Dynamic regulation of polysialylated neural cell adhesion molecule in the suprachiasmatic nucleus.

The suprachiasmatic nucleus (SCN) prominently expresses polysialic acid (PSA), a carbohydrate polymer that is attached to neural cell adhesion molecule (NCAM) and promotes changes in cell interactions. Previous studies have shown that expression of PSA is important for circadian rhythm stability under constant darkness, and for photic entrainment of the SCN circadian clock. In the present study, immunoblot analyses of the Syrian hamster SCN revealed marked diurnal fluctuations in PSA under a 24-h light/dark cycle. PSA levels were reduced by >90% during the mid-to-late dark phase, and were elevated to maximal daytime levels approximately 1 h after lights-on. A similar pattern of PSA fluctuation persisted under constant darkness. Exposure of animals under a 24-h light/dark cycle to a 30-min light pulse during the late dark phase dramatically increased SCN contents of PSA within 60 min, and these values returned to basal levels 1-2 h later. There was no effect of light-on expression of PSA in the hippocampus. Parallel studies revealed changes in the NCAM-180 isoform that carries PSA in the brain, suggesting that regulation of PSA may include protein as well as carbohydrate-associated mechanisms. Immunohistological analysis revealed light-induced enhancement of PSA in the SCN subregion containing calbindin D(28K) cells. PSA staining was also closely associated with the majority of SCN cells expressing light-inducible Fos protein. This rhythmic, light-inducible expression of PSA within the SCN suggests that dynamic cell interactions are important for the photic regulation of circadian clock phase.

Estrogen modulates responses of striatal dopamine neurons to MPP(+): evaluations using in vitro and in vivo techniques.

In vitro superfusion and in vivo electrochemistry were used to investigate the role of estrogen in modulating MPP(+)-induced dopamine output in the corpus striatum and nucleus accumbens of ovariectomized female rats. For in vitro superfusion experiments, dopamine and dihydroxyphenylacetic acid release were determined using HPLC with electrochemical detection from superfusion of corpus striatum fragments with Kreb's ringer phosphate buffer pulsed with MPP(+) alone or MPP(+) with estrogen. The in vivo electrochemistry experiments recorded the dopamine signal from carbon fiber microelectrodes stereotaxically passed through the corpus striatum and nucleus accumbens. Dopamine release was stimulated by pressure ejection of MPP(+) alone or in combination with estrogen through glass micropipettes fastened to the electrodes. Dopamine output from superfusion chambers which received infusion of MPP(+) with estrogen showed significantly lower output of dopamine compared with chambers which received MPP(+) alone. Outputs of dihydroxyphenylacetic acid did not increase following MPP(+) infusions. Data from the electrochemistry experiments demonstrated that estrogen significantly reduced both the amplitude and clearance rates of the MPP(+)-evoked dopamine signal in both the corpus striatum and nucleus accumbens. Results of this study demonstrate that: (1) MPP(+) evokes striatal dopamine release and this effect is significantly reduced in the presence of estrogen as determined by both in vivo electrochemistry and in vitro superfusion: (2) similar, albeit attenuated effects are observed in the nucleus accumbens as determined with in vivo electrochemistry; (3) estrogen acts to inhibit the clearance of dopamine in both the striatum and nucleus accumbens; and (4) estrogen may function as a neuroprotectant by reducing the uptake of neurotoxin into dopaminergic neurons.

Polysialylated neural cell adhesion molecule modulates photic signaling in the mouse suprachiasmatic nucleus.

Polysialic acid (PSA), a sialic acid polymer that regulates plasticity and cell-cell interactions in neural tissues, is expressed in the mammalian circadian clock located in the suprachiasmatic nucleus (SCN). In vivo enzymatic removal of PSA from the mouse SCN significantly impaired both the photic induction of Fos protein in SCN cells and light-induced phase-resetting of the circadian locomotor activity rhythm. Genetic deletion of PSA and it's neural cell adhesion molecule (NCAM) carrier correspondingly attenuated light-induced circadian phase-shifting. Comparison of PSA levels between young and old mice revealed a large aging-related reduction in SCN PSA content that accompanies the diminished capacity for circadian photic response reported in old rodents. Collectively these data support the contention that PSA modulates photic signaling in the SCN, and that normal reductions in the cell surface molecule contribute to aging-related deficits in SCN circadian clock function.