EFFECT OF NMDA RECEPTOR BLOCKADE ON MELATONIN AND ACTIVITY RHYTHM RESPONSES TO A LIGHT-PULSE IN RATS

The possible role of the excitatory amino acids as mediators of the ac ute suppression and subsequent delay by light of pineal melatonin prod uction was studied in rats using the NMDA receptor antagonist MK-801, Saline or MK-801 in doses up to 3 mg/kg (IF), was administered 15 min before a 15-min light pulse (200 Ix), 4 h after dark onset, and the ex cretion of 6-sulphatoxymefatonin (aMT.6S) determined. Under these cond itions saline injected/light exposed animals exhibited an acute, total but transient suppression of urinary aMT.6S excretion and a delay in the onset of aMT.6S the following night of 1.5 +/- 0.2 h, MK-801 faile d to block either the acute or phase delaying effect of light (onset d elayed by 2.2 +/- 0.4 h). Pretreatment with MK-801 (3 mg/kg) failed to block the effects of shorter, less intense light pulses 15 min before the pulse (e,g., 1 min/2 Ix; onset delayed by 2.0 +/- 0.4 h following saline, 1,5 +/- 0.1 h following MK-801) or 60 min before a short dura tion low intensity pulse. In other experiments MK-801 (1 and 3 mg/kg) failed to affect aMT.6S excretion when injected in the dark at the tim e of lights out or 4 h after dark onset. NMDA (10 and 30 mg/kg) inject ion at the time of lights out or 4 h after darkness did not mimic the effects of a tight pulse by decreasing aMT.6S excretion or causing a d elay in the onset of excretion the following night. Finally MK-801 (3 mg/kg) injected 4 h after dark failed to block the phase delaying effe cts of a 15 min light pulse (200 Ix) on running activity in rats, Thes e results do not support the hypothesis that excitatory amino acids in the retino-hypothalamic tract acting on the NMDA receptor subtype and terminating in the suprachiasmatic nucleus mediate the photic influen ces upon rat pineal melatonin and activity rhythms. Copyright (C) 1996 Elsevier Science Inc.