Distribution of angiotensin IV binding sites (AT4 receptor) in the human forebrain, midbrain and pons as visualised by in vitro receptor autoradiography.

Angiotensin IV and other AT4 receptor agonists, improve memory retention and retrieval in the passive avoidance and swim maze learning paradigms. Angiotensin IV binding sites (also known as the AT4 receptors) are widely distributed in guinea pig and monkey (Macaca fascicularis) brains where high densities of the binding sites have been detected in the hippocampus, neocortex and motor nuclei. However, the distribution of the binding sites in the human brain is not known. We have recently localised the angiotensin IV binding sites (AT4 receptors) in post-mortem human brain using iodinated Nle-angiotensin IV, a higher affinity and more stable analogue of angiotensin IV. This radioligand bound with relatively high affinity and specificity to angiotensin IV binding sites. In competition studies on consecutive sections through the prefrontal cortex and claustrum, angiotensin IV, Nle-angiotensin IV and LVV-hemorphin 7 competed for the binding of 125I[Nle]-angiotensin IV with nanomolar affinities. Angiotensin II and the AT1 and AT2 receptor antagonists were ineffective in competing for the binding at concentrations of up to 10 microM. We found high densities of 125I[Nle]-angiotensin IV binding sites throughout the cerebral cortex including the insular, entorhinal, prefrontal and cingulate cortices. Very high densities of the binding sites were observed in the claustrum, choroid plexus, hippocampus and pontine nucleus. Some thalamic nuclei displayed high densities of binding including the anteroprincipal, ventroanterior, anteromedial, medial dorsal and ventrolateral nuclei. The caudate nucleus, putamen, many amygdaloid nuclei and the red nucleus all displayed moderate densities of binding with a higher level detected in the substantia nigra pars compacta. In the hypothalamus, high densities binding sites were found in the ventromedial nucleus with lower levels in the dorsomedial and paraventricular nuclei. The distribution of 125I[Nle]-angiotensin IV binding sites in the human brain is similar to that found in other species and supports multiple roles for the binding sites in the central nervous system, including facilitation of memory retention and retrieval.

Increased ACTH levels do not alter renal 11beta-hydroxysteroid dehydrogenase type 2 gene expression in the sheep.

The regulation of renal 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2) gene expression is poorly understood. Inhibition of expression can result in hypertension. An example of this is in ectopic adrenocorticotropin (ACTH) syndrome (EAS). Inhibition of 11betaHSD2 activity is suggested by the observed increased ratio of cortisol to cortisone in both plasma and urine. To investigate whether ACTH or ACTH-dependent steroids can modulate renal 11betaHSD2 gene expression we analysed renal 11betaHSD2 mRNA levels after treatment with ACTH of 1 H and 24 H and demonstrated no change in the levels of gene expression. We have demonstrated in this study that the expression of 11betaHSD2 in the kidney is unaltered by ACTH. The reduced inactivation of cortisol by 11betaHSD2 observed in EAS is likely to be in part due to end product inhibition or substrate overload of the enzyme by endogenous substrates (cortisol, corticosterone, etc) rather than inhibition of 11betaHSD2 at the transcriptional level by either ACTH or ACTH regulated steroids.

Evidence that the angiotensin IV (AT(4)) receptor is the enzyme insulin-regulated aminopeptidase.

Central infusion of angiotensin IV or its more stable analogues facilitates memory retention and retrieval in normal animals and reverses amnesia induced by scopolamine or by bilateral perforant pathway lesions. These peptides bind with high affinity and specificity to a novel binding site designated the angiotensin AT(4) receptor. Until now, the AT(4) receptor has eluded molecular characterization. Here we identify the AT(4) receptor, by protein purification and peptide sequencing, to be insulin-regulated aminopeptidase (IRAP). HEK 293T cells transfected with IRAP exhibit typical AT(4) receptor binding characteristics; the AT(4) receptor ligands, angiotensin IV and LVV-hemorphin 7, compete for the binding of [(125)I]Nle(1)-angiotensin IV with IC(50) values of 32 and 140 nm, respectively. The distribution of IRAP and its mRNA in the brain, determined by immunohistochemistry and hybridization histochemistry, parallels that of the AT(4) receptor determined by radioligand binding. We also show that AT(4) receptor ligands dose-dependently inhibit the catalytic activity of IRAP. We have therefore demonstrated that the AT(4) receptor is IRAP and propose that AT(4) receptor ligands may exert their effects by inhibiting the catalytic activity of IRAP thereby extending the half-life of its neuropeptide substrates.