Molecular biology and electrophysiology of neuronal nicotinic receptors of rat chromaffin cells

Pre- and postsynaptic modulation of glycinergic and gabaergic transmission by muscarinic receptors on rat hypoglossal motoneurons in vitro

The motor output of hypoglossal motoneurons to tongue muscles takes place in concert with the respiratory rhythm and is determined by the balance between excitatory glutamatergic transmission and inhibitory transmission mediated by glycine or GABA. The relative contribution by these transmitters is a phasic phenomenon modulated by other transmitters. We examined how metabotropic muscarinic receptors, widely expressed in the brainstem where they excite cranial motor nuclei, might influence synaptic activity mediated by GABA or glycine. For this purpose, using thin slices of the neonatal rat brainstem, we recorded (under whole-cell patch clamp) glycinergic or GABAergic responses from visually identified hypoglossal motoneurons after pharmacological block of glutamatergic transmission. Muscarine inhibited spontaneous and electrically induced events mediated by GABA or glycine. The amplitude of glycinergic miniature inhibitory postsynaptic currents was slightly reduced by muscarine, while GABAergic miniature inhibitory postsynaptic currents were unaffected. Motoneuron currents induced by focally applied GABA and glycine were depressed by muscarine with stronger reduction in glycine-mediated responses. Histochemical observations indicated the presence of M1, M2 and M5 subtypes of muscarinic receptors in the neonatal hypoglossal nucleus. These results suggest that muscarine potently depressed inhibitory neurotransmission on brainstem motoneurons, and that this action was exerted via preterminal and extrasynaptic receptors. Since the large reduction in inhibitory neurotransmission may contribute to overall excitation of brainstem motoneurons by muscarinic receptors, these data might help to understand the central components of action of antimuscarinic agents in preanesthetic medication or against motion sickness.

Selective blockade of nicotinic acetylcholine receptors by pimobendan, a drug for the treatment of heart failure: reduction of catecholamine secretion and synthesis in adrenal medullary cells

Pimobendan, a Ca(2+) sensitizer, is used clinically in the treatment of chronic heart failure. Although chronic heart failure is associated with activation of the sympathetic nervous system, it remains unknown whether pimobendan affects the function of sympathetic neurons and the adrenal medulla. Here, we report the inhibitory effects of pimobendan on catecholamine secretion and synthesis in cultured bovine adrenal medullary cells. Pimobendan decreased the catecholamine secretion (IC(50)=29.5 microM) elicited by carbachol, an agonist at nicotinic acetylcholine receptors, but not that elicited by veratridine, an activator of voltage-dependent Na(+) channels, or by high K(+), an activator of voltage-dependent Ca(2+) channels. Pimobendan also inhibited carbachol-induced influx of (22)Na(+) (IC(50)=25.9 microM) and (45)Ca(2+) (IC(50)=26.0 microM), but not veratridine-induced (22)Na(+) influx or high K(+)-induced (45)Ca(2+) influx. The reduction of catecholamine secretion caused by pimobendan was not overcome by increasing the concentration of carbachol. UD-CG 212, an active metabolite of pimobendan, lowered carbachol-induced catecholamine secretion with a concentration/inhibition curve similar to that of pimobendan. In experiments in situ, pimobendan suppressed both basal and carbachol-stimulated (14)C-catecholamine synthesis (IC(50)=5.3 and 4.9 microM) from [(14)C] tyrosine [but not from L: -3, 4-dihydroxyphenyl [3-(14)C] alanine ([(14)C]DOPA)], as well as tyrosine hydroxylase activity (IC(50)=3.8 and 4.3 microM). These findings suggest that pimobendan inhibits carbachol-induced catecholamines secretion and synthesis through suppression of nicotinic acetylcholine receptors.

Modulation of cerebral microvascular permeability by endothelial nicotinic acetylcholine receptors

Nicotine increases the permeability of the blood-brain barrier in vivo. This implies a possible role for nicotinic acetylcholine receptors in the regulation of cerebral microvascular permeability. Expression of nicotinic acetylcholine receptor subunits in cerebral microvessels was investigated with immunofluorescence microscopy. Positive immunoreactivity was found for receptor subunits alpha3, alpha5, alpha7, and beta2, but not subunits alpha4, beta3, or beta4. Blood-brain barrier permeability was assessed via in situ brain perfusion with [14C]sucrose. Nicotine increased the rate of sucrose entry into the brain from 0.3 +/- 0.1 to 1.1 +/- 0.2 microl.g(-1).min(-1), as previously described. This nicotine-induced increase in blood-brain barrier permeability was significantly attenuated by both the blood-brain barrier-permeant nicotinic antagonist mecamylamine and the blood-brain barrier-impermeant nicotinic antagonist hexamethonium to 0.5 +/- 0.2 and 0.3 +/- 0.2 microl.g(-1).min(-1), respectively. These data suggest that nicotinic acetylcholine receptors expressed on the cerebral microvascular endothelium mediate nicotine-induced changes in blood-brain barrier permeability.

Alpha-conotoxins as tools for the elucidation of structure and function of neuronal nicotinic acetylcholine receptor subtypes

Cone snails comprise approximately 500 species of venomous molluscs, which have evolved the ability to generate multiple toxins with varied and often exquisite selectivity. One class, the alpha-conotoxins, is proving to be a powerful tool for the differentiation of nicotinic acetylcholine receptors (nAChRs). These comprise a large family of complex subtypes, whose significance in physiological functions and pathological conditions is increasingly becoming apparent. After a short introduction into the structure and diversity of nAChRs, this overview summarizes the identification and characterization of alpha-conotoxins with selectivity for neuronal nAChR subtypes and provides examples of their use in defining the compositions and function of neuronal nAChR subtypes in native vertebrate tissues.

Nicotinic and muscarinic acetylcholine receptors are essential for the long-term response of tyrosine hydroxylase gene expression to chronic nicotine treatment in rat adrenal medulla

Nicotine induces tyrosine hydroxylase (TH) mRNA by interacting with nicotinic acetylcholine receptors (nAChRs) in cultured adrenal medullary cell systems; however, the mechanisms responsible for the induction of adrenal TH in response to systemically administered nicotine under in vivo conditions are more complex. In the present study, we tested whether nAChRs and muscarinic acetylcholine receptors (mAChRs) participate in the induction of adrenal TH observed after long-term treatment with nicotine. Chronic nicotine treatment (1.6 mg/kg, two daily injections spaced 12 h apart for 7 days) induced TH mRNA, TH protein and TH activity in rat adrenal medulla. This induction of TH gene expression was totally blocked when an antagonist of either nAChRs or mAChRs was administered prior to each nicotine injection. Repeated injections of the mAChR agonist bethanechol (5 mg/kg injected twice per day for 7 days) also produced increases in TH mRNA levels; however, TH protein levels and TH activity did not increase in response to bethanechol. In denervated adrenal glands chronic nicotine treatment did not lead to induction of either TH mRNA, TH protein or TH activity, whereas chronic bethanechol treatment led to induction of TH mRNA, but not TH protein or activity. These results suggest that agonist occupation of both nAChRs and mAChRs are essential for the complete response of TH gene expression to chronic nicotine treatment in rat adrenal medulla, but that stimulation of either cholinergic receptor by itself is not sufficient to elicit a full response. The results also suggest that both transcriptional and post-transcriptional mechanisms may potentially need to be regulated to induce TH protein in response to some stimuli.

Regulation of the neuronal nicotinic acetylcholine receptor by SRC family tyrosine kinases

Src family kinases (SFKs) are abundant in chromaffin cells that reside in the adrenal medulla and respond to cholinergic stimulation by secreting catecholamines. Our previous work indicated that SFKs regulate acetylcholine- or nicotine-induced secretion, but the site of modulatory action was unclear. Using whole cell recordings, we found that inhibition of SFK tyrosine kinase activity by PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine) treatment or expression of a kinase-defective c-Src reduced the peak amplitude of nicotine-induced currents in chromaffin cells or in human embryonic kidney cells ectopically expressing functional neuronal alpha3beta4alpha5 acetylcholine receptors (AChRs). Conversely, the phosphotyrosine phosphatase inhibitor, sodium vanadate, or expression of mutationally activated c-Src resulted in enhanced current amplitudes. These results suggest that SFKs and putative phosphotyrosine phosphatases regulate the activity of AChRs by opposing actions. This proposed model was supported further by the findings that SFKs physically associate with the receptor and that the AChR is tyrosine-phosphorylated.

Modulation of neuronal nicotinic receptor function by the neuropeptides CGRP and substance P on autonomic nerve cells

1. One classical example of how neuropeptides can affect the function of ligand-gated receptors is the modulation of neuronal nicotinic receptors (nAChRs) by substance P. The present review updates current understanding of this action by substance P and compares it with other neuropeptides more recently found to modulate nAChRs in the autonomic nervous system. 2. Calcitonin gene-related peptide (CGRP) and its N-terminal fragments have been shown to exert complex inhibitory as well facilitatory actions on nAChRs. Fragments such as CGRP(1-4), CGRP(1-5) and CGRP(1-6) rapidly and reversibly enhance agonist sensitivity of nAChRs without directly activating those receptors. Longer fragments or the full-length peptide potently inhibit responses mediated by nAChRs via an apparently competitive-type antagonism. This phenomenon differs from the substance P-induced block, which is agonist use-dependent and preferential towards large nicotinic responses. 3. It is argued that the full-length peptides CGRP and substance P might play distinct roles in the activity-dependent modulation of cholinergic neurotransmission, by inhibiting background noise in the case of CGRP or by reducing excessive excitation in the case of substance P. Hence, multiple neuropeptide mechanisms may represent a wide array of fine-tuning processes to regulate nicotinic synaptic transmission. 4. The availability of novel CGRP derivatives with a strong enhancing action on nAChRs may offer new leads for the drug design targeted for potentiation of nAChRs in the autonomic nervous system as well as in the brain, a subject of interest to counteract the deficit of the nAChR function associated with neurodegenerative diseases like Alzheimer's and Parkinson's diseases.