Blocking NMDA-receptors in the pigeon's "prefrontal" caudal nidopallium impairs appetitive extinction learning in a sign-tracking paradigm

Molecular biology of serotonergic systems in avian brains

Serotonin (5-hydroxytryptamine, 5-HT) is a phylogenetically conserved neurotransmitter and modulator. Neurons utilizing serotonin have been identified in the central nervous systems of all vertebrates. In the central serotonergic system of vertebrate species examined so far, serotonergic neurons have been confirmed to exist in clusters in the brainstem. Although many serotonin-regulated cognitive, behavioral, and emotional functions have been elucidated in mammals, equivalents remain poorly understood in non-mammalian vertebrates. The purpose of this review is to summarize current knowledge of the anatomical organization and molecular features of the avian central serotonergic system. In addition, selected key functions of serotonin are briefly reviewed. Gene association studies between serotonergic system related genes and behaviors in birds have elucidated that the serotonergic system is involved in the regulation of behavior in birds similar to that observed in mammals. The widespread distribution of serotonergic modulation in the central nervous system and the evolutionary conservation of the serotonergic system provide a strong foundation for understanding and comparing the evolutionary continuity of neural circuits controlling corresponding brain functions within vertebrates. The main focus of this review is the chicken brain, with this type of poultry used as a model bird. The chicken is widely used not only as a model for answering questions in developmental biology and as a model for agriculturally useful breeding, but also in research relating to cognitive, behavioral, and emotional processes. In addition to a wealth of prior research on the projection relationships of avian brain regions, detailed subdivision similarities between avian and mammalian brains have recently been identified. Therefore, identifying the neural circuits modulated by the serotonergic system in avian brains may provide an interesting opportunity for detailed comparative studies of the function of serotonergic systems in mammals.

Characterisation of the neural basis underlying appetitive extinction & renewal in Cacna1c rats

Recent studies have revealed impairments in Cacna1c ± heterozygous animals (a gene that encodes the Cav 1.2 L-type voltage-gated calcium channels and is implicated in risk for multiple neuropsychiatric disorders) in aversive forms of learning, such as latent inhibition, reversal learning or context discrimination. However, the role of Cav 1.2 L-type voltage-gated calcium channels in extinction of appetitive associations remains under-investigated. Here, we used an appetitive Pavlovian conditioning task and evaluated extinction learning (EL) with a change of context from that of training and test (ABA) and without such a change (AAA) in Cacna1c ± male rats versus their wild-type (WT) littermates. In addition, we used fluorescence in situ hybridization of somatic immediate early genes (IEGs) Arc and Homer1a expression to scrutinize associated changes in the medial prefrontal cortex and the amygdala. Cacna1c ± animals successfully adapt their responses by engaging in appetitive EL and renewal. However, the regional IEG expression profile changed. For the EL occurring in the same context, Cacna1c ± animals presented higher IEG expression in the infralimbic cortex and the central amygdala than controls. The prelimbic region presented a larger neural ensemble in Cacna1c ± than WT animals, co-labelled for the time window of EL in the original context and prolonged exposure to the unrewarded context. With a context change, the Cacna1c ± infralimbic region displayed higher IEG expression during renewal than controls. Taken together, our findings provide novel evidence of distinct brain activation patterns occurring in Cacna1c ± rats after appetitive extinction and renewal despite preserved behavioral responses. This article is part of the Special Issue on "L-type calcium channel mechanisms in neuropsychiatric disorders".

Can we build a neuroecology of innovativeness similar to that pioneered by David Sherry for spatial memory?

David Sherry's pioneering work on the neuroecology of spatial memory has three characteristics that could inspire studies on other cognitive processes: it was grounded in a robust prior literature in psychology and neuroscience; it identified several natural history contexts in which repeated independent evolution of spatial memory differences had occurred in different clades; it involved a precise cognitive ability with a precise neural substrate. We discuss the application of these three principles to a more domain-general trait-innovation. We argue that targeting the caudolateral nidopallium and its connected areas, favoring problem-solving over reversal learning as an experimental assay, and focusing on situations that involve environmental change, such as urbanization and invasion, can help the study of innovation progress, like the field of spatial memory has since 1989.

Trial-by-trial dynamics of reward prediction error-associated signals during extinction learning and renewal

Reward prediction errors (RPEs) have been suggested to drive associative learning processes, but their precise temporal dynamics at the single-neuron level remain elusive. Here, we studied the neural correlates of RPEs, focusing on their trial-by-trial dynamics during an operant extinction learning paradigm. Within a single behavioral session, pigeons went through acquisition, extinction and renewal - the context-dependent response recovery after extinction. We recorded single units from the avian prefrontal cortex analogue, the nidopallium caudolaterale (NCL) and found that the omission of reward during extinction led to a peak of population activity that moved backwards in time as trials progressed. The chronological order of these signal changes during the progress of learning was indicative of temporal shifts of RPE signals that started during reward omission and then moved backwards to the presentation of the conditioned stimulus. Switches from operant choices to avoidance behavior (and vice versa) coincided with changes in population activity during the animals' decision-making. On the single unit level, we found more diverse patterns where some neurons' activity correlated with RPE signals whereas others correlated with the absolute value during the outcome period. Finally, we demonstrated that mere sensory contextual changes during the renewal test were sufficient to elicit signals likely associated with RPEs. Thus, RPEs are truly expectancy-driven since they can be elicited by changes in reward expectation, without an actual change in the quality or quantity of reward.

Neurobiological and Ecological Correlates of Avian Innovation

In the wild, particularly in rapidly changing conditions, being capable of solving new problems can increase an animal's chances of survival and reproduction. In the current context of widespread habitat destruction and increasing urbanization, innovativeness might be a crucial trait. In the past few decades, birds have proven to be a model taxon for the study of innovation, thanks to the abundant literature on avian innovation reports. Innovation databases in birds have been successfully employed to assess associations between innovativeness and other traits such as invasion success, life history, generalism, and brain encephalization. In order to more directly assess the causes of variation in innovation, a complementary approach consists in measuring innovativeness in wild-caught animals using problem-solving tasks that mimic innovations in the field. This method can allow for finer scale evaluation of ecological and neural correlates of innovation. Here, I review some of the most important findings on the correlates of innovation, with a particular focus on neural ones. I conclude by discussing avenues for future research, which I suggest should focus on neurobiology.

Immediate early gene fingerprints of multi-component behaviour

The ability to execute different responses in an expedient temporal order is central for efficient goal-directed actions and often referred to as multi-component behaviour. However, the underlying neural mechanisms on a cellular level remain unclear. Here we establish a link between neural activity at the cellular level within functional neuroanatomical structures to this form of goal-directed behaviour by analyzing immediate early gene (IEG) expression in an animal model, the pigeon (Columba livia). We focus on the group of zif268 IEGs and ZENK in particular. We show that when birds have to cascade separate task goals, ZENK expression is increased in the avian equivalent of the mammalian prefrontal cortex, i.e. the nidopallium caudolaterale (NCL) as well as in the homologous striatum. Moreover, in the NCL as well as in the medial striatum (MSt), the degree of ZENK expression was highly correlated with the efficiency of multi-component behaviour. The results provide the first link between cellular IEG expression and behavioural outcome in multitasking situations. Moreover, the data suggest that the function of the fronto-striatal circuitry is comparable across species indicating that there is limited flexibility in the implementation of complex cognition such as multi-component behaviour within functional neuroanatomical structures.

Sign-tracking behavior is difficult to extinguish and resistant to multiple cognitive enhancers

The attribution of incentive-motivational value to drug-related cues underlies relapse and craving in drug addiction. One method of addiction treatment, cue-exposure therapy, utilizes repeated presentations of drug-related cues in the absence of drug (i.e., extinction learning); however, its efficacy has been limited due to an incomplete understanding of extinction and relapse processes after cues have been imbued with incentive-motivational value. To investigate this, we used a Pavlovian conditioned approach procedure to screen for rats that attribute incentive-motivational value to reward-related cues (sign-trackers; STs) or those that do not (goal-trackers; GTs). In Experiment 1, rats underwent Pavlovian extinction followed by reinstatement and spontaneous recovery tests. For comparison, a separate group of rats underwent PCA training followed by operant conditioning, extinction, and tests of reinstatement and spontaneous recovery. In Experiment 2, three cognitive enhancers (sodium butyrate, D-cycloserine, and fibroblast growth factor 2) were administered following extinction training to facilitate extinction learning. STs but not GTs displayed enduring resistance to Pavlovian, but not operant, extinction and were more susceptible to spontaneous recovery. In addition, none of the cognitive enhancers tested affected extinction learning. These results expand our understanding of extinction learning by demonstrating that there is individual variation in extinction and relapse processes and highlight potential difficulties in applying extinction-based therapies to drug addiction treatment in the clinic.

Cortical inter-hemispheric circuits for multimodal vocal learning in songbirds

Vocal learning in songbirds and humans is strongly influenced by social interactions based on sensory inputs from several modalities. Songbird vocal learning is mediated by cortico-basal ganglia circuits that include the SHELL region of lateral magnocellular nucleus of the anterior nidopallium (LMAN), but little is known concerning neural pathways that could integrate multimodal sensory information with SHELL circuitry. In addition, cortical pathways that mediate the precise coordination between hemispheres required for song production have been little studied. In order to identify candidate mechanisms for multimodal sensory integration and bilateral coordination for vocal learning in zebra finches, we investigated the anatomical organization of two regions that receive input from SHELL: the dorsal caudolateral nidopallium (dNCL_SHELL ) and a region within the ventral arcopallium (Av). Anterograde and retrograde tracing experiments revealed a topographically organized inter-hemispheric circuit: SHELL and dNCL_SHELL , as well as adjacent nidopallial areas, send axonal projections to ipsilateral Av; Av in turn projects to contralateral SHELL, dNCL_SHELL , and regions of nidopallium adjacent to each. Av on each side also projects directly to contralateral Av. dNCL_SHELL and Av each integrate inputs from ipsilateral SHELL with inputs from sensory regions in surrounding nidopallium, suggesting that they function to integrate multimodal sensory information with song-related responses within LMAN-SHELL during vocal learning. Av projections share this integrated information from the ipsilateral hemisphere with contralateral sensory and song-learning regions. Our results suggest that the inter-hemispheric pathway through Av may function to integrate multimodal sensory feedback with vocal-learning circuitry and coordinate bilateral vocal behavior.

Integration of contextual cues into memory depends on "prefrontal" N-methyl-D-aspartate receptors

Every learning event is embedded in a context, but not always does the context become an integral part of the memory; however, for extinction learning it usually does, resulting in context-specific conditioned responding. The neuronal mechanisms underlying contextual control have been mainly investigated for Pavlovian fear extinction with a focus on hippocampal structures. However, the initial acquisition of novel responses can be subject to contextual control as well, although the neuronal mechanisms are mostly unknown. Here, we tested the hypothesis that contextual control of acquisition depends on glutamatergic transmission underlying executive functions in forebrain areas, e.g. by shifting attention to critical cues. Thus, we antagonized N-methyl-D-aspartate (NMDA) receptors with 2-amino-5-phosphonovaleric acid (AP5) in the pigeon nidopallium caudolaterale, the functional analogue of mammalian prefrontal cortex, during the concomitant acquisition and extinction of conditioned responding to two different stimuli. This paradigm has previously been shown to lead to contextual control over extinguished as well as non-extinguished responding. NMDA receptor blockade resulted in an impairment of extinction learning, but left the acquisition of responses to a novel stimulus unaffected. Critically, when responses were tested in a different context in the retrieval phase, we observed that NMDA receptor blockade led to the abolishment of contextual control over acquisition performance. This result is predicted by a model describing response inclination as the product of associative strength and contextual gain. In this model, learning under AP5 leads to a change in the contextual gain on the learned association, possibly via the modulation of attentional mechanisms.

Neurons in the pigeon caudolateral nidopallium differentiate Pavlovian conditioned stimuli but not their associated reward value in a sign-tracking paradigm

Animals exploit visual information to identify objects, form stimulus-reward associations, and prepare appropriate behavioral responses. The nidopallium caudolaterale (NCL), an associative region of the avian endbrain, contains neurons exhibiting prominent response modulation during presentation of reward-predicting visual stimuli, but it is unclear whether neural activity represents valuation signals, stimulus properties, or sensorimotor contingencies. To test the hypothesis that NCL neurons represent stimulus value, we subjected pigeons to a Pavlovian sign-tracking paradigm in which visual cues predicted rewards differing in magnitude (large vs. small) and delay to presentation (short vs. long). Subjects' strength of conditioned responding to visual cues reliably differentiated between predicted reward types and thus indexed valuation. The majority of NCL neurons discriminated between visual cues, with discriminability peaking shortly after stimulus onset and being maintained at lower levels throughout the stimulus presentation period. However, while some cells' firing rates correlated with reward value, such neurons were not more frequent than expected by chance. Instead, neurons formed discernible clusters which differed in their preferred visual cue. We propose that this activity pattern constitutes a prerequisite for using visual information in more complex situations e.g. requiring value-based choices.

Context specificity of both acquisition and extinction of a Pavlovian conditioned response

It is widely held that the extinction of a conditioned response is more context specific than its initial acquisition. One proposed explanation is that context serves to disambiguate the meaning of a stimulus. Using a procedure that equated the learning histories of the contexts, we show that the memory of an appetitive Pavlovian association can be highly context specific despite being unambiguous. This result is inconsistent with predictions of the Rescorla–Wagner model of learning but in line with configural accounts of contextual control of behavior. We propose an explanatory model in which context serves to modulate the gain of associative strength and which expands upon the configural idea of unitary representations of context and conditioned stimuli.

Relative Brain Size and Its Relation with the Associative Pallium in Birds

Despite growing interest in the evolution of enlarged brains, the biological significance of brain size variation remains controversial. Much of the controversy is over the extent to which brain structures have evolved independently of each other (mosaic evolution) or in a coordinated way (concerted evolution). If larger brains have evolved by the increase of different brain regions in different species, it follows that comparisons of the whole brain might be biologically meaningless. Such an argument has been used to criticize comparative attempts to explain the existing variation in whole-brain size among species. Here, we show that pallium areas associated with domain-general cognition represent a large fraction of the entire brain, are disproportionally larger in large-brained birds and accurately predict variation in the whole brain when allometric effects are appropriately accounted for. While this does not question the importance of mosaic evolution, it suggests that examining specialized, small areas of the brain is not very helpful for understanding why some birds have evolved such large brains. Instead, the size of the whole brain reflects consistent variation in associative pallium areas and hence is functionally meaningful for comparative analyses.

Does Early Environmental Complexity Influence Tyrosine Hydroxylase in the Chicken Hippocampus and "Prefrontal" Caudolateral Nidopallium?

In adult chickens, the housing system influences hippocampal morphology and neurochemistry. However, no work has been done investigating the effects of the early life environment on chicken brain development. In the present study, we reared 67 commercial laying hens (Gallus gallus domesticus) in two environments that differed in the degree of complexity (aviary or cage system). These two groups were further divided into two age groups. At 20 weeks of age, 18 aviary-reared birds and 15 cage-reared birds were humanely euthanized and their brains dissected. At 24 weeks of age, a further 16 brains from aviary-reared birds and 18 brains from cage-reared birds were collected. These brains were prepared for immunohistochemical detection of tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of dopamine, in the hippocampus and the caudolateral nidopallium (NCL). There were no differences between the treatment groups in TH staining intensity in the hippocampus or the NCL. In the medial hippocampus, the right hemisphere had higher TH staining intensity compared to the left hemisphere. The opposite was true for the NCL, with the left hemisphere being more strongly stained compared to the right hemisphere. The present study supports the notion that the hippocampus is functionally lateralized, and our findings add to the body of knowledge on adult neural plasticity of the avian brain.