Norepinephrine and learning-induced plasticity in infant rat olfactory system

Non-specific olfactory aversion induced by intrabulbar infusion of the GABA(A) receptor antagonist bicuculline in young rats

On postnatal day 12, young rats show an aversion to an odor to which they had been exposed along with presentations of foot shock on postnatal day 11. The acquisition of this aversive learning involves and requires disinhibition of the mitral/tufted cells induced by centrifugal noradrenergic activation during somatosensory stimulation. This olfactory learning is established only for the odor to which the rat has been exposed during conditioning. Infusion of the GABA(A) receptor antagonist bicuculline at a high dose (2.0 nmol/each olfactory bulb) into the olfactory bulb in the presence of an odor is capable of developing olfactory aversive responses without somatosensory stimulation in young rats. The purpose of this study is to characterize the properties of bicuculline-induced aversive responses. In contrast to the odor specificity of aversive learning produced by odor-shock conditioning, bicuculline-induced aversive responses lack odor specificity. Namely, bicuculline infusion in the presence of a citral odor results, in a dose-dependent manner, in subsequent aversive responses to strange odors (benzaldehyde and vanillin) that have never been presented. Moreover, bicuculline infusion alone is sufficient to produce dose-dependent aversive responses to strange odors (citral, benzaldehyde and geraniol). From these results we suggest that disinhibition of mitral/tufted cells from granule cells by bicuculline infusion makes young rats aversive to strange odors non-specifically, as if the rats had learned the odor aversion as a result of odor exposure paired with foot shock. Different mechanisms of disinhibition of the mitral/tufted cells may underlie both the pharmacological manipulation and noradrenergic activation by somatosensory stimulation.

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.

Social and individual recognition in rodents: Methodological aspects and neurobiological bases

What animals know about each other, and how they construct and use knowledge of their social world involves at least an ability to recognise different social categories. Although much evidence has accumulated that animals are able to identify and classify other individuals into different categories, few studies have definitively demonstrated true individual recognition, i.e. discrimination between individuals on the basis of their idiosyncratic characteristics. Furthermore, the neural structures and pathways involved in social and, a fortiori, individual recognition have as yet been poorly investigated. This paper discusses various methods and measures currently used to assess different forms of social categorisations in animals, with special reference to rodents. Recent progress concerning the neurobiological bases involved in social recognition is also discussed. Finally, integrative perspectives for studying individual recognition in the context of social cognition is underlined in relation to different approaches investigating rodents' ability to use learned olfactory information.

The effect of labor on olfactory exposure learning within the first postnatal hour

Thirty-one neonates delivered by cesarean section were exposed to an odor for 30 min shortly after birth. Fifteen births had uterine labor contractions before delivery; 16 were without contractions. All babies were later tested (median age = 80 hr) for their responses to the familiar exposure odor and a novel odor presented on either side of the face. Overall, the babies spent more time turned toward the exposure odor than toward the novel scent. Babies in the labor condition, but not those bom without labor, displayed a significant preference for the exposure odor. Norepinephrine (NE) levels were higher in babies who oriented preferentially toward the exposure odor. Brief exposure immediately after birth is sufficient for the development of olfactory learning. Heightened learning by neonates from births with contractions may reflect locus coeruleus and NE activation. Olfactory learning may therefore be particularly efficient shortly after birth.

Direct excitation of mitral cells via activation of alpha1-noradrenergic receptors in rat olfactory bulb slices

The main olfactory bulb receives a significant modulatory noradrenergic input from the locus coeruleus. Previous in vivo and in vitro studies showed that norepinephrine (NE) inputs increase the sensitivity of mitral cells to weak olfactory inputs. The cellular basis for this action of NE is not understood. The goal of this study was to investigate the effect of NE and noradrenergic agonists on the excitability of mitral cells, the main output cells of the olfactory bulb, using whole cell patch-clamp recording in vitro. The noradrenergic agonists, phenylephrine (PE, 10 microM), isoproterenol (Isop, 10 microM), and clonidine (3 microM), were used to test for the functional presence of alpha1-, beta-, and alpha2-receptors, respectively, on mitral cells. None of these agonists affected olfactory nerve (ON)-evoked field potentials recorded in the glomerular layer, or ON-evoked postsynaptic currents recorded in mitral cells. In whole cell voltage-clamp recordings, NE (30 microM) induced an inward current (54 +/- 7 pA, n = 16) with an EC(50) of 4.7 microM. Both PE and Isop also produced inward currents (22 +/- 4 pA, n = 19, and 29 +/- 9 pA, n = 8, respectively), while clonidine produced no effect (n = 6). In the presence of TTX (1 microM), and blockers of excitatory and inhibitory fast synaptic transmission [gabazine 5 microM, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) 10 microM, and (+/-)-2-amino-5-phosphonopentanoic acid (APV) 50 microM], the inward current induced by PE persisted (EC(50) = 9 microM), whereas that of Isop was absent. The effect of PE was also observed in the presence of the Ca(2+) channel blockers, cadmium (100 microM) and nickel (100 microM). The inward current caused by PE was blocked when the interior of the cell was perfused with the nonhydrolyzable GDP analogue, GDPbetaS, indicating that the alpha1 effect is mediated by G-protein coupling. The current-voltage relationship in the absence and presence of PE indicated that the current induced by PE decreased near the equilibrium potential for potassium ions. In current-clamp recordings from bistable mitral cells, PE shifted the membrane potential from the downstate (-52 mV) toward the upstate (-40 mV), and significantly increased spike generation in response to perithreshold ON input. These findings indicate that NE excites mitral cells directly via alpha1 receptors, an effect that may underlie, at least in part, increased mitral cell responses to weak ON input during locus coeruleus activation in vivo.

Infusion of the metabotropic receptor agonist, DCG-IV, into the main olfactory bulb induces olfactory preference learning in rat pups

DCG-IV, a type 2 metabotropic glutamate receptor (mGluR2) agonist, was infused into the main olfactory bulb of 1-week-old pups exposed to peppermint odor. A preference for peppermint was demonstrated 24 h later. The data support the proposal that disinhibition at dendrodendritic synapses between granule cells and mitral cells is a critical component of olfactory memory formation in the rat pup olfactory bulb as well as in the accessory olfactory bulb of adult rodents.

Serotonin and NO complementarily regulate generation of oscillatory activity in the olfactory CNS of a terrestrial mollusk

Synchronous oscillation of membrane potentials, generated by assemblies of neurons, is a prominent feature in the olfactory systems of many vertebrate and invertebrate species. However, its generation mechanism is still controversial. Biogenic amines play important roles for mammalian olfactory learning and are also implicated in molluscan olfactory learning. Here, we investigated the role of serotonin, a biogenic amine, in the oscillatory dynamics in the procerebrum (PC), the molluscan olfactory center. Serotonin receptor blockers inhibited the spontaneous synchronous oscillatory activity of low frequency (approximately 0.5 Hz) in the PC. This was due to diminishing the periodic slow oscillation of membrane potential in bursting (B) neurons, which are essential neuronal elements for the synchronous oscillation in the PC. On the other hand, serotonin enhanced the amplitude of the slow oscillation in B neurons and subsequently increased the number of spikes in each oscillatory cycle. These results show that the extracellular serotonin level regulates the oscillation amplitude in B neurons and thus serotonin may be called an oscillation generator in the PC. Although nitric oxide (NO) is known to also be a crucial factor for generating the PC oscillatory activity and setting the PC oscillation frequency, the present study showed that NO only regulates the oscillation frequency in B neurons but could not increase the spikes in each oscillatory cycle. These results suggest complementary regulation of the PC oscillatory activity: NO determines the probability of occurrence of slow potentials in B neurons, whereas serotonin regulates the amplitude in each cycle of the oscillatory activity in B neurons.

Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles

Norepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles. In order to distinguish unambiguously between mitral cells and granule cells of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electrophysiological criteria. At NE concentrations lower than 5 microM there was only one effect of NE upon voltage-gated conductances; NE blocked a high-voltage-activated Ca(2+)-current in mitral cells of both the main and the accessory olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca(2+)-currents was mimicked by the alpha(2)-receptor agonists clonidine and alpha-methyl-NE. As a second effect, NE or clonidine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the spontaneous synaptic activity in mitral cells of either olfactory bulb. The amplitude of glutamate-induced currents in granule cells was modulated neither by clonidine nor by alpha-methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha(2)-receptor-mediated block of Ca(2)+-currents in mitral cells appeared to be a wide-spread disinhibition of mitral cells in the accessory olfactory bulb as well as in the main olfactory bulb.

Isoproterenol increases CREB phosphorylation and olfactory nerve-evoked potentials in normal and 5-HT-depleted olfactory bulbs in rat pups only at doses that produce odor preference learning

Norepinephrine (NE) and serotonin (5-HT) are important modulators of early odor preference learning. NE can act as an unconditioned stimulus (UCS), whereas 5-HT facilitates noradrenergic actions. In this study, we examined the phosphorylation of an important transcription factor, cAMP response element binding protein (CREB), which has been implicated in long-term-memory formation (McLean et al. 1999) during NE-induced odor preference learning in normal and olfactory bulb 5-HT-depleted rat pups. We also examined NE modulation of olfactory nerve-evoked field potentials (ON-EFPs) in anesthetized normal and bulbar 5-HT depleted pups. Systemic injection of 2 mg/kg isoproterenol (beta-adrenoceptor agonist) induced odor preference learning, enhanced pCREB expression in the olfactory bulbs at 10 min after odor pairing, and increased ON-EFPs in normal rat pups but not in bulbar 5-HT-depleted rat pups. A dose of 6 mg/kg isoproterenol, which was ineffective in modulating these measures in normal rat pups, induced odor preference learning, enhanced phosphorylated CREB (pCREB) expression, and increased ON-EFPs in bulbar 5-HT-depleted pups. These outcomes suggest that NE and 5-HT promote specific biochemical and electrophysiological changes that may critically underlie odor preference learning.

Association of an odor with activation of olfactory bulb noradrenergic beta-receptors or locus coeruleus stimulation is sufficient to produce learned approach responses to that odor in neonatal rats

These experiments examined the sufficiency of pairing an odor with either intrabulbar activation of noradrenergic beta-receptors or pharmacological stimulation of the locus coeruleus to support learned odor preferences in Postnatal Day 6-7 rat pups. The results showed that pups exposed to odor paired with beta-receptor activation limited to the olfactory bulb (isoproterenol, 50 microM) displayed a conditioned approach response on subsequent exposure to that odor. Furthermore, putative stimulation of the locus coeruleus (2 microM idazoxan or 2 mM acetylcholine) paired with odor produced a subsequent preference for that odor. The effects of locus coeruleus stimulation could be blocked by a pretraining injection of the beta-receptor antagonist propranolol (20 mg/kg). Together these results suggest that convergence of odor input with norepinephrine release from the locus coeruleus terminals within the olfactory bulb is sufficient to support olfactory learning.

Octopamine reverses the isolation-induced increase in trophallaxis in the carpenter ant Camponotus fellah

Social deprivation is an unusual situation for ants that normally maintain continuous contact with their nestmates. When a worker was experimentally isolated for 5 days and then reunited with a nestmate, she engaged in prolonged trophallaxis. It is suggested that trophallaxis allows her to restore a social bond with her nestmates and to re-integrate into the colony, particularly via the exchange of colony-specific hydrocarbons. Octopamine reduced trophallaxis in these workers as well as hydrocarbon transfer between nestmates, but not hydrocarbon biosynthesis. Administration of serotonin to such 5-day-isolated ants had no effect on the percentage of trophallaxis. Administration of phentolamine alone, an octopamine antagonist, had no effect, but when co-administrated with octopamine it reduced the effect of octopamine alone and restored trophallaxis to control levels. Moreover, the observed effect of octopamine was not due to a non-specific effect on locomotor activity. Therefore, we hypothesise that octopamine mediates behaviour patterns linked to social bonding, such as trophallaxis. On the basis of an analogy with the role of norepinephrine in vertebrates, we suggest that the levels of octopamine in the brain of socially deprived ants may decrease, together with a concomitant increase in their urge to perform trophallaxis and to experience social contacts. Octopamine administration may reduce this social deprivation effect, and octopamine could therefore be regarded as being partly responsible for the social cohesion between nestmates in ant colonies.

Oxytocin induces preservation of social recognition in male rats by activating alpha-adrenoceptors of the olfactory bulb

In this report, a series of four experiments was performed to evaluate the relationship between the olfactory bulb norepinephrine system and intra-olfactory bulb infusion of oxytocin in the preservation of social memory responses. The present data indicate that oxytocin exerts this preservation of social recognition through a specific, receptor-mediated mechanism within the olfactory bulb (experiment 1). The involvement of the olfactory bulb norepinephrine system is revealed by the demonstration that retrodialysis of oxytocin into the olfactory bulb increases norepinephrine release (experiment 4). Our data suggest that the increased output of olfactory bulb norepinephrine resulting from oxytocin appears to activate alpha-adrenoceptors to produce this preservation in recognition because infusions of clonidine into the olfactory bulb preserve recognition responses in a manner similar to that observed with oxytocin (experiment 2). In addition, a co-infusion of oxytocin with phentolamine abolishes recognition responses (experiment 3). Accordingly, this model affords the opportunity to study neuropeptide-catecholamine interactions, link these interactions with a specific behavioural outcome and identify a novel function/site of action for oxytocin in the male.

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.

Gabaergic control of olfactory learning in young rats

Olfactory learning in young rats correlates with neural plasticity in the olfactory bulb, and involves noradrenergic modulation of reciprocal dendrodendritic synapses between mitral cells and GABAergic granule cells. The purpose of this study was to examine, in vivo, the consequences of manipulating bulbar GABA transmission during training. In the first experiment, postnatal day 11 rat pups were trained in an olfactory associative learning task with citral odor and foot shock as the conditioned and unconditioned stimuli, respectively. The pups received continuous infusion of saline or the GABA(A) receptor agonist muscimol into the olfactory bulbs throughout a 30-min training session. The pups were then tested on postnatal day 12 for a preference for or an aversion to citral odor. Saline-infused control pups developed an aversion to citral odor. The GABA(A) receptor agonist muscimol impaired this aversive learning in a dose-dependent manner. In the second experiment, pups were exposed to the odor for 30 min while receiving continuous intrabulbar infusion of a low or high dose of the GABA(A) receptor antagonist bicuculline, without any other reinforcer. Depending on whether a low (0.2 nmol/bulb) or high (1.0 nmol/bulb) dose of bicuculline was infused, the pups showed a preference or an aversion for citral odor after infusion of low and high doses, respectively. These results indicate that disinhibition of mitral cells in the olfactory bulb is critical for olfactory learning in young rats, and suggest that the degree of disinhibition is an important determinant in acquiring either preference or aversion for the conditioned odor.

Vibrissae-evoked behavior and conditioning before functional ontogeny of the somatosensory vibrissae cortex

The following experiments determined that the somatosensory whisker system is functional and capable of experience-dependent behavioral plasticity in the neonate before functional maturation of the somatosensory whisker cortex. First, unilateral whisker stimulation caused increased behavioral activity in both postnatal day (P) 3-4 and P8 pups, whereas stimulation-evoked cortical activity (14C 2-deoxyglucose autoradiography) was detectable only in P8 pups. Second, neonatal rat pups are capable of forming associations between whisker stimulation and a reinforcer. A classical conditioning paradigm (P3-P4) showed that the learning groups (paired whisker stimulation-shock or paired whisker stimulation-warm air stream) exhibited significantly higher behavioral responsiveness to whisker stimulation than controls. Finally, stimulus-evoked somatosensory cortical activity during testing [P8; using 14C 2-deoxyglucose (2-DG) autoradiography] was assessed after somatosensory conditioning from P1-P8. No learning-associated differences in stimulus-evoked cortical activity were detected between learning and nonlearning control groups. Together, these experiments demonstrate that the whisker system is functional in neonates and capable of experience-dependent behavioral plasticity. Furthermore, in contrast to adult somatosensory classical conditioning, these data suggest that the cortex is not required for associative somatosensory learning in neonates.

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 beta-receptor antagonist propranolol in a dose-dependent manner. In a second study, vibrissa stimulation was paired with systemic injections of the NE beta-receptor agonist isoproterenol. Association of vibrissa stimulation with beta-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.

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 microM) 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 al receptor agonist phenylephrine (1 microM, P<0.001). Both norepinephrine and phenylephrine modulation of mitral cell responses were blocked by the al adrenergic antagonist WB-4101 (1 microM). These findings are consistent with observations that the main olfactory bulb exhibits the highest density of alpha1 receptors in the brain. The alpha2 receptor agonist clonidine (100 nM) and the beta receptor agonist isoproterenol (1 microM) 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 al 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.

Castration differentially alters [3H]nisoxetine binding to norepinephrine uptake sites in olfactory bulb and frontal cortex of male rats

In the present study, [3H]nisoxetine binding to norepinephrine (NE) uptake sites and [3H]norepinephrine uptake were investigated within olfactory bulb (OB) and frontal cortex homogenates from intact and castrated male rats. Statistically significant reductions in the number of [3H]nisoxetine binding sites (Bmax) were found in OB from the castrates, while significantly increased Bmax values were obtained in the frontal cortex. Castration also significantly altered the affinity (Kd) of [3H]nisoxetine binding in the frontal cortex, but not in the OB. Assessment of [3H]norepinephrine uptake showed that in neither brain regions were there any statistically significant differences in Km nor Vmax between the castrated and intact male rats, indicating that the basal uptake process is not changed following castration in either of these brain areas. These results demonstrate the differential effects of castration upon [3H]nisoxetine binding sites between the OB and frontal cortex. Such findings provide new evidence for one of the mechanisms by which androgens may modulate central noradrenergic activity.

Unique salience of maternal breast odors for newborn infants

Human infants are particularly responsive to olfactory cues emanating from their mother's nipple/areola region. Beginning within minutes after birth, maternal breast odors elicit preferential head orientation by neonates and help guide them to the nipple. Such odors also influence babies' general motor activity and arousal, which may contribute further to successful nipple localization and sucking. The role of maternal olfactory signals in the mediation of early breast-feeding is functionally analogous to that of nipple-search pheromone as described in nonhuman mammals. To some extent, the chemical profile of breast secretions overlaps with that of amniotic fluid. Therefore, early postnatal attraction to odors associated with the nipple/areola may reflect prenatal exposure and familiarization. Although newborns are generally attracted to breast odors produced by lactating women, breast-fed infants rapidly learn their mother's characteristic olfactory signature while sucking at her breasts and can subsequently recognize her by that unique scent alone. Early odor-based recognition may be an important factor in the development of the infant-mother bond.

The influence of chemical complexity on the perception of multicomponent odor mixtures

The present study investigates the hypothesis that complex object odors (odors that emanate from flowers, foods, sewage, etc.) that consist of dozens of odorants are processed and encoded as discrete entities, as if each was a single chemical odor. To test this hypothesis, the capacity of trained subjects to discriminate and identify the components of stimuli consisting of one to eight object odors was determined. The results indicated that subjects could only identify up to four object odors in a mixture, which is similar to earlier findings with mixtures that contained only single chemical odors. The limited capacity was also reflected in the number of odors selected, regardless of whether the choices were correct or incorrect, in confidence ratings, and in decision times. The identification of a limited number of object odors in every mixture that was presented suggests that both associative (synthetic) and dissociative (analytic) processes are involved in the perceptual analysis of odor mixtures.

Brain substrates of infant-mother attachment: contributions of opioids, oxytocin, and norepinephrine

The aim of this paper is to review recent work concerning the psychobiological substrates of social bonding, focusing on the literature attributed to opioids, oxytocin and norepinephrine in rats. Existing evidence and thinking about the biological foundations of attachment in young mammalian species and the neurobiology of several other affiliative behaviors including maternal behavior, sexual behavior and social memory is reviewed. We postulate the existence of social motivation circuitry which is common to all mammals and consistent across development. Oxytocin, vasopressin, endogenous opioids and catecholamines appear to participate in a wide variety of affiliative behaviors and are likely to be important components in this circuitry. It is proposed that these same neurochemical and neuroanatomical patterns will emerge as key substrates in the neurobiology of infant attachments to their caregivers.

Intracerebroventricular norepinephrine potentiation of the perforant path-evoked potential in dentate gyrus of anesthetized and awake rats: A role for both alpha- and beta-adrenoceptor activation

Norepinephrine (NE) applied iontophoretically to the dentate gyrus in vivo, and bath applied to hippocampal slices in vitro, produces potentiation of the perforant path-evoked potential. beta-receptors mediate exogenous NE potentiation in vitro, while alpha-receptors are implicated in exogenous effects in vivo. The present study uses intracerebroventricular (i.c.v.) NE to mimic in vitro bath conditions in vivo. Short-term NE potentiation was reliably seen with 10 microg [+/-] NE in 2 microl of 0.9% saline i.c.v. Long-term potentiation occurred with higher doses of NE. The beta-agonist isoproterenol and the alpha-agonist phenylephrine also produced potentiation. Long-term effects were common with isoproterenol. The beta-antagonist metoprolol and the alpha-antagonist phentolamine attenuated NE potentiation. The results suggest that both alpha- and beta-receptors could play a role in NE potentiation in dentate gyrus in vivo. In awake animals, 10 microg NE i.c.v. reproduced the potentiation pattern seen in anesthetized rats. NE potentiation in awake rats was independent of behavioral variation.

Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. In the present review we argue that noradrenaline affects aggression on three different levels: the hormonal level, the sympathetic autonomous nervous system, and the central nervous system (CNS), in different, but functionally synergistic ways. Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. Indirect CNS effects include: the shift of attention towards socially relevant stimuli; the enhancement of olfaction (a major source of information in rodents); the decrease in pain sensitivity; and the enhancement of memory (an aggressive encounter is very relevant for the future of the animal). Concerning more aggression-specific effects one may notice that a slight activation of the central noradrenergic system stimulates aggression, while a strong activation decreases fight readiness. This biphasic effect may allow the animal to engage or to avoid the conflict, depending on the strength of social challenge. A hypothesis is presented regarding the relevance of different adrenoceptors in controlling aggression. It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors. The latter phenomenon may be dependent on a noradrenaline-induced corticosterone secretion. It appears that by activating very different mechanisms the systems working with adrenaline and/or noradrenaline prepare the animal in a very complex way to answer the demands imposed by, and to endure the effects caused by, fights. It is a challenge for future research to elucidate how precisely these mechanisms interact to contribute to functionally relevant and adaptive aggressive behaviour.

Habituation of odor responses in the rat anterior piriform cortex

Simultaneous recordings of main olfactory bulb (MOB) and anterior piriform cortex (aPCX) neuron responses to repeated and prolonged odor pulses were examined in freely breathing, urethan-anesthetized rats. Comparisons of odor responses were made between multi-unit recordings of MOB activity and single-unit extracellular and intracellular recordings of Layer II/III aPCX neurons. Odor stimuli consisted of either 2-s pulses repeated at 30-s intervals or a single, prolonged 50-s stimulus. Respiration rate was monitored throughout. MOB and aPCX neuron responses to odor were quantified both through firing frequency and through the temporal patterning of firing over the respiratory cycle. The results demonstrate that aPCX neurons habituate significantly more (faster) than MOB neurons with both repeated and prolonged stimulation paradigms. This enhanced habituation is expressed as both a decrease in aPCX firing despite maintained odor-evoked MOB input and as a decrease in aPCX respiratory cycle entrainment despite maintained MOB cyclic input. Intracellular aPCX recordings suggest that several mechanisms may be involved in this experience-induced change in aPCX function, including 1) decreased excitatory driveof aPCX neurons, 2) decreased excitability of aPCX neurons,and/or 3) enhancement in odor-evoked inhibition of aPCX neurons. These studies provide the initial basis for understanding the mechanisms of nonassociative plasticity in olfactory cortex.

Anatomical and synaptic substrates for avian song learning

In songbirds, vocal learning occurs during periods of major cellular and synaptic change. This neural reorganization includes massive synaptogenesis associated with the addition of new neurons into the vocal motor pathway, as well as pruning of connections between song regions. These observations, coupled with behavioral evidence that song development requires NMDA receptor activation in specific song nuclei, suggest that experiences associated with vocal learning are encoded by activity driven, Hebbianlike processes of synaptic change akin to those implicated in many other forms of developmental plasticity and learning. In this review we discuss the hypothesis that develpmental and/or seasonal changes in NMDA receptor function and the availability of new synapses may modulate thresholds for plasticity and thereby define sensitive periods for vocal learning.

Controlled targeting of tyrosine hydroxylase protein toward processes of locus coeruleus neurons during postnatal development

Dendrites of locus coeruleus (LC) neurons laying within the pericoerulean neuropil (PCA) organize the major site where tyrosine hydroxylase (TH) is present throughout postnatal development. Those dendrites constitute the neuronal compartment in which TH levels increase beyond postnatal day (P) 21 or after RU24722-induced TH expression. Distal LC dendrites are present in the PCA by at least P20 but are devoid of TH and can rapidly accumulate TH protein when gene induction is triggered. Contrasting with the increase in TH levels within LC perikarya and dendrites, TH-mRNA concentration remains constant in LC perikarya from P4 to P42. Thus, supposing TH synthesis and degradation are also constant, any change in TH levels targeted toward axons might be balanced by a shift in the TH deposition within LC dendrites. This mechanism may be crucial in functions that the different processes of LC neurons have at critical steps of postnatal ontogeny.

Increased beta adrenoceptor activation overcomes conditioned olfactory learning deficits induced by serotonin depletion

It was hypothesized that 5-HT2 receptors in the olfactory bulb prime the bulbar response to a beta adrenoceptor mediated unconditioned stimulus (UCS) during odor preference learning in 1-week-old rat pups. The ability of 4 mg/kg of isoproterenol + stroking and 6 mg/kg of isoproterenol + no stroking to induce normal odor preference learning in pups depleted of bulbar 5-HT in the present study supports the hypothesis. The inverted-U curve relation between UCS strength and learning also appears to occur within the bulb.

A computational model of the response of honey bee antennal lobe circuitry to odor mixtures: overshadowing, blocking and unblocking can arise from lateral inhibition

Recent studies of learning about elements of odorant mixtures in honey bees identified several types of interactions between mixture components, such as overshadowing and blocking. The latter phenomenon in particular indicates at least a limited ability of subjects to identify the most salient element of a binary mixture. Here we show that the circuitry in the antennal lobes, the first neuropil in which synaptic interaction affects sensory processing, could give rise to both effects given the incorporation of modifiable synapses onto inhibitory circuitry. The neural model of the antennal lobe that we present incorporates identified cell types and includes a biologically realistic modulatory neuron with which modifiable Hebb-like synaptic interactions take place. A learning rule that incorporates modifiable connections from output (projection) neurons onto the modulatory neuron is sufficient to account for behavioral results on generalization and overshadowing. A second type of excitatory connection from the modulatory neuron onto local inhibitory interneurons is necessary to reproduce behavioral results from blocking and unblocking. We suggest that the neural representations of odor mixtures in the antennal lobe can be modified by previous exposure to one of the mixture components. These results provide testable hypotheses that will guide future behavioral and physiological analyses.

Modulation of inhibition in a model of olfactory bulb reduces overlap in the neural representation of olfactory stimuli

In a neural model of olfactory bulb processing, we demonstrate the putative role of the modulation of two types of inhibition, inspired by electrophysiological data on the effect of acetylcholine and noradrenaline on olfactory bulb synaptic transmission. Feedback regulation of modulation based on bulbar activity serves to 'normalize' the activity of output neurons in response to different levels of input activities. This mechanism also decreases the overlap between pairs of output patterns (Mitral cell activities), enhancing the discrimination between overlapping olfactory input patterns. The effect of the modulation at the two levels of interneurons is complementary: while an increase in periglomerular inhibition decreases the number of responding output neurons, a decrease in granule cell inhibition increases the firing frequencies of these neurons.

Neural mechanisms of mammalian olfactory learning

In this review, we compare the neural basis of olfactory learning in three specialized contexts that occur during sensitive periods of enhanced neural plasticity. Although they involve very different behavioural contexts, they share several common features, including a dependence on noradrenergic transmission in the olfactory bulb. The most extensively characterized of these examples is the learning of pheromonal information by female mice during mating. While this form of learning is unusual in that the neural changes underlying the memory occur in the accessory olfactory bulb at the first stage of sensory processing, it involves similar neural mechanisms to other forms of learning and synaptic plasticity. The learning of newborn lamb odours after parturition in sheep, and the olfactory conditioning in neonatal animals such as rats and rabbits, are mediated by the main olfactory system. Although the neural mechanisms for learning in the main olfactory system are more distributed, they also involve changes occurring in the olfactory bulb. In each case, odour learning induces substantial structural and functional changes, including increases in inhibitory neurotransmission. In the main olfactory bulb, this probably represents a sharpening of the odour-induced pattern of activity, due to increases in lateral inhibition. In contrast, the different morphology of mitral cells in the accessory olfactory bulb results in increased self-inhibition, disrupting the transmission of pheromonal information. Although these examples occur in highly specialized contexts, comparisons among them can enhance our understanding of the general neural mechanisms of olfactory learning.

Expression of alpha 2 adrenoceptors during rat brain development--I. Alpha 2A messenger RNA expression

The distribution of alpha 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 alpha 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 alpha 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 alpha 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 alpha 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 alpha 2A receptors in the fetal brain is in marked contrast to the postnatal development of the alpha 2C receptor type.

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.

Spatial distribution of [14C]2-deoxyglucose uptake in the glomerular layer of the rat olfactory bulb following early odor preference learning

Previous work has shown that odors induce focal uptake of [14C]2-deoxyglucose (2-DG) within the glomerular layer of the main olfactory bulb and that the amount of 2-DG accumulated in these foci increases after early odor learning. To determine if learning-associated changes in 2-DG uptake occur across the entire glomerular layer, we have mapped uptake throughout the layer at fixed angles in coronal sections through the bulb. Resulting arrays for individual bulbs were corrected for differing bulb size and averaged across experimental groups to address the spatial distribution of uptake. The average arrays revealed at least three discrete fields of uptake in naive, peppermint-exposed rats at postnatal day 19 that were not seen in air-exposed littermates. In agreement with previous studies, early preference training with peppermint odor given on postnatal days 1-18 increased 2-DG uptake at postnatal day 19 within odor-dependent patches of uptake in the posterior half of the midlateral bulb, whereas odor-dependent, ventrolateral patches of uptake did not increase to the same extent. In addition, early preference learning was associated with significantly increased 2-DG uptake average over the entire analyzed glomerular layer. These increases were smaller than those within odor-dependent foci and were distributed widely across the glomerular layer, showing low overlap between trained and control rats in anterior regions where peppermint odor did not stimulate 2-DG uptake. The widely distributed increases in 2-DG uptake after learning may reflect changed activity of centrifugal projections that diffusely innervate the glomerular layer.

Early locus coeruleus lesions increase the density of beta-adrenergic receptors in the main olfactory bulb of rats

Norepinephrine is supplied to both deep and superficial layers of the olfactory bulb through dense projections from the locus coeruleus. Beta-adrenergic receptors are located in nearly all bulb laminae, with high-density foci of beta-1 and beta-2-adrenoceptors present in the glomerular layer. Early olfactory experiences that increase norepinephrine levels in the bulb also decrease the density of beta-1- and beta-2-adrenoceptors, as well as the number of high-density glomerular foci of beta-2-receptors. Changes in bulb norepinephrine levels, therefore, may affect the density of beta-adrenoceptors in the bulb. In the current study, we test this hypothesis by performing unilateral lesions of the locus coeruleus with 6-hydroxydopamine on postnatal day 4, and examining the density of beta-1- and beta-2-adrenergic receptors in the main olfactory bulb of the rat using 125I-labeled iodopindolol receptor autoradiography on postnatal day 19. Locus coeruleus destruction resulted in a statistically significant increase in the density of adrenergic receptors in the ipsilateral bulb compared to the contralateral bulb. Both beta-1- and beta-2-adrenoceptor subtypes increased in density with this manipulation, although the number of glomerular layer high-density beta-2 foci was not significantly different between the two bulbs. These results are consistent with the hypothesis that changes in olfactory bulb norepinephrine can regulate the density of beta-adrenergic receptors in the bulb.

Regulation of c-Fos mRNA and fos protein expression in olfactory bulbs from unilaterally odor-deprived adult mice

Odorant deprivation, produced by unilateral naris closure, profoundly reduces tyrosine hydroxylase (TH) expression within intrinsic olfactory bulb dopamine neurons. The TH gene contains an AP-1 site, which interacts with the product of the immediate early gene, c-fos. c-Fos exhibits activity dependent regulation in the CNS. The hypothesis that odorant stimulation and deprivation might modify c-fos expression in TH neurons was tested in adult CD-1 mice, subjected to unilateral naris closure. After 2 months, naris closed and control mice were exposed to either clean air for 60 min or clean air for 60 min followed by 30 min of alternating exposure to 10% isoamyl acetate (1 min) and air (4 min). A parallel reduction occurred in TH and fos expression (both c-fos mRNA and fos-like immunoreactivity) in the glomerular layer of the odorant-deprived olfactory bulb. Odor stimulation induced a short-lived increase in c-fos mRNA and fos-like immunoreactivity in olfactory bulbs contralateral to naris closure. The increase in fos expression was region-specific in the glomerular layer but more diffuse in mitral and granule cell layers. In olfactory bulbs ipsilateral to naris closure, odor stimulation also induced c-fos mRNA expression in the mitral and granule cell layers and sparsely within limited periglomerular regions. Odor induced expression in mitral and granule cell layers may represent increased centrifugal activity acting on as yet unknown genes. These results suggest a correlation between c-fos mRNA expression and increased neuronal activity in the olfactory bulb which, in turn, acts to regulate TH expression in periglomerular neurons.

Activation of locus coeruleus enhances the responses of olfactory bulb mitral cells to weak olfactory nerve input

The main olfactory bulb (MOB) receives a dense projection from the pontine nucleus locus coeruleus (LC), the largest collection of norepinephrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously applied NE on mitral cells, the principal output neurons of MOB, are contradictory. The effect of synaptically released NE on mitral cell activity is not known, nor is the influence of NE on responses of mitral cells to olfactory nerve inputs. The goal of the present study was to assess the influence of LC activation on spontaneous and olfactory nerve-evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetyicholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharge. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/18 cells (mean Increase = 67%). The discharge rate of mitral cells at the time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activity in LC-NE projections to MOB may enhance detection of relatively weak odors.

Investigation of the role of interneurons and their modulation by centrifugal fibers in a neural model of the olfactory bulb

Olfactory bulb processing results from the interaction of relay neurons with two main categories of interneurons which mediate inhibition in two distinct layers: periglomerular cells and granule cells. We present here a neural model of the mammalian olfactory bulb which allows to separately investigate the functional consequences of the two types of interneurons onto the relay neurons responsiveness to odors. The model, although built with simplified representations of neural elements generates various aspects of neural dynamics from the cellular to the populational level. We propose that the combined action of centrifugal control at two different layers of processing is complementary: reduction of the number of active relay neurons responding to a given odorant through increased activity of periglomerular cells, and an increase of response intensity of active mitral cells through decrease of granule cell inhibition.

Odour-induced c-fos expression in the rat olfactory bulb: involvement of centrifugal afferents

Expression of the proto-oncogene c-fos is known to increase in granule cells of the olfactory bulb following a sustained olfactory stimulation. Most granule cells displaying high levels of Fos accumulation are located in the bulbar columns defined by the odour-induced foci of high 2-deoxyglucose glomerular uptake. The present studies were undertaken in order to assess the possible involvement of centrifugal afferents in the modulation of odour-induced patterns of either 2-deoxyglucose accumulation or c-fos expression in the olfactory bulb. A unilateral olfactory peduncle section had no effect on the odour-induced 2-deoxyglucose foci but induced a significant decrease in the number of Fos-containing neurons in odour-selective areas of both olfactory bulbs, ipsilateral and contralateral to the lesion. This suppressive effect was much more pronounced in the side ipsilateral to the peduncle section. It is concluded that c-fos expression induced by a sustained stimulation with propionic acid vapours is not only determined by the olfactory peripheral input but also by afferents of central origin. In order to estimate the contingent involvements of the cholinergic and noradrenergic afferents in this control of c-fos expression, we attempted to mimic the effects of the surgical deafferentation on odour-induced c-fos expression by using a pharmacological approach with selective cholinergic and noradrenergic antagonists. The beta-adrenergic antagonist propanolol induced a suppression of the odour-related patterns of Fos accumulation similar to the one caused by the surgical deafferentation of the olfactory bulb. The muscarinic antagonist scopolamine did not alter c-fos expression in the odour-selective area but increased significantly Fos labelling in the other bulbar aspects. Pharmacological investigations indicate that the noradrenergic and cholinergic centrifugal systems are likely involved in the central modulation of c-fos expression in the OB. The Fos protein could be expressed as an early nuclear signal triggering further long-term modifications of the neuronal phenotype, in certain conditions of sensory stimulation involving the activation of centrifugal systems.

A learned odor evokes an enhanced Fos-like glomerular response in the olfactory bulb of young rats

Young rats exposed to peppermint odor and reinforcing tactile stimulation from postnatal days (PND) 1-18 increase their preference for that odor relative to controls. This early olfactory memory is accompanied by an 80% increase in the density of glomerular-layer cells displaying Fos-like immunoreactivity in response to the learned odor on PND 19. The difference is observed in midlateral portions of the olfactory bulb that align with foci of 2-deoxyglucose (2-DG) uptake in adjacent sections. Trained and control animals are not different in the Fos-like response of juxtaglomerular cells within ventrolateral 2-DG foci. Ratios of midlateral/ventrolateral response differ significantly between trained and control animals and include differences among cells of three staining intensities. These ratios are correlated with ratios of 2-DG uptake (midlateral/ventrolateral foci), which also differ significantly between trained and control rats. Juxtaglomerular cells associated with 2-DG foci also express Egr-1-like immunoreactivity. However, the midlateral Egr-1 response does not differ between trained and control rats. These results show that early memories can be associated with an increased Fos-like response in a primary sensory area of the CNS. They also suggest that only specific regions within the olfactory bulb are modified following the learning of a given odor in early life.

Early olfactory enrichment and deprivation both decrease beta-adrenergic receptor density in the main olfactory bulb of the rat

The density of noradrenergic locus coeruleus projections and beta-adrenergic receptors in the main olfactory bulb of the rat increases with age. Both beta 1- and beta 2-adrenergic receptor subtypes exhibit laminar distributions, with focal regions of high receptor density present within the neuropil of individual glomeruli. Since the first synaptic contacts between olfactory receptor neurons and bulbar neurons occur within the glomeruli, early olfactory experiences possibly could influence the density or distribution of beta-adrenergic receptors in the bulb. We therefore investigated the effects of olfactory deprivation and early olfactory enrichment on the density and distribution of beta-adrenergic receptors in the main olfactory bulb. Animals were subjected to either unilateral naris closure on postnatal day 1 or odor training from postnatal days 1-18. Bulbs were removed on postnatal day 19 and subjected to quantitative autoradiography using the beta-adrenergic receptor antagonist [125I]iodopindolol and specific receptor subtype antagonists ICI 118,551 (beta 2-antagonist) and ICI 89,406 (beta 1-antagonist). Unilateral naris occlusion decreased both the number of beta 2 glomerular foci and the density of beta 1 and beta 2 receptors in the deprived bulb compared to the nondeprived bulb. Early odor training resulted in a significant decrease in the number, area, and receptor density of beta 2 glomerular foci in the midlateral region of the bulb. The distribution of beta 2 glomerular foci also differs with these two sensory manipulations. Changes in beta-adrenergic receptor density in response to both early learning and olfactory deprivation may be induced by a transient increase in olfactory bulb norepinephrine.

Novelty seeking behavior in the rat is dependent upon the integrity of the noradrenergic system

These experiments were designed to investigate the role of the noradrenergic system in promoting investigation of novelty in rats. Behavior was monitored in a hole board equipped with photoelectric cells strategically placed so that locomotor activity, rearing and investigation of each of the holes could be quantified independently. Specially designed computer software permitted recording of the sequence and cumulative duration of the visits to specific holes throughout the session. Dose-response curves of the sedative effect of the alpha 2 adrenergic receptor agonist clonidine were established, a sedative effect being defined as a decrease in overall horizontal displacements, rearings and hole visits. After a one week interval, the rats were rerun in the holeboard, with novel objects placed in four of the nine holes. Previous experiments had shown that rats spend significantly more time investigating holes containing objects than empty holes in this apparatus and this was replicated here. Doses of clonidine which were below threshold for inducing any sedative effect (10 micrograms/kg) totally eliminated preference for holes with objects while having no effect on total time investigating the holes. A subsequent experiment showed that the beta receptor antagonist propranolol (10 mg/kg) produced a similar effect. These results suggest that the noradrenergic system is implicated in stimulus seeking behavior and the post-synaptic beta receptors are involved in mediating the behavior.

Dissociation of behavioral and neural correlates of early associative learning

Wistar rat pups were trained in an olfactory associative conditioning task on postnatal Day 6, 12, or 20. The training consisted of 20 pairings of a novel odor (peppermint) with footshock (1.5 mA, 1 s) with an intertrial interval of 3 min. Additional pups were trained in either unpaired or naive control conditions. On the day following training, pups were either tested for their behavioral response to the conditioned odor in a two-odor choice test, or injected with 14C-2-deoxyglucose and exposed to the odor for examination of olfactory bulb neural responses to the odor. The results demonstrate that, although pups at all ages learned to avoid the odor, only pups trained during the first postnatal week had a modified olfactory-bulb glomerular-layer response to the odor. These results suggest that although olfactory memory is correlated with modification of olfactory bulb glomerular layer function in newborns, these changes are not required for normal memory in older pups.