effect of methamphetamine on photic entraining responses mediates serotonin release from the suprachiasmatic nucleus. High-performance liquid chromatographic analysis revealed that methamphetamine application increased serotonin release from rat suprachiasmatic nucleus slices in a concentration-dependent manner, but did not affect noradrenaline release. In addition, reduction of serotonin content attenuated the effect of methamphetamine on field potential induced by optic nerve stimulation in vitro and also light-induced phase advances of wheel running activity rhythm in vivo. The present results support the idea that methamphetamine produces an inhibitory effect on photic entrainment in the suprachiasmatic nucleus via serotonin release.

Changes in vasoactive intestinal peptide mRNA levels in the rat suprachiasmatic nucleus following p-chlorophenylalanine (PCPA) treatment under light/dark conditions

Photic stimulus and serotonin (5-hydroxytryptamine; 5-HT) are two factors known to regulate vasoactive intestinal peptide (VIP) synthesis in the suprachiasmatic nucleus (SCN). To explore the role of 5-HT in the photic stimulus-induced change in VIP synthesis, we investigated the changes in level of VIP mRNA under a 12 h light/12 h dark cycle following depletion of 5-HT by intraperitoneal administration of p-chlorophenylalanine (PCPA) methyl ester (200 mg/kg concentration) for 3 successive days. To estimate VIP mRNA expression, we performed in situ hybridization using imaging plates combined with microcomputer-based imaging analysis. In light-phase, total signals of VIP mRNA from the PCPA-treated rats showed a significant decrease compared with those from the saline-treated control rats. However, in dark-phase, there were no significant decreases between the PCPA-treated rats and the saline-control rats. The present results strongly suggest that 5-HT neuronal inputs to the SCN interfere with the effect of photic stimulus on VIP synthesis at the mRNA level.

Circadian change of VIP mRNA in the rat suprachiasmatic nucleus following p-chlorophenylalanine (PCPA) treatment in constant darkness

Neuronal activity of the suprachiasmatic nucleus (SCN) is known to be regulated by two major extrinsic factors conveyed by three anatomically distinct pathways to the SCN: photic stimulus by the direct retinohypothalamic tract (RHT) and the indirect geniculohypothalamic tract (GHT), and information from the brainstem by ascending forebrain serotonergic (5-hydroxytryptamine: 5-HT) tract. It has been shown that VIP mRNA level in neurons of the SCN is altered by external light, but remains stable in constant darkness. In the present study, by using the in situ hybridization technique combined with computer-assisted image analysis, we examined VIP mRNA expression in the SCN of rats in which the two major factors were eliminated, i.e. photic stimulus by exposing animals in total darkness and 5-HT transmission by three-day successive administration of p-chlorophenyl-alanine methylester (an inhibitor of tryptophan hydroxylase, 200 mg/kg, daily). In saline-treated controls, VIP mRNA levels remained almost constant throughout the day. In contrast, in PCPA-treated rats, a significant rhythm of VIP mRNA was observed with a peak at CT 4 and a trough at CT 20. These observations suggest that the removal of photic and 5-HT influence induces VIP mRNA rhythm in the SCN, indicating that VIP mRNA is controlled not only by photic information but also by the circadian clock.