Biomedical subjects

M A Rea

Publications and source records attributed to M A Rea.

At least 19 recordsLinked to original sources

PubMed1995

Serotonergic potentiation of photic phase shifts of the circadian activity rhythm.

Recent evidence suggests that serotonin may function to regulate the sensitivity of the circadian clock to the resetting effect of environmental light. Here we report that systemic administration of NAN-190, a drug that acts at both postsynaptic and somatodendritic serotonin receptors, potentiates light-induced phase shifts by as much as 250%. The effects of the drug are dose-related and are significant at light intensities between 0.2 and 200 lux. It is proposed that drugs with pharmacological properties similar to NAN-190 may prove useful as chronobiologics to adjust the sensitivity of the circadian system to natural 'zeitgebers'.

Animals↗

PubMed1995

Tryptophan loading modulates light-induced responses in the mammalian circadian system.

Enhanced endogenous serotonergic activity, stimulated by L-tryptophan (TRYPT) loading, was found to have a substantial impact on neurochemical and behavioral aspects of the circadian response to light in the male Syrian hamster. An intraperitoneal (i.p.) injection of 150 mg/kg TRYPT significantly stimulated serotonin (5-HT) release in the suprachiasmatic nuclear (SCN) region, as reflected by a 205 +/- 30% maximal increase in the extracellular concentration of 5-HT assessed using microdialysis. Administration of TRYPT 1 h before exposure to a light pulse (30 min, 40 lux) delivered during late subjective night dose-dependently suppressed the number of SCN cells expressing light-induced Fos-like immunoreactivity (Fos-LI; maximal suppression @200 mg/kg was 77 +/- 4%, p < 0.001). This action of TRYPT was attenuated by pretreatment with the 5-HT1a antagonist, NAN-190, and was abolished by the 5-HT2/5-HT7 antagonist, ritanserin, or the nonselective 5-HT antagonist, metergoline (all 10 mg/kg). These antagonists alone had no effect on light-induced Fos. In a second experiment, pretreatment of free-running hamsters housed under constant darkness with 150 mg/kg TRYPT 45-60 min prior to light exposure (10 min, 20 lux) during late subjective night (CT 19) significantly attenuated the light-induced phase advances of the circadian activity rhythm (66 +/- 7 min vs. 100 +/- 6 min for vehicle controls; p < 0.001). The same dose of TRYPT given 1 h before lights-on for 5 consecutive days in hamsters maintained under 14L:10D altered the phase angle of entrainment such that activity onsets were delayed by 36 +/- 8 min relative to controls (p < 0.05). The same dose of TRYPT administered during late subjective night also suppressed the extracellular concentration of glutamate in the SCN region assessed using microdialysis (55 +/- 8% suppression; p < 0.05 vs. baseline). These results support the hypothesis that the ascending serotonergic projection to the SCN modulates photic entrainment processes within the circadian oscillator.

Animals↗

PubMed1995

Twelve-hour phase shifts of hamster circadian rhythms elicited by voluntary wheel running.

Running in a novel wheel can serve as a nonphotic zeitgeber to entrain or phase shift circadian rhythms in hamsters. In this study, hamsters were entrained to a light:dark schedule of 14:10 h but had no access to running wheels. At four different phase points of the light cycle, hamsters were transferred to constant darkness and provided with running wheels. All hamsters began running shortly after transfer and were allowed to continue running at their own volition. Approximately 20% of the hamsters transferred at zeitgeber time (ZT) 23 (ZT 12 = lights out) ran more than 4 h after transfer and showed phase advances of the circadian activity rhythm by as much as 15 h, while hamsters that ran less than 4 h on average did not phase shift. A similar result was observed for hamsters transferred at ZT 2. Hamsters transferred at ZT 5 and 8 also did not phase shift if they ran less than 4 h, although the relation between longer runs and phase shifts became less evident. A sustained run in excess of 4 h appeared to be associated with large phase advances. These results show that under certain conditions, a single sustained bout of wheel-running activity is capable of phase shifting the circadian pacemaker by more than 12 h.

Animals↗

PubMed1994

Resetting the biological clock: mediation of nocturnal circadian shifts by glutamate and NO.

Circadian rhythms of mammals are timed by an endogenous clock with a period of about 24 hours located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Light synchronizes this clock to the external environment by daily adjustments in the phase of the circadian oscillation. The mechanism has been thought to involve the release of excitatory amino acids from retinal afferents to the SCN. Brief treatment of rat SCN in vitro with glutamate (Glu), N-methyl-D-aspartate (NMDA), or nitric oxide (NO) generators produced lightlike phase shifts of circadian rhythms. The SCN exhibited calcium-dependent nitric oxide synthase (NOS) activity. Antagonists of NMDA or NOS pathways blocked Glu effects in vitro, and intracerebroventricular injection of a NOS inhibitor in vivo blocked the light-induced resetting of behavioral rhythms. Together, these data indicate that Glu release, NMDA receptor activation, NOS stimulation, and NO production link light activation of the retina to cellular changes within the SCN mediating the phase resetting of the biological clock.

Amino Acid Oxidoreductases↗

PubMed1994

Serotonergic inhibition of extracellular glutamate in the suprachiasmatic nuclear region assessed using in vivo brain microdialysis.

In previous studies, we showed that localized perfusion of the SCN region with serotonin (5-HT) or the non-selective serotonergic, quipazine, using the microdialysis technique significantly reduced the extracellular concentration of the excitatory amino acid (EAA), glutamate. The present investigation was undertaken to extend these findings by characterizing the effects of various classes of 5-HT receptor ligands on the extracellular glutamate concentration in the SCN. Localized SCN application or i.p. injection of the 5-HT1A receptor agonist, 8-OH-DPAT, during the dark phase (6 h after lights-off) significantly reduced the extracellular glutamate concentration in the SCN region from baseline levels (38.7 +/- 8.7 and 53.4 +/- 11.2%, respectively, of pretreatment values; P < 0.05). The effect of systemically applied 8-OH-DPAT was abolished by i.p. injection of the 5-HT1A receptor antagonist, NAN-190, administered 20 min before the 8-OH-DPAT. Localized perfusion of the SCN with the 5-HT1B receptor agonist, TMFPP, also reduced extracellular glutamate but to a lesser degree than 8-OH-DPAT (80.1 +/- 3.9% of pretreatment levels; P < 0.05). This effect was prevented by i.p. injection of the non-selective 5-HT receptor antagonist, metergoline 20 min before TFMPP perfusion. Localized perfusion of the SCN region with the 5-HT2 and 5-HT3 receptor agonists, alpha-methyl 5-HT and 1-phenylbiguanide, respectively, had little effect on extracellular glutamate (both P > 0.1 vs. baseline). Systemic treatment with NAN-190 alone had little effect on extracellular glutamate, however, similar treatments with metergoline or the 5-HT2 receptor antagonist, ritanserin, induced significant increases extracellular glutamate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Differential timing of amino acid and 5-HIAA rhythms in suprachiasmatic hypothalamus.

In vivo brain microdialysis was used to characterize the daily pattern of extracellular excitatory amino acids (EAA; glutamate and aspartate), glutamine and 5-hydroxyindoleacetic acid (5-HIAA) in the region of the suprachiasmatic nucleus (SCN) in freely behaving male Syrian hamsters. Diurnal variations in the extracellular concentrations of EAA and 5-HIAA observed under 14:10-h light-dark (LD) photoperiod were confirmed by cosinor analysis. Peak levels occurred during the night, with the mean acrophasis for 5-HI-AA concentration preceding that for EAA by 1-2 h. Release of EAA was stimulated by K(+)-induced depolarization in a Ca(2+)-dependent manner and was not affected by tetrodotoxin. In the absence of light cues (constant dim red light, < 0.4 lx; DD) there was a significant time-of-day effect in peak glutamate concentration (occurring during subjective night), but the rhythmic pattern of 5-HIAA output was lost. There was no apparent direct temporal linkage between peaks in extracellular EAA or 5-HIAA and bouts of wheel-running activity under LD or DD. These results are evidence that 1) the daily release pattern of extracellular glutamate, but not 5-HIAA, is circadian in nature; and 2) this rhythm in glutamate is not based on Na+ channel-dependent action potentials. Finally, a discordance in the timing of peak nocturnal extracellular 5-HIAA and EAA concentrations was evident, possibly reflecting an interplay between serotonergic and EAA pathways in the SCN.

Amino Acids↗

PubMed1993

Suprachiasmatic nucleus neurochemistry in the conscious brain: correlation with circadian activity rhythms.

The aim of the research reported here was to provide information on the neurochemical processes that underlie the generation and entrainment of mammalian circadian rhythms. The studies were centered principally around the in vivo brain microdialysis technique for assessing the daily pattern of neurotransmitter activity in the suprachiasmatic hypothalamus of freely behaving Syrian and Siberian hamsters. This approach yielded several findings related to the activities of serotonergic and excitatory amino acid systems in the region of the suprachiasmatic nuclei (SNC). Specifically, we found that (1) there were daily variations in the extracellular concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and glutamate (GLU) in the SCN region (highest levels of 5-HIAA occurred soon after lights-off, whereas GLU peaked later in the dark phase); (2) the daily rhythm of GLU, but not serotonin, in the SCN region appeared to be circadian in nature and was not driven by an external influence; (3) the rhythm in GLU measured in SCN microdialysate involved a tetrodotoxin-insensitive mechanism and did not appear to be directly linked to the expression of locomotor behavior; and (4) direct application of serotonin receptor agonists via the microdialysis probe suppressed the concentration of extracellular GLU in the SCN region, suggesting that serotonin may modulate GLU release in the SCN.

Animals↗

PubMed1992

Different populations of cells in the suprachiasmatic nuclei express c-fos in association with light-induced phase delays and advances of the free-running activity rhythm in hamsters.

Circadian rhythmicity is controlled by a light-entrainable pacemaker located in the suprachiasmatic nuclei (SCN) of the mammalian hypothalamus. Brief light exposure during the subjective night causes phase shifts of the free-running activity rhythm and expression of c-fos-related proteins (Fos) among a population of cells in the hamster SCN. Light exposure (30 lux for 15 min) during the early subjective night (CT13) causes phase delays (-60 +/- 12 min), while exposure at mid-subjective night (CT18) causes phase advances (114 +/- 48 min) of the free-running activity rhythm. Light exposure at mid-subjective day (CT6) does not cause phase alterations of the rhythm. Similarly, only light exposure at CT13 or CT18 induces Fos expression in the SCN. The distribution of Fos-immunoreactive cells in the SCN is more widespread in animals stimulated with light at CT18. In addition, a group of cells located dorsal and anterior to the SCN express Fos only after stimulation at CT18. The data are consistent with the hypothesis that Fos expression represents an event in the signal transduction pathway leading to light-induced alterations in circadian pacemaker function. Furthermore, the data raise the possibility that different populations of cells in the suprachiasmatic hypothalamus may participate in light-induced phase advances and delays of the circadian pacemaker.

Animals↗

PubMed1992

Diurnal variation in 5-hydroxyindole-acetic acid output in the suprachiasmatic region of the Siberian hamster assessed by in vivo microdialysis: evidence for nocturnal activation of serotonin release.

In vivo brain microdialysis was used to characterize the daily pattern of 5-hydroxyindole-acetic acid (5-HIAA) release in the region of the suprachiasmatic nuclei (SCN) in freely behaving male Siberian hamsters housed under 16L:8D. A marked diurnal variation in the concentration of extracellular 5-HIAA was apparent, with peak levels (147 +/- 5% of the daily mean; p < 0.05) occurring 2-3 h after lights-off. Smaller nocturnal rises in extracellular 5-HIAA were observed in the posterior hypothalamus and preoptic area (128 +/- 4 and 123 +/- 8% of the daily mean, respectively; both p < 0.05 vs. average daytime levels). Tryptophan loading increased 5-HIAA in SCN microdialysates by 44 +/- 6%, and this response was enhanced by localized perfusion with tetrodotoxin (TTX; 5 microM). Localized applications of KCl (150 mM) or veratridine (100 microM) decreased 5-HIAA by 62 +/- 5 or 49 +/- 11%, respectively. The effect of KCl was not significantly affected by specific calcium channel blockers. Perfusion with TTX markedly decreased SCN 5-HIAA during the dark phase, but had little effect during the light phase (42 +/- 8 vs. 12 +/- 5% suppression, respectively; p < 0.01). Addition of serotonin (3 microM) to the perfusate significantly stimulated 5-HIAA output. This treatment increased the release of 5-HIAA more during the dark than during the light phase (61 +/- 8 vs. 25 +/- 5%, respectively; p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Ultrasound and the blood-brain barrier.

High intensity focused ultrasound was employed to modify the permeability of the normal feline and canine blood-brain barrier (BBB) to a circulating vital dye--Evans blue (EB). The threshold doses (W sec/cm2) for focally increasing the permeability of the BBB in white matter (WM) and gray matter (GM) were as follows: internal capsule (WM)--340 to 680; thalamus (GM)--approximately 1326; and caudate nucleus (GM)--2284 to 2952. In the presence of supralesioning doses of ultrasound, the cross sectional area occupied by the EB was consistently greater than that of the attendant nonhemorrhagic lesion--thus suggesting that BBB changes may be inducible at sublesioning doses. These findings, in conjunction with those of others, suggest that high intensity focused ultrasound may have a role in the treatment of brain tumors based on cell destruction by two mechanisms: (a) direct, by the ultrasound and (b) indirect, by an antineoplastic agent which is delivered via an ultrasonically modified BBB.

Animals↗

PubMed1990

Glutamate is the endogenous amino acid selectively released by rat hippocampal mossy fiber synaptosomes concomitantly with prodynorphin-derived peptides.

The release of endogenous amino acids from depolarized rat hippocampal mossy fiber synaptosomes was investigated to assess the possible role(s) of glutamate and aspartate in mediating the excitatory mossy fiber synaptic input. The relative proportions of prodynorphin-derived peptides concomitantly released with amino acids were also determined to further characterize the biochemical basis for mossy fiber synaptic transmission. Of the 18 amino acids shown to be present in superfusate fractions by liquid chromatographic analysis, only glutamate was released at a significantly enhanced rate from K(+)-stimulated (35 mM KCl) mossy fiber nerve endings. The rates of glutamate and aspartate release were increased by 360 +/- 27% and 54 +/- 12% over baseline, respectively. However, the K(+)-evoked release of glutamate was substantially more Ca2(+)-dependent (80%) than was the release of aspartate (49%). The veratridine (45 microM)-evoked release of both acidic amino acids was entirely blocked by 1 microM tetrodotoxin. Depolarization (45 mM KCl) also stimulated the release of the four prodynorphin (Dyn) products examined, in a rank order of Dyn B much greater than Dyn A(1-17) greater than Dyn A(1-8) much greater than Dyn A(1-13), with Dyn B efflux increasing by more than 5-fold over baseline values. These results suggest that the predominant excitatory amino acid in hippocampal mossy fiber synaptic transmission may be glutamate and that this synaptic input may be modulated by at least four different products of prodynorphin processing.

Action Potentials↗

PubMed1990

VIP-stimulated cyclic AMP accumulation in the suprachiasmatic hypothalamus.

The effect of exogenous vasoactive intestinal peptide (VIP) on the accumulation of cyclic AMP in the isolated suprachiasmatic hypothalamus prepared from rats sacrificed at midsubjective day (CT6), subjective dusk (CT12) and midsubjective night (CT18) was determined. VIP (2 microM) caused a transient, 2.5- to 3.4-fold increase in cyclic AMP which peaked 5 minutes after the addition of VIP and returned to prestimulation values by 10 minutes. The effect of VIP was dose dependent between 0.1 and 10 microM. Cyclic AMP content, both before and after VIP stimulation, was slightly but significantly higher in rats sacrificed at CT6 compared to those sacrificed at CT18; however, the degree of stimulation was similar at all three circadian times.

Animals↗

PubMed1989

Detection of zinc in isolated nerve terminals using a modified Timm's sulfide-silver method.

An ultrastructural method for detecting the presence of zinc in isolated nerve terminals from the mammalian brain is described. This method is based on the well-known Timm's sulfide-silver technique that has been used by many investigators to detect and localize zinc-containing pathways in sections of intact brain tissue. We report here a modification of this technique that we have used to assess the homogeneity, at the electron microscopic level, of a zinc-enriched synaptosomal fraction from the rat hippocampus. This technique allows biochemical assays to be performed on samples of the same tissue if desired, and also provides the large amounts of tissue needed for synaptosomal isolation. Results indicated that all of the mossy fiber synaptosomes, identified on the basis of their large size and characteristic morphology, stained for zinc using this method, as did about one-third of the smaller synaptic profiles present in the same fraction. The method described here should be useful for determining zinc retention and localization in isolated synaptosomes from other regions of the mammalian central nervous system.

Animals↗

PubMed1989

Light increases Fos-related protein immunoreactivity in the rat suprachiasmatic nuclei.

Fifteen minutes of bright, white light exposure at midsubjective night resulted in a marked increase in both the number and stain density of c-fos protein (Fos)-immunoreactive neurons in the suprachiasmatic nuclei (SCN). In all cells, peroxidase reaction product was confined to the nucleus. Most Fos-immunoreactive cells were concentrated in the ventrolateral third of the SCN, although a few immunoreactive cells were also observed diffusely distributed along the dorsal border of the nucleus and into the surrounding hypothalamus. Along the rostro-caudal extent of the SCN, the greatest density of Fos-immunoreactive cells was found at a level approximately 300-400 microns caudal of the rostral pole of the nucleus. The population of Fos-immunoreactive cells in the SCN lies within the terminal fields of both the retinohypothalamic and geniculohypothalamic tracts. In addition, a few Fos-immunoreactive cells were observed in the ventral lateral geniculate nucleus. The results suggest that retinal illumination induces transsynaptic c-fos expression in a select population of SCN neurons.

Animals↗

PubMed1989

ATP release, adenosine formation, and modulation of dynorphin and glutamic acid release by adenosine analogues in rat hippocampal mossy fiber synaptosomes.

Using a hippocampal subcellular fraction enriched in mossy fiber synaptosomes, evidence was obtained indicating that adenosine derived from a presynaptic pool of ATP may modulate the release of prodynorphin-derived peptides. and glutamic acid from mossy fiber terminals. Synaptosomal ATP was released in a Ca2+-dependent manner by K+-induced depolarization. The rapid hydrolysis of extracellular [14C]ATP in the presence of intact mossy fiber synaptosomes resulted in the production of [14C]adenosine. Micromolar concentrations of a stable adenosine analogue, 2-chloroadenosine, inhibited the K+-stimulated release of both dynorphin B and dynorphin A(1-8). 2-Chloroadenosine failed to suppress the evoked release of glutamic acid, measured in these same superfusates, unless the mossy fiber synaptosomes were pretreated with D-aspartic acid to deplete the cytosolic, Ca2+-independent, pool of this acidic amino acid. In synaptosomes pretreated in this manner, release of the remaining Ca2+-dependent pool of glutamic acid was significantly inhibited by NiCl2, 2-chloroadenosine, 5'-N-ethylcarboxamidoadenosine, cyclohexyladenosine, and R(-)-N6(2-phenylisopropyl)adenosine, but not by ATP. 2-Chloroadenosine-induced inhibition was reversed when the external CaCl2 concentration was raised from 1.8 mM to 6 mM. 8-Phenyltheophylline, an adenosine receptor antagonist, effectively blocked the inhibitory effects of 2-chloroadenosine on mossy fiber synaptosomes and significantly enhanced the K+-evoked release of both glutamic acid and dynorphin A(1-8) when added alone to the superfusion medium. These results support the proposition that depolarized hippocampal mossy fiber synaptosomes release endogenous ATP and are capable of forming adenosine from extracellular ATP, and that endogenous adenosine may act at a presynaptic site to inhibit the further release of glutamic acid and the prodynorphin-derived peptides.

Adenosine↗

PubMed1988

Glutamate and dynorphin release from a subcellular fraction enriched in hippocampal mossy fiber synaptosomes.

A procedure is described for the isolation of intact hippocampal mossy fiber synaptosomes. Electron microscopic examination revealed numerous synaptosomal profiles which are clearly of mossy fiber origin, indicated by their large size (2-6 micron diameter) and characteristic morphology. Furthermore, this fraction is enriched in zinc and dynorphin B which appear to be concentrated in mossy fiber terminals in vivo. Synaptosomes isolated by this procedure accumulated 2-deoxyglucose and retained 88% of total lactate dehydrogenase activity after incubation at 30 degrees C for 60 minutes, indicating a high degree of membrane integrity. Oxygen consumption was stimulated 4-fold by veratridine (0.1 mM) and inhibited 90% by ouabain (1 mM), suggesting that synaptosomal metabolism remained tightly coupled to ouabain-sensitive ATPase activity. Potassium-stimulated (45 mM) release of dynorphin B was completely dependent upon the presence of extrasynaptosomal calcium, while only 30% of the evoked release of glutamate was calcium-dependent. D-aspartate, which exchanges glutamate out of the cytoplasmic pool, virtually eliminated the calcium-independent component of glutamate release. This synaptosomal preparation will be useful in identifying the factors that modulate the release of amino acid and opioid neurotransmitters from hippocampal nerve terminals and in the investigation of their presynaptic mechanisms of action.

Animals↗

PubMed1988

Relationship between prostaglandin synthesis and release of acidic amino acid neurotransmitters.

The importance of glutamate as an excitatory neurotransmitter in the central nervous system has become increasingly clear. However, the presynaptic mechanism of amino acid neurotransmitter release does not appear to be consistent with existing models. A major source of controversy has been the finding that a significant amount of the amino acid release evoked by membrane depolarization is calcium-independent. It is often implied that this component of release is of cytosolic origin and subserves no neurotransmitter function. In this report, an alternative model is presented which suggests that the depolarization-induced movement of calcium ions into the nerve terminal acts, simultaneously, to stimulate the release of acidic amino acid neurotransmitters from both a vesicular and cytosolic compartment. According to this model, the influx of calcium may indirectly stimulate the biosynthesis of prostaglandins which lower the plasma membrane potential and reverse the electrogenic transport of amino acids to cause a net efflux. Such a mechanism could explain how the vesicular and cytosolic neurotransmitter pools of release are functionally related.

Animals↗

PubMed1986

Evidence for developmental synaptic regression of cholinergic afferents to the rat main olfactory bulb.

Choline acetyltransferase (ChAT) activity, acetylcholinesterase (AChE) activity and muscarinic cholinergic receptor binding were determined in homogenates of olfactory bulbs from rats killed at intervals from 4 days before through 60 days after birth. In addition, the localization of muscarinic receptors was determined using an in vitro autoradiographic technique in 6-millimicrons thick coronal sections of olfactory bulbs from rats killed at similar intervals after birth. All 3 cholinergic parameters were present in measurable quantities at birth and showed substantial increases between 1 and 20 days after birth. The most rapid increase in cholinergic parameters occurred between days 10 and 20 after birth. ChAT activity and muscarinic receptor binding decreased between days 20 and 35 and increased again between postnatal days 35 and 60. A similar developmental pattern was observed for autoradiographic grain density overlying the granule cell layer of the neonatal bulb. These data suggest that (1) centrifugal cholinergic afferents are present in the rat olfactory bulb at birth, (2) during the early postnatal period (between 10 and 20 days) synaptogenesis occurs resulting in an overproduction of cholinergic synapses and (3) between postnatal days 20 and 35, a period of synaptic reorganization occurs characterized by substantial regression.

Acetylcholinesterase↗