Immunohistochemical distribution of AMPA-type label in the pigeon (C. livia) hippocampus

Behavioral Training Related Neurotransmitter Receptor Expression Dynamics in the Nidopallium Caudolaterale and the Hippocampal Formation of Pigeons

Learning and memory are linked to dynamic changes at the level of synapses in brain areas that are involved in cognitive tasks. For example, changes in neurotransmitter receptors are prerequisite for tuning signals along local circuits and long-range networks. However, it is still unclear how a series of learning events promotes plasticity within the system of neurotransmitter receptors and their subunits to shape information processing at the neuronal level. Therefore, we investigated the expression of different glutamatergic NMDA (GRIN) and AMPA (GRIA) receptor subunits, the GABAergic GABARG2 subunit, dopaminergic DRD1, serotonergic 5HTR1A and noradrenergic ADRA1A receptors in the pigeon's brain. We studied the nidopallium caudolaterale, the avian analogue of the prefrontal cortex, and the hippocampal formation, after training the birds in a rewarded stimulus-response association (SR) task and in a simultaneous-matching-to-sample (SMTS) task. The results show that receptor expression changed differentially after behavioral training compared to an untrained control group. In the nidopallium caudolaterale, GRIN2B, GRIA3, GRIA4, DRD1D, and ADRA1A receptor expression was altered after SR training and remained constantly decreased after the SMTS training protocol, while GRIA2 and DRD1A decreased only under the SR condition. In the hippocampal formation, GRIN2B decreased and GABARG2 receptor expression increased after SR training. After SMTS sessions, GRIN2B remained decreased, GABARG2 remained increased if compared to the control group. None of the investigated receptors differed directly between both conditions, although differentially altered. The changes in both regions mostly occur in favor of the stimulus response task. Thus, the present data provide evidence that neurotransmitter receptor expression dynamics play a role in the avian prefrontal cortex and the hippocampal formation for behavioral training and is uniquely, regionally and functionally associated to cognitive processes including learning and memory.

Neural Substrates of Homing Pigeon Spatial Navigation: Results From Electrophysiology Studies

Over many centuries, the homing pigeon has been selectively bred for returning home from a distant location. As a result of this strong selective pressure, homing pigeons have developed an excellent spatial navigation system. This system passes through the hippocampal formation (HF), which shares many striking similarities to the mammalian hippocampus; there are a host of shared neuropeptides, interconnections, and its role in the storage and manipulation of spatial maps. There are some notable differences as well: there are unique connectivity patterns and spatial encoding strategies. This review summarizes the comparisons between the avian and mammalian hippocampal systems, and the responses of single neurons in several general categories: (1) location and place cells responding in specific areas, (2) path and goal cells responding between goal locations, (3) context-dependent cells that respond before or during a task, and (4) pattern, grid, and boundary cells that increase firing at stable intervals. Head-direction cells, responding to a specific compass direction, are found in mammals and other birds but not to date in pigeons. By studying an animal that evolved under significant adaptive pressure to quickly develop a complex and efficient spatial memory system, we may better understand the comparative neurology of neurospatial systems, and plot new and potentially fruitful avenues of comparative research in the future.

Dissociation of spatial and object memory in the hippocampal formation of Japanese quail

The mammalian temporal cortex can be functionally segregated into regions that encode spatial information and others that are predominantly responsible for object recognition. In the present study, we report comparable functional segregation in the avian brain. Using Japanese quail, we find that bilateral lesions of the hippocampus (Hp) produce robust deficits in performance in a foraging array (FA) spatial memory task, while sparing spontaneous object recognition (SOR). In contrast, lesions to the adjacent area parahippocampalis (APH) compromise both SOR and FA. These observations demonstrate a functional dissociation between Hp and APH that is comparable to the distinctions seen in mammals between the hippocampus and surrounding temporal cortex.

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Are spatial and object information separable in the avian hippocampal formation?

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Quail with lesions to the hippocampus are impaired in a spatial foraging task

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Lesions to area parahippocampalis also selectively impair object recognition

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Like mammals, bird hippocampus shows functional gradients in information processing

Differential projections of the densocellular and intermediate parts of the hyperpallium in the pigeon (Columba livia)

The visual Wulst in birds shows a four-layered structure: apical part of the hyperpallium (HA), interstitial part of HA (IHA), intercalated part of hyperpallium (HI), and densocellular part of hyperpallium (HD). HD also connects with the hippocampus and olfactory system. Because HD is subjacent to HI, the two have been treated as one structure in many studies, and the fiber connections of HD have been examined by afferents and efferents originating outside HD. However, to clarify the difference between these two layers, they need to be treated separately. In the present study, the fiber connections of HD and HI were analyzed with tract-tracing techniques using a combination of injections of cholera toxin subunit B (CTB) for retrograde tracing and biotinylated dextran amine (BDA) for anterograde tracing. When the two tracers were bilaterally injected in HD, a major reciprocal connection was seen with the dorsolateral subdivision (DL) of the hippocampal formation. When CTB and BDA were bilaterally injected in HI, strong reciprocal connections were found between HI and HA. Next, projection neurons in HD and HI were examined by double staining for CTB combined with vesicular glutamate transporter 2 (vGluT2) mRNA in situ hybridization. After CTB was injected in DL or HA, many neurons revealed CTB+/vGluT2+ in HD or HI, respectively. Furthermore, in situ hybridization showed that DL and HA contained neurons expressing various subunits of ionotropic glutamate receptors: AMPA, kainate, and NMDA types. These results suggest that glutamatergic neurons in HD and HI project primarily to DL and HA, respectively.

Distribution of neurotransmitter receptors and zinc in the pigeon (Columba livia) hippocampal formation: A basis for further comparison with the mammalian hippocampus

The avian hippocampal formation (HF) and mammalian hippocampus share a similar functional role in spatial cognition, but the underlying neuronal mechanisms allowing the functional similarity are incompletely understood. To understand better the organization of the avian HF and its transmitter receptors, we analyzed binding site densities for glutamatergic AMPA, NMDA, and kainate receptors; GABAA receptors; muscarinic M1 , M2 and nicotinic (nACh) acetylcholine receptors; noradrenergic α1 and α2 receptors; serotonergic 5-HT1A receptors; dopaminergic D1/5 receptors by using quantitative in vitro receptor autoradiography. Additionally, we performed a modified Timm staining procedure to label zinc. The regionally different receptor densities mapped well onto seven HF subdivisions previously described. Several differences in receptor expression highlighted distinct HF subdivisions. Notable examples include 1) high GABAA and α1 receptor expression, which rendered distinctive ventral subdivisions; 2) high α2 receptor expression, which rendered distinctive a dorsomedial subdivision; 3) distinct kainate, α2 , and muscarinic receptor densities that rendered distinctive the two dorsolateral subdivisions; and 4) a dorsomedial region characterized by high kainate receptor density. We further observed similarities in receptor binding densities between subdivisions of the avian and mammalian HF. Despite the similarities, we propose that 300 hundred million years of independent evolution has led to a mosaic of similarities and differences in the organization of the avian HF and mammalian hippocampus and that thinking about the avian HF in terms of the strict organization of the mammalian hippocampus is likely insufficient to understand the HF of birds.

Daily cycling of nitric oxide synthase (NOS) in the hippocampus of pigeons (C. livia)

Background

Nitric oxide synthase (NOS) is essential for the synthesis of nitric oxide (NO), a non-conventional neurotransmitter with an important role in synaptic plasticity underlying processes of hippocampus-dependent memory and in the regulation of biological clocks and circadian rhythms. Many studies have shown that both the NOS cytosolic protein content and its enzymatic activity present a circadian variation in different regions of the rodent brain, including the hippocampus. The present study investigated the daily variation of NOS enzymatic activity and the cytosolic content of nNOS in the hippocampus of pigeons.

Results

Adult pigeons kept under a skeleton photoperiod were assigned to six different groups. Homogenates of the hippocampus obtained at six different times-of-day were used for NOS analyses. Both iNOS activity and nNOS cytosolic protein concentrations were highest during the subjective light phase and lowest in the subjective dark phase of the circadian period. ANOVA showed significant time differences for iNOS enzymatic activity (p < 0.05) and for nNOS protein content (p < 0.05) in the hippocampus. A significant daily rhythm for both iNOS and nNOS was confirmed by analysis with the Cosinor method (p < 0.05). The present findings indicate that the enzymatic activity of iNOS and content of nNOS protein in the hippocampus of pigeons exhibit a daily rhythm, with acrophase values occurring during the behavioral activity phase.

Conclusions

The data corroborate the reports on circadian variation of NOS in the mammalian hippocampus and can be considered indicative of a dynamic interaction between hippocampus-dependent processes and circadian clock mechanisms.

GluR2/3 label expression of the AMPA-type glutamate receptor in the hippocampal formation of the homing pigeon stabilizes just after birth

The compositions of the glutamate AMPA-type receptors influence the neural response and the subunits GluR2/3 has been referred to as essential for receptor trafficking and synapse consolidation. We investigate the GluR2/3 occurrence and expression in the hippocampal formation of newly born homing pigeons by a semi-quantitative approach, the Western-blotting technique and by immunohistochemistry. Immunoreactivity for GluR2/3 occurs before hatching has been evident in neuropil that was fully dispersed over the hippocampus proper (HP) and the area parahippocampalis (APH). Although many HP cells are NeuN-positives, a specific neuronal protein indicating that they are already differentiated as neurons while not one contains GluR2/3 at the hatching day (P0). Few neurons at the APH seem to express GluR2/3 at P0, but 3 days later (P3) the GluR2/3 labeling can be recognized in many HP neurons, showing a distribution pattern that resembles the adult, gradually increasing in intensity until P10. Also, the Western-blot shows an augment between P0 and P3, remaining stable after that. The enhancement of the neuronal label at P3 coincides with the retraction of the GluR2/3 label in neuropil, reducing their occurrence during the maturational period to become restricted to the dorsomedial portion as reported for adults. As the HP GluR2/3-containing cells are supposedly projecting neurons, taking together, the results signalize the relevance of the GluR2/3 in post-hatch formation of avian hippocampal circuitry in which the third day seems to be the critical period.

Chronic stress alters glucocorticoid receptor and mineralocorticoid receptor mRNA expression in the European starling (Sturnus vulgaris) brain

Although the glucocorticoid response to acute short-term stress is an adaptive physiological mechanism that aids in the response to and survival of noxious stimuli, chronic stress is associated with a negative impact on health. In wild-caught European starlings (Sturnus vulgaris), chronic stress alters the responsiveness of hypothalamic-pituitary-adrenal (HPA) axis as measured by the acute corticosterone response. In the present study, we investigated potential underlying neuroendocrine mechanisms by comparing glucocorticoid receptor and mineralocorticoid receptor mRNA expression in the brains of chronically and nonchronically-stressed starlings. Hypothalamic paraventricular nucleus, but not hippocampal, glucocorticoid receptor mRNA expression in chronically-stressed birds was significantly lower compared to controls, suggesting changes in the efficacy of corticosterone negative feedback. In addition, chronically-stressed birds showed a significant decrease in hippocampal MR mRNA expression. Together, these results suggest that chronic stress changes the brain physiology of wild birds and provides important information for the understanding of the underlying mechanisms that result in dysregulation of the HPA axis in wild animals by chronic stress.

Distribution of glutamate transporter 1 mRNA in the central nervous system of the pigeon (Columba livia)

Glutamate transporter 1 (GLT1) in glial cells removes glutamate that diffuses from the synaptic cleft into the extracellular space. Previously, we have shown the distribution of glutamatergic neurons in the central nervous system (CNS) of the pigeon. In the present study, we identified cDNA sequence of the pigeon GLT1, and mapped the distribution of the mRNA-expressing cells in CNS to examine whether GLT1 is associated with glutamatergic terminal areas. The cDNA sequence of the pigeon GLT1 consisted of 1889bp nucleotides and the amino acids showed 97% and 87% identity to the chicken and human GLT1, respectively. In situ hybridization autoradiograms revealed GLT1 mRNA expression in glial cells and produced regional differences of GLT1 mRNA distribution in CNS. GLT1 mRNA was expressed preferentially in the pallium than the subpallium. Moderate expression was seen in the hyperpallium, Field L, mesopallium, and hippocampal formation. In the thalamus, moderate expression was found in the ovoidal nucleus, rotundal nucleus, triangular nucleus, and lateral spiriform nucleus, while the dorsal thalamic nuclei were weak. In the brainstem, the isthmic nuclei, optic tectum, vestibular nuclei, and cochlear nuclei expressed moderately, but the cerebellar cortex showed strong expression. Bergmann glial cells expressed GLT1 mRNA very strongly. The results indicate that cDNA sequence of the pigeon GLT1 is comparable with that of the mammalian GLT1, and a large number of GLT1 mRNA-expressing areas correspond with areas where AMPA-type glutamate receptors are located. Avian GLT1 in glial cells probably maintain microenvironment of glutamate concentration around synapses as in mammalian GLT1.