Interaction of chlorphenvinphos with cholinergic receptors in the rabbit hypothalamus

The purpose of this study was to find out whether chlorphenvinphos (CVP), an organophosphorous pesticide, interacts with the muscarinic cholinergic receptors in CNS. To attain this goal, the effects of intrahypothalamic injections of oxotremorine (Ox), a muscarinic agonist, and physostigmine (Phys), a carbamate anticholinesterase, were compared with those produced by intrahypothalamic injections of CVP in the rabbit. It was found that the infusion of Ox (20 micrograms) as well as Phys (200 micrograms) into the anterior hypothalamus leads to an increase in the 4-7 Hz theta rhythm in the hippocampus and to the appearance of behavioral symptoms suggestive of a threat response. In the case of Ox, the effects could be prevented by injections of 20 micrograms scopolamine, a muscarinic antagonist. Pretreatment of the hypothalamus with 100 micrograms hemicholinium (HC-3) did not prevent the effects of Phys injected 2 h later. (HC-3 prevents the resynthesis of acetylcholine by blocking choline reuptake. This leads to a gradual depletion of ACh stores and to an inhibition of the cholinergic transmission). It suggests that Phys activates directly postsynaptic muscarinic receptors. Intrahypothalamic injections of CVP in doses of up to 1360 micrograms produced no overt changes in behavior nor in the hippocampal EEG of the rabbit and did not prevent the effect of subsequent injections of Ox. This suggests that CVP is neither an agonist nor antagonist of the muscarinic receptors in the rabbit hypothalamus.

Reversal of hemorrhagic shock in rats by oxotremorine: the role of muscarinic and nicotinic receptors, and AV3V region

In an experimental model of hemorrhagic shock resulting in the death of almost all rats within 20-30 min, centrally active cholinomimetic drugs are reported to induce a prompt, sustained and dose-dependent improvement in blood pressure and survival rate claimed to be due to nicotinic, but not muscarinic actions. In the present study, cholinergic receptor agonist, oxotremorine (50 micrograms/kg, i.v.) increased mean arterial pressure (from 22 +/- 1 to 123 +/- 3 mm Hg) and 60 min-survival rate (from 0% to 92%) in rats bled to hypovolemic shock. Atropine (2 mg/kg, i.v.) pretreatment inhibited the pressor effect of oxotremorine significantly, but did not modify its effect on survival rate. On the other hand, pretreatment with mecamylamine (50 micrograms, i.c.v.) almost abolished the reduction in mortality rate, but inhibited the pressor effect of oxotremorine, partially. These results indicate that oxotremorine-induced pressor response and decrease in mortality in rats with severe hemorrhagic shock are primarily mediated via central muscarinic and nicotinic receptors, respectively. AV3V region was previously reported to be involved in pressor and natriuretic effects of i.c.v. carbachol in normotensive rats. In the present study, the electrolytic lesions of AV3V region significantly inhibited oxotremorine-induced increases in both blood pressure and survival rate in rats subjected to hemorrhagic shock. These findings indicate that AV3V region plays a major role in cholinergic cardiovascular control in hypotensive animals as well as normotensives.

Acetylcholine release in the hippocampus: effects of cholinergic and GABAergic compounds in the medial septal area

The medial septal area (MSA) contains cholinergic and GABAergic neurons that send projections to the hippocampus. These neurons have both cholinergic and GABAergic receptors. This study was designed to determine the effects of intraseptal infusions of cholinergic and GABAergic drugs, which alter mnemonic processes, on hippocampal acetylcholine (ACh) release. Hippocampal ACh release was assessed using in vivo microdialysis and HPLC-EC. Oxotremorine and scopolamine produced a dose-dependent decrease in hippocampal ACh release. Muscimol decreased hippocampal ACh release at both high and low doses, although not in a dose-dependent manner. The effects of scopolamine and muscimol are consistent with a role of ACh in mnemonic processing.

Activation of the medial septal area attenuates LTP of the lateral perforant path and enhances heterosynaptic LTD of the medial perforant path in aged rats

Age-related memory impairments may be due to dysfunction of the septohippocampal system. The medial septal area (MSA) provides the major cholinergic projection to the hippocampus and is critical for memory. Knowledge of the neurobiological mechanisms by which the cholinergic system can attenuate age-related memory loss can facilitate the development of effective cognitive enhancers. At present, one of the best neurobiological models of memory formation is long-term potentiation/long-term depression (LTP/LTD). In previous studies, intraseptal infusion of the muscarinic agonist oxotremorine, which excites MSA neurons, improved memory in aged rats. The present study examined LTP and LTD in aged Fisher 344 rats following intraseptal infusion of oxotremorine. LTP and LTD were assessed using the slope of the EPSP recorded from the hilar region of the dentate gyrus. Induction of LTP was blocked in the lateral perforant path, but not in the medial perforant path, following intraseptal infusions of oxotremorine. The generation and amplitude of heterosynaptic LTD was enhanced in the medial perforant path, but not in the lateral perforant path. The results provide evidence that pharmacological activation of the MSA can modulate LTP and LTD in the hippocampus of aged rats. The implications of these results with respect to memory and synaptic plasticity in the hippocampus are discussed.

Effect on vasopressin release of microinjection of cholinergic agonists into the rat supraoptic nucleus

It is likely that central cholinergic pathways to the paraventricular and supraoptic nuclei participate in the control of vasopressin release. We have shown previously that this is due, in part, to activation of muscarinic, but not nicotinic, receptors in the paraventricular nucleus. There is, however, reason to believe that this cholinergic effect in the supraoptic nucleus may be the result of activation of nicotinic receptors. To test this possibility, we have studied in conscious unrestrained rats the effect of microinjection of muscarinic and nicotinic agonists into the supraoptic nucleus on vasopressin release, mean arterial blood pressure, and heart rate. Under ether anesthesia, a stainless steel guide cannula was placed in the supraoptic nucleus 5-7 days before the experiment, and femoral, arterial, and venous catheters were implanted 1 day before the experiment. Microinjection of nicotine into the supraoptic nucleus at doses of 1 and 10 micrograms resulted in transient increases in the plasma vasopressin concentration that were 7-fold and 11-fold greater, respectively, than control values at 3 min. There were also small transient increases in mean arterial blood pressure, but heart rate was unchanged. The microinjection of 2 and 20 ng of oxotremorine, a muscarinic agonist, into the supraoptic nucleus had no effect on the plasma vasopressin concentration, mean arterial blood pressure, or heart rate. These doses of oxotremorine were previously shown to have potent stimulatory effects on vasopressin release when microinjected into the paraventricular nucleus. These findings suggest that the central cholinergic stimulation of vasopressin release is due, in part, to activation of muscarinic receptors in the paraventricular nucleus and nicotinic receptors in the supraoptic nucleus.

The pretectal cholinergic system is involved through two opposite ways in frog monocular OKN asymmetry

Frog monocular horizontal optokinetic nystagmus (OKN) has been studied by coil recordings, before and after unilateral microinjection of cholinergic drugs into the pretectum. The recorded eye was either contralateral or ipsilateral to the injected structure. Before injection, monocular OKN displayed a directional asymmetry, reacting only to stimulations in the temporonasal (T-N) direction. The intrapretectal administration of a cholinergic muscarinic agonist (oxotremorine), as well as that of a nicotinic antagonist (D-tubocurarine), abolished the monocular OKN asymmetry, inducing the appearance of the naso-temporal (N-T) component; the difference between the slow phase velocity gain of both components was no longer significant. These data suggest that acetylcholine (ACh), at the level of the pretectum, acts in opposite ways through muscarinic and nicotinic binding sites; monocular OKN asymmetry could result, at least partially, from a facilitating nicotinic effect and an inhibitory muscarinic effect. Possible interactions with other transmitter systems are discussed.

Inhibition of rubral neurons by noxious and non-noxious pressure

The role of the red nucleus (RN) in nociception was investigated in this study. Extracellular recordings from spontaneously active RN neurons were conducted in the rat while noxious pressure was delivered to the hindpaws or tail. Cells in the RN were predominantly inhibited by the stimuli. The units were most responsive when noxious pressure was applied to the contralateral hindpaw. Furthermore, more cells in the magnocellular division of the RN responded to the stimuli than cells in the parvocellular division. Delivery of a graded pressure stimulus to the contralateral hindpaw revealed 4 cell types in the RN: non-responsive cells; cells only responsive during the early, non-noxious portion of the stimulus; cells only responsive during the later, noxious portion of the stimulus; and cells that showed an initial response during the non-noxious part of the stimulus and a second, later response during the noxious portion of the stimulus. To further examine the putative role of the RN in nociception, oxotremorine, gamma-aminobutyric acid (GABA), serotonin, glutamate, and morphine were unilaterally microinjected into the RN and the responses of the animals in the tail flick test were assessed. Only morphine produced a significant antinociception in the animals following intrarubral microinjection. However, it is unclear whether this alteration was mediated through the RN because an antinociception of equal magnitude could be elicited from the reticular formation surrounding the RN and lesions of the RN did not alter the antinociception produced by systemic administration of morphine. Although other explanations cannot be ruled out, it appears that the RN may be involved in coordinating the motor response to pain rather than modulating sensory transmission.

An M3-like muscarinic autoreceptor regulates the in vivo release of acetylcholine in rat striatum

Selective muscarinic antagonists were used in an attempt to characterize the muscarinic autoreceptor modulating the release of acetylcholine in the striatum of the rat. In vivo microdialysis was applied to infuse atropine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine), pirenzepine or AF-DX 116 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5, 11-dihydro[2,3-b][1,4]benzodiazepine-6-one), leading to a dose-dependent increase in the overflow of acetylcholine, the order of potency being: atropine greater than 4-DAMP greater than pirenzepine greater than AF-DX 116. We conclude from these data that the muscarinic receptor modulating release in the striatum is of the M3 type.

Differential effects of M1- and M2-muscarinic drugs on striatal dopamine release and metabolism in freely moving rats

A dialysis loop cannula was implanted into rat striatum under anesthetized condition, and the area was perfused with Ringer's solution under freely moving condition after 3 days for surgical recovery. Dopamine (DA) and 3,4-dihydroxyphenylacetic acid recovered in the dialysate were measured by high-performance liquid chromatography with electrochemical detection. The effects of M1- and M2-muscarinic receptor agents, which were perfused continuously into the striatum through the dialysis membrane, were investigated. Continuous perfusion of AF102B, an M1-selective agonist, and oxotremorine, a non-selective agonist, resulted in a dose-dependent increase in the striatal DA release. Pirenzepine (10(-5) and 10(-7) M), an M1-selective antagonist, decreased the release of DA, and the stimulatory effect of AF102B (10(-5) M) was completely inhibited by 10(-5) and 10(-7) M pirenzepine, while the stimulatory effect of oxotremorine (10(-4) M) was only partly inhibited by 10(-5) M pirenzepine. AF-DX116 (10(-5) M), an M2-selective antagonist, increased the DA release, and showed an additive effect on the DA release evoked by AF102B (10(-5) M), whereas it produced no significant effect on oxotremorine (10(-5) M)-evoked DA release. These results suggest that in vivo DA release in the rat striatum is modulated by different subtypes of muscarinic receptors; i.e., the stimulatory effect is mainly mediated by M1-sites and inhibitory effect is mainly mediated by M2-sites. The changes in the DA release induced by the various drugs were prevented by pretreatment with tetrodotoxin (TTX). Since action potential-dependent DA release (exocytosis) is blocked by the pretreatment with TTX, those drugs affect DA release by means of action potential-dependent processes.

[Analysis of sleep-wakefulness structure in parkinsonian syndrome in rats induced by 1-methyl-4-phenyl-1,2,2,6-tetrahydropyridine and oxotremorine]

The structure of the sleep-wakefulness cycle on the white rats with experimental parkinsonian syndrome was investigated, which was induced by administration of MPTP and oxotremorine. The EEG was recorded during 6 hours. The recordings were made 1.5 hours after the administration of the drugs. It was established that MPTP and oxotremorine broke the structure of the sleep-wakefulness cycle almost with complete reduction of paradoxal sleep and the decrease of SWS. The number of sleep spindles was reduced and changed the characteristics of the hippocampal theta rhythms. MPTP induces weakness of the phasic compound and strengthens the tonic compound.

Effect of vasopressin antagonist on antidiuresis by oxotremorine microinjected into the hypothalamic supraoptic and paraventricular nuclei in a water-loaded and ethanol-anesthetized rat

Microinjection of the muscarinic agonist oxotremorine into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei which contain cell bodies of vasopressinergic neurons induced potent antidiuretic effects in water-loaded and ethanol-anesthetized rats. The effects included both decreases in urine outflow and increases in urine osmotic pressure. However, no significant changes in various visceral functions other than antidiuresis such as mean blood pressure, heart rate, respiration rate and rectal temperature were observed when oxotremorine was microinjected into the SON. Only a slight change in mean blood pressure (approx. 10 mmHg decrease) was observed by the microinjection into the PVN. Intravenous preinjection of a vasopressin (AVP) V1 V2 antagonist that has one of the most potent V2 (antidiuretic)-antagonist activities, d(CH2)5-D-Tyr(Et)VAVP, inhibited nearly completely the antidiuretic effects induced by the microinjection of oxotremorine. The results demonstrated that oxotremorine stimulated muscarinic receptors in the hypothalamic SON and PVN, released AVP and induced an antidiuretic effect through AVP-receptors in the kidney.

Two-drug combinations of memory enhancers: effect of dose ratio upon potency and therapeutic window, in mice

Two-drug combinations have been reported to enhance retention more effectively than when either drug was administered alone at the same dose. Some combinations of cholinergic drugs enhance retention even though the total drug dosage is reduced by as much as 97% compared to the dose needed to improve retention when the same drugs are administered singly. The choice of dose ratio is usually arbitrary or based on empirical results. The present study systematically varied the ratio of two drugs in a combination and at the same time varied the dosage of each drug. The drug combinations were administered to mice immediately after training on T-maze footshock avoidance task. Retention was tested one week later. Three two-drug combinations were selected for presentation because they differed considerably as to (a) the lowest effective total dose that improved memory-retention, (b) the optimal ratio that improved retention and (c) the width of the therapeutic window. The effect of a drug combination on retention was found to be dependent on the particular drugs in the combination, the ratio and the dose administered.

Effects of diazepam on muscarinic acetylcholine receptor binding in vivo and on oxotremorine-induced tremor and hypothermia in mice

Diazepam has previously been shown to affect the acetylcholine synthesizing system in mouse brain. This paper reports studies on the effect of diazepam on muscarinic receptor density and on pharmacological effects of oxotremorine. The receptor density was studied using a new technique that allows such studies to be performed in vivo under physiological conditions. The method is based on the fact that L-hyoscyamine, the active antipode of atropine, binds specifically to muscarinic receptors in the brain, and can be measured with high sensitivity by gas chromatography--mass spectrometry. Diazepam was found to modify the binding properties of muscarinic receptors in CNS, thereby decreasing the functional receptor pool. It also prevented tremor induced by the muscarinic agonist oxotremorine. Diazepam could however not prevent the hypothermia induced, but rather accentuated this effect of oxotremorine. It is concluded that diazepam, directly or indirectly, influences the effect of cholinergic stimulators by modulating the size of the muscarinic receptor pool.

Antidiuretic effects of oxotremorine microinjected into the hypothalamic supraoptic and paraventricular nuclei in a water-loaded and ethanol-anesthetized rat

The effect of oxotremorine, a muscarinic agonist, on urine outflow compared with the effects of other cholinergic agonists and inhibitory effects of cholinergic antagonists upon the cholinergic actions were studied by microinjecting drugs stereotaxically, unilaterally into the supraoptic (SON) or paraventricular nuclei (PVN) in the hypothalamus of the rat which was loaded with water and anesthetized with ethanol. Oxotremorine decreased the urine outflow in dose- and time-dependent manners when microinjected into these nuclei. The median effective doses (ED50) were approx. 0.3 and 0.2 nmol in SON an PVN, respectively, being much less than ED50 values for nicotine. The time course of the antidiuretic effects was relatively slow, with the minimal urine outflow at approx. 30 min and the duration of one or longer hours. The antidiuretic effects of oxotremorine in these nuclei as well as the effects of acetylcholine and nicotine were completely blocked by pretreatment with atropine. The pretreatment with hexamethonium inhibited partially the effects of nicotine, but was unable to inhibit the effects of oxotremorine and acetylcholine. The data suggest that the antidiuretic effects of cholinergic agonists in SON and PVN are predominantly mediated through a muscarinic type of acetylcholine receptor. A possible mechanism for the antidiuretic effects is discussed.

Possible interaction between central cholinergic muscarinic and opioid peptidergic systems during memory consolidation in mice

Naloxone (0.01-1.00 mg/kg, ip) facilitated retention of a one-trial inhibitory avoidance task, when administered to male Rockland mice immediately after training, as indicated by performance on a retention test 48 hr later. The dose-response curve was an inverted U in this range of dose. In these conditions naloxone did not lengthen latencies to step-through during the retest of unshocked mice. Higher doses of naloxone (3.00 and 10.00 mg/kg, ip) tended to increase latencies to step-through of both shocked and unshocked mice. These facts rule out an aversive effect of naloxone for low and moderate doses but not for high doses. The influence of naloxone (0.10 mg/kg, ip) on retention was time dependent, which suggests that naloxone facilitated memory consolidation processes. The effects of naloxone were prevented by morphine in both an amnesic and a nonamnesic dose (1.0 and 0.5 mg/kg, ip, respectively). Therefore, naloxone probably facilitated retention as a function of its opiate antagonist properties. The memory facilitation induced by naloxone (0.10 mg/kg, ip) was antagonized by atropine (0.5 mg/kg, ip) but not by methylatropine (0.5 mg/kg, ip), mecamilamine (5 mg/kg, ip), or hexametonium (5 mg/kg, ip). Further, there was a mutual potentiation for both naloxone (0.01 mg/kg, ip) and the muscarinic agonist oxotremorine (6.25 and 12.5 micrograms/kg, ip) administered simultaneously, in doses which had no effect on their own. Moreover, an amnesic dose of atropine (10.00 mg/kg, ip) prevented the enhancement of retention induced by naloxone, while an amnesic dose of morphine (1.00 mg/kg, ip) did not modify the facilitatory effect of oxotremorine (50 micrograms/kg, ip) on retention. An inhibitory modulatory role for endogenous opioid systems on the activity of central cholinergic muscarinic systems during memory consolidation is suggested.

Psychopharmacological profile of diphenylhydantoin in mice

The pharmacological profile of diphenylhydantoin has some common features with that of classical antidepressants: it decreases motor activity in mice it shortens the period of immobility of mice in the "behavioral despair" test it antagonizes palpebral ptosis induced by reserpine in mice.

Quantitation of tolerance development after chronic oxotremorine treatment

A new procedure was developed to quantitate the tolerance which develops as mice are chronically infused with the muscarinic agonist, oxotremorine. Cumulative dose-response curves were constructed for the effects of oxotremorine on body temperature and rotarod performance by administering sequential injections to individual animals. These dose-response curves compare favorably to those constructed by injecting individual animals with one of several doses. The sequential injection technique was used to assess the magnitude of tolerance development to oxotremorine. A linear relationship between oxotremorine infusion rate (dose) and magnitude of change of the ED50 value for impairment of rotarod performance was observed, with animals receiving an infusion rate of 1.0 mg/kg/hr showing a 24-fold increase in ED50. Dose-response curves for tolerant animals were parallel to those constructed for naive animals. The oxotremorine dose required to decrease body temperature to 35 degrees C (ED35 degrees) was 80-fold greater than control in the group treated with 1.0 mg/kg/hr. The dose-response curves for tolerant animals were not parallel to those seen in naive animals. Time courses of recovery from a challenge dose of oxotremorine suggest little change in metabolism occurred during chronic infusion. Chronic oxotremorine infusion resulted in a decrease in the total number of QNB binding sites. Both high- and low-affinity sites were reduced in number. Since no change in K1 for the muscarinic agonist, carbamylcholine, was observed, it seems unlikely that a change occurs in the affinity of the muscarinic receptor for agonists. Significant change in receptor number was detected only in animals that received higher doses of oxotremorine. Chronic oxotremorine treatment had no effect on choline uptake by synaptosomes prepared from any of five brain regions.

[Effect of kynurenines on the actions of acetylcholine, oxotremorine, and nicotine]

Kynurenine, 3-hydroxyanthranilic, anthranilic and nicotinic acids at concentrations 10(-4) and 10(-5) M potentiated contractions of the isolated rat rectum produced by acetylcholine. Contractile response of the phrenico-diaphragmatic preparation of young rats to acetylcholine was diminished by kynurenine, 3-hydroxykynurenine and by quinolinic acid. In rats of both sexes, 3-hydroxyanthranilic, anthranilic, picolinic and nicotinic acids in doses of 10 and 20 mg/kg shortened the latency of oxotremorine tremor. Quinolinic and nicotinic acids reduced the latency of nicotinic tremor and potentiated hypothermia.

Brain catecholamines modifications. The effects on memory facilitation induced by oxotremorine in mice

the immediate posttrial injection of oxotremorine (0.250 mu Mol/kg, IP) can facilitate the retention of a passive-avoidance response in mice. After the administration of alfa-methyl-p-tyrosine methylester (alpha-MPT) by intracerebroventricular injection at doses that had no effect on retention (100 microgram, 10 microliter, 60 min before trial), the immediate posttrial injection of oxotremorine did not enhance retention. The employed dose of alpha-MPT reduced brain levels of norepinephrine by about 40% and those of dopamine by about 25%. Pretreatment with nialamide (30 mg/kg, 20h IP), which prevents the catecholamine depletion induced by alpha-MPT, counteracted the effects depletion induced by alpha-MPT, counteracted the effects of alpha-MPT on the actions of oxotremorine on retention. These results suggest a participation of brain catecholamines on the actions of oxotremorine on retention and a possible interaction of cholinergic neurons with catecholaminergic system in memory processes.

Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination

The posttraining intrahippocampal injection of oxotremorine revealed an improvement of the retention performance in a brightness discrimination task. The oxotremorine effect seemed to be dependent on distinct variables of training and was restricted to rats exhibiting a good acquisition performance. Scopolamine impaired the retention performance of animals with few training errors. The role of hippocampal cholinergic synapses for consolidation was discussed.

Application of principles of steady-state kinetics to the estimation of brain acetylcholine turnover rate: effects of oxotremorine and physostigmine

We have measured the turnover rate of acetylcholine (ACh) in the brains of mice injected with doses of oxotremorine and physostigmine that cause a prolonged increase of ACh concentration in brain. The method used to measure turnover rate of ACh is an application of principles of steady-state kinetics to the change with time of brain choline (Ch) and ACh specific radioactivities after an intravenous pulse injection of phosphorylcholine. We have found that when the concentration of brain ACh and Ch is increased to a new steady state as a result of oxotremorine and physostigmine injections the turnover rate of brain ACh decreases from 0.34 mumol/g/hr (in saline-treated mice) to 0.12 and 0.061 mumol/g/hr, respectively. The possibility that an increase of brain Ch or ACh concentrations plays a role in the control of brain ACh turnover rate is discussed.

Drugs affecting the cholinergic system in the intact mammalian eye. I. Evaluation of the miotic activity of acetylcholine-like drugs in the mouse eye

The effect of concentration and pH on the miotic activity of five acetylcholine-like drugs were studied in the intact mouse eye. The data suggests that it is mainly the nonionized form of the drug which contributes to its miotic activity. Acetylcholine-like drugs, degradable by cholinesterases, exert their miotic activity at higher concentrations than do those which are resistant to enzymatic hydrolysis. More active acetylcholine-like drugs appear to be less sensitive to concentration changes.