5-HT_1BReceptor–Mediated Presynaptic Inhibition of Retinal Input to the Suprachiasmatic Nucleus

Abstract

The suprachiasmatic nucleus (SCN) receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain, and serotonin (5-HT) can modify the response of the SCN circadian oscillator to light. 5-HT 1B receptor-mediated presynaptic inhibition has been proposed as one mechanism by which 5-HT modifies retinal input to the SCN (Pickard et al., 1996). This hypothesis was tested by examining the subcellular localization of 5-HT 1B receptors in the mouse SCN using electron microscopic immunocytochemical analysis with 5-HT 1B receptor antibodies and whole-cell patch-clamp recordings from SCN neurons in hamster hypothalamic slices. 5-HT 1B receptor immunostaining was observed associated with the plasma membrane of retinal terminals in the SCN. 1-[3-(Trifluoromethyl)phenyl]-piperazine HCl (TFMPP), a 5-HT 1B receptor agonist, reduced in a dose-related manner the amplitude of glutamatergic EPSCs evoked by stimulating selectively the optic nerve. Selective 5-HT 1A or 5-HT 7 receptor antagonists did not block this effect. Moreover, in cells demonstrating an evoked EPSC in response to optic nerve stimulation, TFMPP had no effect on the amplitude of inward currents generated by local application of glutamate. The effect of TFMPP on lightinduced phase shifts was also examined using 5-HT 1B receptor knock-out mice. TFMPP inhibited behavioral responses to light in wild-type mice but was ineffective in inhibiting light-induced phase shifts in 5-HT 1B receptor knock-out mice. The results indicate that 5-HT can reduce retinal input to the circadian system by acting at presynaptic 5-HT 1B receptors located on retinal axons in the SCN. Key words: circadian rhythms; serotonin; 5-HT 1B receptor knock-out mice; retinal ganglion cells; presynaptic; hypothalamic sliceThe hypothalamic suprachiasmatic nucleus (SC N) is a circadian oscillator and an important component of the mammalian circadian system responsible for the generation of behavioral and physiological circadian rhythms (see K lein et al., 1991; van den Pol and Dudek, 1993). The SC N receives a direct input from the retina via the retinohypothalamic tract (RHT) that arises from a small subset of retinal ganglion cells (Hendrickson et al., 1972;Moore and Lenn, 1972;Pickard, 1982;Moore et al., 1995). RHT afferents serve to entrain the endogenous SC N oscillator to the 24 hr environmental day -night cycle (Johnson et al., 1988). In addition, the SC N receives a dense serotonergic input from the midbrain raphe (Azmitia and Segal, 1978;Moore et al., 1978;Meyer-Bernstein and Morin, 1996). Although serotonergic input to the SC N is not required for the expression of circadian behavior (Block and Zucker, 1976;Morin and Blanchard, 1991), serotonin (5-HT) and 5-HT agonists can modif y the response of the SCN to light (Miller and Fuller, 1990;Selim et al., 1993;Rea et al., 1994Rea et al., , 1995Pickard et al., 1996; Pickard and Rea, 1997a,b;Ying and Rusak, 1997;Weber et al., 1998).At present, 14 distinct 5-HT receptor subtypes are recognized Hoyer and Martin, 1997). Bi...

Pseudorabies virus expressing enhanced green fluorescent protein: A tool for in vitro electrophysiological analysis of transsynaptically labeled neurons in identified central nervous system circuits

“…For acute hypothalamic slices, the brains were rapidly removed from the skull and immersed in ice-cold, oxygenated (95% O 2 ͞5% CO 2 ) artificial cerebrospinal fluid (ACSF) containing (in mM): 124 NaCl, 3 KCl, 26 NaHCO 3 , 1.4 NaH 2 PO 4 , 11 glucose, 1.3 CaCl 2 , 1.3 MgCl 2 , pH ϭ 7.3-7.4, with an osmolality of 290-305 mosmol͞kg. Brains were blocked and mounted on a vibrating microtome, and horizontal slices (400 m) of the ventral hypothalamus containing the SCN and the proximal optic nerves were prepared, as described (17). Slices were transferred to an interface-style recording chamber, and allowed to equilibrate while continuously perfused with warmed (32-34°C) and oxygenated ACSF for 1-2 h before whole-cell recording.…”

“…Recordings of synaptic activity were made by using patch pipettes constructed of borosilicate glass capillaries as described (17). Pipettes were filled with (in mM): 130 potassium gluconate, 1 NaCl, 5 EGTA, 10 Hepes, 1 MgCl 2 , 1 CaCl 2 , 3 KOH, 2-4 Mg-ATP and 0.2% biocytin (Sigma), pH 7.2.…”

Serotonin as a Modulator of Glutamate- and GABA-Mediated Neurotransmission: Implications in Physiological Functions and in Pathology

“…At a pre-synaptic level, 5-HT can modulate neurotransmitter release: for example, in various brain regions glutamate release is reduced by 5-HT 1A [162,164,177], 5-HT 1B [13,130,151,167] and 5-HT 6 receptors [31]. Similarly, presynaptic inhibition of GABA release is mediated by 5-HT 1A [90,95,96] and 5-HT 1B receptors [88,117].…”

“…For example, in tractus solitarius nucleus [7], dorsal vagal motor nucleus [185] and area postrema [57] 5-HT stimulates glutamate release by activation of pre-synaptic 5-HT 3 receptors. In suprachiasmatic nucleus, pre-synaptic 5-HT 1B receptors inhibit retinal input [151] whereas 5-HT 7 receptors reduce glutamate effects at a post-synaptic level [155]; by these effects, serotonin changes neuronal responsiveness to light stimuli and participates to the regulation of circadian rhythms.…”

“…The modulatory control of 5-HT over glutamate-and/or GABA-mediated transmission also plays a role in other functions, among which the regulation of circadian rhythms [151,155] and may be involved in pathological states linked to circadian system malfunctioning, such as depression, shift work or jet lag. 1A and GABA A receptors interact in the ventromedial nucleus of female rat hypothalamus to regulate lordosis and thus modulate sexual behavior [67].…”