Immediate-early genes and the neural bases of photic and non-photic entrainment

The expression of immediate-early genes (IEGs) within the mammalian suprachiasmatic nucleus (SCN) identifies individual light-responsive cells of the circadian system. Cells immunoreactive for products of IEGs form a neurochemically heterogeneous population, of which a few are VIP (vasoactive intestinal peptide)-immunoreactive or GRP (gastrin-releasing peptide)-immunoreactive, although the phenotypes of most of the others have yet to be determined. Dual-labelling experiments with anatomical tracers reveal that only a minority of efferent projection neurons of the SCN are immunoreactive for IEG products, and it is likely that the majority of the immunoreactive cells are interneurons or glia. Photic induction of IEGs is mediated via NMDA (N-methyl-D-aspartate) and non-NMDA glutamatergic receptors, the SCN expressing a topographically specific complement of subtypes of the NMDA receptor. Non-photic cues (arousal) can shift the clock but this is not associated with expression of IEGs, demonstrating that the proteins encoded by IEGs are probably involved in transducing photic cues, rather than shifting the clock per se. Their induction provides an anatomically explicit marker for circadian phase and photic sensitivity and so is useful in analyses of circadian function, for example, in the tau mutant hamster. Non-photic phase shifts are accompanied by adrenocortical activation, confirming the importance of arousal in shifting of the clock. The phase-shifting effect of arousal can be blocked by treatment with the serotonin receptor antagonist ketanserin, suggesting that ascending serotonergic input to the forebrain, possibly directly to the SCN, is an important mediator of entrainment by arousal.

[Metastases of perianal gland carcinoma in a dog: palliative tumor therapy]

Palliative radiation therapy may provide significant relief in symptoms in pets with incurable cancer. Therapy is applied over a short period of time, using larger than normal fractional doses. Palliative radiation leads to minimal or no side effects. This case report describes the situation of a miniature poodle that was presented with severe dyschezia as well as fresh blood in the feces. The dog had a large abdominal mass which was diagnosed as lymph-node metastasis of a perianal gland carcinoma. Therapy included palliative radiation as well as chemotherapy. Six months after initial presentation the dog is free of clinical symptoms.

Melatonin inhibits GnRH-induced increase of cFOS immunoreactivity in neonatal rat pituitary

In neonatal rat gonadotrophs, melatonin inhibits several GnRH-induced effects: stimulation of LH release as well as the increase of several second messengers as cAMP, diacylglycerol and [Ca2+]i . Recently, GnRH has been shown to induce expression of immediate early genes of fos and jun family in adult rat gonadotrophs. The purpose of this study was to determine, whether melatonin inhibits the GnRH-induced induction of cFos in neonatal rat pituitary cells. The effects of GnRH and/or melatonin on cFos immunoreactivity was determined in primary cultures of neonatal rat pituitary cells attached to the coverslip. GnRH (3 nM) induced a time-dependent increase of cFos immunoreactivity in about 10 to 15% of the cultured cells. Significant increase was observed already 30 min after GnRH administration, the maximal increase occurred after about 60 min and then gradually decreased. Melatonin (100 nM) markedly attenuated the GnRH-induced increase. GnRH increased cFos immunoreactivity in the cells in a dose-dependent manner (EC50=36 pM) and melatonin (100 nM) attenuated the response at all GnRH concentrations tested. Melatonin had no effect on basal cFos immunoreactivity, but it inhibited the GnRH-induced (10 nM) increase of cFos in dose-dependent manner (EC50=12 pM). In conclusion, this is the first report showing the inhibitory effect of melatonin on gene transcriptional activity in gonadotrophs.

Serotonergic antagonists impair arousal-induced phase shifts of the circadian system of the syrian hamster

Single episodes of arousal of Syrian hamsters 2 h before projected activity onset (i.e., CT 10) phase-advanced their free-running circadian rhythm of wheel-running. Serial arousal once every 23 h or once every 23.5 h for 7 days caused large composite phase-advances to the wheel-running rhythm, the latter period being more effective in supporting an interval of stable entrainment. Pre-treatment of hamsters at CT 6 with the serotonergic antagonist ritanserin (1-5 mg/kg, which acts at both 5-HT2 and the putative 5-HT7 receptor, impaired the phase-advancing response to arousal at CT 10 but the drug was without effect on phase advances induced by exposure to light. Pre-treatment with a second serotonergic antagonist, ketanserin (1-5 mg/kg), which is without effect at 5-HT7 but has high affinity for 5-HT2 receptors, was also effective in attenuating the phase advancing effect of arousal at CT 10. However, neither agent was able to achieve complete blockade of the phase advances. These results are discussed in relation to in vitro and in vivo studies in the rat which have identified a role for 5-HT7 receptors in serotonin-mediated circadian entrainment.

Non-photic circadian entrainment in the Syrian hamster is not associated with phosphorylation of the transcriptional regulator CREB within the suprachiasmatic nucleus, but is associated with adrenocortical activation

The mechanisms which mediate non-photic entrainment of the circadian clock are not known. The free-running circadian rhythm of the wheel-running activity of Syrian hamsters was phase advanced by subcutaneous injection of saline and the associated handling at circadian time (CT) 10 (2 h before activity onset, CT 12). Over several experiments, approximately 80% of the animals were sensitive to this non-photic cue and were, therefore, designated as 'shifters'. Injection at CT 6 had no phase-shifting effect in any animals. A series of six injections presented to shifters at CT 10 over 14 circadian cycles produced a cumulative phase advance to the activity rhythm which free ran from the phase established by the final injection, demonstrating that the non-photic stimulus caused true entrainment of the circadian clock, rather than simply masking its expression. Entrainment by light is associated with the rapid phosphorylation of the transcription regulatory factor, cyclic adenosine monophosphate response element binding protein (CREB), in the suprachiasmatic nuclei (SCN), the principal circadian clock. The induction of immunoreactivity to the phosphorylated form of CREB (P-CREB-ir) was used to determine whether phosphorylation is also activated by a non-photic phase-shifting cue. No significant P-CREB-ir was found in shifters maintained under constant dim red light and sampled at either CT 10.05 or 14.05, i.e., times when animals are sensitive to non-photic stimuli and light pulses, respectively. Exposure of animals to light for 5 min at CT 14 caused a dramatic increase in P-CREB-ir within the SCN at CT 14.05. However, despite being sensitive to the phase-shifting effects of handling and injection at CT 10, none of the shifters sampled 5 min after injection of saline at CT 10 exhibited P-CREB-ir within the SCN. These data demonstrate that non-photic cues do not activate a cellular pathway (phosphorylation of CREB) known to be activated within the SCN by light. To assess the importance of arousal in non-photic entrainment, the effect of injection of saline upon activation of the adrenocortical axis, a peripheral index of arousal, was investigated. As anticipated, there was a marked circadian variation in serum cortisol concentrations. In animals identified as shifters, the levels were low during mid-subjective day (CT 6) and rose in advance of activity onset (CT 10). Injection and handling at CT 6 did not have a did not have a significant effect upon serum cortisol levels. In contrast, injection of shifters at CT 10 was associated with a pronounced rise in serum cortisol levels.(ABSTRACT TRUNCATED AT 400 WORDS)