Effects of electrical stimulation on acetylcholine levels in central cholinergic nerve terminals

Reticulo-cortical activity and behavior: A critique of the arousal theory and a new synthesis

It is traditionally believed that cerebral activation (the presence of low voltage fast electrical activity in the neocortex and rhythmical slow activity in the hippocampus) is correlated with arousal, while deactivation (the presence of large amplitude irregular slow waves or spindles in both the neocortex and the hippocampus) is correlated with sleep or coma. However, since there are many exceptions, these generalizations have only limited validity. Activated patterns occur in normal sleep (active or paradoxical sleep) and during states of anesthesia and coma. Deactivated patterns occur, at times, during normal waking, or during behavior in awake animals treated with atropinic drugs. Also, the fact that patterns characteristic of sleep, arousal, and waking behavior continue in decorticate animals indicates that reticulo-cortical mechanisms are not essential for these aspects of behavior.

These puzzles have been largely resolved by recent research indicating that there are two different kinds of input from the reticular activating system to the hippocampus and neocortex. One input is probably cholinergic; it may play a role in stimulus control of behavior. The second input is noncholinergic and appears to be related to motor activity; movement-related input to the neocortex may be dependent on a trace amine.

Reticulo-cortical systems are not related to arousal in the traditional sense, but may play a role in the control of adaptive behavior by influencing the activity of the cerebral cortex, which in turn exerts control over subcortical circuits that co-ordinate muscle activity to produce behavior.

Different modulation of cholinergic neuronal systems by dynorphin A (1-13) in carbon monoxide-exposed mice

The effects of dynorphin A (1-13), a kappa-opioid receptor agonist, on the content of acetylcholine (ACh) and high K+-induced release of endogenous ACh were studied in mice exposed to carbon monoxide (CO). Mice were exposed to CO 3 times at 1-hr intervals and used 7 days after CO exposure. Administration of dynorphin A (1-13) (1.5 and 5.0 nmol/mouse, intracerebroventricularly) 15 min before killing significantly increased the ACh content in the striatum and hippocampus of control mice, but had no effect on the ACh content in CO-exposed mice. Dynorphin A (1-13) did not change the choline acetyltransferase (EC 2.3.1.6) activity in control or CO-exposed mice. The high K+-induced endogenous ACh release from hippocampal slices in CO-exposed mice was significantly lower than that of controls, although exposure to CO did not affect the basal release of endogenous ACh from hippocampal slices compared with controls. Dynorphin A (1-13) caused dose-dependent decreases in high K+-induced release of endogenous ACh from hippocampal slices in control mice. This inhibitory effect of dynorphin A (1-13) was blocked by co-perfusion with nor-binaltorphimine, a selective K-opioid receptor antagonist. On the other hand, dynorphin A (1-13) did not decrease high K+-induced release of endogenous ACh from hippocampal slices in CO-exposed mice. These results suggest that dysfunction of the cholinergic system occurred after exposure to CO, and as a result the inhibitory effects of dynorphin A (1-13) may be blocked in CO-exposed mice.

Increases in cerebral blood flow in rat hippocampus after medial septal injection of naloxone

BACKGROUND AND PURPOSE

In a previous study, we occasionally found that the rat given naloxone in the preoptic region develops behavioral seizures. In view of knowledge that the forebrain including the medial septal nucleus provides cholinergic projections to the hippocampal formation, the present study examined the effects of naloxone injected into the medial septal nucleus on the local blood flow in the hippocampus.

METHODS

A polyurethane-coated platinum electrode with a 1-mm bare tip for measurement of blood flow and a guide cannula made of stainless steel tube for naloxone injection were implanted chronically into the brain. The cerebral blood flow was measured by the hydrogen clearance method in freely moving rats.

RESULTS

The injection of 50 micrograms naloxone caused a significant increase in hippocampal blood flow, with its peak at 20 minutes. Twenty micrograms naloxone caused a similar increase, but 10 micrograms caused only a slight increase that peaked at 30 minutes, suggesting a dose-response of naloxone effect. Hippocampal blood flow was not changed after the injection of saline into the medial septal nucleus and after the injection of naloxone into the caudate nucleus.

CONCLUSIONS

Taken together with previous findings, the results suggest that endogenous opioids exert a decreasing effect on the local blood flow in the hippocampus, probably mediated by the magnocellular cholinergic neurons projecting to the hippocampus.

Acetylcholine release in the rat hippocampus as measured by the microdialysis method correlates with motor activity and exhibits a diurnal variation

Extracellular levels of acetylcholine were measured by the microdialysis method coupled to high performance liquid chromatography in the dorsal hippocampus of freely moving rats over a period of 24 h to examine whether the acetylcholine release in the hippocampus exhibited a diurnal variation. Spontaneous motor activity was simultaneously measured with an automatic animal activity monitor. The amount of acetylcholine collected per 20-min sample varied markedly, in a range from about 5 to 90 pmol. There appeared to be variations in the amount with a 2-4 h periodicity as well as an apparent diurnal periodicity. In all five rats studied, the overall mean value for the dark cycle (11.1-34.5, average 20.9 pmol/20 min) was significantly greater than that for the light cycle (5.1-21.3, average 12.3 pmol/20 min), showing a 70% average increase. Cross-correlation analysis performed between the amount of acetylcholine and the motor activity count for the animal during the sampling revealed a significant positive correlation coefficient in four rats studied. The present study demonstrates for the first time that the acetylcholine release shows a diurnal variation.

Cyclic nucleotide response of the hippocampal formation to septal stimulation in naive and kindled rats

Rats were kindled through nonmagnetic electrodes stereotaxically implanted into the medial septum. Concentrations of cyclic AMP and cyclic GMP were measured by radioimmunoassay in seven brain regions after microwave fixation during the development and expression of kindled seizures. Hippocampal concentrations were similar to untreated controls (cyclic GMP level in the left and right hippocampus, 0.66 +/- 0.04 and 0.68 +/- 0.07 pmol/mg of protein, respectively; cyclic AMP, 9.4 +/- 0.9 and 9.6 +/- 0.8 pmol/mg of protein, respectively), in kindled animals that were not stimulated, and in naive animals in response to septal stimulation, in spite of the presence in the latter group of bilateral hippocampal afterdischarges. Animals that failed to develop kindling and kindled animals that failed to have a seizure in response to stimulation also showed no change in cyclic nucleotide concentrations in any brain region. Kindled animals that developed a seizure following stimulation showed significant elevations in levels of both cyclic GMP and cyclic AMP in hippocampus and in several other brain regions. A single naive animal that had a seizure in response to its first stimulation also appeared to have elevated concentrations of both cyclic nucleotides in hippocampus. These data suggest that the elevation in levels of both cyclic GMP and cyclic AMP during kindled seizures is associated with seizure development rather than with the generation of afterdischarges or with the kindling engram.

Investigation of the rate-limiting step in the synthesis of acetylcholine by the human placenta

The uptake of [3H]choline and its conversion to [3H]acetylcholine were investigated in term human placental tissue in vitro. Although the net uptake of [3H]choline increased throughout a 45 min incubation period, intracellular [3H]choline levels reach a plateau after 2 min. There was a constant increase in [3H]acetylcholine levels throughout the incubation period. After 45 min, 36.5 per cent of the total intracellular tritium was recovered as acetylcholine by high-voltage electrophoresis. The effects of the choline acetyltransferase inhibitors, 2-benzoylethyltrimethyl-ammonium chloride (BETA) and 4-naphthylvinyl pyridine (NVP), and an inhibitor of choline uptake, hemicholinium-3 (HC-3), were also investigated for their influence on the uptake and metabolism of [3H]choline. A significant depression in both [3H]choline uptake and [3H]acetylcholine synthesis could be demonstrated with all three compounds, although with somewhat different time courses and activities. An analysis of the accumulation of [3H]acetylcholine in relation to the uptake and intracellular levels of [3H]choline as well as the patterns of inhibition produced by the inhibitors indicates that, unlike nervous tissue, the rate-limiting step in the synthesis of acetylcholine in human placental tissue is the transacetylation reaction catalysed by choline acetyltransferase.

Chronic phencyclidine increases methionine-enkephalin level in mouse striatum

The chronic administration of phencyclidine-HCl (PCP-HCl), 10 mg/kg, for 6-7 days resulted in a significant increase in the striatal methionine-enkephalin level, although acute administration induced no change of methionine-enkephalin level in this area. The methionine-enkephalin levels in other areas investigated, i.e. the medulla oblongata/pons, the midbrain, the hypothalamus and the cortex, were unchanged after chronic PCP treatment. These results suggest that chronic administration of PCP alters the enkephalinergic neuronal activity in the striatum.

The effect of the scorpion venom, tityustoxin, on high-affinity choline uptake in rat brain cortical slices

Tityustoxin (TsTx) inhibited high affinity choline uptake (HAChU) in cortical slices of the rat brain. The effect was dependent on the concentration of tityustoxin, energy source, incubation time, temperature, and the pH of the incubation medium. The inhibitory effect was dependent upon the presence of sodium and calcium ions in the incubation medium; barium ions could not replace calcium. Both tetrodotoxin and ethyleneglycol-tetra-acetic acid (EGTA) blocked the inhibitory effect of tityustoxin on high affinity choline uptake. On this evidence, it is suggested that the effect of tityustoxin might be related to its action on cell depolarization, causing an increase in the release of acetylcholine (ACh).

Phencyclidine-induced decrease of methionine-enkephalin levels in mouse brain

The effects of acute administration of phencyclidine (PCP) on the steady state levels of methionine-enkephalin in discrete brain areas were investigated in mice. The methionine-enkephalin levels in the medulla oblongata-pons and the midbrain were decreased by the administration of PCP. However, PCP induced no change of the methionine-enkephalin levels in other brain areas at the dose range of 5-20 mg/kg. These results suggest that the pharmacological effects of PCP may involve changes in enkephalinergic neuronal activity.

Septal modulation of the population spike in the fascia dentata produced by perforant path stimulation in the rat

Electrical stimulation of the perforant path, which originates in the entorhinal cortex, produces a characteristic excitatory postsynaptic field potential (extracellular EPSP) which can be recorded in the fascia dentata. This evoked response may include a population spike, if stimulation is sufficient. In the anaesthetized rat, stimulation of the medial septum, when paired with perforant path stimulation, was found to augment the population spike component of the evoked field potential. Stimulation of the septum alone produced no apparent field potential. The augmentation effect was found to have a rapid onset (4 ms), which is sufficient for the participation of interneurons, and a relatively long time course (150 ms). Presynaptic mechanisms of facilitation were ruled out as there was no concurrent alteration of the extracellular EPSP. A change in population spike threshold, compatible with a postsynaptic mechanism, was observed and some possible models of action discussed. Augmentation survived depletion of hippocampal norepinephrine caused by injections of 6-hydroxydopamine into the dorsal noradrenergic bundle, indicating that the facilitation was not due to an activation of the ascending noradrenergic fibres of passage originating from the locus coeruleus. The cholinergic septo-hippocampal pathway was ruled out as a likely candidate for the modulation as the augmentation survived injections of the muscarinic antagonists atropine and scopolamine and the nicotinic antagonists tubocurarine and dihydro-beta-erythroidine. A relationship between the septal modulation and hippocampal theta was suggested.

Potassium activation of [3H]-choline accumulation by isolated sympathetic ganglia of the rat

1 The effect of K-depolarization on the uptake of low and high concentrations of [3H]-choline by isolated superior sympathetic ganglia of the rat has been studied. 2 In unstimulated ganglia, the uptake of [3H]-choline (0.1 microM) ('high affinity uptake') was unaffected by denervation or by hemicholinium-3 (HC-3), suggesting uptake by structures other than cholinergic nerve terminals. 3 K-depolarization of the ganglia increased [3H]-choline accumulation by the high affinity uptake process but in contrast the 'low affinity' accumulation of [3H]-choline (100 microM) was decreased. 4 The K-activated, 'high affinity' component of choline uptake was highly sodium-dependent, inhibited by HC-3, and was abolished by denervation. 5 In incubation conditions designed to prevent transmitter release (Ca-free medium and high-Mg medium), the K-activated uptake of [3H]-choline was abolished. 6 It is concluded that in unstimulated ganglia, there is little choline uptake by nerve terminals. However, when the terminals are depolarized, choline uptake is increased by the activation of a sodium-dependent, HC-3-sensitive transport process. The activation of this uptake process is apparently associated with the release of acetylcholine from the terminals, or by changes in ionic fluxes, and not by the depolarization per se.

Cholinergic pharmacology of mammalian hippocampal pyramidal cells

Responses of CAl pyramidal cells to cholinergic compounds were recorded with intracellular microelectrodes in guinea-pig hippocampal slices. Perfusion of slices with medium containing the muscarinic antagonists atropine or scopolamine (10(-7)-10(-6)M) blocked all actions of acetylcholine. Properties of control neurons and those from separate populations of neurons impaled in slices exposed to muscarinic blocking agents were compared. 1-2 h of perfusion with atropine-containing media significantly decreased membrane input resistance from 37.6 +/- 8.7 (S.D.) M omega (n = 74) to 21.9 +/- 7.7 (S.D.) M omega (n = 24) without producing significant changes in membrane potential. Muscarinic antagonists also reduced or eliminated the anomalous inward rectification normally seen in hippocampal pyramidal neurons. Exposure of slices to 10(-5)-10(-6)M eserine for about 1 h produced changes in neuronal membrane input resistance and potential and slow after hyperpolarizations similar to those elicited by application of acetylcholine. Bethanechol mimicked the actions of acetylcholine but was effective at lower concentrations and had longer lasting effects on afterhyperpolarizations. Nicotine produced an excitatory response in only one of 7 neurons. These experiments demonstrate that the actions of acetylcholine on hippocampal CAl neurons result from interaction with muscarinic receptors. Acetylcholine has modulatory effects on cell membrane properties which may be mediated through tonic release mechanisms.

The excitatory action of acetylcholine on hippocampal neurones of the guinea pig and rat maintained in vitro

In preliminary experiments on 39 identified pyramidal cells in the in vitro slice preparation of the guinea-pig hippocampus the depolarization evoked by acetylcholine (ACh) applied by microiontophoresis was always associated with an increase in membrane resistance. In 9 slices cut from the rat hippocampus similar results were obtained from 24 cells. In a more detailed analysis on 13 cells from the rat hippocampus, whose mean resting potential was -74 mV and mean resting input resistance 33 M omega, the mean peak depolarization evoked by ACh was 11.6 mV and the mean increase in membrane resistance 12 M omega. The reversal potential for the excitatory action of ACh was 29 mV more hyperpolarizing than the resting membrane potential. The depolarization evoked by ACh was linearly related to the corresponding increase in membrane resistance expressed as a fraction of the resting membrane resistance determined before and after the application of ACh. This was true throughout each of the individual applications of ACh and of the peak response evoked by each of the 13 applications. The constancy of this relationship is compatible with the usual model used to describe synaptic events thought to be mediated by the closure of ionic channels which are open in the absence of the transmitter. The onset of the response to ACh was always approximately 4 times slower than that evoked by a near equipotent microiontophoretic application of glutamate from an adjacent barrel of the same multibarrelled micropipette. Following the application of ACh, recovery was also slow and, on average, was approximately 10 times longer than that following a near equipotent application of glutamate. It is suggested that the slow onset and offset of the responses evoked by ACh are not compatible with models based on diffusion and are best explained by postulating a sequential generation of one or more intermediates.

Choline uptake and permanent memory storage

The uptake of 3H-choline and its incorporation into 3H-acetylcholine was studied in vitro on hippocampus slices obtained from animals showing a good or poor long-term memory. The animals were selected on the basis of their retention performance when tested by a brightness discrimination model. The 3H-choline uptake and the incorporation of 3H-choline into 3H-acetylcholine was higher in hippocampus slices from animals showing good retention compared to those from animals with poor retention. The level of high affinity uptake of choline into hippocampus slices may serve as an indicator of the cholinergic activity in this structure under in vitro conditions. The present findings suggest that individual differences in the activity level of the hippocampal cholinergic system do exist and are capable of influencing the retention of the individual animals to a variable degree.

Hippocampal rhythmic slow activity (RSA; theta): a critical analysis of selected studies and discussion of possible species-differences

The literature concerning the correlates of hippocampal RSA (theta) has seen a wealth of hypotheses generated from seemingly contradictory data. Two possible reasons for this are examined here. (1) An analysis of the maximum published RSA amplitudes in over 70 papers shows that there is enormous variation in how effective various research groups have been in tapping the hippocampal RSA generator zones. It is suggested that this variation is a major source of 'contradictory data'. The enormous variability is probably due to the fact that the laminar structure of the hippocampus, and the location of two seemingly independent 180 degrees out-of-phase RSA generators, results in very disparate signals being recorded by electrodes of different configurations. Electrodes which are not optimally placed result in records which may provide misinformation as to whether or not the hippocampus is in the RSA 'mode'. The results of studies with less than adequate records must therefore be viewed with great caution. (2) An explanation often evoked to account for much of the controversy is that of species differences. This idea is examined and it is suggested that there are probably not major species differences in that all of the species appropriately examined thus far have neural systems capable of producing both an atropine-sensitive and an atropine-resistant form of RSA. All species (with the exception of primates) also show relations of RSA to ongoing motor behavior. However, there are definitely species differences in the neural mechanisms underlying the production of atropine-sensitive, immobility-related RSA. Although all species appear to be capable of producing immobility-related RSA some do so only rarely (e.g. rats), while others do so frequently, particularly in response to sensory stimulation (e.g. rabbits, cats, guinea pigs). Therefore, the answer to the question as to whether there are species differences in the occurrence of RSA may be yes, or not, depending upon how specifically the question is posed.

High affinity choline transport and acetylCoA production in brain and their roles in the regulation of acetylcholine synthesis

This review describes recent advances made in the understanding of the regulation of acetylcholine synthesis in brain with regard to the availability of its two precursors, choline and acetylCoA. Choline availability appears to be regulated by the high affinity choline transport system. Investigations of the localization and inhibition of this system are reviewed. Procedures for measuring high affinity choline transport and their shortcomings are described. The kinetics and effects of previous in vivo and in vitro treatments on high affinity choline transport are reviewed. Kinetic and direct coupling of the transport and acetylation of choline are discussed. Recent investigations of the source of acetylCoA used for the synthesis of acetylcholine are reviewed. Three sources of acetylCoA have recently received support: citrate conversion catalyzed by citrate lyase, direct release of acetylCoA from mitochondria following its synthesis from pyruvate catalyzed by pyruvate dehydrogenase, and production of acetylCoA by cytoplasmic pyruvate dehydrogenase. Investigations indicating that acetylCoA availability may limit acetylcholine synthesis are reviewed. A model for the regulation of acetylcholine synthesis which incorporates most of the reviewed material is presented.

Development of behavioral tolerance: a search for subcellular mechanisms

Development of behavioral tolerance is one of the processes by which living organisms adjust to changes in their internal and external environments. The search for neurochemical mechanisms underlying such processes requires the testing of many hypotheses. The present study was designed to examine the possible involvement of certain subcellular events. The concentrations of acetylcholine (ACh) and choline (Ch), the high-affinity transport of Ch, and the rate of synthesis of ACh were measured in synaptosomes prepared from the brains of rats. The assays were made at critical times during the acute changes in behavior induced by administration of the anticholinesterase, di-isopropylfluorophosphate, and during the development of behavioral tolerance to this compound as chronicity of administration continued. No statistically significant differences were found among treatment groups in the total concentration of ACh or Ch, the synthesis of ACh, or the high-affinity transport of Ch. These results, plus evidence from previous experiments, indicate that the development of behavioral tolerance does not relate to the factors studied. Consequently, alternative mechanisms should be considered. In addition to changes in cholinergic (muscarinic) receptors already shown to occur concomitantly with the development of behavioral tolerance, it is suggested that the possible involvement of mechanisms controlling release of ACh should be studied.

Rapid changes in enkephalin levels in rat striatum and hypothalamus induced by diazepam

The acute treatment of rats with diazepam induces pronounced changes in brain enkephalin concentrations, as was estimated for methionine(met)-enkephalin and in some representative experiments for leucine(leu)-enkephalin, employing highly specific radioimmunoassays. Diazepam selectively increased the enkephalin concentrations in the hypothalamus by about 35%, and lowered it in the corpus striatum by roughly 25%; no changes could be detected in the medulla oblongata/pons or midbrain. The drug-induced changes displayed a rapid onset. Peak effects were reached by 2 to 5 min after injection. Changes observed in the hypothalamus were only short lasting and were apparently parallelled by diazepam concentrations in the brain, whereas the decrease in the striatum was of markedly longer duration. Presently, the mechanism underlying all these changes is unknown. Whereas an increase in enkephalin concentrations in the hypothalamus may be discussed in terms of the anti-stress effect of benzodiazepines, the observed drop in striatal enkephalin is not obviously to be correlated to behavioural changes induced by these drugs.

The influence of cholinergic transmitter substances on the incorporation of (14C)-leucine and (3H)-fucose into the total proteins of hippocampus in vivo and in vitro

The incorporation of (14C)-leucine into the total proteins of the hippocampus is inhibited by high concentrations of cholinergic agonists, with nicotinic substances (such as 1,1-dimethyl-4-phenyl-piperazine) being more effective than muscarinic compounds (such as arecoline and pilocarpine). Under these conditions the incorporation of (3H)-fucose is not influenced.

Modulation of ACh turnover in the septal-hippocampal pathway by electrical stimulation and lesioning

The septal-hippocampal cholinergic pathway of the rat was either electrically stimulated or lesioned in order to study whether or not acetylcholine turnover rate (TRACh) changes with the activity of the cholinergic neurons. Appropriate electrical stimulation of the septum selectively increased the TRACh in the hippocampus in nonanesthetized and in barbiturate-treated animals. The ACh content of the hippocampus increased by approximately 30% 1 h after fimbria lesions, but decreased by about 80% 9 days after fimbria lesions. Acute fimbria lesions decreased the TRACh in the lesioned side by approximately 85%, but the TRACh in the intact side and in the cortex was unchanged. The same was true in rats with chronic fimbria lesions. In conclusion, the hippocampal TRACh increases or decreases proportionally to the activity of the cholinergic neurons; therefore the measurement of this parameter is of particular value in understanding how postynaptic cholinergic neurons are modulated by putative neurotransmitter released from afferent nerve terminals.