The action of carbon dioxide on synaptic transmission in the cuneate nucleus

1. The excitability of synaptic structures in the cuneate nucleus was studied in eighteen decerebrate, unanaesthetized cats during acute changes in inspired P(CO2).2. Micro-electrode stimulation in the caudal half of the cuneate nucleus evoked antidromic and orthodromic responses which were recorded simultaneously from a forelimb nerve and from the medial lemniscus at the transected mid-brain surface.3. Increases in the concentration of inspired CO(2) (2-20%) progressively decreased the direct excitability of both the afferent fibre terminals (reflected in the antidromic potentials) and the cuneate relay neurones (reflected in the alpha wave of the orthodromic lemniscal response). Synaptically mediated responses, recorded as the beta component of the lemniscal potentials, were also depressed.4. The relation between input and output at the cuneate was determined by plotting antidromic against trans-synaptic (beta lemniscal) responses for different intensities of stimulation. The mean slope for logarithmic values of control potential amplitudes was 1.17 (+/- S.E. 0.13). It therefore appears that the transfer function for the cuneate is linear over a wide range.5. In the majority of experiments the input-output relation was unchanged or increased by raising P(CO2). It was concluded that the efficiency of synaptic transmission and release of transmitter appeared to be well maintained or possibly increased at individual active synapses during hypercarbia.6. The depressant action of CO(2) on afferent transmission can therefore be attributed largely to a block of impulse conduction in the primary afferent fibres.