High- and low-affinity binding of [3H]acetylcholine at nicotinic cholinergic receptors in rat brain

An integrated approach to study the molecular aspects of regulatory peptides biological mechanism

An integrated methodological approach to study the molecular aspects of short regulatory neuropeptides biological mechanism is proposed. The complex research is based on radioligand-receptor method of analysis and covers such points of peptides molecular activity as: specific binding of peptides to brain cells plasmatic membranes, formation of tissue specific synacton, influence of peptides (as allosteric modulators) on functionality of different neuroreceptors as well as delayed in time effects of peptides on receptor-binding activity of well-known neuroreceptor systems. Radiolabeled ligands in such complex study are the one of the best and precision instruments to uncover the molecular mechanism of multiple and multitarget biological effects of regulatory peptides. In this issue we used heptapeptide Semax as a model regulatory peptide, [³ H]Ach and [³ H]GABA as an effector molecules, and the rat model of stress-induced memory and behavior impairment as a morbid state. We showed the ability of Semax to modulate in a dose-dependent manner [³ H]Ach and [³ H]GABA specific binding to some of its corresponding receptors as well as to affect the number of [³ H]GABA specific binding places on rat neurons plasmatic membranes after complex stress exposure.

Binding of [3H]m-aminolevamisole to receptors in levamisole-susceptible and -resistant Haemonchus contortus

M-aminolevamisole, a potent analogue of the commercial anthelmintic levamisole, was used to investigate ligand-binding properties of homogenates of larval and parasitic stages of the nematode parasite of sheep, Haemonchus contortus. Kinetics of the binding of [3H]m-aminolevamisole to homogenates was measured in a drug-susceptible isolate and compared with a levamisole-resistant isolate. Equilibrium binding studies and kinetic studies revealed a high affinity binding component with a KD of 3 nM. A low affinity component (KD = 2.4 microM) was also apparent in equilibrium studies. High affinity [3H]m-aminolevamisole binding was displaced in a concentration-dependent manner by levamisole analogues and cholinergic agonists. Compared with the susceptible isolate, binding in a levamisole-resistant isolate of the parasite, was quantitatively similar over a range of developmental stages and binding conditions. However, under the conditions of binding there was a reduced affinity (larger KD) and more binding sites (larger Bmax) at the low affinity site in the resistant compared with the susceptible isolate. It was concluded that the ligand was binding to acetylcholine receptor populations of the nematode and that resistance may be associated with alterations in the low affinity site of this receptor.

Nicotinic cholinergic normalization of amphetamine-induced loss of auditory gating in freely moving rats

The impairment in normal sensory processing which is usually observed in schizophrenics has been demonstrated using a paired-stimulus paradigm. Normal individuals show a diminished midlatency evoked potential response to the second of a pair of clicks given at a 0.5-s interval. This phenomenon is termed auditory "gating". Schizophrenics routinely fail to suppress their response to the second click in this paradigm; thus, they do not gate. Heavy tobacco use is common among schizophrenics and it has recently been shown that nicotine causes a transient normalization of auditory gating in these individuals. Our laboratory has been utilizing animal models to investigate the sensory deficit observed in schizophrenia. In the present study, rats were administered amphetamine to produce a schizophrenia-like loss of auditory gating. They were then given nicotine, which resulted in a dose-dependent normalization of the amphetamine-induced loss of gating. This effect was blocked by concurrent central administration of d-tubocurarine. Neither nicotine nor d-tubocurarine had any effect on auditory gating when administered alone. These data are in agreement with the human studies showing normalization of auditory gating with nicotine administration and suggest a possible role for the nicotinic cholinergic receptor in the modulation of auditory gating in the rat model.

Distribution of nicotinic cholinergic receptors in the rat tel- and diencephalon: a quantitative receptor autoradiographical study using [3H]-acetylcholine, [alpha-125I]bungarotoxin and [3H]nicotine

The topographical distribution of [alpha-125I]bungarotoxin [125I]BTX, [3H]nicotine ([3H]Nic), [3H]acetylcholine ([3H]ACh) (in the presence of atropine) binding in rat tel- and diencephalon was investigated using a quantitative receptor autoradiographical technique. With the [3H]ACh and [3H]Nic radioligands, a strong labelling was observed in various thalamic nuclei, including the medial habenula, a moderate labelling in different areas of the cortex cerebri, the nucleus caudatus putamen, the nucleus accumbens and tuberculum olfactorium and a uniform weak labelling in the hypothalamus. When the binding data for [3H]Nic were plotted against binding data for [3H]ACh in various brain nuclei, a significant correlation was obtained. Considering [125I]BTX, the strongest labelling was observed in the lateral mammillary nucleus and the hilus gyrus dentatus of the hippocampal formation. A weak labelling occurred in areas such as the nucleus causatus putamen, the thalamus and the cerebral cortex. No significant correlation was therefore obtained between the degree of [125I]BTX binding in various brain nuclei and the degree of binding observed with [3H]Nic or [3H]ACh. The present results underline the view that the high-affinity [3H]Nic and [3H]ACh binding sites label the same cholinergic nicotinic receptor binding site, while [125I]BTX labels another subpopulation of nicotinic cholinergic receptors, predominantly found in discrete areas of the hypothalamus and the limbic cortex.

In vitro binding of 3H-acetylcholine to nicotinic receptors in rodent and human brain

The binding of 3H-acetylcholine (3H-ACh) to nicotinic receptors in rodent and human brain was measured in the presence of atropine to prevent binding to muscarinic binding sites. 3H-ACh binds specifically and saturably to rodent brain. From saturation binding Kd was 30 nM in rat cerebral cortex, which is close to that calculated from kinetic experiments. The binding was temperature-dependent, being highest at low temperatures and decreasing at higher temperatures. The regional distribution of binding in mouse brain was not uniform. The binding was highest in the midbrain, intermediate in the cerebral cortex and striatum, and lowest in the cerebellum, hippocampus, hypothalamus and medulla oblongata. No significant correlation was found between the regional 3H-ACh binding and the regional binding of 3H-alpha-bungarotoxin (3H-BTX), 3H-nicotine (3H-NIC), 3H-tubocurarine and the endogenous acetylcholine content, although the correlation value for 3H-ACh/3H-NIC was at the limit for significance. 3H-ACh also bound specifically to human cerebral cortical tissue and this binding was approximately three times lower than in rodent brain, when a low 3H-ACh concentration was used. In contrast to rat brain there appears to exist multiple binding sites for 3H-ACh in human cerebral cortex as suggested by the curvelinear nature of the Scatchard plot. It was calculated that 3H-ACh bound with Kd 4 nM and Bmax 8 pmol/g protein and Kd 112 nM and Bmax 67 pmol/g protein. The Hill number of 1.5 for the binding of low concentration and 2.5 for high concentration of 3H-ACh also suggest that the 3H-ACh-binding sites interaction exhibit positive cooperativity.

alpha-Bungarotoxin binds to low-affinity nicotine binding sites in rat brain

Reported differences in the pharmacology and distribution of [3H]nicotine and [125I]alpha-bungarotoxin binding sites in mammalian brain suggest that these ligands label separate receptor sites. Affinity purification of an alpha-bungarotoxin binding protein from rat brain failed to copurify the high-affinity nicotine binding site, which remained in the nonbound soluble fraction after the affinity chromatography step. This confirms the independence of these putative receptor sites. Nevertheless, the binding of [125I]alpha-bungarotoxin to P2 membranes was inhibited by (-)-nicotine (Ki = 9 X 10(-6) M), and this sensitivity was preserved after affinity purification. It is proposed that alpha-bungarotoxin binds to a population of low-affinity nicotine binding sites. Comparison of the enantiomers of nicotine in competition studies at both radioligand binding sites revealed an 80-fold preference for the (-) form at the high-affinity [3H]nicotine binding site, whereas the site labelled by [125I]alpha-bungarotoxin displayed little stereoselectivity. In this respect, the brain alpha-bungarotoxin binding site resembles the nicotinic acetylcholine receptor from Torpedo electric organ.