THE RHYTHMIC GABAERGIC SYSTEM

GABA is the major inhibitory neurotransmitter in the mammalian brain, and has been implicated in the regulation of a variety of behavioral f unctions, including biological rhythms. The focus of this minireview i s the rhythmic variation of the central GABAergic system, comprising f luctuations of GABA levels and turnover, GABA receptor affinity and po stsynaptic activity on the chloride ionophore in rodent's brain. Neuro chemical rhythms correlated with diurnal and circadian changes in seve ral behaviors associated with the GABA(A) receptor, e.g., anxiolysis-r elated behavior. GABA is considered to be the principal neurotransmitt er of the mammalian circadian system, being present in the suprachiasm atic nuclei and the intergeniculate leaflet. Pharmacological manipulat ions of GABA(A) receptors phase shift circadian rhythms and alter circ adian responses to light. Administration of putative modulators of GAB A function, like melatonin or neuroactive steroids, affects the timing of biological rhythms. Therefore, not only does the GABAergic system exhibit strong diurnal and circadian variations, but it also serves as one of the key modulators of the circadian apparatus.