Regional distribution of catecholaminergic terminals in the pigeon visual system

Evolution of slow-wave sleep and palliopallial connectivity in mammals and birds: a hypothesis

Mammals and birds are the only animals that exhibit rapid eye-movement (REM) sleep and slow-wave sleep (SWS). Whereas the electroencephalogram (EEG) during REM sleep resembles the low-amplitude, high-frequency EEG of wakefulness, the EEG during SWS displays high-amplitude, slow-waves (1-4Hz). The absence of similar slow-waves (SWs) in sleeping reptiles suggests that the neuroanatomical and neurophysiological traits necessary for the genesis of SWs evolved independently in the mammalian and avian ancestors. Advances in our understanding of comparative neuroanatomy and the genesis of mammalian SWs suggest that the absence of SWs in reptiles is due to limited connectivity within the pallium, the dorsal portion of the telencephalon that includes the mammalian neocortex, reptilian dorsal cortex and avian Wulst (hyperpallium), as well as the dorsal ventricular ridge in birds and reptiles and the mammalian claustrum and pallial amygdala. In mammals, the slow oscillation (<1Hz) of cortical neurons acts through reciprocal corticothalamic loops and corticocortical connections to synchronize the 1-4Hz activity of thalamocortical neurons in a manner sufficient to generate SWs detectable in the EEG. Given the role that corticocortical (or palliopallial) connections play in the genesis of SWs in mammals, the degree of palliopallial connectivity might explain why birds show SWs and reptiles do not. Indeed, whereas the mammalian neocortex and avian pallium show extensive palliopallial connectivity, the reptilian pallium exhibits limited intrapallial connections. I thus propose that the evolution of SWs is linked to the independent evolution of extensive palliopallial connectivity in mammals and birds. As suggested by experiments functionally linking SWs to performance enhancements, the palliopallial connections that give rise to SWs might also depend on SWs to maintain their efficacy.

A comparative study of alpha2- and beta-adrenoceptor distribution in pigeon and chick brain

The pharmacological properties and anatomical distribution of alpha2-, beta1- and beta2-adrenoceptors in pigeon and chick brains were studied by both homogenate binding and tissue section autoradiography. [3H]Bromoxidine (alpha2-adrenoceptor-), [3H]CGP 12177 (beta-adrenoceptor) and [125I]cyanopindolol (beta-adrenoceptor) were used as radioligands. In both species, [3H]bromoxidine binding to avian brain tissue showed a pharmacological profile similar to that previously reported for alpha2-adrenoceptors in mammals. Regarding the anatomical distribution, the areas with the highest densities of alpha2-adrenoceptors in the pigeon brain included the hyperstriatum, nuclei septalis, tectum opticum and some brainstem nuclei. Most beta-adrenoceptors found in tissue membranes and sections from chick and pigeon brain were of the beta2 subtype, in contrast to what has been reported in the mammalian brain, where the beta1 subtype is predominant. A striking difference was found between the two species regarding the densities of these receptors: while pigeon brain was extremely rich in [125I]cyanopindolol binding throughout the brain (mainly cerebellum) in the pigeon, the levels of labelling in the chick brain were much lower; the exception was the cerebellum, which displayed a higher density than other parts of the brain in both species. Overall, our results support the proposed anatomical equivalences between a number of structures in the avian and mammalian encephalon.

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.

Laminar distribution and sources of catecholaminergic input to the optic tectum of the pigeon (Columbia livia)

A combined immunohistochemical and retrograde tracing approach was used to characterize the catecholaminergic innervation of the optic tectum (TeO), the major target of retinal projections in many avian species. Giemsa counterstaining was employed to determine precisely the laminar localization of immunoreactive fibers and presumptive terminals. The TeO of the pigeon is densely innervated by fibers immunoreactive for tyrosine hydroxylase (TH), which are most heavily distributed to the superficial layers of its dorsal and anterior portions. Within the dorsal-anterior tectum, TH-immunoreactive processes are particularly dense in retinorecipient layers 4 and 7 and in layer 5a. As in the mammalian superior colliculus, the bulk of the catecholaminergic innervation of the pigeon TeO reflects inputs, presumably noradrenergic, originating in the locus coeruleus and nucleus subcoeruleus. However, the catecholaminergic innervation of the pigeon TeO shows several features distinct from those reported for the mammalian superior colliculus. These include an input from a pretectal TH-positive cell group unknown in mammals and the presence of residual TH immunoreactivity after administration of the noradrenergic neurotoxin DSP-4. Moreover, the pattern of TH-immunoreactive fibers in pigeon TeO indicates more laminar and regional specialization within this structure than has been reported for the catecholaminergic innervation of the superior colliculus in mammals.

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.

Distribution of neuropeptide Y, substance P, and choline acetyltransferase in the developing visual system of the pigeon and effects of unilateral retina removal

The distribution of three neuroactive substances, neuropeptide Y, substance P, and choline acetyltransferase, was studied by immunocytochemical methods in central visual regions of adult, developing, and ablated pigeon brains. In normal adult brains, neuropeptide Y-positive cells and processes were present in the nucleus pretectalis, the nucleus of the basal optic root, the nucleus of the marginal optic tract, and the visual Wulst. Substance P-positive cells and processes were found in the optic tectum and in the visual Wulst. Stained fibers and terminal-like processes, but no cells, were also observed in several visual thalamic nuclei. Choline acetyltransferase-positive cells and processes were located in the optic tectum, visual Wulst, the nucleus isthmo opticus, nucleus isthmi and certain visual thalamic nuclei. Cholinergic fibers and processes, but no cells, were present in the nucleus principalis precommissuralis, the supraoptic decussation, and the nucleus lentiformis mesencephali, pars magnocellularis. In the course of development, the distribution of immunoreactivity for all three substances was found to vary. These changes often involved either progressive increases or decreases in the density of labeled cells, neuropil and/or terminal-like profiles. Experiments with retina ablated pigeons clearly demonstrated that changes in the normal pattern of immunoreactivity distribution only occurred if the retina was removed immediately after hatching, i.e., before retinofugal connections have been established. The adult pattern of immunoreactivity for all three substances appears to be reached at about the same time that the anatomical and functional maturation of the pigeon visual system is completed. The present results suggest that this temporal correlation reflects the important role that retinal afferents play in the development of these putative peptidergic and cholinergic systems.

An immunocytochemical analysis of the lateral geniculate complex in the pigeon (Columba livia)

The lateral geniculate complex (GL) of pigeons was investigated with respect to its immunohistochemical characteristics, retinal afferents, and the putative transmitters/modulators of its neurons. The distributions of serotonin-, choline acetyltransferase-, glutamic acid decarboxylase-, tyrosine hydroxylase-, neuropeptide Y- (NPY), substance P- (SP), neurotensin- (NT), cholecystokinin- (CCK), and leucine-enkephalin- (L-ENK) like immunoreactive perikarya and fibers were mapped. Retinal projections were studied following injections of Rhodamine-B-isothiocyanate into the vitreous. Transmitter-specific projections onto the visual Wulst and the optic tectum were studied by simultaneous double-labelling of retrograde tracer molecules and immunocytochemical labelling. The GL can be divided into three major subdivisions, the n. geniculatus lateralis, pars dorsalis (GLd; previously designated as the n. opticus principalis thalami, OPT), the n. marginalis tractus optici (nMOT), and the n. geniculatus lateralis, pars ventralis (GLv). All three subdivisions are retinorecipient. The GLd can be further subdivided into at least five components differing in their immunohistochemical characteristics: n. lateralis anterior (LA); n. dorsolateralis anterior thalami, pars lateralis (DLL), n. dorsolateralis anterior thalami, pars magnocellularis (DLAmc); n. lateralis dorsalis nuclei optici principalis thalami (LdOPT); and n. suprarotundus (SpRt). The LdOPT consists of an area of dense CCK-like and NT-like terminals of probable retinal origin. Three subnuclei (DLL, DLAmc, SpRt) were shown to project to the visual Wulst. Cholinergic and cholecystokinergic relay neurons participated in this projection. The nMOT occupies a position between the GLd and GLv and encircles the rostral pole of n. rotundus and the LA. It is characterized mainly by medium sized NPY-like perikarya which were shown to project onto the ipsilateral optic tectum. Bands of NPY-like fibers in the tectal layers 2, 4, and 7 could at least in part be due to this projection of the nMOT. Most of the antisera used revealed transmitter/modulator-specific fiber systems in the GLv which often showed a layer-specific distribution. Perikaryal labelling was only obtained with glutamic acid decarboxylase. On the basis of its chemoarchitectonics, topography, and connectional pattern, the GLd complex of pigeons is most directly equivalent to the mammalian GLd. However, although the different subdivisions of the avian GLd may represent functionally different channels within the thalamofugal pathway similar to the lamina-specific differentiation within the mammalian geniculostriate projection, direct comparison of subnuclei of birds and mammals is not justified at this time. The nMOT appears similar to the intergeniculate leaflet (IGL) and the avian GLv clearly corresponds in many features to the mammalian GLv.

Ultrastructural study of the avian ventral lateral geniculate nucleus

The ultrastructure of the pigeon and quail ventral lateral geniculate nucleus was analyzed with standard electron microscopy and horseradish peroxidase tracing of its retinal and tectal afferents. Six types of neurons were distinguished: two large, two medium-sized, and two small types. The latter do not project to the optic tectum and appear to be interneurons. Large and medium-sized neurons project to the optic tectum and are thus relay neurons. Profiles with round, large synaptic vesicles were identified as retinal axon terminal afferents and those with pleomorphic, loosely grouped synaptic vesicles as tectal afferents. Gap junctions were seen between perikarya of small neurons and also with unidentified profiles.

Immunohistochemical analysis of the visual wulst of the pigeon (Columba livia)

The avian wulst, a laminated "bulge" in the dorsal telencephalon, contains several distinct regions. The posterolateral portion (visual wulst) has been proposed to be an avian equivalent of the mammalian striate cortex. The present study examines specific neurotransmitters and neuropeptides within the visual wulst by immunohistochemical techniques. Antisera and monoclonal antibodies against choline acetyltransferase (ChAT), nicotinic acetylcholine receptor (nAChR), tyrosine hydroxylase (TH), serotonin (5-HT), glutamic acid decarboxylase (GAD), gamma-aminobutyric acid A receptor (GABAAR), cholecystokinin (CCK), substance P (SP), leucine-enkephalin (L-ENK), neurotensin (NT), neuropeptide Y (NPY), somatostatin (SRIF), corticotropin-releasing factor (CRF), and vasoactive intestinal polypeptide (VIP) were used. Somata and neuropil displaying specific immunoreactivity were generally distributed in accordance with the laminar cytoarchitectonic organization of the wulst. The superficial layer of the wulst, the hyperstriatum accessorium, contained the highest densities of TH-, 5-HT-, SP-, NPY-, SRIF-, CRF-, and VIP-positive neuropil in the wulst, whereas the highest density of CCK- and NT-staining was found in the deepest layer of the wulst, the hyperstriatum dorsale. In addition to the traditionally defined four laminae of the wulst, the immunoreactive staining revealed several subregions within each lamina. The most dorsolateral portion of the wulst contained the highest densities of ChAT- and L-ENK-stained fibers in the wulst, as well as moderately dense staining of neuropil for 5-HT-, TH-, SP-, and CCK-like immunoreactivity. The nAChR-immunoreactivity was faint and distributed rather uniformly throughout the wulst. The results suggest that the wulst consists of multiple regional variations within layers comparable to laminar variations found within different cytoarchitectonic areas of the mammalian neocortex.

Iontophoresis of norepinephrine onto neurons of the pigeon's lateral geniculate nucleus: characterization of an inhibitory response

A group of neurons in the pigeon's lateral geniculate equivalent nucleus (LGNe) shows associative enhancement of their response to light during visually conditioned heart rate change. The source of the relevant unconditioned stimulus input to LGNe for this enhancement has been identified as the locus coeruleus (LC). Thus, we have begun to examine neurotransmitters synthesized in LC for possible involvement in associative modification of neuronal discharge in LGNe. As a first step, we have examined the responses of LGNe neurons to iontophoretic application of norepinephrine (NE) and identified the receptor mediating one response class. The majority of neurons in LGNe show inhibition of maintained activity in response to iontophoretic application of NE or its agonists. The potency of the NE agonists is alpha-methyl NE greater than epinephrine greater than NE greater than phenylephrine greater than isoproterenol. This profile is characteristic of an alpha 2-adrenoceptor. The alpha 2-agonist clonidine also induces inhibition of maintained activity. The alpha 2-antagonists WB-4101 and yohimbine block the NE-inhibition while the alpha 1-antagonist prazosin and beta-antagonist sotalol do not. It is thus suggested that the receptor mediating the NE-inhibition of maintained activity has the characteristics of an alpha 2-adrenoceptor.

Injections of 6-hydroxydopamine in the substantia nigra of the rat brain: morphological and biochemical effects

Rats with unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta were classified as active and inactive according to the intensity of their spontaneous and/or apomorphine-induced turning behavior (TB), and sacrificed at different survival times for morphological and biochemical analysis. In active rats, at any survival time, dopaminergic fluorescence in the nigrostriatal system as well as dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) content of the nucleus caudatus-putamen drastically decreased on the brain side ipsilateral to the injection. Dopaminergic fluorescence as well as DA and DOPAC content of the mesolimbic system ipsilateral to the injection also decreased. In inactive rats, at any survival time, 6-OHDA-induced lesions only partially involved both nigrostriatal and mesolimbic systems. Our results are indicative of a good correlation between the intensity of TB and the extent of 6-OHDA-induced lesions, as assessed by morphological and biochemical analysis.

Organization and actions of the noradrenergic input to the hamster's superior colliculus

Immunocytochemistry using antisera directed against dopamine-beta-hydroxylase (DBH) was used to determine the organization of the noradrenergic (NE) input to the hamster's superior colliculus (SC). Immunocytochemistry for DBH was combined with retrograde transport of fluorogold (FG) to determine the sources of NE input to SC. Microiontophoretic techniques were used together with extracellular single unit recording and receptive field mapping techniques to determine the manner in which NE influenced the responses of individual SC neurons. The hamster's SC contained numerous DBH-positive fibers but no immunopositive cells. These fibers formed a plexus that was most dense in the lower stratum griseum superficiale (SGS). The density of DBH-positive fibers was very low in the stratum opticum (SO) and increased in density in the stratum griseum intermediale (SGI) and the other deep layers. When FG injections into the SC were combined with immunocytochemical detection of DBH, double-labeled cells were observed in the contralateral locus ceruleus. DBH-positive neurons were observed in several other portions of the mesencephalon and pons, but none of these were labelled with FG. The effects of NE iontophoresis were assessed for a total of 135 SC neurons. In 74% (N = 100), NE reduced spontaneous and/or stimulus evoked activity. In 3% (N = 4 cells), NE increased activity, and in 23% (N = 31 cells) it had no effect. These percentages were essentially the same for superficial layer visual cells and somatosensory neurons in the deep laminae. The effect of NE iontophoresis upon signal to noise ratios was assessed for 46 visual and 56 somatosensory neurons. For 54% (N = 25) of the visual cells and 16% (N = 9) of the somatosensory cells, NE iontophoresis decreased signal to noise ratios. For 13% (N = 6) of the visual cells and 21% (N = 12) of the somatosensory cells, NE iontophoresis increased signal to noise ratios. The effects of NE on the responsivity of SC neurons were antagonized by propranolol (86% of the 21 cells tested), sotalol (67% of the six cells tested), and atenolol (effective in the single cell tested). All these agents are beta-adrenergic antagonists. The single alpha-adrenergic antagonist that we evaluated, corynanthine, potentiated the effects of NE on the responsivity of the two SC neurons that we tested.

Neurotransmitters, receptors, and neuropeptides in the accessory optic system: an immunohistochemical survey in the pigeon (Columba livia)

Immunohistochemical techniques were used to survey the distribution of several conventional transmitters, receptors, and neuropeptides in the pigeon nucleus of the basal optic root (nBOR), a component of the accessory optic system. Amongst the conventional neurotransmitters/modulators, the most intense labeling of fibers/terminals within the nBOR was obtained with antisera directed against glutamic acid decarboxylase (GAD) and serotonin (5-HT). Moderately dense fiber plexuses were seen to label with antibodies directed against tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT). GAD-like immunoreactivity (GAD-LI) was found in many small and medium-sized perikarya within the nBOR. Some of the medium-sized cells were occasionally positive for ChAT-LI. Cell body and dendritic staining was also commonly seen with the two tested antisera against receptors-anti-GABA-A receptor and anti-nicotinic acetylcholine receptor. The antisera directed against various neuropeptides produced only fiber labeling within the nBOR. The densest fiber plexus staining was observed with antiserum against neuropeptide Y (NPY-LI), while intermediate fiber densities were seen for substance P (SP-LI) and cholecystokinin (CCK-LI). A few varicose fibers were labeled with antisera against neurotensin (NT), leucine-enkephalin (L-ENK), and the vasoactive intestinal polypeptide (VIP). Unilateral enucleation produced an almost complete elimination of TH-LI in the contralateral nBOR. SP-LI and CCK-LI were also decreased after enucleation. No apparent changes were seen for all other substances. These results indicate that a wide variety of chemically-specific systems arborize within the nBOR. Three of the immunohistochemically defined fiber systems (TH-LI, SP-LI, and CCK-LI fibers) were reduced after removal of the retina, which may indicate the presence of these substances in retinal ganglion cells. In contrast, the fibers exhibiting ChAT-LI, GAD-LI, 5-HT-LI, NPY-LI, NT-LI, L-ENK-LI, and VIP-LI appear to be of nonretinal origin. Two different populations of nBOR neurons exhibited GAD-LI and ChAT-LI. However, these two populations together constituted only about 20% of the nBOR neurons.

p-Chloroamphetamine treatment modifies evoked responses to sinusoidal gratings in the pigeon optic tectum

This study was performed in order to establish whether selective depletion of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the pigeon optic tectum (TeO) induced by p-chloroamphetamine (p-CA) modified tectal evoked potentials (TEPs). TEPs in response to sinusoidal gratings of different contrast, spatial and temporal frequency were recorded in control pigeons and in pigeons intraperitoneally injected with p-CA (10 mg/kg; two administrations in consecutive days). TEPs of p-CA treated pigeons, as compared to those of control pigeons, were reduced in amplitude as a function of contrast, spatial and temporal frequency. In addition, TEPs of p-CA treated pigeons differed from those recorded in controls in their transfer characteristics of contrast and spatial frequency. In particular, TEPs of p-CA treated pigeons did not saturate at moderate contrast, unlike those of controls. Furthermore, the TEP spatial tuning in p-CA treated pigeons is broader than that in controls; it thus suggests a reduction of spatial-frequency selectivity. These findings indicate that a selective neurotoxin for serotonergic systems, such as p-CA, can serve as a useful denervation tool for the study of the serotonergic function in the pigeon TeO. In addition, selective changes of TEP properties suggest the possibility that serotonergic afferents play a modulatory role on the receptive-field characteristics of tectal neurons.

Catecholaminergic subpopulation of retinal displaced ganglion cells projects to the accessory optic nucleus in the pigeon (Columba livia)

In birds, displaced ganglion cells (DGCs) constitute the exclusive source of retinal input to the nucleus of the basal optic root (nBOR) of the accessory optic system. Tyrosine-hydroxylase (TH) immunoreactivity was examined in the pigeon retina after injections of rhodamine-labeled microspheres into the nBOR. A population of about 400 DGCs was observed in each case to exhibit both TH immunoreactivity and rhodamine bead fluorescence. This corresponded to about 10-15% of the total number of identified DGCs in each retina. Double-labeled cells were medium- to large-size (12 to 20 microns in the largest axis) and were always located at the border between the inner nuclear and the inner plexiform layers. Their dendrites could be followed horizontally in lamina 1 of the inner plexiform layer for up to 300 microns from the cell body. The distribution of double-labeled DGCs appeared to be mostly peripheral, matching the overall distribution of identified DGCs. Larger DGCs (21-28 microns) were never seen to contain TH immunoreactivity. Examination of brain sections revealed plexuses of thin varicose TH-positive axons in all subdivisions of the nBOR. Unilateral enucleation produced an almost complete elimination of TH immunoreactivity in the contralateral nucleus. Such results suggest the existence of a population of catecholaminergic DGCs projecting into the accessory optic system of the pigeon. They also support the emerging hypothesis concerning the neurotransmitter heterogeneity of ganglion cells in the vertebrate retina.