The history of neuropeptide research: version 5.a

Discovery of inhibitors of insulin-regulated aminopeptidase as cognitive enhancers

The hexapeptide angiotensin IV (Ang IV) is a metabolite of angiotensin II (Ang II) and plays a central role in the brain. It was reported more than two decades ago that intracerebroventricular injection of Ang IV improved memory and learning in the rat. Several hypotheses have been put forward to explain the positive effects of Ang IV and related analogues on cognition. It has been proposed that the insulin-regulated aminopeptidase (IRAP) is the main target of Ang IV. This paper discusses progress in the discovery of inhibitors of IRAP as potential enhancers of cognitive functions. Very potent inhibitors of the protease have been synthesised, but pharmacokinetic issues (including problems associated with crossing the blood-brain barrier) remain to be solved. The paper also briefly presents an overview of the status in the discovery of inhibitors of ACE and renin, and of AT1R antagonists and AT2R agonists, in order to enable other discovery processes within the RAS system to be compared. The paper focuses on the relationship between binding affinities/inhibition capacity and the structures of the ligands that interact with the target proteins.

Adipokines and the blood-brain barrier

Just as the blood-brain barrier (BBB) is not a static barrier, the adipocytes are not inert storage depots. Adipokines are peptides or polypeptides produced by white adipose tissue; they play important roles in normal physiology as well as in the metabolic syndrome. Adipokines secreted into the circulation can interact with the BBB and exert potent CNS effects. The specific transport systems for two important adipokines, leptin and tumor necrosis factor alpha, have been characterized during the past decade. By contrast, transforming growth factor beta-1 and adiponectin do not show specific permeation across the BBB, but modulate endothelial functions. Still others, like interleukin-6, may reach the brain but are rapidly degraded. This review summarizes current knowledge and recent findings of the rapidly growing family of adipokines and their interactions with the BBB.

Effects of melanocortins on cardiovascular regulation in rats

1. Of the melanocortin peptides, gamma(2)-melanocyte-stimulating hormone (MSH) has been attributed a cardiovascular effect, inducing an increase in blood pressure and heart rate. Although still controversial, this effect, based on pharmacological blockade experiments, is supposed to be mediated through sympathetic activation. 2. The aims of the present study were to identify the N-terminal pro-opiomelanocortin (N-POMC) fragments and melanocortins that influence blood pressure and heart rate and to investigate the real-time changes in baroreflex sensitivity and in sympathetic and vagal modulation underlying cardiovascular effects in conscious rats without the use of pharmacological blockade. 3. Intracerebroventricular administration of different melanocortins and N-POMC induced a long-lasting dose- dependent pressor response from 1 nmol onwards, with only a small initial bradycardic response with the highest dose. 4. Coinciding with this pressor response, an elicitation of the low-frequency (LF) component was observed in spectral analysis of both blood pressure variability (BPV) and heart rate variability (HRV), followed by the high-frequency (HF) component in at least BPV. Baroreflex sensitivity remained unchanged. 5. After intravenous administration, gamma(2)-MSH produced a short-lasting dose-dependent pressor and cardioaccelerator response with very rapid onset with concentrations from 1 nmol onwards. 6. Continuous infusion of gamma(2)-MSH depressed baroreflex sensitivity and simultaneously increased both components of BPV, with a radical reduction of the LF component and a preserved vagal HF component in HRV. 7. Of all the intravenously administered melanocortins, only gamma(2)-MSH was active. The central effect is likely to depend on an increase of (alpha-)sympathetic outflow. 8. For the peripheral effect, gamma(2)-MSH appeared to act as a baroreceptor reflex-blocking agent, being compatible with a role in the acute stress response.

Penetration of neurotrophins and cytokines across the blood-brain/blood-spinal cord barrier

Now that peptides are no longer considered too large to cross the blood-brain barrier, attention has turned to the possibility that larger substances like polypeptides might also enter the central nervous system (CNS). This review summarizes evidence showing that many cytokines and neurotrophins not only enter the brain but also enter the spinal cord, sometimes faster than into the brain.

The rat dermorphin-like immunoreactivity is supported by an aminopeptidase resistant peptide

Site-directed antibodies against synthetic related dermorphin peptides were previously produced and characterized. One of them, which specifically recognizes the crucial 'opioid message' (the N-terminal part of the dermorphin molecule (i.e. Tyr-D-Ala-Phe-Gly) was selected in order to detect and locate endogenous dermorphin-like molecules in rat, mouse and guinea pig tissues. Dermorphin-like peptides were found to be present in tissues known to contain peptides such as neurons in the central nervous system, nerve fibers in the gut and B and T immune cells. With all the tissues assayed, the HPLC profile obtained on the immunoreactive material showed the same main peak eluted at a retention time of 32 +/- 1 min. The results of biochemical experiments in which enzymatic treatments were performed on the dermorphin-like immunoreactivity indicate the immunoreactivity is a peptide resistant to aminopeptidase hydrolysis. This finding suggests the presence of a residue conferring resistance to proteolytic processes of this kind, which is likely to be a D-amino acid residue.

Transport of CRH from mouse brain directly affects peripheral production of beta-endorphin by the spleen

The blood-brain barrier (BBB) regulates the passage of substances between the brain and the periphery. It has not been shown that the secretion from the brain of a small amount of a substance can directly affect the periphery by transport across the BBB. We found that central injection of radioactively labeled corticotropin-releasing hormone (CRH) resulted in the accumulation of intact CRH in the spleen. CRH also increased splenic beta-endorphin, an effect not blocked by pretreatment with dexamethasone. Inhibition of the secretion of CRH from the brain by colchicine resulted in decreased accumulation of CRH in the spleen and also decreased splenic beta-endorphin. Similar findings occurred in the pituitary gland. The results show that the passage of labeled CRH from the brain can directly affect a peripheral organ, thus emphasizing the regulatory function of the BBB.