Automaticity of Central Neurones After Nicotine Block of Synapses

Abstract

The discovery that central neurones manifest a rhythmic electrical potential in the absence of deliberate sensory stimulation suggested that these cells possess an automatic beat homologous to that of the cardiac nodal cells. The possibility of nerve impulses, either from receptors or injured nerves or from closed circuits of interneurones∗ with long maintained activity, had not, however, been excluded. The persistence of a marked potential rhythm in the frog's olfactory bulb for hours after its complete removal and at a time when its only neural connections—the olfactory nerves and cerebral hemispheres—were electrically dead 1 seemed to eliminate any action of impulses from receptors or injured neurones. Since, however, stimulation of the olfactory nerves of such a preparation increases the bulb potentials or even reinitiates them after they have stopped and since this enhancement may persist for many minutes or hours following a brief stimulation 2 the existence of trapped impulses in closed circuits remained a definite possibility. This has been urged especially by Lorente de No. 3 In the present experiments it is shown that when synaptic conduction is blocked by nicotine the rhythmic slow potential waves of the isolated bulb are increased rather than abolished. It may be concluded, therefore, that the activity rhythm originates in single neurones and is immediately independent of any maintained or recurrent bombardment by conducted nerve impulses.

Nicotine has long been used to block synaptic transmission in autonomic ganglia 4 and is also known to exert a curarizing action. 5 , 6 We have found no reference, however, to a similar effect on central synapses, though Langley 7 reported some experiments indicating its occurrence. The following facts evidence such a central blockade at all synapses tested:

1. Local application of 0.1 % nicotine to the exposed spinal cord in the frog abolishes in 15 min. cross reflex responses to stimulation of the central end of a cut sciatic. Peripheral sciatic stimulation remains effective. Langley 7 published a similar observation. Even after strychninization, reflex block is produced by nicotine. Stronger nicotine (0.5%) abolishes motor responses to electrical stimulation of the cord itself though the lower concentrations do not.