Effects of Protein Restriction, Melatonin Administration, and Short Daylength on Brain Benzodiazepine Receptors in Prepubertal Male Rats

Abstract

The possibility that there are changes in brain benzodiazepine binding sites controlled by photoperiod was investigated in two strains of male rats. The hypothesis was tested by 3H-diazepam binding studies in various brain regions of prepubertal rats maintained in 14 or 10 h of light or treated with late-afternoon injections of melatonin (50 micrograms/day). Protein restriction was applied during the experiment to sensitise the animals to the treatments. Under the conditions employed, rats kept in short daylength throughout or kept on long photoperiod and given late-afternoon melatonin injections showed evidence of delayed puberty (seminal vesicle, ventral prostate, and testis weight decreased by 45%, 55%, and 60% respectively, compared to control rats). Binding measurements were made 1 h before and 2 and 5 h after the onset of darkness in the pubertal (42-day-old) or experimentally prepubertal rats. In the rats of the Porton strain (for which protein restriction was obligatory for the gonadal response) there was no consistent treatment or time effects on specific binding of 3H-diazepam to washed membranes of the hypothalamus, midbrain, or striatum. Similarly, there were no differences in the stimulation of 3H-diazepam binding by 100 microM GABA or the inhibition of binding by 50 microM N-acetyl 5 methoxy kynurenamine. By contrast, in Wistar rats, specific binding to midbrain membranes was reduced 5 h after dark compared to 2 h (37% saline; 20% melatonin) and the extent of stimulation by GABA in the hypothalamus was increased 5 h after darkness (35.6% to 46.7% saline; 37.4% to 50% melatonin). Melatonin treatment resulted in significantly higher specific binding in the hypothalamus 2 h after dark (10%, control fed; 20%, protein restricted) but reduced the GABA induced stimulation of binding in the midbrain (35.5% to 25%, control fed; 33.7% to 23.5%, protein restricted). The Bmax of benzodiazepine binding to unwashed cortical P2 synaptosomal membranes has been reported to increase twofold in adult Wistar rats at mid-dark. By contrast the Bmax of juvenile Wistar rats in this study increased only 17% (116 +/- 2.4 fmol/mg protein to 140 +/- 3 fmol/mg protein) between 2 and 5 h after darkness. In melatonin-treated animals the increase in Bmax of 3H-diazepam binding was blocked (124 +/- 5 fmol/mg protein at 2 h; 127 +/- 3 fmol/mg protein at 5 h) and the Kd reduced (4.5 +/- 0.5 to 4.0 +/- 0.2 nM).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of melatonin on the regulation of proenkephalin and prodynorphin mRNA levels induced by kainic acid in the rat hippocampus

“…Although the exact mechanisms underlying the anticonvulsant effect of melatonin in the KA-induced limbic seizure model are not understood, melatonin is believed to influence inhibitory GABAergic neurotransmission (Golombek et al, 1996). Indeed, such an action of melatonin might interfere with KA-induced excitotoxicity and reduce toxic oxidative damage through a mechanism that does not require reactive oxygen species scavenging (Kennaway et al, 1988). Since drugs which are known to enhance GABA-mediated neurotransmission, such as diazepam, pentobarbital, and valproic acid, block both seizure activity and increases in the AP-1 binding factor (Sonnenberg et al, 1989a,b;Pennypacker et al, 1993), the anticonvulsant effect of melatonin may reduce the activation of KA-induced c-Fos, Fra-2, c-Jun, and JunB expression as well as their DNA binding activity to the AP-1 DNA motif.…”

Kynuramines, metabolites of melatonin and other indoles: the resurrection of an almost forgotten class of biogenic amines

“…After the discovery of AFMK and AMK, a number of studies has analyzed the effects of these compounds, mainly but not exclusively with the intention of identifying actions related to the parent compound, melatonin. Binding of AFMK to melatonin receptors [155, 156] and to the benzodiazepine‐binding sites of GABA receptor complexes [157, 158] revealed, however, only moderate affinities. AMK binding to melatonin receptors was also almost two orders of magnitude lower than that of melatonin [159, 160]; such an action had been speculated because of potent inhibition of retinal dopamine release (reported IC 50 = 1 n m ) [130].…”

Melatonin Beyond Its Classical Functions

“…Attempts of identifying actions of AFMK were of limited success, when effects on the reproductive system [224,225] and affinities to benzodiazepine [226,227] and melatonin receptors [228,229] were investigated. However, an interesting chronobiological effect of AFMK was described, which was never followed up later in other systems, namely, an acceleration of the reentrainment of the melatonin rhythm in rats [230].…”