Cells that Express Brain-Derived Neurotrophic Factor mRNA in the Developing Postnatal Rat Brain

Brain-derived neurotrophic factor (BDNF) is a member of a family of related neurotrophic proteins which includes nerve growth factor (NGF) and hippocampus-derived neurotrophic factor/neurotrophin-3 (NT-3). To obtain information regarding possible roles for BDNF during postnatal brain development, we have examined the temporal and spatial expression of this trophic factor using in situ hybridization. In specific neocortical regions BDNF mRNA-expressing cells were seen at 2 weeks of age and thereafter. One particular neuronal cell type strikingly labelled was the inverted pyramidal cell population in the deep layers of parietotemporal cortex. In pyriform and cingulate cortices, BDNF mRNA was detected at postnatal day 1 and 1 week of age, respectively, with increasing levels during ontogeny. Several forebrain regions, including the thalamic anterior paraventricular nucleus, hypothalamic ventromedial nucleus as well as the preoptic area, contained moderate levels of BDNF mRNA throughout development. BDNF mRNA was detected transiently in several brainstem structures, notably in the substantia nigra and interpeduncular nucleus. Expression of this trophic factor in hippocampus was relatively low in the early neonatal brain, but attained high levels in the CA3 and CA4 regions as well as in the dentate gyrus by 2 weeks of age. At this early age, which is still during the period of neurogenesis in the dentate gyrus, labelling was restricted to the outer layer, which contained cells with a more mature appearance. However, by 3 weeks of age labelling was distributed throughout the granule cell layer. Our results show both transient and persistent expression of BDNF mRNA in various regions of the developing rat brain and suggest that there is a caudal to rostral gradient of BDNF expression during postnatal brain development, which may be correlated to neuronal maturation.     

Localization of trkB mRNA in postnatal brain development

We investigated the localization of trkB mRNA, which encodes a putative component of high-affinity brain-derived neurotrophic factor (BDNF) or the neurotrophin-3 (NT-3) receptor, in the postnatal rat brain by in situ hybridization histochemistry. At birth, trkB mRNA was strongly expressed in various regions with the thalamus and cerebral cortex showing the strongest expression. As the rat grows, expression generally persisted or declined in most regions with the exception of the hippocampus where trkB mRNA expression increased during postnatal development. In the adult brain, trkB mRNA was detected in the olfactory system, cerebral cortex, hippocampal formation, amygdala, and cerebellar cortex. These findings, together with the developmental profiles of BDNF and NT-3 mRNA expressions, suggest that trkB product (gp145^trkB) mainly transduces NT-3 signals early in the postnatal period, and BDNF signals later in the period. © 1993 Wiley-Liss, Inc.     

The gene for heparin-binding epidermal growth factor-like growth factor is stress-inducible: its role in cerebral ischemia.

The functions of the epidermal growth factor (EGF) family members in the adult brain are not known. This study investigated the changes in the expression of members of the EGF family following global ischemia employing in situ hybridization and immunohistochemical techniques to elucidate their roles in pathological conditions. EGF mRNA was not detected in either the control or the postischemic rat brain. Although transforming growth factor-alpha (TGF-alpha) mRNA was widely expressed in the normal brain, its expression did not change appreciably following ischemia. By contrast, heparin-binding EGF-like growth factor (HB-EGF) mRNA expression was rapidly increased in the CA3 sector and the dentate gyrus of the hippocampus, cortex, thalamus, and cerebellar granule and Purkinje cell layers. EGF receptor mRNA, which was widely expressed, also showed an increase in the CA3 sector and dentate gyrus. Conversely, HB-EGF mRNA did not show any increase prior to ischemic neuronal injury in the CA1 sector, the region most vulnerable to ischemia. Immunohistochemical detection of HB-EGF in the postischemic brain suggested a slight increase of immunostaining in the dentate gyrus of the hippocampus and the cortex. These findings showed that the gene encoding HB-EGF is stress-inducible, indicating the likelihood that HB-EGF is a neuroprotective factor in cerebral ischemia.     

Postnatal ontogeny of mineralocorticoid and glucocorticoid receptor gene expression in regions of the rat tel- and diencephalon.

In situ hybridization was used to study the neuroanatomical distribution of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) gene expression during development in the rat. This study was based on incubation of adjacent sections of brains from 2-, 8-, 12-, 16-day-old and adult (3 months) rats with 35S-labelled cRNA probes. These probes are transcribed from 513 and 500 basepair cDNA fragments with little homology from rat brain MR and rat liver GR respectively. Different patterns of expression were found in the brain of MR and GR during ontogeny. At postnatal day (pnd) 2, a high density of labelled MR mRNA was found in all pyramidal (CA1-4) and granular (dentate gyrus) cell fields of the hippocampal structure, the anterior hippocampus and indusium griseum, and cortex layer II. Modest to high labelling of MR mRNA was observed in the subfornical organ and the anterior hypothalamus. A variety of other telencephalic regions anterior and posterior of bregma exhibited modest to weak intensity of labelled MR mRNA. The diencephalon virtually lacked labelled MR mRNA. At older postnatal ages including the adult age, this regional distribution of radiolabelled MR mRNA did not change. At pnd 2, abundant radiolabelled GR mRNA was found widespread over the tel- and diencephalon, with the highest density observed in cell field CA1 and CA2 of the hippocampus and the parvocellular division of the hypothalamic paraventricular nucleus. Modestly labelled GR mRNA was observed in various hypothalamic and thalamic nuclei, basal ganglia, the lateral septum and the amygdala. At older postnatal ages and in adulthood, the intensity of labelled GR mRNA became progressively stronger in the hippocampus. Moreover, we observed a trend towards a more condensed and narrow band of cell bodies in the hippocampus for both MR and GR mRNA during ontogeny. A semi-quantitative comparison of the intensity of both labelled mRNA's performed at each age revealed a significantly lower expression of GR than MR mRNA in the CA3 cell field at pnd 2. At pnd 8 and 12, the amount of GR mRNA was significantly lower in the dentate gyrus and the CA3, whereas in adulthood, less GR mRNA was measured in all pyramidal and granular cell fields. The present study demonstrates that MR and GR genes are expressed in early postnatal development in a pattern resembling that in adulthood. As is the case in the adult brain, there is more MR than GR mRNA in the hippocampus during ontogeny, especially in the CA3 cell field and the DG.     

Adrenalectomy further suppresses the NT-3 mRNA response to traumatic brain injury but this effect is not reversed with corticosterone

Fluid percussion injury (FPI) and in situ hybridisation were used to evaluate the expression of NT-3 mRNA in the hippocampus after traumatic brain injury (TBI) in adrenal-intact and adrenalectomised rats (with or without corticosterone replacement). FPI and adrenalectomy independently significantly reduced the expression of NT-3 mRNA in the dentate gyrus (DG) and CA2 region. The effects of adrenalectomy in the CA2 region were partially reversed with corticosterone. In adrenalectomised animals undergoing FPI, a further significant decrease in NT-3 mRNA was observed in the DG, but this was not reversed by corticosterone. Glucocorticoids may, therefore, play a role in the basal regulation of NT-3 in the hippocampus, but the role of glucocorticoids in the modulation of the NT-3 response to TBI is unclear.     

Hyperbaric oxygen treatment decreases post-ischemic neurotrophin-3 mRNA down-regulation in the rat hippocampus.

The therapeutic effect of hyperbaric oxygen (HBO) on ischemic injury was investigated using in situ hybridization to detect the mRNA expression of neurotrophin-3 (NT-3), which is thought to play a crucial role in protecting against neuronal death induced by brain ischemia. The rats under investigation were subjected to 10 min transient forebrain ischemia, and subsequently exposed to HBO (100% oxygen, 2.5 atm absolute) for 2 h. Levels of NT-3 mRNA in the CA1, CA2 and CA3 regions, and the dentate gyrus of the hippocampus were measured after various reperfusion periods. Neuronal death in the hippocampal CA1 region was also measured by Nissl staining, seven days post ischemia. The results demonstrated that HBO treatment significantly reduced the ischemia-induced down-regulation of the NT-3 mRNA level at 4 h post ischemia, and significantly increased cell survival 7 days after reperfusion. The findings suggest that an HBO treatment maintaining the NT-3 mRNA level in the hippocampus can be beneficial to the ischemic brain within a certain time frame.     

Widespread and Developmentally Regulated Expression of Neurotrophin-4 mRNA in Rat Brain and Peripheral Tissues

The neurotrophin gene family includes four structurally related proteins with neurotrophic activities. Two of them, nerve growth factor and brain-derived neurotrophic factor (BDNF), have been studied in detail and information has recently emerged on the expression and function of the third member, neurotrophin-3. In contrast, little information is available on neurotrophin-4 (NT-4), the most recently isolated member of this family. In this report we have used a sensitive RNAase protection assay to analyse the developmental expression of NT-4 mRNA in the rat brain and in 12 different rat peripheral organs. In heart, liver and muscle plus skin NT-4 mRNA levels were maximal at embryonic day (E) E13 (the earliest time point tested), with reduced levels at later times of development. In lung, kidney and thymus similar levels were seen from E13 to postnatal day (P) 1, with reduced levels in the adult. In testis, ovary and salivary gland NT-4 mRNA was detected at E16 with a peak shortly after birth. During brain development, NT-4 mRNA was maximal at E13 followed by a decrease around birth, after which the level increased. The postnatal increase of NT-4 mRNA was also seen in cerebral cortex and brain stem analysed separately, while in the hippocampus similar levels were found from P1 to adulthood. NT-4 mRNA was detected in all ten adult rat brain regions analysed with only small regional variations, being highest in pons–medulla, hypothalamus, thalamus and cerebellum. Adult rat thymus, thyroid, muscle, lung and ovary contained higher levels of NT-4 mRNA than brain, while all other adult tissues showed similar or lower levels than in the brain. The highest level of NT-4 mRNA overall was found in P1 testis. For comparison, BDNF mRNA was analysed in the same tissues. The expression of BDNF mRNA was in many cases different from that of NT-4 mRNA. The peak of NT-4 mRNA expression in several of the peripheral tissues coincided with the peak of naturally occurring neuronal cell death in peripheral ganglia. This is consistent with the possibility that NT-4 acts as a target-derived trophic factor in vivo. The widespread and increased expression of NT-4 mRNA during postnatal brain development could reflect a trophic role of NT-4 for central nervous system neurons. However, non-neuronal functions of NT-4 are also possible, particularly in male and female reproductive tissues, where the NT-4 protein could function as a growth factor for immature germ cells.     

SCG10 mRNA localization in the hippocampus: comparison with other mRNAs encoding neuronal growth-associated proteins (nGAPs)

SCG10 is a nerve growth factor (NGF)-inducible, neuron-specific protein whose expression is tightly correlated with axonal and/or dendritic growth. We have recently shown that the mRNA encoding SCG10 is expressed at significant levels in certain subsets of neurons in the adult rat brain, while its expression is undetectable or negligible in other non-neuronal tissues. Here we show that regional SCG10 mRNA expression in the adult mouse brain is comparable to that in the rat, however, in the hippocampus its expression profile is distinct. In the mouse, SCG10 mRNA is expressed at high levels in pyramidal cells of CA3–CA4 sub-fields of Ammon's horn and at low levels in the CA1–CA2 sub-fields, while it is found rather uniformly throughout the pyramidal cell layer of the rat hippocampus. SCG10 mRNA is not detectable in the dentate gyrus of the mouse hippocampus, although it is expressed in the rat dentate gyrus. Comparison with other mRNAs encoding neuronal growth-associated proteins (nGAPs) such as GAP-43, MAP2, α1-tubulin and stathmin suggests that dentate granule cells express a different repertoire of neuronal growth-associated genes in mouse and rat.     

Increased adult hippocampal brain-derived neurotrophic factor and normal levels of neurogenesis in maternal separation rats

Repeated maternal separation of rat pups during the early postnatal period may affect brain-derived neurotrophic factor (BDNF) or neurons in brain areas that are compromised by chronic stress. In the present study, a highly significant increase in hippocampal BDNF protein concentration was found in adult rats that as neonates had been subjected to 180 min of daily separation compared with handled rats separated for 15 min daily. BDNF protein was unchanged in the frontal cortex and hypothalamus/paraventricular nucleus. Expression of BDNF mRNA in the CA1, CA3, or dentate gyrus of the hippocampus or in the paraventricular hypothalamic nucleus was not affected by maternal separation. All animals displayed similar behavioral patterns in a forced-swim paradigm, which did not affect BDNF protein concentration in the hippocampus or hypothalamus. Repeated administration of bromodeoxyuridine revealed equal numbers of surviving, newly generated granule cells in the dentate gyrus of adult rats from the 15 min or 180 min groups. The age-dependent decline in neurogenesis from 3 months to 7 months of age did not differ between the groups. Insofar as BDNF can stimulate neurogenesis and repair, we propose that the elevated hippocampal protein concentration found in maternally deprived rats might be a compensatory reaction to separation during the neonatal period, maintaining adult neurogenesis at levels equal to those of the handled rats.     

Contrasting effects of electroconvulsive shock on rnRNAs encoding the high affinity kainate receptor subunits (KA1 and KA2) and cyclophilin in the rat

Kainate-preferring glutamate receptors may contribute to the glutamatergic responses to seizures. The cloning of their encoding genes overcomes limitations of the receptor ligands available for their investigation. We have examined the expression of the high affinity kainate receptor subunits KA1 and KA2 mRNAs in the rat hippocampus, using electroconvulsive shock (ECS) as a seizure paradigm not confounded by neurotoxicity. A single shock reduced the levels of KAI mRNA in the CA3c region, while increasing the expression of KA2 mRNA in the dentate gyrus. Following repeated ECS (5 shocks over 10 days), KAI mRNA was reduced in CA3c and in CA3a-b but was unchanged in dentate gyrus. KA2 mRNA, on the other hand, significantly increased in dentate gyrus, and to a lesser extent in CA3c and CA1. All changes in KAI and KA2 mRNAs had returned to baseline 3 weeks after the last shock. We also measured the expression of cyclophilin mRNA, and found it to be reduced in all hippocampul subfields, and in the parietal cortex, after a single ECS. It returned to control levels after repeated ECS but was again reduced following 3 weeks recovery from repeated ECS. These results indicate that the expression of KA1 and KA2 not only change in opposite directions in the rat hippocampus after ECS, but that the alterations are anatomically and temporally regulated. In the respect that cyclophilin is regarded as a housekeeping gene, the reduction in its mRNA suggests that ECS may have more persistent and widespread effects on brain gene expression than previously suspected.     

Hyperthermia induces age-dependent changes in rat hippocampal excitability

The mechanisms underlying the generation of febrile seizures are poorly understood. This study investigated hyperthermia-induced changes in the hippocampus, a structure implicated in febrile seizures. It was hypothesized that neuronal excitability in the hippocampus changes with increasing temperature, and that this change is different in adult as compared with immature rats. Adult and immature (15-17 days postnatal) male rats were studied under urethane anesthesia during normothermia, moderate hyperthermia (38-39.5 degrees C), and severe hyperthermia (>39.5 degrees C). Paired-pulse inhibition of the orthodromically activated population spikes in the dentate gyrus and cornu ammonis 1 region of the hippocampus (CA1), two structures within the hippocampus, was measured after stimulation of the medial perforant path and Schaffer collaterals, respectively. In the adult rat, paired-pulse inhibition was increased in the dentate gyrus during moderate and severe hyperthermia but decreased in CA1 during severe hyperthermia (all p values < 0.05). In the immature rat, paired-pulse inhibition was unchanged in the dentate gyrus but decreased in CA1 during moderate hyperthermia (p < 0.05). We suggest that hyperthermia contributes to seizure susceptibility in the immature hippocampus by decreasing CA1 inhibition. In the adult rat, a decrease in CA1 inhibition requires a higher degree of hyperthermia, and hippocampal seizure generation is opposed by an increase in dentate gyrus inhibition.     

Distribution of pituitary adenylate cyclase activating polypeptide mRNA in the developing rat brain

PACAP is a member of the secretin/vasoactive intestinal peptide (VIP) family, isolated from hypothalamus. Recent studies have shown that PACAP is expressed in many parts of adult brain. We have studied the precise distribution of PACAP mRNA in developing rat brain, employing in situ hybridisation. PACAP mRNA is expressed in distinct parts of the embryonic rat brain from embryonic day 13, with a robust expression in developing cortex, hippocampus, amygdala and hypothalamus as well as in spinal cord and dorsal root ganglia. The expression in hippocampus and cortex diminishes towards adulthood, compared to new-born rat brain. In the mature brain, PACAP mRNA is located in alternating layers of cerebral cortex (layers I, III and V), in the dentate gyrus, in CA4 and CA1 regions, but not in CA2 or CA3 of the hippocampus. The presence of PACAP mRNA in different structures of developing rat brain suggests an important function for this peptide during brain development.     

Expression of trkB and trkC receptors and their ligands brain-derived neurotrophic factor and neurotrophin-3 in the murine amygdala

The neurotrophin brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their cognate receptors, trkB and trkC, have a variety of physiological brain functions, ranging from cell survival to mechanisms involved in learning and memory and long-term potentiation (LTP). LTP can be induced in the cortex and hippocampus, as well as within the amygdala. However, the role of neurotrophins in amygdalar LTP is largely unknown. Expression patterns of BDNF and NT-3 and their cognate receptors in the adult mouse amygdala have not been analyzed in detail. We have therefore examined the expression of trkB, trkC, BDNF, and NT-3 mRNA and protein in different amygdalar nuclei as well as in the hippocampal areas CA1-CA3 and the dentate gyrus. The distribution pattern of trkB, trkC, BDNF, and NT-3 mRNA in the murine hippocampus is comparable to that seen in rats. Within most amygdalar nuclei, a moderate BDNF mRNA expression was found; however, BDNF mRNA was virtually absent from the central nucleus. No expression of NT-3 mRNA was found within the amygdala, but trkC mRNA-expressing cells were widely distributed within this brain region. trkB mRNA was strongly expressed in the amygdala. Because trkB is expressed in a full-length and a truncated form (the latter form is also expressed by nonneuronal cells), we also investigated the distribution of full-length trkB mRNA-expressing cells and could demonstrate that this version of trkB receptors is also widely expressed in the amygdala. These results can serve as a basis for studies elucidating the physiological roles of these receptors in the amygdala.     

Ontogeny of brain-derived neurotrophic factor gene expression in the forebrain of prairie and montane voles

Brain-derived neurotrophic factor (BDNF) plays an important role in normal brain development. In the present study, we examined the ontogenetic pattern of BDNF gene expression in both monogamous prairie voles (Microtus ochrogaster) and promiscuous montane voles (M. montanus); two closely related microtine rodents that differ in life strategy and social behavior. In both species, BDNF mRNA showed an early appearance and a transient expression in a regionally specific manner. In the dentate gyrus and CA3 region of the hippocampus, BDNF mRNA was found neonatally, increased gradually during development, and reached a peak at weaning, followed by a subsequent decline to the adult level. In the paraventricular nucleus of the hypothalamus, levels of BDNF mRNA persisted until weaning, followed by a significant increase to the adult levels at 3 months of age. BDNF mRNA also demonstrated a species-specific developmental pattern. In the cingulate cortex, BDNF mRNA labeling displayed a transient increase in the second and third postnatal weeks followed by a subsequent decrease to the adult level in prairie voles, but persisted throughout the course of development in montane voles. In general, montane voles achieved an adult pattern of BDNF mRNA expression earlier than did prairie voles. Together, these data indicate that BDNF may function differently in infant and adult brains, and that the two species of voles differ in the ontogenetic pattern of BDNF mRNA expression in a regional-specific manner, which may be associated with their different life strategy and brain and behavioral development.     

Postnatal development of 11beta-hydroxysteroid dehydrogenase type 1 in the rat hippocampus

Glucocorticoids (GCs) have important actions in the hippocampus of the brain, where their access to glucocorticoid receptor (GR) is increased by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). 11beta-HSD1 converts biologically inactive 11-dehydrocorticosterone into active corticosterone. However, the postnatal development of 11beta-HSD1 in the hippocampus is not properly understood. In this study, the postnatal distribution and development of 11beta-HSD1 in the hippocampus of the rat brain was studied with immunohistochemistry and Western blot analysis. Results showed that abundant 11beta-HSD1 immunoreactive substance (ir-11beta-HSD1) was present in the hippocampus. There were homogeneous distributions of 11beta-HSD1 in the hippocampal CA1, CA2, CA3, CA4 regions and the dentate gyrus at postnatal days 1, 3, and 7. Interestingly, the developmental distribution of GR in the hippocampus followed the same pattern as 11beta-HSD1. Western blot analysis demonstrated that a higher level of expression of 11beta-HSD1 in the hippocampus was found in the first 2 weeks of life. The expressions of 11beta-HSD1 started to drop to adult levels at about postnatal day 15 both in the hippocampus and in other brain areas. These results suggest that the higher expression of 11beta-HSD1 in the neonatal hippocampus may be important for the maturation of the central nervous system mediated by GCs through GR.     

Temporal changes in the expression of brain-derived neurotrophic factor mRNA in the ventromedial nucleus of the hypothalamus of the developing rat brain

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family, which is important for the growth, differentiation, and survival of neurons during development. We have performed a detailed mapping of BDNF mRNA in the neonatal rat brain using a quantitative in situ hybridization technique. At postnatal day (PND) 4, hypothalamic structures showed only modest expression of BDNF mRNA, with the exception of the ventromedial nucleus (VMN), where expression was higher than that detected in the hippocampus. Abundant BDNF mRNA was also found in the bed nucleus of the anterior commissure, retrosplenial granular cortex, and the posteroventral part of the medial amygdaloid nucleus. Messenger RNAs encoding other neurotrophins, including nerve growth factor (NGF) and neurotrophin-3 (NT-3) and the BDNF receptor trkB, were not selectively localized in neonatal VMN. During subsequent developmental stages, BDNF mRNA expression in the VMN changed dynamically, peaking at PND 4 and falling to minimal levels in the adult brain. In contrast, the low levels of BDNF mRNA observed in the CA3 region of the hippocampus increased to adult levels following PND 10. As the VMN undergoes sexual differentiation, we compared BDNF, NGF, NT-3, and trkB mRNA expression in the VMN in males and females at embryonic day 20 and PND 4, but found no differences between them. These results suggest that localized and high level expression of BDNF mRNA in the neonatal VMN plays an important role in its neural organization and functional development.     

GABA receptor rho subunit expression in the developing rat brain

Ionotropic GABA(C) receptors are composed of rho1, rho2 and rho3 subunits. Although the distribution of rho subunit mRNAs in the adult brain has been studied, information on the developmental regulation of different rho subunits in the brain is scattered and incomplete. Here, GABA(C) receptor rho subunit expression was studied in the developing rat brain. In situ hybridization on postnatal brain slices showed rho2 mRNA expression from newborn in superficial gray layer (SGL) of superior colliculus (SuC), and from the first postnatal week in the hippocampal CA1 region and pretectal nucleus of the optic tract. rho2 mRNA was also expressed in the adult dorsal lateral geniculate nucleus. Quantitative RT-PCR revealed expression of all three rho subunits in the hippocampus and superior colliculus from the first postnatal day. In the hippocampus, rho2 mRNA expression clearly dominated over rho1 and rho3, whereas in the superior colliculus, rho1 mRNA expression levels were similar to rho2. In both areas, a clear up-modulation of rho2 and rho3 mRNA during the first postnatal week was detected. GABA(C) receptor protein expression was confirmed in adult hippocampus, superior colliculus and dorsal lateral geniculate nucleus by immunohistochemistry. Our results demonstrate for the first time the expression of all three rho subunit mRNAs in several regions of the developing and adult rat brain. Our quantitative data allows assessment of putative subunit combinations in the superior colliculus and hippocampus. From the selective distribution of rho subunits, it may be hypothesized that GABA(C) receptors are specifically involved in aspects of visual image motion processing in the rat brain.     

Temporal changes in expression of neuronal nitric oxide synthase mRNA in the rat hippocampus associated with kainate-induced seizures

We studied the temporal changes in expression of neuronal nitric oxide (NO) synthase (nNOS) mRNA in the hippocampus of rats treated with kainic acid by use of in situ hybridization technique. Intraperitoneal injection of 10 mg kg-1 kainic acid decreased expression of nNOS mRNAs in the dentate gyrus and CA3 region of the hippocampus at 3 h and 8 h and increased it in the dentate gyrus and CA1 at one week after treatment. Although our previous study indicated that administration of kainic acid increased NO generation in the rat hippocampus, present results suggest that the injection of kainic acid results in differential regulation of nNOS mRNA and NO formation in the rat hippocampus.