Tatsuo Akema

Effects of preoptic injections of gastrin, cholecystokinin, secretin, vasoactive intestinal peptide and PHI on the secretion of luteinizing hormone and prolactin in ovariectomized estrogen-primed rats

The effects of intracerebral injections of brain-gut peptides in the preoptic region on the secretion of luteinizing hormone (LH) and prolactin (PRL) were examined in ovariectomized, estrogen-primed rats. Gastrin or cholecystokinin-octapeptide injection in the preoptic region induced a moderate increase of circadian rise of LH secretion in some animals but no appreciable change in PRL secretion. Secretin injection was strikingly effective in increasing circadian rise of LH and PRL secretion. Vasoactive intestinal peptide injection completely abolished the occurrence of circadian rise of LH secretion whereas PHI injection facilitated LH secretion undergoing the circadian rhythm. The results suggest that those peptides act as neurotransmitters or neuromodulators in the preoptic region and are implicated in the regulation of LH and PRL secretion.

On the Relationship between Noradrenergic Stimulatory and GABAergic Inhibitory Systems in the Control of Luteinizing Hormone Secretion in Female Rats

The relationship between noradrenergic (NA) stimulatory and gamma-aminobutyric acid (GABA)-mediated inhibitory systems in the control of luteinizing hormone (LH) secretion was examined in ovariectomized rats. Stimulation of the GABAA receptor by an intraventricular (i.c.v.) administration of a GABAA agonist, muscimol, significantly attenuated the LH secretory response to the subsequent i.c.v. injection of NA in estrogen-primed rats. On the other hand, blockade of alpha-adrenergic receptors by phenoxybenzamine did not interfere with the stimulatory effect of an i.c.v. injection of a GABAA antagonist bicuculline. In a different series of experiments, ovariectomized animals had been treated with i.c.v. injections of 6-hydroxydopamine (6-OHDA) or its vehicle. An i.c.v. injection of muscimol or phentolamine significantly inhibited the estrogen-induced surge of LH secretion in vehicle-treated rats. Muscimol also inhibited the existing pulsatile LH secretion in vehicle-treated, estrogen-unprimed animals. In animals in which hypothalamic NA terminals were presumed to be destroyed by 6-OHDA, the inhibitory effect of phentolamine was significantly diminished while that of muscimol was unaltered. These results permit the following conclusions: (1) the central GABAergic system inhibits LH secretion via GABAA receptors; (2) this inhibitory GABA system operates without mediation by the NA system, and (3) the GABAergic non-NA-mediated system can affect physiological patterns of pituitary LH secretion in female rats.

Lipopolysaccharide acutely inhibits proliferation of neural precursor cells in the dentate gyrus in adult rats.

Lipopolysaccharide (LPS), a bacterial endotoxin released during infection, is known to suppress neurogenesis in the dentate gyrus (DG) in mature rats. The present study aimed to elucidate acute effect of LPS, as well as possible mechanisms involved in the effect, on the neurogenesis in the DG of adult rats. In the first experiment, proliferating cells in the DG were labeled with bromodeoxyuridine (BrdU). Double-labeled immunohistochemistry performed 28 days after the BrdU incorporation revealed co-expression of NeuN, a marker of mature neurons, in most of the BrdU-positive cells in the DG. The rat was injected intraperitoneally with LPS or saline at various intervals after the BrdU incorporation, and BrdU-positive cells were examined 24h thereafter. The endotoxin reduced the number of BrdU-positive cells that were labeled 24h before, but not 7 or 28 days before sacrifice, suggesting rapid LPS actions on precursor cells during proliferation, but not after mitosis. In the second experiment, cells in the DG positively stained with BrdU or serine10 phosphorylated histone H3 (pHH3) were examined 5h after the injection of LPS or saline. BrdU was incorporated 2h before sacrifice. In these rats, LPS reduced the number of BrdU- or pHH3-positive cells. LPS did not affect the number of terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL)-positive cells within 5, 8 or 24h. These results indicate that the endotoxin acutely suppresses neurogenesis in the DG in adult rats, presumably by inhibiting proliferation of neural precursor cells, but not by increasing cell death.

Specific role of the posterior dorsal hippocampus–prefrontal cortex in short-term working memory

Working memory in rats involves neural projections from the hippocampus (HP) to the prefrontal cortex (PFC), based on delayed task experiments in a radial‐arm maze, in which the time span of working memory is longer than seconds. To determine whether the HP–PFC pathway is involved in short‐term (on the order of seconds) working memory function, we lesioned the PFC and/or HP, and measured performance in an operant delayed alternation task. The posterior dorsal (pdHP) and ventral HP (vHP) were assessed separately. The bilateral PFC and bilateral pdHP ibotenate lesions produced significant working memory deficits, but the vHP lesion did not. Unilateral pdHP lesions combined with a PFC lesion in the opposite hemisphere reproduced the effects of bilaterally symmetrical lesions. By contrast, unilateral lesions of the pdHP combined with a PFC lesion in the same hemisphere had no effect on delayed alternation. These results indicate that the pdHP–PFC pathway is essential for working memory on the order of seconds in rats, and suggest that the pdHP and vHP pathways to the PFC play different behavioral roles.

Single bout of running exercise changes LC3-II expression in rat cardiac muscle.

Macroautophagy (autophagy) is an intracellular catalytic process. We examined the effect of running exercise, which stimulates cardiac work physiologically, on the expression of microtubule-associated protein 1 light chain 3 (LC3)-II, an indicator of autophagy, as well as some autophagy-related proteins in rat cardiac muscle. The left ventricles were taken from rats immediately (0 h), and at 0.5h, 1h or 3h after a single bout of running exercise on a treadmill for 30 min and also from rats in a rest condition. In these samples, we evaluated the level of LC3-II and p62, and the phosphorylation level of mammalian target of rapamycin (mTOR), Akt and AMP-activated protein kinase alpha (AMPKα) by Western blotting. The exercise produced a biphasic change in LC3-II, with an initial decrease observed immediately after the exercise and a subsequent increase 1h thereafter. LC3-II then returned to the rest level at 3h after the exercise. A negative correlation was found between the LC3-II expression and mTOR phosphorylation, which plays a role in inhibiting autophagy. The exercise increased phosphorylation of AMPKα, which stimulates autophagy via suppression of mTOR phosphorylation, immediately after exercise. The level of p62 and phosphorylated Akt was not altered significantly by the exercise. These results suggest for the first time that a single bout of running exercise induces a biphasic change in autophagy in the cardiac muscle. The exercise-induced change in autophagy might be partially mediated by mTOR in the cardiac muscle.

Regional Specificity in the Effect of Estrogen Implantation within the Forebrain on the Frequency of Pulsatile Luteinizing Hormone Secretion in the Ovariectomized Rat

Pulsatile secretion of luteinizing hormone (LH) in ovariectomized rats was evaluated before and after local implantation of crystalline estradiol benzoate (EB) into various regions within the forebrain. Serum concentrations of LH were measured by radioimmunoassay in samples collected at 6-min intervals through indwelling cardiac catheters. In rats with EB implanted in the preoptic suprachiasmatic area (POSC), and in the nucleus of the tractus diagonalis and the diagonal band of Broca (DBB) to a lesser extent, the mean LH concentration and LH pulse frequency decreased rapidly while the pulse amplitude did not change for 3 h after implantation. Rats with the EB implant in the bed nucleus of stria terminalis, the medial preoptic area, the medial septal nucleus, the anterior hypothalamic area or the third ventricle showed unchanged frequencies of LH secretory pulses. Implantation of progesterone into the POSC of ovariectomized rats produced no significant change in LH secretory profiles. It was suggested that the sites of action of estrogen in decreasing the LH pulse frequency are not widespread but rather restricted within a small part of the brain, including the POSC and DBB, in the ovariectomized rat.

Lipopolysaccharide inhibits luteinizing hormone release through interaction with opioid and excitatory amino acid inputs to gonadotropin-releasing hormone neurones in female rats: Possible evidence for a common mechanism involved in infection and immobilization stress

Acute immobilization stress suppresses naloxone‐ and N‐methyl‐d‐aspartate (NMDA)‐induced, but not gonadotropin‐releasing hormone (GnRH)‐induced, luteinizing hormone (LH) release in ovariectomized oestrogen‐primed rats. To explore whether a common mechanism may underlie inhibition of gonadotropin secretion by various stressors, we examined in the present study the effect of lipopolysaccharide (LPS) on LH release induced by progesterone, GnRH, naloxone and NMDA. The effect of LPS on Fos expression in GnRH neurones was also examined in association with its effect on steroid‐induced LH release. Injection of progesterone (1 mg/rat) at noon induced an LH surge in the afternoon in ovariectomized rats pretreated with oestradiol benzoate. In these rats, the majority of hypothalamic GnRH neurones expressed Fos in the evening. Intravenous (i.v.) administration of LPS (10 µg/rat) inhibited steroid‐induced LH release and also reduced the Fos expression in GnRH neurones. In separate experiments, an i.v. injection of GnRH (50 ng/kg), naloxone (10 mg/kg) or NMDA (20 mg/kg) significantly elevated serum LH concentrations within 10 min. Pretreatment with LPS, which did not affect basal LH release or GnRH‐induced LH release, inhibited naloxone‐induced and NMDA‐induced LH release. These results show that LPS has a suprapituitary site(s) of action to suppress the activity of GnRH neurones in female rats, and suggest that LPS affects the opioid, as well as the excitatory amino acidergic regulation of GnRH neurones. The similarity of effects of LPS and immobilization stress further suggests that a common mechanism is involved in inhibition of GnRH neurones by different stressors.

Gravitational unloading inhibits the regenerative potential of atrophied soleus muscle in mice

Aim:  The present study was performed to investigate the influence of unloading on the regeneration of atrophied and injured skeletal muscle.

Differences between paired-pulse facilitation and long-term potentiation in the dorsal and ventral hippocampal CA1-prefrontal pathways of rats

We studied the interaction between paired-pulse facilitation (PPF) and long-term potentiation (LTP) in the hippocampo-prefrontal cortex (PFC, prelimbic area) pathway, stimulating the ventral or posterior dorsal CA1 region (vCA1 or pdCA1). In the vCA1-PFC, the group averaged PPF did not change after the LTP induction, and there was a negative correlation between the post-LTP PPF change and LTP magnitude. In contrast, the post-LTP PPF of the pdCA1-PFC appeared to decrease significantly, and the PPF change was independent of the LTP magnitude. We found that there were at least two mechanisms of PPF regulation following LTP induction in the pathway resulting from extensive CA1 projections into the prelimbic area. The results imply that the CA1-PFC pathway regulates the PFC PPF quantitatively in LTP-dependent and independent manners, which depend on the local properties of the CA1 regions.

Mechanism of egg envelope digestion by hatching enzymes, HCE and LCE in medaka, Oryzias latipes

Hatching of medaka embryos from the fertilized egg envelope involves two enzymes, HCE and LCE. HCE swells the envelope and then LCE completely dissolves it. We determined HCE and LCE cleavage sites on the egg envelope that are primarily constructed of two groups of subunit proteins, ZI-1,2 and ZI-3. HCE and LCE cleaved different target sequences on the egg envelope proteins but shared one common cleavage site. HCE cleaved the N-terminal region of ZI-1,2 and ZI-3, mainly the Pro-Xaa-Yaa repeat sequence of ZI-1,2 into hexapeptides, but not the site within a zona pellucida (ZP) domain that is considered to be the core structure of the egg envelope. The cleavage of these N-terminal regions results in swelling and softening of the envelope. LCE cleaved the middle of the ZP domain of ZI-1,2, in addition to the upstream of the trefoil domain of ZI-1,2 and the ZP domain of ZI-3. This middle site is in the intervening sequence connecting two subdomains of the ZP domain. Cleaving this site would result in the solubilization of the swollen egg envelope by the disruption of the filamentous structure that is thought to be formed by the non-covalent polymerization of ZP domains.