Cytidine (Cytosine-1-β-D-ribofuranoside) is a pyrimidine nucleoside comprised of a cytosine bound to ribose via a beta-N1-glycosidic bond. Cytidine is a precursor for uridine. Both cytidine and uridine are utilized in RNA synthesis.

D-cycloserine (DCS) facilitates extinction of conditioned freezing to the light CS when no drug pre-exposure has occurred, but pre-exposure to DCS just prior to conditioning disrupted the facilitation of extinction effect in mice. [1] D-cycloserine (DCS) which has a high affinity for the glycine modulatory site in the NMDA receptor complex modulated memory processing in a dose-dependent manner. DCS also facilitates retention in 'senescence-accelerated mice' in which impairment of learning andmemory increases with age. [2] D-cycloserine (DCS) exhibits facilitated extinction of fear but are able to reacquire fear of that conditioned stimulus (CS) in a similar manner as saline-treated control rats. DCS-treated rats exhibits generalized extinction (i.e., they are less fearful of a non-extinguished CS) in comparison to controls. [3] D-cycloserine (DCS), an antimycobacterial agent known to cross the blood-brain barrier, binds with high affinity to this glycine modulatory site, functions as a positive modulator, and facilitates performance of learningtasks in rats. DCS appears to be a potent cognitive enhancer at doses lower than those required for antibacterial activity. [4] D-cycloserine injections (3.25, 15, or 30 mg/kg) before 30 non-reinforced light exposures dose-dependently enhances extinction but does not influence fear-potentiated startle in rats that does not receive extinction training. [5]