Avian homologues of mammalian intralaminar, mediodorsal and midline thalamic nuclei: immunohistochemical and hodological evidence.

This paper presents and reviews data suggesting that the dorsal thalamic zone (abbreviated DTZ) in birds is homologous to the intralaminar, midline, and mediodorsal thalamic nuclear complex (abbreviated IMMC) in mammals. The DTZ is located dorsomedially in the diencephalon of birds and consists of several subnuclei: nucleus dorsomedialis anterior thalami (DMA), nucleus dorsomedialis posterior thalami (DMP), nucleus dorsolateralis anterior thalami, pars medialis (DLM), nucleus dorsointermedius posterior thalami (DIP), nucleus dorsolateralis posterior thalami (DLP), and nucleus subhabenularis lateralis (SHL). Our immunohistochemical studies show that: (1) SHL and medial and dorsal parts of DMA and DMP are relatively rich in GABAergic, enkephalin-containing, substance P-containing, and cholinergic fibers; (2) lateral parts of DMA and DMP are relatively poor in these neurotransmitters; and (3) DIP, DLP, and DLM are moderately rich in cholinergic and substance P-containing fibers. Our retrograde pathway tracing studies indicate that the DIP and DLP in the more lateral parts of DTZ project to somatic striatum, while the DMA, DMP, and SHL located more medially in the DTZ project to visceral/limbic striatum. Our anterograde tracing studies indicate that DIP receives afferents from the dorsal pallidum, whereas DMA and DMP appear to receive afferents from both the ventral striatum and ventral pallidum. Diverse prior studies have shown that in general medial and lateral components of DTZ are connected with visceral/ limbic and somatic brain regions, respectively. These characteristics indicate that: (1) SHL and medial and dorsal parts of DMA and DMP are comparable to mammalian midline thalamic nuclei, including the medial components of the intralaminar nuclei; (2) lateral parts of DMA and DMP are comparable to the mediodorsal nucleus in mammals; (3) DIP is comparable to the parafascicular nucleus in mammals; and (4) DLM and DLP are comparable to the laterally located intralaminar nuclei in mammals. The comparability of avian DTZ and mammalian IMMC suggests that they evolved from thalamic precursor nuclei present in the common reptilian ancestors and that they may perform similar roles in the movement control function of the basal ganglia.