Nicotinic cholinergic receptor binding sites in the brain: regulation in vivo.

Tritiated acetylcholine was used to measure binding sites with characteristics of nicotinic cholinergic receptors in rat brain. Regulation of the binding sites in vivo was examined by administering two drugs that stimulate nicotinic receptors directly or indirectly. After 10 days of exposure to the cholinesterase inhibitor diisopropyl fluorophosphate, binding of tritiated acetylcholine in the cerebral cortex was decreased. However, after repeated administration of nicotine for 10 days, binding of tritiated acetylcholine in the cortex was increased. Saturation analysis of tritiated acetylcholine binding in the cortices of rats treated with diisopropyl fluorophosphate or nicotine indicated that the number of binding sites decreased and increased, respectively, while the affinity of the sites was unaltered.

A strong influence of serotonin axons on beta-adrenergic receptors in rat brain.

The role of serotonin axons in modulating the norepinephrine neurotransmission system in rat brain was investigated. Selective lesions of the forebrain serotonergic system were made by injecting 5,7-dihydroxytryptamine into the midbrain raphe nuclei. Four to six weeks after the lesion, the uptake of 3H-labeled serotonin in the frontal cortex and the hippocampus was reduced by more than 90 percent, while neither the uptake of 3H-labeled norepinephrine nor the content of norepinephrine was affected in either tissue. The number of beta-adrenergic receptors, as measured by radioligand binding with 3H-labeled dihydroalprenolol, was increased in the frontal cortex and hippocampus of rats with lesions. Similarly, specific lesions of central serotonin axons produced by systemically administered p-chloramphetamine resulted in an increase in the binding of 3H-labeled dihydroalprenolol to beta-adrenergic receptors and in the production of adenosine 3',5'-monophosphate in response to isoproterenol. These results indicate that serotonin axons may regulate beta-adrenergic receptor number and function in brain.

Ventral medial hypothalamus: involvement in hypoglycemic convulsions.

After the ventral medial hypothalamus of mice was lesioned with gold thioglucose, the dose of insulin required to produce convulsions in 50 percent of the animals was doubled compared to that in nonlesionad controls. No dose of insulin, up to 50 milliunits per gram, produced convulsions in more than 60 percent of the lesioned mice, even though blood glucose levels fell to approximately 24 milligram percent.

Lithium increases serotonin release and decreases serotonin receptors in the hippocampus.

The effects of long-term lithium administration on pre- and postsynaptic processes involved in serotonergic neurotransmission were measured in rat hippocampus and cerebral cortex. Long-term lithium administration increased both basal and potassium chloride-stimulated release of endogenous serotonin from the hippocampus but not from the cortex. Serotonergic receptor binding was reduced in the hippocampus but not in the cortex. These results suggest a mechanism by which lithium may stabilize serotonin neurotransmission.

Reductions in acetylcholine and nicotine binding in several degenerative diseases.

Alzheimer's disease, Parkinson's disease, and progressive supranuclear palsy are all characterized by loss of neurons in the basal forebrain cholinergic system and by associated reductions in cortical presynaptic cholinergic markers, such as choline acetyltransferase. In this report, we identify that a major cortical receptor alteration in these disorders is a reduction in nicotinic receptors measured using both tritiated acetylcholine and levorotatory tritiated nicotine binding.

Reductions in [3H]nicotinic acetylcholine binding in Alzheimer's disease and Parkinson's disease: an autoradiographic study.

In Alzheimer's disease (AD) and Parkinson's disease (PD), dysfunction in the basal forebrain cholinergic system is accompanied by a consistent loss of presynaptic cholinergic markers in cortex, but changes in cholinergic receptor binding sites are poorly understood. In the present study, we used receptor autoradiography to map the distribution of nicotinic [3H]acetylcholine binding sites in cortices of individuals with AD and PD and matched control subjects. In both diseases, a profound loss of nicotinic receptors occurs in all cortical layers, particularly the deepest layers.

Comparative effects of urapidil, prazosin, and clonidine on ligand binding to central nervous system receptors, arterial pressure, and heart rate in experimental animals.

We studied the effects of urapidil, clonidine, and prazosin on ligand binding to central nervous system receptors in rats and on arterial pressure and heart rate in chloralose-anesthetized cats. Ligand binding studies indicated that urapidil had 90 times greater affinity for alpha 1 than for alpha 2 adrenergic receptors. Administration of urapidil (129 micrograms) into the cerebroventricles of cats revealed no effect after lateral ventricle injection, a decrease of 9.7 +/- 3.0 mm Hg after fourth ventricle injection, and an increase of 10.8 +/- 2.2 mm Hg after restriction of the drug in the forebrain ventricles. Clonidine (30 micrograms) produced hypotension and bradycardia after injection into the lateral ventricle. Prazosin was ineffective after cerebroventricular injection. Intravenous administration of 0.22, 0.67, and 2.00 mg/kg urapidil produced dose-dependent decreases in arterial pressure that were associated with blockade of alpha 1 adrenergic receptors. Intravenous administration of prazosin elicited the same response. Clonidine (10 micrograms/kg, intravenously produced an initial increase in arterial pressure that was unaffected by pretreatment with urapidil or prazosin. These results suggest that urapidil produces hypotension by an action on the peripheral vasculature and in the hindbrain. The peripheral effect involves blockade of alpha 1 adrenoceptors.

Electroconvulsive shock: effects on biochemical correlates of neurotransmitter receptors in rat brain.

The effects of electroconvulsive shock on neurotransmitter receptor binding sites in rat brain have been studied. Electroconvulsive shock decreased beta-adrenergic receptor binding sites in the cerebral cortex and hippocampus but not in the striatum, hypothalamus or cerebellum. The decrease in beta-adrenergic receptor binding sites was highly selective and was similar to the decrease in these receptors seen after chronic treatment of rats with tricyclic antidepressant drugs. Electroconvulsive shock selectively increased serotonin-2 receptor binding sites in the cerebral cortex. This effect was opposite to that of antidepressant drugs.

Tetrahydro-beta-carbolines: affinities for tryptamine and serotonergic binding sites.

The affinities of four tetrahydro-beta-carbolines at [3H]tryptamine, [3H]5-HT (5-HT1), and [3H]spiperone (5-HT2) binding sites in rat cerebral cortex were investigated. The unsubstituted tetrahydro-beta-carboline was the most potent of the four compounds at all three binding sites, but was 200-400 times more potent at the tryptamine site than at either of the serotonin sites.

Exposure to nicotine enhances the behavioral stimulant effect of nicotine and increases binding of [3H]acetylcholine to nicotinic receptors.

Rats were given daily injections of nicotine sulfate in doses ranging from 0.1 to 0.4 mg/kg. The behavioral effect of these injections was measured as locomotor activity in photocell cages. Repeated administration of the same dose to each rat resulted in an enhancement of the stimulant effect of nicotine. This enhanced behavioral effect was quite pronounced within 5 days of repeated injection. Tissue from the cerebral cortex of these rats, exposed to nicotine for 5 days, was assayed for binding of [3H]acetylcholine to nicotinic receptors. These relatively small doses of nicotine resulted in 18-26% increases in cortical nicotinic receptors, compared to saline-treated rats. Rats exposed to 0.2 mg/kg of nicotine for 5 days and then given saline for 7 days still showed an enhanced behavioral response to nicotine on the eighth day after exposure, and nicotinic binding in the cortex was still elevated. However, 21 days after exposure to nicotine both the behavioral response to nicotine and the binding values had returned to the same values as those of saline-treated rats. These data imply that increased binding of [3H]acetylcholine to nicotinic sites and the enhanced behavioral effect of nicotine are functionally linked.

Naltrexone blocks nicotine-induced prolactin release.

Two receptor populations involved in the release of prolactin were examined in conscious, freely moving, male rats bearing indwelling jugular cannulae. The intravenous administration of either nicotine or morphine increased plasma prolactin levels. Pretreatment with the nicotinic antagonist mecamylamine blocked the prolactin response to nicotine only. In contrast, the opiate antagonist naltrexone blocked the prolactin response to both nicotine and morphine. These findings indicate that the nicotine stimulated release of prolactin is dependent not only on functional nicotinic cholinergic receptors but on opiate receptors as well. This suggests that nicotine and morphine release prolactin via a common pathway containing nicotinic cholinergic and opiate synapses in series.

Stimulation of serotonin1A receptors increases release of prolactin in the rat.

The effect of serotonin1A receptor agonists on release of prolactin was examined in awake, freely-moving male rats in which a catheter in the jugular vein allowed samples of blood to be collected periodically after intravenous injection of the agonist. The serotonin1A receptor agonist, 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT) increased concentrations of prolactin in plasma rapidly and in a dose-related manner. Concentrations of prolactin peaked within 9 min after intravenous injection of 8-OHDPAT and returned to baseline values within 30 min. Another serotonin1A receptor agonist, 5-methylurapidil (5-MeU), produced a similar response of prolactin. The effects of these agonists on release of prolactin were completely blocked by pretreatment with the serotonin receptor antagonists, methysergide and metergoline, administered 1 or 2 hr before the agonist. These results demonstrated that serotonin1A receptors can mediate the effects of serotonin on release of prolactin in the male rat.

Nicotinic receptor mediates spontaneous GABA release in the rat dorsal motor nucleus of the vagus.

Spontaneous postsynaptic currents were investigated in neurons of the caudal portion of the dorsal motor nucleus of the vagus using the patch-clamp technique to study the effect of neuronal nicotinic acetylcholine receptor activation on synaptic transmission. In voltage-clamped neurons, bath application of nicotine (1-30 microM) elicited a concentration-dependent increase in the frequency of the spontaneous synaptic currents. The effect was also observed with application of the nicotinic receptor agonists epibatidine (10 nM) and cytisine (10 microM). Mecamylamine (20 microM) and curare (50 microM), two nicotinic receptor antagonists, both decreased the effect of 3 microM nicotine on the frequency of the spontaneous postsynaptic currents. This effect of 3 microM nicotine was also blocked by 20 microM bicuculline, a competitive antagonist of the GABA(A) receptor; in contrast, it was not affected by 1 mM kynurenic acid, an antagonist of the ionotropic glutamate receptor. In the presence of 1 microM tetrodotoxin, 3 microM nicotine was unable to affect the synaptic activity. Our findings suggest the existence of nicotinic receptors on GABAergic axons projecting to the vagal motoneurons. Because the effect is completely abolished by 1 microM tetrodotoxin, the nicotinic receptors are not localized on the presynaptic nerve terminal and their action on the GABA release requires the propagation of an action potential from their location to the synaptic terminal. This effect of nicotinic receptor activation on spontaneous GABA release in the dorsal motor nucleus of the vagus may have an important role in the regulation of gastrointestinal motility.

Chronic reserpine administration selectively up-regulates beta 1- and alpha 1b-adrenergic receptors in rat brain: an autoradiographic study.

Rats were treated for 15 days with reserpine or vehicle. One day after the last treatment, animals were killed and frozen brain sections were prepared for in vitro autoradiography. Binding to beta-adrenergic receptors was measured with [125I]iodocyanopindolol, and binding selective for beta 1 and beta 2 subtypes was assessed by including non-radioactive drugs that selectively mask beta receptor subtypes. Total alpha 1-adrenergic receptor binding was measured with [3H]prazosin, while alpha 1a binding was measured with [3H]WB4101 (in the presence of unlabeled serotonin). Quantitative densitometric analysis revealed that chronic reserpine treatment caused an increase in beta binding throughout the brain, including the cortex, thalamus, amygdala, hippocampus, caudate-putamen and hypothalamus. This effect of reserpine was entirely confined to the beta 1 subtype in all regions examined. [3H]Prazosin binding (alpha 1a plus alpha 1b) was also increased after chronic reserpine in several regions of the cortex and thalamus, as well as the ventral hippocampus and caudal amygdala. No effect of chronic reserpine was seen on [3H]WB4101 binding, indicating that the effect of reserpine on alpha 1 receptors is limited to the alpha 1b subtype. The increase in alpha 1b binding after reserpine administration in rats was generally smaller and less widespread than that seen with beta 1 binding. Thus the effect of reserpine upon noradrenergic neurotransmission demonstrates a high degree of receptor specificity and regional selectivity.

Experimental studies on the neurocardiovascular effects of urapidil.

The major purpose of our study was to determine whether urapidil acts in the central nervous system (CNS) to lower arterial blood pressure. Once demonstrating a CNS antihypertensive action of urapidil we further set out to determine: (1) the relative role of a CNS antihypertensive action to the total antihypertensive effect of urapidil; (2) the brain site of action for the antihypertensive effect of urapidil; and, (3) the receptor mechanism whereby urapidil acts in the CNS to lower arterial blood pressure. Studies were conducted in chloralose-anaesthetised cats, and arterial blood pressure and heart rate were monitored. Drugs were administered intravenously (IV), into the cerebral ventricles (ICV), topically by application to the ventral surface of the medulla and by microinjection into specific nuclei. Receptor binding studies were also conducted using rat cerebral cortex homogenates. We found that injection of urapidil into the fourth ventricle decreased arterial pressure. Local application of urapidil to the ventral medullary surface also decreased arterial blood pressure. Microinjection of urapidil into one of the nuclei associated with the ventral surface of the medulla, the rostral part of the nucleus reticularis lateralis (rLRN), produced a similar degree of antihypertensive effect. The effect of urapidil was not altered by alpha 1-receptor blockade. Instead, the urapidil effect resembled that produced by drugs that stimulate serotonin (5-hydroxytryptamine)-1A receptors (B695-40 and 8-OH-DPAT). Furthermore, urapidil was found to have the highest potency for binding to serotonin-1A receptor sites (as compared to alpha 1- and alpha 2-receptor sites). Urapidil administered IV was shown to lower arterial blood pressure in part by blocking peripheral alpha 1-adrenoceptors but also, in high doses, by acting in the CNS to decrease central sympathetic outflow. These data indicate that urapidil is a unique drug, possessing both peripheral and CNS actions which contribute to its antihypertensive effect. Urapidil may also be unique in that its central action may involve activation of serotonin-1A receptors.

Nicotinic receptor binding sites in rat primary neuronal cells in culture: characterization and their regulation by chronic nicotine.

We have characterized high affinity neuronal nicotinic acetylcholine receptors labeled by [3H]cytisine in primary neuronal cell cultures from fetal rat brains. After 15 days in culture, the highest density of [3H]cytisine binding sites (Bmax approximately 57 fmol/mg protein) was found in cells from the brainstem, which includes the following subcortical brain areas: the septum, thalamus, hypothalamus, midbrain, pons and medulla. A lower density of sites was found in cells from the cerebral cortex, hippocampus, and caudate nucleus. [3H]Cytisine binds to receptors in primary cells from the brainstem and cerebral cortex with a Kd of approximately 0. 5 nM, and the binding is inhibited by the agonists nicotine, acetylcholine, and epibatidine with IC50 values of 1 to 20 nM, and by carbachol and the antagonist dihydro-beta-erythroidine with IC50 values of 0.5 to 1.5 microM. Chronic treatment of neuronal cultures with nicotine for 7 days differentially affected the number of nicotinic receptors in cells from different brain areas; it significantly increased the number of nicotinic binding sites in cells from the cerebral cortex, hippocampus, and caudate, but not in cells from the brainstem. The nicotine-induced increase of receptors in cerebral cortical cultures was not blocked by either mecamylamine or dihydro-beta-erythroidine. These results indicate that primary cultures of rat neuronal cells provide a good model system in which to study and compare the properties and regulation of native neuronal nicotinic acetylcholine receptors.

Chronic nicotine and locomotor activity: influences of exposure dose and test dose.

Repeated exposure to nicotine increases both the number of central nicotinic receptors and the behavioral stimulant effect of nicotine. In the present experiments, the behavioral response to nicotine was examined in photocell activity cages. Groups of rats were tested using doses from 0.1 to 1.6 mg/kg both before and after all rats were exposed for 5 days to a common dose of 0.2 mg/kg/day. Prior to the 5-day exposure, there was a dose-related stimulant response to nicotine, with a maximum response seen at 0.4 mg/kg. After the 5-day exposure, the dose-effect curve was shifted upward, so that greater stimulation was produced at each test dose of nicotine. Other groups of rats were exposed for 5 days to doses of nicotine ranging from 0.01 to 0.30 mg/kg/day. On the 6th day all rats received a common test dose of 0.2 mg/kg and their response was measured in the activity cages. In animals exposed to 0.01 mg/kg/day, the test day response was not different from saline controls, but the groups exposed to higher doses showed increased stimulation in response to the common test dose. Measurements of nicotinic receptor binding using [3H]-acetylcholine found increased binding in groups receiving 0.03 mg/kg/day or more, but not in the group that received 0.01 mg/kg/day. The correspondence between the doses that increase behavioral stimulant reactions to nicotine and the doses that increase nicotinic binding suggest that increased receptor numbers may be responsible for the increased behavioral stimulation. However, rats given high doses (1.6 mg/kg, twice per day) did not show increased behavioral stimulation to a test dose of 0.2 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Electroconvulsive shocks increase the concentration of neocortical and hippocampal neuropeptide Y (NPY)-like immunoreactivity in the rat.

Studies on humans and rats have suggested that neuropeptide Y (NPY) is involved in major depression and anxiety. Therefore, we conducted the present study in order to elucidate the effect of repeated (13 or 14 days) treatment of rats with electroconvulsive shocks (ECS) on the concentration of NPY-like immunoreactivity (-LI) in various brain regions, adrenals and plasma. In addition, the effect of ECS on 125I-NPY binding was studied in 3 brain regions. The effects of ECS were compared to effects of 3 control treatments: one group not being handled at all during the time period, one group handled like the ECS-group but not receiving shocks, and one group receiving shocks below the threshold for induction of convulsions. The latter group developed behavioral signs reminiscent of the inescapable shock-induced 'learned helplessness' syndrome (a proposed animal model of depression). We found that the concentration of NPY-LI in the frontal and parietal cortex and in the hippocampus were approximately doubled in the ECS-group as compared to the 3 control groups. No changes in NPY-LI were detected in the striatum, hypothalamus, pons, olfactory bulbs or cerebellum, nor in plasma or adrenals. In spite of the marked changes in NPY-LI concentration, the binding characteristics of 125I-NPY in the frontal and parietal cortex and in the hippocampus were similar in all 4 groups of rats. Finally, we confirmed the previous observation that ECS increase [3H]prazosin binding in cortex. In conclusion, ECS treatment increases neocortical and hippocampal NPY-LI concentrations, while leaving 125I-NPY binding unaffected. Subconvulsive shocks were without effect.

Muscarinic and nicotinic cholinergic binding sites in Alzheimer's disease cerebral cortex.

The total population of muscarinic receptors and a subpopulation of muscarinic receptors with high affinity for agonists were measured with [3H]quinuclidinyl benzilate and [3H]acetylcholine, respectively, in homogenates of cerebral cortex from control and Alzheimer's disease brains. No significant differences between control and Alzheimer's diseased cortex were found in either the total population of receptors or the subpopulation with high affinity for agonists in either the frontal or temporal poles. Nicotinic cholinergic receptors labeled by [3H]acetylcholine were measured in homogenates and by autoradiography in the same brain areas. In contrast to muscarinic sites, binding to nicotinic sites was markedly decreased in Alzheimer's disease cortex. Autoradiography of [3H]acetylcholine binding to nicotinic sites indicated that in control cortex these sites are more concentrated in laminae IV-VI than in the superficial laminae, and that in Alzheimer's disease there is loss of these sites in all cortical laminae.

Bilateral changes in neocortical [3H]pirenzepine and [3H]oxotremorine-M binding following unilateral lesions of the rat nucleus basalis magnocellularis: an autoradiographic study.

Neocortical choline acetyltransferase (ChAT) activity and muscarinic [3H]pirenzepine, [3H]oxotremorine-M, [3H]N-methylscopolamine ([3H]NMS; both high- and low-affinity agonist (carbachol) sites) and nicotinic [3H]acetylcholine binding were assessed both ipsi- and contralaterally 1 week and 13 weeks after unilateral ibotenic acid lesions of the rat nucleus basalis magnocellularis (NBM). Ipsilateral ChAT activity was reduced to 49% of control values 1 week postlesion but by 13 weeks had recovered to 80% of control values. Contralateral ChAT activity did not change significantly at either 1 week or 13 weeks postlesion. At 1 week postlesion, [3H]oxotremorine-M binding was increased by 33% and 54% in ipsilateral and contralateral neocortex, respectively. By week 13, both ipsi- and contralateral [3H]oxotremorine-M binding had returned to normal but [3H]pirenzepine binding was significantly decreased by 31% and 39% in the ipsilateral and contralateral hemispheres, respectively. The binding of [3H]NMS and [3H]acetylcholine did not differ significantly from control values at either 1 week or 13 weeks postlesion. These data suggest that none of the cholinergic binding sites studied is preferentially localized presynaptically and that there may be interhemispheric regulation of neocortical cholinergic binding sites.