A histological and diceCT-derived 3D reconstruction of the avian visual thalamofugal pathway

Amniotes feature two principal visual processing systems: the tectofugal and thalamofugal pathways. In most mammals, the thalamofugal pathway predominates, routing retinal afferents through the dorsolateral geniculate complex to the visual cortex. In most birds, the thalamofugal pathway often plays the lesser role with retinal afferents projecting to the principal optic thalami, a complex of several nuclei that resides in the dorsal thalamus. This thalamic complex sends projections to a forebrain structure called the Wulst, the terminus of the thalamofugal visual system. The thalamofugal pathway in birds serves many functions such as pattern discrimination, spatial memory, and navigation/migration. A comprehensive analysis of avian species has unveiled diverse subdivisions within the thalamic and forebrain structures, contingent on species, age, and techniques utilized. In this study, we documented the thalamofugal system in three dimensions by integrating histological and contrast-enhanced computed tomography imaging of the avian brain. Sections of two-week-old chick brains were cut in either coronal, sagittal, or horizontal planes and stained with Nissl and either Gallyas silver or Luxol Fast Blue. The thalamic principal optic complex and pallial Wulst were subdivided on the basis of cell and fiber density. Additionally, we utilized the technique of diffusible iodine-based contrast-enhanced computed tomography (diceCT) on a 5-week-old chick brain, and right eyeball. By merging diceCT data, stained histological sections, and information from the existing literature, a comprehensive three-dimensional model of the avian thalamofugal pathway was constructed. The use of a 3D model provides a clearer understanding of the structural and spatial organization of the thalamofugal system. The ability to integrate histochemical sections with diceCT 3D modeling is critical to better understanding the anatomical and physiologic organization of complex pathways such as the thalamofugal visual system.

Intra-"cortical" activity during avian non-REM and REM sleep: variant and invariant traits between birds and mammals

Several mammalian-based theories propose that the varying patterns of neuronal activity occurring in wakefulness and sleep reflect different modes of information processing. Neocortical slow-waves, hippocampal sharp-wave ripples, and thalamocortical spindles occurring during mammalian non-rapid eye-movement (NREM) sleep are proposed to play a role in systems-level memory consolidation. Birds show similar NREM and REM (rapid eye-movement) sleep stages to mammals; however, it is unclear whether all neurophysiological rhythms implicated in mammalian memory consolidation are also present. Moreover, it is unknown whether the propagation of slow-waves described in the mammalian neocortex occurs in the avian "cortex" during natural NREM sleep. We used a 32-channel silicon probe connected to a transmitter to make intracerebral recordings of the visual hyperpallium and thalamus in naturally sleeping pigeons (Columba livia). As in the mammalian neocortex, slow-waves during NREM sleep propagated through the hyperpallium. Propagation primarily occurred in the thalamic input layers of the hyperpallium, regions that also showed the greatest slow-wave activity (SWA). Spindles were not detected in both the visual hyperpallium, including regions receiving thalamic input, and thalamus, using a recording method that readily detects spindles in mammals. Interestingly, during REM sleep fast gamma bursts in the hyperpallium (when present) were restricted to the thalamic input layers. In addition, unlike mice, the decrease in SWA from NREM to REM sleep was the greatest in these layers. Taken together, these variant and invariant neurophysiological aspects of avian and mammalian sleep suggest that there may be associated mechanistic and functional similarities and differences between avian and mammalian sleep.

Representation of time interval entrained by periodic stimuli in the visual thalamus of pigeons

Animals use the temporal information from previously experienced periodic events to instruct their future behaviors. The retina and cortex are involved in such behavior, but it remains largely unknown how the thalamus, transferring visual information from the retina to the cortex, processes the periodic temporal patterns. Here we report that the luminance cells in the nucleus dorsolateralis anterior thalami (DLA) of pigeons exhibited oscillatory activities in a temporal pattern identical to the rhythmic luminance changes of repetitive light/dark (LD) stimuli with durations in the seconds-to-minutes range. Particularly, after LD stimulation, the DLA cells retained the entrained oscillatory activities with an interval closely matching the duration of the LD cycle. Furthermore, the post-stimulus oscillatory activities of the DLA cells were sustained without feedback inputs from the pallium (equivalent to the mammalian cortex). Our study suggests that the experience-dependent representation of time interval in the brain might not be confined to the pallial/cortical level, but may occur as early as at the thalamic level.

Involvement of the Avian Dorsal Thalamic Nuclei in Homing Pigeon Navigation

The navigational ability of birds has been a focus of popular and scientific interest for centuries, but relatively little is known about the neuronal networks that support avian navigation. In the brain, regions like the piriform cortex, olfactory bulbs, hippocampal formation, vestibular nuclei, and the wulst, are among the brain regions often discussed as involved in avian navigation. However, despite large literature showing a prominent role of some anterior and dorsal thalamic nuclei in mammalian spatial navigation, little is known about the role of the thalamus in avian navigation. Here, we analyzed a possible role of the dorsal anterior thalamic nuclei in avian navigation by combining olfactory manipulations during the transport of young homing pigeons to a release site and c-Fos immunohistochemistry for the mapping brain activity. The results reveal that odor modulated neurons in the avian dorsolateral lateral (DLL) subdivision of the anterior thalamic nuclei are actively involved in processing outward journey, navigational information. Outward journey information is used by pigeons to correctly determine the homeward direction. DLL participation in acquiring path-based information, and its modulation by olfactory exposure, broadens our understanding of the neural pathways underlying avian navigation.

Distribution of calcium-binding proteins in the pigeon visual thalamic centers and related pretectal and mesencephalic nuclei. Phylogenetic and functional determinants

Multichannel processing of environmental information constitutes a fundamental basis of functioning of sensory systems in the vertebrate brain. Two distinct parallel visual systems - the tectofugal and thalamofugal exist in all amniotes. The vertebrate central nervous system contains high concentrations of intracellular calcium-binding proteins (CaBPrs) and each of them has a restricted expression pattern in different brain regions and specific neuronal subpopulations. This study aimed at describing the patterns of distribution of parvalbumin (PV) and calbindin (CB) in the visual thalamic and mesencephalic centers of the pigeon (Columba livia). We used a combination of immunohistochemistry and double labeling immunofluorescent technique. Structures studied included the thalamic relay centers involved in the tectofugal (nucleus rotundus, Rot) and thalamofugal (nucleus geniculatus lateralis, pars dorsalis, GLd) visual pathways as well as pretectal, mesencephalic, isthmic and thalamic structures inducing the driver and/or modulatory action to the visual processing. We showed that neither of these proteins was unique to the Rot or GLd. The Rot contained i) numerous PV-immunoreactive (ir) neurons and a dense neuropil, and ii) a few CB-ir neurons mostly located in the anterior dorsal part and associated with a light neuropil. These latter neurons partially overlapped with the former and some of them colocalized both proteins. The distinct subnuclei of the GLd were also characterized by different patterns of distribution of CaBPrs. Some (nucleus dorsolateralis anterior, pars magnocellularis, DLAmc; pars lateralis, DLL; pars rostrolateralis, DLAlr; nucleus lateralis anterior thalami, LA) contained both CB- and PV-ir neurons in different proportions with a predominance of the former in the DLAmc and DLL. The nucleus lateralis dorsalis of nuclei optici principalis thalami only contained PV-ir neurons and a neuropil similar to the interstitial pretectal/thalamic nuclei of the tectothalamic tract, nucleus pretectalis and thalamic reticular nucleus. The overlapping distribution of PV and CB immunoreactivity was typical for the pretectal nucleus lentiformis mesencephali and the nucleus ectomamillaris as well as for the visual isthmic nuclei. The findings are discussed in the light of the contributive role of the phylogenetic and functional factors determining the circuits׳ specificity of the different CaBPr types.

Crocodilian Forebrain: Evolution and Development

Organization and development of the forebrain in crocodilians are reviewed. In juvenile Caiman crocodilus, the following features were examined: identification and classification of dorsal thalamic nuclei and their respective connections with the telencephalon, presence of local circuit neurons in the dorsal thalamic nuclei, telencephalic projections to the dorsal thalamus, and organization of the thalamic reticular nucleus. These results document many similarities between crocodilians and other reptiles and birds. While crocodilians, as well as other sauropsids, demonstrate several features of neural circuitry in common with mammals, certain striking differences in organization of the forebrain are present. These differences are the result of evolution. To explore a basis for these differences, embryos of Alligator misissippiensis were examined to address the following. First, very early development of the brain in Alligator is similar to that of other amniotes. Second, the developmental program for individual vesicles of the brain differs between the secondary prosencephalon, diencephalon, midbrain, and hindbrain in Alligator. This is likely to be the case for other amniotes. Third, initial development of the diencephalon in Alligator is similar to that in other amniotes. In Alligator, alar and basal parts likely follow a different developmental scheme.

Hippocampal memory consolidation during sleep: a comparison of mammals and birds

The transition from wakefulness to sleep is marked by pronounced changes in brain activity. The brain rhythms that characterize the two main types of mammalian sleep, slow-wave sleep (SWS) and rapid eye movement (REM) sleep, are thought to be involved in the functions of sleep. In particular, recent theories suggest that the synchronous slow-oscillation of neocortical neuronal membrane potentials, the defining feature of SWS, is involved in processing information acquired during wakefulness. According to the Standard Model of memory consolidation, during wakefulness the hippocampus receives input from neocortical regions involved in the initial encoding of an experience and binds this information into a coherent memory trace that is then transferred to the neocortex during SWS where it is stored and integrated within preexisting memory traces. Evidence suggests that this process selectively involves direct connections from the hippocampus to the prefrontal cortex (PFC), a multimodal, high-order association region implicated in coordinating the storage and recall of remote memories in the neocortex. The slow-oscillation is thought to orchestrate the transfer of information from the hippocampus by temporally coupling hippocampal sharp-wave/ripples (SWRs) and thalamocortical spindles. SWRs are synchronous bursts of hippocampal activity, during which waking neuronal firing patterns are reactivated in the hippocampus and neocortex in a coordinated manner. Thalamocortical spindles are brief 7-14 Hz oscillations that may facilitate the encoding of information reactivated during SWRs. By temporally coupling the readout of information from the hippocampus with conditions conducive to encoding in the neocortex, the slow-oscillation is thought to mediate the transfer of information from the hippocampus to the neocortex. Although several lines of evidence are consistent with this function for mammalian SWS, it is unclear whether SWS serves a similar function in birds, the only taxonomic group other than mammals to exhibit SWS and REM sleep. Based on our review of research on avian sleep, neuroanatomy, and memory, although involved in some forms of memory consolidation, avian sleep does not appear to be involved in transferring hippocampal memories to other brain regions. Despite exhibiting the slow-oscillation, SWRs and spindles have not been found in birds. Moreover, although birds independently evolved a brain region--the caudolateral nidopallium (NCL)--involved in performing high-order cognitive functions similar to those performed by the PFC, direct connections between the NCL and hippocampus have not been found in birds, and evidence for the transfer of information from the hippocampus to the NCL or other extra-hippocampal regions is lacking. Although based on the absence of evidence for various traits, collectively, these findings suggest that unlike mammalian SWS, avian SWS may not be involved in transferring memories from the hippocampus. Furthermore, it suggests that the slow-oscillation, the defining feature of mammalian and avian SWS, may serve a more general function independent of that related to coordinating the transfer of information from the hippocampus to the PFC in mammals. Given that SWS is homeostatically regulated (a process intimately related to the slow-oscillation) in mammals and birds, functional hypotheses linked to this process may apply to both taxonomic groups.

Synaptic circuitry in the retinorecipient layers of the optic tectum of the lamprey (Lampetra fluviatilis). A combined hodological, GABA and glutamate immunocytochemical study

The ultrastructure of the retinorecipient layers of the lamprey optic tectum was analysed using tract tracing techniques combined with GABA and glutamate immunocytochemistry. Two types of neurons were identified; a population of large GABA-immunonegative cells, and a population of smaller, highly GABA-immunoreactive interneurons, some of whose dendrites contain synaptic vesicles (DCSV). Five types of axon terminals were identified and divided into two major categories. The first of these are GABA-immunonegative, highly glutamate-immunoreactive, contain round synaptic vesicles, make asymmetrical synaptic contacts, and can in turn be divided into AT1 and AT2 terminals. The AT1 terminals are those of the retinotectal projection. The origin of the nonretinal AT2 terminals could not be determined. AT1 and AT2 terminals establish synaptic contacts with DCSV, with dendrites of the retinopetal neurons (DRN), and with conventional dendritic (D) profiles. The terminals of the second category are GABA-immunoreactive and can similarly be divided into AT3 and AT4 terminals. The AT3 terminals contain pleiomorphic synaptic vesicles and make symmetrical synaptic contacts for the most part with glutamate-immunoreactive D profiles. The AT4 terminals contain rounded synaptic vesicles and make asymmetrical synaptic contacts with DRN, with DCSV, and with D profiles. A fifth, rarely observed category of terminals (AT5) contain both clear synaptic vesicles and a large number of dense-core vesicles. Synaptic triads involving AT1, AT2 or AT4 terminals are rare. Our findings are compared to these of previous studies of the fine structure and immunochemical properties of the retinorecipient layers of the optic tectum or superior colliculus of Gnathostomes.