Contact call-driven Zenk protein induction and habituation in telencephalic auditory pathways in the Budgerigar (Melopsittacus undulatus): implications for understanding vocal learning processes

Memory-specific correlated neuronal activity in higher-order auditory regions of a parrot

Male budgerigars (Melopsittacus undulatus) are open-ended learners that can learn to produce new vocalisations as adults. We investigated neuronal activation in male budgerigars using the expression of the protein products of the immediate early genes zenk and c-fos in response to exposure to conspecific contact calls (CCs: that of the mate or an unfamiliar female) in three subregions (CMM, dNCM and vNCM) of the caudomedial pallium, a higher order auditory region. Significant positive correlations of Zenk expression were found between these subregions after exposure to mate CCs. In contrast, exposure to CCs of unfamiliar females produced no such correlations. These results suggest the presence of a CC-specific association among the subregions involved in auditory memory. The caudomedial pallium of the male budgerigar may have functional subdivisions that cooperate in the neuronal representation of auditory memory.

Social calls provide novel insights into the evolution of vocal learning

Learned song is among the best-studied models of animal communication. In oscine songbirds, where learned song is most prevalent, it is used primarily for intrasexual selection and mate attraction. Learning of a different class of vocal signals, known as contact calls, is found in a diverse array of species, where they are used to mediate social interactions among individuals. We argue that call learning provides a taxonomically rich system for studying testable hypotheses for the evolutionary origins of vocal learning. We describe and critically evaluate four nonmutually exclusive hypotheses for the origin and current function of vocal learning of calls, which propose that call learning (1) improves auditory detection and recognition, (2) signals local knowledge, (3) signals group membership, or (4) allows for the encoding of more complex social information. We propose approaches to testing these four hypotheses but emphasize that all of them share the idea that social living, not sexual selection, is a central driver of vocal learning. Finally, we identify future areas for research on call learning that could provide new perspectives on the origins and mechanisms of vocal learning in both animals and humans.

Sex differences in behavioural and neural responsiveness to mate calls in a parrot

Vocalisation in songbirds and parrots has become a prominent model system for speech and language in humans. We investigated possible sex differences in behavioural and neural responsiveness to mate calls in the budgerigar, a vocally-learning parrot. Males and females were paired for 5 weeks and then separated, after which we measured vocal responsiveness to playback calls (a call of their mate versus a call of an unfamiliar conspecific). Both sexes learned to recognise mate calls during the pairing period. In males, but not females, mate calls evoked significantly fewer vocal responses than unfamiliar calls at one month after separation. Furthermore, in females, there was significantly greater molecular neuronal activation in response to mate calls compared to silence in the caudomedial mesopallium (CMM), a higher-order auditory region, in both brain hemispheres. In males, we found right-sided dominance of molecular neuronal activation in response to mate calls in the CMM. This is the first evidence suggesting sex differences in functional asymmetry of brain regions related to recognition of learned vocalisation in birds. Thus, sex differences related to recognition of learned vocalisations may be found at the behavioural and neural levels in avian vocal learners as it is in humans.

Differences in number and distribution of striatal calbindin medium spiny neurons between a vocal-learner (Melopsittacus undulatus) and a non-vocal learner bird (Colinus virginianus)

Striatal projecting neurons, known as medium spiny neurons (MSNs), segregate into two compartments called matrix and striosome in the mammalian striatum. The matrix domain is characterized by the presence of calbindin immunopositive (CB+) MSNs, not observed in the striosome subdivision. The existence of a similar CB+ MSN population has recently been described in two striatal structures in male zebra finch (a vocal learner bird): the striatal capsule and the Area X, a nucleus implicated in song learning. Female zebra finches show a similar pattern of CB+ MSNs than males in the developing striatum but loose these cells in juveniles and adult stages. In the present work we analyzed the existence and allocation of CB+ MSNs in the striatal domain of the vocal learner bird budgerigar (representative of psittaciformes order) and the non-vocal learner bird quail (representative of galliformes order). We studied the co-localization of CB protein with FoxP1, a transcription factor expressed in vertebrate striatal MSNs. We observed CB+ MSNs in the medial striatal domain of adult male and female budgerigars, although this cell type was missing in the potentially homologous nucleus for Area X in budgerigar. In quail, we observed CB+ cells in the striatal domain at developmental and adult stages but they did not co-localize with the MSN marker FoxP1. We also described the existence of the CB+ striatal capsule in budgerigar and quail and compared these results with the CB+ striatal capsule observed in juvenile zebra finches. Together, these results point out important differences in CB+ MSN distribution between two representative species of vocal learner and non-vocal learner avian orders (respectively the budgerigar and the quail), but also between close vocal learner bird families.

Localized brain activation related to the strength of auditory learning in a parrot

Parrots and songbirds learn their vocalizations from a conspecific tutor, much like human infants acquire spoken language. Parrots can learn human words and it has been suggested that they can use them to communicate with humans. The caudomedial pallium in the parrot brain is homologous with that of songbirds, and analogous to the human auditory association cortex, involved in speech processing. Here we investigated neuronal activation, measured as expression of the protein product of the immediate early gene ZENK, in relation to auditory learning in the budgerigar (Melopsittacus undulatus), a parrot. Budgerigar males successfully learned to discriminate two Japanese words spoken by another male conspecific. Re-exposure to the two discriminanda led to increased neuronal activation in the caudomedial pallium, but not in the hippocampus, compared to untrained birds that were exposed to the same words, or were not exposed to words. Neuronal activation in the caudomedial pallium of the experimental birds was correlated significantly and positively with the percentage of correct responses in the discrimination task. These results suggest that in a parrot, the caudomedial pallium is involved in auditory learning. Thus, in parrots, songbirds and humans, analogous brain regions may contain the neural substrate for auditory learning and memory.

Contact-call driven and tone-driven zenk expression in the nucleus ovoidalis of the budgerigar (Melopsittacus undulatus)

The effectiveness of species-typical contact calls and a 3-kHz pure tone to induce zenk gene protein expression in the primary thalamic auditory relay nucleus ovoidalis was compared in budgerigars (Melopsittacus undulatus), a parrot species capable of lifelong vocal learning. Ovoidalis consists of a core which projects topographically to field L of the telencephalon and a ventromedial shell containing many calcitonin-gene-related peptide neurons that project throughout field L as well as to an adjacent field receiving visual input. Tone-induced and call-induced zenk expression in the ovoidalis core were similar; however, call-induced zenk expression in ventromedial ovoidalis shell was significantly greater than tone-induced expression. These results support the idea that the ovoidalis shell may contain neurons specialized to process complex sounds including species-typical communication sounds.

Maternal aggression: New insights from Egr-1

Lactating mice display fierce aggression towards novel, male mice. This study compares neuronal activity in the brains of aggression-tested (T) and -untested (U) mice using early growth response factor 1 (Egr-1; also known as Krox 24, NGFI-A, Zif268, Tis8, and ZENK) as a measure of neuronal activity. Animals were sampled 90 min after either a sham or real 7-min test with a male intruder, after which their brains were examined for immunoreactivity to Egr-1 (Egr-IR). Significant increases in Egr-IR in T mice were identified in 11 of 40 brain regions, including paraventricular nucleus of the hypothalamus; anterior and lateral hypothalamus (both posterior portion); ventromedial hypothalamus; lateral periaqueductal gray; and medial, central, and basolateral amygdala. Posterodorsal (MePD) and posteroventral medial amygdala were examined for the first time in association with maternal aggression. MePD, a region associated with both sexual and aggressive behaviors in rats, hamsters, and mice, showed increased Egr-IR in association with testing. Taken together, the results from this study provide new insights into the neural circuits regulating maternal behaviors.

Feeding and contact call stimulation both induce zenk and cfos expression in a higher order telencephalic area necessary for vocal learning in budgerigars

Stimulation with natural contact calls and feeding were used to assess zenk and fos protein expression in budgerigars (Melopsittacus undulatus), a vocal learning parrot species in which feeding and physical contact often occur in conjunction with vocalization. Although only calls induced gene expression in Field L, the primary telencephalic auditory area, both calls and feeding induced gene expression in the frontal lateral nidopallium (NFl), a brain area in receipt of input from Field L which projects to areas afferent to vocal control nuclei and which is necessary for new call learning. NFl thus appears poised to provide both non-auditory as well as auditory feedback to the vocal system.

Expression of the immediate-early gene-encoded protein Egr-1 (zif268) during in vitro classical conditioning

Expression of the immediate-early genes (IEGs) has been shown to be induced by activity-dependent synaptic plasticity or behavioral training and is thought to play an important role in long-term memory. In the present study, we examined the induction and expression of the IEG-encoded protein Egr-1 during an in vitro neural correlate of eyeblink classical conditioning. The results showed that Egr-1 protein expression as determined by immunocytochemistry and Western blot analysis rapidly increased during the early stages of conditioning and remained elevated during the later stages. Further, expression of Egr-1 protein required NMDA receptor activation as it was blocked by bath application of AP-5. These findings suggest that the IEG-encoded proteins such as Egr-1 are activated during relatively simple forms of learning in vertebrates. In this case, Egr-1 may have a functional role in the acquisition phase of conditioning as well as in maintaining expression of conditioned responses.

Sexual dimorphism of vocal control nuclei in budgerigars (Melopsittacus undulatus) revealed with Nissl and NADPH-d staining

Nissl staining and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry were used to explore the existence of sexual dimorphism in vocal control nuclei of adult budgerigars (Melopsittacus undulatus), a parrot species capable of lifelong vocal learning. Behavioral studies indicate that adult males possess larger vocal repertoires than adult females and learn new calls more quickly. The results of the present study show that the volumes of all vocal nuclei, as measured using both Nissl-stained and NADPH-d-stained material, as well as the total numbers of NADPH-d neurons, were 35-110% greater in males. Furthermore, all vocal nuclei exhibit conspicuous NADPH-d staining compared to surrounding fields in both adult males and females. Nevertheless, there were no significant gender differences in either the intensity of neuropil staining or the densities of NADPH-d neurons in vocal nuclei. Moreover NADPH-d neuron somal shapes were similar in males and females. Diameters of NADPH-d neurons in vocal nuclei were 8.5-32% larger in males than in females. Greater size of NADPH-d neuronal somata in males may be a general property of this cell type in budgerigars because a similar gender difference was found in a visual nucleus, the entopallium, which is not directly associated with the vocal control system and does not exhibit sexual dimorphism in total volume or total NADPH-d neuron numbers. Taken together, the results of the present study favor the hypothesis that superior lifelong vocal learning ability in male budgerigars rests largely on larger volumes of vocal control nuclei in males rather than on sexual dimorphism in the internal composition of vocal nuclei.

Contact call-driven zenk mRNA expression in the brain of the budgerigar (Melopsittacus undulatus)

Contact call-driven zenk (zif268, egr1, NGF1A, Krox 24) mRNA expression was mapped with in situ hybridization histochemistry in a vocal learning parrot, the budgerigar (M. undulatus). Relative to controls, call stimulation induced high zenk mRNA expression in all auditory areas including those closely associated with the vocal system within the anterior forebrain (Brauth et al. (2001) J. Comp. Neurol. 432, 481; (2002) Learn. Memory 9, 76). Thus there is a high correspondence between the distributions of neurons exhibiting contact call-driven zenk protein and mRNA expression in budgerigars. Field L2a, an area reported previously to express only perinucleolar zenk protein localization (Brauth et al. (2002) Learn. Memory 9, 76) also showed zenk mRNA expression.

Neuronal activation in female budgerigars is localized and related to male song complexity

Females of several songbird species have been shown to respond preferentially to a more complex song. The male budgerigar (Melopsittacus undulatus) sings complex songs consisting of discrete components, known as syllables. We exposed female budgerigars to either standard male song, complex song, or simple song, the iteration of only one syllable (either frequency-modulated or unmodulated). Using immunocytochemistry, we analysed the expression of the protein product of the immediate early gene ZENK in a number of forebrain regions. The level of Zenk protein expression caused by song stimuli varied among each of the brain regions. Expression was highest in the caudomedial neostriatum (NCM), lower in the caudomedial hyperstriatum ventrale (CMHV), and lowest in the hippocampus. There was a significant effect of song complexity on the number of Zenk-immunoreactive cells in the NCM, but not in the hippocampus. Zenk protein expression correlated significantly and positively with the number of different syllables to which the females were exposed in the NCM and to a lesser extent in the CMHV, but not in the hippocampus. For the NCM this correlation was also significant within the group exposed to natural song. These results suggest that the NCM is involved in the perception of song complexity in female budgerigars.