INHIBITORY TRANSMITTERS — A REVIEW 357 



behaviour on paper chromatograms is markedly different. One of these 

 fractions was purified to the point that its specific activity towards the cray- 

 fish neuron was three times higher than that of GAB A, but the material was 

 not further characterized. The constituent of Factor I extracts active on the 

 motor neurons of the mammalian spinal cord remains to be identified ; it should 

 perhaps be added that it is not identical with any of the other commonly 

 considered compounds hkely to be present in an extract of brain acetyl- 

 choline, histamine, 5-hydroxytryptamine, adrenahne, etc. 



Other substances known to occur and to be metabohzed in the brain have 

 been reported to have effects which may possibly be on synaptic structures, 

 or have depressant actions on neurons similar to those described above for 

 GAB A. A brief discussion only of these other potential candidates will there- 

 fore be made. 



Sympathin {Adrenaline and Noradrenaline) 



Vogt (1954) has pubfished figures showing the distribution of noradrenaline 

 within the central nervous system which indicate its widespread occurrence, 

 and markedly higher concentration in the hypothalamus and area postrema. 

 Rather variable effects of adrenaline on cord reflexes have been reported in 

 the past, with depressant actions predominating. Recently Cranmer et al. 

 (1959) have shown that inhibition of spinal reflexes brought about by stimu- 

 lation of the bulbar reticular formation can likewise be induced by adrenaline, 

 which observation may therefore serve to explain the results of earlier workers. 

 The "anaesthetic" effect of adrenaline administered through the carotid 

 artery or into the cerebrospinal fluid has long been known, and Feldberg 

 (1958) has argued that the result points to a neuronal inhibition as the cause. 

 The fact, however, that Curtis et al. (1957) found no electrical effects which 

 could be interpreted as due to synaptic action suggest that the results which 

 have been described may be due to "depressor" effects or are even secondary 

 to vascular changes. 



5-Hydroxytryptamine 



The statements made above respecting sympathin are to a considerable 

 extent repeatable for 5-hydroxytryptamine (5HT). Its distribution in the 

 brain is similar to that of sympathin, being highest in the hypothalamus and 

 area postrema, but as much may be extracted also from the mesencephahc 

 grey matter (Amin et al, 1954). It can be shown to be released from the 

 frog's spinal cord (Angelucci, 1956) and the amount is increased by reflex 

 excitation. Intraventricular administration of the substance again leads to a 

 lethargic or stuporous reaction of the animal which, as in the case of nor- 

 adrenaUne above, may well be due to neuronal depression. Evidence for an 

 inhibitory synaptic action of 5HT is, however, completely lacking. 



