Septum volume and food-storing behavior are related in parids

Neuronal diversity and reciprocal connectivity between the vertebrate hippocampus and septum

A hallmark of the nervous system is the precision with which myriad cell types are integrated into functional networks that control complex behaviors. The limbic system governs evolutionarily conserved processes essential for survival. The septum and the hippocampus are central to the limbic system, and control not only emotion-related behaviors but also learning and memory. Here, we provide a developmental and evolutionary perspective of the hippocampus and septum and highlight the neuronal diversity and circuitry that connects these two central components of the limbic system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles Comparative Development and Evolution > Regulation of Organ Diversity.

The expression of tyrosine hydroxylase and DARPP-32 in the house crow (Corvus splendens) brain

Birds of the family Corvidae which includes diverse species such as crows, rooks, ravens, magpies, jays, and jackdaws are known for their amazing abilities at problem-solving. Since the catecholaminergic system, especially the neurotransmitter dopamine, plays a role in cognition, we decided to study the distribution of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines in the brain of house crows (Corvus splendens). We also studied the expression of DARPP-32 (dopamine and cAMP-regulated phosphoprotein), which is expressed in dopaminoceptive neurons. Our results demonstrated that as in other avian species, the expression of both TH and DARPP-32 was highest in the house crow striatum. The caudolateral nidopallium (NCL, the avian analogue of the mammalian prefrontal cortex) could be differentiated from the surrounding pallial regions based on a larger number of TH-positive "baskets" of fibers around neurons in this region and greater intensity of DARPP-32 staining in the neuropil in this region. House crows also possessed distinct nuclei in their brains which corresponded to song control regions in other songbirds. Whereas immunoreactivity for TH was higher in the vocal control region Area X compared to the surrounding MSt (medial striatum) in house crows, staining in RA and HVC was not as prominent. Furthermore, the arcopallial song control regions RA (nucleus robustus arcopallialis) and AId (intermediate arcopallium) were strikingly negative for DARPP-32 staining, in contrast to the surrounding arcopallium. Patterns of immunoreactivity for TH and DARPP-32 in "limbic" areas such as the hippocampus, septum, and extended amygdala have also been described.

Concerted and mosaic evolution of functional modules in songbird brains

Vertebrate brains differ in overall size, composition and functional capacities, but the evolutionary processes linking these traits are unclear. Two leading models offer opposing views: the concerted model ascribes major dimensions of covariation in brain structures to developmental events, whereas the mosaic model relates divergent structures to functional capabilities. The models are often cast as incompatible, but they must be unified to explain how adaptive changes in brain structure arise from pre-existing architectures and developmental mechanisms. Here we show that variation in the sizes of discrete neural systems in songbirds, a species-rich group exhibiting diverse behavioural and ecological specializations, supports major elements of both models. In accordance with the concerted model, most variation in nucleus volumes is shared across functional domains and allometry is related to developmental sequence. Per the mosaic model, residual variation in nucleus volumes is correlated within functional systems and predicts specific behavioural capabilities. These comparisons indicate that oscine brains evolved primarily as a coordinated whole but also experienced significant, independent modifications to dedicated systems from specific selection pressures. Finally, patterns of covariation between species and brain areas hint at underlying developmental mechanisms.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.

Differences in relative hippocampus volume and number of hippocampus neurons among five corvid species

The relative size of the avian hippocampus (Hp) has been shown to be related to spatial memory and food storing in two avian families, the parids and corvids. Basil et al. [Brain Behav Evol 1996;47:156-164] examined North American food-storing birds in the corvid family and found that Clark's nutcrackers had a larger relative Hp than pinyon jays and Western scrub jays. These results correlated with the nutcracker's better performance on most spatial memory tasks and their strong reliance on stored food in the wild. However, Pravosudov and de Kort [Brain Behav Evol 2006;67:1-9] raised questions about the methodology used in the 1996 study, specifically the use of paraffin as an embedding material and recalculation for shrinkage. Therefore, we measured relative Hp volume using gelatin as the embedding material in four North American species of food-storing corvids (Clark's nutcrackers, pinyon jays, Western scrub jays and blue jays) and one Eurasian corvid that stores little to no food (azure-winged magpies). Although there was a significant overall effect of species on relative Hp volume among the five species, subsequent tests found only one pairwise difference, blue jays having a larger Hp than the azure-winged magpies. We also examined the relative size of the septum in the five species. Although Shiflett et al. [J Neurobiol 2002;51:215-222] found a difference in relative septum volume amongst three species of parids that correlated with storing food, we did not find significant differences amongst the five species in relative septum. Finally, we calculated the number of neurons in the Hp relative to body mass in the five species and found statistically significant differences, some of which are in accord with the adaptive specialization hypothesis and some are not.

Brain activation pattern depends on the strategy chosen by zebra finches to solve an orientation task

Zebra finches (Taeniopygia guttata) were trained to find food in one of four feeders on the floor of an aviary. This feeder was always in the same place during training and was additionally marked by a distinct pattern. In the test trial the distinctly patterned feeder was interchanged with one of the other feeders, so that the birds had to decide to use either the pattern or the original location for finding food. Half of the birds used one strategy and half used the other. According to the strategy applied, different brain areas were activated, as demonstrated by c-Fos immunohistochemistry. The hippocampus was activated when spatial cues were used, while in birds orienting using the pattern of the feeder, part of the collothalamic (tectofugal) visual system showed stronger activation. The visual wulst of the lemnothalamic (thalamofugal) visual system was activated with both strategies, indicating an involvement in both spatial and pattern-directed orientation. Because the experimental situation was the same for all zebra finches, the activation pattern was only dependent on the strategy that was voluntarily chosen by each of the birds.

Seasonal and individual variation in singing behavior correlates with α2-noradrenergic receptor density in brain regions implicated in song, sexual, and social behavior

In seasonally breeding male songbirds, both the function of song and the stimuli that elicit singing behavior change seasonally. The catecholamine norepinephrine (NE) modulates attention and arousal across behavioral states, yet the role of NE in seasonally-appropriate vocal communication has not been well-studied. The present study explored the possibility that seasonal changes in alpha 2-noradrenergic receptors (α(2)-R) within song control regions and brain regions implicated in sexual arousal and social behavior contribute to seasonal changes in song behavior in male European starlings (Sturnus vulgaris). We quantified singing behavior in aviary housed males under spring breeding season conditions and fall conditions. α(2)-R were identified with the selective ligand [(3)H]RX821002 using autoradiographic methods. The densities of α(2)-R in song control regions (HVC and the robust nucleus of the arcopallium [RA]) and the lateral septum (LS) were lower in Spring Condition males. α(2)-R densities in the caudal portion of the medial preoptic nucleus (POM) related negatively to singing behavior. Testosterone concentrations were highest in Spring Condition males and correlated with α(2)-R in LS and POM. Results link persistent seasonal alterations in the structure or function of male song to seasonal changes in NE α(2)-Rs in HVC, RA, and LS. Individual differences in α(2)-R in the POM may in part explain individual differences in song production irrespective of the context in which a male is singing, perhaps through NE modification of male sexual arousal.

Using ecology to guide the study of cognitive and neural mechanisms of different aspects of spatial memory in food-hoarding animals

Understanding the survival value of behaviour does not tell us how the mechanisms that control this behaviour work. Nevertheless, understanding survival value can guide the study of these mechanisms. In this paper, we apply this principle to understanding the cognitive mechanisms that support cache retrieval in scatter-hoarding animals. We believe it is too simplistic to predict that all scatter-hoarding animals will outperform non-hoarding animals on all tests of spatial memory. Instead, we argue that we should look at the detailed ecology and natural history of each species. This understanding of natural history then allows us to make predictions about which aspects of spatial memory should be better in which species. We use the natural hoarding behaviour of the three best-studied groups of scatter-hoarding animals to make predictions about three aspects of their spatial memory: duration, capacity and spatial resolution, and we test these predictions against the existing literature. Having laid out how ecology and natural history can be used to predict detailed cognitive abilities, we then suggest using this approach to guide the study of the neural basis of these abilities. We believe that this complementary approach will reveal aspects of memory processing that would otherwise be difficult to discover.

Hippocampal lesions impair spatial memory performance, but not song--a developmental study of independent memory systems in the zebra finch

Songbirds demonstrate song- and spatial-learning, forms of memory that appear distinct in formal characteristics and fitting the descriptions and criteria of procedural and episodic-like memory function, respectively. As in other vertebrates, the neural pathways underlying these forms of memory may also be dissociable, and include the corresponding song circuit and hippocampus (HP). Whether (or not) these two memory systems interact is unknown. Interestingly, the HP distinguishes itself as a site of immediate early gene expression in response to song and as a site of estrogen synthesis, a steroid involved in song learning. Thus, an interaction between these memory systems and their anatomical substrates appears reasonable to hypothesize, particularly during development. To test this idea, juvenile male or female zebra finches received chemical lesions of the HP at various points during song learning, as did adults. Song structure, singing behavior, song preference, and spatial memory were tested in adulthood. Although lesions of the HP severely compromised HP-dependent spatial memory function across all ages and in both sexes, we were unable to detect any effects of HP lesions on song learning, singing, or song structure in males. Interestingly, females lesioned as adults, but not as juveniles, did lose the characteristic preference for their father's song. Since compromise of the neural circuits that subserve episodic-like memory does very little (if anything) to affect procedural-like (song learning) memory, we conclude that these memory systems and their anatomical substrates are well dissociated in the developing male zebra finch.

The septal complex of the fire-bellied toad Bombina orientalis: Chemoarchitecture

In order to investigate whether chemoarchitecture would support the subdivision of the anuran septum based on cytoarchitectonic and hodological studies, we performed enzyme-histochemical detection of NADPH-diaphorase and immunohistological demonstration of choline-acetyl transferase (ChAT), aspartate, calretinin, gamma-aminobutyric acid (GABA), 5-hydroxy-tryptamine, tyrosine hydroxylase, neuropeptide Y (NPY), somatostatin, Leu- and Leu + Met-enkephalin, and substance P in the fire-bellied toad Bombina orientalis. Labeling of cell bodies matched well the previously defined subnuclei: The dorsolateral septal nucleus contains enkephalin-immunoreactive (-ir) and weakly stained GABA-ir neurons; calretinin-ir and weakly labeled GABA-ir neurons are found in the ventrolateral septal nucleus. The medial septal nucleus is characterized by the presence of numerous ChAT-ir and some tyrosine hydroxylase-ir neurons, while the dorsal septal nucleus is outlined by its NPY-ir neurons. Many ChAT-ir and some aspartate-ir and somatostatin-ir neurons are found in the diagonal band of Broca, and the central septal nucleus contains some GABA-ir and ChAT-ir neurons. In contrast, labeled fibers form a pattern which does not match the boundaries of septal subnuclei. Comparing the anuran septal complex with that of other vertebrates reveals that the complexity of the lateral septum has increased during the evolution from anamniote to amniote vertebrates. In spite of this fact, many similarities in chemoarchitecture between anurans and other vertebrates are evident. Some basal septal functions such as involvement in learning and memory formation or inhibition of sexual behavior appear to have persisted during vertebrate evolution.

Failure to detect seasonal changes in the song system nuclei of the black-capped chickadee (Poecile atricapillus)

Most temperate songbird species sing seasonally, and the brain areas involved in producing song (the song system) vary in size alongside the changes in behavior. Black-capped chickadees (Poecile atricapillus) also sing seasonally, and we find that there are changes in the stereotypy and the length of the fee-bee song from the nonbreeding to the breeding season. Yet despite these changes, we fail to find any evidence of seasonal changes in the song system. The song system of males is larger than that of females, as is typical in songbirds, but the ratio between the sexes is small compared to other species. We suggest three hypotheses to explain our failure to find seasonal variation in the chickadee song system.

A Multi-disciplinary Approach to Understanding Hippocampal Function in Foodhoarding Birds

Spatial memory and the hippocampal formation (HF) of food-hoarding birds have been put forward as a prime example of how natural selection has shaped a cognitive system and its neural underpinnings. Here, I review what we know about the HF of hoarding birds and lay out the work that is currently underway to use this system to obtain a better understanding of hippocampal function in general. This interdisciplinary programme includes evolutionary, ecological, psychological, ethological, and neuroscientific approaches to the study of behaviour and cognition. Firstly, we need to understand the behaviour of the birds in their natural environment, and identify the aspects of cognition and behaviour that may be especially valuable for the species under study. Secondly, these cognitive and behavioural traits are compared to closely-related non-hoarding species. Thirdly, we also compare HF anatomy between closely-related hoarding and non-hoarding species, identifying possible neural mechanisms underlying behavioural differences. Finally, behavioural and neuroscientific approaches are combined in experiments directly investigating the involvement of the HF or any of its anatomical and physiological aspects in the behaviours under study. This process loops back upon itself in many different ways, with all the different approaches informing each other. In this way we are making progress in understanding the functioning of the HF, not only in food-hoarding birds, but in all vertebrates.

Anatomy of the Avian Hippocampal Formation

Increasing knowledge of the avian hippocampal formation (hippocampus and parahippocampal area) suggests that it plays a role in a variety of behaviors, such as homing, cache retrieving, visual discrimination, imprinting, and sexual behavior. Knowledge of the neural circuits in the hippocampal formation and its related areas or nuclei is important for the understanding of these functions. This review therefore describes the functional neuroanatomy of the avian hippocampal formations, i.e., its subdivisions, cytoarchitecture, and afferent and efferent connections. Evidence obtained by a combination of Nissl staining and tract-tracing shows that the pigeon hippocampal formation can be divided into seven subdivisions: dorsolateral (DL), dorsomedial (DM), triangular (Tr), V-shaped (V), magnocellular (Ma), parvocellular, and cell-poor regions. DL and DM can be further divided into dorsal and ventral, and lateral and medial portions, respectively. In the hippocampal formation, reciprocal connections are found between DL-DM, DL-Tr, DL-Ma, DM-Ma, DM-V, and Tr-V. Neurons in the V-shaped layer appear to be intrinsic neurons. Sensory inputs from higher order visual and olfactory stations enter DL and DM, are modified or integrated by intrinsic hippocampal circuitry, and the outputs are sent, via DL and DM, to various telencephalic nuclei, septum, and hypothalamus. The neural pathways indicate that the hippocampal formation plays a central role in the limbic system, which also includes the dorsolateral corticoid area, nucleus taeniae of the amygdala, posterior pallial amygdala, septum, medial part of the anterior dorsolateral nucleus of the thalamus, and the lateral mammillary nucleus. Connectional and comparative studies, including the use of kainic acid excitotoxicity, suggest that the V-shaped layer is comparable to the dentate gyrus of the mammalian hippocampal formation and DM to Ammon's horn and subiculum.

Cannabinoid inhibition improves memory in food-storing birds, but with a cost

Food-storing birds demonstrate remarkable memory ability in recalling the locations of thousands of hidden food caches. Although this behaviour requires the hippocampus, its synaptic mechanisms are not understood. Here we show the effects of cannabinoid receptor (CB1-R) blockade on spatial memory in food-storing black-capped chickadees (Poecile atricapilla). Intra-hippocampal infusions of the CB1-R antagonist SR141716A enhanced long-term memory for the location of a hidden food reward, measured 72 h after encoding. However, when the reward location changed during the retention interval, birds that had received SR141716A during initial learning showed impairments in recalling the most recent reward location. Thus, blocking CB1-R activity may lead to more robust, long-lasting memories, but these memories may be a source of proactive interference. The relationship between trace strength and interference may be important in understanding neural mechanisms of hippocampal function in general, as well as understanding the enhanced memory of food-storing birds.

Neuron production, neuron number, and structure size are seasonally stable in the hippocampus of the food-storing black-capped chickadee (Poecile atricapillus)

Neuronal recruitment in the black-capped chickadee (Poecile atricapillus) hippocampus occurs at a higher rate in the fall than at other times of the year. As a means of determining whether this increase in recruitment results from greater neuron production, chickadees were caught in the wild between October and March and injected with the cell-birth marker 5-bromo-2'-deoxyuridine. Two weeks later, birds were killed by overdose, and hippocampal neuron production, apoptosis, neuron number, and hippocampal volume were determined. Chickadees collected in October, November, January, February, and March did not differ in neuron production, apoptosis, hippocampal volume, or neuron number. These findings indicate that increases in neuronal recruitment in the chickadee hippocampus in the fall do not result from increased neuron production, but instead, enhanced survival of new neurons.

Efferent connections of septal nuclei of the domestic chick (Gallus domesticus): An anterograde pathway tracing study with a bearing on functional circuits

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in different subregions of the septum of domestic chicks. The main targets of septal projections comprised the ipsi- and contralateral septal nuclei, including the nucleus of the diagonal band, basal ganglia, including the ventral paleostriatum, lobus parolfactorius, nucleus accumbens, and olfactory tubercle, archistriatum, piriform cortex, and anterior neostriatum. Further diencephalic and mesencephalic septal projections were observed in the ipsilateral preoptic region, hypothalamus (the main regions of afferentation comprising the lateral hypothalamic nuclei, ventromedial, paraventricular and periventricular nuclei, and the mammillary region), dorsal thalamus, medial habenular and subhabenular nuclei, midbrain central gray, and ventral tegmental area. Contralateral projections were also encountered in the septal nuclei, ventral paleostriatum, periventricular and anteromedial hypothalamic nuclei, suprachiasmatic nucleus, and the lateral hypothalamic area. Avian septal efferents are largely similar to those of mammals, the main differences being a relatively modest hippocampal projection arising mainly from the nucleus of the diagonal band (as confirmed by a specific experiment with the retrograde pathway tracer True blue), the lack of interpeduncular projection, and a greater contingent of amygdalar efferents arising from the lateral septum rather than the nucleus of the diagonal band. This pattern of connectivity is likely to reflect an important role of the avian septal nuclei in the coordination of limbic circuits and the integration of a wide variety of information sources modulating the appropriate behavioral responses: attention and arousal level, memory formation, hormonally mediated behaviors, and their affective components (such as ingestive, reproductive, and parental behaviors), social interaction, locomotor modulation, and circadian rhythm.

Fiber connections of the hippocampal formation and septum and subdivisions of the hippocampal formation in the pigeon as revealed by tract tracing and kainic acid lesions

The organization of the pigeon hippocampal formation was examined by tract tracing by using biotinylated dextran amine (BDA) and cholera toxin B subunit (CTB) and by injections of kainic acid to produce excitotoxic lesions. The hippocampal formation was divided into seven subdivisions based on Nissl staining and intrinsic and septal connections: dorsomedial (DM), dorsolateral (DL), triangular (Tr), V-shaped layer, magnocellular (Ma), parvocellular, and cell-poor regions. DL was composed of dorsal and ventral portions and sent associational fibers to DM, the V-shaped layer, and Tr. DL had strong reciprocal connections with the densocellular part of the hyperpallium (HD) and projected to the dorsolateral corticoid area. DM had reciprocal fiber connections with the V-shaped layer, Ma, and DL as well as with several subdivisions of the arcopallium. DL and DM, but not the V-shaped layer, projected fibers to the septum where those from DM exceeded in number those from DL. These projections further extended to the hypothalamus, particularly the lateral hypothalamic area. The lateral and medial septal nuclei projected back a very small number of ascending fibers to the hippocampal formation. Intraventricular injections of kainic acid induced neuronal loss widely in the hippocampal formation and subsequently produced gliosis in DM. These results indicate that DL receives its main afferents from HD and in turn sends inputs to an intrinsic circuit composed of hippocampal subdivisions DM, Ma, Tr, and the V-shaped layer; and also that DM is the main exit to the septum and hypothalamus. It is suggested that neurons in the V-shaped layer are intrinsic. Together, the results suggest that the V-shaped layer is comparable to the dentate gyrus of the mammalian hippocampal formation and that DM incorporates components comparable to both Ammon's horn and the subiculum.

Long-Term Memory for Spatial Locations in a Food-Storing Bird (Poecile atricapilla) Requires Activation of NMDA Receptors in the Hippocampal Formation During Learning

Food-storing birds use a form of long-term memory to recover their hidden food caches that depends on the hippocampal formation (HF). The authors assessed whether food-storing birds' long-term memory for spatial locations requires N-methyl-D-aspartate receptor (NMDA-R)-dependent synaptic plasticity. Black-capped chickadees (Poecile atricapilla) were given bilateral infusions of the NMDA-R antagonist AP5 into the hippocampus, and their memory on a spatial reference memory task was assessed. NMDA-R inactivation during learning prevented formation of long-term spatial memories but did not affect short-term memory and retrieval processes. NMDA-R inactivation immediately following learning did not disrupt long-term memory formation. NMDA-R inactivation disrupted the learning of multiple serially encoded reward locations when a 180-min delay separated successive learning episodes, suggesting that NMDA-R activity has a role in the incorporation of new information into existing long-term memory, as well as in forming unitary long-term memories.