Connections of the auditory forebrain in the pigeon (Columba livia)

Cloning and expression analysis of cadherin-10 in the CNS of the chicken embryo

A full-length cDNA of a novel cadherin of chicken (cad10) was cloned. The deduced amino acid sequence of the putative cytoplasmic domain of this molecule is highly homologous to a previously published cytoplasmic fragment of human cadherin-10, a type II cadherin. An in situ hybridization analysis in chicken embryos shows that cad10 expression starts at about 4 days' incubation (E4) and persists at least until the hatching stage. In the central nervous system (CNS), cad10 expression is spatially restricted at all stages of development. At early stages, expression reflects the neuromeric organization of the brain. For example, in the alar plate of the diencephalon, cad10 expression is restricted to the dorsal thalamic neuromere. A number of cad10-expressing brain nuclei are formed in this neuromeric domain during later development. Specific cad10-expressing gray matter structures are also found in all other major divisions of the brain. Many of these structures are known to be functionally connected to each other. The cad10 expression pattern is distinct from that of other cadherins. These results support the idea that cadherins provide a molecular code for the regionalization of the embryonic CNS at the different stages of development.

Copulation activates Fos-like immunoreactivity in the male quail forebrain

It has been demonstrated using Fos immunocytochemistry that copulation activates specific cell populations in the mammalian brain. Prior to this study, no similar work has been carried out in birds. In mammals, Fos has identified brain circuits activated by genital (penile)/somatosensory and by olfactory/vomeronasal stimuli. Such inputs, of course, should play little or no role in birds (no penis, little or no role for olfaction) and a differential responsiveness could therefore be expected. Male Japanese quail (Coturnix japonica) were allowed to interact freely with adult females and the presence of active sexual behavior, including cloacal contact movements, was confirmed in each case. Control subjects were exposed to a domestic chick (same size as an adult quail) and no sexual behavior was observed. Copulation induced the appearance of Fos-like immunoreactive (FLI) cells in the preoptic area, the hyperstriatum ventrale, parts of the archistriatum, and the nucleus intercollicularis. Induction of FLI cells was observed throughout the rostral to caudal extent of the preoptic region of males from the level of the tractus septomesencephalicus to the level of the anterior commissure, and in the rostral part of the hypothalamus to the level of the supraoptic decussation. The FLI cells did not lie directly adjacent to the third ventricle, but were located 500-1000 microns from the ventricle wall at the level of the lateral edge of the medial preoptic nucleus or, in more caudal sections, in a position ventrolateral to the bed nucleus striae terminalis. It is unlikely that the Fos induction in males resulted from copulation-induced endocrine changes because copulation did not affect plasma levels of luteinizing hormone or testosterone. It is concluded that the responses were due to copulation-associated somatosensory inputs and/or to stimuli originating from the female.

Projections of the dorsomedial nucleus of the intercollicular complex (DM) in relation to respiratory-vocal nuclei in the brainstem of pigeon (Columba livia) and zebra finch (Taeniopygia guttata)

Injections of neuronal tracers were made into the dorsomedial nucleus of the intercollicular complex (DM) of pigeons and zebra finches in order to investigate the projections of this nucleus which has long been implicated in respiratory-vocal control. Despite the fact that pigeons are nonsongbirds and zebra finches are songbirds, the projections were very similar in both species. Most descended throughout the brainstem, taking ventral and dorsal trajectories, which merged in the medulla. Those descending ventrally terminated upon the ventrolateral parabrachial nucleus (PBvl), the nucleus infraolivaris superior, a nucleus of the rostral ventrolateral medulla (RVL), and the nucleus retroambigualis (RAm). Those taking a dorsal trajectory via the occipitomesencephalic tract terminated in the tracheosyringeal part of the hypoglossal nucleus (XIIts), the suprahypoglossal region, and nucleus retroambigualis. There were also substantial projections throughout an arc extending between XIIts and RVL rostrally, and XIIts and RAm caudally. Neurons throughout this arc, which include inspiratory premotor neurons at levels straddling the obex and expiratory premotor neurons more caudally (in RAm), were retrogradely labeled from spinal injections. The DM projections were predominantly ipsilateral, but there were distinct contralateral projections to all the homologous nuclei in both species. All but the projections to PBvl and XIIts were reciprocal. In summary, the projections of DM suggest that it is able to influence all the key motor and premotor nuclei involved in patterned respiratory-vocal activity.

Vocal control pathways through the anterior forebrain of a parrot (Melopsittacus undulatus)

A feature of the telencephalic vocal control system in the budgerigar (Melopsittacus undulatus) that has been hypothesized to represent a profound difference in organization from the oscine vocal system is its reported lack of an inherent circuit through the anterior forebrain. The present study reports anatomical connections that indicate the existence of an anterior forebrain circuit comparable in important ways to the "recursive" pathway of oscine songbirds. Results from anterograde and retrograde tracing experiments with biocytin and fluorescently labeled dextran amines indicate that the central nucleus of the anterior archistriatum (AAc) is the source of ascending projections upon the oval nuclei of the anterior neostriatum and ventral hyperstriatum (NAo and HVo, respectively). Efferent projections from the latter nuclei terminate in the lateral neostriatum afferent to AAc, thereby forming a short recurrent pathway through the pallium. Previously reported projections from HVo and NAo upon the magnocellular nucleus of the lobus parolfactorius (LPOm), and after LPOm onto the magnocellular nucleus of the dorsal thalamus (DMm; G.F. Striedter [1994] J. Comp. Neurol. 343:35-56), are confirmed. A specific projection from DMm onto NAom is also demonstrated; therefore, a recurrent pathway through the basal forebrain also exists in the budgerigar vocal system that is similar to the anterior forebrain circuit of oscine songbirds. Parallels between these circuits and mammalian basal ganglia-thalamo-cortical circuits are discussed. It is hypothesized that vocal control nuclei of the avian anterior neostriatum may perform a function similar to the primate supplemental motor area.

Changes of neuronal responsiveness in the mediorostral neostriatum/hyperstriatum after auditory filial imprinting in the domestic chick

Auditory filial imprinting in the domestic chick is an established experimental model for investigating basic mechanisms of learning-related synaptic plasticity. In in vivo electrophysiological studies, we analysed whether imprinting alters the responsiveness of neurons to acoustic imprinting stimuli in the mediorostral neostriatum/hyperstriatum ventrale. We compared the response characteristics of neurons in the mediorostral neostriatum/hyperstriatum ventrale in freely behaving or anesthetized acoustically imprinted, non-imprinted (naive controls) and passive control chicks (stimulus-exposed) during presentation of either the imprinting stimulus or an unfamiliar discrimination stimulus. In acoustically imprinted chicks, the multiunit activity in anesthetized chicks and the fast Fourier transform power spectrum in freely behaving chicks in the mediorostral neostriatum/hyperstriatum ventrale were significantly changed during playback of the learned stimulus in comparison to spontaneous activity and compared to the activity during playback of the unfamiliar discrimination stimulus. In anesthetized non-imprinted and passive control chicks, the multiunit activity showed slightly enhanced activity during playback of either the imprinting or the discrimination stimulus in comparison to spontaneous activity. However, in both control groups there were no significant differences between the responses towards the imprinting and the discrimination stimuli. These results indicate that neurons in the mediorostral neostriatum/hyperstriatum ventrale change their responsiveness towards learned, behaviorally relevant stimuli during auditory filial imprinting.

Organization of the dopaminergic innervation of forebrain areas relevant to learning: a combined immunohistochemical/retrograde tracing study in the domestic chick

The mediorostral neostriatum/hyperstriatum ventrale (MNH) and neostriatum dorsocaudale (Ndc) of the domestic chick are crucially involved in auditory filial imprinting, whereas the lobus parolfactorius (LPO) seems to be involved in the emotional modulation of behavior. Because there is evidence that MNH and Ndc are akin to higher association areas in mammals, the present study evaluates the dopaminergic and thalamic input to these areas, as well as to the avian caudate/putamen homologue LPO, by using retrograde pathway tracing, together with dopamine (DA) and tyrosine hydroxylase (TH) immunohistochemistry. By combining DA immunohistochemistry with retrograde fluorescent tracing, we demonstrated that dopaminergic afferents to the MNH and Ndc arise mainly from the area ventralis, whereas the main dopaminergic input to the LPO arises from the substantia nigra. The main thalamic input to the MNH and LPO arises from the dorsal thalamic nuclei, n. dorsomedialis anterior and n. dorsolateralis anterior, whereas the thalamic input to the Ndc arises from the n. dorsolateralis posterior and n. subrotundus. Furthermore, there are reciprocal intratelencephalic connections between distinct parts of the neostriatum caudale and the mediorostral neostriatum. DA-immunoreactive (ir) fibers are present at moderate densities in the MNH and Ndc and at high densities in the LPO. At the ultrastructural level, DA- and TH-ir axon terminals in the MNH and Ndc form predominantly symmetric synaptic contacts with dendritic shafts, which are often situated in close vicinity to unstained terminals. These results indicate that the general organization of dopaminergic afferents to the chick telecephalon is similar to that of the mesotelencephalic dopaminergic subsystems in mammals such as the mesostriatal and mesolimbocortical DA system.

Restricted expression of R-cadherin by brain nuclei and neural circuits of the developing chicken brain

Cadherins are a family of Ca(2+)-dependent cell-cell adhesion molecules regulating morphogenesis by a preferentially homophilic binding mechanism. We have previously shown that the expression of R-cadherin in the early chicken forebrain (embryonic days E3-E6) is restricted to particular neuromeres or parts of neuromeres. R-cadherin-expressing neuroblasts born in these areas accumulate in the mantle zone and aggregate in particular (pro-) nuclei (Gänzler and Redies [1995] J. Neurosci. 15:4157-4172). In the present study, these findings are extended to later developmental stages (embryonic days E8, E11, and E15). By immunohistochemical and in situ hybridization techniques, we show that, at these stages of development, R-cadherin expression remains restricted to particular developing gray matter regions and fiber tracts. The R-cadherin-positive fiber tracts connect some of the R-cadherin-positive gray matter areas to form parts of particular neural circuits in the visual, auditory, somatosensory, and motor systems. Moreover, R-cadherin expression reflects the morphologic differentiation of gray matter regions. As brain nuclei become morphologically more distinct, the expression of R-cadherin shows a clearer demarcation of the nuclear boundaries. In addition, R-cadherin expression in some nuclei becomes restricted to particular subregions or to clusters of neurons. In the cerebellum, R-cadherin is expressed in parasagittal stripes. These results suggest that R-cadherin expression reflects the functional and morphologic maturation of gray matter structures and of information processing circuits in the embryonic chicken brain.

Telencephalic afferents to the caudolateral neostriatum of the pigeon

The pigeon caudolateral neostriatum (NCL) shares a dopaminergic innervation with mammalian frontal cortical areas and is implicated in the regulation of avian cognitive behavior. Retrograde tracing methods were used to identify forebrain projections to NCL and to suggest a possible role of this area in mediating spatial behavior. NCL receives telencephalic projections from the hyperstriatum accessorium, cells along the border of hyperstriatum dorsale and hyperstriatum ventrale, anterolateral hyperstriatum adjacent to the vallecula, confined cell groups within the anterior neostriatum, and subdivisions of the archistriatum. In addition, labeling of a small number of large cells near the fasciculus prosencephali lateralis was observed at the level of the anterior commissure. In accordance with previous studies, projections of subtelencephalic areas were revealed to originate from the thalamic posterior dorsolateral nucleus and nucleus subrotundus, as well as from the tegmental nucleus pedunculopontinus and locus coeruleus. Forebrain connections of NCL show that somatosensory, visual, and olfactory information can combine in this division of the neostriatum. NCL is therefore suited to participate in a neural circuit that regulates spatial behavior. Moreover, the present study reveals that NCL is reached by a limbic projection from the nucleus taeniae. This projection also suggests similarity between NCL and mammalian frontal cortical areas.

Influence of urethane anesthesia on neural processing in the auditory cortex analogue of a songbird

Functional maps of auditory response areas were derived from multi-unit recordings in the caudal telencephalon of the starling (Sturnus vulgaris L.). A regular grid of recording sites with distances of 200 microns horizontally and 100 microns vertically was placed over the auditory cortex analogue. Within one plane, mapping of auditory responses was first performed in the awake bird and then repeated under urethane anesthesia. The data from both experimental approaches differ considerably. Urethane reduces the spontaneous discharge rate significantly. Under anesthesia, inhibition decreases in all auditory subunits. Excitation is less affected. Eight auditory subcenters were divided into three groups according to the changes in their excitatory responses. In the first group 'on' and sustained excitation changed only weakly. These areas are thought to receive direct inputs from the diencephalon. In the second group, 'on' and sustained excitation are substantially reduced. These subcenters seem to receive projections from other forebrain areas. In subunits of the third group, an increase in sustained excitation is correlated to a decrease in inhibition. Within some specific centers, distinct natural calls, for example the bird's own song, elicit stronger responses under anesthesia than other stimuli.

Auditory pathways of caudal telencephalon and their relation to the song system of adult male zebra finches

Auditory information is critical for vocal imitation and other elements of social life in song birds. In zebra finches, neural centers that are necessary for the acquisition and production of learned vocalizations are known, and they all respond to acoustic stimulation. However, the circuits by which conspecific auditory signals are perceived, processed, and stored in long-term memory have not been well documented. In particular, no evidence exists of direct connections between auditory and vocal motor pathways, and two newly identified centers for auditory processing, caudomedial neostriatum (Ncm) and caudomedial hyperstriatum ventrale (cmHV), have no documented place among known auditory circuits. Our goal was to describe anatomically the auditory pathways in adult zebra finch males and, specifically, to show the projections by which Ncm and vocal motor centers may receive auditory input. By using injections of different kinds of neuroanatomical tracers (biotinylated dextran amines, rhodamine-linked dextran amines, biocytin, fluorogold, and rhodamine-linked latex beads), we have shown that, as in other avian groups, the neostriatal field L complex in caudal telencephalon is the primary forebrain relay for pathways originating in the auditory thalamus, i.e., the nucleus ovoidalis complex (Ov). In addition, Ncm and cmHV also receive input from the Ov complex. Ov has been broken down into two parts, the Ov "core" and "shell," which project in parallel to different targets in the caudal telencephalon. Parts of the field L complex are connected among themselves and to Ncm, cmHV, and caudolateral Hv (clHV) through a complex web of largely reciprocal pathways. In addition, clHV and parts of the field L complex project strongly to the "shelf" of neostriatum underneath the song control nucleus high vocal center (HVC) and to the "cup" of archistriatum rostrodorsal to another song-control nucleus, the robust nucleus of the archistriatum (RA). We have documented two points at which the vocal motor pathway may pick up auditory signals: the HVC-shelf interface and a projection from clHV to the nucleus interfacialis (NIf), which projects to HVC. These data represent the most complete survey to date of auditory pathways in the adult male zebra finch brain, and of their projections to motor stations of the song system.

Organization of afferent and efferent projections of the nucleus basalis prosencephali in a passerine, Taeniopygia guttata

The connections of nucleus basalis (NB) of the rostral forebrain of the zebra finch were investigated electrophysiologically and with anterograde and retrograde tracing methods to determine their functional organization, the sources of their pontine afferents, and the targets of their telencephalic efferents. The nucleus was found to be partitioned into three major components, a rostral lingual part that received a hypoglossal projection via a lateral subnucleus of the principal sensory trigeminal nucleus (PrV), a middle beak part that received a trigeminal projection via a medial subnucleus of PrV, and a caudal auditory part that received a short latency auditory projection via the intermediate nucleus of the lateral lemniscus. Beak NB also received a projection from a paralateral lemniscal nucleus, and the dorsocaudal part of auditory NB and the medially adjacent neostriatum also received a projection from a lateral subnucleus of the superior vestibular nucleus (VS). The efferent projections of each of the three major parts of NB were mainly to the adjacent neostriatum frontale (NF), which then provided projections to the lobus parolfactorius (exclusive of area X), the lateral archistriatum intermedium (Ail), and the lateral neostriatum caudale (NCl). Ail received a projection from NCl and provided terminal fields to the contralateral NCl and the NF. The major projections of Ail, however, descended bilaterally through the brainstem via the occipitomesencephalic tracts, with dense terminations in the medial spiriform nucleus and with extensive bilateral terminations throughout the lateral reticular formation of the pons and medulla. For the most part, jaw, tongue, and tracheosyringeal motor nuclei did not receive terminations. The results suggest that NB in zebra finch, like NB in pigeon and duck, is likely to be a major component of trigeminal sensorimotor circuitry involved in feeding and in other oral-manipulative behaviors. Results also show that the auditory component of NB is not directly linked to the vocal control system at telencephalic levels, but the possibility remains that the lingual, beak, and auditory parts of NB play a role in vocalization by multisynaptic influences on cranial nerve motor nuclei innervating various parts of the vocal tract.

Evidence for muscarinic acetylcholine receptor subtypes in the pigeon telencephalon

At least five subtypes of muscarinic acetylcholine receptors are expressed in various mammalian tissue preparations. The following experiment, through the use of direct binding assays (using tritiated quinuclidinyl benzilate), competitive binding assays (using tritiated quinuclidinyl benzilate and unlabeled pirenzepine or AF-DX 116), and autoradiographic techniques, examined whether two of these five putative muscarinic acetylcholine receptor subtypes can be found in avian brain. Accordingly, autoradiographic mapping of pirenzepine-sensitive (M1-like) and AF-DX 116-sensitive (M2-like) muscarinic acetylcholine receptor subtypes in the pigeon telencephalon was conducted. Although both ligands bound throughout the brain, most telencephalic regions, including the archistriatum, the neostriatum, and basal ganglia structures like lobus paraolfactorius, nucleus accumbens, and paleostriatum, showed a higher density of M1-like sites. The exception to this finding was the nucleus basalis which appeared as a region where M2-like sites predominated. Moreover, the telencephalic region with the largest ratio of M1-like to M2-like sites was the lateral portion of the parahippocampus; a characteristic shared with the mammalian dentate gyrus. The findings reported here are generally consistent with previous reports of mammalian M1/M2 receptor distributions.

Parallel pathways and convergence onto HVc and adjacent neostriatum of adult zebra finches (Taeniopygia guttata)

The structure and connectivity of the forebrain nucleus HVc, a site of sensorimotor integration in the song control system of oscine birds, were investigated in adult zebra finches. HVc in males comprises three cytoarchitectonic subdivisions: the commonly recognized central region with large and medium-sized darkly staining cells, a ventral caudomedial region with densely packed small and medium-sized cells, and a dorsolateral region with oblong cells and rows of cells. All three subdivisions project to area X and the robust nucleus of the archistriatum, with more complexity in the classes and distribution of cells than previously reported. In females, HVc is very small and has a cytoarchitecture distinct from that of the three male subdivisions. The structure of HVc in females treated with estradiol at 15 days of age is similar to male HVc. Tracer studies in males with fluorescent and biotinylated dextrans demonstrate non-topographic projections onto HVc that may carry auditory information, including type 1 and type 2 neurons in subdivisions L1 and L3 of the field L complex, a class of neurons in nucleus interface, nucleus uvaeformis, the caudal neostriatum ventral to HVc, and intrinsic HVc connections. These data are interpreted in terms of HVc's functional properties. Additionally, the neostriatum immediately ventral to HVc receives projections from field L, ventral hyperstriatum, and caudal neostriatum, and projects to a region surrounding RA and near to or into area X. The similarity of the connectivity of HVc and adjacent neostriatum suggests the possibility that they share a common origin.

Intratelencephalic projections of the visual wulst in pigeons (Columba livia)

The visual wulst is the telencephalic target of the thalamofugal visual pathway of birds, and thus the avian equivalent of the striate cortex of mammals. The anterograde tracer Phaseolus vulgaris leucoagglutinin was used to follow the intratelencephalic connections of the major constituents of the visual wulst in pigeons. In particular, efferent pathways from the granular layer (Intercalated nucleus of the hyperstriatum accessorium, IHA), supragranular layer (hyperstriatum accessorium, HA), and infragranular layers (hyperstriatum intercalatus superior and/or hyperstriatum dorsale, HIS/HD) were investigated. These efferent projections were confirmed by injections of the retrograde tracer cholera toxin subunit B into their terminal fields. When a deposit of the anterograde tracer was centered in IHA, which receives the visual thalamic input, efferent fibers were seen mainly dorsomedially to IHA. When a deposit of the anterograde tracer was centered in HA, efferent fibers were seen to extend mainly in three directions: 1) medially to the tractus septomesencephalicus, which sends projections to extratelencephalic visual nuclei: 2) ventrolaterally to the lateral portion of the neostriatum frontale, where there were also labeled cells after the retrograde tracer was injected in HA; and 3) ventromedially to the paleostriatal complex, which is the avian equivalent of the mammalian caudale, 5) neostriatum intermedium, 6) archistriatum intermedium, and 7) hyperstriatum laterale. Finally, HIS/HD have projections predominantly to HA and the dorsocaudal telencephalon (area corticoidea dorsolateralis and area parahippocampalis), as well as relatively minor projections to the areas which also receive projections from HA. No anterogradely labeled fibers were seen in the tractus septomesencephalicus following the tracer injections in HIS/HD. These results indicate that the visual information from the granular layer is distributed via the supragranular layer HA to multiple areas within the telencephalon, such as the neostriatum frontale and paleostriatal complex. In addition, HA is the source of an extratelencephalic projection via the tractus septomesencephalicus. Thus, the avian supragranular layer HA contains neurons which are the source of both intratelencephalic and extratelencephalic projections, whereas neurons of the mammalian cortex are segregated into two distinct layers, supragranular and infragranular layers, based on the targets of their projections. The findings are further discussed and compared to the mammalian striate cortex.

Convergence of somatosensory and auditory projections in the avian torus semicircularis, including the central auditory nucleus

Projections of dorsal column, spinal, and cochlear nuclei upon the central nucleus of the torus semicircularis (otherwise known as nucleus mesencephalicus lateralis, pars dorsalis, or MLd) and upon other toral nuclei were investigated in pigeon by anterograde and retrograde tracing and electrophysiological methods. The anatomical results showed that caudal regions of the dorsal column nuclei and medial lamina V of the upper four cervical spinal segments have extensive projections upon the contralateral central auditory nucleus and upon other nuclei of the torus, in particular the core portion of the preisthmic superficial area of Puelles et al. (L. Puelles, C. Rrobles, M. Martiez-de-la-Torre, and S. Martinez, 1994, J. Comp. Neurol. 340:98-125). The projections of nucleus angularis were found to terminate throughout most of the contralateral central nucleus except the dorsomedial portion at rostral levels, where the majority of the projections of nucleus laminaris were concentrated. Nucleus angularis (and to a lesser extent nucleus laminaris) was also found to have substantial projections to certain noncentral toral nuclei, in particular to the caudomedial shell nucleus of Puelles et al. (1994). As shown positively with both Nissl and cytochrome oxidase staining and negatively with substance P labeling, this nucleus is a medial extension of more caudal regions of the central nucleus, and it is suggested that it should be included as part of the auditory midbrain. The electrophysiological results confirmed the anatomical findings by showing that evoked potentials and multiunit activity can be recorded throughout the central and noncentral toral nuclei by using electrical stimulation of the radial nerve and auditory click stimuli. The core portion of the preisthmic superficial area, however, can be regarded as a distinct somatosensory nucleus of the midbrain. It is concluded that there is substantial convergence of somatosensory and auditory inputs within both central auditory and noncentral nuclei of the torus semicircularis in pigeon.

Dopaminergic innervation of the telencephalon of the pigeon (Columba livia): a study with antibodies against tyrosine hydroxylase and dopamine

The dopaminergic structures in the telencephalon of the pigeon were investigated with antisera against glutaraldehyde-conjugated dopamine (DA) and tyrosine hydroxylase (TH). Our goal was to describe the morphological patterns of the labelled axons and to provide a detailed map of the density and regional distribution of the dopaminergic innervation in relation to cytoarchitectonic areas. DA- and TH-like fibers reached their highest density in the paleostriatum augmentatum and the lobus parolfactorius of the basal ganglia. The paleostriatum primitivum was characterized by a dichotomous DA-positive innervation with a diffuse fiber network contacting enpassant granular cells and a more specific input that completely wrapped up large cells, which probably represent relay neurons. Two distinct DA-positive pathways could be followed back from the forebrain leading to the dopaminergic cell groups of the nucleus tegmenti pedunuculopontinus pars lateralis and the area ventralis tegmentalis. The primary sensory areas of the visual, auditory, somatosensory, and trigeminal systems within the forebrain of the pigeon were virtually devoid of DA-like fibers and demonstrated only TH-positive axons, probably of a noradrenergic nature. Among the limbic structures, the neostriatum caudolaterale (a possible equivalent of the mammalian prefrontal cortex), the septum, the nucleus accumbens, and parts of the archistriatum were heavily labelled by DA-like axons. A highly characteristic morphological feature of the catecholaminergic innervation was the presence of "baskets," which are constituted by TH- and DA-positive fibers coiled up around large perikarya, so that the surrounded somata were virtually visible by the presence of labelled axons. The density of basket and nonbasket type innervations seemed to be independently regulated, so that each forebrain structure could be characterized by a mixture consisting of the individual degrees of these two features. Our results demonstrate that the dopaminergic innervation of the forebrain of the pigeon is widespread but shows important regional variations. Similar to mammals, associative and motor structures are heavily innervated by dopaminergic fibers, whereas sensory areas are dominated by their noradrenergic input. The basket and nonbasket type innervations observed in virtually all of these subdivisions of the telencephalon may indicate the presence of two main classes of catecholaminergic afferents with different mechanisms of modulation of forebrain activity patterns.

Thalamostriatal projection neurons in birds utilize LANT6 and neurotensin: a light and electron microscopic double-labeling study

Based on its location, connectivity and neurotransmitter content, the dorsal thalamic zone in birds appears to be homologous to the intralaminar, midline, and mediodorsal nuclear complex in the thalamus of mammals. We investigated the neuroactive substances used by thalamostriatal projection neurons of the dorsal thalamic zone in the pigeon. Single-labeling experiments showed that many neurons in the dorsal thalamic zone are immunoreactive for neurotensin and the neurotensin-related hexapeptide, (Lys8,Asn9)NT(8-13) (LANT6). Double-labeling experiments, using the retrograde fluorescent tracer, FluoroGold, combined with fluorescence immunocytochemistry for either LANT6 or neurotensin, showed that neurotensin- and LANT6-containing neurons in the dorsal thalamic zone project to the striatum of the basal ganglia. Immunofluorescence double-labeling experiments showed that neurotensin and LANT6 are often (possibly always) co-expressed in neurons in the dorsal thalamic zone. Electron microscopic immunohistochemical double-labeling showed that LANT6 terminals in the striatum make asymmetric contacts with heads of spines labeled for substance P and heads of spines not labeled for substance P, suggesting that these terminals synapse with both substance P-containing and non-substance P-containing medium spiny striatal projection neurons. These findings indicate that LANT6 and neurotensin may be utilized as neurotransmitters in thalamostriatal projections in birds and raise the possibility that this may also be the case in other amniotes.

Organization of the avian "corticostriatal" projection system: a retrograde and anterograde pathway tracing study in pigeons

Birds have well-developed basal ganglia within the telencephalon, including a striatum consisting of the medially located lobus parolfactorius (LPO) and the laterally located paleostriatum augmentatum (PA). Relatively little is known, however, about the extent and organization of the telencephalic "cortical" input to the avian basal ganglia (i.e., the avian "corticostriatal" projection system). Using retrograde and anterograde neuroanatomical pathway tracers to address this issue, we found that a large continuous expanse of the outer pallium projects to the striatum of the basal ganglia in pigeons. This expanse includes the Wulst and archistriatum as well as the entire outer rind of the pallium intervening between Wulst and archistriatum, termed by us the pallium externum (PE). In addition, the caudolateral neostriatum (NCL), pyriform cortex, and hippocampal complex also give rise to striatal projections in pigeon. A restricted number of these pallial regions (such as the "limbic" NCL, pyriform cortex, and ventral/caudal parts of the archistriatum) project to such ventral striatal structures as the olfactory tubercle (TO), nucleus accumbens (Ac), and bed nucleus of the stria terminalis (BNST). Such "limbic" pallial areas also project to medialmost LPO and lateralmost PA, while the hyperstriatum accessorium portion of the Wulst, the PE, and the dorsal parts of the archistriatum were found to project primarily to the remainder of LPO (the lateral two-thirds) and PA (the medial four-fifths). The available evidence indicates that the diverse pallial regions projecting to the striatum in birds, as in mammals, are parts of higher order sensory or motor systems. The extensive corticostriatal system in both birds and mammals appears to include two types of pallial neurons: 1) those that project to both striatum and brainstem (i.e., those in the Wulst and the archistriatum) and 2) those that project to striatum but not to brainstem (i.e., those in the PE). The lack of extensive corticostriatal projections from either type of neuron in anamniotes suggests that the anamniote-amniote evolutionary transition was marked by the emergence of the corticostriatal projection system as a prominent source of sensory and motor information for the striatum, possibly facilitating the role of the basal ganglia in movement control.

Visual and somatosensory inputs to the avian song system via nucleus uvaeformis (Uva) and a comparison with the projections of a similar thalamic nucleus in a nonsongbird, Columba livia

Nucleus uvaeformis (Uva), previously identified as a component of song control circuitry in songbirds, and nucleus dorsolateralis posterior thalami, pars caudalis (DLPc) in pigeon, were compared with respect to their relative positions in the dorsolateral part of the posterior thalamus, their cell types, and their afferent and efferent projections. Both nuclei are closely related to the habenulointerpeduncular tract, have similar cell types, and receive a dense projection from deep layers of the optic tectum, predominantly ipsilaterally, and a distinct projection from the dorsal column and external cuneate nuclei, predominantly contralaterally. Recordings of multiple unit activity evoked by visual and somatosensory stimuli were used to guide injections of tracer into either DLPc or Uva, and the projections to the telencephalon were charted. Both nuclei were found to have a major terminal field in the medial part of the ipsilateral neostriatum intermedium (NI), known as nucleus interfacialis (NIf) in songbirds, and a minor terminal field in the roof of the neostriatum caudale (NC). In pigeon, the DLPc terminations in NC were within a region known as neostriatum dorsale (Nd), and, in male songbirds, the Uva terminations were in the high vocal center (HVC). Recordings of visual and somatosensory evoked activity were then used to guide injections of tracer into NI, and the afferent and efferent projections were again compared in pigeon and songbirds. The projections from either DLPc or Uva were confirmed, and terminal fields were observed either in Nd in pigeon, the dorsolateral part of NC in female songbirds, or HVC in male songbirds. Injections of tracer into either Nd or HVC confirmed their sources of afferents in DLPc or Uva, respectively, and in NI, but there was incomplete overlap of the distribution of retrogradely labelled cells in NI and the terminal fields of DLPc or Uva. It is concluded that DLPc and Uva are comparable nuclei having similar afferent and efferent projections relaying visual and somatosensory information to the telencephalon. The possible role of this information in vocal control is discussed.