Changes in density of brainstem afferents in ferret primary auditory cortex (AI) during postnatal development.

Histochemical methods were used to assess the distribution of 4 neurotransmitters thought to be involved in cortical plasticity. They were measured in the primary auditory cortex of the ferret from just before the onset of hearing. Acetylcholinesterase staining was strongest in layers I, IV and VI and there was a gradual increase in the amount of staining from postnatal day (PND) 21 through to adulthood. Serotonin fibres were located mainly in layers I-III and their density increased gradually over the same time period. Noradrenergic fibres were sparsely scattered throughout the cortex but their density and distribution showed little change over the age range studied. Dopaminergic fibres were densest in layers V and VI at all ages. However, there was a transient doubling in their density that started round about the onset of hearing at PND 28, peaked at PND 35 and had returned to baseline levels by 2 wk later. This transient peak in density did not occur in the adjacent suprasylvian gyrus and did not appear to be a general phenomenon. The local transient increase in dopaminergic fibres implies that they may have an important role during a short period in auditory cortical development. This role may involve modifying the cortical circuitry that is involved in analysing the input from the auditory periphery.