Dopamine D2 receptor expression in hippocampus and parahippocampal cortex of rat,cat, and human in relation to tyrosine hydroxylase-immunoreactive fibers

A detailed study comparing the distribution of D2 receptors and tyrosine hydroxylase-immunoreactive fibers in the hippocampus and parahippocampal cortices of the rat, cat, and human was conducted. The distribution of [¹²⁵I]epidepride binding to D2 receptors along the transverse and longitudinal axes of the hippocampus and parahippocampus differed among the species. In rat hippocampus, the number of sites was highest in septal portions of lacunosum-moleculare of CA1 and stratum moleculare of the subiculum. Virtually no binding to D2 receptors existed in the temporal hippocamps. For the cat hippocampus, the highest binding existed in the inner one-third of the molecular layer of the dentate gyrus (DG). There were also significant numbers of D2 receptors in strata radiatum and oriens of the CA subfields, with almost undetectable levels in lacunosum moleculare and subiculum. The number of sites was higher in the septal than temporal hippocampus. In the human hippocampus, highest binding was observed in the molecular layer of DG and the subiculum, with lower levels in strata oriens and lacunosum-moleculare of CA3, and very low binding in CA1. The histochemical demonstration of the pattern of mossy fibers revealed an organization complementary to that of D2 receptors in cat and human. In none of the species was there significant expression of D2 receptors in the entorhinal cortex, except in the caudal extreme of this region in the rat. In that region a trilaminar pattern was exhibited that continued into the perirhinal cortex. A trilaminar pattern of D2 receptor expression was observed in the perirhinal cortex of all species, with the highest values in the external and deep laminae and low expression in the middle laminae. The organization of dopamine fibers was assessed by comparing the distribution of tyrosine hydroxylase-positive and dopamine β-hydroxylase-immunoreactive fibers in these same regions. It revealed consistent mismatches between the pattern of D2 receptor expression and dopaminergic innervation in all three species. The implications for this mismatch are discussed. It is hypothesized that the distribution of D2 receptors, and not of dopamine fibers, determines what neural systems dopamine influences in the hippocampal complex. © 1994 Wiley-Liss, Inc.     

Effects of exercise on NMDA receptor subunit contributions to bidirectional synaptic plasticity in the mouse dentate gyrus

We examined synaptic plasticity in the dentate gyrus (DG) of the hippocampus in vitro in juvenile C57Bl6 mice (28-40 days of age), housed in control conditions with minimal enrichment (Controls) or with access to an exercise wheel (Runners). LTP expression was significantly greater in slices from Runners than in those from Controls, but could be blocked by APV in both groups. LTP was significantly reduced by NR2B subunit antagonists in both groups. NVP-AAM077, an antagonist with a higher preference for NR2A subunits over NR2B subunits, blocked LTP in slices from Runners and produced a slight depression in Control animals. LTD in the DG was also blocked by APV, but not by either of the NR2B specific antagonists. Strikingly, NVP-AAM077 prevented LTD in Runners, but not in Control animals, suggesting an increased involvement of NR2A subunits in LTD in animals that exercise. NVP-AAM077 did not block LTD in NR2A Knock Out (KO) animals that exercised, as expected. In an attempt to discern whether NMDA receptors located at extrasynaptic sites could play a role in the induction of LTD, DL-TBOA was used to block excitatory amino acid transport and increase extracellular glutamate levels. Under these conditions, LTD was not blocked by the co-application of a specific NR2B subunit antagonist in either group, but NVP-AAM077 again blocked LTD selectively in Runners. These results indicate that NR2A and NR2B subunits play a significant role in LTP in the DG, and that exercise can significantly alter the contribution of NMDA NR2A subunits to LTD.     

The 5‐HT4 receptor levels in hippocampus correlates inversely with memory test performance in humans

The cerebral serotonin (5‐HT) system is involved in cognitive functions such as memory and learning and animal studies have repeatedly shown that stimulation of the 5‐HT type 4 receptor (5‐HT₄R) facilitates memory and learning and further that the 5‐HT₄R modulates cellular memory processes in hippocampus. However, any associations between memory functions and the expression of the 5‐HT₄R in the human hippocampus have not been investigated. Using positron emission tomography with the tracer [¹¹C]SB207145 and Reys Auditory Verbal Learning Test we aimed to examine the individual variation of the 5‐HT4R binding in hippocampus in relation to memory acquisition and consolidation in healthy young volunteers. We found significant, negative associations between the immediate recall scores and left and right hippocampal BP_ND, (p = 0.009 and p = 0.010 respectively) and between the right hippocampal BP_ND and delayed recall (p = 0.014). These findings provide evidence that the 5‐HT₄R is associated with memory functions in the human hippocampus and potentially pharmacological stimulation of the receptor may improve episodic memory. Hum Brain Mapp 34:3066–3074, 2013. © 2012 Wiley Periodicals, Inc.     

Luminal 5‐HT4 receptors—A successful target for prokinetic actions

The prokinetic effects of 5‐HT₄ receptor (5‐HT₄R) agonists have been utilized clinically for almost three decades to relieve symptoms of constipation. Surprisingly, the mechanism(s) of action of these compounds is still being debated. Recent studies highlight luminal 5‐HT₄Rs as an alternative and effective target for these prokinetic agents. These include the study by Shokrollahi et al (2019, Neurogastroenterol Motil, e13598) published in the current issue of Neurogastroenterology and Motility, who found that activation of mucosal 5‐HT₄Rs by intraluminal prucalopride, significantly enhanced propulsive motor patterns in rabbit colon. The authors highlight the idea that development of agonists targeting luminal 5‐HT₄Rs in the colonic mucosa might be more effective and safer in achieving prokinetic effects on intestinal motility. The purpose of this mini‐review is to discuss the evidence for luminal 5‐HT₄Rs as an emerging target for prokinetic agents in facilitating propulsive motor patterns in the colon.     

Characterization of dopamine D1 and D2 receptor‐expressing neurons in the mouse hippocampus

The hippocampal formation is part of an anatomical system critically involved in learning and memory. Increasing evidence suggests that dopamine plays an important role in learning and memory as well as in several forms of synaptic plasticity. However, the precise identification of neuronal populations expressing D1 or D2 dopamine receptors within the hippocampus is still lacking. To clarify this issue, we used BAC transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the promoter of dopamine D1 or D2 receptors. In Drd1a‐EGFP mice, sparse GFP‐expressing neurons were detected among glutamatergic projecting neurons of the granular layer of the dentate gyrus and GABAergic interneurons located in the hilus. A dense immunofluorescence was observed in the outer and medial part of the molecular layer of the dentate gyrus as well as in the inner part of the molecular layer of CA1 corresponding to the terminals of pyramidal neurons of the entorhinal cortex defining the perforant and the temporo‐ammonic pathway respectively. Finally, scattered D1 receptor‐expressing neurons were also identified as GABAergic interneurons in the CA3/CA1 fields of the hippocampus. In Drd2‐EGFP transgenic mice, GFP was exclusively detected in the glutamatergic mossy cells located in the polymorphic layer of the dentate gyrus. This pattern was confirmed in Drd2‐Cre mice crossed with NLS‐LacZ‐Tau^mGFP:LoxP and RCE:LoxP reporter lines. Our results demonstrate that D1 and D2 receptor‐expressing neurons are strictly segregated in the mouse hippocampus. By clarifying the identity of D1 and D2 receptor‐expressing neurons in the hippocampus, this study establishes a basis for future investigations aiming at elucidating their roles in the hippocampal network. © 2012 Wiley Periodicals, Inc.     

Differential expression and localization of the phosphorylated and nonphosphorylated neurofilaments during the early postnatal development of rat hippocampus

Neurofilament (NF) proteins are expressed in most mature neurons in the central nervous system. Although they play a crucial role in neuronal growth, organization, shape, and plasticity, their expression pattern and cellular distribution in the developing hippocampus remain unknown. In the present study, we have used Western blotting and immunocytochemistry to study the low- (NF-L), medium- (NF-M), and high- (NF-H) molecular-weight NF proteins; phosphorylated epitopes of NF-M and NF-H; and a nonphosphorylated epitope of NF-H in the early postnatal (through P1-P21) development of the rat hippocampus. During the first postnatal week, NF-M was the most abundantly expressed NF, followed by NF-L, whereas the expression of NF-H was very low. Through P7-P14, the expression of NF-H increased dramatically and later began to plateau, as also occurred in the expression of NF-M and NF-L. At P1, no NF-M immunopositive cell bodies were detected, but cell processes in the CA1-CA3 fields were faintly immunopositive for NF-M and for the phosphorylated epitopes of NF-M and NF-H. At P7, CA3 pyramidal neurons were strongly immunopositive for NF-L and NF-H, but not for NF-M. The axons of granule cells, the mossy fibers (MFs), were NF-L and NF-M positive through P7-P21 but were NF-H immunonegative at all ages. Although they stained strongly for the phosphorylated NF-M and NF-H at P7, the staining intensity sharply decreased at P14 and remained so at P21. The cell bodies of CA1 pyramidal neurons and granule cells remained immunonegative against all five antibodies in all age groups. Our results show a different time course in the expression and differential cell type and cellular localization of the NF proteins in the developing hippocampus. These developmental changes could be of importance in determining the reactivity of hippocampal neurons in pathological conditions in the immature hippocampus.     

Volumes of the components of the hippocampus in the aging F344 rat

Much of the recent data on cells, synapses, and other structures in the dentate gyrus and hippocampus as a function of age are packing density or volume fraction data. In order to estimate total numbers, volumes, or surface areas of cells, synapses, vessels, etc., as a function of age, the total volumes of the subregions of the dentate gyrus and hippocampus must be known. The volumes of these subregions, visualized with the Timm stain, have been determined in 24 F344 rats from 4 to 37 months of age. Volumes of the various structures showed age-related increases which were statistically significant for the perforant path zone of the dentate gyrus molecular layer, as well as the total molecular layer, the hilus, and regio inferior and total mossy fiber systems. If the 4-month age group is eliminated from consideration, only the ratio of the volume of the mossy fiber zones to the volume of the perforant path zones of the dentate molecular layer increases significantly with age. Our general finding of lack of volumetric reorganization of the subdivisions of the hippocampal region between 12 and 37 months suggests that studies of the packing densities of structures in most of these zones may be considered comparable across ages, assuming comparability of sampling regions.     

On the role of 5‐HT1A receptor gene in behavioral effect of brain‐derived neurotrophic factor

Experiments were made on a congenic AKR.CBA‐D13Mit76C (76C) mouse strain created by transferring a chromosome 13 fragment containing the 5‐HT_1A receptor gene from a CBA strain to an AKR background. It was shown that 76C mice differed from AKR mice by decreased 5‐HT_1A receptor and tryptophan hydroxylase‐2 (tph‐2) genes expression in the midbrain. Functional activity of 5‐HT_2A receptors and 5‐HT_2A receptor mRNA levels in the midbrain and hippocampus of 76C mice were decreased compared with AKR mice. Central brain‐derived neurotrophic factor (BDNF) administration (300 ng i.c.v.) reduced 5‐HT_1A and 5‐HT_2A receptor mRNA levels in the frontal cortex and tph‐2 mRNA level in the midbrain of AKR mice. However, BDNF failed to produce any effect on the expression of 5‐HT_1A, 5‐HT_2A, and tph‐2 genes in 76C mice but decreased functional activity of 5‐HT_2A receptors in 76C mice and increased it in AKR mice. BDNF restored social deficiency in 76C mice but produced asocial behavior (aggressive attacks towards young mice) in AKR mice. The data indicate that a small genetic variation altered the response to BDNF and show an important role of 5‐HT_1A receptor gene in the 5‐HT system response to BDNF treatment and in behavioral effects of BDNF. © 2014 Wiley Periodicals, Inc.     

Integrity of Cajal–Retzius cells in the reeler‐mouse hippocampus

Cajal–Retzius (CR) cells are early‐born glutamatergic neurons that are primarily known as the early main source of the signal protein Reelin. In the reeler mutant, the absence of Reelin causes severe defects in the radial migration of neurons, resulting in abnormal cortical layering. To date, the exact morphological properties of CR‐cells independent of Reelin are unknown. With this in view, we studied the ontogenesis, density, and distribution of CR‐cells in reeler mice that were cross‐bred with a CXCR4‐EGFP reporter mouse line, thus enabling us to clearly identify CR‐cells positions in the disorganized hippocampus of the reeler mouse. As evidenced by morphological analysis, differences were found regarding CR‐cell distribution and density: generally, we found fewer CR‐cells in the developing and adult reeler hippocampus as compared to the hippocampus of wild‐type animals (WT); however, in reeler mice, CR‐cells were much more closely associated to the hippocampal fissure (HF), resulting in relatively higher local CR‐cell densities. This higher local cell density was accompanied by stronger immunoreactivity of the CXCR4 ligand, stroma‐derived factor‐1 (SDF‐1) that is known to regulate CR‐cell positioning. Importantly, confocal microscopy indicates an integration of CR‐cells into the developing and adult hippocampal network in reeler mice, raising evidence that network integration of CR‐cells might be independent of Reelin.     

Adaptations in 5‐HT receptor expression and function: Implications for treatment of cognitive impairment in aging

Malfunction of the serotonin system may contribute to memory deficits during aging. We evaluated the 5‐HT₆ antagonist RO4368554 in two models of learning and memory in aged rats. Male rats (18 months) were assigned to two groups of equal cognitive performance. After 2 weeks of 5‐HT₆ antagonist RO4368554 (5 mg/kg, i.p.) treatment, rats showed significant improvement in object recognition and social discrimination compared with rats given chronic vehicle. Brains from these animals were examined for changes in plasticity‐associated proteins Ki‐67 and PCNA. No differences were seen between groups in any of these markers. We also measured mRNA expression of 5‐HT₆, along with 5‐HT_1A, 5‐HT_1B, and tryptophan hydroxylase‐2 mRNAs in 4‐month‐old and 24‐month‐old F344 rats. Decreases in 5‐HT_1B expression were observed in several forebrain regions in the old rats. These results demonstrate that 5‐HT₆ and 5‐HT_1B receptors are potential targets for treatment of age‐related memory disorders. © 2009 Wiley‐Liss, Inc.     

Serotonin (5‐HT) regulates neurite outgrowth through 5‐HT1A and 5‐HT7 receptors in cultured hippocampal neurons

Serotonin (5‐HT) production and expression of 5‐HT receptors (5‐HTRs) occur early during prenatal development. Recent evidence suggests that, in addition to its classical role as a neurotransmitter, 5‐HT regulates neuronal connectivity during mammalian development by modulating cell migration and neuronal cytoarchitecture. Given the variety of 5‐HTRs, researchers have had difficulty clarifying the specific role of each receptor subtype in brain development. Signalling mediated by the G‐protein‐coupled 5‐HT_1AR and 5‐HT₇R, however, has been associated with neuronal plasticity. Thus, we hypothesized that 5‐HT promotes neurite outgrowth through 5‐HT_1AR and 5‐HT₇R. The involvement of 5‐HT_1AR and 5‐HT₇R in the morphology of rat hippocampal neurons was evaluated by treating primary cultures at 2 days in vitro with 5‐HT and specific antagonists for 5‐HT_1AR and 5‐HT₇R (WAY‐100635 and SB269970, respectively). The stimulation of hippocampal neurons with 100 nM 5‐HT for 24 hr produced no effect on either the number or the length of primary neurites. Nonetheless, after 5HT₇R was blocked, the addition of 5‐HT increased the number of primary neurites, suggesting that 5HT₇R could inhibit neuritogenesis. In contrast, 5‐HT induced secondary neurite outgrowth, an effect inhibited by 1 μM WAY‐100635 or SB269970. These results suggest that both serotonergic receptors participate in secondary neurite outgrowth. We conclude that 5‐HT_1AR and 5‐HT₇R regulate neuronal morphology in primary hippocampal cultures by promoting secondary neurite outgrowth. © 2014 Wiley Periodicals, Inc.