IDENTIFICATION OF MAMMALIAN INHIBITORY TRANSMITTERS 343 



particular mode of depression. It has been established for spinal motoneurons 

 (Coombs et al., 1955) that the alteration in the permeabihty of the receptor 

 membrane produced by the interaction between the inhibitory transmitter 

 and the specialized subsynaptic receptors involves both potassium and 

 chloride ions. For the motoneuron, the resultant redistribution of these ions 

 results in a flow of current through this membrane which hyperpolarizes the 

 rest of the postsynaptic membrane (cf. Eccles, 1957, 1959). This hyperpolariza- 

 tion or inhibitory postsynaptic potential (i.p.s.p.) has been detected by 

 intracellular recording not only from motoneurons but also from cells of 

 Clarke's column (Curtis et al., 1958) and from neurons of the cerebral cortex 

 (Albe-Fessard and Buser. 1955; Phillips, 1956). The transient increase in 

 membrane conductance produced by the transmitter-receptor complex, in 

 association with the longer lasting hyperpolarization, accounts satisfactorily 

 for the depression of the activity of neurons by inhibitory synaptic action 

 (cf. Eccles, 1957). 



Therefore, in order that a substance be considered as an inhibitory trans- 

 mitter, it must also interact with these specialized transmitter-receptors to 

 produce the same permeability change as does the naturally occurring 

 transmitter. The importance of the method of applying the substance to 

 neurons then becomes apparent since methods depending upon administration 

 via the blood stream or by surface apphcation to neuronal tissue may fail to 

 produce an adequate concentration of the substance at its site of presumed 

 action. Not only may the agent fail to penetrate the blood-brain or cerebro- 

 spinal fluid-brain barriers but it may be partially inactivated by enzymic 

 action during its passage through the blood stream or nervous tissue. In 

 addition, even though observations may be made of the behavior of a single 

 cell, it may be impossible to determine whether the administered compound 

 is depressing the excitability of this cell by a direct action upon its membrane 

 or by an indirect action due to the stimulation of cells or sensory receptors 

 having inhibitory synaptic connections with the cell under observation. 



The necessity for recording intracellularly from the observed neuron is also 

 obvious since extracellular recording may not be adequate to distinguish 

 between depression in the absence of a membrane potential change and 

 inhibition associated with an hyperpolarization. The requirements for 

 applying suspected transmitter agents to neurons and recording the effects 

 upon the membrane are most conveniently satisfied by the use of co-axial 

 electrodes (Curtis et al., 1959) since the location of these cells precludes the 

 use of the separate recording and drug-applying electrodes which have been 

 used in investigations of the neuromuscular junction (Nastuk, 1953; del 

 Castillo and Katz, 1955). These electrodes consist of two glass microelectrodes, 

 the inner, of tip diameter approximately 0-5 /m, projecting 40-60 /x beyond the 

 10-12 )u. orifice of the outer barrel with which it is co-axial. It is usually 

 possible to penetrate the surface membrane of a motoneuron with the inner 



