PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Eur J Neurosci. Author manuscript; available in PMC 2009 October 13.
Published in final edited form as:
PMCID: PMC2761206
NIHMSID: NIHMS145257

Age matters

Abstract

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty in using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasise that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Keywords: development, adult, LTP, hippocampus, dopamine

Introduction

Imagine if a pharmaceutical company registered a novel analgesic or antipsychotic drug intended for adult patients, and then announced that it would conduct its clinical trials in children. Without question, this proposal would be thought absurd, not only ethically but because the results might not be scientifically valid for adults. Clinical trials must recruit study subjects that represent the intended patient population.

Although this scenario seems extraordinary, it is in fact common in neuroscience to study young animals in which the brain has not fully developed. These studies can be informative and provide valuable information, but the age of animals needs to be considered as a factor that may influence the results. In addition, such young animals should not be erroneously defined as “adults”. In this commentary we argue that as practitioners and consumers of science, it is necessary to pay close attention to the age of animals used in experiments. We review the stages of rodent maturation and discuss key principles arising from our own data and a review of the literature. We highlight that brain development continues well after puberty, that quantitative changes may occur gradually or rapidly, and that the mechanisms underlying certain neurobiological processes change as an animal matures.

Why and to what extent is age overlooked?

Neuroscientists have at their disposal numerous model systems with which to carry out their research. A key decision in designing an experiment is choosing the best model with which to tackle the question at hand. In simpler model systems such as cell lines or in those that more closely resemble neural systems, such as embryonic or postnatal neuronal cultures, the focus of the study can be reductionist, allowing consideration of aspects of complex systems in narrower terms. Such models are extremely useful for the study of cellular and molecular mechanisms, and the limitations of these simplified systems are recognized by most readers and authors. For example, those studying the biophysical properties of ion channels in cell lines generally recognize that they are not modelling the intact nervous system.

Over the last decade there has been a noticeable shift from a ‘reductionist’ to a ‘systems’ perspective and with this has come a substantial growth in the use of ex vivo brain slices - in which neuronal architecture is better preserved - for the study of synaptic and cellular biology. This trend has been facilitated by technological advances. For example, techniques such as patch-clamp electrophysiology and cellular imaging, once only possible in isolated cells, are now routinely practiced in tissue slices. However, such studies are performed using animals of various ages and it is seldom acknowledged that they are often limited to very young animals. This is important because findings in young animals do not necessarily extrapolate to adults. Although this limitation is not confined to slice physiology, it is here that the use of young animals is most evident.

To assess the extent to which animals of different ages are being used in neuroscience research, we performed a systematic review of the literature in which we tabulated the ages at which rodents were tested. We examined (i) studies on hippocampal long-term potentiation (LTP) in rodents and (ii) studies using the Morris water maze test of spatial memory (see supporting information for methodology and references). In rodents, hippocampal LTP has been extensively investigated as a mechanism for spatial memory (Morris et al., 2003).

In our review on hippocampal LTP, we first examined studies that used ex vivo brain tissue preparations (see supporting information). Figure 1A shows that only a minority of studies (34%) used adult animals, while the majority (59%) used animals that had not yet reached adulthood (see next section for age definitions). The remaining 7% of studies did not specify an age or weight. Overall, the studies used a wide range of ages, from postnatal day 1 (P1) to P915 (i.e. 30 months). Furthermore, the age ranges at which animals were tested did not just vary between studies, but also within. Approximately 30% of experiments pooled animals whose ages varied by 3–4 weeks, often spanning critical age periods, such as puberty. We also found that 42% of studies defined animals as “adults”, although the methods section or supplementary online material clearly indicated that the average age of the animals was not within an adult range. When we repeated this analysis using the lowest age in each study (see supporting information for detailed explanation) we found that 60% of studies incorrectly defined young animals as “adults”. To highlight the problems in controlling for the developmental stage of rodents we looked at the various definitions of “adult” (or mature) in this cohort of papers and found that this varied across studies from a low of P24-25 to a high of P549 (18 months) (Larson et al., 2005; Fukata et al., 2006; Sim et al., 2006). The definition of “young adult” varied from a low of P21-22 to a high of P92 (3 months) (Xu et al., 2000; Meredith et al., 2003; Miyamoto et al., 2005; Sim et al., 2006; Lauterborn et al., 2007).

Figure 1
Results of a systematic review examining age distribution of rodents used in neuroscience experiments. A, Age distribution of subjects used in studies on hippocampal LTP using ex vivo (solid line, n=336) and in vivo (dashed line, n=112) preparations. ...

When we confined our analysis of ex vivo studies to experiments using voltage- or current-clamp slice electrophysiology (patch clamp or sharp electrodes) we found that 75% of studies were using young animals and only 20% examined adult animals; the rest (5%) did not specify the age of the experimental subjects (table 1).

Table 1
Studies on hippocampal LTP and Morris water maze (Morris WM) analysed with respect to experimental approaches and ages at which animals were tested.

Possible reasons for this bias towards young animals include improved cell visibility and identification in young tissue, primarily due to less advanced myelination. Although there are newer methods available to conduct such studies in older animals, they are more challenging. Interestingly though, the scarce use of adult animals was not just limited to voltage- and current-clamp studies; only 38% of experiments using field recordings in the tissue slice, and 45% of those using other techniques (e.g. western blots, immunohistochemistry, electron microscopy, etc.) examined adult animals (table 1). Such techniques do not rely on good individuation of cells, and could thus be performed easily in adult animals. Although a few of these ex vivo studies may have been specifically examining development and could therefore justify this choice of age, this is unlikely to be the case for the majority. Thus, studies of the same phenomenon (hippocampal LTP) in the live animal show quite a different age profile. As our next analysis shows (figure 1A and table 1), studies of hippocampal LTP using in vivo electrophysiology and/or behavioural tests, focused mostly on adult animals (65%); only 26% used young animals, and the rest (9%) did not report age. Likewise, studies using the Morris water maze to test spatial memory (figure 1B, table 1) were performed mainly on adult subjects (77%); only 17% of the studies were performed in young animals and in 5%, age was not specified. The lack of significant overlap between these different techniques makes it difficult to extrapolate between them and means that information gleaned from ex vivo work often does not directly complement studies of in vivo physiology and behaviour.

From birth to adulthood – the development of a lab rat

To determine when an animal is an adult, it is important to review the developmental stages the animal progresses through to reach adulthood. Both rats and mice show a similar developmental profile (figure 2). At P21, rodents are weaned i.e. separated from their mother, after which they begin to undergo sexual maturation (Sisk & Zehr, 2005). Sexual maturity is generally defined by vaginal opening (females) or balano-preputial separation (males). This point is reached in female rats at approximately P32-34 (Lewis et al., 2002) but in males, maturity occurs much later at around P45 to P48 (Lewis et al., 2002). However, the age of sexual maturity varies considerably between individuals, ranging from as young as P40 to as old as P76 in male rats (Lewis et al., 2002).

Figure 2
Different developmental stages of a rat. The body weight of a rat changes greatly during the first two postnatal months. Male rats weigh approximately 14 g at P7, 45 g at weaning (P21), 115 g as adolescents (P35) and 300 g by young adulthood (>P63). ...

It is also important to note that sexual maturity itself does not mark the beginning of adulthood, but rather denotes the beginning of adolescence. Like humans, rats progress through a period of adolescence characterised by behaviours such as increased risk-taking and social play. These behaviours extend well beyond the pubertal period through the transition to adulthood (Spear, 2000), which begins after the eighth week of postnatal life (~P63).

The body weight of an animal is sometimes considered an indicator of its age. However, weight is not an accurate surrogate marker for age. A comparison of the weights and ages from websites of two major vendors revealed that there is considerable variability both between vendors and between different colonies from the same vendor. In fact, Sprague Dawley rats are available in two sub-strains, one (originated by Charles River Laboratories, Wilmington, MA), which gains less weight than the other (originated by the Sprague Dawley Company, Madison, WI). Thus, a 300 g male Sprague Dawley rat can be between 57 and 70 days of age, depending on the vendor and colony (Charles River SD SAS, 67 days; Harlan SD, 57 days, Harlan CD, 70 days). Data from our own institution show that large variability exists even when rats are obtained from the same colony. As figure 3A shows, male rats weighing between 250 g and 274 g (a weight range commonly provided by vendors) differed in age by three weeks, from P49 (periadolescent) to P70 (young adulthood). In addition, male rats of the same exact age showed up to 100 g variation in body weight (figure 3B). Weight is, therefore, only an approximate marker of age.

Figure 3
Body weights and ages of male Sprague-Dawley rats received in our institution over the past four years. A, rats are divided in weight-ranges, corresponding to those provided by vendors. Within each weight-range, animals vary considerably in age. B, rats ...

Neural circuits change at different stages of postnatal development

A vast number of developmental processes are required to produce an adult animal from an embryo and many maturational changes occur during the early postnatal period. A number of these developmental changes are well known and extensively characterised. For example, the neurotransmitter γ-aminobutyric acid (GABA) switches from being depolarising to hyperpolarising during the first few postnatal days (Ben Ari et al., 1989; Rivera et al., 1999; Ben Ari, 2002). Likewise, NMDA receptor subunit composition changes gradually during the first few postnatal weeks (Monyer et al., 1994; Sheng et al., 1994; Zhong et al., 1995; Dumas, 2005a). Such changes radically alter the consequences of neurotransmitter release and receptor activation over early stages of development. However, even after these early stages, changes continue to occur and these are often less well characterised and recognised.

In the next sections we provide examples to show how such changes can impact experimental results.

Dopaminergic transmission – changes occurring during the periadolescent period

Some developmental changes are so abrupt that they render the adult system qualitatively different from its juvenile form. For example, electrophysiological studies in the prefrontal cortex (PFC) show that inhibitory fast-spiking GABAergic interneurons are unresponsive to dopamine D2 receptor activation when rats are prepubertal, aged P14-35 (Seamans et al., 2001; Gorelova et al., 2002; Tseng & O’Donnell, 2007). However, when rats reach P50 and start to approach adulthood, dopamine D2 receptor activation becomes excitatory in these neurons (figure 4; Tseng & O’Donnell, 2007). This is an example of a change occurring at a late time period (~P50-60), when many might have believed developmental processes had finished. In fact, those studies performed only at young ages erroneously concluded that dopamine D2 receptor activation does not modulate the activity of fast-spiking GABAergic interneurons in the PFC. This event is notable not only for its clear implications for adult neuromodulation of the PFC, but also because its sudden occurrence is indicative of a specific developmental shift, rather than a slower consequence of ageing.

Figure 4
Age-dependent responses to dopamine D2 receptor stimulation in prefrontal cortex fast-spiking interneurons. In ex vivo cortical slices, cells from pre-pubertal rats do not respond to quinpirole (1 μm), a D2 agonist whereas in adult rats quinpirole ...

Interestingly, another study of this system found that two populations of PFC interneurons develop dopamine D1 receptor-mediated excitation at different time points (Tseng & O’Donnell, 2007). Specifically, fast spiking interneurons show increased excitation to the application of a D1 dopamine receptor agonist before puberty whereas non-fast spiking interneurons only develop this response after P50. Thus, even within the same brain region, specific cell types can show different rates of development.

Dopamine in the PFC has been heavily implicated in schizophrenia, a disorder that typically develops during late adolescence and early adulthood. The significance of the late maturation of the PFC is heightened in this case; studying its role in schizophrenia using young animals would almost certainly result in misleading conclusions. Dopamine in the PFC is also involved in mediating a number of other traits and behaviours such as impulsivity and executive control and so the implications of these findings extend beyond schizophrenia research. In the examples above, early studies, performed only in young rats, led to the wrong assumptions about the role of dopamine in the PFC and this could have substantial, deleterious consequences on our understanding of disease processes and on drug development.

The ontogeny of dopamine receptor expression is another example of a system that changes during adolescence and provides an example of a developmental change that occurs gradually with an inverted U-shaped trajectory. Several studies have shown that D1 and D2 dopamine receptors increase gradually after birth, and peak peripubertally, before receptor number declines in later life. Notably, for different brain regions this peak in receptor expression differs, reflecting the timing of maturation for each structure. For example, in the striatum, dopamine receptor levels peak between P28 and P40 (Noisin & Thomas, 1989; Andersen et al., 2000), before declining to stable levels during P60-120 (Andersen et al., 2000; Tarazi & Baldessarini 2000). In contrast, in the PFC, in which maturation is delayed, maximal receptor expression does not occur until about P60 and then declines to a stable level between P80-120. Of note, is that in studies where the authors considered rats to be adult at P45-60, analysis of receptor expression was not continued beyond this age, and thus the decline that occurs in true adulthood was missed (Sales et al., 1989; Chen & Weiss, 1991; Tarazi & Baldessarini, 2000).

Another example of a U-shaped trajectory comes from our data on dopamine cell firing in the midbrain. Figure 5 shows that dopamine cell activity changes over age. Activity is low after weaning, peaks at about P45, and declines thereafter. Adolescence, thus, seems to be a distinct developmental stage, with respect to dopamine cell firing and dopamine receptor expression. The biological relevance of this peak may be related to certain distinctive behavioural characteristics associated with adolescence in many species including humans, such as increased risk-taking, impulsivity, and social play. Without examining later time points, however, this peak would not have been observed. In addition, these studies demonstrate that drawing conclusions from only a limited number of time points that do not include the adolescent period would be misleading; i.e. it would have been assumed that firing rate is constant from the weanling period to adulthood. It is also important to note that, although cells in juvenile rats (P24-27) and adults (>P70) have similar firing rates, this does not mean that other characteristics of these cells are similar. To give just one example, dopamine neurons from juvenile rats are typically excited by nicotine, whereas in adults they are primarily inhibited (Marinelli, 2008).

Figure 5
Age-dependent changes on firing rate of midbrain dopamine neurons. Dopamine neuron activity is low shortly after weaning, it increases during the adolescent period and peaks around P45, and it decreases thereafter. Dopamine neuron activity was assessed ...

These findings show that although some systems achieve features of their “adult” phenotype at ages preceding true adulthood, others do not. They also emphasise that maturational changes do not necessarily follow a simple linear course characterised by progressive increases or decreases. In addition to the PFC and midbrain, age-related changes in neuronal activity and reactivity to dopamine also occur in other brain areas, such as the arcuate nucleus and the striatum (Coyle et al., 1985; Napier et al., 1985; Arbogast & Voogt, 1991; Andersen et al., 2000). Hence, there is compelling evidence that throughout the brain dopamine circuits are not mature in young animals, and so any study of this system that uses young animals should be interpreted with caution.

Developmental changes in the induction and expression of LTP

LTP is one of the most studied phenomena in neuroscience and so provides a good illustrative case for this commentary. The above literature review (figure 1A) shows that studies of LTP use animals of many different ages ranging from before, during, and after puberty. LTP at different ages is likely to serve different functions and may well arise from different mechanisms (for review see Dumas, 2005b). Despite this, as shown in figure 1A, few studies of LTP have considered age to be a relevant factor. However, the few papers that have looked across different ages have identified important developmental differences.

Yasuda et al. (2003) examined changes in the way LTP is regulated in the first four postnatal weeks. Consistent with studies showing the essential role of calcium/calmodulin-dependent protein kinase II (CaMKII) for the induction of LTP (Lisman et al., 2002), Yasuda et al. (2003) found that CaMKII inhibition did indeed block LTP in hippocampal slices from three-week old animals (P21). In younger (P17-18) animals, however, inhibition of CaMKII had a far weaker effect and in P7-8 rats there was no effect at all. In contrast, inhibition of protein kinase A (PKA) had no effect on LTP in animals >P27 but blocked LTP in young animals (P7-8). Lu et al. (2007) extended this work showing that a second critical period for PKA emerges after P49, concluding that PKA is required for LTP induction at both early (<P27) and late (>P49) time points but not in between. These careful studies showed that what appears to be a unitary phenomenon at all ages – synaptic strengthening induced by the same pairing protocol – actually relies on different molecular mechanisms at different ages. Whether these particular characteristics change further as animals go further through adulthood, remains to be tested.

Lu et al. (2007) also demonstrated that a requirement for calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors for LTP induction - currently a controversial topic in the field - is also age-dependent. Specifically, these receptors were shown to be required in 2 week old (P14) and 8 week old (P56) mice but, crucially, not in 3 week old mice (P21). This age-dependency may explain why some authors have found calcium-permeable AMPA receptors to be critical for LTP induction whereas others have not. Lu et al. (2007) point out that two critical experiments with contradictory findings (Plant et al., 2006; Adesnik & Nicoll, 2007) may have been conducted with animals at different ages, spanning the time between when calcium-permeable AMPA receptors are required (P14) and when they are not (P21). Jensen et al. (2003) also showed a developmental shift in the requirement for putative calcium-permeable AMPA receptors for the induction of LTP.

The efficacy of different pairing protocols at inducing LTP during development was also examined across ages. Meredith et al. (2003) found that the effectiveness of pairing pre-synaptic stimulation with single post-synaptic action potentials (30 pairings) at CA3-CA1 synapses gradually decreases with advancing age until, in the “adult”, a paired burst of post-synaptic activity is required to elicit LTP (they considered their animals adults at P30-45, hence the quote marks). By manipulating GABA-A receptor-mediated inhibition the authors were able to reestablish the effectiveness of single spikes in the adult and so suggested that differences across age may reflect maturation of the GABA system. Whether further changes continue to occur after P45 remains unknown.

Age continues to be important factor in neurobiological studies of adult animals as well. LTP has been examined in mature animals to study age-related memory decline. As adult animals grow older, there is an increase in the induction threshold and potentiation becomes less stable (Barnes, 1979), and there is growing literature on how synaptic plasticity changes during both adulthood (Lynch et al., 2006) and old age (Foster, 1999). These studies provide another rich source of data about age-related changes in neural function, even after the animal has reached adulthood.

Development of spinal sensory processing

The examples presented above relate mostly to higher cortical functions, such as memory and cognition. Similar age-related factors may influence functions that involve subcortical neural structures. For example, although the neural circuitry for pain signalling in the spinal cord emerges at birth or shortly after, the type of afferent input to the spinal cord dorsal horn differs substantially between P23 rats and P60 rats (Park et al., 1999). Similarly, the expression of glutamatergic and GABAergic receptors in the spinal cord differs between immature and mature rats (Pattinson & Fitzgerald, 2004). Whether such reorganization and maturation in spinal cord dorsal horn participates in age-related differences in pain sensitivity (Pattinson & Fitzgerald, 2004; Howard et al., 2005) or if parallel changes are of greater import remains under investigation.

Summary and conclusions

Neuroscience studies, in particular those involving ex vivo electrophysiology, are performed on animals of all ages, mostly young animals. In addition, definitions of adulthood vary widely across studies, with the most concerning trend being the categorization of young rodents as adults. We have presented a number of examples that illustrate several reasons why it is essential to avoid this error, and why it is important to consider age as a critical experimental variable.

First, developmental changes continue far beyond the first few postnatal weeks. Therefore, accurate characterisation of the true adult phenotype requires the study of adult animals.

Second, developmental changes occur with different patterns or trajectories. For example, certain characteristics change gradually over time while others show much more sudden shifts. Additionally, certain phenotypes like dopamine receptor expression and dopamine cell firing rate show an inverted U-shaped trajectory, the peak of which could potentially define a critical period for a certain behaviour or phenomenon (Paus et al., 2008).

Third, phenomena that appear to be unitary in their form may be governed by different mechanisms at different ages. Here, we discussed work on LTP that showed a differing cohort of molecular effectors and rules for induction of synaptic potentiation at different ages.

The major reason that people have used younger animals for ex vivo electrophysiology studies is that, historically, many experiments have been difficult to perform in older animals. Now, however, techniques for using older animals in patch-clamp recordings are readily available and should not be considered overly daunting. Simple protocol adaptations, such as modifying visualisation methods, make recordings from adult animals attainable (Moyer & Brown, 1998; Moyer & Brown, 2002). In addition to cortex, hippocampus, and spinal cord, scientists have become proficient in obtaining patch-clamp recordings in adults from other structures including the striatum (Martin et al., 2005), midbrain (Fagen et al., 2007; Chen et al., 2008), and amygdala (Tye et al., 2008). An additional benefit of using older animals is that it is possible to collect data from animals that have been subjected to repeated manipulations, such as drug administrations or learning tasks. However, care should be taken that manipulations are performed in adult animals, because there is abundant evidence that the brain reacts differently to perturbations across ages (Andersen et al., 2000; Andersen, 2003; Bolaños et al., 2003; Black et al., 2006).

Because age-related changes have now been observed in numerous systems, we believe that it is essential for investigators to identify the developmental stage of their experimental subjects and to give due consideration to whether it is likely that their results can be generalised to adult ages. Although we have focused largely on rats, the same age-related issues apply to studies using different species including primates and mice. Thus, for each species, it is critical to be aware of the developmental period that subjects occupy. Such considerations are most important when the phenomenon to be modelled is known to be age-dependent. One of the best examples involves the link between PFC function and schizophrenia, a disease that does not emerge until late adolescence or early adulthood (Paus et al., 2008). Other traits and pathologies are age-dependent, for example, diseases associated with ageing such as Parkinson’s or Alzheimer’s Diseases, and these too may require animal models of appropriate age to be of relevance.

In conclusion then, stark developmental changes appear to be the rule rather than the exception. In all the examples considered herein, when investigators have conducted thorough investigations over long developmental time spans they have either overturned false assumptions in the field (e.g the essential role of CaMKII in LTP or the insensitivity of PFC interneurons to D2 dopamine receptor activation) or they have clarified discrepancies in the subject (e.g. whether calcium permeable AMPA receptors are required for LTP). Thus, giving proper consideration to matters of age has the potential to radically improve our understanding of brain function.

Supplementary Material

Acknowledgments

We would like to thank Kuei Yuang Tseng and Patricio O’Donnell for providing the figure on PFC function during development, K.Y.T for data on rat body weights, Nicole Tucci and Wai Chong Wong for help with the systematic review of the literature, Claire Cannon for producing the artwork in figure 2, Robert Messing, Liam Drew, Harriet de Wit, Mark Farrant, and Kuei Yuan Tseng for helpful comments on the manuscript, and Carrie Ferrario for suggesting the title of the manuscript. This work was supported by USPHS grant from the National Institutes of Health DA020654 to M.M.

Abbreviations

AMPA
α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
GABA
γ-aminobutyric acid
LTP
long term potentiation
P
postnatal day
PKA
protein kinase A
PFC
prefrontal cortex

References

  • Adesnik H, Nicoll RA. Conservation of glutamate receptor 2-containing AMPA receptors during long-term potentiation. J Neurosci. 2007;27:4598–4602. [PubMed]
  • Andersen SL. Trajectories of brain development: point of vulnerability or window of opportunity? Neurosci Biobehav Rev. 2003;27:3–18. [PubMed]
  • Andersen SL, Thompson AT, Rutstein M, Hostetter JC, Teicher MH. Dopamine receptor pruning in prefrontal cortex during the periadolescent period in rats. Synapse. 2000;37:167–169. [PubMed]
  • Arbogast LA, Voogt JL. Ontogeny of tyrosine hydroxylase mRNA signal levels in central dopaminergic neurons: development of a gender difference in the arcuate nuclei. Brain Res Dev Brain Res. 1991;63:151–161. [PubMed]
  • Barnes CA. Memory deficits associated with senescence: a neurophysiological and behavioral study in the rat. J Comp Physiol Psychol. 1979;93:74–104. [PubMed]
  • Ben-Ari Y. Excitatory actions of gaba during development: the nature of the nurture. Nat Rev Neurosci. 2002;3:728–739. [PubMed]
  • Ben-Ari Y, Cherubini E, Corradetti R, Gaiarsa JL. Giant synaptic potentials in immature rat CA3 hippocampal neurones. J Physiol. 1989;416:303–325. [PubMed]
  • Black YD, Maclaren FR, Naydenov AV, Carlezon WA, Baxter MG, Konradi C. Altered attention and prefrontal cortex gene expression in rats after binge-like exposure to cocaine during adolescence. J Neurosci. 2006;26:9656–9665. [PubMed]
  • Bolaños CA, Barrot M, Berton O, Wallace-Black D, Nestler EJ. Methylphenidate treatment during pre- and periadolescence alters behavioral responses to emotional stimuli at adulthood. Biol Psychiatry. 2003;54:1317–1329. [PubMed]
  • Chen JF, Weiss B. Ontogenetic expression of D2 dopamine receptor mRNA in rat corpus striatum. Brain Res Dev Brain Res. 1991;63:95–104. [PubMed]
  • Chen BT, Bowers MS, Martin M, Hopf FW, Guillory AM, Carelli RM, Chou JK, Bonci A. Cocaine but not natural reward self-administration nor passive cocaine infusion produces persistent LTP in the VTA. Neuron. 2008;59:288–297. [PMC free article] [PubMed]
  • Coyle S, Napier TC, Breese GR. Ontogeny of tolerance to haloperidol: behavioral and biochemical measures. Brain Res. 1985;355:27–38. [PubMed]
  • Fagen ZM, Mitchum R, Vezina P, McGehee DS. Enhanced nicotinic receptor function and drug abuse vulnerability. J Neurosci. 2007;27:8771–8778. [PubMed]
  • Foster TC. Involvement of hippocampal synaptic plasticity in age-related memory decline. Brain Res Brain Res Rev. 1999;30:236–249. [PubMed]
  • Fukata Y, Adesnik H, Iwanaga T, Bredt DS, Nicoll RA, Fukata M. Epilepsy-related ligand/receptor complex LGI1 and ADAM22 regulate synaptic transmission. Science. 2006;313:1792–1795. [PubMed]
  • Gorelova N, Seamans JK, Yang CR. Mechanisms of dopamine activation of fast-spiking interneurons that exert inhibition in rat prefrontal cortex. J Neurophysiol. 2002;88:3150–3166. [PubMed]
  • Howard RF, Walker SM, Mota PM, Fitzgerald M. The ontogeny of neuropathic pain: postnatal onset of mechanical allodynia in rat spared nerve injury (SNI) and chronic constriction injury (CCI) models. Pain. 2005;115:382–389. [PubMed]
  • Jensen V, Kaiser KMM, Borchardt T, Adelmann G, Rozov A, Burnashev N, Brix C, Frotscher M, Andersen P, Hvalby III, Sakmann B, Seeburg PH, Sprengel R. A juvenile form of postsynaptic hippocampal long-term potentiation in mice deficient for the AMPA receptor subunit GluR-A. J Physiol. 2003;553:843–856. [PubMed]
  • Larson J, Jessen RE, Kim D, Fine AKS, du Hoffmann J. Age-dependent and selective impairment of long-term potentiation in the anterior piriform cortex of mice lacking the fragile X mental retardation protein. J Neurosci. 2005;25:9460–9469. [PubMed]
  • Lauterborn JC, Rex CS, Kramár E, Chen LY, Pandyarajan V, Lynch G, Gall CM. Brain-derived neurotrophic factor rescues synaptic plasticity in a mouse model of fragile X syndrome. J Neurosci. 2007;27:10685–10694. [PubMed]
  • Lewis EM, Barnett JF, Freshwater L, Hoberman AM, Christian MS. Sexual maturation data for Crl Sprague-Dawley rats: criteria and confounding factors. Drug Chem Toxicol. 2002;25:437–458. [PubMed]
  • Lisman J, Schulman H, Cline H. The molecular basis of CaMKII function in synaptic and behavioural memory. Nat Rev Neurosci. 2002;3:175–190. [PubMed]
  • Lu Y, Allen M, Halt AR, Weisenhaus M, Dallapiazza RF, Hall DD, Usachev YM, McKnight GS, Hell JW. Age-dependent requirement of AKAP150-anchored PKA and GluR2-lacking AMPA receptors in LTP. EMBO J. 2007;26:4879–4890. [PubMed]
  • Lynch G, Rex CS, Gall CM. Synaptic plasticity in early aging. Ageing Res Rev. 2006;5:255–280. [PubMed]
  • Marinelli M. Dopamine neuron activity and relationship to addiction. In: Georges F Chair, editor. Drugs of abuse: what dopamine neurons do and don’t do!; Winter Conference on Brain Research 41st Annual Meeting; UT: Snowbird; 2008.
  • Marinelli M, White FJ. Enhanced vulnerability to cocaine self-administration is associated with elevated impulse activity of midbrain dopamine neurons. J Neurosci. 2000;20:8876–8885. [PubMed]
  • Martin M, Chen BT, Hopf FW, Bowers MS, Bonci A. Cocaine self-administration selectively abolishes LTD in the core of the nucleus accumbens. Nat Neurosci. 2006;9:868–869. [PubMed]
  • Meredith RM, Floyer-Lea AM, Paulsen O. Maturation of long-term potentiation induction rules in rodent hippocampus: role of GABAergic inhibition. J Neurosci. 2003;23:11142–11146. [PubMed]
  • Miyamoto Y, Chen L, Sato M, Sokabe M, Nabeshima T, Pawson T, Sakai R, Mori N. Hippocampal synaptic modulation by the phosphotyrosine adapter protein ShcC/N-Shc via interaction with the NMDA receptor. J Neurosci. 2005;25:1826–1835. [PubMed]
  • Monyer H, Burnashev N, Laurie DJ, Sakmann B, Seeburg PH. Developmental and regional expression in the rat brain and functional properties of four NMDA receptors. Neuron. 1994;12:529–540. [PubMed]
  • Morris RGM, Moser EI, Riedel G, Martin SJ, Sandin J, Day M, O’Carroll C. Elements of a neurobiological theory of the hippocampus: the role of activity-dependent synaptic plasticity in memory. Philos Trans R Soc Lond B Biol Sci. 2003;358:773–786. [PMC free article] [PubMed]
  • Moyer JR, Brown TH. Patch-clamp techniques applied to brain slices. In: Walz W, Boulton AA, Baker GB, editors. Advanced Techniques for Patch-Clamp Analysis. Humana Press; Totawa, NJ: 2002. pp. 135–194.
  • Moyer JR, Brown TH. Methods for whole-cell recording from visually preselected neurons of perirhinal cortex in brain slices from young and aging rats. J Neurosci Methods. 1998;86:35–54. [PubMed]
  • Napier TC, Coyle S, Breese GR. Ontogeny of striatal unit activity and effects of single or repeated haloperidol administration in rats. Brain Res. 1985;333:35–44. [PubMed]
  • Noisin EL, Thomas WE. Ontogeny of dopaminergic function in the rat midbrain tegmentum, corpus striatum and frontal cortex. Brain Res. 1988;469:241–252. [PubMed]
  • Park JS, Nakatsuka T, Nagata K, Higashi H, Yoshimura M. Reorganization of the primary afferent termination in the rat spinal dorsal horn during post-natal development. Brain Res Dev Brain Res. 1999;113:29–36. [PubMed]
  • Pattinson D, Fitzgerald M. The neurobiology of infant pain: development of excitatory and inhibitory neurotransmission in the spinal dorsal horn. Reg Anesth Pain Med. 2004;29:36–44. [PubMed]
  • Paus T, Keshavan M, Giedd JN. Why do many psychiatric disorders emerge during adolescence? Nat Rev Neurosci. 2008;9:947–957. [PMC free article] [PubMed]
  • Plant K, Pelkey KA, Bortolotto ZA, Morita D, Terashima A, McBain CJ, Collingridge GL, Isaac JTR. Transient incorporation of native GluR2-lacking AMPA receptors during hippocampal long-term potentiation. Nat Neurosci. 2006;9:602–604. [PubMed]
  • Rivera C, Voipio J, Payne JA, Ruusuvuori E, Lahtinen H, Lamsa K, Pirvola U, Saarma M, Kaila K. The K+/Cl− co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation. Nature. 1999;397:251–255. [PubMed]
  • Sales N, Martres MP, Bouthenet ML, Schwartz JC. Ontogeny of dopaminergic D-2 receptors in the rat nervous system: characterization and detailed autoradiographic mapping with [125I]iodosulpride. Neuroscience. 1989;28:673–700. [PubMed]
  • Seamans JK, Gorelova N, Durstewitz D, Yang CR. Bidirectional dopamine modulation of GABAergic inhibition in prefrontal cortical pyramidal neurons. J Neurosci. 2001;21:3628–3638. [PubMed]
  • Sheng M, Cummings J, Roldan LA, Jan YN, Jan LY. Changing subunit composition of heteromeric NMDA receptors during development of rat cortex. Nature. 1994;368:144–147. [PubMed]
  • Sim JA, Chaumont S, Jo J, Ulmann L, Young MT, Cho K, Buell G, North RA, Rassendren F. Altered hippocampal synaptic potentiation in P2X4 knock-out mice. J Neurosci. 2006;26:9006–9009. [PubMed]
  • Sisk CL, Zehr JL. Pubertal hormones organize the adolescent brain and behavior. Front Neuroendocrinol. 2005;26:163–174. [PubMed]
  • Spear LP. The adolescent brain and age-related behavioral manifestations. Neurosci Biobehav Rev. 2000;24:417–463. [PubMed]
  • Tarazi FI, Baldessarini RJ. Comparative postnatal development of dopamine D(1), D(2) and D(4) receptors in rat forebrain. Int J Dev Neurosci. 2000;18:29–37. [PubMed]
  • Tseng KY, O’Donnell P. Dopamine modulation of prefrontal cortical interneurons changes during adolescence. Cereb Cortex. 2007;17:1235–1240. [PMC free article] [PubMed]
  • Tye KM, Stuber GD, de Ridder B, Bonci A, Janak PH. Rapid strengthening of thalamo-amygdala synapses mediates cue-reward learning. Nature. 2008;453:1253–1257. [PMC free article] [PubMed]
  • Xu B, Gottschalk W, Chow A, Wilson RI, Schnell E, Zang K, Wang D, Nicoll RA, Lu B, Reichardt LF. The role of brain-derived neurotrophic factor receptors in the mature hippocampus: modulation of long-term potentiation through a presynaptic mechanism involving TrkB. J Neurosci. 2000;20:6888–6897. [PMC free article] [PubMed]
  • Yasuda H, Barth AL, Stellwagen D, Malenka RC. A developmental switch in the signaling cascades for LTP induction. Nat Neurosci. 2003;6:15–16. [PubMed]
  • Zhong J, Carrozza DP, Williams K, Pritchett DB, Molinoff PB. Expression of mRNAs encoding subunits of the NMDA receptor in developing rat brain. J Neurochem. 1995;64:531–539. [PubMed]