Catalyzes one step in the degradation of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). H2O + NAD(+) + succinate semialdehyde = 2 H(+) + NADH + succinate Redox-regulated. Inhibited under oxydizing conditions (By similarity). Amino-acid degradation; 4-aminobutanoate degradation. Homotetramer. Brain, pancreas, heart, liver, skeletal muscle, kidney. Lower in spleen, lung, kidney and testis. Belongs to the aldehyde dehydrogenase family.