Forebrain connections of medial agranular cortex in the prairie vole, Microtus ochrogaster

Fluorescent axonal tracers were used to investigate the connections of medi al agranular cortex (frontal area 2, Fr2) in male prairie voles. The rostra l and caudal portions of Fr2 (rFr2 and cFr2) have distinct but partially ov erlapping patterns of connections. Thalamic labeling after cFr2 injections was present in anteromedial nucleus (AM), ventrolateral nucleus (VL), later al segment, mediodorsal nucleus (MD1), centrolateral nucleus (CL), ventrome dial nucleus (VM), posterior nucleus (Po) and lateral posterior nucleus (LP ). A band of labeled cells involving CL, central medial nucleus (CM) and rh omboid nucleus (Rh) formed a halo around the periphery of submedial (gelati nosus) nucleus (Sm). Within cFr2 there is a rostrocaudal gradient whereby p rojections from VL and MDI become progressively sparser caudally, whereas t hose from LP and Po become denser. Rostral Fr2 receives afferents from a si milar group of thalamic nuclei, but has denser innervation from VL and MDI, lacks afferents from LP, and receives less input from nuclei around the pe riphery of Sm. Caudal Fr2 has extensive cortical connections including orbi tal cortex, rostral Fr2, Fr1, caudal parietal area 1 (Par1), parietal area 2 (Par2), and posterior parietal, retrosplenial and visual areas. Rostral F r2 has similar connections with areas Fr1, Par1 and Par2; orbital connectio ns focused in ventrolateral orbital cortex (VLO); connections with caudal F r2; greatly reduced connections with posterior parietal cortex and the visu al areas; and no connections with retrosplenial cortex. The axone linking r Fr2 and cFr2 with each other and with other cortical areas travel predomina tely in the deep gray matter of layers VI and VII rather than in the white matter. Projections to the dorsal striatum from rFr2 are widespread in the head of the caudate, become progressively restricted to a dorsocentral focu s more caudally, and disappear by the level of the anterior commissure. The projections from cFr2 are largely restricted to a focal dorsocentral regio n of the striatum and to the dorsolateral margin of the caudatoputamen. In comparison-to area Fr2, the laterally adjacent area Fr1 has thalamic and co rtical connections which are markedly restricted. Area Fr1 receives thalami c input from nuclei VL, anteroventral nucleus (AV), CL and Po, but none fro m mediodarsal nucleus (MD) or LP, and its input from VM is reduced. Cortica l afferents to Fr1 originate from areas Fr2, caudal Par1 and Par2., Medial agranular cortex of prairie voles has a pattern of connections largely simi lar to that seen in rats, suggesting that area Fr2 in prairie voles is part of a cortical network that may mediate complex behaviors involving spatial orientation.