Research reportZENK expression in a restricted forebrain area correlates negatively with preference for an imprinted stimulus
Introduction
Sexual imprinting is an early learning process by which a stable preference for a sexual partner is established. Immelmann [21], [22] was the first to show that young zebra finch males, which were reared by Bengalese finches (Lonchura striata) as foster parents, showed a strong preference for females of this species. He demonstrated that sexual imprinting is restricted to a sensitive period early in development, and that the preference, once being established, cannot be altered again.
Subsequent research revealed more details of this learning process, and also allowed to begin with investigations of its physiological basis. It was shown that sexual imprinting is a two-stage process [3], [6], [23], [24], [31]. In the first stage, called acquisition, the young zebra finch male develops a social preference for the individuals of its social environment. This first stage starts at about day 10 after hatching and ends in most cases between days 20 and 40. The second stage, called consolidation, begins at the time when the zebra finch male becomes sexually mature and starts courting females. The social preference obtained during the acquisition phase guides the young male's choice of a mating partner so that it courts preferentially females of the species which he was reared with. After the first courtship events have occurred, an irreversible preference for a mate is established.
Our experiments have shown that the consolidation process can be experimentally controlled. If a male reared by foster parents is isolated after reaching nutritional independence at day 40 and kept in isolation until day 100, a short exposure to a female (from 1 week down to hours) [7], [23], [24], [30] can either modify or consolidate the previously acquired preference. If the female, to which a Bengalese finch reared male is exposed, is of the own species, part of the birds modify their preference towards zebra finch females, as can be shown in subsequent choice tests. With appropriate choice of the length of exposure to the zebra finch female, it is possible to “produce” males with mixed preferences. These males, if exposed to a Bengalese finch and a zebra finch female in a choice test, direct their courtship behaviour to both females with a constant relation. For example, a male directing 30% of its courtship bouts to the zebra finch and 70% to the Bengalese finch female does not change this relation in subsequent tests [30]. No further modification of the preference is possible after this first exposure to a female. We therefore presume that it has a stabilizing (consolidating) effect on a previously labile preference. Because in the experimental setup the exposure to a female is the first opportunity for a male to practice directed courtship behaviour, we call it “first courtship”. We believe this “first courtship” to be the key event in sexual imprinting because at this time the stable preference for a sexual partner is established.
Because “first courtship” can be controlled experimentally, it was possible to investigate the physiological basis of the consolidation process. Experiments using the incorporation of radioactive, modified glucose (14-C-2-deoxyglucose, 2-DG) into active neurons showed a strong activation of four forebrain areas (Fig. 1) as a consequence of the “first courtship” situation [9].
These areas were, in the traditional avian forebrain nomenclature, the hyperstriatum accessorium/dorsale (HAD), the archi-neostriatum caudale (ANC), the medial neo-hyperstriatum (MNH), and the lateral neo-hyperstriatum (LNH). We have used this nomenclature, which was introduced by Wallhäusser and Scheich [45] in our previous publications (reviews: [4], [5]). In this study, we used the altered nomenclature according to Reiner et al. [32] HAD is now called hyperpallium accessorium/mesopallium dorsale (HAMD), the ANC is called arcopallium-neopallium caudale (abbreviation ANC preserved). The medial neo-hyperstriatum is now the medial nido-mesopallium (MNM), and the lateral part of the same two areas is called LNM (lateral nido-mesopallium).
Within these four areas, subsequent investigations demonstrated that structural changes of neuronal connections, as measured by alterations of the density of dendritic spines, occurred as a consequence of consolidation in these four forebrain areas [33], [34], [35], [36], [37], [38].
Only two of these, MNM and LNM, are thought to be involved in imprinting because the reduction of spine density, which can be observed here as a consequence of imprinting, is irreversible. Because irreversibility is one of the key features of imprinting, we suggested that this irreversible reduction of dendritic spines within LNM and MNM, which is also possible only during a sensitive period [8], might be the anatomical correlate of this learning process. Further experiments (review: [10]) substantiated these findings and provided supporting evidence. It could be shown for example that long-term potentiation (LTP) and/or long-term depression (LTD), which both have been shown to be involved in plastic changes of neuronal wiring, play also a role in the consolidation event of sexual imprinting [36].
Immediate early genes (IEGs) and their proteins have been speculated to be involved in the mediation between synaptic activation and the activation of late response genes which induce reconstruction of membrane structures. The best investigated examples of IEGs are c-fos and zenk, which is also known as zif 268, egr-1, NGFI-A or krox 24 (the notation zenk will be used in this article when referring to the gene, ZENK when referring to the protein). They are observed in mammals as well as in birds [15], [28], [29], and their expression has been studied with two main purposes, namely as markers for activated neurons, similar to the above mentioned 2-DG experiments, and in the context of learning and memory.
IEG's should therefore be also expressed by sexual imprinting. Indeed, experiments using the “first courtship” paradigm revealed enhanced levels of Fos and ZENK within the imprinting relevant areas MNM and LNM [39], [26], [27] and in other areas, for example hippocampus [40]. In contrast to the 2-DG-study which showed a similar activation also in arousing situations without imprinting, the IEG's in the above mentioned areas were much stronger activated in the “first courtship” situation.
If LNM and MNM are specifically activated in the course of sexual imprinting, these areas may, when the learning process is finished, be involved in processing information related to the imprinting object. If so, one could further speculate that it could also be involved later in the recognition of the imprinted object. Thus, exposing the imprinted, adult bird to the imprinted object should alter the activation of these areas.
In the context of song learning, Bolhuis et al. [13], [14] and Terpstra et al. [44], [45] performed an experiment based on the same assumption. They showed that an area which had previously been identified as a candidate for the storage of learned song, expressed the more ZENK immunoreactive neurons, the more elements were shared between the song a given experimental bird had learned and the song which it heard in the ZENK experiment.
We have previously described that in the “first courtship” experiments we can produce zebra finch males with mixed preference. This means, the birds show a fixed ratio of courtship elements directed to zebra finch females and Bengalese females, respectively, in a choice test. Analogous to the experiment of Bolhuis et al., one could predict that zebra finch males with a strong preference for Bengalese finch females should show less activation of the LNM if exposed to a zebra finch female in the ZENK experiment, compared to a zebra finch male with a less strong preference for the Bengalese finch (or a stronger zebra finch preference). To test this, we designed an experiment with two rearing conditions, one with male zebra finches raised by own species parents, the other with Bengalese finch parents. The first group was either exposed to zebra finch or to Bengalese finch females as adults, after they had been tested for their preference and before being processed to visualize the ZENK expression within LNM and MNM. The second group was exposed with 100 days to zebra finch females according to the above described “first courtship” design, and the males, which showed mixed preferences in subsequent choice tests, were as adults exposed to zebra finch females preceding ZENK immunohistochemistry. The results described here were partly congruent with our predictions, but partly quite surprising.
Section snippets
Material and methods
The experiments were performed on 23 zebra finch males raised from the Institute's stock. The experimental time table is presented in Fig. 2 under the following conditions. Males of group 1 (n = 20) were raised by their own parents and isolated at day 40. Until day 100 they were kept in cages (40 cm × 30 cm × 28 cm) where they could hear, but not see other zebra finch males. Males of group 2 (n = 12) were reared by Bengalese finch foster parents which received zebra finch eggs as replacement of own eggs.
Results
Labeled neurons were clearly to be identified by a blueish-black signal, background staining was minimal. In general, ZENK-ir neurons were apparent in almost every region of the brain, but in most cases the density was very low. An exception were the primary sensory areas of the forebrain, Entopallium and Field L. An overview of the distribution of ZENK immunoreactivity within the forebrain is given in Lieshoff et al. [27], where in spite of the different experimental conditions, the same brain
Discussion
Our results clearly support earlier findings indicating that the lateral and the medial part of the nido- and mesopallium may be involved in the storage and processing of information acquired by sexual imprinting. If an adult zebra finch male is exposed either to a female of its own or to another species, the activation of the lateral nido-mesopallium depends on the preference for one of these stimuli which is established in the course of sexual imprinting. In contrast to the results of
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Filial responses as predisposed and learned preferences: Early attachment in chicks and babies
2017, Behavioural Brain ResearchCitation Excerpt :This is particularly relevant for precocial avian species endowed with the dedicated system of filial imprinting [24,37–40], a learning mechanism that restricts subsequent affiliative responses to the first conspicuous objects experienced (in the case of acoustic stimuli, this process can start even before hatching, during the last days of incubation [41]). After a brief exposure, chicks learn the features of their imprinting object and develop a social preference for it (this learning mechanism is important for the development of sexual preferences too, a phenomenon called sexual imprinting, see [42–46]. Crucially, naïve chicks do not merely absorb information from the environment but actively search for stimulation from conspicuous objects (e.g. [47]).
Social predisposition dependent neuronal activity in the intermediate medial mesopallium of domestic chicks (Gallus gallus domesticus)
2016, Behavioural Brain ResearchCitation Excerpt :Encoding the properties of this object can require the storing of additional information for the aspects that do not fit the pre-wired template. A similar phenomenon has been found in sexual-imprinting in zebra finches [65]. In this species sexual imprinting on conspecifics is facilitated, but cross-species rearing can originate also sexual preferences for non-conspecifics.
Functional organization of telencephalic visual association fields in pigeons
2016, Behavioural Brain ResearchCitation Excerpt :Therefore, we assume that the reduced ZENK-expression in our experimental groups reflects a learning-related buildup of stimulus familiarity in these regions [19,11,95,38]. Consistent with this, Huchzermeyer et al. [36] showed that a stimulus imprinted in juvenile zebra finches induced lower levels of ZENK-expression than a non-imprinted stimulus, when presented again at adult stages. Birds are visually lateralized with a left hemispheric superiority for visual discrimination of form details [28,23,86,54].
Spatial memory and the avian hippocampus: Research in zebra finches
2013, Journal of Physiology ParisCitation Excerpt :Previous experiments had already shown that this could not be the entopallium, because all primary sensory areas of the telencephalon do not show expression of c-Fos and Zenk. Instead, we proposed that the so called LNM (lateral nido-mesopallium) which receives visual input from the entopallium and has already been shown to express both IEG products (Lieshoff et al., 2004; Huchzermeyer et al., 2006), would be activated if the birds performed visual tasks. Zebra finches were again trained to find food in one of four feeders in the “dry Morris water maze”.
Neural-activity mapping of memory-based dominance in the crow: Neural networks integrating individual discrimination and social behaviour control
2011, NeuroscienceCitation Excerpt :The present results regarding neural correlates of submission behaviour in mM and mN suggest a novel function for these nuclei in social behaviour control. As in previous studies on hippocampus, neural activity in mM and mN has been investigated with an emphasis on the cognitive function such as visual recognition and memory (McCabe and Horn, 1994; Huchzermeryer et al., 2006; Patton et al., 2009), but little effort has been made to discriminate between the components of cognition (i.e. social-stimulus discrimination) and behaviour control. However, we must be cautious about attributing a role of mM and mN in behaviour control per se, in as much as there is no anatomical evidence of a direct descending pathway from these pallial nuclei to motor control nuclei.