Pier Francesco Mannaioni

Relaxin‐induced increased coronary flow through stimulation of nitric oxide production

1 Relaxin (RLX) is a multifunctional hormone which, besides its role in pregnancy and parturition, has also been shown to influence the cardiovascular system. In this study, we investigated the effect of RLX on coronary flow of rat and guinea‐pig hearts, isolated and perfused in a Langendorff apparatus. RLX was either added to the perfusion fluid at a concentration of 5 × 10−9 M for a 20‐min perfusion, or given as a bolus into the aortic cannula at concentrations of 10−9 M, 5 × 10−9 M and 10−8 M dissolved in 1 ml of perfusion fluid. 2 RLX, given either for a 20‐min perfusion or as a bolus in the aortic cannula to guinea‐pig and rat isolated hearts, increased the coronary flow and the amount of nitrite, a stable end‐product of nitric oxide (NO) metabolism, that appeared in the perfusates in a concentration‐dependent fashion. 3 The increase in coronary flow and in nitrite in the perfusates induced by RLX was significantly reduced by pretreatment with the nitric oxide synthase (NOS) inhibitor, NG‐monomethyl‐L‐arginine (L‐NMMA, 10−4 M). 4 The effects of RLX on coronary flow and nitrite amounts in the perfusates were compared with those induced by the endothelium‐dependent vasodilator agent, acetylcholine (ACh, 10−8 − 10−7 M), and by the endothelium‐independent vasodilator agent, sodium nitroprusside (SNP, 10−7‐10−6M). The results obtained show that RLX is more effective than ACh and SNP in increasing coronary flow. 5 The results of this study show that RLX increases coronary flow through stimulation of NO production; hence this hormone should be regarded as a novel agent capable of improving myocardial perfusion.

Central histaminergic system and cognition.

The neurotransmitter histamine is contained within neurons clustered in the tuberomammillary nuclei of the hypothalamus. These cells give rise to widespread projections extending through the basal forebrain to the cerebral cortex, as well as to the thalamus and pontomesencephalic tegmentum. These morphological features suggest that the histaminergic system acts as a regulatory center for whole-brain activity. Indeed, this amine is involved in the regulation of numerous physiological functions and behaviors, including learning and memory, as indicated by extensive research reviewed in this paper. Histamine effects on cognition might be explained by the modulation of the cholinergic system. However, interactions of histamine with any transmitter system, and/or a putative intrinsic procognitive role cannot be excluded. Furthermore, although experimental evidence indicates that attention-deficit hyperactivity disorder symptoms arise from impaired dopaminergic and noradrenergic transmission, recent research suggests that histamine is also involved. The possible relevance of histamine in disorders such as age-related memory deficits, Alzheimers disease and attention-deficit hyperactivity disorder is worth of consideration, and awaits validation with clinical trials that will prove the beneficial effects of histaminergic drugs in the treatment of these diseases.

Nitric Oxide: A Regulatory Mediator of Mast Cell Reactivity

Rat serosal mast cells were tested for their ability to generate a nitric oxide (NO)-like factor by two bioassay systems: inhibition of platelet aggregation and stimulation of mast cell guanylate cyclase. The relationship between histamine release and NO-like activity from these cells was also investigated. Incubation with human washed platelets of rat serosal mast cells treated with indomethacin resulted in an inhibition of thrombin-induced platelet aggregation proportional to the number of cells.

Activation of histaminergic H3 receptors in the rat basolateral amygdala improves expression of fear memory and enhances acetylcholine release

The basolateral amygdala (BLA) is involved in learning that certain environmental cues predict threatening events. Several studies have shown that manipulation of neurotransmission within the BLA affects the expression of memory after fear conditioning. We previously demonstrated that blockade of histaminergic H3 receptors decreased spontaneous release of acetylcholine (ACh) from the BLA of freely moving rats, and impaired retention of fear memory. In the present study, we examined the effect of activating H3 receptors within the BLA on both ACh release and expression of fear memory. Using the microdialysis technique in freely moving rats, we found that the histaminergic H3 agonists R‐α‐methylhistamine (RAMH) and immepip, directly administered into the BLA, augmented spontaneous release of ACh in a similar manner. Levels of ACh returned to baseline on perfusion with control medium. Rats receiving intra‐BLA, bilateral injections of the H3 agonists at doses similar to those enhancing ACh spontaneous release, immediately after contextual fear conditioning, showed stronger memory for the context–footshock association, as demonstrated by longer freezing assessed at retention testing performed 72u2003h later. Post‐training, bilateral injections of 15u2003ng oxotremorine also had a similar effect on memory retention, supporting the involvement of the cholinergic system. Thus, our results further support a physiological role for synaptically released histamine, that in addition to affecting cholinergic transmission in the amygdala, modulates consolidation of fear memories

Interactions between histaminergic and cholinergic systems in learning and memory.

The aim of this review is to survey biochemical, electrophysiological and behavioral evidence of the interactions between the cholinergic and histaminergic systems and evaluate their possible involvement in cognitive processes. The cholinergic system has long been implicated in cognition, and there is a plethora of data showing that cholinergic deficits parallel cognitive impairments in animal models and those accompanying neurodegenerative diseases or normal aging in humans. Several other neurotransmitters, though, are clearly implicated in cognitive processes and interact with the cholinergic system. The neuromodulatory effect that histamine exerts on acetylcholine release is complex and multifarious. There is clear evidence indicating that histamine controls the release of central acetylcholine (ACh) locally in the cortex and amygdala, and activating cholinergic neurones in the nucleus basalis magnocellularis (NBM) and the medial septal area-diagonal band that project to the cortex and to the hippocampus, respectively. Extensive experimental evidence supports the involvement of histamine in learning and memory and the procognitive effects of H(3) receptor antagonists. However, any attempt to strictly correlate cholinergic/histaminergic interactions with behavioral outcomes without taking into account the contribution of other neurotransmitter systems is illegitimate. Our understanding of the role of histamine in learning and memory is still at its dawn, but progresses are being made to the point of suggesting potential treatment strategies that may produce beneficial effects on neurodegenerative disorders associated with impaired cholinergic function.

Histamine H3 receptor-mediated impairment of contextual fear conditioning and in-vivo inhibition of cholinergic transmission in the rat basolateral amygdala

We investigated the effects of agents acting at histamine receptors on both, spontaneous release of ACh from the basolateral amygdala (BLA) of freely moving rats, and fear conditioning. Extensive evidence suggests that the effects of histamine on cognition might be explained by the modulation of cholinergic systems. Using the microdialysis technique in freely moving rats, we demonstrated that perfusion of the BLA with histaminergic compounds modulates the spontaneous release of ACh. The addition of 100u2003mm KCl to the perfusion medium strongly stimulated ACh release, whereas, 0.5u2003µm tetrodotoxin (TTX) inhibited spontaneous ACh release by more than 50%. Histaminergic H3 antagonists (ciproxifan, clobenpropit and thioperamide), directly administered to the BLA, decreased ACh spontaneous release, an effect fully antagonized by the simultaneous perfusion of the BLA with cimetidine, an H2 antagonist. Local administration of cimetidine alone increased ACh spontaneous release slightly, but significantly. Conversely, the administration of H1 antagonists failed to alter ACh spontaneous release. Rats receiving intra‐BLA, bilateral injections of the H3 antagonists at doses similar to those inhibiting ACh spontaneous release, immediately after contextual fear conditioning, showed memory consolidation impairment of contextual fear conditioning. Post‐training, bilateral injections of 50u2003µg scopolamine also had an adverse effect on memory retention. These observations provide the first evidence that histamine receptors are involved in the modulation of cholinergic tone in the amygdala and in the consolidation of fear conditioning.

Protective effects of M40403, a selective superoxide dismutase mimetic, in myocardial ischaemia and reperfusion injury in vivo.

Myocardial injury caused by ischaemia and reperfusion comes from multiple pathogenic events, including endothelial damage, neutrophil extravasation into tissue, mast cell activation, and peroxidation of cell membrane lipids. These events are followed by myocardial cell alterations resulting eventually in cell necrosis. An enhanced formation of reactive oxygen species is widely accepted as a stimulus for tissue destruction and cardiac failure. In this study, we have investigated the cardioprotective effects of M40403 in myocardial ischaemia‐reperfusion injury. M40403 is a low molecular weight, synthetic manganese containing superoxide dismutase mimetic (SODm) that selectively removes superoxide anion. Ischaemia was induced in rat hearts in vivo by ligating the left anterior descending coronary artery. Thirty minutes after the induction of ischaemia, the ligature was removed and reperfusion allowed to occur for at least 60 min. M40403 (0.1–1 mg kg−1) was given intravenously 15 min before ischaemia. The results obtained in this study showed that M40403 significantly reduced the extent of myocardial damage, mast cell degranulation and the incidence of ventricular arrhythmias. Furthermore, M40403 significantly attenuated, in a dose‐dependent manner, neutrophil infiltration in the myocardium as well as the associated induction of lipid peroxidation. Calcium overload seen post‐reperfusion of the ischaemic myocardium was also reduced by M40403. Immunohistochemical analysis for nitrotyrosine revealed a positive staining in cardiac tissue taken after reperfusion: this was attenuated by M40403. Moreover reperfused cardiac tissue sections showed positive staining for P‐selectin and for anti‐intercellular adhesion molecule (ICAM‐1) in the vascular endothelial cells. M40403 treatment markedly reduced the intensity and degree of P‐selectin and ICAM‐1 in these tissues. No staining for nitrotyrosine, P‐selectin or ICAM‐1 was found in cardiac tissue taken at the end of the ischaemic period. Overall, M40403 treatment reduced the morphological signs of myocardial cell injury and significantly improved survival. Taken together, these results clearly indicate that M40403 treatment exerts a protective effect against ischaemia‐reperfusion‐induced myocardial injury, supporting a key role for superoxide anion in reperfusion injuries. This suggests that synthetic enzymes of SOD such as M40403, offer a novel therapeutic approach for the treatment of ischaemic heart disease where superoxide anion plays a dominant role.

The release of histamine by free radicals

An extensive discussion of the available evidence about the release of histamine by free radicals is presented. In addition, the involvement of the free-radical driven release of histamine in human pathophysiology is debated.

Endogenous histamine in the medial septum–diagonal band complex increases the release of acetylcholine from the hippocampus: a dual‐probe microdialysis study in the freely moving rat

The effects of histaminergic ligands on both ACh spontaneous release from the hippocampus and the expression of c‐fos in the medial septum–diagonal band (MSA‐DB) of freely moving rats were investigated. Because the majority of cholinergic innervation to the hippocampus is provided by MSA‐DB neurons, we used the dual‐probe microdialysis technique to apply drugs to the MSA‐DB and record the induced effects in the projection area. Perfusion of MSA‐DB with high‐KCl medium strongly stimulated hippocampal ACh release which, conversely, was significantly reduced by intra‐MSA‐DB administration of tetrodotoxin. Histamine or the H2 receptor agonist dimaprit, applied directly to the hippocampus, failed to alter ACh release. Conversely, perfusion of MSA‐DB with these two compounds increased ACh release from the hippocampus. Also, thioperamide and ciproxifan, two H3 receptor antagonists, administered into MSA‐DB, increased the release of hippocampal ACh, whereas R‐α‐methylhistamine, an H3 receptor agonist, produced the opposite effect. The blockade of MSA‐DB H2 receptors, caused by local perfusion with the H2 receptor antagonist cimetidine, moderated the spontaneous release of hippocampal ACh and antagonized the facilitation produced by H3 receptor antagonists. Triprolidine, an H1 receptor antagonist, was without effect. Moreover, cells expressing c‐fos immunoreactivity were significantly more numerous in ciproxifan‐ or thioperamide‐treated rats than in controls, although no colocalization of anti‐c‐fos and anti‐ChAT immunoreactivity was observed. These results indicate a role for endogenous histamine in modulating the cholinergic tone in the hippocampus.

Cortical acetylcholine release elicited by stimulation of histamine H1 receptors in the nucleus basalis magnocellularis: a dual‐probe microdialysis study in the freely moving rat

Perfusion of the nucleus basalis magnocellularis (NBM) with histamine agonists and antagonists modulates the spontaneous release of cortical acetylcholine (ACh) in freely moving rats. Perfusion of the NBM with Ringer solution containing 100u2003mm K+ strongly stimulated the spontaneous release of cortical ACh in freely moving rats, whereas perfusion with 1u2003µm tetrodotoxin reduced cortical ACh spontaneous release by more than 50%. Administration of histamine to the NBM concentration‐dependently increased the spontaneous release of cortical ACh. Administration of H1 (methylhistaprodifen) but not H2 (dimaprit) or H3 (R‐α‐methylhistamine) receptor agonists to the NBM mimicked the effect of histamine. Perfusion of the NBM with either H1 (mepyramine or triprolidine) or H2 (cimetidine) receptor antagonists failed to alter ACh spontaneous release from the cortex, however, H1 but not H2 receptor antagonists antagonized the releases of cortical ACh elicited by histamine and methylhistaprodifen. Local administration of H3 receptor antagonists (clobenpropit and thioperamide) to the NBM increased the spontaneous release of ACh from the cortex; this effect was antagonized by H1 receptor antagonism. Conversely local administration of MK‐801, a noncompetitive receptor antagonist of the N‐methyl‐d‐aspartate receptor, to the NBM failed to alter ACh spontaneous release from the cortex and to antagonize ACh release elicited by histamine. This study demonstrates that activation of histamine H1 receptors in the NBM increases ACh spontaneous release from the cortex.