Early Olfactory Learning Research Articles

The maturational characteristics of the GABA input in the anterior piriform cortex may also contribute to the rapid learning of the maternal odor during the sensitive period.

During the first ten postnatal days (P), infant rodents can learn olfactory preferences for novel odors if they are paired with thermo-tactile stimuli that mimic components of maternal care. After P10, the thermo-tactile pairing becomes ineffective for conditioning. The current explanation for this change in associative learning is the alteration in the norepinephrine (NE) inputs from the locus coeruleus (LC) to the olfactory bulb (OB) and the anterior piriform cortex (aPC). By combining patch-clamp electrophysiology and computational simulations, we showed in a recent work that a transitory high responsiveness of the OB-aPC circuit to the maternal odor is an alternative mechanism that could also explain early olfactory preference learning and its cessation after P10. That result relied solely on the maturational properties of the aPC pyramidal cells. However, the GABAergic system undergoes important changes during the same period. To address the importance of the maturation of the GABAergic system for early olfactory learning, we incorporated data from the GABA inputs, obtained from in vitro patch-clamp experiment in the aPC of rat pups aged P5–P7 reported here, to the model proposed in our previous publication. In the younger than P10 OB-aPC circuit with GABA synaptic input, the number of responsive aPC pyramidal cells to the conditioned maternal odor was amplified in 30% compared to the circuit without GABAergic input. When compared with the circuit with other younger than P10 OB-aPC circuit with adult GABAergic input profile, this amplification was 88%. Together, our results suggest that during the olfactory preference learning in younger than P10, the GABAergic synaptic input presumably acts by depolarizing the aPC pyramidal neurons in such a way that it leads to the amplification of the pyramidal neurons response to the conditioned maternal odor. Furthermore, our results suggest that during this developmental period, the aPC pyramidal cells themselves seem to resolve the apparent lack of GABAergic synaptic inhibition by a strong firing adaptation in response to increased depolarizing inputs.

Maturation of pyramidal cells in anterior piriform cortex may be sufficient to explain the end of early olfactory learning in rats.

Studies have shown that neonate rodents exhibit high ability to learn a preference for novel odors associated with thermo-tactile stimuli that mimics maternal care. Artificial odors paired with vigorous strokes in rat pups younger than 10 postnatal days (P), but not older, rapidly induce an orientation-approximation behavior toward the conditioned odor in a two-choice preference test. The olfactory bulb (OB) and the anterior olfactory cortex (aPC), both modulated by norepinephrine (NE), have been identified as part of a neural circuit supporting this transitory olfactory learning. One possible explanation at the neuronal level for why the odor-stroke pairing induces consistent orientation-approximation behavior in <P10 pups, but not in >P10, is the coincident activation of prior existent neurons in the aPC mediating this behavior. Specifically, odor-stroke conditioning in <P10 pups may activate more mother/nest odor's responsive aPC neurons than in >P10 pups, promoting orientation-approximation behavior in the former but not in the latter. In order to test this hypothesis, we performed in vitro patch-clamp recordings of the aPC pyramidal neurons from rat pups from two age groups (P5–P8 and P14–P17) and built computational models for the OB-aPC neural circuit based on this physiological data. We conditioned the P5–P8 OB-aPC artificial circuit to an odor associated with NE activation (representing the process of maternal odor learning during mother–infant interactions inside the nest) and then evaluated the response of the OB-aPC circuit to the presentation of the conditioned odor. The results show that the number of responsive aPC neurons to the presentation of the conditioned odor in the P14–P17 OB-aPC circuit was lower than in the P5–P8 circuit, suggesting that at P14–P17, the reduced number of responsive neurons to the conditioned (maternal) odor might not be coincident with the responsive neurons for a second conditioned odor.

The Role of L-type Calcium Channels in Olfactory Learning and Its Modulation by Norepinephrine

L type calcium channels (LTCCs) are prevalent in different systems and hold immense importance for maintaining/performing selective functions. In the nervous system, CaV1.2 and CaV1.3 are emerging as critical modulators of neuronal functions. Although the general role of these calcium channels in modulating synaptic plasticity and memory has been explored, their role in olfactory learning is not well understood. In this review article we first discuss the role of LTCCs in olfactory learning especially focusing on early odor preference learning in neonate rodents, presenting evidence that while NMDARs initiate stimulus-specific learning, LTCCs promote protein-synthesis dependent long-term memory (LTM). Norepinephrine (NE) release from the locus coeruleus (LC) is essential for early olfactory learning, thus noradrenergic modulation of LTCC function and its implication in olfactory learning is discussed here. We then address the differential roles of LTCCs in adult learning and learning in aged animals.

Chemical Identification of "Maternal Signature Odors" in Rat.

Newborn altricial mammals need just after birth to locate their mother's nipples for suckling. In this precocious behavior, including for the human baby, maternal odor via the olfactory process plays a major role. Maternal odor emitted by lactating females or by amniotic fluid (AF) attracts pups, but the chemical identity of this attractant has not yet been elucidated. Here, using behavioral tests and gas chromatography coupled with mass spectrometry (GC-MS) techniques, we show that AF extracts from rat pregnant female, nipples, ventral skin, milk, and nest extracts of mother contained 3-6 active substances. AF extracts contained 3 active compounds: ethylbenzene, benzaldehyde, and benzyl alcohol, and their mixture in similar proportions to those found in AF extracts, in a ratio, respectively, of 1:1:12 (700 ng), attracts pups as putative maternal attractant substances (MAS). These 3 AF substances have already been identified in milk, nipples, ventral wash, and nest extracts of mother, but not in feces. Moreover, anethole flavor incorporated in pregnant rat and mother's diet is also detected in AF, nipples, milk, and nest extracts and the pups are attracted to anethole odor, but not in the case of the no-anethole pups. MAS, combined with diet flavors present in the AF bath, represent olfactory signals as "maternal signature odors" (MSO) that are learned by fetus and pups. These findings open the way to improved understanding of the neurobiology of early olfactory learning and of the importance of evolutionarily conserved survival behavior in many mammal species.

Maternal Deprivation in Early Ontogeny Impairs Olfactory Learning with Mother’s Grooming Imitation in 129Sv Mice

Early experience, mediated by mother's care, exerts long-term effects on the formation of behavioral phenotype. However, there are no published data on the effects of such experience on the results of early learning. We investigated the effects of maternal deprivation associated with handling in 129Sv mice during postnatal days 3-6 on the results of olfactory learning with mother's grooming imitation used as the reinforcement on postnatal day 8. Mother deprivation and handling procedure are shown to impair early olfactory learning.

Early olfactory experience induces structural changes in the primary olfactory center of an insect brain

The antennal lobe (AL) is the first olfactory center of the insect brain and is constituted of different functional units, the glomeruli. In the AL, odors are coded as spatiotemporal patterns of glomerular activity. In honeybees, olfactory learning during early adulthood modifies neural activity in the AL on a long-term scale and also enhances later memory retention. By means of behavioral experiments, we first verified that olfactory learning between the fifth and eighth day of adulthood induces better retention performances at a late adult stage than the same experience acquired before or after this period. We checked that the specificity of memory for the odorants used was improved. We then studied whether such early olfactory learning also induces long-term structural changes in the AL consistent with the formation of long-term olfactory memories. We also measured the volume of 15 identified glomeruli in the ALs of 17-day-old honeybees that either experienced an odor associated with sucrose solution between the fifth and eighth day of adulthood or were left untreated. We found that early olfactory experience induces glomerulus-selective increases in volume that were specific to the learned odor. By comparing our volumetric measures with calcium-imaging recordings from a previous study, performed in 17-day-old bees subjected to the same treatment and experimental conditions, we found that glomeruli that showed structural changes after early learning were those that exhibited a significant increase in neural activity. Our results make evident a correlation between structural and functional changes in the AL following early olfactory learning.

Olfactory learning in the rat immediately after birth: Unique salience of first odors.

An infant rat's chance of survival is increased when it remains close to the nest. Early olfactory learning supports such adaptive behavior. Previous experiments indicated that non-associative odor exposure immediately after birth promoted later attachment to a similarly scented artificial nipple. The goal of the current experiments was to extend these findings on olfactory learning in the hours after birth by: exposing pups to more than one odor exposure (Experiment 1), dissecting the role of timing versus order of odor exposure (Experiment 2), testing the odor specificity of these effects (Experiments 3 and 4), and evaluating associative odor conditioning soon after birth (Experiment 5). Without explicit prior odor experience, pups only hours old do not respond much to a novel odor. Prior non-associative odor experience increases later motor activity to that same odor and to novel odors. Furthermore, these findings may be specific to certain amodal dimensions of the (in our case) lemon odor exposure. Single odor non-associative and associative conditioning was equally effective immediately after birth and during the third postnatal hour. Nevertheless, pups given two mere odor exposures responded to the first one more than the second at test, regardless of whether the exposures began immediately or 2 hr after birth. Possible mechanisms for these findings concerning early olfactory learning are discussed.

A Pheromone That Rapidly Promotes Learning in the Newborn

Mammalian neonates depend on their mother's food supply and use a defined sequence of actions to find her mammary area. Their behavior is initially uncertain and demanding but rapidly becomes optimal. Efficient learning is thus operating in newborns. For instance, European rabbit (Oryctolagus cuniculus) pups localize the nipples through typical orocephalic movements. These movements are released by the mammary pheromone secreted in milk or by prenatally learned odor cues. During daily nursing, they also learn odors associated with the mother, supposedly with sucking as the main reinforcer. We here investigate the role of the mammary pheromone as an enforcer of early olfactory learning in newborn rabbits. In testing more than 950 pups, we show that the mammary pheromone promotes learning of neutral odorants paired with the pheromone in single and short trials. The pheromone-induced learning is efficient from birth and supports successive acquisition of distinct odorants. This reveals that a mammalian pheromone can function as a "cognitive organizer" that promotes early learning of relevant environmental cues.

Sniffing Out NMDA Receptors in the Olfactory Cortex

Selective olfactory learning is essential for survival in most newborn mammals. Findings by Franks and Isaacson in this issue of Neuron suggest that early olfactory learning might be selective, in part because olfactory experience downregulates NMDA receptors at primary inputs to the olfactory cortex.

Optical imaging of odor preference memory in the rat olfactory bulb.

Early olfactory preference learning in rat pups occurs when novel odors are paired with reinforcing tactile stimulation that activate the noradrenergic locus coeruleus. Pairing of odor and a noradrenergic agonist in the olfactory bulb is both necessary and sufficient for odor preference learning. This suggests the memory change occurs in the olfactory bulb. Previous electrophysiological experiments demonstrated that odor preference training induces an increase in the field excitatory postsynaptic potential to olfactory nerve input and an alteration, after training, in glomerular [14C]2- deoxyglucose uptake and in single-unit responses of principal cells. We investigate here whether, 24 h after olfactory preference training, there is an alteration in intrinsic optical signals at the glomerular level. Six-day-old rat pups were trained, as previously, for a peppermint odor preference. Trained pups and control littermates were subjected to imaging of odor-induced intrinsic optical signals 1 day after the training session. Trained pups exhibited significantly larger responses to the peppermint compared with untrained littermates previously exposed to the same odor. The response of trained pups to a control odor (amyl acetate) was, however, not significantly different from that of untrained littermates. These observations demonstrate that odor preference memory can be read-out by optical imaging techniques.

Sucking, not milk, is important for the rapid learning of nipple-search odors in newborn rabbits.

Newborn rabbits are only nursed once a day for 3 to 4 min and are completely dependent on a pheromone on the mother's ventrum for the release of nipple-search behavior and sucking. However, if the mother's ventrum is perfumed, pups can be conditioned in just one 3-min nursing session to respond with nipple searching to the novel odorant. To define more precisely the reinforcing properties of the nursing situation supporting such rapid learning, odor conditioning was conducted in independent groups of 2-day-old pups after successively eliminating potential reinforcing stimuli such as the doe's behavior, milk ingestion, nipple-search behavior, or sucking nipples. All experimental groups showed significant conditioning compared to control groups subjected to similar treatments on unscented does, with the strength of conditioning indicating that behavior of the doe, milk consumption, or arousal during performance of the search behavior were not major reinforcers, but rather the opportunity to suck nipples. Thus, this study suggests intraoral stimulation associated with sucking to be an important reinforcer in this paradigm, as for early olfactory learning in other young mammals.

Endogenous opioids and their role in odor preference acquisition and consolidation following odor-shock conditioning in infant rats.

We assessed the neurochemical basis of olfactory learning induced by presentations of odor and moderate shock in infant rats. Paradoxically, shock conditioning produces an odor preference in 8-day-olds, but an odor aversion in 12-day-olds. Studies have demonstrated the importance of opioids in early olfactory learning; their specific role remains undefined. In this study, postnatal Days 8 and 12 pups were systemically injected with naltrexone, a nonspecific opioid antagonist, or saline and received either paired or backward presentations of odor-moderate shock or odor-only presentations. Blocking the opioid system during conditioning disrupted acquisition of the Day 8 odor preference, but not the Day 12 odor aversion. Additional Day 8 pups were given naltrexone posttraining. Naltrexone not only blocked consolidation of an odor preference but also yielded an odor aversion. These results suggest that the opioid system has a critical role in both olfactory learning and consolidation of odor preferences during the sensitive period.

Facilitatory effect of ritanserin is mediated by dopamine D1 receptors on olfactory learning in young rats

The olfactory bulb is critically involved in early olfactory learning. In this study, we examined the effect of intrabulbar infusion of ritanserin, a 5-hydroxytryptamine(2) (5-HT(2)) receptor antagonist on a one-trial aversive olfactory learning in young rats. Ritanserin, a 5-HT(2) receptor antagonist, was continuously infused into the olfactory bulb of postnatal day-11 (PND 11) rat pups during a 30-min training session of pairing citral odor and foot shock. On the following day, the time spent in the part of the apparatus where the odor was present was measured as an index of odor aversion. Consistent with a previous study on olfactory preference learning, 1 nM ritanserin, but not 10 nM, blocked the olfactory aversive learning. We further examined the ability of 10 nM ritanserin to induce olfactory learning in the absence of the unconditioned stimulus foot shock. Pups that received intrabulbar infusion of 10 nM ritanserin in the presence of citral odor developed an aversion to the odor without foot shock. Since ritanserin has been shown to have an affinity for dopamine receptors, we examined the effect of dopamine antagonists on the ritanserin-induced aversive olfactory learning. Co-infusion of the dopamine D(1) receptor antagonist (+/-)-SKF-83566 with ritanserin dose-dependently prevented induced learning. In contrast, the D(2) receptor antagonist spiperone was without effect. These results extend the previous finding on the role of bulbar 5-HT(2) receptors in early olfactory learning and suggest that high concentration of ritanserin facilitates aversive olfactory learning through D(1) receptors in the olfactory bulb.

Facilitatory effect of ritanserin is mediated by dopamine D1 receptors on olfactory learning in young rats

The olfactory bulb is critically involved in early olfactory learning. In this study, we examined the effect of intrabulbar infusion of ritanserin, a 5-hydroxytryptamine2 (5-HT2) receptor antagonist on a one-trial aversive olfactory learning in young rats. Ritanserin, a 5-HT2 receptor antagonist, was continuously infused into the olfactory bulb of postnatal day-11 (PND 11) rat pups during a 30-min training session of pairing citral odor and foot shock. On the following day, the time spent in the part of the apparatus where the odor was present was measured as an index of odor aversion. Consistent with a previous study on olfactory preference learning, 1 nM ritanserin, but not 10 nM, blocked the olfactory aversive learning. We further examined the ability of 10 nM ritanserin to induce olfactory learning in the absence of the unconditioned stimulus foot shock. Pups that received intrabulbar infusion of 10 nM ritanserin in the presence of citral odor developed an aversion to the odor without foot shock. Since ritanserin has been shown to have an affinity for dopamine receptors, we examined the effect of dopamine antagonists on the ritanserin-induced aversive olfactory learning. Co-infusion of the dopamine D1 receptor antagonist (±)-SKF-83566 with ritanserin dose-dependently prevented induced learning. In contrast, the D2 receptor antagonist spiperone was without effect. These results extend the previous finding on the role of bulbar 5-HT2 receptors in early olfactory learning and suggest that high concentration of ritanserin facilitates aversive olfactory learning through D1 receptors in the olfactory bulb. © 2000 John Wiley & Sons, Inc. Dev Psychobiol 37: 246–252, 2000

PCREB in the neonate rat olfactory bulb is selectively and transiently increased by odor preference-conditioned training.

Early olfactory preference learning in rat pups occurs when novel odors are paired with tactile stimulation, for example stroking. cAMP-triggered phosphorylation of cAMP response element binding protein (pCREB) has been implicated as a mediator of learning and memory changes in various animals (Frank and Greenberg 1994). In the present study we investigate whether CREB is phosphorylated in response to conditioned olfactory training as might be predicted given the proposed role of the phosphorylated protein in learning. On postnatal day 6, pups were trained for 10 min using a standard conditioned olfactory learning paradigm in which a conditioned stimulus, Odor, was either used alone or paired with an unconditioned stimulus, Stroking (using a fine brush to stroke the pup). In some instances stroking only was used. The pups were sacrificed at 0, 10, 30, or 60 min after the training. Using Western blot analysis, we observed that the majority of olfactory bulbs in conditioned pups (Odor + Stroking) had a greater increase in pCREB activation at 10 min after training than pups given nonlearning training (Odor only or Stroking only). The phosphorylated protein levels were low at 0 min and at 60 min after training. This is in keeping with the slightly delayed and short-lived activation period for this protein. The localization of pCREB increases within the olfactory bulb as seen by immunocytochemistry. Naive pups were not exposed to odor or training. There was a significantly higher level of label in mitral cell nuclei within the dorsolateral quadrant of the bulb of pups undergoing odor-stroke pairing. No significant differences were observed among nonlearning groups (Naive, Odor only, or Stroking only) or among any training groups in the granule or periglomerular cells of the dorsolateral region. The localized changes in the nuclear protein are consistent with studies showing localized changes in the bulb in response to a learned familiar odor. The present study demonstrates that selective increases in pCREB occur as an early step following pairing procedures that normally lead to the development of long-term olfactory memories in rat pups. These results support the hypothesized link between pCREB and memory formation.

Norepinephrine and associative conditioning in the neonatal rat somatosensory system

Neonatal rats, aged postnatal days 3–4, were trained in a somatosensory associative conditioning task involving temporal correlation of facial vibrissa stimulation and aversive shock. This training resulted in a subsequent conditioned behavioral activation/arousal response to vibrissa stimulation alone, compared to non-learning control pups trained with random vibrissa–shock presentations. The acquisition of the conditioned response was blocked by systemic injections of the NE β-receptor antagonist propranolol in a dose-dependent manner. In a second study, vibrissa stimulation was paired with systemic injections of the NE β-receptor agonist isoproterenol. Association of vibrissa stimulation with β-receptor activation resulted in subsequent conditioned responses to vibrissa stimulation alone, in a dose-dependent manner. Together, these results suggest that early associative somatosensory conditioning requires and involves NE in a manner similar to that previously demonstrated for early olfactory learning.

Inhibition of Nitric Oxide Synthase Impairs Early Olfactory Associative Learning in Newborn Rats

The present experiments examined the role of nitric oxide ( NO) in early associative olfactory learning in rats. A preference for peppermint odor was induced by pairing peppermint odor with tactile stimulation in Wistar rat pups, in either a repetitive training paradigm or in a one-trial olfactory learning paradigm. In a first experiment we studied the effect of nitric oxide synthase (NOs) inhibition on early olfactory learning in a repetitive paradigm, by systemic daily injections of NG-nitro-l-arginine methyl ester (l-NAME, 50 mg/kg, i.p.). In order to exclude possible deleterous effects of repeated injections ofl-NAME, we explored in a second experiment the effect of a single inhibitor injection in a one-trial olfactory learning paradigm. Inhibition of NOs was performed by either administration ofl-NAME (50 mg/kg, i.p.), or 7-nitroindazole (7-NI, 30 mg/kg, i.p.), a more selective inhibitor of the neuronal NOs. We showed that bothl-NAME and 7-NI impaired early olfactory associative learning when given before training but not before subsequent testing. Considering that NOs neurons are already widespread in the central nervous system (the olfactory bulb included) during the first postnatal week, the sites where NO inhibition may have acted to impair olfactory learning are discussed. The mechanisms of action of NO in relation with other neurotransmitters known to be necessary for olfactory conditioning in rat pups remain to be established. Impairment by NO synthesis inhibition of the acquisition during the first postnatal week of an olfactory conditioning, but not its recall, suggests a role for NO at synapses involved in that learning.

Norepinephrine increases rat mitral cell excitatory responses to weak olfactory nerve input via alpha-1 receptors in vitro

A rat olfactory bulb in vitro slice preparation was used to investigate the actions of norepinephrine on spontaneous and afferent (olfactory nerve) evoked activity of mitral cells. Single olfactory nerve shocks elicited a characteristic mitral cell response consisting of distinct, early and late spiking components separated by a brief inhibitory epoch. Bath-applied norepinephrine (1 μM) increased the early spiking component elicited by perithreshold (79% increase, P<0.02), but not by suprathreshold (3% decrease, P>0.05), intensity olfactory nerve shocks. The facilitatory effect of norepinephrine was due to a reduction in the incidence of response failures to perithreshold intensity shocks. Norepinephrine also decreased the inhibitory epoch separating the early and late spiking components by 44% ( P<0.05). By contrast, norepinephrine had no consistent effect on the spontaneous discharge rate of the mitral cells. The effects of norepinephrine were mimicked by the α1 receptor agonist phenylephrine (1 μM, P<0.001). Both norepinephrine and phenylephrine modulation of mitral cell responses were blocked by the α1 adrenergic antagonist WB-4101 (1 μM). These findings are consistent with observations that the main olfactory bulb exhibits the highest density of α1 receptors in the brain. The α2 receptor agonist clonidine (100 nM) and the β receptor agonist isoproterenol (1 μM) had inconsistent effects on mitral cell spontaneous and olfactory nerve-evoked activity. These results indicate that norepinephrine increases mitral cell excitatory responses to weak but not strong olfactory nerve inputs in vitro via activation of α1 receptors. This is consistent with recent findings in vivo that synaptically released norepinephrine preferentially increases mitral cell excitatory responses to weak olfactory nerve inputs. Taken together, these results suggest that the release of norepinephrine in the olfactory bulb may increase the sensitivity of mitral cells to weak odors. Olfactory cues evoke norepinephrine release in the main olfactory bulb, and norepinephrine plays important roles in early olfactory learning and reproductive/maternal behaviors. By increasing mitral cell responses to olfactory nerve input, norepinephrine may play a critical role in modulating olfactory function, including formation and/or recall of specific olfactory memories.

Fetal Alcohol Syndrome: Early Olfactory Learning as a Model System to Study Neurobehavioral Deficits

The goal of basic research examining the deficits underlying fetal alcohol syndrome is to develop an animal model which allows investigation and assessment of the neural and cognitive impairments resulting from prenatal alcohol exposure. The following review focuses on animal models and their relationship to human deficits following prenatal alcohol exposure. In addition, this review examines a unique, well-established model system which may permit an increased understanding of the role of alcohol on the developing brain and cognitive behavior. Specifically, large metabolic, neurochemical, neuropharmacological, morphological and neurophysiological changes in young rats have been reported as a consequence of early olfactory preference conditioning, a form of learning that normally occurs during both human and rat development. This olfactory odor preference training paradigm can be used to assess changes in learning as well as the neural substrates underlying this learning. Olfactory preference training has been used to examine: 1) learning, as demonstrated by a behavioral preference for an odor previously paired with stimulation which mimics maternal care; 2) metabolism, by measuring 2-deoxyglucose uptake and distribution in response to the trained odor; 3) neurotransmitter levels, by using in vivo microdialysis, to examine changes in neurotransmitter levels in the olfactory bulb in response to a trained odor. Using in vivo microdialysis enables measurement of both baseline responsiveness of alcohol-exposed pups as well as learned responses at several different developmental ages. The established neural features of this olfactory model include an increase in behavioral preference for a trained odor, increases in 2-DG uptake in specific foci within the olfactory bulb in response to the odor, and increases in dopamine in response to olfactory preference training stimuli, as well as conditioned increases in norepinephrine following olfactory preference training. Using these known behavioral, metabolic and neurochemical indices in control pups allows identification of some of the neurotransmitter systems involved in deficits and the neurobiological basis for impairments induced by prenatal alcohol exposure.

Expression of α2 adrenoceptors during rat brain development—I. α2A messenger RNA expression

The distribution of α 2A adrenoceptor messenger RNA expression in developing rat brain was characterized using in situ hybridization with 35S-labeled riboprobes. Intense hybridization signal was detected as early as embryonic day 14 in several areas adjacent to the forebrain and hindbrain germinal zones and in central noradrenergic neurons. A marked increase in messenger RNA expression was observed throughout the brain during late prenatal development, consistent with the migration and maturation of neurons in developing brain structures. In embryonic brain, there was a temporal and spatial correspondence in the appearance of α 2A messenger RNA expression and binding sites labeled with [ 3H]idazoxan or p-[ 125I]iodoclonidine, indicating translation into receptor protein at an early stage of development. Whereas the presynaptic expression remained constant throughout development, there was an early postnatal decline of α 2A receptor expression in many brain regions, including the olfactory bulb, cortex, caudate–putamen, hippocampus, thalamus, hypothalamus and medulla. Thereafter, messenger RNA expression increased, establishing an adult-like pattern during the second postnatal week, but remained low in areas such as the caudate–putamen, thalamus and hippocampus, which do not exhibit extensive expression in the adult. The transient perinatal expression of this α 2 adrenoceptor type, which coincides with a period of hyperreactivity to sensory stimuli in the locus coeruleus, may indicate a specific functional role for the α 2A receptor in the developing rat brain. The early and intense expression in olfactory structures suggests an involvement in early olfactory learning. The pattern of widespread, transient expression of α 2A receptors in the fetal brain is in marked contrast to the postnatal development of the α 2C receptor type.