Serotonin regulates the phase of the rat suprachiasmatic circadian pacemaker in vitro only during the subjective day.

Abstract

SUMMARY1. The suprachiasmatic nucleus (SCN) of the hypothalamus is the primary pacemaker for circadian rhythms in mammals. The 24 h pacemaker is endogenous to the SCN and persists for multiple cycles in the suprachiasmatic brain slice.2. While serotonin is not endogenous to the SCN, a major midbrain hypothalamic afferent pathway is serotonergic. Within this tract the dorsal raphe nucleus sends direct projections to the ventrolateral portions of the SCN. We investigated a possible regulatory role for serotonin in the mammalian circadian system by examining its effect, when applied at projection sites, on the circadian rhythm of neuronal activity in rat SCN in vitro.3. Eight-week-old male rats from our inbred colony, housed on a 12 h light: 12 h dark schedule, were used. Hypothalamic brain slices containing the paired SCN were prepared in the day and maintained in glucose and bicarbonate-supplemented balanced salt solution for up to 53 h. 4. A 1011 ml drop of 10-6 M-serotonin (5-hydroxytryptamine (5-HT) creatinine sulphate complex) in medium was applied to the ventrolateral portion of one of the SCN for 5 min on the first day in vitro. The effect of the treatment at each of seven time points across the circadian cycle was examined. The rhythm of spontaneous neuronal activity was recorded extracellularly on the second and third days in vitro. Phase shifts were determined by comparing the time-of-peak of neuronal activity in serotonin-vs. media-treated slices.5. Application of serotonin during the subjective day induced significant advances in the phase of the electrical activity rhythm (n = 11). The most sensitive time of treatment was CT 7 (circadian time 7 is 7 h after 'lights on' in the animal colony), when a 70 + 0-1 h phase advance was observed (n = 3). This phase advance was perpetuated on day 3 in vitro without decrement. Serotonin treatment during the subjective night had no effect on the timing of the electrical activity rhythm (n= 9).6. The specificity of the serotonin-induced phase change was assessed by treating t To whom correspondence should be addressed. MS 9615M. MEDANIC AND M. U. GILLETTE slices in the same manner with a microdrop of serotonergic agonists, 5-carboxamidotryptamine, that targets the 5-HT1 class of receptors, or 8-hydroxydipropylaminotetralin (8-OH DPAT), that acts on the 5-HTlA receptor subtype. Daytime (CT 9) application of 5-carboxamidotryptamine resulted in a 6-0+001 h phase advance (n = 3), while treatment during the subjective night (CT 15, n = 2) had little observable effect. Similarly, treatment with 8-OH DPAT at CT 9 induced a phase advance of 69 + 01 h (n = 3) in the rhythm of electrical activity.7. These results demonstrate that serotonin can induce large phase changes in the SCN circadian pacemaker and that the SCN undergoes endogenous changes in sensitivity to serotonin. The data suggest that serotonergic inputs to the ventrolateral SCN can regulate the circadian pacemaker during the daytime.

A New Concept for Melatonin Deficit On Pineal Calcification and Melatonin Excretion

“…There is overwhelming evidence that melatonin exerts its chronobiotic effects via the SCN (Cassone et al 1986). On the other hand, there are many hints pointing to the assumption that the SCN`s sensitivity for light and melatonin varies with time (Thompson et al 1990;Owen and Arendt 1992;Dubocovich et al 1996) as well as under the influence of various modulators (Medanic and Gillette 1992;Rea et al 1994;Duncan et al 1998). It also varies between states (McIntyre et al 1990), but moreover between individuals (Lewy et al 1985;McArthur et al 1996).…”

Circadian actions of melatonin at the suprachiasmatic nucleus

“…Phase advances were also induced at CT 10 by 2-[125I]-iodomelatonin [23]. However, application of other neurochemicals [ 11 ], including serotonin [25], neuropeptide Y [26] and glutamate [4], at this time did not induce the same pattern of phase shifts as melatonin. The melatonin-sensitive periods appear to bracket the transition points in the entrained lighting cycle.…”

Pituitary adenylyl cyclase-activating peptide: A pivotal modulator of glutamatergic regulation of the suprachiasmatic circadian clock

“…Perfusion was stopped temporarily during drug administration. All testing agents were dissolved in fresh medium, the pH was adjusted to 7.2, the medium was reoxygenated, and 1 l was applied directly to the SCN by micropipette (19). The effective concentration is diluted 10-100 times as the microdrop diffuses into the brain slice.…”