Monoaminergic modulation of the sensitivity of neurones in the cingulate cortex to iontophoretically applied substance P

The interactions of noradrenaline (NA) and 5-hydroxytryptamine (5-HT) with substance P (SP) were studied on single neurones in the anterior cingulate cortex of the rat. Iontophoretic application of 5-HT potentiated the excitatory responses of some neurones to SP and reduced responses of others. However these effects were usually accompanied by parallel changes in baseline firing rate i.e. increase and decrease respectively. In studies where carbachol (CCh) was used as a control the excitatory responses to this substance were always altered in a similar fashion to those to SP. The effects of NA on SP-responses were more consistent. This amine caused a reduction of response to SP regardless of whether there was an increase, decrease, or no change in baseline firing rate. Responses to SP could be reduced on many cells in the absence of changes in response to CCh and even on some cells where CCh responses were concurrently enhanced. Lesions of the locus coeruleus which resulted in a depletion of NA in the ipsilateral cingulate cortex gave rise to a substantial increase in sensitivity of neurones to SP two weeks later. However, lesions of the median raphe(MR)-nucleus which strongly reduced cortical 5-HT had no detectable effect on SP-responses. The data indicate that both NA and 5-HT can alter cortical neurone-sensitivity to SP but that the former amine may be involved in a more specific and possibly a functional interaction.