Muscarinic type 1 receptors mediate part of nitric oxide’s vagal facilitatory effect in the isolated innervated rat right atrium

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Rational Design of Photochromic Analogues of Tricyclic Drugs

Tricyclic chemical structures are the core of many important drugs targeting all neurotransmitter pathways. These medicines enable effective therapies to treat from peptic ulcer disease to psychiatric disorders. However, when administered systemically, they cause serious adverse effects that limit their use. To obtain localized and on-demand pharmacological action using light, we have designed photoisomerizable ligands based on azobenzene that mimic the tricyclic chemical structure and display reversibly controlled activity. Pseudo-analogues of the tricyclic antagonist pirenzepine demonstrate that this is an effective strategy in muscarinic acetylcholine receptors, showing stronger inhibition upon illumination both in vitro and in cardiac atria ex vivo. Despite the applied chemical modifications to make pirenzepine derivatives sensitive to light stimuli, the most potent candidate of the set, cryptozepine-2, maintained a moderate but promising M₁ vs M₂ subtype selectivity. These photoswitchable "crypto-azologs" of tricyclic drugs might open a general way to spatiotemporally target their therapeutic action while reducing their systemic toxicity and adverse effects.

Muscarinic toxins

Muscarinic toxins isolated from the venom of Dendroaspis snakes may interact with a high affinity, large selectivity and various functional properties with muscarinic receptors. Therefore, these toxins are invaluable tools for studying the physiological role, molecular functioning and structural organization of the five subtypes of these G-Protein Coupled Receptors. We review the data on the most relevant results dealing with the isolation/identification, mode of action, structure/function and exploitation of these toxins and finally highlight the unresolved issues related to their pharmacological studies.

N-desmethylclozapine an M1 receptor agonist enhances nitric oxide's cardiac vagal facilitation in the isolated innervated rat right atrium

We have previously determined that neuronal nitric oxide (NO) may partly mediate its established cholinergic effect via activation of muscarinic type 1 (M1) receptors located at the preganglionic/postganglionic synapse. In this series of experiments we set out to confirm this finding using an M1 agonist. Experiments were carried out on the isolated vagally innervated right atrium in the presence of atenolol (4 microM). The right vagus was stimulated at 4, 8, 16, 32 Hz; pulse duration 1 ms at 20 V for 20 s and the effect on cardiac interval (ms) assessed. N-desmethylclozapine (100 nM), a potent M1 agonist, enhanced the vagally induced increase in cardiac interval, a lower concentration of 50 nM had no significant effect on cardiac interval. This effect was prevented by pre-treatment of the atria with the neuronal NO synthase inhibitor 1 (2-trifluoromethylphenyl)imidazole (TRIM) at 0.14 mM. The vagal stimulation protocol was repeated in order to rule out a reduction in vagal effectiveness which may have been due to the experimental stimulation protocol used in this study. TRIM (0.14 mM) alone causes a small but significant attenuation of the vagally induced increase in cardiac interval. These results show that agonism of M1 receptors on cardiac vagal preganglionic fibres enhances vagal cardiac effects which can be prevented by a neuronal NO inhibitor.

Subtype-selective blockade of cardiac muscarinic receptors inhibits vagal chronotropic responses in cats

This study was designed to determine if chronotropic responses induced by neurally released acetylcholine are modified by subtype-selective blockade of cardiac muscarinic cholinoreceptors. In anesthetized cats, a single burst of vagal stimulation was generated with an incremental time delay after the P wave of the atrial electrogram (P-Stimulus interval). The slope of the relationships between P-Stimulus and P-P intervals was used to assess changes in responsiveness of cardiac pacemaker to vagal effects throughout the cardiac cycle. An increase in P-Stimulus interval over the initial portion (approximately 120 ms) of the cardiac cycle produced a significant increment in lengthening of the P-P interval. Once the maximal negative chronotropic response was achieved, a further increase in P-Stimulus interval by only approximately 25 ms resulted in profound (by 80-90%) reductions in vagal effects, thus yielding a bimodal vagal phase response curve. Antagonists of M1 (pirenzepine), M2 (methoctramine and gallamine), and M3 (4-DAMP) muscarinic cholinoreceptors produced a reduction in the magnitude of maximal lengthening of cardiac cycle as well as an increase in latency of vagal effects. However, the increment in prolongation of P-P interval induced by a given change in timing of vagal stimulation during cardiac cycle was reduced by M1 and M2 muscarinic receptor blockers, but was unaffected by 4-DAMP. None of the antagonists modified the range of P-Stimulus intervals over which the maximum-to-minimum change of vagal responses occurred. Taken together, these data suggest different contribution of various subtypes of cardiac muscarinic receptors into the negative chronotropic responses induced by brief bursts of vagal stimulation.