- Email: [email protected]
Circadian changes of type II adenylyl cyclase mRNA in the rat suprachiasmatic nuclei
Brain Research 810 Ž1998. 279–282
Short communication
Circadian changes of type II adenylyl cyclase mRNA in the rat suprachiasmatic nuclei Felino Ramon A. Cagampang a , Ferenc A. Antoni b , Susan M. Smith b , Hugh D. Piggins a , Clive W. Coen a, ) a
Anatomy and Human Biology, DiÕision of Biomedical Sciences, King’s College London, London, WC2R 2LS, UK b MRC Brain Metabolism Unit, UniÕersity of Edinburgh, Edinburgh, EH8 9JZ, UK Accepted 25 August 1998
Abstract Circadian functions of the suprachiasmatic nuclei ŽSCN. are influenced by cyclic AMP ŽcAMP.. Adenylyl cyclase type II ŽAC-II. is a cAMP-generating enzyme which, in the context of activation by Gsa , is further stimulated by protein kinase C or G protein bg subunits. Using in situ hybridization we have found a biphasic variation in AC-II mRNA within the rat SCN during the light–dark cycle Žpeaks at Zeitgeber time 6 and 18. and also in constant darkness Žpeaks at circadian time 2 and 14.. The cingulate cortex showed no such variation. These findings suggest that circadian changes in AC-II expression may be pertinent to the rhythmic functions of the SCN. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Suprachiasmatic nuclei; Circadian rhythm; Adenylyl cyclase; Cyclic AMP; In situ hybridization histochemistry
The intracellular second messenger cyclic adenosine 3X ,5X-monophosphate ŽcAMP. has been implicated in the circadian clock function of the SCN w5,7x. An increase in cAMP during the middle of the subjective day can advance the endogenous clock in vitro; at other phases of the cycle it is without effect on the rhythm w7,8x. Currently nine different isoforms of the cAMP-synthesizing enzyme adenylyl cyclase ŽAC; EC 4.6.1.1. are known; these have distinct functional properties and tissue distribution patterns w2,6,10,16x. The Ca2q-insensitive AC isoform AC-II is a positive integrator of signals derived from different membrane receptors w2,16x; once stimulated by Gs a –GTP it can be activated further by Gb g and by protein kinase C ŽPKC. w2,16x. AC-II is the only AC thus far identified in the SCN w6x. We have previously demonstrated that mRNAs for five PKC isoforms are present in the SCN and that the mRNAs for the four Ca2q-dependent PKC isoforms show a circadian rhythm at this site w3x. Given earlier reports of a circadian rhythm in the content of cAMP within the SCN w12,17x, we have investigated whether AC-II mRNA is
)
Corresponding author. [email protected]
Fax:
q44-171-873-2285;
E-mail:
differentially expressed in the SCN across the 24-h cycle in the presence and absence of photic cues. Adult male Wistar strain rats Ž220–300 g; Charles River, UK. were maintained in a 12:12 h light–dark ŽLD. schedule Žlights on at 07.00 h, designated as Zeitgeber time wZTx 0; temperature at 21 " 28C; food and water ad libitum.; ‘darkness’ consisted of constant dim red light providing - 2 lux at cage level with an emission spectrum between 600 and 700 nM, a range outside the spectral sensitivity curve for the circadian phase-shifting response w15x. To establish whether AC-II mRNA changes across the 24-h LD cycle, rats were killed at ZT 0, 2, 6, 10, 12, 14, 18 and 22. To test for such variation in the absence of the LD entrainment cues, lights were not turned on at the usual time of transition from dark to light Ždesignated as circadian time wCTx 0.; on day 2 of constant darkness ŽDD. the animals in this group were killed over the following 24 h at CT 0, 2, 6, 10, 12, 14, 18 and 22. Five animals were killed at each time point, with a maximum of 3 animals killed on any single occasion in order to reduce variation within a time point Ž- "5 min.. Brains were removed from the cranium, frozen in dry ice and stored at y708C. The cDNA corresponding to nucleotides 1755–2106 of the rat AC-II cDNA ŽGenBank M80550, courtesy of Dr. R.R. Reed, John Hopkins University, Baltimore, MD, USA. was ligated into pcDNA3 ŽInVitrogen. by standard proce-
0006-8993r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 6 - 8 9 9 3 Ž 9 8 . 0 0 9 3 4 - 2
280
F.R.A. Cagampang et al.r Brain Research 810 (1998) 279–282
dures. 35 S-labeled sense- and antisense-riboprobes were synthesized using the requisite RNA polymerases w4x. The specificity of the probe was established by RNA protection assay. In situ hybridization histochemistry was carried out as described previously w4x. All of the sections used for a particular study were processed concurrently and exposed to the film for the same duration; the LD and DD studies were carried out independently. The hybridization signals from film autoradiographs at the mid-rostrocaudal level of the SCN were analyzed with the Visilog program ŽData Cell, UK. as described previously w4x. Intense labelling for AC-II mRNA was found in the SCN ŽFig. 1. as well as in the cerebral cortex, supraoptic nuclei, caudate putamen and anterior thalamic nuclei; this is consistent with previous observations w6,10x. In RNAsetreated sections and in sections hybridized with the sense probe, no signals were generated Žresults not shown.. Quantitative analysis of the autoradiographs Ž n s 5 animals at each time point. indicates a significant variation of AC-II mRNA expression within the SCN ŽOne-way ANOVA: LD: F7,32 s 22.489, p - 0.001; DD: F7,32 s 24.518, p - 0.001.. There is a significant biphasic variation in AC-II mRNA in LD ŽFig. 2. with peaks during the day ŽZT 6. and night ŽZT 18.. A similar biphasic variation in the mRNA is found in DD ŽFig. 2. with peaks during the subjective day ŽCT 2. and subjective night ŽCT 14.. In the cingulate cortex ŽFig. 2. no variation in AC-II mRNA was found ŽLD: F7,32 s 0.501, p s 0.827; DD: F7,32 s 0.895, p s 0.522..
This study demonstrates that the expression of AC-II mRNA in the SCN varies across the 24-h cycle under both LD and DD conditions. In the LD condition, there is a biphasic variation with peak AC-II mRNA levels at midday and 12 h later at mid-night. Two peaks also occur in DD; one during the early part of the subjective day ŽCT 2. and the other 12 h later ŽCT 14.. It remains to be established whether the results reflect changes in gene transcription, mRNA degradation or both of these processes. AC-II mRNA in the cingulate cortex showed no significant variation across the circadian cycle under either LD or DD. Because the experiments were carried out independently, comparison of the signal levels under the two lighting conditions is not justified. The biphasic rhythm of AC-II mRNA on the second day in DD suggests that this is determined by the circadian pacemaker. The pattern of expression of AC-II mRNA is similar to the biphasic variation that has been found in the level of cAMP within the SCN of rats kept in constant darkness w17x or in the SCN maintained in vitro w12x; in both of those studies the level of cAMP in the SCN was found to be elevated during the late subjective day ŽCT 10. and late subjective night ŽCT 22.. Those findings together with the present results indicate that each of the cAMP peaks in the SCN occurs with a consistent latency of 4 to 8 h after a peak in the level of AC-II mRNA Žin either the LD or DD condition.. It should be noted that biphasic changes in cAMP-hydrolysis have also been observed in the SCN w12x; whether these andror the changes in AC-II
Fig. 1. Film autoradiograph generated from a coronal section of rat brain hybridized to the riboprobe for the AC-II mRNA. The SCN are located above the arrows. Bar s 1 mm.
F.R.A. Cagampang et al.r Brain Research 810 (1998) 279–282
281
Fig. 2. Expression of the mRNA for AC-II in the suprachiasmatic nuclei ŽSCN, left panels. and cingulate cortex ŽCCX, right panels. of animals killed during a light–dark cycle Žtop panels. or during constant dim red light Žbottom panels.. Open horizontal bars represent the light period; the dark period is indicated by the closed horizontal bars. Values are expressed as the mean Ž"S.E.M.. difference Ž D . in the relative optical density from film background Ž n s 5 animals per time point with 2 sections analyzed from each animal.. The data at Zeitgeber time ŽZT. 0 and circadian time ŽCT. 0 have been plotted again at ZT 24 and CT 24, respectively. Asterisks indicate peak values that are not significantly different from each other but differ significantly Ž p - 0.05. from the intervening levels.
mRNA expression actively contribute to the local variation in cAMP content remains to be investigated. AC-II functions as a positive integrator of Gs a , Gb g and PKC and appears largely resistant to inhibition by Gi a w2,16x. Thus, co-incidental activation of Gs and either Gi , Go , Gq or PKC may stimulate AC-II. This may occur through distinct membrane receptors activated by a single neurotransmitter Že.g., serotonin. or require coordinated release of two or more mediators. Alternatively, receptors that activate both Gs and Gq, such as those for VIP and PACAP w13x, may produce greatly amplified cAMP responses through AC-II. We have recently found a biphasic variation in the mRNAs for VIP and PACAP receptors within the SCN during constant darkness ŽRef. w4x, additional unpublished observations.. These peptides can reset the phase of the SCN clock in vitro w9,14x and in vivo
w1,11x, but only at restricted points in the circadian cycle. We have also observed biphasic expression of mRNAs for the Ca2q-dependent b1, b2 and g PKC isoforms in the SCN w3x; the peaks of expression occur at the beginning of both the subjective day and the subjective night as in the case of AC-II mRNA. On the basis of these findings we may speculate that coordinated changes in the expression of proteins within the cAMP and PKC signaling pathways may underlie some of the phase-dependent actions of neuroactive substances on the SCN.
Acknowledgements We are grateful to Dr. R.R. Reed for the rat AC-II plasmid cDNA, to Mr. T. Kalamatianos for his technical
282
F.R.A. Cagampang et al.r Brain Research 810 (1998) 279–282
assistance and to Prof. I.C. Campbell for his comments on the manuscript. This work was supported by grants from the BBSRC.
References w1x H.E. Albers, S.-Y. Liou, E.G. Stopa, R.T. Zoeller, Interaction of colocalized neuropeptides: functional significance in the circadian timing system, J. Neurosci. 11 Ž1991. 846–851. w2x F.A. Antoni, Calcium regulation of adenylyl cyclase: relevance for endocrine control, Trends Endo. Metab. 8 Ž1997. 7–14. w3x F.R.A. Cagampang, M. Rattray, I.C. Campbell, J.F. Powell, C.W. Coen, Variation in the expression of the mRNA for protein kinase C isoforms in the rat suprachiasmatic nuclei, caudate putamen and cerebral cortex, Mol. Brain Res. 53 Ž1998. 277–284. w4x F.R.A. Cagampang, W.J. Sheward, A.J. Harmar, H.D. Piggins, C.W. Coen, Circadian changes in the expression of vasoactive intestinal peptide 2 receptor mRNA in the rat suprachiasmatic nuclei, Mol. Brain Res. 54 Ž1998. 108–112. w5x L.N. Edmunds, I.A. Carre, C. Tamponnet, J. Tong, The roles of ions and second messengers in circadian clock function, Chronobiol. Int. 9 Ž1992. 180–200. w6x T. Furuyama, S. Inagaki, H. Takagi, Distribution of type II adenylyl cyclase mRNA in the rat brain, Mol. Brain Res. 19 Ž1993. 165–170. w7x M.U. Gillette, Regulation of entrainment pathways by the suprachiasmatic circadian clock: sensitivities to second messengers, Prog. Brain Res. 111 Ž1996. 121–132.
w8x M.U. Gillette, R.A. Prosser, Circadian rhythm of the rat suprachiasmatic brain slice is rapidly reset by daytime application of cAMP analogs, Brain Res. 474 Ž1988. 348–352. w9x J. Hannibal, J.M. Ding, D. Chen, J. Fahrenkrug, P.J. Larsen, M.U. Gillette, J.D. Mikkelsen, Pituitary adenylate cyclase-activating peptide ŽPACAP. in the retinohypothalamic tract: a potential daytime regulator of the biological clock, J. Neurosci. 17 Ž1997. 2637–2644. w10x N. Mons, D.M.F. Cooper, Adenylate cyclases—critical foci in neuronal signaling, Trends Neurosci. 18 Ž1995. 536–542. w11x H.D. Piggins, M.C. Antle, B. Rusak, Neuropeptides phase shift the mammalian circadian pacemaker, J. Neurosci. 15 Ž1995. 5612–5622. w12x R.A. Prosser, M.U. Gillette, Cyclic changes in cAMP concentration and phosphodiesterase activity in a mammalian circadian clock studied in vitro, Brain Res. 568 Ž1991. 185–192. w13x S.R. Rawlings, M. Hezareh, Pituitary adenylate cyclase-activating polypeptide ŽPACAP. and PACAPrVIP receptors: action on the anterior pituitary gland, Endocr. Rev. 17 Ž1996. 4–29. w14x S. Shibata, M. Ono, K. Tominaga, T. Hamada, A. Watanabe, S. Watanabe, Involvement of vasoactive intestinal polypeptide in NMDA-induced phase delay of firing activity rhythm in the suprachiasmatic nucleus in vitro, Neurosci. Biobehav. Rev. 18 Ž1994. 591–595. w15x J.S. Takahashi, P.J. DeCoursey, L. Bauman, M. Menaker, Spectral sensitivity of a novel photoreceptive system mediating entrainment of circadian rhythms, Nature 308 Ž1984. 186–188. w16x R. Taussig, A.G. Gilman, Mammalian membrane-bound adenylyl cyclases, J. Biol. Chem. 270 Ž1995. 1–4. w17x S. Yamazaki, M. Maruyama, F.R.A. Cagampang, S-I.T. Inouye, Circadian fluctuation of cAMP in the suprachiasmatic nucleus and the anterior hypothalamus of the rat, Brain Res. 651 Ž1994. 329–331.
Short communication
Circadian changes of type II adenylyl cyclase mRNA in the rat suprachiasmatic nuclei Felino Ramon A. Cagampang a , Ferenc A. Antoni b , Susan M. Smith b , Hugh D. Piggins a , Clive W. Coen a, ) a
Anatomy and Human Biology, DiÕision of Biomedical Sciences, King’s College London, London, WC2R 2LS, UK b MRC Brain Metabolism Unit, UniÕersity of Edinburgh, Edinburgh, EH8 9JZ, UK Accepted 25 August 1998
Abstract Circadian functions of the suprachiasmatic nuclei ŽSCN. are influenced by cyclic AMP ŽcAMP.. Adenylyl cyclase type II ŽAC-II. is a cAMP-generating enzyme which, in the context of activation by Gsa , is further stimulated by protein kinase C or G protein bg subunits. Using in situ hybridization we have found a biphasic variation in AC-II mRNA within the rat SCN during the light–dark cycle Žpeaks at Zeitgeber time 6 and 18. and also in constant darkness Žpeaks at circadian time 2 and 14.. The cingulate cortex showed no such variation. These findings suggest that circadian changes in AC-II expression may be pertinent to the rhythmic functions of the SCN. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Suprachiasmatic nuclei; Circadian rhythm; Adenylyl cyclase; Cyclic AMP; In situ hybridization histochemistry
The intracellular second messenger cyclic adenosine 3X ,5X-monophosphate ŽcAMP. has been implicated in the circadian clock function of the SCN w5,7x. An increase in cAMP during the middle of the subjective day can advance the endogenous clock in vitro; at other phases of the cycle it is without effect on the rhythm w7,8x. Currently nine different isoforms of the cAMP-synthesizing enzyme adenylyl cyclase ŽAC; EC 4.6.1.1. are known; these have distinct functional properties and tissue distribution patterns w2,6,10,16x. The Ca2q-insensitive AC isoform AC-II is a positive integrator of signals derived from different membrane receptors w2,16x; once stimulated by Gs a –GTP it can be activated further by Gb g and by protein kinase C ŽPKC. w2,16x. AC-II is the only AC thus far identified in the SCN w6x. We have previously demonstrated that mRNAs for five PKC isoforms are present in the SCN and that the mRNAs for the four Ca2q-dependent PKC isoforms show a circadian rhythm at this site w3x. Given earlier reports of a circadian rhythm in the content of cAMP within the SCN w12,17x, we have investigated whether AC-II mRNA is
)
Corresponding author. [email protected]
Fax:
q44-171-873-2285;
E-mail:
differentially expressed in the SCN across the 24-h cycle in the presence and absence of photic cues. Adult male Wistar strain rats Ž220–300 g; Charles River, UK. were maintained in a 12:12 h light–dark ŽLD. schedule Žlights on at 07.00 h, designated as Zeitgeber time wZTx 0; temperature at 21 " 28C; food and water ad libitum.; ‘darkness’ consisted of constant dim red light providing - 2 lux at cage level with an emission spectrum between 600 and 700 nM, a range outside the spectral sensitivity curve for the circadian phase-shifting response w15x. To establish whether AC-II mRNA changes across the 24-h LD cycle, rats were killed at ZT 0, 2, 6, 10, 12, 14, 18 and 22. To test for such variation in the absence of the LD entrainment cues, lights were not turned on at the usual time of transition from dark to light Ždesignated as circadian time wCTx 0.; on day 2 of constant darkness ŽDD. the animals in this group were killed over the following 24 h at CT 0, 2, 6, 10, 12, 14, 18 and 22. Five animals were killed at each time point, with a maximum of 3 animals killed on any single occasion in order to reduce variation within a time point Ž- "5 min.. Brains were removed from the cranium, frozen in dry ice and stored at y708C. The cDNA corresponding to nucleotides 1755–2106 of the rat AC-II cDNA ŽGenBank M80550, courtesy of Dr. R.R. Reed, John Hopkins University, Baltimore, MD, USA. was ligated into pcDNA3 ŽInVitrogen. by standard proce-
0006-8993r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 6 - 8 9 9 3 Ž 9 8 . 0 0 9 3 4 - 2
280
F.R.A. Cagampang et al.r Brain Research 810 (1998) 279–282
dures. 35 S-labeled sense- and antisense-riboprobes were synthesized using the requisite RNA polymerases w4x. The specificity of the probe was established by RNA protection assay. In situ hybridization histochemistry was carried out as described previously w4x. All of the sections used for a particular study were processed concurrently and exposed to the film for the same duration; the LD and DD studies were carried out independently. The hybridization signals from film autoradiographs at the mid-rostrocaudal level of the SCN were analyzed with the Visilog program ŽData Cell, UK. as described previously w4x. Intense labelling for AC-II mRNA was found in the SCN ŽFig. 1. as well as in the cerebral cortex, supraoptic nuclei, caudate putamen and anterior thalamic nuclei; this is consistent with previous observations w6,10x. In RNAsetreated sections and in sections hybridized with the sense probe, no signals were generated Žresults not shown.. Quantitative analysis of the autoradiographs Ž n s 5 animals at each time point. indicates a significant variation of AC-II mRNA expression within the SCN ŽOne-way ANOVA: LD: F7,32 s 22.489, p - 0.001; DD: F7,32 s 24.518, p - 0.001.. There is a significant biphasic variation in AC-II mRNA in LD ŽFig. 2. with peaks during the day ŽZT 6. and night ŽZT 18.. A similar biphasic variation in the mRNA is found in DD ŽFig. 2. with peaks during the subjective day ŽCT 2. and subjective night ŽCT 14.. In the cingulate cortex ŽFig. 2. no variation in AC-II mRNA was found ŽLD: F7,32 s 0.501, p s 0.827; DD: F7,32 s 0.895, p s 0.522..
This study demonstrates that the expression of AC-II mRNA in the SCN varies across the 24-h cycle under both LD and DD conditions. In the LD condition, there is a biphasic variation with peak AC-II mRNA levels at midday and 12 h later at mid-night. Two peaks also occur in DD; one during the early part of the subjective day ŽCT 2. and the other 12 h later ŽCT 14.. It remains to be established whether the results reflect changes in gene transcription, mRNA degradation or both of these processes. AC-II mRNA in the cingulate cortex showed no significant variation across the circadian cycle under either LD or DD. Because the experiments were carried out independently, comparison of the signal levels under the two lighting conditions is not justified. The biphasic rhythm of AC-II mRNA on the second day in DD suggests that this is determined by the circadian pacemaker. The pattern of expression of AC-II mRNA is similar to the biphasic variation that has been found in the level of cAMP within the SCN of rats kept in constant darkness w17x or in the SCN maintained in vitro w12x; in both of those studies the level of cAMP in the SCN was found to be elevated during the late subjective day ŽCT 10. and late subjective night ŽCT 22.. Those findings together with the present results indicate that each of the cAMP peaks in the SCN occurs with a consistent latency of 4 to 8 h after a peak in the level of AC-II mRNA Žin either the LD or DD condition.. It should be noted that biphasic changes in cAMP-hydrolysis have also been observed in the SCN w12x; whether these andror the changes in AC-II
Fig. 1. Film autoradiograph generated from a coronal section of rat brain hybridized to the riboprobe for the AC-II mRNA. The SCN are located above the arrows. Bar s 1 mm.
F.R.A. Cagampang et al.r Brain Research 810 (1998) 279–282
281
Fig. 2. Expression of the mRNA for AC-II in the suprachiasmatic nuclei ŽSCN, left panels. and cingulate cortex ŽCCX, right panels. of animals killed during a light–dark cycle Žtop panels. or during constant dim red light Žbottom panels.. Open horizontal bars represent the light period; the dark period is indicated by the closed horizontal bars. Values are expressed as the mean Ž"S.E.M.. difference Ž D . in the relative optical density from film background Ž n s 5 animals per time point with 2 sections analyzed from each animal.. The data at Zeitgeber time ŽZT. 0 and circadian time ŽCT. 0 have been plotted again at ZT 24 and CT 24, respectively. Asterisks indicate peak values that are not significantly different from each other but differ significantly Ž p - 0.05. from the intervening levels.
mRNA expression actively contribute to the local variation in cAMP content remains to be investigated. AC-II functions as a positive integrator of Gs a , Gb g and PKC and appears largely resistant to inhibition by Gi a w2,16x. Thus, co-incidental activation of Gs and either Gi , Go , Gq or PKC may stimulate AC-II. This may occur through distinct membrane receptors activated by a single neurotransmitter Že.g., serotonin. or require coordinated release of two or more mediators. Alternatively, receptors that activate both Gs and Gq, such as those for VIP and PACAP w13x, may produce greatly amplified cAMP responses through AC-II. We have recently found a biphasic variation in the mRNAs for VIP and PACAP receptors within the SCN during constant darkness ŽRef. w4x, additional unpublished observations.. These peptides can reset the phase of the SCN clock in vitro w9,14x and in vivo
w1,11x, but only at restricted points in the circadian cycle. We have also observed biphasic expression of mRNAs for the Ca2q-dependent b1, b2 and g PKC isoforms in the SCN w3x; the peaks of expression occur at the beginning of both the subjective day and the subjective night as in the case of AC-II mRNA. On the basis of these findings we may speculate that coordinated changes in the expression of proteins within the cAMP and PKC signaling pathways may underlie some of the phase-dependent actions of neuroactive substances on the SCN.
Acknowledgements We are grateful to Dr. R.R. Reed for the rat AC-II plasmid cDNA, to Mr. T. Kalamatianos for his technical
282
F.R.A. Cagampang et al.r Brain Research 810 (1998) 279–282
assistance and to Prof. I.C. Campbell for his comments on the manuscript. This work was supported by grants from the BBSRC.
References w1x H.E. Albers, S.-Y. Liou, E.G. Stopa, R.T. Zoeller, Interaction of colocalized neuropeptides: functional significance in the circadian timing system, J. Neurosci. 11 Ž1991. 846–851. w2x F.A. Antoni, Calcium regulation of adenylyl cyclase: relevance for endocrine control, Trends Endo. Metab. 8 Ž1997. 7–14. w3x F.R.A. Cagampang, M. Rattray, I.C. Campbell, J.F. Powell, C.W. Coen, Variation in the expression of the mRNA for protein kinase C isoforms in the rat suprachiasmatic nuclei, caudate putamen and cerebral cortex, Mol. Brain Res. 53 Ž1998. 277–284. w4x F.R.A. Cagampang, W.J. Sheward, A.J. Harmar, H.D. Piggins, C.W. Coen, Circadian changes in the expression of vasoactive intestinal peptide 2 receptor mRNA in the rat suprachiasmatic nuclei, Mol. Brain Res. 54 Ž1998. 108–112. w5x L.N. Edmunds, I.A. Carre, C. Tamponnet, J. Tong, The roles of ions and second messengers in circadian clock function, Chronobiol. Int. 9 Ž1992. 180–200. w6x T. Furuyama, S. Inagaki, H. Takagi, Distribution of type II adenylyl cyclase mRNA in the rat brain, Mol. Brain Res. 19 Ž1993. 165–170. w7x M.U. Gillette, Regulation of entrainment pathways by the suprachiasmatic circadian clock: sensitivities to second messengers, Prog. Brain Res. 111 Ž1996. 121–132.
w8x M.U. Gillette, R.A. Prosser, Circadian rhythm of the rat suprachiasmatic brain slice is rapidly reset by daytime application of cAMP analogs, Brain Res. 474 Ž1988. 348–352. w9x J. Hannibal, J.M. Ding, D. Chen, J. Fahrenkrug, P.J. Larsen, M.U. Gillette, J.D. Mikkelsen, Pituitary adenylate cyclase-activating peptide ŽPACAP. in the retinohypothalamic tract: a potential daytime regulator of the biological clock, J. Neurosci. 17 Ž1997. 2637–2644. w10x N. Mons, D.M.F. Cooper, Adenylate cyclases—critical foci in neuronal signaling, Trends Neurosci. 18 Ž1995. 536–542. w11x H.D. Piggins, M.C. Antle, B. Rusak, Neuropeptides phase shift the mammalian circadian pacemaker, J. Neurosci. 15 Ž1995. 5612–5622. w12x R.A. Prosser, M.U. Gillette, Cyclic changes in cAMP concentration and phosphodiesterase activity in a mammalian circadian clock studied in vitro, Brain Res. 568 Ž1991. 185–192. w13x S.R. Rawlings, M. Hezareh, Pituitary adenylate cyclase-activating polypeptide ŽPACAP. and PACAPrVIP receptors: action on the anterior pituitary gland, Endocr. Rev. 17 Ž1996. 4–29. w14x S. Shibata, M. Ono, K. Tominaga, T. Hamada, A. Watanabe, S. Watanabe, Involvement of vasoactive intestinal polypeptide in NMDA-induced phase delay of firing activity rhythm in the suprachiasmatic nucleus in vitro, Neurosci. Biobehav. Rev. 18 Ž1994. 591–595. w15x J.S. Takahashi, P.J. DeCoursey, L. Bauman, M. Menaker, Spectral sensitivity of a novel photoreceptive system mediating entrainment of circadian rhythms, Nature 308 Ž1984. 186–188. w16x R. Taussig, A.G. Gilman, Mammalian membrane-bound adenylyl cyclases, J. Biol. Chem. 270 Ž1995. 1–4. w17x S. Yamazaki, M. Maruyama, F.R.A. Cagampang, S-I.T. Inouye, Circadian fluctuation of cAMP in the suprachiasmatic nucleus and the anterior hypothalamus of the rat, Brain Res. 651 Ž1994. 329–331.