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

Abstract

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.

Fig. 1

The effects of aging and time of day on SCN expression of GRP and VIP mRNAs. (A) Aging but not time of day decreased SCN expression of GRP mRNA. (B) SCN VIP mRNA was lower in old hamsters and was not affected by time of day. Bars are the mean + SEM (n = 5–8 per group).

Fig. 2

Photomicrographs depicting in situ hybridization of GRP mRNA in the SCN. Scale bar, 1 mm (refers to all panels).

Fig. 3

Photomicrographs depicting in situ hybridization of VIP mRNA in the SCN. Scale bar, which represents 1 mm, refers to all panels.

Fig. 4

The effects of aging and fluoxetine administration (15 mg/kg per day) on SCN expression of VIP mRNA and specific [¹²⁵I]iodocyanopindolol (ICYP) binding to 5-HT_1B receptors at ZT6. (A) Aging significantly decreased SCN VIP mRNA expression but fluoxetine treatment had no effect. (B) Aging increased specific [¹²⁵I]ICYP binding in the SCN; fluoxetine administration blocked this age-related increase but did not alter specific [¹²⁵I]ICPY binding in young hamsters. Bars represent the mean + SEM (n = 7–12 per group). *P < 0.05 vs. young vehicle-treated group.

Fig. 5

Photomicrographs depicting specific [¹²⁵I]ICYP binding in the SCN. Scale bar, 1 mm (refers to all panels).

Fig. 6

The effects of aging and fluoxetine administration (12 mg/kg per day) on SCN expression of VIP mRNA and specific [¹²⁵I]ICYP binding to 5-HT_1B receptors at ZT13. (A) Aging decreased SCN VIP mRNA expression at ZT13; this effect was not attenuated by fluoxetine. (B) Fluoxetine but not aging decreased specific [¹²⁵I]ICYP binding in the SCN at ZT13. Bars are the mean + SEM (n = 8–15 per group).

Fig. 7

The effects of aging and time of day on specific [¹²⁵I]ICYP binding in the SCN and specific [³H]paroxetine (PRX) binding in the SCN. (A) Specific [¹²⁵I]ICYP binding in the SCN was greater in old hamsters than in young hamsters at ZT6 but not at ZT13. *P < 0.05 vs. young at same timepoint. (B) Aging increased specific [³H]paroxetine binding sites in the SCN while time of day had no effect.

Fig. 8

Photomicrographs depicting specific [³H]paroxetine binding in the SCN. Scale bar, 1 mm (refers to all panels).