Dorsal pallidal neurons directly link the nidopallium and midbrain in the zebra finch (Taeniopygia guttata)

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The basal ganglia (BG) of vertebrates are considered to have evolved along very conservative lines (Reiner, Medina, & Veenman, 1998). Thus, within the subpallium of both birds and mammals, an outer striatum receives glutamatergic projections from most areas of the pallium (McGeorge & Faull, 1989; Veenman, Wild, & Reiner, 1995), and an inner globus pallidus (GP or pallidum) receives GABAergic projections from the striatum by way of spiny projection neurons and projects out of the BG, also by way of GABAergic neurons, to the thalamus and brainstem (Person, Gale, Farries, & Perkel, 2008). Although the cortico‐striato‐pallidal projection system is generally considered to be “one way” (Shepherd, 2013), striato‐cortical, pallido‐striatal, and pallido‐cortical projections are known in mammals (e.g., Chen, 2015; Jayaraman, 1980; Voorn, 2010). Of these, only a striato‐pallial projection has been noted thus far in birds (Wild, unpublished observations).
Despite fundamental similarities in the organization of the avian and mammalian BG, other differences should not be overlooked (Doupe, Perkel, Reiner, & Stern, 2005; Person et al., 2008; Reiner, 2010). One of these differences is that the GP in primates generally possess separate external and internal divisions (or comparable separate nuclei in rodents), whereas the GP in birds is not thus subdivided, yet possesses projections similar to those of both GPi and GPe segments of the mammalian GP (Karten & Dubbeldam, 1973; Medina & Reiner, 1997; Medina, Jiao, & Reiner, 1999; Person et al., 2008). Another difference is that in songbirds, such as the zebra finch, some striatal and pallidal neurons are found intermixed within a rostral component of the BG known as area X and surrounding regions of the medial striatum (Person et al., 2008; Reiner, Laverghetta, Meade, Cuthbertson, & Bottjer, 2004), an unusual neuronal admixture that might provide functional advantages in terms of song learning (Luo & Perkel, 1999; Person et al., 2008; Reiner et al., 2004). In other, more caudal parts of the songbird BG, striatal and pallidal neurons are arranged in separate compartments, as they are in nonsongbirds and mammals. However, our knowledge of specific, topographic projections of these more caudal parts of the dorsal pallidum is limited (Jiao et al., 2000; Karten & Dubbeldam, 1973; Kitt & Brauth, 1981; Medina & Reiner, 1997). The present study in the zebra finch focusses on one small part of the most caudal GP, which is shown to project to the intercollicular nucleus, where terminations partially surround the central nucleus of the inferior colliculus (MLd). These pallidal neurons also have central processes (presumptive dendrites) that extend up to 2 mm into the nidopallium, thereby potentially linking sensory (auditory and/or somatosensory) regions of the telencephalon and midbrain directly.
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