- Email: [email protected]
The effect of streptozotocin-induced diabetes on cell proliferation in the rat dentate gyrus
Neuroscience Letters 293 (2000) 91±94
www.elsevier.com/locate/neulet
The effect of streptozotocin-induced diabetes on cell proliferation in the rat dentate gyrus Joy Jackson-Guilford, J. David Leander, Laura K. Nisenbaum* Neuroscience Research Division, Lilly Research Laboratories, Lilly Corporate Center, DC 0510, Eli Lilly and Company, Indianapolis, IN 46285, USA Received 12 June 2000; accepted 21 August 2000
Abstract Recent evidence has shown an association between diabetes mellitus and de®cits in learning and memory. However, the mechanism by which cognitive abilities are impaired in diabetes has not been identi®ed. The dentate gyrus of the hippocampus plays a signi®cant role in spatial learning and memory. Studies in rodents show that learning tasks enhance neurogenesis in the dentate gyrus of the adult hippocampus. To investigate whether cognitive de®cits in diabetes may be related to alterations in hippocampal neurogenesis, we measured the number of 5-bromo-2 0 -deoxyuridine (BrdU)-positive cells, an indicator of cell proliferation, in the dentate gyrus in an animal model of diabetes. Streptozotocin-induced diabetes produced a dramatic decrease in cell proliferation in the dentate gyrus as compared to controls. The results from this study suggest a potential role for alterations in neurogenesis in the cognitive decline observed in diabetes mellitus. q 2000 Published by Elsevier Science Ireland Ltd. Keywords: Diabetes mellitus; Learning; Memory; Dentate gyrus; Hippocampus; BrdU; Neurogenesis; Cell proliferation
Diabetes mellitus is one of the most common serious metabolic disorders in humans. In addition to the diabetic condition itself, numerous secondary complications are associated with the illness. Increasing evidence has shown that diabetes may be associated with de®cits in learning and memory [2]. Studies examining the effects of streptozotocin-induced diabetes on memory function in mice and rat models have also shown de®cits in memory retention and retrieval as compared to non-diabetic controls [1]. However, the mechanism by which cognitive abilities are impaired in diabetes has not been identi®ed. The hippocampal formation is clearly recognized as being involved in learning and memory. However, the cellular mechanisms underlying this association are not well understood. Recent evidence has shown that the process of neurogenesis, which includes cell proliferation, survival, migration and differentiation continues in the hippocampal formation well into adulthood in a variety of species, including rodents, non-human primates as well as humans [9,12]. Moreover, training in hippocampal-dependent tasks, such as the Morris water maze or trace-eye blink conditioning * Corresponding author. Tel.: 11-317-433-2851; fax: 11-317276-5546. E-mail address: [email protected] (L.K. Nisenbaum).
increases cell proliferation in the subgranular proliferative zone of the dentate gyrus in rodents [11]. Thus, enhanced cell proliferation may be important for hippocampal-dependent learning and memory. In contrast, several factors such as age, excitotoxicity, brain diseases and injury decrease the rate of cell proliferation in this area [5]. Given the cognitive de®cits previously demonstrated to occur in diabetes, we examined whether streptozotocin-induced diabetes would alter the number of proliferating cells in the rat dentate gyrus. Male, Sprague±Dawley rats weighing 150±175 g were used in this study. The animals were housed individually and maintained on Purina Lab Diet rodent chow and water ad libitum. The rats were randomly assigned to one of two groups (four animals/group). Diabetes mellitus was induced in the experimental group by a single i.v. injection of streptozotocin (45 mg/kg, 1.0 ml/kg) into the tail vein. Control animals received a similar injection into the tail vein of the vehicle (citric acid buffer, pH 4.5). Blood glucose levels were determined 2 days after the injection of streptozotocin using the Accu-Chek Advantage Glucose Monitor (Boehringer Mannheim, Indianapolis, IN). Only animals that exhibited a blood glucose level of 300 mg/dl or higher were used in these studies. Two days after the streptozotocin administration, animals
0304-3940/00/$ - see front matter q 2000 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 0) 01 50 2- 0
92
J. Jackson-Guilford et al. / Neuroscience Letters 293 (2000) 91±94
were given a series of four injections of BrdU (100 mg/kg, i.p., dissolved in 0.9% saline with 0.007 N NaOH) one injection every 2 h during a period of 6 h to label mitotically active cells. Twenty-four hours after the ®rst BrdU injection, animals were anesthetized with Nembutal (50 mg/kg, i.p.) and perfused transcardially with 4% paraformaldehyde in 0.1 M phosphate buffer. Brains were removed and post®xed for 2 h in paraformaldehyde and then transferred to 30% sucrose in PBS for 2 days. Coronal sections (20 mm)
were cut in a cryostat and mounted on Superfrost Plus slides. For BrdU immunohistochemistry, slides were rinsed with PBS and placed in a 658C water bath for 10 min, permeabilized with 0.1% trypsin in 0.1 M Tris buffer for 6 min, denatured with 2 N HCl for 30 min and incubated in 0.3% H2O2 for 5 min. The sections were incubated with 3% normal horse serum, and mouse monoclonal antibody against BrdU (Novocastra Laboratories, US; 1:100 in PBS). After rinsing in PBS, slides were reacted immunohis-
Fig. 1. The number of proliferating cells in the dentate gyrus is decreased in streptozotocin-treated diabetic animals. Immunohistochemical visualization of BrdU-labeled cells reveals a difference in number between control (A) and streptozotocin-treated (B) adult rats. Arrows indicate the location of BrdU-labeled cells in the subgranular layer. Scale bar 50 mm.
J. Jackson-Guilford et al. / Neuroscience Letters 293 (2000) 91±94
tochemically using a Vectastain Elite ABC kit (Vector Laboratories, CA). The sections were counter-stained using cresyl violet. Sections were visualized using a Leica DMR microscope with a 40£ objective. Stereological analysis of the number of BrdU-labeled cells was performed using a modi®ed version of the optical fractionator method [20]. Brie¯y, the number of BrdU-labeled cells was counted in the rostral dentate gyrus in every fourth section taken between 21.80 and 23.14 mm according to Paxinos and Watson [15]. Cells in the outermost plane of focus were omitted to avoid duplicate cell counts. Data were analyzed using an unpaired t-test to test signi®cance of difference between means of the treatment groups. The level of signi®cance was set at P , 0:05. In control animals, immunohistochemical visualization of BrdU produced labeling of clusters and strings of positive cells in the subgranular region of the dentate gyrus (Fig. 1A). In contrast, there was a dramatic reduction in the number of BrdU-positive cells in the diabetic animals (Fig. 1B). Quanti®cation of BrdU-labeled cells using stereological analysis revealed a four-fold decrease in the number of proliferating cells in the rostral dentate gyrus of the streptozotocin-treated group as compared to controls (Fig. 2; t 4:797; P , 0:01). The results from the present study demonstrate that streptozotocin-induced diabetes produces a signi®cant reduction in the number of proliferating cells within the dentate gyrus. In the adult rat, granule cells are continually produced from a population of precursor cells located within the subgranular zone of the dentate gyrus [7]. The majority of these cells eventually differentiate into granule neurons [7]. Alterations in neurogenesis within the adult hippocampus have been shown to occur through a change in either the number of proliferating cells or the survival of newly generated cells [12]. Thus, the observed decrease in proliferating cells within the dentate gyrus of the diabetic animals suggests
Fig. 2. The effect of streptozotocin-induced diabetes on the number of proliferating cells in the rat dentate gyrus. Treatment with streptozotocin (STZ) produced a four fold decrease in the number of BrdU-labeled cells in the dentate gyrus relative to control animals. Bars represent mean ^ SEM for four animals per experimental group. *Signi®cantly different from control, P , 0:01.
93
that a reduction in newly formed granule neurons may occur. Although the mechanism underlying this reduction in granule cell production is not well understood, previous studies suggest several factors may be involved. One factor that has been shown to alter the production of granule neurons is the level of circulating adrenal steroids. For example, administration of corticosterone to rats decreases cell proliferation in the dentate gyrus whereas removal of endogenous adrenal steroids by adrenalectomy increases the number of newly formed neurons [6]. Given that streptozotocin-induced diabetes can lead to an increase in plasma corticosterone levels [16], enhanced glucocorticoid receptor activation could mediate the observed reduction in dentate gyrus cell proliferation in the present study. Another factor known to in¯uence dentate gyrus cell production is serotonin (5-HT). Several lines of evidence suggest that 5-HT stimulates neurogenesis in the dentate gyrus [10], whereas a depletion of 5-HT decreases neurogenesis in this region [4]. Streptozotocin-induced diabetes has been shown to reduce 5-HT synthesis and metabolism in a time-dependent manner [8,17,18]. Thus, it is possible that reductions in hippocampal 5-HT could underlie the observed decrease in dentate gyrus cell proliferation. Neurogenesis has been implicated in hippocampaldependent learning. In general, factors that reduce neurogenesis (glucocorticoids, stress, aging) decrease learning, whereas factors that enhance neurogenesis (estrogen, enriched environment) increase learning [12]. Long-term potentiation (LTP), an activity-dependent form of synaptic plasticity, has been hypothesized to be related to the cellular mechanisms of learning and memory [3]. Running, which increases LTP in the dentate gyrus, enhances both neurogenesis as well as spatial learning in mice [19]. With respect to diabetes, formation of LTP is impaired in the dentate gyrus of streptozotocin-treated rats [14]. Furthermore, streptozotocin-induced diabetes produces a de®cit in hippocampal-dependent learning [1]. Together, these ®ndings suggest that the observed reduction in cell proliferation in the dentate gyrus may be related to the cognitive de®cits associated with streptozotocin-induced diabetes. In conclusion, recent ®ndings suggest that neurogenesis continues throughout adulthood naturally in the brains of primates, including humans [9,13]. Neurogenesis within the dentate gyrus may be important for ongoing learning and memory [12]. Increasing evidence indicates that cognitive impairment is associated with diabetes mellitus [2]. The present demonstration of a reduction in cell proliferation in an animal model of diabetes provides evidence for a potential substrate underlying these cognitive de®cits. [1] Biessels, G.J., Kamal, A., Ramakers, G.M., Urban, I.J., Spruijt, B.M., Erkelens, D.W. and Gispen, W.H., Place learning and hippocampal synaptic plasticity in streptozotocininduced diabetic rats, Diabetes, 45 (1996) 1259±1266. [2] Biessels, G.J., Kappelle, A.C., Bravenboer, B., Erkelens,
94
[3] [4]
[5] [6] [7]
[8]
[9]
[10] [11]
J. Jackson-Guilford et al. / Neuroscience Letters 293 (2000) 91±94 D.W. and Gispen, W.H., Cerebral function in diabetes mellitus, Diabetologia, 37 (1994) 643±650. Bliss, T.V. and Collingridge, G.L., A synaptic model of memory: long-term potentiation in the hippocampus, Nature, 361 (1993) 31±39. Brezun, J.M. and Daszuta, A., Depletion in serotonin decreases neurogenesis in the dentate gyrus and the subventricular zone of adult rats, Neuroscience, 89 (1999) 999±1002. Cameron, H.A., Hazel, T.G. and McKay, R.D., Regulation of neurogenesis by growth factors and neurotransmitters, J. Neurobiol., 36 (1998) 287±306. Cameron, H.A. and Gould, E., Adult neurogenesis is regulated by adrenal steroids in the dentate gyrus, Neuroscience, 61 (1994) 203±209. Cameron, H.A., Woolley, C.S., McEwen, B.S. and Gould, E., Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat, Neuroscience, 56 (1993) 337±344. Chaouloff, F., Laude, D., Merino, D., Serrurier, B., Baudrie, V. and Elghozi, J.L., Duration of streptozotocin diabetes in¯uences the response of hypothalamic serotonin metabolism to immobilization stress, Neuroendocrinology, 50 (1989) 344±350. Eriksson, P.S., Per®lieva, E., Bjork-Eriksson, T., Alborn, A.M., Nordberg, C., Peterson, D.A. and Gage, F.H., Neurogenesis in the adult human hippocampus, Nat. Med., 4 (1998) 1313±1317. Gould, E., Serotonin and hippocampal neurogenesis, Neuropsychopharmacology, 21 (1999) 46S ±51S. Gould, E., Beylin, A., Tanapat, P., Reeves, A. and Shors, T., Learning enhances adult neurogenesis in the hippocampal formation, Nat. Neurosci., 2 (1999) 260±265.
[12] Gould, E., Tanapat, P., Hastings, N.B. and Shors, T.J., Neurogenesis in adulthood: a possible role in learning, Trends Cog. Sci., 3 (1999) 186±192. [13] Gould, E., Tanapat, P., McEwen, B.S., Flugge, G. and Fuchs, E., Proliferation of granule cell precursors in the dentate gyrus of adult monkeys is diminished by stress, Proc. Nat. Acad. Sci., 95 (1998) 3168±3171. [14] Kamal, A., Biessels, G.J., Urban, I.J. and Gispen, W.H., Hippocampal synaptic plasticity in streptozotocin-diabetic rats: impairment of long-term potentiation and facilitation of long-term depression, Neuroscience, 90 (1999) 737±745. [15] Paxinos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, Academic Press, New York, 1998. [16] Schwartz, M.W., Strack, A.M. and Dallman, M.F., Evidence that elevated plasma corticosterone levels are the cause of reduced hypothalamic corticotrophin-release hormone gene expression in diabetes, Regul. Pept., 72 (1997) 105± 112. [17] Thorre, K., Chaouloff, F., Sarre, S., Meeusen, R., Ebinger, G. and Michotte, Y., Differential effects of restraint stress on hippocampal 5-HT metabolism and extracellular levels of 5-HT in streptozotocin-diabetic rats, Brain Res., 772 (1997) 209±216. [18] Trulson, M.E., Jacoby, J.H. and MacKenzie, R.G., Streptozotocin-induced diabetes reduces brain serotonin synthesis in rats, J. Neurochem., 46 (1986) 1068±1072. [19] van Praag, H., Kempermann, G. and Gage, F.H., Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus, Nat. Neurosci., 2 (1999) 266±270. [20] West, M.J., Slomianka, L. and Gunderson, H.J.G., Unbiased stereological estimation of the total number of neurons in the subdivisions of the rat hippocampus using the optional fractionator, Anat. Rec., 231 (1991) 482±497.
www.elsevier.com/locate/neulet
The effect of streptozotocin-induced diabetes on cell proliferation in the rat dentate gyrus Joy Jackson-Guilford, J. David Leander, Laura K. Nisenbaum* Neuroscience Research Division, Lilly Research Laboratories, Lilly Corporate Center, DC 0510, Eli Lilly and Company, Indianapolis, IN 46285, USA Received 12 June 2000; accepted 21 August 2000
Abstract Recent evidence has shown an association between diabetes mellitus and de®cits in learning and memory. However, the mechanism by which cognitive abilities are impaired in diabetes has not been identi®ed. The dentate gyrus of the hippocampus plays a signi®cant role in spatial learning and memory. Studies in rodents show that learning tasks enhance neurogenesis in the dentate gyrus of the adult hippocampus. To investigate whether cognitive de®cits in diabetes may be related to alterations in hippocampal neurogenesis, we measured the number of 5-bromo-2 0 -deoxyuridine (BrdU)-positive cells, an indicator of cell proliferation, in the dentate gyrus in an animal model of diabetes. Streptozotocin-induced diabetes produced a dramatic decrease in cell proliferation in the dentate gyrus as compared to controls. The results from this study suggest a potential role for alterations in neurogenesis in the cognitive decline observed in diabetes mellitus. q 2000 Published by Elsevier Science Ireland Ltd. Keywords: Diabetes mellitus; Learning; Memory; Dentate gyrus; Hippocampus; BrdU; Neurogenesis; Cell proliferation
Diabetes mellitus is one of the most common serious metabolic disorders in humans. In addition to the diabetic condition itself, numerous secondary complications are associated with the illness. Increasing evidence has shown that diabetes may be associated with de®cits in learning and memory [2]. Studies examining the effects of streptozotocin-induced diabetes on memory function in mice and rat models have also shown de®cits in memory retention and retrieval as compared to non-diabetic controls [1]. However, the mechanism by which cognitive abilities are impaired in diabetes has not been identi®ed. The hippocampal formation is clearly recognized as being involved in learning and memory. However, the cellular mechanisms underlying this association are not well understood. Recent evidence has shown that the process of neurogenesis, which includes cell proliferation, survival, migration and differentiation continues in the hippocampal formation well into adulthood in a variety of species, including rodents, non-human primates as well as humans [9,12]. Moreover, training in hippocampal-dependent tasks, such as the Morris water maze or trace-eye blink conditioning * Corresponding author. Tel.: 11-317-433-2851; fax: 11-317276-5546. E-mail address: [email protected] (L.K. Nisenbaum).
increases cell proliferation in the subgranular proliferative zone of the dentate gyrus in rodents [11]. Thus, enhanced cell proliferation may be important for hippocampal-dependent learning and memory. In contrast, several factors such as age, excitotoxicity, brain diseases and injury decrease the rate of cell proliferation in this area [5]. Given the cognitive de®cits previously demonstrated to occur in diabetes, we examined whether streptozotocin-induced diabetes would alter the number of proliferating cells in the rat dentate gyrus. Male, Sprague±Dawley rats weighing 150±175 g were used in this study. The animals were housed individually and maintained on Purina Lab Diet rodent chow and water ad libitum. The rats were randomly assigned to one of two groups (four animals/group). Diabetes mellitus was induced in the experimental group by a single i.v. injection of streptozotocin (45 mg/kg, 1.0 ml/kg) into the tail vein. Control animals received a similar injection into the tail vein of the vehicle (citric acid buffer, pH 4.5). Blood glucose levels were determined 2 days after the injection of streptozotocin using the Accu-Chek Advantage Glucose Monitor (Boehringer Mannheim, Indianapolis, IN). Only animals that exhibited a blood glucose level of 300 mg/dl or higher were used in these studies. Two days after the streptozotocin administration, animals
0304-3940/00/$ - see front matter q 2000 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 0) 01 50 2- 0
92
J. Jackson-Guilford et al. / Neuroscience Letters 293 (2000) 91±94
were given a series of four injections of BrdU (100 mg/kg, i.p., dissolved in 0.9% saline with 0.007 N NaOH) one injection every 2 h during a period of 6 h to label mitotically active cells. Twenty-four hours after the ®rst BrdU injection, animals were anesthetized with Nembutal (50 mg/kg, i.p.) and perfused transcardially with 4% paraformaldehyde in 0.1 M phosphate buffer. Brains were removed and post®xed for 2 h in paraformaldehyde and then transferred to 30% sucrose in PBS for 2 days. Coronal sections (20 mm)
were cut in a cryostat and mounted on Superfrost Plus slides. For BrdU immunohistochemistry, slides were rinsed with PBS and placed in a 658C water bath for 10 min, permeabilized with 0.1% trypsin in 0.1 M Tris buffer for 6 min, denatured with 2 N HCl for 30 min and incubated in 0.3% H2O2 for 5 min. The sections were incubated with 3% normal horse serum, and mouse monoclonal antibody against BrdU (Novocastra Laboratories, US; 1:100 in PBS). After rinsing in PBS, slides were reacted immunohis-
Fig. 1. The number of proliferating cells in the dentate gyrus is decreased in streptozotocin-treated diabetic animals. Immunohistochemical visualization of BrdU-labeled cells reveals a difference in number between control (A) and streptozotocin-treated (B) adult rats. Arrows indicate the location of BrdU-labeled cells in the subgranular layer. Scale bar 50 mm.
J. Jackson-Guilford et al. / Neuroscience Letters 293 (2000) 91±94
tochemically using a Vectastain Elite ABC kit (Vector Laboratories, CA). The sections were counter-stained using cresyl violet. Sections were visualized using a Leica DMR microscope with a 40£ objective. Stereological analysis of the number of BrdU-labeled cells was performed using a modi®ed version of the optical fractionator method [20]. Brie¯y, the number of BrdU-labeled cells was counted in the rostral dentate gyrus in every fourth section taken between 21.80 and 23.14 mm according to Paxinos and Watson [15]. Cells in the outermost plane of focus were omitted to avoid duplicate cell counts. Data were analyzed using an unpaired t-test to test signi®cance of difference between means of the treatment groups. The level of signi®cance was set at P , 0:05. In control animals, immunohistochemical visualization of BrdU produced labeling of clusters and strings of positive cells in the subgranular region of the dentate gyrus (Fig. 1A). In contrast, there was a dramatic reduction in the number of BrdU-positive cells in the diabetic animals (Fig. 1B). Quanti®cation of BrdU-labeled cells using stereological analysis revealed a four-fold decrease in the number of proliferating cells in the rostral dentate gyrus of the streptozotocin-treated group as compared to controls (Fig. 2; t 4:797; P , 0:01). The results from the present study demonstrate that streptozotocin-induced diabetes produces a signi®cant reduction in the number of proliferating cells within the dentate gyrus. In the adult rat, granule cells are continually produced from a population of precursor cells located within the subgranular zone of the dentate gyrus [7]. The majority of these cells eventually differentiate into granule neurons [7]. Alterations in neurogenesis within the adult hippocampus have been shown to occur through a change in either the number of proliferating cells or the survival of newly generated cells [12]. Thus, the observed decrease in proliferating cells within the dentate gyrus of the diabetic animals suggests
Fig. 2. The effect of streptozotocin-induced diabetes on the number of proliferating cells in the rat dentate gyrus. Treatment with streptozotocin (STZ) produced a four fold decrease in the number of BrdU-labeled cells in the dentate gyrus relative to control animals. Bars represent mean ^ SEM for four animals per experimental group. *Signi®cantly different from control, P , 0:01.
93
that a reduction in newly formed granule neurons may occur. Although the mechanism underlying this reduction in granule cell production is not well understood, previous studies suggest several factors may be involved. One factor that has been shown to alter the production of granule neurons is the level of circulating adrenal steroids. For example, administration of corticosterone to rats decreases cell proliferation in the dentate gyrus whereas removal of endogenous adrenal steroids by adrenalectomy increases the number of newly formed neurons [6]. Given that streptozotocin-induced diabetes can lead to an increase in plasma corticosterone levels [16], enhanced glucocorticoid receptor activation could mediate the observed reduction in dentate gyrus cell proliferation in the present study. Another factor known to in¯uence dentate gyrus cell production is serotonin (5-HT). Several lines of evidence suggest that 5-HT stimulates neurogenesis in the dentate gyrus [10], whereas a depletion of 5-HT decreases neurogenesis in this region [4]. Streptozotocin-induced diabetes has been shown to reduce 5-HT synthesis and metabolism in a time-dependent manner [8,17,18]. Thus, it is possible that reductions in hippocampal 5-HT could underlie the observed decrease in dentate gyrus cell proliferation. Neurogenesis has been implicated in hippocampaldependent learning. In general, factors that reduce neurogenesis (glucocorticoids, stress, aging) decrease learning, whereas factors that enhance neurogenesis (estrogen, enriched environment) increase learning [12]. Long-term potentiation (LTP), an activity-dependent form of synaptic plasticity, has been hypothesized to be related to the cellular mechanisms of learning and memory [3]. Running, which increases LTP in the dentate gyrus, enhances both neurogenesis as well as spatial learning in mice [19]. With respect to diabetes, formation of LTP is impaired in the dentate gyrus of streptozotocin-treated rats [14]. Furthermore, streptozotocin-induced diabetes produces a de®cit in hippocampal-dependent learning [1]. Together, these ®ndings suggest that the observed reduction in cell proliferation in the dentate gyrus may be related to the cognitive de®cits associated with streptozotocin-induced diabetes. In conclusion, recent ®ndings suggest that neurogenesis continues throughout adulthood naturally in the brains of primates, including humans [9,13]. Neurogenesis within the dentate gyrus may be important for ongoing learning and memory [12]. Increasing evidence indicates that cognitive impairment is associated with diabetes mellitus [2]. The present demonstration of a reduction in cell proliferation in an animal model of diabetes provides evidence for a potential substrate underlying these cognitive de®cits. [1] Biessels, G.J., Kamal, A., Ramakers, G.M., Urban, I.J., Spruijt, B.M., Erkelens, D.W. and Gispen, W.H., Place learning and hippocampal synaptic plasticity in streptozotocininduced diabetic rats, Diabetes, 45 (1996) 1259±1266. [2] Biessels, G.J., Kappelle, A.C., Bravenboer, B., Erkelens,
94
[3] [4]
[5] [6] [7]
[8]
[9]
[10] [11]
J. Jackson-Guilford et al. / Neuroscience Letters 293 (2000) 91±94 D.W. and Gispen, W.H., Cerebral function in diabetes mellitus, Diabetologia, 37 (1994) 643±650. Bliss, T.V. and Collingridge, G.L., A synaptic model of memory: long-term potentiation in the hippocampus, Nature, 361 (1993) 31±39. Brezun, J.M. and Daszuta, A., Depletion in serotonin decreases neurogenesis in the dentate gyrus and the subventricular zone of adult rats, Neuroscience, 89 (1999) 999±1002. Cameron, H.A., Hazel, T.G. and McKay, R.D., Regulation of neurogenesis by growth factors and neurotransmitters, J. Neurobiol., 36 (1998) 287±306. Cameron, H.A. and Gould, E., Adult neurogenesis is regulated by adrenal steroids in the dentate gyrus, Neuroscience, 61 (1994) 203±209. Cameron, H.A., Woolley, C.S., McEwen, B.S. and Gould, E., Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat, Neuroscience, 56 (1993) 337±344. Chaouloff, F., Laude, D., Merino, D., Serrurier, B., Baudrie, V. and Elghozi, J.L., Duration of streptozotocin diabetes in¯uences the response of hypothalamic serotonin metabolism to immobilization stress, Neuroendocrinology, 50 (1989) 344±350. Eriksson, P.S., Per®lieva, E., Bjork-Eriksson, T., Alborn, A.M., Nordberg, C., Peterson, D.A. and Gage, F.H., Neurogenesis in the adult human hippocampus, Nat. Med., 4 (1998) 1313±1317. Gould, E., Serotonin and hippocampal neurogenesis, Neuropsychopharmacology, 21 (1999) 46S ±51S. Gould, E., Beylin, A., Tanapat, P., Reeves, A. and Shors, T., Learning enhances adult neurogenesis in the hippocampal formation, Nat. Neurosci., 2 (1999) 260±265.
[12] Gould, E., Tanapat, P., Hastings, N.B. and Shors, T.J., Neurogenesis in adulthood: a possible role in learning, Trends Cog. Sci., 3 (1999) 186±192. [13] Gould, E., Tanapat, P., McEwen, B.S., Flugge, G. and Fuchs, E., Proliferation of granule cell precursors in the dentate gyrus of adult monkeys is diminished by stress, Proc. Nat. Acad. Sci., 95 (1998) 3168±3171. [14] Kamal, A., Biessels, G.J., Urban, I.J. and Gispen, W.H., Hippocampal synaptic plasticity in streptozotocin-diabetic rats: impairment of long-term potentiation and facilitation of long-term depression, Neuroscience, 90 (1999) 737±745. [15] Paxinos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, Academic Press, New York, 1998. [16] Schwartz, M.W., Strack, A.M. and Dallman, M.F., Evidence that elevated plasma corticosterone levels are the cause of reduced hypothalamic corticotrophin-release hormone gene expression in diabetes, Regul. Pept., 72 (1997) 105± 112. [17] Thorre, K., Chaouloff, F., Sarre, S., Meeusen, R., Ebinger, G. and Michotte, Y., Differential effects of restraint stress on hippocampal 5-HT metabolism and extracellular levels of 5-HT in streptozotocin-diabetic rats, Brain Res., 772 (1997) 209±216. [18] Trulson, M.E., Jacoby, J.H. and MacKenzie, R.G., Streptozotocin-induced diabetes reduces brain serotonin synthesis in rats, J. Neurochem., 46 (1986) 1068±1072. [19] van Praag, H., Kempermann, G. and Gage, F.H., Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus, Nat. Neurosci., 2 (1999) 266±270. [20] West, M.J., Slomianka, L. and Gunderson, H.J.G., Unbiased stereological estimation of the total number of neurons in the subdivisions of the rat hippocampus using the optional fractionator, Anat. Rec., 231 (1991) 482±497.