Shuichi Tanabe

Effects of prolyl endopeptidase inhibitors and neuropeptides on delayed neuronal death in rats.

We investigated the effects of the prolyl endopeptidase inhibitors 1-[1-(Benzyloxycarbonyl)-L-prolyl]prolinal (Z-Pro-Prolinal) and N-benzyloxycarbonyl-thioprolyl-thioprolinal-dimethylaceta l (ZTTA) on delayed neuronal death induced by four-vessel-occlusion transient ischemia in rats. We also examined the effects of [pGlu4, Cyt6, ArgS]vasopressin (vasopressin-(4-9)) and thyrotropin-releasing hormone (TRH) on the delayed neuronal death. Furthermore, we investigated the role of vasopressin receptors in the effects of vasopressin and prolyl endopeptidase inhibitors. Z-Pro-Prolinal, vasopressin-(4-9) and TRH protected pyramidal cells in the CA1 subfield of the rat hippocampus from delayed neuronal death after 10-min ischemia. The effect of vasopressin-(4-9) was abolished by vasopressin receptor antagonists. The effect of Z-Pro-Prolinal was also abrogated by the antagonists. These results suggest that the neuroprotective effect of prolyl endopeptidase inhibitors is mediated by neuropeptides such as [Arg8]vasopressin and TRH, and indicate the involvement of vasopressin receptors in the neuroprotective effect of vasopressin-(4-9) and prolyl endopeptidase inhibitors.

ZTTA, a postproline cleaving enzyme inhibitor, improves cerebral ischemia-induced deficits in a three-panel runway task in rats.

We investigated the effect of N-benzyloxycarbonyl-thioprolyl-thioprolinal-dimethylaceta l (ZTTA), a novel postproline cleaving enzyme (prolyl endopeptidase, PPCE) inhibitor, on the in vitro activity of rat brain PPCE and memory impairment induced by cerebral ischemia. ZTTA noncompetitively inhibited rat brain PPCE (ki = 2.9 microM). Cerebral ischemia for 5 min increased the number of errors in a working memory task with a three-panel runway paradigm. ZTTA at 6 mg/kg, administered immediately after blood flow reperfusion, significantly reduced the increase in working memory errors expected to occur 24 h after 5 min of ischemia. The antiamnesic action of ZTTA may be ascribable to a neuroprotective effect on the central nervous system due to some neuropeptides that are substrates of PPCE in the brain.

Effects of Arginine-Vasopressin Fragment 4–9 on Rodent Cholinergic Systems

Arginine-vasopressin fragment 4-9 (AVP4-9) has been demonstrated in animal studies to facilitate learning and memory. To clarify the mechanisms of this facilitation, we focused on the effects of AVP4-9 on rodent cholinergic systems. AVP4-9 (0.1 microM) enhanced the basal and the high-potassium-evoked acetylcholine (ACh) release from rat hippocampal slices (122.4 and 120.0% of control, respectively) in the presence of 1.3 mM calcium (physiological level) at 60 min after the incubation at 37 degrees C. The AVP4-9-stimulated basal ACh release was inhibited by a V1-selective antagonist ([(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)1, O-methyl-Tyr2, Arg8] vasopressin), but not by a V2-selective antagonist ([adamantaneacetyl1, O-ethyl-D-Tyr2, Val4, aminobutyryl6, Arg8,9]-vasopressin). In addition, AVP4-9 did not affect the basal ACh release under the calcium-free condition at 37 degrees C or in the presence of 1.3 mM calcium at 4 degrees C. However, AVP4-9 facilitated the passive-avoidance response of scopolamine (a cholinergic blocker)-induced memory-deficient mice. These findings demonstrate that AVP4-9 stimulates ACh release via mediation by V1-like vasopressin receptors, and shows dependence on calcium ion and temperature. The results also suggest that the mechanism of the facilitative effects of AVP4-9 on learning and memory consist of the observed stimulation of cholinergic systems and other parallel pathways that would not be inhibited by cholinergic blocking.

Facilitation of Passive Avoidance Response by Newly Synthesized Cationized Arginine Vasopressin Fragment 4-9 in Rats

The effects of a newly synthesized cationized arginine vasopressin fragment 4-9 analogue (C-AVP-(4-9)) on learning and memory in rats were studied by the passive avoidance test. C-AVP-(4-9) and its parent peptide, arginine vasopressin fragment 4-9 (AVP-(4-9)), a well known potent neuropeptide, were subcutaneously injected 1.5 hr prior to the retention test. The most effective doses of C-AVP-(4-9) and AVP-(4-9) were 8.6 x 10(-2) and 1.3 nmol/kg, respectively. To evaluate the distribution of C-AVP-(4-9) in the control nervous system (CNS), apparent tissue-plasma concentration rations (Kp.app) of intravenously administered radioiodinated C-AVP-(4-9) (125I-C-AVP-(4-9)) in the CNS in mice were determined. At the apparent steady state of plasma concentration of 125I-C-AVP-(4-9), the Kp.app values of the 125I-C-AVP-(4-9) in the cerebrum, cerebellum and spinal cord were over 12 times higher than that of the vascular space marker which slightly penetrates the BBB. Moreover, the rat cerebral homogenate converted C-AVP-(4-9) into its parent peptide AVP-(4-9). These results suggest that the potent effects of C-AVP-(4-9) on learning and memory may be due to AVP-(4-9) generated as a result of distribution and metabolism of peripherally administered C-AVP-(4-9) in the CNS.

Arginine8-vasopressin metabolite, AVP4–9, stimulates calcium signals in rat hippocampus

In the present study. w,e examined the et’fects of AVPJ-!, on signal transduction in tat hippocampus. In analysis of competitions studies of hippocampal membrane preparations, [“S]AVP4-9 binding was inhibited by vasopressin and \‘I antagonist. but not completely. In addition. WC did not detected the specific [‘“S]AVP+9 binding site of the clor~td receptor expressed cells. In dispersed cells of rat hippocampus, AVP4-Y caused increase\ in intracellular calcium concentration ([Ca”]i). whereas in the case of AVPI-U, higher concentrations were qece.qsary to obtain the same responses. I’nder the extracellu!ar Ca” -fret condition . :\VPU caused a slight elcvatlon of [Ca”),. Pretreatment :vith the vasopressin VI receptor antagonist partially blocked the AVPcY-induced increases of [Ca”]i. The AVPd-9 also promoted inositol I, 4. 5 .triphosphate (IP3) accumulation in hippocampal cell\. The >c results suggest that A%‘~-~ causes an increase in [Ca’-]i via VI-like but not traditional VI vasopressin receptor.