AXONAL CONNECTIONS OF THE MEDIAL MAGNOCELLULAR NUCLEUS OF THE ANTERIOR NEOSTRIATUM IN ZEBRA FINCHES

The medial magnocellular nucleus of the anterior neostriatum (mMAN) is a small cortical nucleus which was previously identified as a compone nt of the neural circuitry controlling vocal behavior in songbirds bas ed on its efferent connection to the High Vocal Center (HVC), a major song control nucleus (Nottebohm et al. [1982] J. Comp. Neurol. 207:344 -357; Bottjer et al. [1989] J. Comp. Neurol. 279:312-326). We have con ducted tract tracing experiments (using wheat-germ agglutinin-horserad ish peroxidase (WGA-HRP), the carbocyanine dye DiI, and biocytin) to d etermine the complete pattern of afferent and efferent connections of mMAN in adult male zebra finches. We confirmed the existence of an eff erent projection from mMAN to HVC and discovered a novel projection to the region medial to caudal HVC called paraHVC (pHVC). Injections of retrograde tracers into mMAN showed that afferent input to mMAN origin ates from the dorsomedial nucleus of the posterior thalamus (DMP). Inj ections of DiI into DMP produced anterograde label over mMAN, thus con firming the DMP-to-mMAN projection. Interestingly, this anterograde la bel extended beyond the region of mMAN defined by HVC-projecting neuro ns into the immediately surrounding cortex. This extended terminal fie ld of DMP efferents indicates that mMAN encompasses a core population of projection neurons surrounded by a shell of non-HVC-projecting neur ons, both of which receive input from the dorsal thalamus. Analysis of retrograde DiI label resulting from DMP injections revealed two major sources of afferent input to DMP originating in regions of the archis triatum and hypothalamus. Inputs to DMP were distributed throughout th e dorsal archistriatum and included the area that receives a projectio n from the parvicellular shell region of the lateral magnocellular nuc leus of the anterior neostriatum, a song control nucleus, as well as t he dorsal portion of the robust nucleus of the archistriatum, the moto r-cortical output of the song control system. The projections from son g control regions of the archistriatum to DMP may feed information bac k into telencephalic song control circuitry via the DMP --> mMAN --> H VC/pHVC pathway. The other source of afferent input to DMP is located in the external cellular stratum of the lateral hypothalamus (SCE). Th is newly delineated SCE --> DMP --> mMAN --> HVC/pKVC pathway is the f irst report of a hypothalamic brain region neuroanatomically integrate d with song control circuitry. Because hypothalamic brain regions are important for homeostasis and regulating behavior, the trans-synaptic circuitry of mMAN may help to integrate information about the bird's i nternal state, such as sexual maturation, with song learning and produ ction. J. Comp. Neurol. 382:364-381, 1997. (C) 1997 Wiley-Liss, Inc.