Distribution of low Km GTPase activity in mouse CNS: effect of chronic morphine.

The low Km GTPase displayed an apparent Km value of 0.2-0.4 microM in P2 fractions from whole mouse brain. The activity of this enzyme ranged from 102 (pmols of GTP hydrolysed per micrograms of protein per min) in the striatum to 39 in the pons-medulla oblongata. Intermediate values were found in other structures, 74-62 in thalamus, hypothalamus, periaqueductal gray matter (PAG), rest of mesencephalon, cortex and spinal cord. The Km also varied throughout the mouse CNS: the spinal cord, striatum and PAG exhibited Km values (0.308-0.271 microM) higher than cortex, thalamus, pons-medulla, hypothalamus and remaining mesencephalon (0.239-0.193 microM). Chronic morphine (3 days) decreased the low Km GTPase activity of PAG (42), whereas it increased the one of thalamus (99). After chronic exposure to the opioid the Km values of the enzyme in striatum (0.193), PAG (0.192) and spinal cord (0.201) diminished, and the ones of hypothalamus (0.357) and rest of mesencephalon (0.287) augmented. The herein reported diversity of low GTPase activity might be due to the presence of different ratios of G alpha types/subtypes in the neural structures studied. As a result of chronic morphine the ratio and/or the functionality of G proteins would be altered in particular areas of mouse CNS.