GABAergic control of Arg-vasopressin release from suprachiasmatic nucleus slice culture

GABA transporters regulate tonic and synaptic GABAA receptor-mediated currents in the suprachiasmatic nucleus neurons

GABA is a principal neurotransmitter in the hypothalamic suprachiasmatic nucleus (SCN) that contributes to intercellular communication between individual circadian oscillators within the SCN network and the stability and precision of the circadian rhythms. GABA transporters (GAT) regulate the extracellular GABA concentration and modulate GABAA receptor (GABAAR)-mediated currents. GABA transport inhibitors were applied to study how GABAAR-mediated currents depend on the expression and function of GAT. Nipecotic acid inhibits GABA transport and induced an inward tonic current in concentration-dependent manner during whole cell patch-clamp recordings from SCN neurons. Application of either the selective GABA transporter 1 (GAT1) inhibitors NNC-711 or SKF-89976A, or the GABA transporter 3 (GAT3) inhibitor SNAP-5114, produced only small changes of the baseline current. Coapplication of GAT1 and GAT3 inhibitors induced a significant GABAAR-mediated tonic current that was blocked by gabazine. GAT inhibitors decreased the amplitude and decay time constant and increased the rise time of spontaneous GABAAR-mediated postsynaptic currents. However, inhibition of GAT did not alter the expression of either GAT1 or GAT3 in the hypothalamus. Thus GAT1 and GAT3 functionally complement each other to regulate the extracellular GABA concentration and GABAAR-mediated synaptic and tonic currents in the SCN. Coapplication of SKF-89976A and SNAP-5114 (50 µM each) significantly reduced the circadian period of Per1 expression in the SCN by 1.4 h. Our studies demonstrate that GAT are important regulators of GABAAR-mediated currents and the circadian clock in the SCN.NEW & NOTEWORTHY In the suprachiasmatic nucleus (SCN), the GABA transporters GAT1 and GAT3 are expressed in astrocytes. Inhibition of these GABA transporters increased a tonic GABA current and reduced the circadian period of Per1 expression in SCN neurons. GAT1 and GAT3 showed functional cooperativity: inhibition of one GAT increased the activity but not the expression of the other. Our data demonstrate that GABA transporters are important regulators of GABAA receptor-mediated currents and the circadian clock.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Exposure to fipronil elevates systolic blood pressure and disturbs related biomarkers in plasma of rats

Recent reports show that fipronil affects non-target organisms, including environmental species populations and potentially humans. We aimed to examine if fipronil exposure affects the systolic blood pressure and related biomarkers. Thus, fipronil was orally administered to rats (30 mg/kg/day) during 15 days (Fipronil group) or physiological solution (Control group). While fipronil increased significantly the systolic blood pressure (158±13 mmHg), no significant changes were observed in Control group (127±3 mmHg). Significantly, higher levels of fipronil in plasma were observed in Fipronil group (0.46±0.09 μg/mL versus 0.17±0.11 μg/mL in Control group). Fipronil group showed lower weight gain compared with Control group. While fipronil resulted in higher concentrations of endothelin-1, reduced antioxidant capacity and lower levels of circulating matrix metalloproteinase 2 (MMP-2) and nitric oxide (NO) metabolites compared to Control group, no alteration was observed in serum biomarkers of renal and hepatic/biliary functional abilities. Therefore, this study suggests that fipronil causes hypertension and endothelin-1 plays a key role. Also, these findings suggest that reductions of both MMP-2 and NO may contribute with the elevation of systolic blood pressure observed with fipronil.

Localization and expression of GABA transporters in the suprachiasmatic nucleus

GABA is a principal neurotransmitter in the suprachiasmatic hypothalamic nucleus (SCN), the master circadian clock. Despite the importance of GABA and GABA uptake for functioning of the circadian pacemaker, the localization and expression of GABA transporters (GATs) in the SCN has not been investigated. The present studies used Western blot analysis, immunohistochemistry and electron microscopy to demonstrate the presence of GABA transporter 1 (GAT1) and GAT3 in the SCN. By using light microscopy, GAT1 and GAT3 were co-localized throughout the SCN, but were not expressed in the perikarya of arginine vasopressin- or vasoactive intestinal peptide-immunoreactive (-ir) neurons of adult rats, nor in the neuronal processes labelled with the neurofilament heavy chain. Using electron microscopy, GAT1- and GAT3-ir was found in glial processes surrounding unlabelled neuronal perikarya, axons, dendrites, and enveloped symmetric and asymmetric axo-dendritic synapses. Glial fibrillary acidic protein-ir astrocytes grown in cell culture were immunopositive for GAT1 and GAT3 and both GATs could be observed in the same glial cell. These data demonstrate that synapses in the SCN function as 'tripartite' synapses consisting of presynaptic axon terminals, postsynaptic membranes and astrocytes that contain GABA transporters. This model suggests that astrocytes expressing both GATs may regulate the extracellular GABA, and thereby modulate the activity of neuronal networks in the SCN.

Dexamethasone induces different wheel running activity than corticosterone through vasopressin release from the suprachiasmatic nucleus

During the analysis of wheel running activity, we found that corticosterone (1 mg/100 g BW) injection decreased wheel activity, while dexamethasone (0.1 mg/100 g) increased the activity. To clarify the functional differences between corticosterone and dexamethasone, we measured Arg-vasopressin (AVP) release from the suprachiasmatic nucleus (SCN) slice culture in vitro and AVP coding mRNA in the SCN in vivo. The corticosterone (0.2 and 2 microg/ml, final concentration in medium) decreased the AVP release, while it increased by dexamethasone (0.2 and 2 microg/ml). An AVP mRNA in the SCN was decreased by both corticosterone (1 mg/100 g) and dexamethasone (0.1 mg/100 g). The differences in wheel activity by corticosterone and dexamethasone are discussed from the changes of AVP in the SCN.

Serotonin-stimulated glutamate release from an SCN explant culture was higher during light period

Serotonin (5-HT) and glutamate are concerned with the input pathways to the suprachiasmatic nucleus (SCN), a center of biological rhythms in mammals. Arg-vasopressin (AVP) is one of the output paths from the SCN to other brain areas. Functional relations of 5-HT on glutamate release, which stimulates AVP release, were therefore analyzed in an SCN slice culture using a glutamate biosensor. Spontaneous release of glutamate from the SCN culture was higher during the light period than the dark period. The 5-HT-stimulated increase in glutamate release was also larger at 0900 h than at 2000 h. In addition, glutamate, but not 5-HT, increased the AVP release. These findings suggest the possibility that the 5-HT has no direct facilitatory functions in AVP release via liberation of glutamate from the SCN.

Melatonin inhibits Arg-vasopressin release via MT(2) receptor in the suprachiasmatic nucleus-slice culture of rats

The effect of melatonin on the release of Arg-vasopressin (AVP) was analyzed in a suprachiasmatic nucleus-slice explant culture. The release of AVP into the culture medium exhibited a circadian rhythm, with higher level during the subjective day and lower level during the subjective night. Melatonin (500 nM) inhibited the release of AVP. Luzindole, a MT(2) (Mel 1b) melatonin receptor antagonist, attenuated the effect of melatonin on the AVP release. Results indicate that the inhibition of AVP release by melatonin in the suprachiasmatic nucleus-slice culture depends at least in part on the melatonin MT(2) receptor.

Lipopolysaccharide increases arginine-vasopressin release from rat suprachiasmatic nucleus slice cultures

The aim of this study was to determine whether Escherichia coli lipopolysaccharide at the doses of 25, 50 and 100 microM influences arginine-vasopressin (AVP) secretion in young rat suprachiasmatic nucleus (SCN) neurons. Lipopolysaccharide administered in the medium for 3 h increased significantly the arginine-vasopressin release lasting up to 6 h after treatment. These results provide the first evidence that lipopolysaccharide influences AVP secretion in SCN neurons. Moreover, these findings may explain some central effects observed in vivo after lipopolysaccharide administration.

Pituitary adenylate cyclase-activating polypeptide and melatonin in the suprachiasmatic nucleus: effects on the calcium signal transduction cascade

The suprachiasmatic nucleus (SCN) harbors an endogenous oscillator generating circadian rhythms that are synchronized to the external light/dark cycle by photic information transmitted via the retinohypothalamic tract (RHT). The RHT has recently been shown to contain pituitary adenylate cyclase-activating polypeptide (PACAP) as neurotransmitter/neuromodulator. PACAPergic effects on cAMP-mediated signaling events in the SCN are restricted to distinct time windows and sensitive to melatonin. In neurons isolated from the SCN of neonatal rats we investigated by means of the fura-2 technique whether PACAP and melatonin also influence the intracellular calcium concentration ([Ca2+]i). PACAP elicited increases of [Ca2+]i in 27% of the analyzed neurons, many of which were also responsive to the RHT neurotransmitters glutamate and/or substance P. PACAP-induced changes of [Ca2+]i were independent of cAMP, because they were not mimicked by forskolin or 8-bromo-cAMP. PACAP caused G-protein- and phospholipase C-mediated calcium release from inositol-trisphosphate-sensitive stores and subsequent protein kinase C-mediated calcium influx, demonstrated by treatment with GDP-beta-S, neomycin, U-73122, calcium-free saline, thapsigargin, bisindolylmaleimide, and chelerythrine. The calcium influx was insensitive to antagonists of voltage-gated calcium channels of the L-, N-, P-, Q- and T-type (diltiazem, nifedipine, verapamil, omega-conotoxin, omega-agatoxin, amiloride). Immunocytochemical characterization of the analyzed cells revealed that >50% of the PACAP-sensitive neurons were GABA-immunopositive. Our data demonstrate that in the SCN PACAP affects the [Ca2+]i, suggesting that different signaling pathways (calcium as well as cAMP) are involved in PACAPergic neurotransmission or neuromodulation. Melatonin did not interfere with calcium signaling, indicating that in SCN neurons the hormone primarily affects the cAMP signaling pathway.

Circadian rhythm of Arg-vasopressin contents in the suprachiasmatic nucleus in relation to corticosterone

Circadian rhythms of locomotor activity and adrenal glucocorticoid are controlled by the suprachiasmatic nucleus (SCN), the center of a biological clock, in mammals. Arg-vasopressin (AVP) contents in the SCN play a role in endogenous circadian rhythm during the absence of time cues. The AVP-containing neurons in the SCN are considered to transmit a circadian signal to the other parts of the brain. The circadian rhythms of AVP in the SCN in relation to the plasma corticosterone and locomotor activity were investigated. Under the light-dark cycle, plasma corticosterone levels were reciprocally correlated with the AVP content in the SCN. Under free-running conditions with constant dim light, AVP rhythms were reciprocally synchronized with the locomotor activity. The correlation of AVP with plasma corticosterone is different at different times of the day both under the LD cycle and constant dim light. Dexamethasone (i. p., 0.1 mg/100) increased the AVP contents, and this tendency was significantly greater during the dark period. These results indicate that corticosterone in the blood may regulate the circadian rhythm through AVP variation in the SCN.