INHIBITORY TRANSMITTERS — A REVIEW 



355 



transmitter at motor neurons, but is perhaps acting as a "depressor" at other 

 sites. 



Of the numbered conditions listed above then. Factor I seems to satisfy 

 nos. 4 and 7. Florey (1954) reported the inactivation of Factor I by homo- 

 genates of brain (point no. 6), and Florey and McLennan (1955a) showed 

 that Factor I activity appeared in an exudate from cerebral cortex (point no. 

 5), although they did not show an increase in its appearance during inhibitory 

 stimulation. 



Recently McLennan (unpublished obser\ ations) has attempted to assess 

 the degree to which Factor I can also satisfy the three criteria for an inhibitory 

 transmitter detailed by Curtis et al. (1959). Intracellular records were made 

 from motor neurons of the cat's spinal cord, and subjected to the action of 

 Factor I by topical application of the material to the cord. As expected, ortho- 

 dromic action potentials recorded from the cells were blocked ; the effect of 

 the Factor I could be reversed by washing or was sometimes spontaneously 

 reversed. Concomitant with the disappearance of action potentials the mem- 

 brane potential of the cell was raised by 2-5 mV and i.p.s.p.'s reduced or, 

 rarely, entirely abohshed (Fig. 4). E.p.s.p.'s showed relatively little change 



lOntsec. 



Fig. 4. Inhibitory postsynaptic potentials recorded from a quadriceps motor 

 neurone in response to dorsal root stimulation. Factor I solution was applied 

 to the exposed spinal cord between the two records. Note the increased polariza- 

 tion of the membrane (displacement downwards) and reduced amplitude of the 

 i.p.s.p. (McLennan, unpublished observations). 



except when the alteration of the membrane potential was comparatively 

 large, in which event the e.p.s.p.'s were somewhat reduced (Fig. 5). These 

 actions on the membrane potential and on evoked postsynaptic potentials 

 are in accord with the requirements for the action of an inhibitory trans- 

 mitter set out above. 



Considerable work has been devoted to attempts to isolate the active in- 

 gredients in Factor I-containing extracts. Using the original definition of 

 Factor I activity (see above) as the basis for their assay, Bazemore et al. 

 (1957) isolated from the extracts y-aminobutyric acid (GAB A") wliich they 



