Acetylcholine as an excitatory and inhibitory transmitter in the mammalian central nervous system

Norepinephrine May Oppose Other Neuromodulators to Impact Alzheimer's Disease

While much of biomedical research since the middle of the twentieth century has focused on molecular pathways inside the cell, there is increasing evidence that extracellular signaling pathways are also critically important in health and disease. The neuromodulators norepinephrine (NE), serotonin (5-hydroxytryptamine, 5HT), dopamine (DA), acetylcholine (ACH), and melatonin (MT) are extracellular signaling molecules that are distributed throughout the brain and modulate many disease processes. The effects of these five neuromodulators on Alzheimer's disease (AD) are briefly examined in this paper, and it is hypothesized that each of the five molecules has a u-shaped (or Janus-faced) dose-response curve, wherein too little or too much signaling is pathological in AD and possibly other diseases. In particular it is suggested that NE is largely functionally opposed to 5HT, ACH, MT, and possibly DA in AD. In this scenario, physiological "balance" between the noradrenergic tone and that of the other three or four modulators is most healthy. If NE is largely functionally opposed to other prominent neuromodulators in AD, this may suggest novel combinations of pharmacological agents to counteract this disease. It is also suggested that the majority of cases of AD and possibly other diseases involve an excess of noradrenergic tone and a collective deficit of the other four modulators.

Delayed apoptotic pyramidal cell death in CA4 and CA1 hippocampal subfields after a single intraseptal injection of kainate

We have performed a detailed time-course analysis of cell death in the hippocampal formation, basal forebrain and amygdala following a single intraseptal injection of kainate in adult rats. Acetylcholinesterase histochemistry revealed a profound loss of staining in the medial septum but not in the diagonal band, and cholinergic fiber density was highly reduced in the hippocampus and amygdala at 10 days postinjection. Terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphatebiotin nick end labeling (TUNEL) histochemistry was performed for precise location of apoptotic cells. Both the medial septum and amygdala exhibited numerous TUNEL-positive nuclei after the intraseptal injection of kainate, while the lateral septum exhibited a lower but significant incidence in terms of apoptotic cells. In the medial septum, the presence of apoptotic cells was at a location displaying acetylcholinesterase staining. TUNEL histochemistry revealed a time-dependent sequential apoptotic cell death in hippocampal pyramidal cells. During the first two days postinjection, apoptosis in the hippocampus was only evident in the CA3 region. At five days postinjection, the entire CA4 region became apoptotic. At 10 days postinjection, the whole extent of the CA1 pyramidal cell layer exhibited numerous TUNEL-positive nuclei. The time-course of kainate-induced apoptosis in Ammons's horn correlated with the disappearance of hippocampal pyramidal neurons as detected by Nissl staining, which is suggestive of a prominent apoptotic death for these cells. The temporal delayed distant damage to CA4 and CA1 hippocampal subfields after a single intraseptal kainate injection is not seen in other models employing kainate and may be a valuable tool for exploring the cellular mechanisms leading to cell death in conditions of status epilepticus.

SLOW and FAST lateral geniculate neurons are differently influenced by acetylcholine

In rats anesthetized with urethane, potentials of 108 neurons were recorded extracellularly in the dorsal part of the lateral geniculate body (dLGB). Neuronal responses to diffuse light stimuli were studied before and during the iontophoretic application of acetylcholine (ACh). Although individual cells of all groups of functionally different neuron types could be influenced by ACh, responses to flashes were most pronounced and uniformly enhanced in the groups of SLOW ON-like cells located in the dorsolateral and caudal parts of the dLGB. The activity in primary response phases to light flashes increased also in caudally located SLOW OFF-like cells. In the group of ventromedially located FAST OFF-like cells the postinhibitory offdischarge in the response to flash was significantly augmented. Only few cells of FAST ON-like groups were affected and some of them inhibited by ACh. off

Cholinergic synapses in the central nervous system: studies of the immunocytochemical localization of choline acetyltransferase

Cholinergic synapses can be identified in immunocytochemical preparations by the use of monoclonal antibodies and specific antisera to choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh) and a specific marker for cholinergic neurons. Electron microscopic studies demonstrate that the fibers and varicosities observed in light microscopic preparations of many brain regions are small-diameter unmyelinated axons and vesicle-containing boutons. The labeled boutons generally contain clear vesicles and one or more mitochondrial profiles. Many of these boutons form synaptic contacts, and the synapses are frequently of the symmetric type, displaying thin postsynaptic densities and relatively short contact zones. However, ChAT-labeled synapses with asymmetric junctions are also observed, and their frequency varies among different brain regions. Unlabeled dendritic shafts are the most common postsynaptic elements in virtually all regions examined although other neuronal elements, including dendritic spines and neuronal somata, also receive some cholinergic innervation. ChAT-labeled boutons form synaptic contacts with several different types of unlabeled neurons within the same brain region. Such findings are consistent with a generally diffuse pattern of cholinergic innervation in many parts of the central nervous system. Despite many similarities in the characteristics of ChAT-labeled synapses, there appears to be some heterogeneity in the cholinergic innervation within as well as among brain regions. Differences are observed in the sizes of ChAT-immunoreactive boutons, the types of synaptic contacts, and the predominant postsynaptic elements. Thus, the cholinergic system presents interesting challenges for future studies of the morphological organization and related function of cholinergic synapses.

Demarcations of the mechanosensory projection zones in the raccoon thalamus, shown by cytochrome oxidase, acetylcholinesterase, and Nissl stains

To determine anatomically the boundaries and internal organization of the kinesthetic and cutaneous mechanosensory regions of the ventrobasal thalamus, alternate section series from electrophysiologically mapped tissues from 14 raccoons were stained for cytochrome oxidase, myelinated fibers, acetylcholinesterase, and Nissl substance. Microelectrode tracks, along with electrolytic lesions placed as tissue markers, reveal that the mechanoreceptor projection zones have higher cytochrome oxidase and lower acetylcholinesterase staining than some neighboring regions. Both these enzymatic stains reveal particularly sharp boundaries separating the mechanoresponsive region, from the lateral posterior nucleus dorsally and from the ventroposterior inferior nucleus ventrally. The kinesthetic projection zone is often separated from other mechanoreceptor projections by bundles as well as laminae of myelinated fibers, similar to those separating cutaneous projections from distinct body parts. These subdivisions are particularly well marked by the cytochrome oxidase stain. The combination, in neighboring sections, of the use of the several stains adds considerably to the visible delineation of these functionally distinct regions, beyond what can be seen in Nissl-stained sections.

Arecoline-induced elevations of regional cerebral metabolism in the conscious rat

Local cerebral glucose utilization (LCGU) was measured, using the quantitative [14C]2-deoxy-D-glucose ([14C]DG) method, at 3 min after administration to 3-month-old, awake Fischer rats of the muscarinic agonist arecoline (AREC) 0.05, 0.5, 5, 15 or 50 mg/kg or saline i.p. Animals were pretreated with methylatropine (a cholinergic antagonist which does not enter the brain and has no effect on cerebral metabolism) 4 mg/kg s.c. to prevent parasympathomimetic side-effects of AREC. Tremor produced by AREC was rated subjectively. Intensity of tremor was dose-related, peaked at 2-5 min after AREC, and abated within 30 min. Elevations in LCGU (measured after [14C]DG injection during peak behavior) in extrapyramidal regions, which mediate tremor, were related to the intensity of tremor. The lowest dose of AREC selectively increased LCGU in the hippocampus and median raphe; higher doses produced more generalized metabolic enhancement. In the hippocampus and cortex, LCGU rose in layers in which cholinoceptive cells are located. Regions of the auditory pathway and superficial neocortical layers (I-III) were generally unaffected by AREC, but LCGU did not decrease in any region. The selective increase in LCGU produced by low doses of AREC in the hippocampus presumably is due to a specific action of AREC, and demonstrates the high sensitivity of this region to cholinomimetic stimulation.

Cytophotometric analyses of thalamic neuronal RNA in soman intoxicated rats

Quantitative azure B-RNA cytophotometry was used to monitor metabolic responses of individual neurons within the ventrobasal nuclear complex (VBC) and nucleus reticularis (NR) of the rat thalamus following administration of soman (0.5, 0.9 or 1.5 LD50, sc). A dose-dependent depression in brain acetylcholinesterase (AChE) was evidenced. With respect to thalamic RNA responses, a complex pattern of RNA alterations was evidenced, with these two regions generally exhibiting opposite patterns of dose-related RNA changes. With sub-lethal dosages of soman, RNA accumulation was evidenced in the acetylcholine (ACh) mediated excitatory VBC region and RNA depletion in the ACh mediated inhibitory NR neurons. With a lethal dose, an opposite RNA response pattern was observed in both thalamic regions. It is postulated that the observed RNA response pattern with sub-lethal dosages of soman is what one would anticipate with cholinergic brainstem reticular formation activation. The absence of such a response with lethal doses strongly suggests some disruption of functional excitatory cholinergic activity and perhaps also an impairment of inhibitory cholinergic synaptic activity.

Asymmetric distribution of acetylcholine receptors and M channels on prepyriform neurons

The responses of pyramidal neurons of rat prepyriform cortex to ionophoretic application of acetylcholine (ACh) were studied in a submerged, perfused brain slice. ACh excited some neurons but only if applied to an area near to the cut surface of the slice. This area contained the basal dendrites of the pyramidal cells and some cell bodies. No excitation was seen if ACh was applied at depths of 250 microns or more from the cut surface, an area which contained only apical dendrites, although the apical dendrites were very sensitive to excitatory amino acids such as aspartate (Asp) and glutamate (Glu). On all neurons which did not discharge to ionophoretic application of ACh, ACh potentiated the response to Glu and Asp. No potentiation of amino acid responses was obtained on apical dendrites. The potentiation had a time course similar to that of the discharge of neurons which fired to ACh. This observation suggests that pyramidal neurons have receptors for ACh on basal but not apical dendrites. The ACh response in the basal dendrite-soma region was elicted by pilocarpine and blocked by atropine but not curare. This was true whether the response studied was direct excitation or potentiation of the response to an amino acid. The ACh response was associated with a voltage-dependent increase in membrane resistance which had a slow time course and appeared to be due to a turning off of an M current, as described by Brown and Adams (1980) in sympathetic ganglion cells. The effects of ACh were minimal at the resting potential but increased with depolartization. ACh had no effect on the current-voltage relation of the cell, except at depolarized potentials of less than -60 mV. Ionophoretic application of Ba2+ to the basal dendritic region resulted in potentiation of the amino acid responses and sometimes induced a discharge similar to that of ACh. Since Ba2+ mimics the ACh response, presumably by a direct blockade of the M channel, the effects of Ba2+ on apical dendrites were tested to determine whether these dendrites contain M channels associated with a transmitter receptor other than ACh. However, Ba2+ did not induce potentiation in apical dendrites, suggesting that M channels are also restricted to the basal dendrites or cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Scanning-integrating cytophotometric analyses of brain neuronal RNA and acetylcholinesterase in acute soman toxicated rats

Cytophotometric analyses of RNA and acetylcholinesterase responses of caudate and cerebrocortical neurons of soman toxicated rats were conducted to characterize impairments in regulatory aspects of neuronal metabolism occurring in the acute phase of cholinesterase impairment. There was a severe and dose-dependent suppression (20-60%) in neuronal acetylcholinesterase activity in both a.m. and p.m.-treated rats; no diurnal differences were apparent in control acetylcholinesterase levels or neuronal acetylcholinesterase responsiveness to soman toxication. RNA levels, however, were markedly higher in p.m. than in a.m. saline-treated controls. Soman depressed caudate neuron RNA contents in the afternoon, but not in the morning. Cerebrocortical neuron RNA levels were suppressed in both a.m. and p.m.-toxicated rats, although this RNA depletion was more severe in the afternoon. These results indicate that soman can elicit marked alterations in neuronal transcriptional-translational capabilities and that there are diurnal variations in cellular metabolic responsiveness to soman toxication. Although functional relationships between soman-induced cholinesterase inhibition and RNA depletion remain to be elucidated, depressed RNA metabolism appears to be a maladaptive response preventing rapid regeneration of cholinesterase following poisoning.

Multiple actions of acetylcholine on hippocampal pyramidal cells in organotypic explant cultures

Hippocampal cultures were prepared from 7- to 10-day-old rats by means of the roller-type technique. The preservation of the characteristic hippocampal cytoarchitecture allowed, after many weeks in vitro, impalement of pyramidal cells by microelectrodes under visual control. Application of 10(-7) to 10(-5) M acetylcholine to the bath depolarized hippocampal pyramidal cells, strongly increased their rate of firing and induced paroxysmal depolarization shifts. This depolarizing action was accompanied by a reduction in the amplitude of evoked postsynaptic potentials. Whereas it was not clear whether the decrease in the amplitude of the excitatory postsynaptic potentials was only a result of membrane depolarization, acetylcholine clearly and reversibly reduced the potency of evoked inhibitory postsynaptic potentials. Iontophoresis of acetylcholine to the perisomatic region of pyramidal neurons, like acetylcholine applied to the bath, increased their firing rate and powerfully decreased the amplitude and duration of spontaneous and evoked inhibitory postsynaptic potentials. In contrast, iontophoresis of acetylcholine in the pyramidal cell layer at a distance from the recorded neuron generated a hyperpolarizing response associated with a reduction in firing rate. At high current strength, the initial hyperpolarization was (often) followed by a paroxysmal depolarization shift. High frequency electrical stimulation with electrodes located close to the acetylcholine pipette in the pyramidal cell layer (i.e. about 1 mm away from the recorded neuron) mimicked the acetylcholine effect. Resistance measurements indicated that membrane input resistance was decreased in the majority of cells during application of acetylcholine. This decrease in membrane resistance may result from a direct action of acetylcholine or from an increased synaptic activity. Synaptic alterations induced by acetylcholine were quick in onset and in recovery, while the increase in the rate of firing occurred somewhat later. Atropine (10(-5) M), which had no significant action by itself, completely abolished the action of acetylcholine applied to the bath or by iontophoresis. In contradistinction, naloxone did not influence the acetylcholine effects, although opiates and opioid peptides produce paroxysmal depolarization shifts in pyramidal cells which resemble those induced by acetylcholine. Addition of 8-16 mM magnesium to the bathing solution or exposure of the cultures to a calcium-free solution containing 1 mM cobalt abolished the effects of acetylcholine. In the presence of 10(-6) g/ml tetrodotoxin, 10(-5) M acetylcholine decreased the membrane input resistance of pyramidal cells, reduced their threshold for the generation of tetrodotoxin-resistant spikes and generated paroxysmal depolarization shifts in a proportion of pyramidal cells...