Brain proteolysis of oxytocin in vitro and in vivo changes during aging in male rats

Antidepressant-like effects of oxytocin in mice are dependent on the presence of insulin-regulated aminopeptidase

Oxytocin is a neuromodulator with antidepressant-like effects. In vitro, oxytocin is rapidly cleaved by insulin-regulated aminopeptidase (IRAP). Oxytocin metabolites are known to exert strong central activities that are different from the effects of the parent molecule. Our goal is to investigate in vivo whether IRAP deletion modifies the antidepressant-like effects of oxytocin. Male and female C57Bl/6 mice, IRAP wild-type (IRAP(+/+)) and knock-out (IRAP(-/-)) mice were injected subcutaneously with saline, oxytocin or oxytocin combined with angiotensin IV. One hour after injection, immobility was timed during a 5 min forced swim that was preceded by an open field to study locomotor behaviour. Oxytocin induced antidepressant-like effects in male (0.25 mg/kg oxytocin) and female (0.15 mg/kg oxytocin) C57Bl/6 mice subjected to the forced swim test. Oxytocin did not influence locomotor behaviour in mice, as shown with the open field. These findings were reproduced in transgenic male (aged 3-6 months) and female (aged 12-18 months) IRAP(+/+) mice. However, the major findings of our study were that the antidepressant-like effect was reversed in angiotensin IV treated IRAP(+/+) mice and was completely absent in age- and gender-matched IRAP(-/-) mice. The lack of an antidepressant-like effect of oxytocin in young male and middle-aged female IRAP(-/-) mice attributes an important role to IRAP in mediating this effect.

Prolyl, cystyl and pyroglutamyl peptidase activities in the hippocampus and hypothalamus of streptozotocin-induced diabetic rats

Prolyl, cystyl and pyroglutamyl peptidases are emerging targets for diabetes and cognitive deficit therapies. The present study is focused on the influence of diabetes mellitus induced by streptozotocin on levels of representative hydrolytic activities of these enzymes in the rat hypothalamus and hippocampus. Streptozotocin-diabetic rats presented about 348mg glucose/dL blood, and a slightly increased hematocrit and plasma osmolality. The activities of soluble and membrane-bound dipeptidyl-peptidase IV, and soluble cystyl aminopeptidase did not differ between diabetic and control rats in both brain areas. Hippocampal soluble prolyl oligopeptidase presented similar activities between diabetic and controls. Increased activities in diabetics were observed for soluble prolyl oligopeptidase (1.78-fold) and membrane-bound cystyl aminopeptidase (2.55-fold) in the hypothalamus, and for membrane-bound cystyl aminopeptidase (5.14-fold) in the hippocampus. In both brain areas, the activities of membrane-bound and soluble pyroglutamyl aminopeptidase were slightly lower (<0.7-fold) in diabetics. All modifications (except hematocrit) observed in streptozotocin-treated rats were mitigated by the administration of insulin. Glucose and/or insulin were shown to alter in vitro the hypothalamic activities of soluble pyroglutamyl aminopeptidase and prolyl oligopeptidase, as well as membrane-bound cystyl aminopeptidase. These data provide the first evidence that diabetes mellitus generates direct and indirect effects on the activity levels of brain peptidases. The implied regional control of regulatory peptide activity by these peptidases suggests novel potential approaches to understand certain disruptions on mediator and modulatory functions in diabetes mellitus.

Ontogeny of Soluble and Particulate Prolyl Endopeptidase Activity in Several Areas of the Rat Brain and in the Pituitary Gland

We have analyzed the activity of prolyl endopeptidase (PEP) in several areas of the rat brain (brain cortex, striatum, brain stem, cerebellum and hypothalamus) and in the pituitary gland during ontogeny. In all of these areas, we observed a reduction in PEP activity during development. However, the temporal profile of these alterations was found to be area specific and differences in the ontogeny of the soluble and particulate forms of PEP were observed. Thus, by postnatal day 20 (PD20), soluble PEP activity had began to decrease in the brain cortex and striatum, whereas decreased soluble PEP activity was observed earlier, at PD15, in the brain stem and cerebellum. Changes in the particulate fraction were even more pronounced. Senescence was associated with decreased soluble PEP activity in the striatum, but in contrast, particulate PEP activity was found to be increased in the senescent brain stem. The present results indicate that alterations in the levels of activity of PEP may represent an important event in the development and aging of the central nervous system.

Estimation of oxytocin mRNA in the human paraventricular nucleus in AIDS by means of quantitative in situ hybridization

Recently, a 40% reduction in the total of oxytocin immunoreactive neurons of the paraventricular nucleus of the hypothalamus in AIDS patients was reported. In the present study, we determined whether this decrease is associated with a diminished amount of oxytocin mRNA. We used in situ hybridization combined with densitometric image analysis for the quantitative assessment of oxytocin gene expression in the paraventricular nucleus of a group of AIDS patients (n=10) and a carefully matched control group (n=8). We found no significant difference (P=0.08) in the amount of oxytocin mRNA per total paraventricular nucleus between the two groups. In addition, no significant differences were found in the part of the volume of the paraventricular nucleus that was occupied by hybridized cells (P=0.12) or in the mean signal density (P=0.08). The findings do not support the hypothesis that the extensive decrease in oxytocin immunoreactive neurons of the paraventricular nucleus in AIDS is associated with a decrease in total oxytocin mRNA content in this nucleus. The data are compatible with the suggestion that in AIDS the biosynthesis of oxytocin is changed in an unknown way at the (post)transcriptional level.

Initiating and responsible enzyme of arginine vasopressin degradation in human placenta and pregnancy serum

The hydrolysis of arginine vasopressin (AVP) by human placental subcellular fractions and pregnancy sera was studied in the presence of selective inhibitors and the antibody against pregnancy serum oxytocinase (P-LAP) (EC 3.4.11.3) by measuring liberated amino acids by high-performance liquid chromatography (HPLC). AVP degradation by placental subcellular fractions and pregnancy sera was inhibited by bestatin. The IC50 values of bestatin on AVP degradation by placental subcellular fractions and pregnancy sera were similar to that of this inhibitor on the P-LAP measured by L-Leu-p-nitroamnilide as a substrate (LAP activity), which we reported previously. Our immunotitration study clearly showed that the initiating and responsible protease in AVP degradation in human placenta and pregnancy serum is P-LAP. Since N-benzylcarbonyl-valyl-prolinal (Z-Val-prolinal), a selective inhibitor of post-proline endopeptidase, and phosphoramidon, a putative endopeptidase-24.11 inhibitor, could not significantly influence the degradation of AVP by placental microsomal fractions. Neither enzyme seems to be actively involved in AVP degradation.