Wnt-2b in the intermediate hyperpallium apicale of the telencephalon is critical for the thyroid hormone-mediated opening of the sensitive period for filial imprinting in domestic chicks (Gallus gallus domesticus).

Filial imprinting is the behavior observed in chicks during the sensitive or critical period of the first 2-3 days after hatching; however, after this period they cannot be imprinted when raised in darkness. Our previous study showed that temporal augmentation of the endogenous thyroid hormone 3,5,3'-triiodothyronine (T3) in the telencephalon, by imprinting training, starts the sensitive period just after hatching. Intravenous injection of T3 enables imprinting of chicks on days 4 or 6 post-hatching, even when the sensitive period has ended. However, the molecular mechanism of how T3 acts as a determinant of the sensitive period is unknown. Here, we show that Wnt-2b mRNA level is increased in the T3-injected telencephalon of 4-day old chicks. Pharmacological inhibition of Wnt signaling in the intermediate hyperpallium apicale (IMHA), which is the caudal area of the telencephalon, blocked the recovery of the sensitive period following T3 injection. In addition, injection of recombinant Wnt-2b protein into the IMHA helped chicks recover the sensitive period without the injection of T3. Lastly, we showed Wnt signaling to be involved in imprinting via the IMHA region on day 1 during the sensitive period. These results indicate that Wnt signaling plays a critical role in the opening of the sensitive period downstream of T3.

Positive feedback of NR2B-containing NMDA receptor activity is the initial step toward visual imprinting: a model for juvenile learning.

Imprinting in chicks is a good model for elucidating the processes underlying neural plasticity changes during juvenile learning. We recently reported that neural activation of a telencephalic region, the core region of the hyperpallium densocellulare (HDCo), was critical for success of visual imprinting, and that N-Methyl-D-aspartic (NMDA) receptors containing the NR2B subunit (NR2B/NR1) in this region were essential for imprinting. Using electrophysiological and multiple-site optical imaging techniques with acute brain slices, we found that long-term potentiation (LTP) and enhancement of NR2B/NR1 currents in HDCo neurons were induced in imprinted chicks. Enhancement of NR2B/NR1 currents as well as an increase in surface NR2B expression occurred even following a brief training that was too weak to induce LTP or imprinting behavior. This means that NR2B/NR1 activation is the initial step of learning, well before the activation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors which induces LTP. We also showed that knockdown of NR2B/NR1 inhibited imprinting, and inversely, increasing the surface NR2B expression by treatment with a casein kinase 2 inhibitor successfully reduced training time required for imprinting. These results suggest that imprinting stimuli activate post-synaptic NR2B/NR1 in HDCo cells, increase NR2B/NR1 signaling through up-regulation of its expression, and induce LTP and memory acquisition. The study investigated the neural mechanism underlying juvenile learning. In the initial stage of chick imprinting, NMDA receptors containing the NMDA receptor subunit 2B (NR2B) are activated, surface expression of NR2B/NR1 (NMDA receptor subunit 1) is up-regulated, and consequently long-term potentiation is induced in the telencephalic neurons. We suggest that the positive feedback in the NR2B/NR1 activation is a unique process of juvenile learning, exhibiting rapid memory acquisition.

Impaired imprinting and social behaviors in chicks exposed to mifepristone, a glucocorticoid receptor antagonist, during the final week of embryogenesis.

UNLABELLED: The effects of glucocorticoid receptor dysfunction during embryogenesis on the imprinting abilities and social behaviors of hatchlings were examined using "fertile hen's egg-embryo-chick" system. METHODS AND RESULTS: Of embryos treated with mifepristone (0.4µmol/egg) on day 14, over 75% hatched a day later than the controls (day 22) without external anomalies. The mifepristone-treated hatchlings were assayed for imprinting ability on post-hatching day 2 and for social behaviors on day 3. The findings were as follows: imprinting ability (expressed as preference score) was significantly lower in mifepristone-treated hatchlings than in controls (0.65±0.06 vs. 0.92±0.02, P<0.005). Aggregation tests to evaluate the speed (seconds) required for four chicks, individually isolated with cardboard dividers in a box, to form a group after removal of the barriers showed that aggregation was significantly slower in mifepristone-treated hatchlings than in controls (8.7±1.1 vs. 2.6±0.3, P<0.001). In belongingness tests to evaluate the speed (seconds) for a chick isolated at a corner to join a group of three chicks placed at the opposite corner, mifepristone-treated hatchlings took significantly longer than controls (4.5±0.4/40 cm vs. 2.4±0.08/40 cm, P<0.001). In vocalization tests, using a decibel meter to measure average decibel level/30s (chick vocalization), mifepristone-treated hatchlings had significantly weaker vocalizations than controls (14.2±1.9/30s vs. 26.4±1.3/30s P<0.001). In conclusion, glucocorticoid receptor dysfunction during the last week embryogenesis altered the programming of brain development, resulting in impaired behavioral activities in late life.

Neuroendocrine contributions to sexual partner preference in birds.

A majority of birds are socially monogamous, providing exceptional opportunities to discover neuroendocrine mechanisms underlying preferences for opposite-sex partners where the sexes form extended affiliative relationships. Zebra finches have been the focus of the most systematic program of research to date in any socially monogamous animal. In this species, sexual partner preference can be partially or largely sex reversed with hormone manipulations during early development, suggesting a role for organizational hormone actions. This same conclusion emerges from research with Japanese quail, which do not form long-term pairs. In zebra finches, social experience manipulations during juvenile development also can sex reverse partner preference, either alone or in combination with an early hormone manipulation. Although there are several candidate brain regions where neural mechanisms could underlie these effects of hormones or social experience, the necessary research has not yet been done to determine their involvement. The neuroendocrinology of avian sexual partner preference is still frontier territory.

Molecular function of microtubule-associated protein 2 for filial imprinting in domestic chicks (Gallus gallus domesticus).

RNA interference (RNAi)-mediated gene-silencing can be a tool for elucidating the role of genes in the neural basis of behavioral plasticity. Previously, we reported that exogenous DNA could be successfully delivered into newly-hatched chick brains via electroporation. Here, we used this in vivo gene-transfer technique and showed that transfected microRNA vectors preferentially silence exogenous DNA expression in neuronal cells. Using this system, the up-regulation of microtubule-associated protein 2 (MAP2) accompanying filial imprinting was suppressed in vivo, which impaired the filial imprinting in chicks. In addition, the phosphorylation of MAP2 was found to increase in parallel with filial imprinting, and lithium chloride, an inhibitor of glycogen synthase kinase 3 (GSK3), was found to impair filial imprinting. Our results suggest that the regulation of MAP2 expression and its phosphorylation are required for filial imprinting and may modify microtubule stability, thereby leading to cytoskeletal reorganization during imprinting. This in vivo RNAi-mediated gene-silencing system will facilitate the analysis of gene function in the living chick brain and provides further clues regarding the molecular mechanisms underpinning avian learning.

Effects of prenatal exposure to antithyroid drugs on imprinting behavior in chicks.

Thyroid hormones play important roles in vertebrate brain development. However, there is little understanding of the direct effects of fetal thyroid dysfunction, i.e., not acquired through the mother, on learning ability. In the present study, we use a chick embryo as a fetal model to investigate the effects of prenatal exposure to antithyroid drugs on imprinting behavior in hatched chicks. Methimazole (MMI) at 20micromol/egg or 5micromol/egg of propylthiouracil (PTU) was administered to eggs on day 14 while the control was given only a vehicle. An imprinting test was conducted after the chicks hatched. Day-old chicks were exposed to a rotating training object for 150min. The next day, the trained chicks were exposed to the training object and a novel object. The imprinting preference was represented as a preference score (PS) calculated as the rate of following the training object to following the training and novel objects. In the MMI-treated chicks, the PS was 0.68+/-0.06 (range, 0.38-0.88), which was significantly lower than that in the control chicks (0.86+/-0.04, p<0.01). In the PTU-treated chicks, the PS was 0.69+/-0.04 (range, 0.52-0.89), which was also significantly lower than that in the control (0.88+/-0.02, p<0.001). The present findings suggested that fetal thyroid dysfunction inhibited brain development, leading to impaired learning and memory. Our chick model can be considered useful for investigating the direct effects of prenatal exposure to antithyroid drugs or substances in the environment on learning ability after birth.

Demonstration of a neural circuit critical for imprinting behavior in chicks.

Imprinting behavior in birds is elicited by visual and/or auditory cues. It has been demonstrated previously that visual cues are recognized and processed in the visual Wulst (VW), and imprinting memory is stored in the intermediate medial mesopallium (IMM) of the telencephalon. Alteration of neural responses in these two regions according to imprinting has been reported, yet direct evidence of the neural circuit linking these two regions is lacking. Thus, it remains unclear how memory is formed and expressed in this circuit. Here, we present anatomical as well as physiological evidence of the neural circuit connecting the VW and IMM and show that imprinting training during the critical period strengthens and refines this circuit. A functional connection established by imprint training resulted in an imprinting behavior. After the closure of the critical period, training could not activate this circuit nor induce the imprinting behavior. Glutamatergic neurons in the ventroposterior region of the VW, the core region of the hyperpallium densocellulare (HDCo), sent their axons to the periventricular part of the HD, just dorsal and afferent to the IMM. We found that the HDCo is important in imprinting behavior. The refinement and/or enhancement of this neural circuit are attributed to increased activity of HDCo cells, and the activity depended on NR2B-containing NMDA receptors. These findings show a neural connection in the telencephalon in Aves and demonstrate that NR2B function is indispensable for the plasticity of HDCo cells, which are key mediators of imprinting.

Neurobehavioral teratogenicity of perfluorinated alkyls in an avian model.

Perfluorinated alkyls are widely-used agents that accumulate in ecosystems and organisms because of their slow rate of degradation. There is increasing concern that these agents may be developmental neurotoxicants and the present study was designed to develop an avian model for the neurobehavioral teratogenicity of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). Fertilized chicken eggs were injected with 5 or 10mg/kg of either compound on incubation day 0. On the day of hatching, imprinting behavior was impaired by both compounds. We then explored underlying mechanisms involving the targeting of protein kinase C (PKC) isoforms (alpha, beta, gamma) in the intermedial part of the hyperstriatum ventrale, the region most closely associated with imprinting. With PFOA exposure, cytosolic PKC concentrations were significantly elevated for all three isoforms; despite the overall increase in PKC expression, membrane-associated PKC was unaffected, indicating a defect in PKC translocation. In contrast, PFOS exposure evoked a significant decrease in cytosolic PKC, primarily for the beta and gamma isoforms, but again without a corresponding change in membrane-associated enzyme; this likely partial, compensatory increases in translocation to offset the net PKC deficiency. Our studies indicate that perfluorinated alkyls are indeed developmental neurotoxicants that affect posthatch cognitive performance but that the underlying synaptic mechanisms may differ substantially among the various members of this class of compounds, setting the stage for disparate outcomes later in life.

A mechanism-based complementary screening approach for the amelioration and reversal of neurobehavioral teratogenicity.

The identification of mechanisms and outcomes for neurobehavioral teratogenesis is critical to our ability to develop therapies to ameliorate or reverse the deleterious effects of exposure to developmental neurotoxicants. We established mechanistically-based complementary models for the study of cholinergic systems in the mouse and the chick, using both environmental neurotoxicants (chlorpyrifos, perfluoroalkyls) and drugs of abuse (heroin, nicotine, PCP). Behavioral evaluations were made using the Morris maze in the mouse, evaluating visuospatial memory related to hippocampal cholinergic systems, and imprinting in the chick, examining behavior dependent on cholinergic innervation of the IMHV. In both models we demonstrated the dependence of neurobehavioral deficits on impairment of cholinergic receptor-induced expression, and translocation of specific PKC isoforms. Understanding this mechanism, we were able to reverse both the synaptic and behavioral deficits with administration of neural progenitors. We discuss the prospects for clinical application of neural progenitor therapy, emphasizing protocols for reducing or eliminating immunologic rejection, as well as minimizing invasiveness of procedures through development of intravenous administration protocols.

Prenatal lipopolysaccharide exposure affects maternal behavior and male offspring sexual behavior in adulthood.

OBJECTIVE: This study investigates the effects of prenatal lipopolysaccharide (LPS) exposure on the maternal behavior of pregnant rats and the physical development and sexual behavior of their male offspring in adulthood. METHODS: For two experiments, pregnant rats were injected with LPS (250 microg/kg, i.p.) on gestation day (GD) 21. In the first experiment, the maternal behavior (postnatal day, PND, 6) and the dam's open-field general activity (PND7) were evaluated. In the second experiment, the maternal pre- and postnatal parameters, the pup's development, the offspring's sexual behavior in adulthood, and the pup's organ weights were assessed. RESULTS: Compared to the control group, the LPS-treated dams presented reduced maternal behavior, decreased general activity, a smaller body weight difference between GD21 and PND1, a greater number of perinatal deaths, and smaller litters. For the male pups, LPS treatment resulted in a decreased body weight on PND2, whereas the anogenital distance and the day of testis descent were not modified. The male sexual behavior was impaired by prenatal LPS. Particularly the number of ejaculating animals was reduced. The testis weight was also lower in the prenatally LPS-treated rats than in the control rats. CONCLUSION: We propose that prenatal LPS exposure on GD21 acts as an imprinting factor that interferes with the programming of brain sexual determination in offspring.

Neurobehavioral teratogenicity of sarin in an avian model.

Nerve gas organophosphates like sarin are likely to be used in urban terrorism, leading to widespread exposures of pregnant women and young children. Here, we established a model for sarin neurobehavioral teratogenicity in the developing chick so as to explore the consequences of apparently subtoxic sarin exposure and the mechanisms underlying synaptic and behavioral deficits. Chicken eggs were injected with sarin (2, 6 and 12 microg/kg) on incubation days 2 and 6, treatments that did not decrease hatching and did not evoke dysmorphology. After hatching the chicks were tested for filial imprinting and neurochemical markers known to be critical for imprinting. Imprinting was reduced at 2 and 6 microg/kg but not at the highest dose. Acetylcholinesterase and choline acetyltransferase were unaffected but sarin reduced the concentration of the high-affinity choline transporter, the rate-limiting factor in acetylcholine utilization. The concentration of PKC isoforms was assessed in the imprinting-related intermediate part of the medial hyperstriatum ventrale, the region most closely associated with cholinergic function in imprinting behavior. Sarin reduced the concentration of all isoforms (alpha, beta, gamma) with a similar, biphasic dose-response curve to that seen for behavioral performance, a relationship noted in previous work with organophosphate pesticides. Our results indicate that otherwise subtoxic exposures to sarin produce neurodevelopmental deficits; since we utilized a chick model, which is devoid of maternal confounds that are present in mammalian development, the adverse effects of sarin are mediated directly in the developing organism.

[Selective disruption of memory consolidation in chicks produced by 5'-iodo-2'-deoxyuridine].

We tested the hypothesis that DNA synthesis is involved in molecular mechanisms of memory consolidation. Nucleotide analogs 5'-iodo- and 5'-bromo-2'-deoxyuridine impair DNA functions being incorporated into elongated DNA chain and cause amnesia in a number of training models in mice. We studied possible amnestic effects of 5'-iodo-2'-deoxyuridine (IdU) in different training models in newborn chicks--in passive avoidance, taste aversion, imprinting and spatial learning in a maze. In the taste aversion model injection of IdU (10 mg/kg 5 min before or 50 min after training) produced amnesia at test 1-2 days after training, at the same time it had no effect on memory retention in test 6 h after training. IdU injection 2 h after training produced no amnesia. Similar amnestic effect in taste aversion model was found for 5'-bromo-2'-deoxyuridine (BrdU). In models of imprinting, passive avoidance and spatial learning IdU injection before or after training had no effect on memory retention. These data presuppose that brain DNA synthesis might play a critical role in mechanisms of memory consolidation in taste aversion learning in chicks.

Perinatal alcohol exposure enhances nocistatin levels in adulthood.

In earlier experiments perinatal hormonal imprinting by alcohol decreased the hormone content of immune cells for life. In the present study, both a single day (15% on the third postnatal day) and a long-term treatment schedule of alcohol exposure (3% for 21 days) of dams during lactation significantly (P < 0.01) enhanced endogenous levels of nocistatin in the blood plasma as well as in the cerebrospinal fluid of the offspring, measured in 3-month-old rats. Our data suggest that alcohol consumption during lactation can cause a life-long influence on nocistatin levels in the offspring and most likely modify nocistatin-related functions such as pain tolerance.

Regional and cellular distribution of the extracellular matrix protein tenascin-C in the chick forebrain and its role in neonatal learning.

The juvenile brain's pronounced synaptic plasticity in response to early experience and learning events is related to the fact that the genetically pre-programmed molecular machinery mediating neuronal development and synapse formation, is activated throughout postnatal brain development and thereby can be recruited for learning and long-term memory formation. In situ hybridization and immunocytochemistry experiments revealed that tenascin-C, one candidate molecule which we suspect to be involved in neonatal learning, is expressed in the forebrain of domestic chicks around the sensitive period during which auditory filial imprinting takes place. The involvement of tenascin-C in this juvenile learning task was tested by injections of monoclonal antibodies directed to distinct domains of the tenascin-C molecule into the avian prefrontal cortex analog, the medio-rostral nidopallium/mesopallium (formerly termed medio-rostral neostriatum/hyperstriatum ventrale), a forebrain area which has been shown to be critically involved in auditory filial imprinting. Injections of monoclonal antibody Tn 68, which is directed against a cell-binding domain of the tenascin-C molecule, strongly reduced the imprinting rate, as opposed to injections of the monoclonal antibody Tn 578, which binds to a domain involved in neurite outgrowth. Double labeling immunohistochemistry revealed that tenascin-C is associated with neurons which express the Ca(2+)-binding protein parvalbumin, and displays a staining pattern highly reminiscent of perineuronal nets of the extracellular matrix. These results indicate that a distinct domain of tenascin-C is functionally involved in the juvenile learning process of filial imprinting and further suggest a critical role of a specific neuronal subpopulation.

A chick model for the mechanisms of mustard gas neurobehavioral teratogenicity.

The chemical warfare blistering agent, sulfur mustard (SM), is a powerful mutagen and carcinogen. Due to its similarity to the related chemotherapy agents nitrogen mustard (mechlorethamine), it is expected to act as a developmental neurotoxicant. The present study was designed to establish a chick model for the mechanisms of SM on neurobehavioral teratogenicity, free of confounds related to mammalian maternal effects. Chicken eggs were injected with SM at a dose range of 0.0017-17.0 microg/kg of egg, which is below the threshold for dysmorphology, on incubation days (ID) 2 and 7, and then tests were conducted posthatching. Exposure to SM elicited significant deficits in the intermedial part of the hyperstriatum ventrale (IMHV)-related imprinting behavior. Parallel decreases were found in the level of membrane PKCgamma in the IMHV, while eliciting no net change in cytosolic PKCgamma. The chick, thus, provides a suitable model for the rapid evaluation of SM behavioral teratogenicity and elucidation of the mechanisms underlying behavioral anomalies. The results obtained, using a model that controls for confounding maternal effects, may be replicated in the mammalian model and provide the groundwork for studies designed to offset or reverse the SM-induced neurobehavioral defects in both avian and mammals.

Convergent effects on cell signaling mechanisms mediate the actions of different neurobehavioral teratogens: alterations in cholinergic regulation of protein kinase C in chick and avian models.

Although the actions of heroin on central nervous system (CNS) development are mediated through opioid receptors, the net effects converge on dysfunction of cholinergic systems. We explored the mechanisms underlying neurobehavioral deficits in mouse and avian (chick, Cayuga duck) models. In mice, prenatal heroin exposure (10 mg/kg on gestation days 9-18) elicited deficits in behaviors related to hippocampal cholinergic innervation, characterized by concomitant pre- and postsynaptic hyperactivity, but ending in a reduction of basal levels of protein kinase C (PKC) isoforms betaII and gamma and their desensitization to cholinergic receptor-induced activation. PKCalpha, which is not involved in the behaviors studied, was unaffected. Because mammalian models possess inherent confounding factors from maternal effects, we conducted parallel studies using avian embryos, evaluating hyperstriatal nucleus (intermedial part of the hyperstriatum ventrale, IMHV)-related, filial imprinting behavior. Heroin injection to the eggs (20 mg/kg) on incubation days 0 and 5 diminished the post-hatch imprinting ability and reduced PKCg and bII content in the IMHV membrane fraction. Two otherwise unrelated agents that converge on cholinergic systems, chlorpyrifos and nicotine, elicited the same spectrum of effects on PKC isoforms and imprinting but had more robust actions. Pharmacological characterization also excluded direct effects of opioid receptors on the expression of imprinting; instead, it indicated participation of serotonergic innervation. The avian models can provide rapid screening of neuroteratogens, exploration of common mechanisms of behavioral disruption, and the potential design of therapies to reverse neurobehavioral deficits.

Cholinergic synaptic signaling mechanisms underlying behavioral teratogenicity: effects of nicotine, chlorpyrifos, and heroin converge on protein kinase C translocation in the intermedial part of the hyperstriatum ventrale and on imprinting behavior in an avian model.

A wide variety of otherwise unrelated neuroteratogens elicit a common set of behavioral defects centering around cholinergic contributions to cognitive function. We utilized the developing chick to overcome confounds related to maternal effects and compared the actions of nicotine, chlorpyrifos, and heroin on cholinergic signaling in the intermedial part of the hyperstriatum ventrale (IMHV), which controls imprinting behavior. Chicken eggs were injected with nicotine (10 mg/kg of egg), chlorpyrifos (10 mg/kg of egg), or heroin (20 mg/kg of egg; all doses below the threshold for dysmorphology) on incubation days (ID) 0 and 5, and then tests were conducted posthatching. All three compounds elicited significant deficits in imprinting behavior. We also found defects in cholinergic synaptic signaling specifically involving the muscarinic receptor-mediated membrane translocation of protein kinase C (PKC)-gamma and in the basal levels of both PKCgamma and PKCbetaII, the two isoforms known to be relevant to behavioral performance. In contrast, there were no alterations in the response of PKCalpha, an isoform that does not contribute to the behavior, nor were cytosolic levels of any of the isoforms affected. Taken together with similar results obtained in rodents, our findings suggest that disparate neuroteratogens all involve signaling defects centering on the ability of cholinergic receptors to elicit PKCgamma translocation/activation and that this effect is direct, i.e., not mediated by maternal confounds. The chick thus provides a suitable model for the rapid screening of neuroteratogens and elucidation of the mechanisms underlying behavioral anomalies.

Effect of treatment at weaning with the serotonin antagonist mianserin on the brain serotonin and cerebrospinal fluid nocistatin level of adult female rats: a case of late imprinting.

Four weeks old (weanling) female rats were treated with the tricyclic antidepressant and histamine/serotonin receptor blocker mianserin for studying its faulty hormonal imprinting effect. Measurements were done four months later. Brain serotonin levels significantly decreased in four regions (hippocampus, hypothalamus, striatum and brainstem), without any change in the cortex. Sexual activity of the treated and control rats was similar. Cerebrospinal fluid nocistatin level was one magnitude higher in the treated rats, than in the controls. The density of uterine estrogen receptors was significantly reduced, while binding capacity of glucocorticoid receptors of liver and thymus remained at control level. The results call attention to the possibility of 1. a broad spectrum imprinting at the time of weaning by a receptor level acting non-hormone molecule 2. imprinting of the brain in a non-neonatal period of life and 3. a very durable (lifelong?) effect of the late imprinting with an antidepressant.

Haloperidol impairs auditory filial imprinting and modulates monoaminergic neurotransmission in an imprinting-relevant forebrain area of the domestic chick.

In vivo microdialysis and behavioural studies in the domestic chick have shown that glutamatergic as well as monoaminergic neurotransmission in the medio-rostral neostriatum/hyperstriatum ventrale (MNH) is altered after auditory filial imprinting. In the present study, using pharmaco-behavioural and in vivo microdialysis approaches, the role of dopaminergic neurotransmission in this juvenile learning event was further evaluated. The results revealed that: (i) the systemic application of the potent dopamine receptor antagonist haloperidol (7.5 mg/kg) strongly impairs auditory filial imprinting; (ii) systemic haloperidol induces a tetrodotoxin-sensitive increase of extracellular levels of the dopamine metabolite, homovanillic acid, in the MNH, whereas the levels of glutamate, taurine and the serotonin metabolite, 5-hydroxyindole-3-acetic acid, remain unchanged; (iii) haloperidol (0.01, 0.1, 1 mm) infused locally into the MNH increases glutamate, taurine and 5- hydroxyindole-3-acetic acid levels in a dose-dependent manner, whereas homovanillic acid levels remain unchanged; (iv) systemic haloperidol infusion reinforces the N-methyl-d-aspartate receptor-mediated inhibitory modulation of the dopaminergic neurotransmission within the MNH. These results indicate that the modulation of dopaminergic function and its interaction with other neurotransmitter systems in a higher associative forebrain region of the juvenile avian brain displays similar neurochemical characteristics as the adult mammalian prefrontal cortex. Furthermore, we were able to show that the pharmacological manipulation of monoaminergic regulatory mechanisms interferes with learning and memory formation, events which in a similar fashion might occur in young or adult mammals.

Effect of neonatal beta-endorphin imprinting on sexual behavior and brain serotonin level in adult rats.

A single dose (3 microg) beta-endorphin was administered to newborn female and male rats (hormonal imprinting). In adult age (at 5 months) sexual behavior, steroid hormone binding capacity and brain serotonin content was studied. Females' sexual activity (lordosis quotient) significantly decreased and more animals protested against mounting (ratio of kicking and crying 21/24 vs. 8/24; p < 0.001). Males' sexual activity did not change, however more males were aggressive (4/10 vs. 1/10). Uterine estrogen receptor density significantly increased and affinity decreased. There was no change in the binding capacity of thymic glucocorticoid receptors. In the brain, five regions were studied for serotonin content. There was a gender difference in serotonin level and the intragroup differences were also high. In the endorphin treated males the serotonin level was significantly lower than in the controls. In the endorphin treated females the intragroup scattering has been significantly reduced. Nociceptin content of the cerebrospinal fluid was not changed. The experiments call attention to the possibility of adjustment of sexual and behavioral sphere by the individually different endorphin surge during labor.