Gastrin-releasing peptide phase-shifts suprachiasmatic nuclei neuronal rhythms in vitro.

The main mammalian circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Gastrin-releasing peptide (GRP) and its receptor (BB(2)) are synthesized by rodent SCN neurons, but the role of GRP in circadian rhythm processes is unknown. In this study, we examined the phase-resetting actions of GRP on the electrical activity rhythms of hamster and rat SCN neurons in vitro. In both rat and hamster SCN slices, GRP treatment during the day did not alter the time of peak SCN firing. In contrast, GRP application early in the subjective night phase-delayed, whereas similar treatment later in the subjective night phase-advanced the firing rate rhythm in rat and hamster SCN slices. These phase shifts were completely blocked by the selective BB(2) receptor antagonist, [d-Phe(6), Des-Met(14)]-bombesin 6-14 ethylamide. We also investigated the temporal changes in the expression of genes for the BB(1) and BB(2) receptors in the rat SCN using a quantitative competitive RT-PCR protocol. The expression of the genes for both receptors was easily detected, but their expression did not vary over the diurnal cycle. These data show that GRP phase-dependently phase resets the rodent SCN circadian pacemaker in vitro apparently via the BB(2) receptor. Because this pattern of phase shifting resembles that of light on rodent behavioral rhythms, these results support the contention that GRP participates in the photic entrainment of the rodent SCN circadian pacemaker.

Expression of mt(1) melatonin receptor subtype mRNA in the entrained rat suprachiasmatic nucleus: a quantitative RT-PCR study across the diurnal cycle.

Melatonin acts on specific receptors in the suprachiasmatic nuclei (SCN) to phase-dependently regulate the phase of the circadian clock. How the gating of melatonin's effect is restricted to particular times of day is not known, but may be related to temporal differences in receptor availability. In the present study, we used a competitive reverse transcription-polymerase chain reaction (RT-PCR) method to determine if the expression of mt(1) melatonin receptor subtype mRNA in rat SCN varied across the 12:12 light-dark (LD) cycle. Measurement of core body temperature using radiotelemetry confirmed that the male Wistar rats used exhibited a robust diurnal rhythm. mt(1) receptor mRNA was readily detected in reduced SCN slices at all times of day. However, there was no significant variation in the amount of mt(1) mRNA with time of day. Expression of MT(2) melatonin receptor subtype mRNA in reduced SCN slices was confirmed by nested PCR. These results indicate that changes in the level of mt(1) mRNA do not underlie the diurnal and/or circadian variation in the response of the SCN circadian clock to the phase-resetting effects of melatonin.

Sexual dimorphism in N-acetyltransferase and melatonin levels in the giant freshwater prawn Macrobrachium rosenbergii de Man.

N-acetyltransferase (NAT) and melatonin were determined in the optic lobe of the giant freshwater prawn Macrobrachium rosenbergii de Man. The prawns were divided into three groups: fast-growing "jumper" males; slow-growing "laggard" males; and females. Both NAT and melatonin levels in the jumper and laggard males were comparable, whereas those of the female were significantly lower. The results suggested a sexual dimorphism in the NAT and melatonin in the optic lobe of this species. It was also found that when one optic lobe was isolated, the level of NAT and melatonin in the contralateral optic lobe did not show a compensatory increase in either males or females. On the contrary, melatonin was suppressed in the remaining optic lobes in both sexes.

N-acetyltransferase, hydroxyindole-O-methyltransferase and melatonin in the optic lobes of the giant tiger shrimp Penaeus monodon.

The activities of the enzymes N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) and the hormone melatonin were studied in the optic lobe of the subadult giant tiger shrimp Penaeus monodon. Compared with the level in other species, a relatively high level of NAT activity that was temperature- and pH-dependent were observed. The NAT enzyme had a relatively high maximum velocity (Vmax, 100 pmol/hr/micrograms protein) and low Michaelis constant (Km, 22 microM), when tryptamine is used as substrate. In contrast to the high level of NAT activity, HIOMT activity and melatonin levels were low in the optic lobe of the giant tiger shrimp. Sex differences in the levels of NAT activity and melatonin, which are observed in a freshwater species Macrobrachium rosenbergii, were not noticeable in the saltwater species P. monodon, at least not when they were in their subadult stage.

Continuous light increases N-acetyltransferase activity in the optic lobe of the giant freshwater prawn Macrobrachium rosenbergii de Man (Crustacea: Decapoda).

Giant freshwater prawns, Macrobrachium rosenbergii de Man, were reared under three different lighting conditions: continuous darkness (DD), 12 hr of light and 12 hr of darkness (LD 12:12) and continuous light (LL). After one month, the prawns were sacrificed and optic lobes isolated from the eyestalks were determined for N-acetyltransferase (NAT) activities and melatonin concentrations. Gonads were weighed and examined under light microscopy. The optic lobes from LL prawns contained significantly higher activities of NAT than those from LD 12:12 prawns. The melatonin concentrations and size and histological features of the gonads from the three groups of prawns did not differ. The results indicate that continuous light increases NAT activities in the optic lobe of M. rosenbergii but has no drastic effect on gonadal growth.