Modulation of visual cortical plasticity by acetylcholine and noradrenaline

During a critical period of postnatal development, the temporary closure of one eye in kittens will permanently shift the ocular dominance (OD) of neurones in the striate cortex to the eye that remains open. The OD plasticity can be substantially reduced if the cortex is infused continuously with the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) during the period of monocular deprivation, an effect that has been attributed to selective depletion of cortical noradrenaline. However, several other methods causing noradrenaline (NA) depletion leave the plasticity intact. Here we present a possible explanation for the conflicting results. Combined destruction of the cortical noradrenergic and cholinergic innervations reduces the physiological response to monocular deprivation although lesions of either system alone are ineffective. We also find that 6-OHDA can interfere directly with the action of acetylcholine (ACh) on cortical neurones. Taken together, our results suggest that intracortical 6-OHDA disrupts plasticity by interfering with both cholinergic and noradrenergic transmission and raise the possibility that ACh and NA facilitate synaptic modifications in the striate cortex by a common molecular mechanism.     

Substance P in the ascending cholinergic reticular system

The neocortex receives a major cholinergic innervation from magnocellular neurones in the basal forebrain. However, an ascending cholinergic reticular system has also been postulated to arise from acetylcholinesterase (AChE)-containing neurones in the midbrain and pontine tegmentum. Lesions of this region decrease both AChE and choline acetyltransferase (ChAT) in various forebrain areas, and recent immunohistochemical studies have identified a group of ChAT-containing cell bodies in the midbrain reticular formation and dorsolateral pontine tegmentum. Here we have combined retrograde tracing with ChAT immunohistochemistry to demonstrate that this tegmental cholinergic cell group also directly innervates the cerebral cortex. Other immunohistochemical studies have indicated that the neuropeptide substance P is also present in certain cells in the laterodorsal tegmentum, and these too appear to project to the forebrain. We have therefore performed immunohistochemistry for both ChAT and substance P and have discovered that a subpopulation of the ascending cholinergic reticular neurones contains substance P. Thus, peptide-cholinergic coexistence, previously noted in peripheral neurones, also occurs in the brain.     

Acetylcholine induces burst firing in thalamic reticular neurones by activating a potassium conductance

Recent studies have emphasized the role of acetylcholine (ACh) as an excitatory modulator of neuronal activity in mammalian cortex and hippocampus. Much less is known about the mechanism of direct cholinergic inhibition in the central nervous system or its role in regulating neuronal activities. Here we report that application of ACh to thalamic nucleus reticularis (nRt) neurones, which are known to receive a cholinergic input from the ascending reticular system of the brain stem, causes a hyperpolarization due to a relatively small (1-4 nS) increase in membrane conductance to K+. This cholinergic action appears to be mediated by the M2 subclass of muscarinic receptors and acts in conjunction with the intrinsic membrane properties of nucleus reticularis neurones to inhibit single spike activity while promoting the occurrence of burst discharges. Thus, cholinergic inhibitory mechanisms may be important in controlling the firing pattern of this important group of thalamic neurones.     

Somatostatin immunoreactivity in neuritic plaques of Alzheimer's patients

Senile dementia of the Alzheimer's type can be diagnosed with certainty only by examining neurofibrillary tangles and neuritic plaques under the microscope. Recently, it has been suggested that the condition is linked to specific neurotransmitter systems, with a decline of cortical acetylcholine, choline acetyltransferase, cholinergic neurones projecting to the cortex, cortical noradrenaline content, locus coeruleus neurones and cortical somatostatic content. Using immunocytochemical methods, we here report that somatostatin-immunoreactive processes are present in neuritic plaques in human Alzheimer's specimens. These data, as well as other reports of non-cholinergic changes, strongly imply that Alzheimer's disease cannot be linked exclusively to cortical cholinergic elements, as proposed previously. Rather, our data on plaque and somatostatin co-localization and distribution patterns suggest that Alzheimer's neuropathology may involve primarily the loss of selective cortical neurones that are targets of the implicated transmitter systems and that plaque formation may result from the degeneration of presynaptic and postsynaptic neurites of large projection neurones in layers III and V. Given the neurochemically heterogeneous input to these cells, it is not surprising that several neurotransmitter systems, one of which is somatostatin, are implicated in the pathology of Alzheimer's disease.     

Location of neuronal tangles in somatostatin neurones in Alzheimer's disease

Senile dementia of the Alzheimer type is a chronic, progressive neuropsychiatric condition characterized clinically by global intellectual impairment and neuropathologically by the presence of numerous argyrophilic plaques and tangles. Neurochemical investigations have established loss of the cholinergic and aminergic projections to the cerebral cortex and a loss of the content of somatostatin, with preservation of cholecystokinin and vasoactive intestinal polypeptide, neuropeptides also located in cells intrinsic to the cortex. We describe here the relationship between cortical somatostatin immunoreactivity and the plaques and tangles of diseased tissue by immunocytochemical and silver impregnation techniques on paraffin-embedded tissue. In sections of Alzheimer's tissue, cortical somatostatin-immunoreactive perikarya exhibited morphological changes consistent with neuronal degeneration. Silver-stained material immunostained subsequently showed that many neurones containing tangles were also somatostatin positive. No such colocalization was observed using antisera to other neuropeptides. Our findings indicate that a subclass of somatostatin-positive neurones are affected selectively in Alzheimer's disease and that these neurones also contain neuronal tangles. Thus, destruction of somatostatin-containing neurones is an early and perhaps critical event in the disease process.     

Reduced somatostatin-like immunoreactivity in cerebral cortex from cases of Alzheimer disease and Alzheimer senile dementa

Both Alzheimer's disease and senile dementia of the Alzheimer type (AD/SDAT) are progressive dementias characterized neuropathologically by the presence in the cerebral cortex of numerous neurofibrillary tangles and neuritic plaques. We use the abbreviation AD/SDAT to denote all such cases, irrespective of age of onset. Studies of neurotransmitter-related parameters in autopsied brain tissues from patients with AD/SDAT have, to date, been confined to five putative transmitter systems. Acetycholine-releasing neurones seem to be most markedly and consistently affected, as judged by the extensive reductions in choline acetyltransferase (ChAT) and acetylcholinesterase activities that have been reported. Despite numerous studies, there is no consistent evidence for the involvement of neurones releasing dopamine, noradrenaline, serotonin, or gamma-aminobutyric acid in AD/SDAT, nor for loss of muscarinic cholinergic receptors. Thus, the involvement of cholinergic neurones in AD/SDAT seems to be specific. However, the possible involvement of neurones using other chemicals as transmitters has yet to be explored. The recent recognition of the existence of so-called 'peptidergic neurones' in the mammalian brain (for review see ref. 8) and the availability of radioimmunoassay (RIA) techniques for studying these peptides, have led us to begin a systematic investigation of neuropeptides in autopsied brain tissue from cases of AD/SDAT, and from neurologically normal individuals. We report here results obtained with a RIA for somatostatin, showing that somatostatin-like immunoreactivity in the cerebral cortex is reduced in tissue from AD/SDAT patients.     

Retrograde axonal transport of target tissue-derived macromolecules

Neurones depend on contact with their target tissues for survival and subsequent development. The protein, nerve growth factor (NGF), can be selectively taken up by sympathetic nerve terminals and reaches the neuronal perikaryon by a process of retrograde intra-axonal transport, suggesting that its role in vivo is to act as a target tissue-derived trophic factor. The development of the neurones of the chick ciliary ganglion requires the presence of structures derived from the optic cup. Several studies in vitro have shown that media conditioned by non-neuronal cells contain factors that result in the survival of neurones from ciliary ganglia. In particular, chick embryo iris, ciliary body and choroid contained large amounts of these factors indicating the presence of a target tissue-derived trophic factor for the cholinergic ciliary ganglion. This study demonstrates that neurones of the ciliary ganglion accumulate, by retrograde intra-axonal transport, proteins synthesized and released by optic tissues in culture.     

A subpopulation of rat dorsal root ganglion neurones is catecholaminergic

The neurotransmitters used by the sensory neurones of the dorsal root ganglia (DRG) are unknown. A proportion of these cells contain physiologically active peptides; for example, subpopulations of small-diameter neurones contain substance P or somatostatin. Although these peptides probably have some influence on synaptic transmission in the dorsal horn of the spinal cord, their status as neurotransmitters is uncertain and it is possible that they coexist with conventional neurotransmitters. In addition, the neurones containing identified peptides account for only a fraction of the DRG sensory neurones. There is evidence that the DRG contain catecholamines within fibres thought to be autonomic, but these substances have not been found within the sensory cell bodies themselves. Moreover, the apparently inappropriate, inhibitory physiological effect of catecholamines in the dorsal horn has argued against their being primary sensory neurotransmitter molecules. We have used here antisera against tyrosine hydroxylase (TH; EC 1.14.16.2) and dopamine-beta-hydroxylase (DBH; EC 1.14.17.1), two enzymes specific to catecholaminergic cells, to show that a subpopulation of rat DRG neurones is catecholaminergic and that the neurotransmitter they make is probably dopamine. We believe this to be the first report of catecholaminergic sensory neurones.     

Acetylcholine hyperpolarizes central neurones by acting on an M2 muscarinic receptor

Acetylcholine (ACh) is considered to act as a neurotransmitter in the mammalian brain by binding to membrane receptors and bringing about a change in neurone excitability. In the case of muscarinic receptors, cell excitability is usually increased; this effect results from a closure of membrane potassium channels in cortical cells. However, some central neurones are inhibited by ACh, and we hypothesized that these two opposite effects of ACh resulted from interactions with different subtypes of muscarinic receptor. We made intracellular recordings from neurones in the rat nucleus parabrachialis, a group of neurones in the upper pons some of which themselves synthesize ACh. ACh and muscarine caused a membrane hyperpolarization which resulted from an increase in the membrane conductance to potassium ions. The muscarinic receptor subtype was characterized by determining the dissociation equilibrium constant (KD) for pirenzepine during the intracellular recording; the value of approximately 600 nM indicates a receptor in the M2 class. This muscarinic receptor is quite different from that which brings about a decrease in potassium conductance in other neurones, which has a pirenzepine KD of approximately 10 nM (M1 receptors). It is possible that antagonists selective for this kind of M2 receptor would be useful in the management of conditions, such as Alzheimer's disease, which are associated with a reduced effectiveness of cholinergic neurones.     

Substance P raises neuronal membrane excitability by reducing inward rectification

Much interest has recently centred on the properties of peptides that modulate the excitability of nerve cells. Such compounds include the undecapeptide substance P, which is particularly well established as an excitatory neurotransmitter, and we examine here its effects on magnocellular cholinergic neurones taken from the medial and ventral aspects of the globus pallidus of newborn rats and grown in dissociated culture. These neurones have previously been shown to respond to substance P3 and are analogous to the nucleus basalis of Meynert in man, which gives a diffuse projection to the cerebral cortex and whose degeneration is the likely cause of Alzheimer's disease. Substance P depolarizes these cultured neurones by reducing an inwardly rectifying potassium conductances; this conductance has been found in several neuronal types and has similar properties to those of certain other cells. As discussed below, modulation of inward (or anomalous) rectification by substance P implies a self-reinforcing element to the depolarization caused by the peptide.     

Age-impaired impulse flow from nucleus basalis to cortex

Recent studies have renewed interest in the role of acetylcholine (ACh) in the cognitive changes associated with ageing and dementia. Deficits in cortical choline acetyltransferase (ChAT) in Alzheimer's disease have been consistently demonstrated, while other research has suggested a connection between deterioration of cortical ACh fibres and dementia. However, despite clear biochemical and anatomical evidence for a fall in ACh in dementia, results of therapeutic trials with cholinergic agonists, precursors and cholinesterase inhibitors have been inconsistent. Such findings suggest that cortical cholinergic disorders are not wholly a function of simple biochemical change; alterations of impulse flow along cholinergic fibres could well be as debilitating. An important extrinsic source of cortical ACh innervation derives from neurones diffusely located in rat basal forebrain, denoted the nucleus basalis (NB). We have now investigated the impulse conduction properties of cortically projecting, putatively cholinergic NB axons in adult and aged rats and have found that conduction latencies from NB to frontal cortex are significantly longer (by 51%) in aged animals. In addition, systematic analysis varying cortical stimulation depth revealed that these longer latencies are due entirely to decreased conduction velocities in the subcortical fibre projections. Indeed, intracortical velocities were virtually identical in the two groups. Our results indicate that ageing occasions a decrease in the temporal fidelity of impulse flow in the cholinergic input to the cortex from the NB, a previously overlooked but potentially important element in cognitive deficits that occur with age.     

Corticotropin regulates transsynaptically the activity of septo-hippocampal cholinergic neurones

The activity of septo-hippocampal neurones is affected by the action on cholinergic perikarya in the septum of a variety of putative neurotransmitters, including substance P and beta-endorphin. (The latter is released in the septal region from neurones which originate in the medial basal hypothalamus.) It has also been reported that two other neuropeptides, corticotropin (ACTH1-24) and alpha-melanotropin (alpha-MSH), affect acetylcholine turnover in septo-hippocampal neurones in a manner that is not blocked by transection of the afferents to the hippocampus, from which it has been inferred that the neurotransmitters act directly on the hippocampus. We now describe experiments with corticotropin which show that the effect is rather the influence on septo-hippocampal cholinergic neurones of peptidergic neurones within the septum.     

Fetal occipital cortical neurones transplanted to the rostral cortex can extend and maintain a pyramidal tract axon

In adult rats, cortical neurones that send axons through the pyramidal tract are confined to layer V, over the rostral two-thirds of the cerebral hemisphere. However, during the first postnatal week, many neurones in layer V in the occipital cortex (including the visual cortex) also extend axon collaterals through the pyramidal tract and into the spinal cord. These occipital corticospinal collaterals are completely eliminated over the subsequent 2 weeks, although their cells of origin do not die. We now report that when portions of the occipital cortex from fetal rats are transplanted to more rostral cortical regions of newborn rats, some of the transplanted neurones not only extend axons through the pyramidal tract, but also maintain these axons beyond the stage at which they are normally eliminated. These results suggest that normally-eliminated cortical axons can be 'rescued' and, in the case of pyramidal tract neurones, the position of the neurones within the tangential plane of the cortex is a critical factor in determining which neurones retain and which lose their pyramidal tract collaterals.     

Antibodies to paired helical filaments in Alzheimer's disease do not recognize normal brain proteins

During ageing of the human brain, and particularly in senile dementia of the Alzheimer type (AD), many neurones progressively accumulate abnormal cytoplasmic fibres, called paired helical filaments (PHF). Each such fibre consists of a pair of intermediate (10 nm) filaments twisted into a double helix with a periodicity of 160 nm. PHF accumulate in large perikaryal masses, called neurofibrillary tangles, and are also found in degenerating cortical neurites that form neurite plaques. The density of PHF-containing neurites and cell bodies correlates with the degree of dementia and the extent of loss of cholinergic neurotransmitter function in AD. Recently, we demonstrated that PHF from human cerebral cortex are large, rigid polymers with unusual molecular properties, including insolubility in SDS, urea and other denaturing solvents and apparent resistance to protease digestion. These properties have so far prevented complete purification and analysis of the constituents of PHF. Based on their insolubility, we have developed a new method of preparing highly enriched PHF fractions and have raised an antiserum that is highly specific for PHF. We report here that this antiserum specifically labels PHF, free of any associated normal fibrous proteins and, unexpectedly, it reacts with neither neurofilaments nor any other normal cytoskeletal protein in brain sections or on immunoblotted gels. These anti-PHF antibodies have been used for the specific detection of Alzheimer-type PHF and in the search for cross-reacting antigens in various tissues, and are suitable for immunoaffinity purification of PHF. Our results indicate that PHF contain determinants that are not shared with normal neuronal fibrous proteins.     

Effect of impulse blockage on cytochrome oxidase activity in monkey visual system

Cytochrome oxidase (cytochrome c oxidase; ferrocytochrome c: oxygen oxidoreductase, EC 1.9.2.1) has been introduced as an oxidative metabolic marker for neurones in the central nervous system. Previous studies have shown that mature neurones remained sensitive to altered functional demands, and that both developing and adult neurones responded to sensory deprivation or deafferentation by reducing their cytochrome oxidase (Cyt. Ox.) activity. More recently, we showed that the blockage of retinal impulse transmission with tetrodotoxin led to a reversible reduction in Cyt. Ox. staining of affected lateral geniculate (LGN) and striate neurones in adult cats. The present study sought to extend these findings to adult monkeys, where Cyt. Ox. 'puffs' or 'blobs' are uniquely present in the visual cortex. We found that, while the retina remained histologically intact, with only moderate decreases in Cyt. Ox. staining of large ganglion cells and the two plexiform layers, subtle changes occurred in the LGN as early as 1 day post-tetrodotoxin injection, and clear reduction in enzyme levels was evident in both the LGN and the visual cortex by 3 days. Changes became progressively more severe up to 4 weeks post-injection. Within area 17, alternating bands of high and low Cyt. Ox. staining occurred in lamina IV, with alternating rows of dark and lightly reactive puffs superimposed in exact register. Thus, the mature visual neurones in the primate remain extremely sensitive to the cessation of retinal impulse transmission, and plastic metabolic changes occur through several synapses along the sensory pathway.     

Neurotransmitter synthesis and uptake by isolated sympathetic neurones in microcultures.

Assays of isolated single sympathetic neurones show that their transmitter functions can be either adrenergic or cholinergic depending on growth conditions. The data suggest that the number of transmitters made by most mature individual neurones is restricted.     

A dopamine- and cyclic AMP-regulated phosphoprotein enriched in dopamine-innervated brain regions

Several mammalian neurotransmitter candidates, for example, serotonin, dopamine and noradrenaline, may exert some of their synaptic effects by regulating protein phosphorylation systems. Comparison of the regional distribution of brain phosphoproteins with neurotransmitter systems may help to identify the specific phosphoproteins involved in the functions of particular neurotransmitters. Here we report the association of one such phosphoprotein with the dopamine pathways in brain. This protein, of apparent molecular weight (MW) 32,000 (32K), seems to be present only in nervous tissue. Its regional distribution within the brain is very similar to the pattern of dopamine-containing nerve terminals; more specifically, the protein appears to be enriched in those dopaminoceptive neurones which possess D-1 receptors (dopamine receptors coupled to adenylate cyclase). The state of phosphorylation of the protein in these dopaminoceptive neurones can be regulated by both dopamine and cyclic AMP. These results suggest that the phosphoprotein may mediate certain of the trans-synaptic effects of dopamine acting on dopaminoceptive neurones.     

Curare can open and block ionic channels associated with cholinergic receptors

Curare has long been regarded as a typical competitive antagonist of acetylcholine (ACh) at the vertebrate neuromuscular junction. Recently, however, it has been shown that curare can also block the channels opened by ACh at the frog neuromuscular junction as well as on rat and Aplysia neurones; moreover, curare is able to depolarize rat myotubes and thus behaves as an agonist for the cholinergic receptor of this preparation (see ref. 6). Using the single channel recording technique, we have now found that, on rat myotubes, curare can both open and block in the same cell the channels controlled by the cholinergic receptor.