Connections of the pigeon dorsomedial forebrain studied with WGA-HRP and 3H-proline

The distribution of neurotransmitters and neurotransmitter-related enzymes in the dorsomedial telencephalon of the pigeon (Columba livia)

Immunoreactivity to four neurotransmitters/transmitter-related enzymes was found in the dorsomedial telencephalon (hippocampal region) of the pigeon. Putative afferent fibers containing choline acetyltransferase-like, serotonin-like, and tyrosine hydroxylase-like immunoreactivity were seen in a fiber tract passing through the septo-hippocampal junction and along the medial wall of the hippocampal region. The most intensive labeling of neuropil and terminals of all four substances was found in the dorsomedial area of the hippocampal region. Glutamic acid decarboxylase-like immunoreactivity was seen in sparsely scattered cells throughout the region. These results are discussed in relation to hypotheses about the boundaries and subdivisions of the hippocampal region of the pigeon.

Parallel evolution in mammalian and avian brains: comparative cytoarchitectonic and cytochemical analysis

Comparative morphology, which is based on the selection theory of evolution, analyses the impact of function upon structure and, therefore, emphasizes the adaptive events and biological advantage during the evolution of organs. A comparison based on analogies is described here as an adequate method. The hypothesis is proposed that the evolution of the brain follows the same trends in birds as in mammals. This hypothesis is proved by (1) allometric studies of brain weight and brain structure volume in relation to body weight in mammals and birds; (2) architectonic studies using image analysis on cell and fibre stains as well as on histochemical preparations and receptor autoradiography; and (3) hodological studies with injections of [3H]leucin, HRP and WGA-HRP. The results reveal a vast amount of structural and functional similarities in avian and mammalian brain organization, especially an expansion of structures that permit multimodal integration capacity in the telencephalon. Thus, a parallel evolution occurred in these two groups of vertebrates. It is argued that this may be a general phenomenon in evolution. A cladistic approach, which is based on the concept of homologies (plesio-, apomorphies), pushes aside the existence of analogies. For this reason, cladism does not seem to be a method to answer questions of evolutionary morphology adequately.

Homing behavior of hippocampus and parahippocampus lesioned pigeons following short-distance releases

The avian hippocampal formation has been proposed to play a critical role in the neural regulation of a navigational system used by homing pigeons to locate their loft once in the familiar area near home. In support of this hypothesis, the homing performance of pigeons with target lesions of either the hippocampus or parahippocampus was found to be impaired compared to controls following releases of about 10 km. Further, radio tracking revealed that the in-flight behavior of the hippocampal lesioned homing pigeons was characterized by numerous direction changes and generally poor orientation with respect to the home loft. The results identify a local navigational impairment on the part of the hippocampal lesioned pigeons in the vicinity of the loft where landmark cues are thought to be important. Additionally, target lesions of the hippocampus or parahippocampus were found to be similarly effective in causing homing deficits.

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.

Differences between ipsilaterally and contralaterally evoked potentials in the visual wulst of the zebra finch

The telencephalic target of the thalamofugal visual pathway in birds, the visual wulst, is part of the hyperstriatum accessorium/dorsale in the bird's brain. In this study, we tried to determine the exact location of the visually responsive area in the zebra finch by recording visually evoked potentials (VEPs) from different sites throughout the hyperstriatum and calculating current source densities (CSDs). In addition, we examined the influence of ipsilateral and contralateral stimuli on stimulus processing within this area, and tried to get insight into the neuronal machinery of the thalamofugal pathway by application of drugs such as tetrodotoxin (TTX) and picrotoxin. About two-thirds of the hyperstriatum is responsive to contralateral stimuli but only a small portion responds to ipsilateral stimuli. Contralateral visual information arrives in the hyperstriatum dorsale (HD) and is processed further to the hyperstriatum accessorium (HA). The small influence of ipsilaterally evoked potentials is not due to inhibition by the activity of the contralateral eye, as could be demonstrated previously for the ectostriatum. Instead, our results show that ipsilaterally evoked potentials are inhibited at least in part by a projection from the contralateral visual wulst.

Ipsilaterally evoked responses of the zebra finch visual wulst are reduced during ontogeny

Visual wulst responses to ipsi- and contralateral visual stimuli were investigated in young zebra finches (Taeniopygia guttata castanotis Gould) of different ages. Contralateral responses in 20, 40, 60 and 80 day old birds do not differ significantly from those in adults. In contrast, ipsilateral responses decrease substantially during development and become very weak and irregular in adult birds.

Hippocampal specialization of food-storing birds

In a study of 52 individuals belonging to 35 species or subspecies of passerine birds it was shown that the volume of the hippocampal complex relative to brain and body size is significantly larger in species that store food than in species that do not. Retrieval of stored food relies on an accurate and long-lasting spatial memory, and hippocampal damage disrupts memory for storage sites. The results suggest, therefore, that food-storing species of passerines have an enlarged hippocampal complex as a specialization associated with the use of a specialized memory capacity. Other life-history variables were examined and found not to be correlated with hippocampal volume.

Endogenous levels of serotonin and 5-hydroxyindoleacetic acid in specific areas of the pigeon CNS: effects of serotonin neurotoxins

Endogenous levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined by high-performance liquid chromatography (HPLC) in specific regions of the pigeon central nervous system (CNS). High endogenous 5-HT levels in the visual wulst and brainstem and medium 5-HT content in the optic lobes were found. The cerebellum and retina showed low endogenous 5-HT levels. Similar endogenous 5-HIAA levels were measured in the visual wulst, optic lobes and brainstem, whereas the 5-HIAA content of the cerebellum and retina was significantly lower. The effects of para-chloroamphetamine (p-CA) and 5,7-dihydroxytryptamine (5,7-DHT) on the 5-HT and 5-HIAA content of the same regions were studied. Six days after p-CA treatment, the 5-HT content of the visual wulst, optic lobes, brainstem and the 5-HIAA content of the optic lobes and cerebellum markedly decreased. Nine days after 5,7-DHT administration, the 5-HT and 5-HIAA content of the visual wulst and optic lobes was significantly reduced. At longer survival times, serotonergic systems were differentially affected depending on both the neurotoxin treatment and the specific brain regions examined. The 5-HT content of the pigeon retina was not modified by p-CA treatment, whereas 5,7-DHT intravitreally injected caused a pronounced 5-HT depletion. Our results demonstrate that selective neurotoxins for serotonergic systems can provide a useful denervation tool for the study of serotonergic function in the pigeon CNS.

Unimpaired acquisition of spatial reference memory, but impaired homing performance in hippocampal-ablated pigeons

Hippocampal ablated homing pigeons have been shown to suffer a retrograde spatial reference memory deficit involving a preoperatively acquired homeward orientation response based on local cues around a previously visited release site. Here we report that the postoperative acquisition of such a response is unimpaired. Initially, 25 hippocampal ablated and 11 sham-operated controls were given 5 training releases from each of two sites. In the subsequent experimental releases from the two training sites, the controls and half the hippocampal-ablated pigeons had their navigational maps rendered dysfunctional via an anosmic procedure. Nonetheless, both groups successfully oriented homeward, indicating that the hippocampal-ablated pigeons were unimpaired in the acquisition and implementation of directionally useful information around the training sites to direct a homeward orientation response. The remaining half of the hippocampal-ablated pigeons who were not rendered anosmic, and thus served as controls, also oriented homeward. The data indicate that, for hippocampal-ablated homing pigeons, postoperative acquisition is unimpaired in the same spatial reference memory task where a robust retrograde impairment was observed. However, the hippocampal-ablated pigeons were impaired in the time required to return home, indicating a deficit in homing performance beyond the initial orientation stage.

Impaired retention of preoperatively acquired spatial reference memory in homing pigeons following hippocampal ablation

Hippocampal-parahippocampal-ablated homing pigeons have been shown to suffer a retrograde loss of information used in the recognition of their home loft. Here we report that the range of retrograde deficits includes spatial reference memory in the form of information gained from repeated training sites that can be used to direct a homeward orientation response. Following 8 training releases from each of two sites, 28 of 42 homing pigeons underwent hippocampal-parahippocampal ablation. In the subsequent test releases from the two sites, untreated controls whose navigational map was rendered temporarily dysfunctional by an anosmic procedure showed no impairment in determining the home direction, indicating the successful retention and utilization of directionally useful information around the release sites. Hippocampal-ablated controls who were not rendered anosmic and thus had access to their navigational map also showed no impairment in determining the home direction, indicating no general impairment in initial orientation as a result of hippocampal ablation. In contrast, hippocampal-ablated pigeons whose navigational map was rendered temporarily dysfunctional failed to successfully orient homeward from the training sites, indicating impairment in the retention and/or implementation of directional information gathered at the release sites during training. The data reveal a spatial reference memory deficit involving pre-ablation acquired directional information in homing pigeons following hippocampal ablation.