Astrocytic involvement in learning and memory consolidation

Ultrastructural characterization of noradrenergic axons and Beta-adrenergic receptors in the lateral nucleus of the amygdala

Norepinephrine (NE) is thought to play a key role in fear and anxiety, but its role in amygdala-dependent Pavlovian fear conditioning, a major model for understanding the neural basis of fear, is poorly understood. The lateral nucleus of the amygdala (LA) is a critical brain region for fear learning and regulating the effects of stress on memory. To understand better the cellular mechanisms of NE and its adrenergic receptors in the LA, we used antibodies directed against dopamine beta-hydroxylase (DβH), the synthetic enzyme for NE, or against two different isoforms of the beta-adrenergic receptors (βARs), one that predominately recognizes neurons (βAR 248) and the other astrocytes (βAR 404), to characterize the microenvironments of DβH and βAR. By electron microscopy, most DβH terminals did not make synapses, but when they did, they formed both asymmetric and symmetric synapses. By light microscopy, βARs were present in both neurons and astrocytes. Confocal microscopy revealed that both excitatory and inhibitory neurons express βAR248. By electron microscopy, βAR 248 was present in neuronal cell bodies, dendritic shafts and spines, and some axon terminals and astrocytes. When in dendrites and spines, βAR 248 was frequently concentrated along plasma membranes and at post-synaptic densities of asymmetric (excitatory) synapses. βAR 404 was expressed predominately in astrocytic cell bodies and processes. These astrocytic processes were frequently interposed between unlabeled terminals or ensheathed asymmetric synapses. Our findings provide a morphological basis for understanding ways in which NE may modulate transmission by acting via synaptic or non-synaptic mechanisms in the LA.

Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

Stimulation of β-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK(1/2) ) via G(s)/G(i) switching or β-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the β(1)/β(2) -adrenergic agonist isoproterenol, that acted on β(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on β(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca(2+) release from intracellular stores and G(iα)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, β-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK(1/2) phosphorylation. ERK(1/2) phosphorylation by Src activation depended on β-arrestin 2, but not β-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.

Blockade of astrocytic glutamate uptake in rats induces signs of anhedonia and impaired spatial memory

Mood disorders are associated with regional brain abnormalities, including reductions in glial cell and neuron number, glutamatergic irregularities, and differential patterns of brain activation. Because astrocytes are modulators of neuronal activity and are important in trafficking the excitatory neurotransmitter glutamate, it is possible that these pathologies are interrelated and contribute to some of the behavioral signs that characterize depression and related disorders. We tested this hypothesis by determining whether depressive-like signs were induced by blocking central astrocytic glutamate uptake with the astrocytic glutamate transporter (GLT-1) inhibitor, dihydrokainic acid (DHK), in behavioral tests that quantify aspects of mood, including reward and euthymia/dysthymia: intracranial self-stimulation (ICSS) and place conditioning. We found that DHK elevated ICSS thresholds, a depressive-like effect that could reflect reduced sensitivity to reward (anhedonia) or increased aversion (dysphoria). However, DHK treatment did not establish conditioned place aversions, suggesting that this treatment does not induce dysphoria. To identify the brain regions mediating the behavioral effects of DHK, we examined c-Fos expression in areas implicated in motivation and emotion. DHK increased c-Fos expression in many of these regions. The dentate gyrus of the hippocampus was robustly activated, which led us to explore whether DHK alters hippocampal learning. DHK impaired spatial memory in the MWM. These findings identify disruption of astrocyte glutamate uptake as one component of the complex circuits that mediate anhedonia and cognitive impairment, both of which are common symptoms of depression. These finding may have implications for the etiology of depression and other disorders that share the features of anhedonia and cognitive impairment.

Noradrenaline release in the locus coeruleus modulates memory formation and consolidation; roles for α- and β-adrenergic receptors

Noradrenaline, essential for the modulation of memory, is released in various parts of the brain from nerve terminals controlled by the locus coeruleus (LoC). Noradrenaline release consequent upon input from higher brain areas also occurs within the LoC itself. We examined the effect of noradrenaline on adrenergic receptors in the LoC on memory processing, using colored bead discrimination learning in the young domestic chick. We have shown previously that the release of noradrenaline in the hippocampus and cortex (mesopallium) is essential for acquisition and consolidation of short-term to intermediate and to long-term memory. Noradrenaline release within the LoC is triggered by the glutamatergic input from the forebrain. Inhibition by LoC injection of NMDA or AMPA receptor antagonists is rescued by injection of β2-and β3-adrenoceptor (AR) agonists in the hippocampus. We show that inhibition of α2A-ARs by BRL44408 in the LoC up to 30 min post-training consolidates weakly-reinforced learning. Conversely activation of α2A-ARs in the LoC at the times of consolidation between short-term and intermediate and long-term memory caused memory loss, which is likely to be due to a decreased release of noradrenaline within these two time windows. The α2A-AR antagonist will block presynaptic inhibitory receptors leading to an increase in extracellular noradrenaline. This interpretation is supported by the actions of noradrenaline uptake blockers that produce the same memory outcome. BRL44408 in the mesopallium also caused memory enhancement. β2-ARs are important in the first time window, whereas α1-, α2C-and β3-ARs are important in the second time window. The results reveal that for successful memory formation noradrenaline release is necessary within the LoC as well as in other brain regions, at the time of consolidation of memory from short-term to intermediate and from intermediate to long-term memory.

Astrocytes and human cognition: modeling information integration and modulation of neuronal activity

Recent research focusing on the participation of astrocytes in glutamatergic tripartite synapses has revealed mechanisms that support cognitive functions common to human and other mammalian species, such as learning, perception, conscious integration, memory formation/retrieval and the control of voluntary behavior. Astrocytes can modulate neuronal activity by means of release of glutamate, d-serine, adenosine triphosphate and other signaling molecules, contributing to sustain, reinforce or depress pre- and post-synaptic membranes. We review molecular mechanisms present in tripartite synapses and model the cognitive role of astrocytes. Single protoplasmic astrocytes operate as a "Local Hub", integrating information patterns from neuronal and glial populations. Two mechanisms, here modeled as the "domino" and "carousel" effects, contribute to the formation of intercellular calcium waves. As waves propagate through gap junctions and reach other types of astrocytes (interlaminar, polarized, fibrous and varicose projection), the active astroglial network functions as a "Master Hub" that integrates results of distributed processing from several brain areas and supports conscious states. Response of this network would define the effect exerted on neuronal plasticity (membrane potentiation or depression), behavior and psychosomatic processes. Theoretical results of our modeling can contribute to the development of new experimental research programs to test cognitive functions of astrocytes.

Alcohol administration during adulthood induces alterations of parvalbumin and glial fibrillary acidic protein immunoreactivity in rat hippocampus and cingulate cortex

Alcohol induces impairment of cognition, learning and memory. Neurotoxic effects of alcohol on the pathology of the hippocampus and the cingulate cortex were investigated in experimental rats. Parvalbumin (PV), a calcium-binding protein, is a crucial component of GABAergic neurons and glial fibrillary acidic protein immunoreactive (GFAP-ir) astrocytes have been used as markers. We investigated the effects of ethanol exposure during adulthood on the PV-ir neurons and GFAP-ir astrocytes in the hippocampus and the cingulate cortex of 3-month-old male Wistar rats. The rats were divided into 2 groups: control (C) and alcohol-exposed groups. The control group received distilled water whereas the alcohol-exposed groups received either a low dose (20%w/v, LD) or high dose (40%w/v, HD) of ethanol for periods of 21 days, 3 or 6 months. The brains of the animals were processed for immunohistochemistry using anti-parvalbumin and anti-GFAP antibodies and the numbers of PV immunoreactive (PV-ir) neurons and GFAP-ir astrocytes were counted/unit area. For each period of administration, the number of PV-ir neurons was significantly reduced for groups exposed to both the low and the high doses of ethanol compared to those of control groups in both the hippocampus and the cingulate cortex (p<0.01). In addition, the number of PV-ir neurons was progressively reduced after prolonged ethanol exposure. In contrast, there was a significantly increased number of GFAP-ir astrocytes observed in the hippocampus and the cingulate cortex in all groups exposed to ethanol and this was a function of both the duration and the dose of ethanol exposure, indicating that PV-ir neurons are as sensitive as the GFAP-ir astrocytes to ethanol exposure. Our data indicate that alcohol exposure induced a reduction of PV-ir neurons and an increase of GFAP-ir astrocytes in the hippocampus and the cingulate cortex and this may be associated with the impairment of cognition, learning and memory after chronic alcohol administration.

Serotonin increases ERK1/2 phosphorylation in astrocytes by stimulation of 5-HT2B and 5-HT2C receptors

We have previously shown that fluoxetine causes ERK(1/2) phosphorylation in cultured mouse astrocytes mediated exclusively by stimulation of 5-HT(2B) receptors (Li et al., 2008b). This raises the question whether this is also the case for serotonin (5-HT) itself. In the present study serotonin was found to induce ERK(1/2) phosphorylation by stimulation of 5-HT(2B) receptors with high affinity (EC(50): 20-30 pM), and by stimulation of 5-HT(2C) receptor with low affinity (EC(50): 1 microM or higher). ERK(1/2) phosphorylation induced by stimulation of either 5-HT(2B) or 5-HT(2C) receptors was mediated by epidermal growth factor (EGF) receptor transactivation (Peng et al., this issue), shown by the inhibitory effect of AG1478, an inhibitor of the EGF receptor tyrosine kinase, and GM6001, an inhibitor of Zn-dependent metalloproteinases, and thus of 5-HT(2B) receptor-mediated EGF receptor agonist release. It is discussed that the high potency of the 5-HT(2B)-mediated effect is consistent with literature data for binding affinity of serotonin to cloned human 5-HT(2B) receptors and with observations of low extracellular concentrations of serotonin in brain, which would allow a demonstrated moderate and modality-dependent increase in specific brain areas to activate 5-HT(2B) receptors. In contrast the relevance of the observed 5-HT(2C) receptors on astrocytes is questioned.

Astrocytic adrenoceptors and learning: alpha1-adrenoceptors

Noradrenergic receptors are expressed on both on astrocytes and neurons and noradrenergic activation of astrocytic beta(2)- and beta(3)-adrenoceptors are necessary for memory consolidation. In this paper, we marshal evidence for astrocytic alpha(1)-adrenoceptor involvement in memory consolidation. We examine the role of alpha(1)-adrenoceptors in hippocampal and mesopallial (cortical) memory processing using a discriminative avoidance task in the day-old chick. The selective alpha(1)-adrenoceptor agonist, methoxamine, caused the consolidation of weakly-reinforced memory at the time of transition of short-term to intermediate memory and at the time of transition of intermediate to long-term memory. The selective antagonist prazosin prevented memory consolidation at these two times. Blockade of memory by injection of an alpha(2)-adrenoceptor agonist into the LoC could be overcome by mesopallial or hippocampal injection of alpha(1)-, beta(2)- and beta(3)-adrenoceptor agonists. The results of studies where we challenged the ability of methoxamine to promote consolidation by pre-administration of astrocytic metabolic inhibitors of glycogenolysis or oxidative metabolism, suggest that the alpha(1)-adrenoceptor effect is astrocytic. This conclusion is supported by the finding that co-administration of suboptimal doses of methoxamine and thrombin have an additive effect on promoting consolidation. Thrombin causes a calcium response in cultured chick astrocytes but not in neurons. Thrombin, like methoxamine, promotes consolidation at the transition points between short-term, intermediate memory and long-term memory stages. Thrombin enhancement of memory consolidation is blocked by an alpha(1)-adrenoceptor antagonist but not by antagonists of beta(2)- or beta(3)-adrenoceptors. In summary, noradrenaline activation of alpha(1)-adrenoceptors is necessary for consolidation from both short-term and intermediate memory in both the hippocampus and the mesopallium in the chick. Evidence is presented suggesting that the memory consolidating action of alpha(1)-adrenoceptor stimulation results from receptors located on astroctyes and involves an increase in free cytosolic calcium from internal stores.

Molluscan memory of injury: evolutionary insights into chronic pain and neurological disorders

Molluscan preparations have yielded seminal discoveries in neuroscience, but the experimental advantages of this group have not, until now, been complemented by adequate molecular or genomic information for comparisons to genetically defined model organisms in other phyla. The recent sequencing of the transcriptome and genome of Aplysia californica, however, will enable extensive comparative studies at the molecular level. Among other benefits, this will bring the power of individually identifiable and manipulable neurons to bear upon questions of cellular function for evolutionarily conserved genes associated with clinically important neural dysfunction. Because of the slower rate of gene evolution in this molluscan lineage, more homologs of genes associated with human disease are present in Aplysia than in leading model organisms from Arthropoda (Drosophila) or Nematoda (Caenorhabditis elegans). Research has hardly begun in molluscs on the cellular functions of gene products that in humans are associated with neurological diseases. On the other hand, much is known about molecular and cellular mechanisms of long-term neuronal plasticity. Persistent nociceptive sensitization of nociceptors in Aplysia displays many functional similarities to alterations in mammalian nociceptors associated with the clinical problem of chronic pain. Moreover, in Aplysia and mammals the same cell signaling pathways trigger persistent enhancement of excitability and synaptic transmission following noxious stimulation, and these highly conserved pathways are also used to induce memory traces in neural circuits of diverse species. This functional and molecular overlap in distantly related lineages and neuronal types supports the proposal that fundamental plasticity mechanisms important for memory, chronic pain, and other lasting alterations evolved from adaptive responses to peripheral injury in the earliest neurons. Molluscan preparations should become increasingly useful for comparative studies across phyla that can provide insight into cellular functions of clinically important genes.

Chronic vs. acute interactions between supraoptic oxytocin neurons and astrocytes during lactation: role of glial fibrillary acidic protein plasticity

In this article, we review studies of astrocytic-neuronal interactions and their effects on the activity of oxytocin (OXT) neurons within the magnocellular hypothalamo-neurohypophysial system. Previous work over several decades has shown that withdrawal of astrocyte processes increases OXT neuron excitability in the hypothalamic supraoptic nucleus (SON) during lactation. However, chronically disabling astrocyte withdrawal does not significantly affect the functioning of OXT neurons during suckling. Nevertheless, acute changes in a cytoskeletal element of astrocytes, glial fibrillary acidic protein (GFAP), occur in concert with changes in OXT neuronal activity during suckling. Here, we compare these changes in GFAP and related proteins with chronic changes that persist throughout lactation. During lactation, a decrease in GFAP levels accompanies retraction of astrocyte processes surrounding OXT neurons in the SON, resulting from high extracellular levels of OXT. During the initial stage of suckling, acute increases in OXT levels further strengthen this GFAP reduction and facilitate the retraction of astrocyte processes. This change, in turn, facilitates burst discharges of OXT neurons and leads to a transient increase in excitatory neurochemicals. This transient neurochemical surge acts to reverse GFAP expression and results in postburst inhibition of OXT neurons. The acute changes in astrocyte GFAP levels seen during suckling likely recur periodically, accompanied by rhythmic changes in glutamate metabolism, water transport, gliotransmitter release, and spatial relationships between astrocytes and OXT neurons. In the neurohypophysis, astrocyte retraction and reversal with accompanying GFAP plasticity also likely occur during lactation and suckling, which facilitates OXT release coordinated with its action in the SON. These studies of the dynamic interactions that occur between astrocytes and OXT neurons mediated by GFAP extend our understanding of astrocyte functions within the central nervous system.

Long-term memory shapes the primary olfactory center of an insect brain

The storage of stable memories is generally considered to rely on changes in the functional properties and/or the synaptic connectivity of neural networks. However, these changes are not easily tractable given the complexity of the learning procedures and brain circuits studied. Such a search can be narrowed down by studying memories of specific stimuli in a given sensory modality and by working on networks with a modular and relatively simple organization. We have therefore focused on associative memories of individual odors and the possible related changes in the honeybee primary olfactory center, the antennal lobe (AL). As this brain structure is organized in well-identified morpho-functional units, the glomeruli, we looked for evidence of structural and functional plasticity in these units in relation with the bees' ability to store long-term memories (LTMs) of specific odors. Restrained bees were trained to form an odor-specific LTM in an appetitive Pavlovian conditioning protocol. The stability and specificity of this memory was tested behaviorally 3 d after conditioning. At that time, we performed both a structural and a functional analysis on a subset of 17 identified glomeruli by measuring glomerular volume under confocal microscopy, and odor-evoked activity, using in vivo calcium imaging. We show that long-term olfactory memory for a given odor is associated with volume increases in a subset of glomeruli. Independent of these structural changes, odor-evoked activity was not modified. Lastly, we show that structural glomerular plasticity can be predicted based on a putative model of interglomerular connections.

What learning in day-old chickens can teach a neurochemist: focus on astrocyte metabolism

The learning process sets in motion a prolonged, reproducible, and complicated pattern of brain activation, which provides information about biochemical reactions in activated brain. Study of this pattern during one-trial aversive bead discrimination in day-old chick is facilitated by precise timing of sequential metabolic events occurring between a 10-s learning period, in which the chicks learn to associate a red bead with aversive taste, and memory consolidation, indicated by unwillingness to peck at untainted red beads while freely pecking at corresponding blue beads. Inhibition of learning by metabolic inhibitors and restoration of memory by specific substrates at specific times allow determination of specific metabolic events and their neuronal or astrocytic localization. Downstream metabolism of glycogen and of glucose to pyruvate/lactate is segregated into separate pools. Glucose metabolism via pyruvate dehydrogenation provides energy in both neurons and astrocytes and may include gap junction-mediated lactate transport into astrocytes. A key role is played by glycogenolysis, stimulated by beta2-adrenergic and/or 5-HT2-receptor stimulation along with alpha2-adrenergic stimulation of glycogen synthesis. The importance of glycogen reflects that it selectively supports de novo synthesis of transmitter glutamate by combined pyruvate dehydrogenation and carboxylation in astrocytes.

On the role of synchrony for neuron-astrocyte interactions and perceptual conscious processing

Recent research on brain correlates of cognitive processes revealed the occurrence of global synchronization during conscious processing of sensory stimuli. In spite of technological progress in brain imaging, an explanation of the computational role of synchrony is still a highly controversial issue. In this study, we depart from an analysis of the usage of blood-oxygen-level-dependent functional magnetic resonance imaging for the study of cognitive processing, leading to the identification of evoked local field potentials as the vehicle for sensory patterns that compose conscious episodes. Assuming the "astrocentric hypothesis" formulated by James M. Robertson (astrocytes being the final stage of conscious processing), we propose that the role of global synchrony in perceptual conscious processing is to induce the transfer of information patterns embodied in local field potentials to astrocytic calcium waves, further suggesting that these waves are responsible for the "binding" of spatially distributed patterns into unitary conscious episodes.

Astrocytes going live: advances and challenges

Astrocytes are one of the most numerous cell types in the CNS. They have emerged as sophisticated cells participating in a large and diverse variety of functions vital for normal brain development, adult physiology and pathology. Recent in vivo studies have provided exciting new insight into astrocyte physiology in the intact healthy brain. This review will summarize some of their most intriguing findings, discuss some of their implications, and look ahead at some of the challenges we face in studying astrocyte function in vivo.

Imbalance between excitatory and inhibitory amino acids at spinal level is associated with maintenance of persistent pain-related behaviors

Although the postsynaptic events responsible for development of pathological pain have been intensively studied, the relative contribution of presynaptic neurotransmitters to the whole process remains less elucidated. In the present investigation, we sought to measure temporal changes in spinal release of both excitatory amino acids (EAAs, glutamate and aspartate) and inhibitory amino acids (IAAs, glycine, ?-aminobutyric acid and taurine) in response to peripheral inflammatory pain state. The results showed that following peripheral chemical insult induced by subcutaneous bee venom (BV) injection, there was an initial, parallel increase in spinal release of both EAAs and IAAs, however, the balance between them was gradually disrupted when pain persisted longer, with EAAs remaining at higher level but IAAs at a level below the baseline. Moreover, the EAAs-IAAs imbalance at the spinal level was dependent upon the ongoing activity from the peripheral injury site. Intrathecal blockade of ionotropic (NMDA and non-NMDA) and metabotropic (mGluRI, II, III) glutamate receptors, respectively, resulted in a differential inhibition of BV-induced different types of pain (persistent nociception vs. hyperalgesia, or thermal vs. mechanical hyperalgesia), implicating that spinal antagonism of any specific glutamate receptor subtype fails to block all types of pain-related behaviors. This result provides a new line of evidence emphasizing an importance of restoration of EAAs-IAAs balance at the spinal level to prevent persistence or chronicity of pain.

Non-invasive quantification of brain glycogen absolute concentration

The only currently available method to measure brain glycogen in vivo is 13C NMR spectroscopy. Incorporation of 13C-labeled glucose (Glc) is necessary to allow glycogen measurement, but might be affected by turnover changes. Our aim was to measure glycogen absolute concentration in the rat brain by eliminating label turnover as variable. The approach is based on establishing an increased, constant 13C isotopic enrichment (IE). 13C-Glc infusion is then performed at the IE of brain glycogen. As glycogen IE cannot be assessed in vivo, we validated that it can be inferred from that of N-acetyl-aspartate IE in vivo: After [1-13C]-Glc ingestion, glycogen IE was 2.2 +/- 0.1 fold that of N-acetyl-aspartate (n = 11, R(2) = 0.77). After subsequent Glc infusion, glycogen IE equaled brain Glc IE (n = 6, paired t-test, p = 0.37), implying isotopic steady-state achievement and complete turnover of the glycogen molecule. Glycogen concentration measured in vivo by 13C NMR (mean +/- SD: 5.8 +/- 0.7 micromol/g) was in excellent agreement with that in vitro (6.4 +/- 0.6 micromol/g, n = 5). When insulin was administered, the stability of glycogen concentration was analogous to previous biochemical measurements implying that glycogen turnover is activated by insulin. We conclude that the entire glycogen molecule is turned over and that insulin activates glycogen turnover.

Common key-signals in learning and neurodegeneration: focus on excito-amino acids, beta-amyloid peptides and alpha-synuclein

In this paper a hypothesis that some special signals ("key-signals" excito-amino acids, beta-amyloid peptides and alpha-synuclein) are not only involved in information handling by the neuronal circuits, but also trigger out substantial structural and/or functional changes in the Central Nervous System (CNS) is introduced. This forces the neuronal circuits to move from one stable state towards a new state, but in doing so these signals became potentially dangerous. Several mechanisms are put in action to protect neurons and glial cells from these potentially harmful signals. However, in agreement with the Red Queen Theory of Ageing (Agnati et al. in Acta Physiol Scand 145:301-309, 1992), it is proposed that during ageing these neuroprotective processes become less effective while, in the meantime, a shortage of brain plasticity occurs together with an increased need of plasticity for repairing the wear and tear of the CNS. The paper presents findings supporting the concept that such key-signals in instances such as ageing may favour neurodegenerative processes in an attempt of maximizing neuronal plasticity.

Importance of adrenergic receptors in prenatally induced cognitive impairment in the domestic chick

In the domestic chick, mild hypoxia (24h of 14% oxygen) at two stages of embryonic development results in post-hatch memory deficiencies tested using a discriminated bead avoidance task. The nature of the memory loss depends on the gestational age at which the hypoxia occurs. Hypoxia on embryonic day 10 (E10) of a 21 day incubation results in chicks with no short-term memory 10 min after training, whereas hypoxia on day 14 (E14) results in chicks with good labile memory 30 min after training but no consolidation of memory into permanent storage (120 min). Hypoxia at E14 is associated with increased plasma levels of noradrenaline and therefore we suggest that altered catecholamine exposure within the brain contributes to cognitive problems by modifying the responsiveness of brain beta-adrenoceptors. In ovo administration of noradrenaline, or the beta(2)-adrenoceptor agonist formoterol, at E14 had the same effect on memory consolidation as hypoxia. Following hypoxia at E14, memory could be rescued after training by central injection of a beta(3)-adrenoceptor agonist, but not by a beta(2)-adrenoceptor agonist. The differences in the responsiveness of memory processing to beta(2)-adrenoceptor agonists suggests alterations to the receptors or downstream of the receptor activation. However, both types of beta-adrenoceptor agonists rescued memory in E10 treated chicks implying that at this age hypoxia does not affect the receptors. In summary, hypoxia or increased levels of stress hormones during incubation alters beta-adrenoceptor responsiveness; the outcome of the insult depends upon the cellular developmental processes at a given embryonic stage.