Catecholaminergic innervation of the suprachiasmatic nucleus in the adult rat: ultrastructural relationships with neurons containing vasoactive intestinal peptide or vasopressin

Adrenergic Agonists Activate Transcriptional Activity in Immortalized Neuronal Cells From the Mouse Suprachiasmatic Nucleus

The suprachiasmatic nucleus of the hypothalamus (SCN) houses the central circadian oscillator of mammals. The main neurotransmitters produced in the SCN are γ-amino-butyric acid, arginine-vasopressin (AVP), vasoactive intestinal peptide (VIP), pituitary-derived adenylate cyclase-activating peptide (PACAP), prokineticin 2, neuromedin S, and gastrin-releasing peptide (GRP). Apart from these, catecholamines and their receptors were detected in the SCN as well. In this study, we confirmed the presence of β-adrenergic receptors in SCN and a mouse SCN-derived immortalized cell line by immunohistochemical, immuno-cytochemical, and pharmacological techniques. We then characterized the effects of β-adrenergic agonists and antagonists on cAMP-regulated element (CRE) signaling. Moreover, we investigated the interaction of β-adrenergic signaling with substances influencing parallel signaling pathways. Our findings have potential implications on the role of stress (elevated adrenaline) on the biological clock and may explain some of the side effects of β-blockers applied as anti-hypertensive drugs.

Circadian Clock Protein Content and Daily Rhythm of Locomotor Activity Are Altered after Chronic Exposure to Lead in Rat

Lead exposure has been reported to produce many clinical features, including parkinsonism. However, its consequences on the circadian rhythms are still unknown. Here we aimed to examine the circadian rhythms of locomotor activity following lead intoxication and investigate the mechanisms by which lead may induce alterations of circadian rhythms in rats. Male Wistar rats were injected with lead or sodium acetate (10 mg/kg/day, i.p.) during 4 weeks. Both groups were tested in the “open field” to quantify the exploratory activity and in the rotarod to evaluate motor coordination. Then, animals were submitted to continuous 24 h recordings of locomotor activity under 14/10 Light/dark (14/10 LD) cycle and in complete darkness (DD). At the end of experiments, the clock proteins BMAL1, PER1-2, and CRY1-2 were assayed in the suprachiasmatic nucleus (SCN) using immunohistochemistry. We showed that lead significantly reduced the number of crossing in the open field, impaired motor coordination and altered the daily locomotor activity rhythm. When the LD cycle was advanced by 6 h, both groups adjusted their daily locomotor activity to the new LD cycle with high onset variability in lead-intoxicated rats compared to controls. Lead also led to a decrease in the number of immunoreactive cells (ir-) of BMAL1, PER1, and PER2 without affecting the number of ir-CRY1 and ir-CRY2 cells in the SCN. Our data provide strong evidence that lead intoxication disturbs the rhythm of locomotor activity and alters clock proteins expression in the SCN. They contribute to the understanding of the mechanism by which lead induce circadian rhythms disturbances.

The noradrenaline reuptake inhibitor atomoxetine phase-shifts the circadian clock in mice

Circadian rhythms are recurring cycles in physiology and behaviour that repeat with periods of near 24 h and are driven by an endogenous circadian timekeeping system with a master circadian pacemaker located in the suprachiasmatic nucleus (SCN). Atomoxetine is a specific noradrenaline reuptake inhibitor that is used in the clinical management of attention-deficit/hyperactivity disorder (ADHD). In the current study we examined the effects of atomoxetine on circadian rhythms in mice. Atomoxetine (i.p.; 3 mg/kg) treatment of mice free-running in constant light (LL) at circadian time (CT) 6 induced large phase delays that were significantly different to saline controls. Treatment of animals with atomoxetine at CT13 or CT18 did not elicit any significant phase shifts. We also examined the effects of atomoxetine treatment of animals free-running in constant darkness (DD). Atomoxetine treatment at CT6 in these animals leads to more modest, but significant, phase advances, whereas treatment at CT18 did not elicit significant phase shifts. The effects of atomoxetine in LL were attenuated by pretreatment with the α-1 adrenoreceptor antagonist prazosin and were mimicked by another noradrenaline reuptake inhibitor, reboxetine. Further, atomoxetine treatment at CT6 induced a downregulation of c-Fos and CLOCK in the SCN, but did not alter the expression of PER2 and BMAL1. Atomoxetine during the night phase did not alter any of these factors. Atomoxetine treatment preceding a light pulse at CT15 enhanced the magnitude of the photic-phase shift, whereas it altered photic induction of the immediate early gene products c-Fos and ARC in the SCN. These data indicate that atomoxetine can reset the circadian clock and indicate that part of the therapeutic profile of atomoxetine may be through circadian rhythm modulation.

Evaluation of serotonin, noradrenaline and dopamine reuptake inhibitors on light-induced phase advances in hamster circadian activity rhythms

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed for the treatment of anxiodepressive states that are often associated with perturbed circadian rhythms including, in certain patients, phase advances. Surprisingly, the influence of SSRIs upon circadian activity rhythms has been little studied in experimental models.

OBJECTIVES

Accordingly, this study examined the ability of SSRIs to modulate the phase-setting properties of light on circadian activity rhythms in hamsters. Their actions were compared to those of the mixed serotonin/noradrenaline reuptake inhibitor (SNRI), venlafaxine, the selective noradrenaline reuptake inhibitor, reboxetine, and the dopamine reuptake inhibitor, bupropion.

MATERIALS AND METHODS

Wheel-running activity rhythms were recorded in male Syrian hamsters. Drugs were administered systemically before a light stimulus that was used to advance the timing of the hamster running rhythms.

RESULTS

Four chemically diverse SSRIs, citalopram (1-10 mg/kg, intraperitoneally), fluvoxamine (1-10), paroxetine (1-10), and fluoxetine (10 and 20), all robustly and significantly inhibited the ability of light to phase advance hamster circadian wheel-running activity rhythms. Their actions were mimicked by venlafaxine (1-10) that likewise elicited a marked reduction in phase advances. Conversely, reboxetine (1-20) and bupropion (1-20) did not exert significant effects.

CONCLUSIONS

These data suggest that suppression of serotonin (but not noradrenaline or dopamine) reuptake by SSRIs and SNRIs modifies circadian locomotor activity rhythms in hamsters. Further, they support the notion that an inhibitory influence upon the early-morning light-induced advance in circadian activity contributes to the therapeutic effects of serotonin uptake inhibitors in certain depressed patients.

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.

Neuronal expression of catechol O-methyltransferase mRNA in neonatal rat suprachiasmatic nucleus

We examined the expression profile of catechol O-methyltransferase (COMT) mRNA and its protein in the neonatal rat hypothalamus by in situ hybridization and immunohistochemistry to clarify the sites of dopamine degradation. Strong COMT mRNA expression was observed in the suprachiasmatic nucleus (SCN) throughout its rostrocaudal extent at postnatal day 1 (P1) and P2, and the mRNA levels decreased gradually until P16. COMT mRNA was predominantly localized to the ventral and medial parts of the SCN. Intense COMT immunoreactivity was demonstrated in the ventral SCN and was detected in neuronal perikarya and processes at P1. Ependymal and microglial cells also exhibited strong COMT immunoreactivity. These results indicate that COMT may directly be involved in dopaminergic signaling in the neonatal SCN.

Late onset of motor neurons in mice overexpressing wild-type peripherin

Peripherin, a type III intermediate filament (IF) protein, upregulated by injury and inflammatory cytokines, is a component of IF inclusion bodies associated with degenerating motor neurons in sporadic amyotrophic lateral sclerosis (ALS). We report here that sustained overexpression of wild-type peripherin in mice provokes massive and selective degeneration of motor axons during aging. Remarkably, the onset of peripherin-mediated disease was precipitated by a deficiency of neurofilament light (NF-L) protein, a phenomenon associated with sporadic ALS. In NF-L null mice, the overexpression of peripherin led to early- onset formation of IF inclusions and to the selective death of spinal motor neurons at 6 mo of age. We also report the formation of similar peripherin inclusions in presymptomatic transgenic mice expressing a mutant form of superoxide dismutase linked to ALS. Taken together, these results suggest that IF inclusions containing peripherin may play a contributory role in motor neuron disease.

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.

Fear and power-dominance motivation: proposed contributions of peptide hormones present in cerebrospinal fluid and plasma

We propose that fear and power-dominance drive motivation are generated by the presence of elevated plasma and cerebrospinal fluid (CSF) levels of certain peptide hormones. For the fear drive, the controlling hormone is corticotropin releasing factor, and we argue that elevated CSF and plasma levels of this peptide which occur as a result of fear-evoking and other stressful experiences in the recent past are detected and transduced into neuronal activities by neurons in the vicinity of the third ventricle, primarily in the periventricular and arcuate hypothalamic nuclei. For the power-dominance drive, we propose that the primary signal is the CSF concentration of vasopressin, which is detected in two circumventricular organs, the subfornical organ and organum vasculosum of the lamina terminalis. We suggest that the peptide-generated signals detected in periventricular structures are transmitted to four areas in which neuronal activities represent fear and power-dominance: one in the medial hypothalamus, one in the dorsolateral quadrant of the periaqueductal gray matter, a third in the midline thalamic nuclei, and the fourth within medial prefrontal cortex. The probable purpose of this system is to maintain a state of fear or anger and consequent vigilant or aggressive behavior after the initial fear- or anger-inducing stimulus is no longer perceptible. We further propose that all the motivational drives, including thirst, hunger and sexual desire are generated in part by non-steroidal hormonal signals, and that the unstimulated motivational status of an individual is determined by the relative CSF and plasma levels of several peptide hormones.

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.

Circadian rhythm of drinking and running-wheel activity in rats with 6-hydroxydopamine lesions of the ventral tegmental area

Circadian rhythms in drinking and running-wheel (locomotor) activity of rats with 6-hydroxydopamine (6-OHDA, 4 microg/2 microl per rat)-induced lesions in the ventral tegmental area (VTA) were examined under a light-dark (LD) cycle and constant dim light (5 lux). Under the LD cycle, the length of the locomotor activity period was decreased during the dark, and increased during the light period in the lesioned rats. Under the constant dim light conditions, the free-running circadian period (tau) of drinking and activity rhythm was longer in lesioned rats than in sham-operated controls. The elongation of the circadian period was accompanied by decrements in activity. These observations suggest that the mesolimbic dopaminergic system modulates rhythms in circadian drinking and locomotor activity.

Disruption of type IV intermediate filament network in mice lacking the neurofilament medium and heavy subunits

To clarify the role of the neurofilament (NF) medium (NF-M) and heavy (NF-H) subunits, we generated mice with targeted disruption of both NF-M and NF-H genes. The absence of the NF-M subunit resulted in a two- to threefold reduction in the caliber of large myelinated axons, whereas the lack of NF-H subunits had little effect on the radial growth of motor axons. In NF-M-/- mice, the velocity of axonal transport of NF light (NF-L) and NF-H proteins was increased by about two-fold, whereas the steady-state levels of assembled NF-L were reduced. Although the NF-M or NF-H subunits are each dispensable for the formation of intermediate filaments, the absence of both subunits in double NF-M; NF-H knockout mice led to a scarcity of intermediate filament structures in axons and to a marked approximately twofold increase in the number of microtubules. Protein analysis indicated that the levels of NF-L and alpha-internexin proteins were reduced dramatically throughout the nervous system. Immunohistochemistry of spinal cord from the NF-M-/-;NF-H-/- mice revealed enhanced NF-L staining in the perikaryon of motor neurons but a weak NF-L staining in axons. In addition, axonal transport studies carried out by the injection of [35S]methionine into spinal cord revealed after 30 days very low levels of newly synthesized NF-L proteins in the sciatic nerve of NF-M-/-;NF-H-/- mice. The combined results demonstrate a requirement of the high-molecular-weight subunits for the assembly of type IV intermediate filament proteins and for the efficient translocation of NF-L proteins into the axonal compartment.

Vasoactive intestinal peptide neurons as synaptic targets for vasopressin neurons in the suprachiasmatic nucleus. Double-label immunocytochemical demonstration in the rat

Cellular relationships between neurons producing vasopressin or vasoactive intestinal peptide in the suprachiasmatic nucleus of the hypothalamus, the main component of the central circadian timing system in mammals, were investigated in the rat using double immunocytochemistry. Analysis of serial confocal images revealed that the vasopressin-synthesizing neurons not only are important targets for the vasoactive intestinal peptide-synthesizing neurons, as previously demonstrated, but also establish reciprocal axosomatic contacts with these neurons, which have never been reported. On average, 5.4 vasoactive intestinal peptide contacts per vasopressin perikaryon and 1.7 vasopressin contacts per vasoactive intestinal peptide perikaryon were counted. That both types of neurons are linked by reciprocal synapses was confirmed at the electron microscopic level using a combination of immunoperoxidase and immunogold-silver labeling. Existence of an anatomical substrate for a vasopressinergic control of the vasoactive intestinal peptide neurons may have important functional consequences. In view (i) of the presumed, direct or indirect, involvement of the vasopressin neurons in relaying pacemaker information within and outside the suprachiasmatic nucleus, and (ii) of the established role of the vasoactive intestinal peptide neurons as the main light-sensitive cells, it provides support for a neuronal mechanism through which the circadian clock may regulate inputs related to environmental messages. Our electron-microscopic data also extended earlier observations, pointing to the involvement of vasopressin and vasoactive intestinal peptide terminals in so-called double synapses that, conceivably, could regulate neuronal synchronization in the suprachiasmatic nucleus. A morphological basis for non-synaptic interactions that could be involved in ephaptic and/or paracrine communication between both types of peptidergic neurons is also reported.

Reduced glucose availability attenuates circadian responses to light in mice

To test whether circadian responses to light are modulated by decreased glucose availability, we analyzed photic phase resetting of the circadian rhythm of locomotor activity in mice exposed to four metabolic challenges: 1) blockade of glucose utilization induced by 2-deoxy-D-glucose (2-DG), 2) fasting (food was removed for 30 h), 3) insulin administration, and 4) insulin treatment after fasting. In mice housed in constant darkness, light pulses applied during early subjective night induced phase delays of the rhythm of locomotor activity, whereas light pulses applied during late subjective night caused phase advances. There was an overall reduction of light-induced phase shifts, with a more pronounced effect for delays, in mice pretreated with 500 mg/kg ip 2-DG compared with mice injected with saline. Administration of glucose with 2-DG prevented the reduction of light-induced phase delays. Furthermore, phase delays were reduced in fed mice pretreated with 5 IU/kg sc insulin and in fasted mice injected with saline or insulin compared with control fed mice. These results show that circadian responses to light are reduced when brain glucose availability is decreased, suggesting a metabolic modulation of light-induced phase shifts.

Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-containing cells in the suprachiasmatic nucleus of the Syrian hamster (Mesocricetus auratus)

Catecholamines, including dopamine (DA), affect the activity of cells in the suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian clock in mammals. This study examined the distribution of dopaminergic cells in the SCN of the male Syrian hamster, using both single- and double-label immunocytochemistry for tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis and for aromatic L-amino acid decarboxylase (AADC), the second enzyme needed to produce DA. Some neurons immunopositive for TH (TH + ) were found in the SCN, but most of the TH + cells of the region were located just outside the borders of the nucleus, as defined by pyronin Y staining. In the SCN, 91% of these cells were also immunopositive for AADC and thus, likely to be dopaminergic. Cells positive for AADC, many of which were not TH +, were found throughout the SCN, with the highest concentration seen in the ventral aspects of the nucleus. Cells containing AADC, but lacking TH may synthesize products other than DA, such as trace amines. These anatomical observations suggest that local neurons that produce DA and perhaps trace amines, may play a role in SCN function and in the neural control of circadian rhythms.

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.

Intrinsic organization and monoaminergic innervation of the suprachiasmatic nucleus transplanted to adult rats. A light- and electron-microscopic study

Light- and electron-microscopic immunocytochemistry was used to investigate grafts of foetal hypothalamic tissue implanted close to the site of the suprachiasmatic nucleus in adult rats with bilateral surgical ablation of this nucleus. The transplants contained vasoactive intestinal peptide and vasopressin cell clusters, which have previously been shown to characterize functional suprachiasmatic nucleus grafts. Vasoactive intestinal peptide and vasopressin neurons presented synaptic features that have not been described in the native suprachiasmatic nucleus. More specifically, their terminals within the graft were involved in 'double' synapses with separate unlabelled dendrites. Moreover, in dually stained sections, an unexpected synaptic investment of vasoactive intestinal peptide neurons by vasopressin endings was detected, which revealed reversed vasoactive intestinal peptide/vasopressin interactions compared to those described in the native nucleus. These observations could reflect some immature features of the grafted neurons. Ultrastructural relationships of monoaminergic fibres arising from host and/or intragraft neurons were also examined. Within the engrafted suprachiasmatic nucleus, tyrosine hydroxylase-labelled fibres, which probably belonged to cografted dopaminergic neurons, showed normal patterns of distribution and synaptic connections, with no preferential relationships with vasoactive intestinal peptide or vasopressin neurons. Serotoninergic axons arborized within transplants but, in agreement with previous data showing an inhibitory influence of the suprachiasmatic nucleus on ingrowing serotoninergic fibres, they had no predilection for the area corresponding to that nucleus. In spite of their relative scarcity, serotoninergic fibres within the engrafted suprachiasmatic nucleus showed an almost normal synaptic incidence, but synapses were not predominantly shared with the vasoactive intestinal peptide neurons, known to be their major targets in the native nucleus. This may contribute not only to the failure of functional grafts to synchronize with environmental conditions, but also to the inability of transplants to restore hormonal rhythms such as estrous cyclicity.