Evidence for a cholinergic influence on catecholaminergic pathways terminating in the anterior and medial basal hypothalamus

Potentiation by DSP-4 of EEG slowing and memory impairment in basal forebrain-lesioned rats

The effects of cholinergic and noradrenergic depletion, alone and in combination, on spatial memory and electroencephalogram (EEG) activity were investigated. Basal forebrain-lesioned rats exhibited a significant decrease in cortical choline acetyltransferase activity and spatial memory impairment. In the cortical EEG, the basal forebrain lesion induced EEG slowing such as an increase in delta power activity and a decrease in beta power activity. Noradrenergic depletion following a treatment with DSP-4 (N-2-(chloroethyl)-N-ethyl-2-bromobenzylamine) had no effect on cortical choline acetyltransferase activity and spatial memory, but it aggravated the cognitive impairment induced by the basal forebrain lesion. DSP-4 itself increased delta power activity in non-lesioned rats, whereas DSP-4 potentiated the EEG slowing induced by the basal forebrain lesions. Systemic administration of tetrahydroaminoacridine at 1 or 3 mg/kg, i.p., ameliorated the memory deficits and EEG slowing induced by the basal forebrain lesion. However, the drug could not attenuate the EEG slowing and memory impairment in rats that had received a combination of DSP-4 and basal forebrain lesion. These results suggest that noradrenergic depletion aggravated the EEG slowing and the spatial memory impairment induced by cholinergic dysfunction and may decrease the efficacy of an anticholinesterase agent in reversing the cortical cholinergic hypofunction.

Plasma corticosterone is increased and correlated with brain acetylcholine in physostigmine- but not in neostigmine-treated rats

Rats were given intraperitoneal injections of physostigmine (PHYSO), neostigmine (NEO) or saline (SAL). Either 15 or 30 min later the number and intensity of observable cholinomimetic effects (OCE) was determined, plasma was collected for corticosterone (Cst) measurement, and the cerebral cortex, striatum, hippocampus and hypothalamus were removed after microwave treatment for the measurement of tissue acetylcholine (ACh) and choline (Ch) concentrations. Plasma Cst correlated with the number of OCEs at both 15 and 30 min in both NEO- and PHYSO-treated animals. Although the number and intensity of OCE were the same in NEO- and PHYSO-treated animals 15 min after injection, plasma Cst was significantly higher in the PHYSO-treated group. ACh levels in the cortex were also increased in PHYSO- compared with NEO-treated animals 15 min after injection. Ch levels remained unchanged. Plasma Cst correlated positively with ACh levels in the cortex and striatum in PHYSO-treated rats both 15 and 30 min after injection. These data support the involvement of central cholinergic mechanisms in the regulation of the HPA axis.

Studies on the role of central catecholaminergic mechanisms in the antidotal effect of the oxime HI 6 in soman poisoned mice

The effects of atropine and the oxime HI 6 on running performance, brain and plasma cholinesterase activity and brain catecholamines were investigated in mice intoxicated with sublethal doses of soman (100 micrograms/kg s.c.). The running time on a rotating mash wire drum (total running time 60 min) after injection of soman was reduced to 17.2 min. Treatment with atropine (10 mg/kg i.p.) or HI 6 (55 mg/kg i.p.) improved the running performance to 48.2 and 44.8 min, respectively. Cholinesterase activity was decreased in soman poisoned mice to 47.3% in plasma and 43.5% in brain. Therapy with the oxime HI 6 resulted in a reactivation of soman-inhibited peripheral cholinesterase to 76.6%, but failed to reactivate central cholinesterase. Dopamine levels in mice brain were elevated in soman poisoning by 23.2%, whereas noradrenaline levels remained unchanged. The increase in brain dopamine levels was antagonized by atropine as well as by HI 6. The results of this study lead to the speculation that central dopaminergic mechanisms may be involved in soman toxicity as well as in the antidotal action of atropine and the mainly peripherally acting oxime HI 6.

Changes in brain monoamine content and metabolism induced by paraoxon and soman intoxication. Effect of atropine

1. Soman induced a decrease in hypothalamic, hippocampal and cortical noradrenaline, an increase in hippocampal and cortical dopamine and an increase in hypothalamic serotonin while paraoxon did not modify these neurotransmitter concentrations. The increases in 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindole acetic acid produced by the two drugs witness an accelerated turnover of dopamine and serotonin. 2. Atropine pretreatment completely antagonized the paraoxon-induced changes but only partially suppressed the soman-induced modifications. 3. These data suggest that soman produces a direct effect on monoamine metabolism which is not related to acetylcholine accumulation.

Further evidence for, and nature of, the facilitatory GABAergic influence on central noradrenergic transmission

In order to explore the nature of the facilitatory GABAergic control of cerebral noradrenergic neurons, we have studied the effect of a variety of GABA mimetics (given systemically or injected locally into brain areas containing noradrenergic cell bodies or terminals) on several indices of noradrenaline turnover in the rat brain. Systemic administration of both direct and indirect acting GABA mimetics enhanced; 1) the pargyline induced accumulation of normetanephrine in the hypothalamus; 2) total DOPEG levels in a number of brain regions innervated by noradrenergic neurons; 3) both DOPAC and MOPEG levels in noradrenergic cell body areas (A1, A2 and A6). These effects are probably mediated by GABAA receptors as specific GABAA or mixed GABAA/GABAB agonists but not the GABAB agonist baclofen enhanced noradrenaline turnover. Interruption of noradrenergic impulse flow (by local injection of tetrodotoxin or by hemitransection) blocked the ability of progabide to increase DOPEG concentrations in the hypothalamus and cerebral cortex. Similarly, the co-administration of clonidine with progabide antagonized the progabide-induced increase in hypothalamic total DOPEG levels. Co-administration of yohimbine with progabide provoked an additive effect on hypothalamic DOPEG levels at moderate but not at high doses of yohimbine. Thus, the acceleration of noradrenaline turnover induced by GABA mimetics appears to depend on ongoing activity in noradrenergic neurons and occurs via an increase in neuronal discharges. Local injection of muscimol into the nucleus accumbens or hypothalamus failed to affect DOPEG levels in these structures; similarly, local injection of muscimol into the locus coeruleus failed to modify DOPEG levels in corresponding noradrenergic projection areas. These data indicate that the GABAergic influence is not exerted via GABA receptors located on noradrenergic cell bodies or nerve endings. Furthermore, since systemically administered progabide still increased hypothalamic DOPEG levels after ibotenate-induced destruction of the hypothalamic neuronal cell bodies, a presynaptic modulation of noradrenergic neurons by local GABAergic interneurons is excluded. Chemical destruction of serotoninergic pathways or enhancement of 5-HT transmission by quipazine failed to alter the ability of progabide to increase cerebral DOPEG levels. Moreover, scopolamine or naloxone also failed to affect the progabide-induced increase in cerebral DOPEG levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of physostigmine, paraoxon and soman on brain GABA level and metabolism

The effects of sublethal doses of physostigmine, paraoxon and soman on GABA levels and metabolism were studied in small rat brain areas (hypothalamus, striatum, cerebellum, rest of the brain). Physostigmine induced a significant decrease in striatal GABA level and a reduction of synthesis index in the rest of the brain while organophosphates have little or no effect on GABA level and metabolism. This work provides new data about the physostigmine effect on brain GABA which could be related to the action of the anticholinesterase agents on other non-cholinergic neurotransmitters.

Vagotomy or atropine blocks hypoglycemic effect of insulin injected into ventromedial hypothalamic nucleus

Stereotaxic microinjections of insulin (100 microU) into the ventromedial hypothalamic nucleus (VMN) resulted in rapid decrease, whereas injection of control saline into the same region caused a slight increase of hepatic venous plasma glucose concentration in rats. The hypoglycemic effect of insulin injected into the VMN was eliminated by pretreatment of the animals with atropine but not with propranolol or with phentolamine. Subdiaphragmatic vagotomy also prevented the decrease of hepatic venous plasma glucose concentration seen after microinjection of insulin into the VMN. These results support the hypothesis that the VMN is an insulin-sensitive glucoregulator center or that it is part of one and that the glucoregulatory impulse that originates in the VMN reaches the effector organ, the liver, through the cholinergic fibers of the vagus nerves.

Brain catecholamine metabolism changes and hypothermia in intoxication by anticholinesterase agents

Sublethal doses of physostigmine, paraoxon and soman induce a short-lasting fall in rat core temperature potentiated by alpha-methyl-para-tyrosine (alpha-MT) (early effects). When the own hypothermic effect of the anticholinesterase agent has disappeared (late effects), alpha-MT induces a new decrease in temperature. Parallel biochemical studies of catecholamine levels and turnover were performed in several brain areas. The norepinephrine (NE) turnover is generally increased particularly in the hypothalamus, suggesting that NE hypothalamic changes might be linked to a latent perturbation of thermoregulatory mechanisms. Furthermore, it was shown that soman acts differently from the other drugs by inducing quite important changes in both NE and dopamine levels.

Influence of intoxication by anticholinesterase agents on core temperature in rats: relationships between hypothermia and acetylcholinesterase inhibition in different brain areas

The early and late effect of three anticholinesterase agents (physostigmine, paraoxon and soman) on core temperature and brain acetylcholinesterase (AcChE) inhibition are compared. The study was performed in adult male rats using sublethal doses of all drugs. AcChE activity was determined by hypothalamus, striatum, hippocampus, medulla oblongata-pons and rest of the brain. The co-existence of intoxication symptoms, hypothermia and AcChE inhibitions was clearly shown by the early effects. However, no relationship was found between the degree of hypothermia and that of inhibition whatever brain area. In contrast, late AcChE inhibition was not accompanied by symptoms and lowering of core temperature. Several hypotheses have been suggested to explain this phenomenon: de novo synthesis of enzyme, decreased sensitivity of neurotransmitter or return to normal of brain acetylcholine level through a negative feed back at the presynaptic level. The present data suggest that this latter assumption is the most likely.