Neurogenesis in the adult is involved in the formation of trace memories

The vertebrate brain continues to produce new neurons throughout life. In the rat hippocampus, several thousand are produced each day, many of which die within weeks. Associative learning can enhance their survival; however, until now it was unknown whether new neurons are involved in memory formation. Here we show that a substantial reduction in the number of newly generated neurons in the adult rat impairs hippocampal-dependent trace conditioning, a task in which an animal must associate stimuli that are separated in time. A similar reduction did not affect learning when the same stimuli are not separated in time, a task that is hippocampal-independent. The reduction in neurogenesis did not induce death of mature hippocampal neurons or permanently alter neurophysiological properties of the CA1 region, such as long-term potentiation. Moreover, recovery of cell production was associated with the ability to acquire trace memories. These results indicate that newly generated neurons in the adult are not only affected by the formation of a hippocampal-dependent memory, but also participate in it.

Regulation of hippocampal neurogenesis in adulthood

A substantial number of new granule neurons are produced in the dentate gyrus in adulthood in a variety of mammalian species, including humans. Numerous studies have demonstrated that the production and survival of new hippocampal neurons can be enhanced or diminished by hormones and experience. Steroid hormones of the ovaries and adrenal glands have been shown to modulate the production of immature neurons by affecting the proliferation of granule cell precursors. Aversive experiences have been demonstrated to decrease the production of immature granule cells, whereas enriching experiences, including learning, have been shown to enhance the survival of new hippocampal cells. These studies indicate that adult-generated neurons represent a unique form of structural plasticity that can be regulated by the environment, and furthermore suggest that new neurons play an important role in hippocampal function.

The modulation of Pavlovian memory

Exposure to stressful experiences as well as sex differences in the brain are known to influence the acquisition of new memories. This review focuses on acquisition of two types of Pavlovian learning paradigms: hippocampal-independent delay conditioning and hippocampal-dependent trace conditioning and their modulation by exposure to stressful experience and sex differences in the brain. We concentrate on two sets of findings: the first is that exposure to an acute stressful experience enhances Pavlovian conditioning in the male rat, while exposure to the very same experience dramatically impairs conditioning in female rat. The sexually-opposed effects of stress on conditioning are mediated by differing hormonal substrates (adrenal versus ovarian steroids) and possibly by differing anatomical and biochemical pathways. The second set of findings is that training with hippocampal-dependent trace conditioning enhances the survival of newly generated neurons in the adult hippocampal formation. The same amount of training with hippocampal-independent delay conditioning does not affect their survival. In addition, females acquire the trace task faster than males and generate more new neurons. As with the stress effects on learning, these sex effects are influenced by hormonal status. It is our contention that identifying the hormonal and neuronal processes that modulate associative memory formation will provide insight into the processes of memory formation itself.

Changes in chalazal cell walls and in the peroxidase enzymes of the crease region during grain development in barley

In an investigation of the role of peroxidase enzymes in the differentiation of the tissues of the crease region of barley, plants of winter barley cv. Halcyon were grown from anthesis onwards in controlled conditions at a constant temperature of 16 degrees C. Four ears were harvested at 2-d intervals from 6 d after anthesis (daa) until 50 daa. Grains from mid-ear were used for (i) fresh and dry weight determinations, (ii) extraction of crease tissue for the determination of peroxidase activity and for the separation of isozymes of peroxidase by isoelectric focusing (IEF) and (iii) detection of lignin and suberin in the tissues of the crease using autofluorescence and cytochemistry. Peroxidase activity was located histochemically in the crease tissue of cv. Chariot. Scanning electron microscopy studies were carried out on developing grains of cv. Blenheim. Maximum grain water content was achieved at 14 daa. Lignin and suberin were detected in the walls of the chalazal cells from 18 daa onwards. No changes in the staining of chalazal cell walls were detected at the end of grain filling (32 daa), but loss of autofluorescence and staining were observed at 42 daa, just prior to the final, rapid phase of grain dehydration. Peroxidase activity per fresh weight of crease tissue was high at 6 daa and low at 22 daa. It was also low between 32 and 40 daa, but it rose again from 42 daa onwards. IEF demonstrated that both anionic and cationic isozymes of peroxidase were present in crease tissue, the pattern of bands showing some marked changes during the course of grain development.

Some toxic and enzymatic activities of Bothrops ammodytoides (yarará ñata) venom

Bothrops ammodytoides, the smallest representative of this genus, is found only in Argentina. Venom was extracted from thirty adult specimens (35-70 cm in length, 90-300 g in weight) captured in the Province of Buenos Aires and kept in captivity. Venom yield was 3-30 mg. SDS-PAGE showed strong bands at 14.0; 23-25; 45; 54 and 63 kDa and weak bands at 17.0; 30.0; 40.0 and 85.0 kDa. Toxic activities were: LD50 (intravenous, mice) 0.5+/-0.2 microg/g; minimal procoagulant dose on human plasma (MPD-P) 35+/-2 mg/l; and minimal defibrinogenating dose (MDD, mice) 6-12 microg. Hemorrhagic and/or necrotic activities appear to play a major role in lethality; minimal hemorrhagic dose (MHD, mice) is 10+/-2 microg/g and minimal necrotizing dose (MND, mice) is 38+/-5 microg. The LD50, MPD-P and MND are among the lowest in venoms from Bothrops species found in Argentina. B. ammodytoides venom exhibited high proteolytic and phospholipase A2 activities. Most of the B. ammodytoides venom components cross-react with Bivalent Bothropic antivenom (Instituto Nacional de Producción de Biológicos ANLIS Dr. G. Malbrin, against B. alternatus and B. neuwiedii venoms). One ml of antivenom neutralizes 1.2 mg of B. ammodytoides venom.

Erratum: rapid extension of axons into the CA3 region by adult-generated granule cells. J comp neurol 413:146-154

Hastings NB, Gould E. 1999. Rapid extension of axons into the CA3 region by adult-generated granule cells. J Comp Neurol 413:146-154. In the Acknowledgments section of the above article, Patima Tanapat's name was incorrectly printed as Patrick Tanapat. The acknowledgment should read as follows: We gratefully acknowledge Patima Tanapat for helpful comments in the manuscript and Joseph Goodhouse for assistance with confocal imaging. The Publisher regrets the error.

Stress and hippocampal neurogenesis

The dentate gyrus of the hippocampal formation develops during an extended period that begins during gestation and continues well into the postnatal period. Furthermore, the dentate gyrus undergoes continual structural remodeling in adulthood. The production of new granule neurons in adulthood has been documented in a number of mammalian species, ranging from rodents to primates. The late development of this brain region makes the dentate gyrus particularly sensitive to environmental and experience-dependent structural changes. Studies have demonstrated that the proliferation of granule cell precursors, and ultimately the production of new granule cells, are dependent on the levels of circulating adrenal steroids. Adrenal steroids inhibit cell proliferation in the dentate gyrus during the early postnatal period and in adulthood. The suppressive action of glucocorticoids on cell proliferation is not direct but occurs through an NMDA receptor-dependent excitatory pathway. Stressful experiences, which are known to elevate circulating levels of glucocorticoids and stimulate hippocampal glutamate release, inhibit the proliferation of granule cell precursors. Chronic stress results in persistent inhibition of granule cell production and changes in the structure of the dentate gyrus, raising the possibility that stress alters hippocampal function through this mechanism. This review considers the unusual developmental profile of the dentate gyrus and its vulnerability to environmental perturbations. The long-term impact of developmental events on hippocampal function is considered.

Neurogenesis in the neocortex of adult primates

In primates, prefrontal, inferior temporal, and posterior parietal cortex are important for cognitive function. It is shown that in adult macaques, new neurons are added to these three neocortical association areas, but not to a primary sensory area (striate cortex). The new neurons appeared to originate in the subventricular zone and to migrate through the white matter to the neocortex, where they extended axons. These new neurons, which are continually added in adulthood, may play a role in the functions of association neocortex.

Rapid extension of axons into the CA3 region by adult-generated granule cells

The dentate gyrus continues to produce granule neurons throughout adulthood. The present study examined the extension of axons by adult-generated granule neurons into hippocampal area CA3. We injected the fluorescent retrograde tracers Fast blue (FB) and FluoroRuby (FR) into area CA3 of adult male rats at various times after the administration of 5'-bromo-2'-deoxyuridine (BrdU), a marker of proliferating cells and their progeny. We report that immature granule cells extend axons into CA3 as rapidly as 4-10 days after mitosis. A significant increase in the percentage of BrdU-labeled cells that were labeled with FB or FR was observed by 2 weeks after BrdU administration. This proportion remained roughly constant up to 3 weeks after BrdU-labeling, a time at which markers of a mature neuronal phenotype are expressed. BrdU-labeled cells that contained either FB or FR often were located far from the tracer injection site, indicating that these cells had extended relatively long axons. Collectively these results suggest that adult-generated granule neurons may influence normal hippocampal function, even at a very early stage after their production.

Serotonin and hippocampal neurogenesis

The dentate gyrus continues to produce new granule neurons well into adulthood. This has been demonstrated for many mammalian species, from rodents to primates. The proliferation of granule cell precursors can be suppressed by stressful experiences, presumably via adrenal steroids. Recent evidence suggests that serotonin can enhance the production of new neurons via activation of the 5HT1A receptor. These results present the possibility that the inhibitory effects of stress on granule cell production may be prevented by 5HT1A receptor agonists.

Estrogen stimulates a transient increase in the number of new neurons in the dentate gyrus of the adult female rat

To determine whether a sex difference exists in the production of hippocampal cells during adulthood, we examined proliferating cells and their progeny in adult rats using the thymidine analog bromodeoxyuridine (BrdU) combined with immunohistochemistry for markers of neurons and glia. Additionally, to determine whether ovarian hormones affect cell proliferation, we examined the numbers of BrdU-labeled cells at different estrous cycle stages and after ovarian steroid manipulation. Stereological analyses of the numbers of BrdU-labeled cells revealed that females produced more cells than males in the dentate gyrus but not in the subventricular zone. The production of new hippocampal cells in females appears to be affected by ovarian hormone levels; ovariectomy diminished the number of BrdU-labeled cells, an effect reversed by estrogen replacement. A natural fluctuation in cell proliferation was also noted; females produced more cells during proestrus (when estrogen levels are highest) compared with estrus and diestrus. Many of these cells acquired neuronal characteristics, including the formation of dendrites and expression of Turned-On-After-Division 64 kDa, a marker of immature granule neurons, and the calcium-binding protein calbindin, a marker of mature granule neurons. However, examination of the numbers of pyknotic cells and the numbers of BrdU-labeled cells at longer survival times revealed that many new cells in the dentate gyrus eventually degenerate. Consistently the number of labeled cells in females is no longer higher than that observed in males by 2 weeks after the last BrdU injection. These findings suggest that estrogen-enhanced cell proliferation during proestrus results in more immature neurons in the hippocampal formation of females compared with males and present the possibility that these new cells exert an important influence on hippocampal function.

Hippocampal neurogenesis in adult Old World primates

The production of new hippocampal neurons in adulthood has been well documented in rodents. Recent studies have extended these findings to other mammalian species, such as tree shrews and marmoset monkeys. However, hippocampal neurogenesis has not been demonstrated in adult Old World primates. To investigate this possibility, we injected 11 adult Old World monkeys of different ages (5-23 years) with the thymidine analog bromodeoxyuridine and examined the fate of the labeled cells at different survival times by using neuronal and glial markers. In the young-adult and middle-aged monkeys, we found a substantial number of cells that incorporated bromodeoxyuridine and exhibited morphological and biochemical characteristics of immature and mature neurons. New cells located in the dentate gyrus expressed a marker of immature granule neurons, Turned On After Division 64 kDa protein, as well as markers of mature granule neurons including neuron specific enolase, neuronal nuclei, and the calcium-binding protein calbindin. Fewer new cells expressed the astroglial marker glial fibrillary acidic protein. Evidence of neurogenesis was observed in the oldest monkeys (23 years) as well, but it appeared to be less robust. These results indicate that the adult brains of Old World monkeys produce new hippocampal neurons. Adult macaque monkeys may provide a useful primate model for studying the functional significance of adult neurogenesis.

Learning enhances adult neurogenesis in the hippocampal formation

Thousands of hippocampal neurons are born in adulthood, suggesting that new cells could be important for hippocampal function. To determine whether hippocampus-dependent learning affects adult-generated neurons, we examined the fate of new cells labeled with the thymidine analog bromodeoxyuridine following specific behavioral tasks. Here we report that the number of adult-generated neurons doubles in the rat dentate gyrus in response to training on associative learning tasks that require the hippocampus. In contrast, training on associative learning tasks that do not require the hippocampus did not alter the number of new cells. These findings indicate that adult-generated hippocampal neurons are specifically affected by, and potentially involved in, associative memory formation.

The cognitive determinants of performance on the Austin Maze

This study aimed to investigate which abilities are measured by the Austin Maze. One hundred and eight university students were administered a battery of eight neuropsychological tests including, the Austin Maze, the Tower of London, the Wisconsin Card Sort Test, Block Design, the Visual Spatial Learning Test, Digit Span Backwards, the Brown-Peterson Task and the Wide Range Achievement Test of Reading. Results indicated that visuospatial ability and memory both significantly contributed to performance on the Austin Maze, but differed in the degree to which they explained the performance depending on which measure of maze performance was employed. It appears that visuospatial ability is measured in early trials of the Austin Maze when individuals are orienting themselves to the path. In later trials individuals must call upon visuospatial memory to consolidate the details of the path. Executive function and working memory were not found to be significantly implicated in performance on the Austin Maze.

A study on the venom yield of venomous snake species from Argentina

A study on the venom yield of snakes from Argentina over a three year period was carried out on adult specimens of Bothrops alternatus (n = 74); Bothrops neuwiedii (n = 127); Bothrops ammodytoides (n = 30); Bothrops moojeni (n = 14); Bothrops jararaca (n = 14); B. jararacussu (n = 6); Crotalus durissus terrificus (n = 120) and Micrurus spp. (n = 6) as well as with 12 specimens of newborn C. d. terrificus kept in captivity. While for each species there was a positive correlation between venom yield and number of snakes milked, the correlation with the snake's body weights after individual milkings was even better, suggesting that the size of the snakes is more important in determining the venom yield than the number of snakes milked or the specimen's sex. Individual milkings indicated that, in addition to the snake size, when the amount of venom is normalized per 100 g body weight there is a species specific difference in venom yield. It follows the order B. jararacussu > B. moojeni approximately = B. jararaca approximately = B. alternatus > B. neuwiedii> Micrurus spp approximately = B. ammodytoides> C. d. terrificus. Although the venom yield per 100 g body weight of newborn C. d. terrificus specimens is 2-fold higher than that of adults, no correlation was observed between venom yield and body weight.

Primary care: differences and similarities

A group of health professionals from the United Kingdom visited New Zealand earlier this year. A nurse in the group shares her impressions.

Stress inhibits the proliferation of granule cell precursors in the developing dentate gyrus

The granule cell population of the dentate gyrus is produced predominantly during the postnatal period in rats. Previous studies have shown that experimental increases in the levels of adrenal steroids suppress the proliferation of granule cell precursors during the first postnatal week, the time of maximal neurogenesis in the dentate gyrus. These findings raise the possibility that stressful experiences that elevate adrenal steroid levels may inhibit the production of granule neurons, and thus alter the development of the dentate gyrus. To test this possibility, we exposed naive rat pups to the odors of a known predator, adult male rats, and examined both plasma corticosterone levels and the number of 3H-thymidine labeled cells in the dentate gyrus. A single exposure of rat pups to adult male rat odor elevated corticosterone levels immediately and diminished the number of 3H-thymidine labeled cells in the granule cell layer by 24 h later. These results suggest that stressful experiences suppress the production of granule neurons in the developing dentate gyrus.

Proliferation of granule cell precursors in the dentate gyrus of adult monkeys is diminished by stress

Although granule cells continue to be added to the dentate gyrus of adult rats and tree shrews, this phenomenon has not been demonstrated in the dentate gyrus of adult primates. To determine whether neurons are produced in the dentate gyrus of adult primates, adult marmoset monkeys (Callithrix jacchus) were injected with BrdU and perfused 2 hr or 3 weeks later. BrdU is a thymidine analog that is incorporated into proliferating cells during S phase. A substantial number of cells in the dentate gyrus of adult monkeys incorporated BrdU and approximately 80% of these cells had morphological characteristics of granule neurons and expressed a neuronal marker by the 3-week time point. Previous studies suggest that the proliferation of granule cell precursors in the adult dentate gyrus can be inhibited by stress in rats and tree shrews. To test whether an aversive experience has a similar effect on cell proliferation in the primate brain, adult marmoset monkeys were exposed to a resident-intruder model of stress. After 1 hr in this condition, the intruder monkeys were injected with BrdU and perfused 2 hr later. The number of proliferating cells in the dentate gyrus of the intruder monkeys was compared with that of unstressed control monkeys. We found that a single exposure to this stressful experience resulted in a significant reduction in the number of these proliferating cells. Our results suggest that neurons are produced in the dentate gyrus of adult monkeys and that the rate of precursor cell proliferation can be affected by a stressful experience.

Adrenal steroids and N-methyl-D-aspartate receptor activation regulate neurogenesis in the dentate gyrus of adult rats through a common pathway

Adrenal steroids and N-methyl-D-aspartate receptor activation have both been shown to regulate the rate of proliferation of granule neuron progenitor cells in the dentate gyrus of adult rats [Cameron H. A. and Gould E. (1994) Neuroscience 61, 203-209; Cameron H. A. et al. (1995) J. Neurosci. 15, 46874692]. Parallels between the actions of these two factors suggest that they may regulate cell division through a common pathway. This hypothesis was tested by altering both of the factors simultaneously and determining whether the effects were additive. The results of this study demonstrate that alterations in N-methyl-D-aspartate receptor activation block the effects of corticosterone level on cell proliferation; N-methyl-D-aspartate blocks the adrenalectomy-induced increase in [3H]thymidine-labelled cell density in the dentate gyrus, whereas the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (MK-801) prevents the corticosterone-induced decrease in proliferating cells. This finding suggests that adrenal steroids and N-methyl-D-aspartate receptor activation regulate granule cell production in the adult rat dentate gyrus through a common pathway and that N-methyl-D-aspartate receptor activation operates downstream of corticosterone in this pathway.

The epidemiology of louping-ill, a tick borne infection of red grouse (Lagopus lagopus scoticus)

The epidemiology of louping-ill in red grouse was studied in northern Britain concentrating on the possible role of other species and mechanisms of disease persistence. This tick borne viral disease caused heavy mortality in red grouse, particularly chicks. Louping-ill induced mortality reduced the strength of the density dependence that generates the tendency of grouse populations to cycle and in some populations may cause population sinks. Four routes of transmission were examined and non-viraemic transmission of virus between ticks cofeeding on hares was considered significant. Field data supported the hypothesis that disease dynamics is influenced greatly by mountain hares, both as passive amplifiers and as hosts for the tick vector. Genetic variation in louping-ill within Britain was small.