Distribution of the mRNA encoding the four dopamine D(1) receptor subtypes in the brain of the european eel (Anguilla anguilla): comparative approach to the function of D(1) receptors in vertebrates.

Four subtypes of D(1) dopamine receptors are expressed in the brain of the European eel (Anguilla anguilla), an elopomorph teleost. To correlate this molecular multiplicity with specific localisation and functions, the distribution of the D(1) receptor transcripts was analysed by in situ hybridisation. The four D(1) receptor transcripts exhibit largely overlapping expression territories. In telencephalon, they are found in the olfactory bulb and the dorsal telencephalon (except its lateral part) but are most abundant in the subpallial areas. More caudally, the entopeduncular nucleus, preoptic nuclei, preglomerular nuclear complex, ventral thalamus, periventricular hypothalamus, optic tectum and cerebellum, all contain various amounts of D(1) receptor transcripts. Finally, D(1) receptor mRNAs are present in nuclei associated with the cranial nerves. The two D(1A) receptor subtypes are generally the most abundant and present a different distribution in several areas. The D(1B) mRNA, although present in fewer areas than D(1A) transcripts, is the most abundant in ventrolateral telencephalon and torus semicircularis. The D(1C) receptor transcript, which has not been found in mammals, is restricted to diencephalon and cerebellum. In view of the expression territories of D(1) receptor transcripts and previous data, some areas of the everted telencephalon of teleost may be homologous to regions of the tetrapod brain. In particular, D(1) expression territories of the ventral telencephalon are likely to be equivalent to striatal areas. These observations suggest an evolutionary scenario in which the D(1A) receptor subtype was highly conserved after the first gene duplication during the evolution of craniates, whereas D(1B) and D(1C), and their associated specific characteristics, appeared later, probably in the gnathostome lineage.