Yoshiko Ohnishi

Effects of ovariectomy and calcium deficiency on learning and memory of eight-arm radial maze in middle-aged female rats

There is increasing evidence that estrogen and calcium ion are involved in learning and memory. In the present study, to examine the effect of estrogen deficiency and low-calcium diet on learning and memory, middle-aged female Wistar rats (50 weeks old) were fed either a low-calcium (0.02% Ca) or a normal-calcium (1.25% Ca) diet throughout the experiment. Rats were ovariectomized (OVX) or sham-operated (Sham). These animals were divided into four groups: 1) Sham group with normal-calcium diet [Sham-normal Ca group], 2) OVX group with normal-calcium diet [OVX-low Ca group], 3) Sham group with low-calcium diet [Sham-low Ca group], 4) OVX group with low-calcium diet [OVX-low Ca group]. Seventy-seven days after the OVX or Sham operation, the learning and memory abilities in the female rats were examined by using a radial maze task according to the method of Olton and Samuelson (regular trials) and using a delay-interposed task following regular trials. During regular trials and delay-interposed tasks, the OVX-low Ca group was inferior to all the other groups in accuracy of choice behavior. Both Sham-normal Ca and Sham-low Ca groups showed more accurate choices than the OVX-low Ca group, but were less accurate than the Sham-normal Ca group. In addition, there was no significant difference in locomotor activity between any of the groups. These results suggest that OVX or low-calcium diet may impair learning and memory, and that the combination of these factors impaired more markedly when the rats were tested in the eight-arm radial maze. These results may also imply the possibility that a woman in menopause or post-menopause suffers impairment of learning and/or memory when intakes low-calcium diet.

Inhibitory effects of group II mGluR-related drugs on memory performance in mice

The cAMP/protein kinase A signaling pathway is negatively modulated by group II metabotropic glutamate receptors (mGluRs), and the cross-talk that occurs between these receptors may modulate learning and memory. To examine the relationship among cAMP/PKA-signaling pathway activity, group II mGluRs, and learning and memory, mice were trained to perform a step-through-type passive avoidance task, and 10 min before each avoidance trial the following drugs were injected intracisternally (i.cist.): vehicle (0.05% dimethylsulfoxide); a specific group II mGluR agonist, DCG-IV (1-50 ng/mouse); a specific group II mGluR antagonist, LY341495 (10-300 ng); a selective inhibitor of cAMP-specific phosphodiesterase, rolipram (100-1000 ng); an activator of adenylyl cyclase, forskolin (25-250 ng); a specific inhibitor of PKA, H-89 (150 or 300 ng) or; an activator of protein kinase C, phorbol 12-myristate 13-acetate (PMA 200 ng). DCG-IV (25 and 50 ng) or LY341495 (150 and 300 ng) reduced the latency in the avoidance task. The reduction of latency by DCG-IV was not observed in mice coinjected with DCG-IV (50 ng) together with rolipram (500 ng) or forskolin (25 ng). Conversely, coinjection of LY341495 with 100 or 1000 ng rolipram, or with 25 or 250 ng forskolin tended to potentiate the LY341495-induced shortening of latency. In addition, the reduction of latency by DCG-IV (50 ng) was not observed in mice coinjected with DCG-IV and PMA together. However, the reduction of latency by LY341495 (300 ng) was potentiated when the drug was coadministered with PMA. These results suggest that changes in the cAMP/PKA-signaling pathway, mediated by group II mGluRs, influence memory in the passive avoidance task, and that both the excessive activation and deactivation of this pathway may induce the impairment of learning and memory.

Effects of estradiol and progesterone on radial maze performance in middle-aged female rats fed a low-calcium diet

There is increasing evidence that ovarian steroids and calcium ions are involved in learning and memory. To examine the effect of ovarian steroids on learning and memory under a low-calcium condition, middle-aged female rats were fed either a low-calcium (0.02% Ca) or a normal-calcium (1.25% Ca) diet. All rats were ovariectomized (OVX), and these animals were divided into eight groups: 1) an OVX group with a normal-calcium diet (OVX-normal-Ca group), 2) an OVX group with 17beta-estradiol treatment and a normal-calcium diet (E2 group), 3) an OVX with progesterone treatment and a normal-calcium diet (P4 group), 4) an OVX with 17beta-estradiol and progesterone treatments and a normal-calcium diet (E2 + P4 group), 5) an OVX group with a low-calcium diet (OVX-low-Ca group), 6) an OVX group with 17beta-estradiol treatment and a low-calcium diet (LE2 group), 7) an OVX group with progesterone treatment and a low-calcium diet (LP4 group), and 8) an OVX group with 17beta-estradiol and progesterone treatments and a low-calcium diet (LE2 + LP4). Seventy-seven days after the OVX operation, the learning and memory abilities of the rats were examined by using an eight-arm radial maze task. E2 and E2 + P4 groups learned in fewer trials, and performed better in the radial maze and the working memory task than the other groups under the normal-calcium condition. Rats in the LE2 group learned in fewer trials, and performed better in the maze and working memory task than the other low-calcium groups, but in combination with progesterone under the low-calcium condition (LE2 + LP4 group), the facilitative effect of estradiol in all the tasks was inhibited. Treatment with progesterone alone did not inhibit the learning and memory task performance. These results suggest the possibility that treatment with estradiol under low-calcium conditions cannot improve impaired learning and memory when progesterone is applied simultaneously, and that the intake of adequate calcium may be necessary and effective for patients with learning and memory hypofunction receiving hormone replacement therapy.

Facilitative effect of a novel AVP fragment analog, NC-1900, on memory retention and recall in mice

In order to determine the mechanism of action of a new AVP(4-9) analog, NC-1900, on memory processes, memory retention and retrieval tests were conducted in a step-through passive avoidance (PA) task in mice. The administration of NC-1900 facilitated memory retention and retrieval in the PA task through vasopressin1A (V1A) receptors but not V2 receptors. The effect of NC-1900 on memory retention test performance appeared to be due to activation of the protein kinase C (PKC) signaling pathway via V1A receptors; however, the modulation of PKC was not essential for the facilitative effect of the new peptide in the retrieval test. The facilitation of memory retrieval by NC-1900 may also be mediated by other non-PKC-dependent signaling pathways, such as the phospholipase C-inositol trisphosphate pathway.

Effect of pretraining administration of NC-1900, a vasopressin fragment analog, on memory performance in non- or CO2-amnesic mice.

In the present study, we investigated the facilitative effect of NC-1900, a new arginine vasopressin (AVP(1-9)) fragment analog, on memory performance in eight-arm radial maze or passive avoidance (PA) tasks in nonamnesic and amnesic (PA tasks only) mice. In the radial maze, all injections (subcutaneous) were given daily 60 min before each trail. NC-1900 (1 ng/kg)-treated animals showed enhancement of performance, and AVP(4-9) (1 microg/kg), an AVP(1-9) fragment, had similar effects, although the effective dose was 1000-fold higher. In the PA task, all drugs were administrated subcutaneously 60 min before the acquisition trial (Acq.), and the amnesic mice were exposed to CO(2) just after the Acq. NC-1900 (1 ng/kg) enhanced the memory performance of nonamnesic mice and ameliorated CO(2)-induced amnesia. AVP(4-9) (1 microg/kg) had a similar effect, although only at higher doses, while AVP(1-9) (0.1-1 microg/kg) had no effect. The facilitating effect of NC-1900 on nonamnesic mice was inhibited by coinjection [Pmp(1)-Tyr(Me)(2)]-AVP (Pmp,Tyr-AVP; 1 microg/kg), a V(1A) antagonist, but not by OPC-31260, a vasopressin(2) (V(2)) antagonist. The effect of NC-1900 on CO(2)-induced amnesia was also decreased by coinjection of Pmp,Tyr-AVP or [deamino-Pen(1), Me-Tyr(2)]-AVP (10 microg/kg), both of which are V(1) antagonists. These results suggested that NC-1900 has a more potent effect on facilitation of memory via the V(1A) receptor than AVP(4-9) in non- and CO(2)-amnesic conditions.

Effect of donepezil on group II mGlu receptor agonist- or antagonist-induced amnesia on passive avoidance in mice.

We examined the effect of the acetylcholinesterase (ACHE) inhibitor, donepezil hydrocloride (DONP), on group II metabotropic glutamate (mGlu) receptor agonist- or antagonist-induced amnesia in the step-through passive avoidance task in male mice. DCG-IV, a group II mGlu receptor agonist, at dose of 50 ng and LY341495, a group II mGlu receptor antagonist, at dose of 300 ng, significantly attenuated the latency on the step-through task. The subcutaneous injection of DONP at dose of 1 mg/kg 1 hour before passive avoidance performance ameliorated the amnesia induced by DCG-IV and LY341495, whereas donepezil alone did not affect task latency. The results suggest that activation of group II mGlu receptors and disinhibition of the cAMP/PKA signaling pathway (caused by group II mGlu receptor antagonist) have a negative action on step-through passive avoidance memory performance, and that group II mGlu receptors and ACh interact to modulate learning and memory function.