Parcellation of the frontal cortex of the New World monkey Callithrix jacchus by eight neurotransmitter-binding sites

The most extensive development during primate brain evolution involves the cortex of the frontal lobe, especially its prefrontal region. The distribution of neurotransmitter receptors is unknown in this part of the cortex of New World monkeys. The respective distributions of eight different receptors for the transmitters l-glutamate (l-glu and NMDA), ?-amino-butyric acid (GABA_A), noradrenaline (a₁), acetylcholine (M₁ and M₂) and serotonin (5-HT₁ and 5-HT₂) were therefore studied in cortical areas of the frontal lobe of the lissencephalic New World monkey, Callithrix jacchus. The results are compared to earlier data on Old World monkeys in order to obtain insight into evolutionary trends at the level of chemical neuroanatomy. Our results indicate that the density and laminar pattern of some receptors change precisely at the cytoarchitectonic boundaries between different cortical areas, while some other receptors do not exhibit measurable changes. For example, the premotor area 6 can be distinguished from prefrontal areas by its high concentration of adrenergic a₁ receptors as labelled with [³H] prazosin, with only the cingulate area 24 showing higher values. In other cases, the receptor distribution changes within a cytoarchitectonically homogeneous area. Thus, area 8 can be subdivided into dorsal and ventral regions on the basis of the distribution of GABA_A, muscarinic and serotonin receptors. Comparison of these results in a New World monkey with receptor distributions in other primate species reveals much larger interspecies differences in the areas of the frontal lobe than e.g. in the primary visual cortex. This is interpreted as an indication of extensive changes in the neuro-chemical organisation of this part of the brain during primate brain evolution.