Dr. Johnson attended Stephen F. Austin University from 1963-1967, graduating with a BS in biology and a minor in chemistry. He then taught physical science in the Houston ISD for two years before enrolling in the graduate program in biophysical sciences at the University of Houston in 1969. After studying the hydrodynamic sedimentation properties of closed and nicked circular DNA under high g forces at UH, he met Dr. Marlyne Kilbey from the Department of Psychology who was studying the effects of Δ9-tetrahydrocannabinol (Δ9-THC) on aggressive behavior in mice. This meeting completely changed his research interests. As a result, he continued his studies with Drs. Kilbey and Beng T. Ho at the Texas Research Institute of Mental Sciences (TRIMS) from neurochemical and behavioral perspectives and earned his PhD in 1974.
He then accepted a postdoctoral fellow with Drs. Wm. L. Dewey and Louis Harris at the Medical College of Virginia (MCV, now merged with Virginia Commonwealth University, or VCU), where he continued his work on the behavioral and neuropharmacological effects of cannabinoids in mice (1975-1977). Following his two and one-half years of training in pharmacology, he moved back to Texas where he began his independent career as an Assistant Professor in the Dept. of Pharmacology and Toxicology at the University of Texas Medical Branch in Galveston, TX.
His early work at UTMB was generally focused on mechanisms of uptake and release of neurotransmitters and how drugs of abuse, particularly cocaine and phencyclidine affected these processes. A related area of research involved understanding the mechanisms by which nitric oxide impacted these processes. This was followed by studies on the mechanisms by which glutamate analogs (particularly N-methyl-D-aspartate, NMDA) as well as glycine and serine altered neurotransmitter release through stimulation of both ionotropic and metabotropic transmitter receptors.