Cholinergic function and intellectual decline in Alzheimer's disease

Unilateral and bilateral nucleus basalis lesions: differences in neurochemical and behavioural recovery

The neurochemical and behavioural recovery following unilateral and bilateral lesions of the nucleus basalis was investigated in adult male Wistar rats 20 days and 6 months after surgery. The lesions were made by stereotaxic injections of ibotenic acid. Twenty days after surgery there was a statistically significant choline acetyltransferase decrease in the frontal and parietal ipsilateral cortex of the unilaterally lesioned rats and in the cortex of both hemispheres after bilateral lesions. Cortical high affinity choline uptake rate was significantly decreased 4 days after lesions but showed a rapid recovery within 20 days post-lesion in unilaterally and bilaterally lesioned rats. However, at this time both groups of lesioned rats showed a marked impairment in the acquisition of passive and active (shuttle-box) avoidance conditioned responses. Six months after surgery the decrease in choline acetyltransferase activity was smaller and statistically significant in the ipsilateral frontal cortex only in the unilaterally lesioned rats and in the frontal and parietal cortex of both hemispheres in the bilaterally lesioned rats. High affinity choline uptake was increased in the contralateral hemispheres of the unilaterally lesioned rats and was significantly larger than in the bilaterally lesioned rats. There was no difference in the acquisition of both passive and active avoidance conditioned responses between the sham operated and unilaterally lesioned rats, while the bilaterally lesioned rats could only negotiate the active avoidance conditioned response. In conclusion, our experiments demonstrate a remarkable neurochemical and behavioural recovery within 6 months in rats with a unilateral lesion of the nucleus basalis and only a limited recovery in the bilateral lesioned rats.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Brain dialysis: detection of acetylcholine in the striatum of unrestrained and unanesthetized rats

A dialysis cannula was implanted into rat striatum while the animals were anesthetized, and after at least one day following the surgery the area was perfused with Ringer solution under the unrestrained and unanesthetized conditions. Concentration of acetylcholine (ACh) in the perfusate was determined by high-performance liquid chromatography (HPLC)-electrochemical detection (ECD) with the enzyme-column on which acetylcholine esterase and choline oxidase were immobilized. ACh in the dialysate was only detectable when the Ringer solution containing eserine, an inhibitor of acetylcholinesterase, was perfused. ACh peak on HPLC-ECD could be detected at least for 4 h under these conditions. The level of ACh increased 2-3 fold with the perfusion of 1 mM atropine sulfate, a blocker of ACh receptor. These data indicate that brain dialysis in the presence of eserine is useful for study on the neurochemical activity of ACh neurons in the brain.

Monoclonal antibodies to monkey brain choline acetyltransferase: production and immunohistochemistry

Two monoclonal antibodies to monkey brain choline acetyltransferase (ChAT) were obtained. Immunoblot analysis indicated that both antibodies revealed two adjacent protein bands on a sodium dodecyl sulfate-polyacrylamide gel electrophoresis. ChAT was demonstrated immunohistochemically by one of the antibodies in the motoneurons in the monkey brainstem and spinal cord. This antibody also revealed ChAT-positive terminal-like structures in the neuropils of lamina IX of the cervical spinal cord and interpeduncular nucleus.

Molecular biology and neurobiology of choline acetyltransferase

In the 45 years since the first description of choline acetyltransferase (ChAT; EC 2.3.1.6.), significant progress has been made in characterizing the molecular properties of this important neurotransmitter synthetic enzyme. We are now on the verge of understanding its genetic regulation and biological function(s). The Drosophila cDNA has been cloned, sequenced, and expressed in both a eucaryotic and a procaryotic system. The levels of ChAT specific mRNA have been determined during Drosophila development. Monoclonal and polyclonal antibodies have been produced to the enzyme from a variety of sources and used for biochemical and immunocytochemical studies. Two well characterized genetic systems have identified the ChAT gene and described a series of useful alleles. As a nervous system specific protein expressed only in the subset of neurons using acetylcholine as a neurotransmitter, ChAT is a good model for uncovering the processes and factors responsible for regulating genes involved in neurotransmitter phenotype selection and maintenance. Recent studies have described the purification of a cholinergic factor from muscle conditioned medium and indicated the potential importance of nerve growth factor (NGF) for regulating ChAT expression in the central nervous system. These factors, or ones remaining to be discovered, may be involved in the etiology or disease process of neurodegenerative nervous system disorders such as Alzheimer's disease.

Effect of phosphatidylserine on acetylcholine release and content in cortical slices from aging rats

Cortical slices were prepared from male rats 3 to 28 months old. The slices were superfused with choline-enriched Krebs solution containing physostigmine and electrically stimulated at frequencies of 1, 2 and 5 Hz for 5 min periods preceded and followed by rest periods. The amount of acetylcholine released during the stimulation periods was quantified by bioassay. In some experiments acetylcholine content was measured at the end of the superfusion period in stimulated and unstimulated slices. The evoked acetylcholine release was constant between 3 and 11 months of age at each frequency tested and showed a 50% decrease between 11 and 14 months of age with no further decrease up to 28 months. No difference in the evoked acetylcholine release was detected between 3 and 16 month old rats if the old rats were pretreated with phosphatidylserine 15 mg/kg IP for at least 7 days. The effect of phosphatidylserine lasted for 5 days after interruption of the treatment. There was no difference in acetylcholine content between the stimulated and unstimulated slices in 3 month old rats. In 16 month old rats stimulation brought about a 44% decrease in acetylcholine content. This decrease did not occur in rats pretreated with phosphatidylserine for 7 days. Phosphatidylserine appears to restore acetylcholine release in aging rats by maintaining an adequate acetylcholine supply in the slices.

Blockade of spatial learning by the M1 muscarinic antagonist pirenzepine

Two experiments were conducted to determine the effects of the M1 muscarinic receptor antagonist pirenzepine on place navigation in a water maze. In the first experiment rats were required to learn the location of a hidden platform following intracerebroventricular injections of equimolar doses of pirenzepine or scopolamine methylbromide. Both drugs dose-dependently impaired spatial learning according to both escape latency data and transfer test analysis. Pirenzepine was approximately 3 times less potent than scopolamine, a potency ratio which suggests M1 receptor mediation of the impairment. In the second experiment pirenzepine (1 approximately 92.3 micrograms/rat ICV) was injected prior to training on a simultaneous place discrimination task in the water maze. Impairments of choice accuracy were found with a dose of 20 micrograms/rat in the absence of any marked increases in either errors of omission or choice latency. These data suggest that M1 receptor blockade impairs processes which are involved in spatial learning.