Effect of I.C.V. injection of AT4 receptor ligands, NLE1-angiotensin IV and LVV-hemorphin 7, on spatial learning in rats.

Central administration of angiotensin IV (Ang IV) or its analogues enhance performance of rats in passive avoidance and spatial memory paradigms. The purpose of this study was to examine the effect of a single bolus injection of two distinct AT4 ligands, Nle1-Ang IV or LVV-haemorphin-7, on spatial learning in the Barnes circular maze. Mean number of days for rats treated with either Nle1-Ang IV or LVV-haemorphin-7 to achieve learner criterion is significantly reduced compared with controls (P < 0.001 and P < 0.05 respectively). This is due to enhanced ability of the peptide-treated rats to adopt a spatial strategy for finding the escape hatch. In all three measures of learning performance, (1) the number of errors made, (2) the distance travelled and (3) the latency in finding the escape hatch, rats treated with either 100 pmol or 1 nmol of Nle1-Ang IV or 100 pmol LVV-haemorphin-7 performed significantly better than the control groups. As early as the first day of testing, the rats treated with the lower dose of Nle1-Ang IV or LVV-haemorphin-7 made fewer errors (P < 0.01 and P < 0.05 respectively) and travelled shorter distances (P < 0.05 for both groups) than the control animals. The enhanced spatial learning induced by Nle1-Ang IV (100 pmol) was attenuated by the co-administration of the AT4 receptor antagonist, divalinal-Ang IV (10 nmol). Thus, administration of AT4 ligands results in an immediate potentiation of learning, which may be associated with facilitation of synaptic transmission and/or enhancement of acetylcholine release.

Ceramide can induce cell death in sensory neurons, whereas ceramide analogues and sphingosine promote survival.

Ceramide has been shown to induce apoptosis in leukaemic cells and some other cell types, but there are few data on its role in neuronal cells. We investigated the effect of ceramide and its analogues in cultured sensory neurons from neonatal mice. These cells undergo apoptosis in the absence of neurotrophins. Treatment with ceramide or its analogues increased survival, both in the presence and absence of NGF. Sphingosine treatment also increased survival. In the presence of the ceramidase inhibitor N-oleoyl ethanolamine, which blocks conversion of ceramide to sphingosine, the addition of natural ceramide-induced cell death, even in the presence of nerve growth factor (NGF). N-oleoyl ethanolamine did not cause cell death by itself. N-oleoyl ethanolamine did not alter the response to ceramide analogues, indicating that they were not ceramidase substrates. These results indicate that, in sensory neurons, exogenous ceramide is converted to sphingosine, which promotes cell survival. When conversion is blocked by ceramidase inhibition, exogenous ceramide causes cell death, presumably due to the high levels of ceramide itself. The ceramide analogues all mimicked the effect of sphingosine rather than ceramide, casting serious doubt on their validity as models of ceramide action. Ceramide analogues could prevent neuronal death even in the combined presence of N-oleoyl ethanolamine and natural ceramide. Surprisingly, dihydro C2-ceramide, which is frequently used as a control for C2-ceramide, had the same effect as ceramide analogues.