A comparison of the postnatal development of post-activation potentiation in the neocortex and dentate gyrus of the rat

Unique infant neurobiology produces distinctive trauma processing

Trauma experienced in early life has unique neurobehavioral outcomes related to later life psychiatric sequelae. Recent evidence has further highlighted the context of infant trauma as critical, with trauma experienced within species-atypical aberrations in caregiving quality as particularly detrimental. Using data from primarily rodent models, we review the literature on the interaction between trauma and attachment in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. Together these data suggest that infant trauma processing and its enduring effects are impacted by both the immaturity of brain areas for processing trauma and the unique functioning of the early-life brain, which is biased towards forming robust attachments regardless of the quality of care. Understanding the critical role of the caregiver in further altering early life brain processing of trauma is important for developing age-relevant treatment and interventions.

The neurobiology of safety and threat learning in infancy

What an animal needs to learn to survive is altered dramatically as they change from dependence on the parent for protection to independence and reliance on self-defense. This transition occurs in most altricial animals, but our understanding of the behavioral neurobiology has mostly relied on the infant rat. The transformation from dependence to independence occurs over three weeks in pups and is accompanied by complex changes in responses to both natural and learned threats and the supporting neural circuitry. Overall, in early life, the threat system is quiescent and learning is biased towards acquiring attachment related behaviors to support attachment to the caregiver and proximity seeking. Caregiver-associated cues learned in infancy have the ability to provide a sense of safety throughout lifetime. This attachment/safety system is activated by learning involving presumably pleasurable stimuli (food, warmth) but also painful stimuli (tailpinch, moderate shock). At about the midway point to independence, pups begin to have access to the adult-like amygdala-dependent threat system and amygdala-dependent responses to natural dangers such as predator odors. However, pups have the ability to switch between the infant and adult-like system, which is controlled by maternal presence and modification of stress hormones. Specifically, if the pup is alone, it will learn fear but if with the mother it will learn attachment (10-15days of age). As pups begin to approach weaning, pups lose access to the attachment system and rely only on the amygdala-dependent threat system. However, pups learning system is complex and exhibits flexibility that enables the mother to override the control of the attachment circuit, since newborn pups may acquire threat responses from the mother expressing fear in their presence. Together, these data suggest that the development of pups' threat learning system is not only dependent upon maturation of the amygdala, but it is also exquisitely controlled by the environment. Most notably the mother can switch pup learning between attachment to threat learning in a moment's notice. This enables the mother to navigate pup's learning about the world and what is threatening and what is safe.

Early life adversity during the infant sensitive period for attachment: Programming of behavioral neurobiology of threat processing and social behavior

Animals, including humans, require a highly coordinated and flexible system of social behavior and threat evaluation. However, trauma can disrupt this system, with the amygdala implicated as a mediator of these impairments in behavior. Recent evidence has further highlighted the context of infant trauma as a critical variable in determining its immediate and enduring consequences, with trauma experienced from an attachment figure, such as occurs in cases of caregiver-child maltreatment, as particularly detrimental. This review focuses on the unique role of caregiver presence during early-life trauma in programming deficits in social behavior and threat processing. Using data primarily from rodent models, we describe the interaction between trauma and attachment during a sensitive period in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. These data suggest that trauma experienced directly from an abusive caregiver and trauma experienced in the presence of caregiver cues produce similar neurobehavioral deficits, which are unique from those resulting from trauma alone. We go on to integrate this information into social experience throughout the lifespan, including consequences for complex scenarios, such as dominance hierarchy formation and maintenance.

Developmental changes in plasticity, synaptic, glia and connectivity protein levels in rat dorsal hippocampus

Thus far the identification and functional characterization of the molecular mechanisms underlying synaptic plasticity, learning, and memory have not been particularly dissociated from the contribution of developmental changes. Brain plasticity mechanisms have been largely identified and studied using in vitro systems mainly derived from early developmental ages, yet they are considered to be general plasticity mechanisms underlying functions -such as long-term memory- that occurs in the adult brain. Although it is possible that part of the plasticity mechanisms recruited during development is then re-recruited in plasticity responses in adulthood, systematic investigations about whether and how activity-dependent molecular responses differ over development are sparse. Notably, hippocampal-dependent memories are expressed relatively late in development, and the hippocampus undergoes and extended developmental post-natal structural and functional maturation, suggesting that the molecular mechanisms underlying hippocampal neuroplasticity may actually significantly change over development. Here we quantified the relative basal expression levels of sets of plasticity, synaptic, glia and connectivity proteins in rat dorsal hippocampus, a region that is critical for the formation of long-term explicit memories, at two developmental ages, postnatal day 17 (PN17) and PN24, which correspond to a period of relative functional immaturity and maturity, respectively, and compared them to adult age. We found that the levels of numerous proteins and/or their phosphorylation, known to be critical for synaptic plasticity underlying memory formation, including immediate early genes (IEGs), kinases, transcription factors and AMPA receptor subunits, peak at PN17 when the hippocampus is not yet able to express long-term memory. It remains to be established if these changes result from developmental basal activity or infantile learning. Conversely, among all markers investigated, the phosphorylation of calcium calmodulin kinase II α (CamKII α and of extracellular signal-regulated kinases 2 (ERK-2), and the levels of GluA1 and GluA2 significantly increase from PN17 to PN24 and then remain similar in adulthood, thus representing correlates paralleling long-term memory expression ability.

Unique neurobiology during the sensitive period for attachment produces distinctive infant trauma processing

BACKGROUND

Trauma has neurobehavioral effects when experienced at any stage of development, but trauma experienced in early life has unique neurobehavioral outcomes related to later life psychiatric sequelae. Recent evidence has further highlighted the context of infant trauma as a critical variable in determining its immediate and enduring consequences. Trauma experienced from an attachment figure, such as occurs in cases of caregiver child maltreatment, is particularly detrimental.

METHODS

Using data primarily from rodent models, we review the literature on the interaction between trauma and attachment in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. We then consider how trauma with and without the caregiver produces long-term changes in emotionality and behavior, and suggest that these experiences initiate distinct pathways to pathology.

RESULTS

Together these data suggest that infant trauma processing and its enduring effects are impacted by both the immaturity of brain areas for processing trauma and the unique functioning of the early-life brain, which is biased toward processing information within the attachment circuitry.

CONCLUSION

An understanding of developmental differences in trauma processing as well as the critical role of the caregiver in further altering early life brain processing of trauma is important for developing age-relevant treatment and interventions.

Contextual and auditory fear conditioning continue to emerge during the periweaning period in rats

Anxiety disorders often emerge during childhood. Rodent models using classical fear conditioning have shown that different types of fear depend upon different neural structures and may emerge at different stages of development. For example, some work has suggested that contextual fear conditioning generally emerges later in development (postnatal day 23–24) than explicitly cued fear conditioning (postnatal day 15–17) in rats. This has been attributed to an inability of younger subjects to form a representation of the context due to an immature hippocampus. However, evidence that contextual fear can be observed in postnatal day 17 subjects and that cued fear conditioning continues to emerge past this age raises questions about the nature of this deficit. The current studies examine this question using both the context pre-exposure facilitation effect for immediate single-shock contextual fear conditioning and traditional cued fear conditioning using Sprague-Dawley rats. The data suggest that both cued and contextual fear conditioning are continuing to develop between PD 17 and 24, consistent with development occurring the in essential fear conditioning circuit.

Translational neuroscience measures of fear conditioning across development: applications to high-risk children and adolescents

Several mental illnesses, including anxiety, can manifest during development, with onsets in late childhood. Understanding the neurobiological underpinnings of risk for anxiety is of crucial importance for early prevention and intervention approaches. Translational neuroscience offers tools to investigate such mechanisms in human and animal models. The current review describes paradigms derived from neuroscience, such as fear conditioning and extinction and overviews studies that have used these paradigms in animals and humans across development. The review also briefly discusses developmental trajectories of the relevant neural circuits and the emergence of clinical anxiety. Future studies should focus on developmental changes in these paradigms, paying close attention to neurobiological and hormonal changes associated with childhood and adolescence.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

Early postnatal stress alters extracellular signal-regulated kinase signaling in the corticolimbic system modulating emotional circuitry in adult rats

The present study elucidated whether early life stress alters the extracellular signal-regulated kinase (ERK) pathway that underlies fear retrieval and fear extinction based on a contextual fear conditioning paradigm, using a juvenile stress model. Levels of phospho-ERK (pERK), the active form of ERK, increased after fear retrieval in the hippocampal CA1 region but not in the medial prefrontal cortex (mPFC). ERK activation in the CA1 following fear retrieval was not observed in adult rats who received aversive footshock (FS) stimuli during the second postnatal period (2wFS), which exhibited low levels of freezing. In fear extinction, pERK levels in the CA1 were increased by repeated extinction trials, but they were not altered after extinction retrieval. In contrast, pERK levels in the mPFC did not change during extinction training, but were enhanced after extinction retrieval. These findings were compatible in part with electrophysiological data showing that synaptic transmission in the CA1 field and mPFC was enhanced during extinction training and extinction retrieval, respectively. ERK activation in the CA1 and mPFC associated with extinction processes did not occur in rats that received FS stimuli during the third postnatal period (3wFS), which exhibited sustained freezing behavior. The repressed ERK signaling and extinction deficit observed in the 3wFS group were ameliorated by treatment with the partial N-methyl-D-aspartate receptor agonist D-cycloserine. These findings suggest that early postnatal stress induced the downregulation of ERK signaling in distinct brain regions through region-specific regulation, which may lead to increased behavioral abnormalities or emotional vulnerabilities in adulthood.

Functional emergence of the hippocampus in context fear learning in infant rats

The hippocampus is a part of the limbic system and is important for the formation of associative memories, such as acquiring information about the context (e.g., the place where an experience occurred) during emotional learning (e.g., fear conditioning). Here, we assess whether the hippocampus is responsible for pups' newly emerging context learning. In all experiments, postnatal day (PN) 21 and PN24 rat pups received 10 pairings of odor-0.5 mA shock or control unpaired odor-shock, odor only, or shock only. Some pups were used for context, cue or odor avoidance tests, while the remaining pups were used for c-Fos immunohistochemistry to assess hippocampal activity during acquisition. Our results show that cue and odor avoidance learning were similar at both ages, while contextual fear learning and learning-associated hippocampal (CA1, CA3, and dentate gyrus) activity (c-Fos) only occurred in PN24 paired pups. To assess a causal relationship between the hippocampus and context conditioning, we infused muscimol into the hippocampus, which blocked acquisition of context fear learning in the PN24 pups. Muscimol or vehicle infusions did not affect cue learning or aversion to the odor at PN21 or PN24. The results suggest that the newly emerging contextual learning exhibited by PN24 pups is supported by the hippocampus.

A review of adversity, the amygdala and the hippocampus: a consideration of developmental timing

A review of the human developmental neuroimaging literature that investigates outcomes following exposure to psychosocial adversity is presented with a focus on two subcortical structures – the hippocampus and the amygdala. Throughout this review, we discuss how a consideration of developmental timing of adverse experiences and age at measurement might provide insight into the seemingly discrepant findings across studies. We use findings from animal studies to suggest some mechanisms through which timing of experiences may result in differences across time and studies. The literature suggests that early life may be a time of heightened susceptibility to environmental stressors, but that expression of these effects will vary by age at measurement.

The effect of temporary amygdala inactivation on extinction and reextinction of fear in the developing rat: unlearning as a potential mechanism for extinction early in development

It is well accepted that fear extinction does not cause erasure of the original conditioned stimulus (CS)-unconditioned stimulus association in the adult rat because the extinguished fear often returns (e.g., renewal and reinstatement). Furthermore, extinction is NMDA and GABA dependent, showing that extinction involves new inhibitory learning. We have recently observed each of these extinction-related phenomena in 24-d-old but not in 17-d-old rats. These results suggest that different neural processes mediate extinction early in development. However, the neural processes underlying extinction in the developing rat are unknown. Therefore, the present study investigated amygdala involvement in extinction and reextinction during development. In experiment 1, temporary inactivation of the amygdala (using bupivacaine, a sodium channel modulator) during extinction training impaired extinction of conditioned fear in 17- and 24-d-old rats. In experiment 2, 17- and 24-d-old rats were conditioned, extinguished, and then reconditioned to the same CS. After reconditioning, the CS was reextinguished; at this time, some rats at each age had their amygdala temporarily inactivated. Reextinction was amygdala independent in 24-d-old rats, as previously shown in adult rats. However, reextinction was still amygdala dependent in 17-d-old rats. In Experiment 3, the age at conditioning, reconditioning, reextinction, and test was held constant, but the age of initial extinction varied across groups; reextinction was found to be amygdala independent if initial extinction occurred at 24 d of age but amygdala dependent if it occurred at 17 d of age. Consistent with previous findings, these results show that there are fundamental differences in the neural mechanisms of fear extinction across development.

A developmental dissociation in reinstatement of an extinguished fear response in rats

Recently, studies from our laboratory have shown that 16-day-old rats, in contrast to 23-day-old rats, fail to show either ABA renewal or recovery of an extinguished fear response following a pre-test injection of FG7142 [Kim, J. H. & Richardson, R. (2007). A developmental dissociation of context and GABA effects on extinguished fear in rats. Behavioral Neuroscience; Yap & Richardson, unpublished data]. The present study, using freezing as a measure of learned fear, extends these findings by examining whether there is a developmental difference in susceptibility to reinstatement following extinction. 16- and 23-day-old rats were trained to fear a white-noise conditioned stimulus (CS) by pairing it with a shock unconditioned stimulus (US). This fear was subsequently extinguished by non-reinforced presentations of the CS. Some rats received a post-extinction Reminder which consisted of a single presentation of a reduced-intensity US. Experiments 1 and 2 demonstrated that this Reminder was effective in reinstating extinguished fear in 23-day-olds, and that this reinstatement effect was context-specific in rats this age. In contrast, 16-day-old rats failed to show the reinstatement effect in either experiment. The failure to observe a post-extinction reinstatement effect in the 16-day-olds was not due to a general ineffectiveness of the Reminder treatment at this age because it did alleviate spontaneous forgetting in rats this age (Experiment 3). Taken together, the results suggest that fundamentally different processes may mediate extinction early in development compared to later in development.

Postnatal development of synaptopodin expression in the rodent hippocampus

Synaptopodin is an actin-binding protein of renal podocytes and dendritic spines. We have recently shown that synaptopodin is localized to the spine apparatus, a characteristic organelle of dendritic spines on forebrain neurons. Synaptopodin-deficient mice do not form spine apparatuses, indicating a role of synaptopodin in the formation of this organelle. Here we studied the development of synaptopodin expression in the postnatal rat hippocampus. At birth, synaptopodin mRNA is mainly expressed in CA3 pyramidal neurons. At postnatal day (P) 6, synaptopodin mRNA expression is still strongest in CA3 but is now also found in CA1 pyramidal neurons and granule cells of the suprapyramidal blade of the dentate gyrus. At P9, an almost adult pattern is seen with synaptopodin mRNA expressed by virtually all principal neurons. While synaptopodin mRNA was restricted to cell somata, immunostaining for synaptopodin protein labeled dendritic layers. At birth, no immunoreactivity was visible, while at P5 a weak staining mainly in stratum oriens was observed. At P9, immunolabeling was still strongest in stratum oriens followed by the molecular layer of the dentate gyrus. The adult pattern with strong labeling of all dendritic layers was reached by P12. Together these findings show that synaptopodin expression follows the well-known sequence of hippocampal principal neuron development. Unexpectedly, we also observed synaptopodin mRNA expression in a small population of interneurons as revealed by double labeling with interneuron markers. However, no immunolabeling for synaptopodin was observed in identified interneurons, confirming that the protein is mainly present in spine-bearing principal cells.

Early eyelid opening enhances spontaneous alternation and accelerates the development of perforant path synaptic strength in the hippocampus of juvenile rats

Development of the hippocampus is not entirely preprogrammed; its structure and function are sensitive to postnatal experience. For instance, neonatal handling/exposure to novelty and peripubertal environmental enrichment enhance hippocampal function and related memory abilities. However, these complex environmental manipulations make it difficult to deduce the primary stimuli that drive more rapid hippocampal maturation, and few experiments have studied the neural mechanisms that support the behavioral modifications. To address these issues, I performed early eyelid opening in rat pups and examined developmental alterations in exploration of a Y-maze and in synaptic transmission measured in hippocampal slices. Early eyelid opening accelerated development of spontaneous alternation. Additionally, early eyelid opening promoted more rapid remodeling of afferent input to the dentate gyrus and area CA1 as well as earlier maturation of perforant path synaptic physiology. These findings implicate visual input as an extrinsic factor that drives hippocampal development and the emergence of hippocampal-dependent behavior.

In vivo recordings of long-term potentiation and long-term depression in the dentate gyrus of the neonatal rat

Previous in vitro studies demonstrated that long-term potentiation (LTP) could be elicited at medial perforant path (MPP) synapses onto hippocampal granule cells in slices from 7-day-old rats. In contrast, in vivo studies suggested that LTP at perforant path synapses could not be induced until at least days 9 or 10 and then in only a small percentage of animals. Because several characteristics of the oldest granule cells are adult-like on day 7, we re-examined the possibility of eliciting LTP in 7-day-old rats in vivo. We also recorded from 8- and 9-day-old rats to further elucidate the occurrence and magnitude of LTP in neonates. With halothane anesthesia, all animals in each age group exhibited synaptic plasticity of the excitatory postsynaptic potential following high-frequency stimulation of the MPP. In 7-day-old rats, LTP was elicited in 40% of the animals and had an average magnitude of 143%. Long-term depression (LTD) alone (magnitude of 84%) was induced in 40% of the animals, while short-term potentiation (STP) alone (magnitude of 123%) was induced in 10%. STP followed by LTD was elicited in the remaining 10%. Data were similar for all ages combined. In addition, the N-methyl-d-aspartate (NMDA) antagonist (R,S)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked the occurrence of LTP at each age and doubled the percentage of animals expressing LTD alone for all ages combined. These results demonstrate that tetanic stimulation can elicit LTP or LTD at MPP synapses in 7-day-old rats, supporting our premise that at least a portion of the dentate gyrus is functional at this early age.

Maturation of granule cell dendrites after mossy fiber arrival in hippocampal field CA3

Most granule neurons in the rat dentate gyrus are born over the course of the first 2 postnatal weeks. The resulting heterogeneity has made it difficult to define the relationship between dendritic and axonal maturation and to delineate a time course for the morphological development of the oldest granule neurons. By depositing crystals of the fluorescent label Dil in hippocampal field CA3, we retrogradely labeled granule neurons in fixed tissue slices from rats aged 2-9 days. The results showed that all labeled granule cells, regardless of the age of the animal, exhibited apical dendrites. On day 2, every labeled neuron had rudimentary apical dendrites, and a few dendrites on each cell displayed immature features such as growth cones, varicosities, and filopodia. Some cells displayed basal dendrites. By day 4, the most mature granule neurons had longer and more numerous apical branches, as well as various immature features. Most had basal dendrites. On days 5 and 6, the immature features and the basal dendrites had begun to regress on the oldest cells, and varying numbers of spines were present. On day 7, the first few adult-like neurons were seen: immature features and basal dendrites had disappeared, all dendrites reached the top of the molecular layer, and the entire dendritic tree was covered with spines. These data show that dendritic outgrowth occurs before, or concurrent with, axon arrival in the CA3 target region, and that adult-like granule neurons are present by the end of the first week.

Nonconvulsive Kainic Acid-Induced Seizures Elicit Age-Dependent Impairment of Memory for the Elevated Plus-Maze

The goal of this study was to evaluate changes in spatial learning in adult and immature rats during and after nonconvulsive seizures. An elevated plus-maze was used in 18- and 25-day-old and adult rats. Kainic acid (KA 6 mg/kg) was administered 60 minutes before the first exposure (Experiment 1) or after a 3-day pretraining (Experiment 2, only adult rats). Animals were retested three times with 24-hour intervals. EEG activity was monitored in 18-day-old rats. KA prolonged the transfer latency (TL) in all age groups. In the youngest group the TL was prolonged 24 hours after KA when epileptic EEG graphoelements were still registered. In both older groups, prolonged TL was measured only 60 minutes after KA. In the pretrained adults, significantly prolonged TLs persisted for 24 hours after KA. KA changed the performance of adult and immature rats in the elevated plus maze not only during nonconvulsive seizures but also 24 hours later.

Kinetic parameters of calcium currents in maturing acutely isolated CA1 cells

Calcium currents were recorded in CA1 hippocampal cells from immature (P(4-10)) and older (P(22-55)) rats, using whole-cell voltage clamp techniques. Parameters defining the voltage-dependence of activation (tau(m)) and inactivation (tau(h)), steady-state inactivation and activation were determined at both stages of maturation. Current density increased with maturation. A transient low voltage activated (l.v.a.) current was found in P(4-10) cells, but not in the older cells. At voltages less negative than -30 mV, current inactivation was best described by two exponentials (tau(hf), tau(hs)); the ratio of the amplitudes of the two components changed with maturation, with a dominance of the faster component (tau(hf)) in the younger cells. The voltage dependence of tau(hf) followed a simple dependence model, decreased with increasing depolarization, in all cells at both stages of maturation. In P(4-10) cells, tau(hs) was voltage insensitive (range -25 to +30 mV); in P(22-55) cells, the voltage dependence of tau(hs) was found to be complex. Two current components were identified from the voltage dependence of the conductance in both groups. The first, more hyperpolarized component, the l.v.a. current found in P(4-10) cells; this was absent in the older cells, in which we found a component with a different voltage dependence. The voltage dependence of the conductance of the second, more depolarized component did not differ in younger and older cells. In the course of maturation, the steady-state inactivation of the second component underwent a hyperpolarizing shift and a decrease in voltage sensitivity.

Impaired cued and contextual memory in NPAS2-deficient mice

Neuronal PAS domain protein 2 (NPAS2) is a basic helix-loop-helix (bHLH) PAS domain transcription factor expressed in multiple regions of the vertebrate brain. Targeted insertion of a beta-galactosidase reporter gene (lacZ) resulted in the production of an NPAS2-lacZ fusion protein and an altered form of NPAS2 lacking the bHLH domain. The neuroanatomical expression pattern of NPAS2-lacZ was temporally and spatially coincident with formation of the mature frontal association/limbic forebrain pathway. NPAS2-deficient mice were subjected to a series of behavioral tests and were found to exhibit deficits in the long-term memory arm of the cued and contextual fear task. Thus, NPAS2 may serve a dedicated regulatory role in the acquisition of specific types of memory.

Choline availability to the developing rat fetus alters adult hippocampal long-term potentiation

Supplementation with choline during pregnancy in rats causes a long-lasting improvement of visuospatial memory of the offspring. To determine if the behavioral effects of choline are related to physiological changes in hippocampus, the effect of perinatal choline supplementation or deficiency on long-term potentiation (LTP) was examined in hippocampal slices of 6-8 and 12-14 month old rats born to dams consuming a control, choline-supplemented, or a choline-free diet during pregnancy. Stimulating and recording electrodes were placed in stratum radiatum of area CA1 to record extracellular population excitatory postsynaptic potentials (pEPSPs). To induce LTP, a theta-like stimulus train was generated. The amplitude of the stimulus pulses was set at either 10% or 50% of the stimulus intensity which had induced the maximal pEPSP slope on the input/output curve. We found that at both ages, a significantly smaller percentage of slices from perinatally choline-deficient rats displayed LTP after 10% stimulus intensity (compared with control and choline-supplemented rats), and a significantly larger percentage of slices from choline-supplemented rats displayed LTP at 50% stimulus intensity (compared with control and choline-deficient rats). Results reveal that alterations in the availability of dietary choline during discrete periods of development lead to changes in hippocampal electrophysiology that last well into adulthood. These changes in LTP threshold may underlie the observed enhancement of visuospatial memory seen after prenatal choline supplementation and point to the importance of choline intake during pregnancy for development of brain and memory function.

Cortistatin is expressed in a distinct subset of cortical interneurons

Cortistatin is a presumptive neuropeptide that shares 11 of its 14 amino acids with somatostatin. In contrast to somatostatin, administration of cortistatin into the rat brain ventricles specifically enhances slow wave sleep, apparently by antagonizing the effects of acetylcholine on cortical excitability. Here we show that preprocortistatin mRNA is expressed in a subset of GABAergic cells in the cortex and hippocampus that partially overlap with those containing somatostatin. A significant percentage of cortistatin-positive neurons is also positive for parvalbumin. In contrast, no colocalization was found between cortistatin and calretinin, cholecystokinin, or vasoactive intestinal peptide. During development there is a transient increase in cortistatin-expressing cells in the second postnatal week in all cortical areas and in the dentate gyrus. A transient expression of preprocortistatin mRNA in the hilar region at P16 is paralleled by electrophysiological changes in dentate granule cells. Together, these observations suggest mechanisms by which cortistatin may regulate cortical activity.

Diet-induced alterations in the ontogeny of long-term potentiation

The ability of prenatally malnourished rats to establish and maintain long-term potentiation (LTP) of the perforant path/dentate granule cell synapse was examined in freely moving rats at 15, 30, and 90 days of age. Measures of the population EPSP slope and population spike amplitude (PSA) were calculated from dentate field potential recordings obtained prior to and at various times following tetanization of the perforant pathway. Significant enhancement of both population EPSP slope and PSA measures was obtained from all animals of both malnourished and well-nourished diet groups at 15 days of age. However, the magnitude of enhancement obtained from 15-day-old prenatally malnourished animals was significantly less than that of age-matched, well-nourished controls. At 30 days of age, PSA measures obtained from approximately 50% of prenatally malnourished 30-day-old rats showed no significant effect of tetanization, while measures obtained from the remaining 50% of these animals did not differ significantly from controls. EPSP slope measures for this age group followed much the same pattern, i.e., malnourished animals showing no significant enhancement of PSA measures exhibited only slight increases in EPSP slope beginning 1 h after tetanization and returned to baseline by 18 h post-tetanization. EPSP slope measures obtained from PSA-enhanced malnourished animals did not differ significantly from controls. At 90 days of age, PSA measures obtained from 50% of malnourished animals declined from pretetanization levels immediately following tetanization. Three hours after tetanization, however, this measure had increased to a level which did not differ significantly from that of the control group. PSA measures obtained from the remaining 50% of 90-day-old malnourished animals showed initial and sustained enhancement which did not differ significantly from those obtained from well-nourished age-matched controls. These results indicate that gestational protein malnutrition significantly affects the magnitude of tetanization-induced enhancement of dentate granule cell response in preweanling rats (15-day-old animals) and significantly alters the time-course and magnitude of potentiation in approximately half of prenatally malnourished animals tested at 30 and 90 days of age. Given the primarily postnatal development of the dentate granule cells, these results may reflect malnutrition-induced delays in the neurogenesis and functional development of granule cells previously reported by our group. Most striking is the fact that significant impairments in LTP establishment were obtained from prenatally malnourished animals at 90 days of age, implying that dietary rehabilitation commencing at birth is an intervention strategy incapable of ameliorating the effects of the gestational insult.

Post-activation potentiation in the neocortex. III. Kindling-induced potentiation in the chronic preparation

Previous experiments have shown the neocortex to be very resistant to the induction of long-term potentiation in chronic preparations. We show here that kindling-induced potentiation effects can be reliably produced in the neocortex of awake, freely moving rats. These effects develop rather slowly. In sites contralateral to the stimulation electrode, potentiation effects did not become clear until the animals had received about 5 days or more of stimulation. Ipsilateral sites required even longer (approximately 10 days), and both sites required more than 13 days to reach asymptotic levels of potentiation. Both monosynaptic and polysynaptic components were present in the neocortical field potentials. When population spikes were absent, the surface negative monosynaptic EPSP component tended to show a potentiation effect. If population spikes were present, they were generally enhanced while the monosynaptic population EPSP tended to be depressed. Consequently, the apparent depression may have been due to competing field currents. The later polysynaptic components (15-28 ms latency to peak) always showed a potentiation effect with 5 or more kindling stimulations and is presumed to result from activation of cortico-cortical associational fibers. All of these effects were long-lasting, showing little decay over a period of several weeks.

Post-activation potentiation in the neocortex. IV. Multiple sessions required for induction of long-term potentiation in the chronic preparation

The neocortex in chronically prepared rats is very resistant to the induction of long-term potentiation (LTP). In the first of two experiments described in this paper, we tried unsuccessfully to induce neocortical LTP within one session by coactivating basal forebrain cholinergic and cortical inputs to our neocortical recording site. In the second experiment, we tested a new procedure which involved the application of repeated conditioning sessions over several days. This procedure was suggested by our finding that kindling-induced potentiation (KIP) of cortical field potentials could be reliably triggered but was slow to develop. We administered 30 high frequency trains per day to the corpus callosum for 25 days. LTP in callosal-neocortical field potentials became clear after about 5 days of stimulation and reached asymptotic levels by about 15 days. After the termination of treatment, LTP persisted for at least 4 weeks, the duration of our post-stimulation test period. As in previous experiments on kindling-induced potentiation, the potentiation effects were clear in both early population spike components and in a late (probably disynaptic) component. The monosynaptic EPSP component was often depressed, but this may have been due to competing field currents generated by the enhanced population spike activity. We discuss these results in the context of theories emphasizing slower but more permanent memory storage in neocortex compared to the hippocampus.

Quantitative analysis of long-term potentiation in the hippocampal dentate gyrus of the freely-moving 15-day-old rat

The magnitude and duration of long-term potentiation (LTP) of perforant path/dentate granule cell synapses was examined in freely moving rats beginning at 15 days of age. Measures of dentate granule cell population EPSP slope and population spike amplitude (PSA) obtained before and after tetanization were used to evaluate the level of LTP. Tetanization resulted in significant enhancement of both the population EPSP slope (approximately +75%) and PSA (approximately +40%) measures. This enhancement was maintained without significant change for 18 h, after which both measures began a steady and continuous rise. Daily input/output response measures from age-matched nontetanized animals were used to factor out enhancement related to normal development. Under this schema, tetanization-induced enhancement of both EPSP slope and PSA measures decayed slowly, beginning 18-24 h after tetanization, returning to baseline 5 days after tetanization. Enhancement obtained from 90-day-old animals decayed to baseline 24 h after tetanization. The longer duration of LTP obtained from preweanlings is discussed with regard to the development of inhibitory systems modulating granule cell excitability.

Sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats: positive correlation between LTP and contextual learning

Three experiments investigated sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats. Experiment 1 revealed a robust sex difference in the magnitude of LTP induced at perforant path synapses in the dentate gyrus of pentobarbital-anesthetized rats. This sex difference in LTP was evident in rats of 35 and 60 days of age and was not the result of pre-LTP sex differences in perforant path synaptic transmission; 20-day-old rats did not show LTP. An analysis of field potentials evoked during LTP induction revealed a sex difference in the magnitude of N-methyl-D-aspartate (NMDA) receptor activation that was highly correlated with the magnitude of LTP. Experiment 2 showed that males condition more fear, measured as freezing, to the contextual conditional stimuli (CSs) of a conditioning chamber compared to their female counterparts. This sex difference in conditional freezing was apparent with both low and high unconditional stimulus (US, footshock) intensities. Experiment 3 revealed that the enhanced fear conditioning in males was specific to contextual CSs, and consisted of a more rapid rate of conditioning. Together, these experiments reveal a positive correlation between the magnitude of hippocampal LTP and a form of learning that depends on the hippocampus. Furthermore, they suggest a neural basis for sex differences in hippocampus-dependent learning tasks.

Development of inhibitory and excitatory synaptic transmission in the rat dentate gyrus

We studied the ontogeny of inhibitory and excitatory processes in the rat dentate gyrus by examining paired-pulse plasticity in the hippocampal slice preparation. The mature dentate gyrus produces characteristic paired-pulse responses across a wide range of interpulse intervals (IPI). Paired-pulse effects on population excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude were analyzed at postnatal day 6 (PN6), PN7/8, PN9/10, PN15/16, and PN > 60. The synaptic paired-pulse profile (10-5,000 ms IPI) matured by PN7/8. The triphasic pattern of short-latency depression, a relative facilitation at intermediate intervals, and long-latency depression was present at all ages tested. Paired-pulse effects on granule cell discharge indicated the presence of weak short-latency (20 ms IPI) inhibition at PN6, the earliest day that a population spike could be evoked. By PN7/8, short-latency inhibition was statistically equivalent to the mature dentate gyrus. Long-latency (500-2,000 ms IPI) PS inhibition was present, and equal to the mature dentate gyrus by PN6. The most consistent difference between the mature and developing dentate gyrus occurred at intermediate IPIs (40-120 ms) where spike facilitation was significantly depressed in the development groups. The studies indicate that short-term plasticity matures rapidly in the dentate gyrus and suggest that the inhibitory circuitry can function at a surprisingly early age.

Maturation of long-term potentiation in the hippocampal dentate gyrus of the freely moving rat

The ability of the perforant path/dentate granule cell synapse of the hippocampal formation to establish and maintain enhanced levels of synaptic transmission in response to tetanization (long-term potentiation, LTP) was investigated in freely moving rats at 15, 30, and 90 days of age. Measures of 1) the slope of the population excitatory postsynaptic potential (EPSP), and 2) the population spike amplitude (PSA) obtained before, and at several times following tetanization, were used to evaluate the magnitude and duration of LTP as a function of age. Significant enhancement of both EPSP slope and PSA measures was obtained from animals of all three ages in response to perforant path tetanization. The initial degree of enhancement was essentially the same across the age groups, ranging from +27% to +38% of pretetanization levels for EPSP slope measures and +60% to +75% of pretetanization levels for PSA measures, obtained 15 min after tetanization. The duration of this enhancement obtained from animals of the preweaning group was significantly longer than that obtained from either 30- or 90-day-old animals. Enhanced measures of both EPSP slope and PSA decayed to baseline levels in these older animals 18 to 24 h after tetanization, while animals tetanized at 15 days of age maintained potentiated levels of both measures for a period of 5 days following tetanization. Tetanization of 15-day-old animals resulted in a significant reduction in the latency to EPSP onset without affecting the time-based relationships among the other measured parameters, which included latency of the population spike onset, population spike minimum, and population spike offset.(ABSTRACT TRUNCATED AT 250 WORDS)

Post-activation potentiation and depression in the neocortex of the rat: II. Chronic preparations

Although long-term potentiation (LTP) has been demonstrated in a number of subcortical sites in chronic preparations, there have been no demonstrations of LTP in the neocortex of chronic preparations. Even neocortical slice and acute preparations often require a drug-induced suppression of inhibition before LTP effects can be reliably induced. We have attempted to induce LTP in neocortical sites in 7 different experiments using chronically prepared adult rats. We were unable to obtain any evidence, even a trend, for the induction of LTP. The following manipulations were tested: (1) standard stimulation train parameters that have been shown to be highly effective in subcortical and hippocampal sites; (2) a 10-fold increase in the intra-train pulse durations; (3) variations in train pulse frequency (1 Hz to 300 Hz) and train duration (100 ms to 15 min); (4) co-activation of multiple inputs by stimulation of combinations of cortical sites or cortical and thalamic sites; (5) reduction of inhibition by administration of picrotoxin; 5) Housing of animals in an enriched environment; (6) utilization of the neocortical stimulation trains as a cue in a learning task; (7) application of pilocarpine to co-activate cholinergic systems. Although none of these manipulations produced LTP, the application of pilocarpine did facilitate the induction of a long-lasting depression effect. These findings contrast with the results obtained from anesthetized rats and from studies using brain slices, where LTP can be reliably induced. These results are discussed in light of other recent findings with respect to LTP and LTD effects.

Post-activation potentiation in the neocortex: I. Acute preparations

Long-term potentiation is widely studied as a memory model, and has been demonstrated in a number of subcortical sites in both acute and chronic preparations. In the neocortex, however, most of the demonstrations of LTP have been in neocortical slice or acute preparations, and even these have often required a drug-induced attenuation of inhibition before the LTP could be reliably expressed. In this paper we show that LTP can be reliably expressed in adult rats in a number of neocortical sites, both ipsilateral and contralateral to the site of callosal stimulation. We also show that, when recording field potentials, LTP is expressed roughly equally at all cortical depths. In a third experiment, we monitored input/output (I/O), paired-pulse inhibition and short-term potentiation effects over the course of LTP induction. The ipsilateral responses were, as expected, of shorter latency and larger amplitude than contralateral responses. They also showed small spike-like components that correlated with cell discharge. Nevertheless, the contralateral responses tended to show the largest LTP effects. The paired pulse effect was mainly depression, lasting for up to 3000 ms, at both ipsilateral and control sites. The short-term potentiation components were best fit by two summed exponentials with time constants of about 70 s and 12 min. The LTP effect lasted at least two h which was the longest period monitored in these experiments.

Neurobiology of associative learning in the neonate: early olfactory learning

Mammalian neonates have been simultaneously described as having particularly poor memory, as evidenced by infantile amnesia, and as being particularly excellent learners with unusually plastic nervous systems that are easily influenced by experience. An understanding of the neurobiological constraints and mechanisms of early learning may contribute to a unified explanation of these two disparate views. Toward that end, we review here our work on the neurobiology of learning and memory in neonates. Specifically, we have examined the neurobiology of early learning using an olfactory classical conditioning paradigm. Olfactory classical conditioning in neonates at the behavioral level conforms well with the requirements and outcomes of classical conditioning described in adults. Furthermore, specific neural correlates of this behavioral conditioning have been described including anatomical and physiological changes, neural pathways, and modulatory systems. In this Review, we outline the behavioral paradigm, the identified neural correlates, and apparent mechanisms of this learning. Finally, we compare the neurobiology of early learning with that reported for mature animals, with specific reference to the role of US-CS convergence, memory modulation, consolidation, and distributed memory.

Paired-pulse and frequency potentiation of cortical responses in developing rats

The postnatal development of paired-pulse and frequency potentiations of the first positive and negative components (P1N1) of the cortical interhemispheric response (IHR) was studied in urethane anesthetized rats aged from 7 to 90 days. The paired-pulse potentiation appeared in the rat sensorimotor cortex starting from the age of 15 days. The magnitude of potentiation increased with age. The interpulse interval inducing maximum potentiation shortened from 125 ms in 15-day-old rats to 70 ms in adult rats. Similar results concerning the paired-pulse responses were found for visual cortex but the maturation was somewhat delayed--potentiation first appeared at postnatal day (PND) 18. The frequency potentiation reached adult properties in the sensorimotor cortex by PND 25. There is no time coincidence in the development of the two potentiation phenomena studied, paired pulse potentiation appeared earlier than frequency potentiation.

Ontogeny of hippocampal afterdischarges in the urethane-anesthetized rat

Experimental studies have shown that seizure manifestations vary as the brain develops. This study investigated the characteristics of afterdischarges in the hippocampal circuits at various ages in the developing rat. Rats from the following post-natal periods were tested: PN 10-11, PN 14-15, PN 17-19, PN 21-23 and PN 25-27. Animals were anesthetized with urethane and recording electrodes placed in the hippocampus bilaterally. Stimulating electrodes were placed in the left CA3 region and in the angular bundle. Afterdischarges were produced in all animals using stimulus trains of 20 or 50 Hz. Rats in the PN 10-11 and 14-15 age groups had afterdischarges that consisted of population spikes in CA1 and broad positive potentials in the dentate gyrus. Between PN 17 and 19, maximal dentate activation, which consists of bursts of large amplitude population spikes in the dentate gyrus, first appeared in response to 20 Hz stimulation to CA3 or either 20 or 50 Hz stimulation to the angular bundle. Rats older than 21 days had afterdischarge patterns like those recorded in the adult. These data indicate that, in the rat, the seizure capabilities of the limbic circuits go through a major transition period around PN 17-19. The appearance of maximal dentate activation marks the ability of the developing rat brain to produce and sustain reverberatory seizure discharges.

Olfactory associative conditioning in infant rats with brain stimulation as reward: II. Norepinephrine mediates a specific component of the bulb response to reward

One of the circuits modified by early olfactory learning is in the olfactory bulb. Specifically, response patterns of mitral-tufted cells are modified by associative conditioning during the early postnatal period. In addition, previous work has demonstrated that mitral-tufted cell single units respond to both olfactory conditioned stimuli and rewarding stimulation of the medial forebrain bundle-lateral hypothalamus (MFB-LH). The present study suggests that norepinephrine beta-receptor activation is required for early olfactory learning using MFB-LH stimulation as reward. Propranolol injected before odor-MFB-LH pairings blocks the acquisition of conditioned behavioral responses and their neural correlates to the conditioned odor. Furthermore, propranolol blocks a specific class of the mitral-tufted cell responses to MFB-LH reward stimulation. The relationship of this response to reward and early learning is discussed.

A comparison of the ontogeny of excitatory and inhibitory neurotransmission in the CA1 region and dentate gyrus of the rat hippocampal formation

In vivo electrophysiological experiments were used to chart the ontogeny of excitatory and inhibitory neurotransmission in the hippocampal formation of rats. Using standardized protocols, responses in the dentate gyrus were quantified and systematically compared to similar measurements obtained in the CA1 region. Measurements were taken at numerous ages, ranging from postnatal day (PN) 6 to adults (PN 60). Excitation was monitored by two parameters recorded with extracellular electrodes in response to monosynaptic inputs to CA1 pyramidal cells or to dentate gyrus granule cells: maximum population spike (PSmax) amplitudes and maximum population excitatory postsynaptic potential slopes (pEPSP slopemax). Inhibition was assessed by a paired-pulse protocol to measure maximal inhibition (the potency of inhibition at an interpulse interval of 20 ms) and duration of inhibition (the interpulse interval at which paired-pulse inhibition changed to paired-pulse facilitation). Excitatory parameters matured later in the dentate gyrus than in CA1, consistent with the later appearance of granule cells. Until PN 21, pEPSPmax values in the dentate gyrus paralleled those in CA1; thereafter they diverged with far larger values in the dentate gyrus. Inhibitory parameters reached adult values between PN 14 and 18. In both regions paired-pulse responses consisted of three phases: (1) an initial inhibition; (2) a second facilitatory phase; and (3) a later inhibition. The maximal inhibition in the initial phase was comparable in both regions, but lasted longer in the CA1 region. The facilitation in the second phase was greater in the dentate gyrus, and the inhibition in the third phase was greater in the dentate gyrus. Results are discussed in terms of neurogenesis of principal cells and GABAergic cells in the regions of interest.

An in vivo study of the ontogeny of long-term potentiation (LTP) in the CA1 region and in the dentate gyrus of the rat hippocampal formation

The influences of stimulus intensity, intratrain frequency, and number of trains were studied for their effects on the development of long-term potentiation (LTP) in the hippocampal formation. LTP was analyzed with full input-output curves for population spike (PS) amplitudes (PS-LTP) calculated from responses elicited in the CA1 region and in the dentate gyrus by monosynaptic activation. A standardized protocol employing a sequence of stimuli was devised to systematically compare LTP in the dentate gyrus to that in the CA1 region in rats of various ages ranging from postnatal day (PN) 6 to adults (PN 60). In adult animals, the degrees of LTP were comparable in the dentate gyrus and CA1 region for the 3 stimulus strengths studied (intensity just subthreshold for PS, intensity giving 1/4 maximal PS, and intensity giving 1/2 maximal PS). LTP developed at different rates in the two regions, reaching adult values about two weeks after birth in CA1 but about 3 weeks after birth in the dentate gyrus. We postulate that differences in the rate of development in CA1 and in the dentate gyrus are related to the later neurogenesis of dentate granule cells and also possibly to a later functional maturation of N-methyl-D-aspartate (NMDA) receptor-channel complexes on these cells.

Afterpotential characteristics and firing patterns in maturing rat hippocampal CA1 neurones in in vitro slices

The postnatal evolution of depolarizing after-potentials (DAPs) and after-hyperpolarizations (AHPs) was studied in rat CA1 hippocampal neurones (5-68 days of age) using in vitro slices. Results were pooled into 4 age groups: P5-9, P10-16, P17-24 and P26-68. In P5-9 cells, DAPs were seen as passive signals, with a time constant similar to the time constant of the membrane. The evolution of the DAP was characterized by a decrease in amplitude, an increase in duration and a change in contour. In P10-16 and P17-24 cells, the DAPs often had a plateau or a hump-like shape which increased the probability of firing and the occurrence of spike doublets. The firing pattern and bursting behaviour of P10-16 CA1 neurones differed from the pattern typical of the adult. P5-9 and P10-16 cells had post-burst AHPs with a smaller amplitude and a more prolonged early phase than at late stages of development.

Horizontal long-term potentiation of responses in rat somatosensory cortex

The search for mechanisms in neocortex that change synaptic efficacy and produce associative learning through activity-dependent processes has focused on the role of glutamate receptors of the N-methyl-D-aspartate (NMDA) type. NMDA receptor activation is necessary for the induction of long-term potentiation (LTP) in hippocampus and in neocortex. The effect of NMDA receptor activation is modulated in several ways, including Mg2+ block of the NMDA-dependent channel which prevents Ca2+ entry until neurons become partially depolarized. We report that when NMDA receptor activation is facilitated by lowering the extracellular [Mg2+] in the bathing medium, a low-frequency train presented in layer VI induces potentiated responses throughout a wide horizontal extent of layer II/III in neocortical slices. The response amplitudes potentiated by 34-200% over baseline values depending on the intensity of the repetitive conditioning stimulus and the distance of the recording site from the stimulus. At the same time that pre-existing evoked responses were potentiated, horizontal spread of activity in layer II/III was facilitated resulting in responses appearing at sites more than 1 mm from the stimulus. This enhanced transmission of responses persisted for greater than 2 h, and its induction was prevented by selective NMDA receptor antagonists. The results show that the horizontal spread of activity can be increased by altering the conditions of the stimulus presentation. We conclude that the mechanisms supporting LTP could determine the area of neocortex that is activated by a sensory input.

Long-term potentiation in intact infant rat hippocampus

In rats 14-28 days of age, high-frequency stimulation of the perforant path granule cell synapse in the dentate gyrus produced long-term potentiation of the population spike that was comparable in magnitude (150-250% of baseline) and duration (120 min) to that produced in adult animals with the same stimulation paradigm. In contrast, potentiation of the excitatory postsynaptic potential occurred inconsistently.

Effect of minaprine on synaptic transmission in the neocortex of the rat in vivo

The effects of minaprine and/or excitatory amino acid antagonists on transcallosal responses were examined in urethane-anesthetized rats. The transcallosal response was recorded from the surface of the anterior neocortex, following electrical stimulation of the contralateral corpus callosum. The transcallosal response consisted of a biphasic positive-negative waveform. Intravenously-administered minaprine increased the amplitude of the positive- and negative-waves, in a dose-dependent manner. Intracortical injection of (+/-)-2-amino-5-phosphonovalerate (APV) and gamma-D-glutamylglycine (DGG) reduced the amplitude of the negative-wave, with no effects on the amplitude of the positive-wave. L-Glutamate diethylester (GDEE) had no effect on the transcallosal response. The minaprine-induced increase in the amplitude of the negative-wave was completely antagonized by simultaneous intracortical injections of APV and DGG which, per se, did not affect that transcallosal response. In contrast, APV and DGG had no effect on the increase in the amplitude of the positive-wave induced by minaprine. The enhancing effect of minaprine on the transcallosal response remained unaltered in case of an intracortical injection of GDEE. These findings indicate that the negative-wave of the transcallosal response may be related to receptors for excitatory amino acids. The possibility that the pharmacological action of minaprine on synaptic transmission in the neocortex may be linked to the excitatory amino acid receptors warrants further attention.

A critical period for long-term potentiation in the developing rat visual cortex

The in vitro rodent visual cortical slice preparation demonstrates a critical period for long-term potentiation (LTP). Current source density (CSD) analysis reveals peak potentiation of both supra-(layers II-III) and infragranular (layers V) layers of visual cortex during the second postnatal week following stimulation of the subadjacent white matter. By day 30 both the supra- and infragranular CSD sinks show only minimal potentiation. In adults there is no change in supragranular response but infragranular layers reveal 177% potentiation. Therefore, we conclude that rodent visual cortex displays a critical period for maximum plasticity of both supra and infra-granular layers. Supragranular visual cortex plasticity ends by day 30 whereas infragranular layers retain plastic qualities into adulthood.

Single-unit analysis of postnatal olfactory learning: modified olfactory bulb output response patterns to learned attractive odors

Neonatal rats learn to approach odors associated with stimulation normally provided by their mother. The present report describes changes in olfactory bulb single-unit activity following olfactory learning in young rats. Rat pups were exposed from postnatal day 1 to 18 to either (1) peppermint-scented air while receiving tactile stimulation (Pepp-Stroked), (2) peppermint-scented air with no tactile stimulation (Pepp-Only), (3) clean air and tactile stimulation (Stroked-Only), or (4) clean air and no tactile stimulation (Naive). On day 19, single-unit activity was recorded from mitral/tufted cells in urethane-anesthetized, freely breathing pups in response to either peppermint or a novel orange odor. Mitral/tufted cell response patterns to peppermint were significantly altered in Pepp-Stroked animals compared to control pups. Peppermint exposure alone, not associated with tactile stimulation (Pepp-Only), did not affect subsequent single-cell response patterns to that odor. In addition, the modification of response patterns was specific to peppermint and was not associated with a change in respiration rate. Furthermore, Pepp-Stroked pups had a relative behavioral preference for peppermint on day 19 compared to control pups. These results demonstrate that postnatal olfactory learning selectively modifies the subsequent response patterns of olfactory bulb output cells to the attractive odor. Furthermore, these results indicate that the initial coding of an odor's attractive value occurs within the olfactory bulb.

Early handling increases hippocampal long-term potentiation in young rats

The hippocampal formation undergoes major anatomical and physiological changes postnatally, and thus might be expected to be particularly sensitive to early handling effects. Long-term potentiation (LTP) in the hippocampal formation, a form of brain plasticity thought to be important in learning and memory, was examined in young rats following early handling or control treatments. The amplitude of LTP was reliably greater in rats receiving the early handling regime. Possible mechanisms and consequences of enhanced LTP were discussed.

Barbiturate-enhanced paired-pulse depression in neonatal rats

Paired-pulse depression of the population spike in the hippocampal formation of barbiturate anaesthetized mature rats is increased compared to that in urethane unanaesthetized rats. This barbiturate-induced increase in depression is greatly enhanced in immature animals, which are also known to be behaviorally more sensitive to barbiturates. These results suggest an increased sensitivity of the recurrent inhibitory (possible GABAergic) system to barbiturates in the immature central nervous system.