Biomedical subjects

B Rusak

Publications and source records attributed to B Rusak.

At least 37 records · Page 2Linked to original sources

PubMed1998

Chronic exposure to melatonin receptor agonists does not alter their effects on suprachiasmatic nucleus neurons.

Previous studies have demonstrated that melatonin and a novel melatonin receptor agonist, S20098 (N-[2-(7-methoxy-1-naphthyl) ethyl] acetamide), regulate neuronal firing activity of photically responsive cells in the suprachiasmatic nucleus in vivo. In the present study, we used several different methods to investigate the effects of chronic daily treatment with melatonin, S20098 (1.0 mg/kg, s.c.) or control vehicle for 14 d on responsiveness of suprachiasmatic nucleus cells to these agonists. Both chronic and acute application of drugs were carried out during the day-night transition period. We confirmed that suprachiasmatic nucleus cells from control animals were most sensitive at this circadian phase. Chronic drug treatments did not alter sensitivity of photically responsive suprachiasmatic nucleus cells to S20098 or melatonin given intraperitoneally (i.p.) or iontophoretically in vivo. Suprachiasmatic nucleus cells studied in brain slice preparations also responded similarly to micropressure ejections of melatonin receptor agonists regardless of drug pretreatment. These results indicate that chronic melatonin receptor agonist pretreatment does not result in desensitization of suprachiasmatic nucleus neuronal responses to these agonists during the daily phase of maximum melatonin sensitivity.

Acetamides↗

PubMed1997

Melatonin does not influence the expression of c-fos in the suprachiasmatic nucleus of rats and hamsters.

We have assessed whether melatonin can induce c-fos expression at various circadian phases, and whether melatonin can inhibit photically induced c-fos expression in the suprachiasmatic nucleus (SCN) in both rats and Syrian hamsters. Subcutaneous administration of melatonin at a dose of 100 microg/kg neither induced expression of Fos, the protein product of the c-fos proto-oncogene, nor inhibited the expression of Fos-like immunoreactivity (Fos-lir) induced by a light pulse in the SCN of rats and hamsters. In situ hybridization studies also demonstrated the absence of induction by acute melatonin treatments of c-fos mRNA in the SCN. Taken together, these results demonstrate that melatonin effects on SCN cells involve signal transduction pathways that do not include regulation of c-fos gene expression.

Animals↗

PubMed1997

Distribution of ionotropic glutamate receptor subunit immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet of the hamster.

Glutamate is thought to mediate the effects of light on the circadian pacemaker contained in the suprachiasmatic nucleus. Glutamate can reset this pacemaker both in vivo and in vitro while glutamate antagonists can reduce photically induced phase shifts in activity rhythms and c-fos expression in the suprachiasmatic nucleus. Most behavioural and gene expression experiments investigating circadian rhythms use hamsters, but the majority of the anatomical data on the presence and distribution of selected glutamate receptor subunits in the suprachiasmatic nucleus has been collected from rat. In the present study, we examined the distribution of ionotropic glutamate receptor subunits in the hamster suprachiasmatic nucleus using mono- and polyclonal antibodies directed against these subunits. In addition, we examined the distribution of immunostaining for these subunits in a second structure of the mammalian circadian system, the intergeniculate leaflet of the thalamus since it also is thought to receive glutamatergic input from the retina and is important in the entrainment of circadian rhythms. The results indicated that all of the subunits investigated (GluR1, GluR2/3, GluR4, GluR5/6/7, and NMDAR1) were present in the suprachiasmatic nucleus and that all but GluR4 were present in the intergeniculate leaflet. Each of the subunits investigated had a unique pattern of distribution and intensity of staining. The distribution of immunoreactivity for these subunits in the hamster suprachiasmatic nucleus and intergeniculate leaflet differed from that reported in the rat. The presence of these subunits in the suprachiasmatic nucleus and intergeniculate leaflet implies the presence of functional NMDA and non-NMDA receptors in these structures that may have a role in photic entrainment of the circadian pacemaker.

Animals↗

PubMed1997

5-HT7 receptors mediate serotonergic effects on light-sensitive suprachiasmatic nucleus neurons.

Serotonin (5-HT) has been shown to phase shift circadian rhythms in mammals and to affect responses of the circadian system to light, but it is not clear which receptors are involved in these actions. We found that drugs which act as 5-HT1A receptor agonists suppressed photic responses of hamster SCN cells, but these drugs also exhibit high affinity for the recently cloned 5-HT7 receptor. We therefore studied the effects of 5-HT agonists and antagonists with differential affinities for 5-HT7 and 5-HT1A receptors on responses of hamster SCN cells to retinal illumination. We confirmed that the 5-HT receptor agonists 5-HT, 8-OH-DPAT and 5-CT, dose-dependently reduced photic activation of SCN cells. These effects could be blocked by co-application of antagonists with high affinities for 5-HT7 receptors: ritanserin or clozapine. The 5-HT1A/B/D antagonist, cyanopindolol, which is inactive at 5-HT7 receptors, did not antagonize the actions of 8-OH-DPAT. Selective 5-HT1A antagonists, WAY100635 and p-MPPI, had weak or no antagonist effects on the responses to 8-OH-DPAT in the SCN, but they effectively antagonized the actions of 8-OH-DPAT in the hippocampus. In the cerebellar cortex where few 5-HT7 receptors are present, ritanserin failed to antagonize the effects of 8-OH-DPAT. Our results indicate that the 5-HT7 receptor subtype plays a major role in mediating the effects of 5-HT on photic responses of SCN cells in the hamster.

Animals↗

PubMed1997

Sources of p75-nerve growth factor receptor-like immunoreactivity in the rat suprachiasmatic nucleus.

In mammals, the suprachiasmatic nucleus is critical for the generation of circadian rhythms and their entrainment to environmental cues. In the rat, the ventrolateral aspect of the suprachiasmatic nucleus receives a robust retinal input. This region also exhibits the most intense immunolabeling for the low-affinity nerve growth factor receptor in the forebrain. Our study was aimed at identifying the sources of this low-affinity nerve growth factor receptor immunoreactivity using immunohistochemistry combined with retrograde tract-tracing, and orbital enucleation. To determine the origin of the low-affinity nerve growth factor receptor immunoreactivity from sources extrinsic to the suprachiasmatic nucleus, unilateral injections of the retrograde tracer, Fluorogold, were made into the suprachiasmatic nucleus. Retrogradely labeled neurons that were also immunopositive for the low-affinity nerve growth factor receptor were found in both the basal forebrain and the retina. In the basal forebrain, such cells were found throughout its rostrocaudal extent, with the majority also being immunoreactive for the cholinergic marker, choline acetyltransferase. In the retina, cells retrogradely labeled with Fluorogold that were immunoreactive for low-affinity nerve growth factor receptor were located in the ganglion cell layer. Orbital enucleations were performed to confirm the findings observed following retrograde labeling in the retina. Unilateral orbital enucleations resulted in a significant reduction in low-affinity nerve growth factor receptor immunoreactivity in the contralateral suprachiasmatic nucleus compared to that seen on the ipsilateral side when examined one week post-surgery. Bilateral enucleations resulted in an equal decrease on both sides of the suprachiasmatic nucleus. Similar low-affinity nerve growth factor-like immunoreactivity was seen in the suprachiasmatic nucleus even two to four weeks after bilateral enucleations. Taken together, these findings suggest that low-affinity nerve growth factor receptors in the suprachiasmatic nucleus derive from multiple sources. While some receptors may be intrinsic to suprachiasmatic nucleus neurons, most appear to be of extrinsic origin and are located on axon terminals of basal forebrain cholinergic neurons and retinal ganglion cells.

Animals↗

PubMed1997

Effects of microinjections of substance P into the suprachiasmatic nucleus region on hamster wheel-running rhythms.

The suprachiasmatic nucleus (SCN) receives a direct retinal projection, which in rats includes substance P (SP)-immunoreactive retinal ganglion cells. While SP has been shown to have neurophysiological effects on SCN cells in Syrian hamsters and rats, it is not known what effects SP in the SCN has on circadian rhythms in hamsters. We examined this question using male Syrian hamsters that were implanted with cannulas aimed at the SCN region and maintained in constant dim red lighting conditions. Hamsters received 0.5 microl microinjections of saline or SP (500 pmol in saline) at a variety of circadian times (CT). Saline injections had little or no phase-shifting effects at any phase tested. SP had no significant effects at CT4-8, 16-20, or 20-24, but did cause small phase delays of -23.7 +/- 7 min (mean +/- sem) at CT12-16. In order to examine the dose-response relations of this effect, hamsters were also microinjected with 50 and 2500 pmol of SP at CT12-16. Both the 50 and 2500 pmol doses induced very small phase delays (-14.2 +/- 7 min and -18.2 +/- 5 min, respectively), indicating no obvious dose dependence within this range. These results do not suggest that SP alone in the SCN mimics light effects on circadian rhythms or is a key neurotransmitter involved in photic entrainment. It remains to be determined whether SP interacts with other transmitters in the SCN to modulate their effects on rhythm phase.

Animals↗

PubMed1996

Muscarinic receptors mediate carbachol-induced phase shifts of circadian activity rhythms in Syrian hamsters.

Carbachol, a non-specific cholinergic agonist, when administered intraventricularly or directly into the suprachiasmatic nucleus (SCN), causes phase-dependent shifts in circadian rhythms of wheel-running activity in rodents. The cholinergic receptor subtype involved in mediating these carbachol-induced phase shifts, however, remains uncertain. In order to investigate this issue we injected carbachol into the SCN through indwelling cannulas at circadian times (CT) 6, 14 and 22 in Syrian hamsters (Mesocricetus auratus) maintained in constant darkness. Carbachol elicited large phase advances at CT 6 (69.8 +/- 15.7 min; mean +/- S.E.M.) and CT 22 (83.9 +/- 24.8 min) and phase delays at CT 14 (59.7 +/- 18 min). We attempted to block the carbachol-induced phase shifts at these three phases using specific antagonists of nicotinic and muscarinic receptors. Mecamylamine, a nicotinic receptor antagonist, did not block carbachol-induced phase shifts at any of the phases tested. Atropine, a muscarinic receptor antagonist, blocked carbachol-induced phase shifts at CT 6 (-11.6 +/- 4.8 min; Mean phase shift +/- S.E.M.) and CT 22 (-20 +/- 6.6 min), suggesting that carbachol mediates its phase-shifting effects at these phases through muscarinic receptors.

Animals↗

PubMed1996

Distribution of pituitary adenylate cyclase activating polypeptide (PACAP) immunoreactivity in the hypothalamus and extended amygdala of the rat.

Pituitary adenylate cyclase activating polypeptide (PACAP) is found in two forms of 27 and 38 amino acids (PACAP-27 and PACAP-38 respectively) in the mammalian central nervous system. Using antibodies to these two forms of PACAP, we examined the distribution of PACAP immunoreactivity in the rat hypothalamus and a number of extrahypothalamic areas. The patterns of immunostaining for PACAP-27 and PACAP-38 were similar: prominent terminal labelling was present in the retrochiasmatic area, median eminence, and posterior periventricular nucleus of the hypothalamus as well as the bed nucleus of the stria terminalis and amygdaloid complex. After colchicine treatment, immunopositive cell bodies were found in the preoptic region of the periventricular zone of the hypothalamus, the suprachiasmatic and paraventricular hypothalamic nuclei, neural structures adjacent to the median eminence (including the retrochiasmatic area, arcuate nucleus, ventromedial hypothalamus, and tuber cinereum), and the lateral mammillary and supramammillary nuclei. In all these areas, immunolabelling appeared specific since it was abolished by preabsorption of primary antisera with the appropriate PACAP peptide. However, the number of immunopositive cells in the suprachiasmatic nucleus was also reduced by preabsorption of PACAP-27/38 antisera with vasoactive intestinal polypeptide, suggesting that a subpopulation of cells in the suprachiasmatic nucleus express a peptide which has significant sequence homology with both PACAP-27/38 and vasoactive intestinal polypeptide. The distribution of PACAP immunoreactivity throughout the hypothalamus, bed nucleus of the stria terminalis, and amygdala suggests the involvement of PACAP in a number of processes including limbic, autonomic, and neuroendocrine functions as well as regulation of the circadian pacemaker.

Amygdala↗

PubMed1996

Expression of fosB mRNA in the hamster suprachiasmatic nucleus is induced at only selected circadian phases.

We have studied the expression of fosB mRNA in the suprachiasmatic nucleus (SCN) of hamsters by in situ hybridization using oligonucleotides with sequences complementary to the C-terminal of the fosB mRNA sequence. In animals exposed for 48 h to darkness, there was little or no background expression in SCN cells of fosB mRNA at any circadian phase. Light pulses (30 min) were able to induce fosB expression only during the subjective night. Transcripts of fosB increased rapidly to peak by the end of a 30-min light pulse. Light-induced increases gradually declined in darkness, but levels were still elevated for up to 150 min after the light pulse. Induction in response to a light pulse was largely restricted to the ventrolateral portion of the nucleus which receives the heaviest retinal projection. The temporal and anatomical pattern of fosB mRNA expression in the hamster SCN therefore resembles that reported previously for other immediate-early genes, such as c-fos.

Analysis of Variance↗

PubMed1996

Spontaneous and light-evoked expression of JunB-like protein in the hamster suprachiasmatic nucleus near subjective dawn.

Some cells in the hamster suprachiasmatic nucleus (SCN) show a circadian rhythm of expression of junB mRNA in constant darkness, while others show junB mRNA only in response to light at night. We found that both the light-induced and spontaneous expressions of junB mRNA are translated into protein in SCN cells. In constant darkness, JunB-like immunoreactivity (lir) appears spontaneously in cells in the dorsal SCN around subjective dawn and persists for at least 4 h into the subjective day. During the subjective night, there is no spontaneous expression, but a light pulse can induce JunB-lir in cells throughout the SCN, and especially in the ventrolateral portion. As a component of AP-1 proteins, JunB may play a role both in mediating circadian responses to photic stimuli and in spontaneous oscillation of elements of the SCN circadian pacemaker.

Analysis of Variance↗

PubMed1996

Spontaneous circadian and light-induced expression of junB mRNA in the hamster suprachiasmatic nucleus.

We examined both spontaneous and light-evoked expression of junB mRNA in the hamster suprachiasmatic nucleus (SCN), an endogenous circadian pacemaker. junB was expressed in the SCN in response to a light pulse during the subjective night and early subjective day as well as spontaneously during the transition from subjective night to subjective day. Light-evoked expression was strongest in the ventral SCN, while spontaneous expression was stronger in the dorsal SCN. Spontaneous expression began around subjective dawn and persisted for at least 4 h into the subjective day. The expression of junB mRNA may play a role in both phase-shifting responses to light and in spontaneous oscillation of the SCN pacemaker.

Analysis of Variance↗

PubMed1996

Nerve growth factor phase shifts circadian activity rhythms in Syrian hamsters.

Nerve growth factor (NGF) receptors are found in high density in the rodent suprachiasmatic nucleus (SCN), a site which regulates mammalian circadian rhythms. We examined the effects of NGF (40 ng) or vehicle injections into the SCN, at circadian times (CT) 6, 14 or 22 on activity rhythms in hamsters maintained in constant darkness. NGF caused phase advances at CT6 (30.9 min) and CT22 (36.9 min), and phase delays at CT14 (31.2 min). Saline and cytochrome-c administration had no phase-shifting effects at CT6 and CT22, but at CT14 cytochrome-c produced large phase delays, implying that NGF-induced delays at this phase may be non-specific. Similarities between NGF-induced shifts and those elicited by the cholinergic agonist carbachol suggest a common mode of action.

Acetylcholine↗

PubMed1996

Melatonin analogues as agonists and antagonists in the circadian system and other brain areas.

We studied the effects of drugs related to melatonin on neuronal firing activity in the suprachiasmatic nucleus, intergeniculate leaflet and other brain areas in urethane-anesthetized Syrian hamsters. We tested melatonin and two naphthalenic derivatives of melatonin, a putative agonist (S20098: N-[2-(7-methoxy-1-naphthyl)ethyl]acetamide), and a putative antagonist (S20928: N-[2-(1-naphthyl)ethyl]cyclobutyl carboxamide). Both melatonin and S20098 given intraperitoneally (i.p.) were able to suppress firing rates of cells in a similar dose-dependent manner, but the effects of S20098 were longer lasting. Iontophoresis of melatonin dose dependently depressed spontaneous and light-evoked activity of cells in the suprachiasmatic nucleus and intergeniculate leaflet, while iontophoresis of S20098 was relatively ineffective, probably because it is a poorly charged compound. S20928 (2.0-10 mg/kg, i.p.) alone decreased firing rates of light-sensitive cells by 25-50% for 5-30 min in the suprachiasmatic nucleus and intergeniculate leaflet; however, low doses (< 2.0 mg/kg) of S20928 partially blocked the effects of melatonin agonists on most cells. The non-selective serotonin antagonist metergoline did not block the effects of either melatonin agonist. Both melatonin agonists and antagonists were less effective when applied to cells in the hippocampus and dorsal lateral geniculate nucleus. These results indicate that S20098 is an agonist acting probably on melatonin receptors in the Syrian hamster brain. S20928 may have mixed agonist/antagonist properties, but at low doses appears to function as an antagonist at melatonin receptors in the suprachiasmatic nucleus and intergeniculate leaflet.

Acetamides↗

PubMed1996

Activation of hamster suprachiasmatic neurons in vitro via metabotropic glutamate receptors.

The neurophysiological effects of ionophoretic application of the metabotropic glutamate receptor agonist (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid on the firing rate of single suprachiasmatic nucleus cells was studied in a hypothalamic slice preparation. In addition, the effects of the phenylglycine derivative (RS)-alpha-methyl-4-carboxyphenylglycine, a selective metabotropic glutamate receptor antagonist, on responses evoked by (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid and N-methyl-D-aspartate were assessed. (1S,3R)-1-Amino-1,3-cyclopentanedicarboxylic acid elicited current-dependent increases in neuronal activity in 65% of all suprachiasmatic nucleus cells studied, while N-methyl-D-aspartate activated 93% of the same cells. Cells in the ventrolateral suprachiasmatic nucleus were more sensitive to (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid application (82% of cells activated) than those in the dorsomedial suprachiasmatic nucleus (28% of cells activated). In addition, responses evoked by (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid in the ventrolateral suprachiasmatic nucleus had a rapid onset and a prolonged recovery from agonist application, whereas responses elicited in the dorsomedial suprachiasmatic nucleus were slower in onset and recovered more quickly from agonist application. Co-application of (RS)-alpha-methyl-4-carboxyphenylglycine selectivity attenuated (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid-evoked responses but had no effect on N-methyl-D-aspartate-evoked activity in the same cells. These results indicate that (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid application activates single cells in the hamster suprachiasmatic nucleus and these responses are selectively attenuated by (RS)-alpha-methyl-4-carboxyphenylglycine. Cells in the ventrolateral suprachiasmatic nucleus are more sensitive to (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid application than those in the dorsomedial suprachiasmatic nucleus. These results are the first demonstration that metabotropic glutamate receptors can modulate spontaneous neuronal activity within the suprachiasmatic nucleus, the predominant mammalian circadian pacemaker.

Animals↗

PubMed1995

Neonatal monosodium glutamate treatment prevents effects of constant light on circadian temperature rhythms of adult rats.

Housing rats under continuous illumination (LL) disrupts circadian rhythms controlled by a pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). The neural mechanisms underlying this effect are not well understood. The present study examined the effects of LL on circadian rhythms and on light-induced expression of Fos protein in the SCN, intergeniculate leaflet (IGL), and ventrolateral geniculate nucleus (vLGN) in adult rats treated neonatally with monosodium glutamate (MSG). Such treatment is known to lead to acute degeneration of retinal ganglion cells. Despite degeneration of the optic nerve, neonatal MSG treatment (2 mg/g SC on postnatal days 1,3,5,7, and 9) had no effect on daily temperature rhythms in the adult animal under a light-dark cycle. However, the disintegration of circadian rhythms under LL conditions observed in adult rats treated neonatally with 10% saline was prevented in MSG-treated rats. Furthermore, neonatal MSG treatment attenuated light-induced expression of Fos protein in the IGL and vLGN, but not in the SCN. These data suggest that neonatal MSG treatment alters the response of the circadian system to LL and that cells within the IGL/vLGN region may mediate this response.

Animals↗

PubMed1995

Ionophoretically applied substance P activates hamster suprachiasmatic nucleus neurons.

ionophoretic ejection of substance P (SP) activated 31% and suppressed 9% of hamster suprachiasmatic nucleus (SCN) cells in vitro. Hamster SCN cells did not demonstrate variation in sensitivity to SP across the circadian phases tested. SP modulated the response of 47% of hamster SCN cells to the excitatory amino acid (EAA) agonists glutamate and N-methyl-D-aspartate (NMDA). The results indicate that SP can alter both the spontaneous and EAA-evoked firing rate characteristics of hamster SCN neurons.

Action Potentials↗

PubMed1995

Two distinct retinal projections to the hamster suprachiasmatic nucleus.

Electrical stimulation of the intergeniculate leaflet (IGL) region in hamsters can induce expression of Fos-like immunoreactivity (lir) in a restricted portion of the dorsolateral suprachiasmatic nucleus (SCN). The authors investigated whether the mechanisms by which stimulation affects SCN cells involves orthodromic activation of IGL cells projecting to the SCN or antidromic activation of retinal ganglion cells that send bifurcating axonal projections to both the IGL and the SCN. Bilateral optic enucleation strongly reduced induction of Fos-lir in SCN cells in response to electrical stimulation of the IGL region, implicating antidromic activation of retinal ganglion cells as the mechanism. This result implies that a class of retinal ganglion cells that project to the IGL also project selectively to the dorsolateral SCN. Earlier pharmacological studies suggest that this anatomically distinct retinal projection to the SCN is also neurochemically different from that innervating the rest of the nucleus.

Animals↗

PubMed1995

Neuropeptides phase shift the mammalian circadian pacemaker.

We studied the influence on circadian rhythms of peptides that have been reported to be colocalized in suprachiasmatic nucleus (SCN) neurons. Gastrin-releasing peptide (GRP1-27), peptide histidine isoleucine (PHI), and vasoactive intestinal polypeptide (VIP) were microinjected into the suprachiasmatic nucleus (SCN) region of Syrian hamsters free running under three different constant lighting conditions. All peptide injections caused phase-dependent phase shifts of hamster locomotor activity rhythms which were unaffected by constant lighting conditions. GRP1-27 (150 pmol) caused large phase delays when injected at circadian times (CT) 12-16, modest phase advances when administered at CT20-24, and few shifts during the subjective day. Injections of saline vehicle at any of these phases caused only very small phase shifts. Phase delays induced by GRP1-27 at CT12-14 were dose dependent, unrelated to injection volume (at a constant dose), and attenuated by pretreatment with the BN/GRP-preferring receptor antagonist BIM 26226. VIP (150 pmol) caused moderate phase delays at CT12-14 and moderate phase advances at CT20-24. PHI (150 pmol) caused moderate phase delays at CT12-14 only. Coadministration of 150 pmol of GRP1-27, PHI, and VIP in an equimolar neuropeptide cocktail (50 pmol of each peptide) caused phase delays at CT12-14 and phase advances at CT20-24 which did not differ from those induced by 150 pmol of GRP1-27 alone at these phases. The shifts induced by 150 pmol of the peptide cocktail were smaller than the sum of the shifts induced by 50 pmol doses of each peptide administered separately at those phases. Since the phase-delaying effects of the cocktail were weaker than the summed effects of the component 50 pmol doses of the peptides, these data demonstrate a lack of synergism among the effects of these peptides. Since GRP1-27 (150 pmol) evoked shifts similar in magnitude to those of the cocktail, there is no evidence that these apparently colocalized neuropeptides must interact to exert maximal effects on the circadian pacemaker.

Analysis of Variance↗