In vivo resetting of the hamster circadian clock by 5-HT7 receptors in the suprachiasmatic nucleus - PubMed (original) (raw)

In vivo resetting of the hamster circadian clock by 5-HT7 receptors in the suprachiasmatic nucleus

J C Ehlen et al. J Neurosci. 2001.

Abstract

Serotonin (5-HT) has been strongly implicated in the regulation of the mammalian circadian clock located in the suprachiasmatic nuclei (SCN); however, its role in behavioral (nonphotic) circadian phase resetting remains elusive. Central to this issue are divergent lines of evidence that the SCN may, or may not, be a target for the phase-resetting effects of 5-HT. We have addressed this question using a novel reverse-microdialysis approach for timed perfusions of serotonergic and other agents to the Syrian hamster SCN with durations equivalent to the increases in in vivo 5-HT release during phase-resetting behavioral manipulations. We found that 3 hr perfusions of the SCN with either 5-HT or the 5-HT(1A,7) receptor agonist 2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydro-naphthalene (8-OH-DPAT) at midday advanced the phase of the free-running circadian rhythm of wheel-running assessed using an Aschoff type II procedure. Phase shifts induced by 8-OH-DPAT were enhanced more than threefold by pretreatment with the 5-HT synthesis inhibitor para-chlorophenylalanine. Phase advances induced by SCN 8-OH-DPAT perfusion were significantly inhibited by the 5-HT(2,7) receptor antagonist ritanserin and by the more selective 5-HT(7) receptor antagonist DR4004, implicating the 5-HT(7) receptor in mediating this phase resetting. Concurrent exposure to light during the 8-OH-DPAT perfusion abolished the phase advances. Furthermore, coperfusion of the SCN with TTX, which blocked in vivo 5-HT release, did not suppress intra-SCN 8-OH-DPAT-induced phase advances. These results indicate that 5-HT(7) receptor-mediated phase resetting in the SCN is markedly influenced by the degree of postsynaptic responsiveness to 5-HT and by photic stimulation. Finally, 5-HT may act directly on SCN clock cells to induce in vivo nonphotic phase resetting.

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Figures

Fig. 1.

Fig. 1.

Diagrammatic coronal sections showing histologically verified locations of the microdialysis probes tips of different groups included in the study. Coronal planes extend from the anterior SCN (top left section) to the caudal hypothalamus (bottom right section).Symbols represent the ventral extent of the probe implants for the treatment groups as follows: *, _p-_CPA plus SCN 8-OH-DPAT perfusion; ▾, no _p-_CPA plus SCN 8-OH-DPAT perfusion; ○, _p-_CPA plus SCN ACSF vehicle perfusion; ●, _p-_CPA plus SCN 8-OH-DPAT perfusion in light; ▴, _p-_CPA plus SCN 5-HT perfusion; ▵, _p-_CPA plus SCN 8-OH-DPAT plus DR4004 perfusion; ■, _p-_CPA plus SCN 8OH-DPAT plus ritanserin perfusion; ▪, _p-_CPA plus 8-OH-DPAT perfusion 2 mm caudal to SCN.OC, Optic chiasma; 3V, third ventricle. Scale bar, 1.0 mm. Insets are photomicrographs of cresyl violet-stained coronal hypothalamic sections showing the location of the probe tip (PT) relative to the SCN in three animals under higher (A, B) and lower (C) magnification. Note the positioning of the microdialysis tip against the lateral margin of the SCN. Values shown above the scale bars are in micrometers.

Fig. 2.

Fig. 2.

Double-plotted wheel-running activity records of two representative profiles from three treatment groups. Days are indicated vertically from top to bottom, and time is indicated horizontally. The shaded horizontal bars represent the 3 hr microdialysis perfusion from ZT 6 to ZT 9. A, B, SCN 8-OH-DPAT perfusion in darkness without _p-_CPA pretreatment; C,D, SCN 8-OH-DPAT perfusion in darkness with_p-CPA pretreatment (2 additional profiles of this treatment group are shown in Fig.4_A,B); E,F, SCN 8-OH-DPAT perfusion in the light with_p-_CPA pretreatment. See Materials and Methods for details of the various treatment protocols.

Fig. 3.

Fig. 3.

Double-plotted wheel-running activity records of two representative profiles from three 5-HT antagonist treatment groups. All were pretreated with _p-CPA. The_shaded horizontal bars represent the 3 hr microdialysis perfusion from ZT 6 to ZT 9. A, B, SCN 8-OH-DPAT perfusion in darkness; C, D, SCN 8-OH-DPAT plus intraperitoneal ritanserin injection in darkness;E, F, SCN 8-OH-DPAT plus DR4004 perfusion in darkness.

Fig. 4.

Fig. 4.

The inhibitory effects of intraperitoneal administration of the 5-HT2,7 receptor antagonist ritanserin or coperfusion with two concentrations of the highly selective 5-HT7 receptor antagonist DR4004 on the phase-advancing effect of SCN perfusion with 8-OH-DPAT. These treatments significantly attenuated the phase-advancing response to the 8-OH-DPAT. Bars with different letters_are significantly different. Numbers in the_bars are the number of animals per group.

Fig. 5.

Fig. 5.

The inhibitory effect of 1 hr microdialysis perfusion with TTX (0.5 μ

m

; shaded region) on in vivo 5-HT release in the SCN, measured from the microdialysis probe. A significant suppression of 5-HT release occurred within 40 min of the onset of TTX perfusion and persisted for at least 1.6 hr after cessation of perfusion. *p < 0.05 versus pretreatment baseline level (n = 4).

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