Joel C. Bornstein

Alzheimer's disease and Aβ toxicity: from top to bottom

Evidence implicating the β-amyloid protein (Aβ) in the pathogenesis of Alzheimers disease has steadily accumulated. However, the mechanism by which Aβ causes dementia is unknown. Here we argue that a more integrated, top–down approach to brain function is needed to assess the role of Aβ in Alzheimers disease, and that more attention should be paid to the effects of Aβ on synaptic function rather than on cell death.

INTRINSIC PRIMARY AFFERENT NEURONS OF THE INTESTINE

After a long period of inconclusive observations, the intrinsic primary afferent neurons of the intestine have been identified. The intestine is thus equipped with two groups of afferent neurons, those with cell bodies in cranial and dorsal root ganglia, and these recently identified afferent neurons with cell bodies in the wall of the intestine. The first, tentative, identification of intrinsic primary afferent neurons was by their morphology, which is type II in the terminology of Dogiel. These are multipolar neurons, with some axons that project to other nerve cells in the intestine and other axons that project to the mucosa. Definitive identification came only recently when action potentials were recorded intracellularly from Dogiel type II neurons in response to chemicals applied to the lumenal surface of the intestine and in response to tension in the muscle. These action potentials persisted after all synaptic transmission was blocked, proving the Dogiel type II neurons to be primary afferent neurons. Less direct evidence indicates that intrinsic primary afferent neurons that respond to mechanical stimulation of the mucosal lining are also Dogiel type II neurons. Electrophysiologically, the Dogiel type II neurons are referred to as AH neurons. They exhibit broad action potentials that are followed by early and late afterhyperpolarizing potentials. The intrinsic primary afferent neurons connect with each other at synapses where they transmit via slow excitatory postsynaptic potentials, that last for tens of seconds. Thus the intrinsic primary afferent neurons form self-reinforcing networks. The slow excitatory postsynaptic potentials counteract the late afterhyperpolarizing potentials, thereby increasing the period during which the cells can fire action potentials at high rates. Intrinsic primary afferent neurons transmit to second order neurons (interneurons and motor neurons) via both slow and fast excitatory postsynaptic potentials. Excitation of the intrinsic primary afferent neurons by lumenal chemicals or mechanical stimulation of the mucosa appears to be indirect, via the release of active compounds from endocrine cells in the epithelium. Stretch-induced activation of the intrinsic primary afferent neurons is at least partly dependent on tension generation in smooth muscle, that is itself sensitive to stretch. The intrinsic primary afferent neurons of the intestine are the only vertebrate primary afferent neurons so far identified with cell bodies in a peripheral organ. They are multipolar and receive synapses on their cell bodies, unlike cranial and spinal primary afferent neurons. They communicate with each other via slow excitatory synaptic potentials in self reinforcing networks and with interneurons and motor neurons via both fast and slow EPSPs.

Projections and chemical coding of neurons with immunoreactivity for nitric oxide synthase in the guinea-pig small intestine

The distribution of nitric oxide synthase (NOS) immunoreactivity was investigated in the guinea-pig small intestine. There were many immunoreactive nerve cell bodies in the myenteric plexus but very few in submucous ganglia. NOS immunoreactivity was not found in non-neuronal cells except for rare mucosal endocrine cells. Abundant immunoreactive nerve fibres in both myenteric and submucous ganglia, and in the circular muscle, arose from myenteric nerve cells whose axons projected anally along the intestine. NOS immunoreactivity coexisted with VIP-immunoreactivity, but not with substance P immunoreactivity. We conclude that nitric oxide synthase is located in a sub-population of enteric neurons, amongst which are inhibitory motor neurons that supply the circular muscle layer.

Plurichemical transmission and chemical coding of neurons in the digestive tract.

The enteric nervous system contains neurons with well-defined functions. However, when neurons of the same function are examined in different regions or species, they are found to show subtle differences in their pharmacologies of transmission and different chemical coding. Individual enteric neurons use more than one transmitter, i.e., transmission is plurichemical. For example, enteric inhibitory neurons have three or more primary transmitters, including nitric oxide, vasoactive intestinal peptide, and possibly adenosine triphosphate and pituitary adenylyl cyclase activating peptide. Primary transmitters are highly conserved, although their relative roles vary considerably between gut regions. Multiple substances, including transmitters and their synthesizing enzymes and nontransmitters (such as neurofilament proteins), provide neurons with a chemical coding through which their functions and projections can be identified. Although equivalent neurons in different regions have the same primary transmitters, other chemical markers differ substantially. Caution must be taken in extrapolating pharmacological and neurochemical observations between species or even between regions in the one species. On the other hand, careful interregion and interspecies comparisons lead to an understanding of the features of enteric neurons that are highly conserved and can be used in valid extrapolation.

Intracellular recording from myenteric neurons of the guinea-pig ileum that respond to stretch.

1 Isolated longitudinal muscle‐myenteric plexus preparations from guinea‐pig ileum were used to investigate the activity of myenteric neurons when the tissue was stretched in the circumferential direction. Membrane potentials were recorded via flexibly mounted intracellular recording electrodes containing Neurobiotin in 1 M KCl. The preparations were stretched to constant widths (+20 % and +40 % beyond slack width). 2 Multipolar neurons (Dogiel type II morphology) discharged spontaneous action potentials and proximal process potentials during maintained stretching, three of twenty‐one at +20 % stretch and seven of nine at +40 % stretch. At the maximum extent of stretch tried, +40 % beyond slack tissue width, action potentials in Dogiel type II neurons occurred at 10‐33 Hz. Neurons with other morphologies were all uniaxonal. Some displayed spontaneous fast EPSPs or action potentials, three of forty‐one at +20 % stretch and seven of nineteen at +40 % stretch. 3 In seven of eight Dogiel type II neurons, action potentials or proximal process potentials persisted when membrane hyperpolarization was imposed via the recording electrode. Action potential discharge was abolished by hyperpolarization in seven of nine uniaxonal neurons; the exceptions were two orally projecting neurons. 4 Dogiel type II and uniaxonal neurons were classified as rapidly accommodating if they discharged action potentials only at the beginning of a 500 ms intracellular depolarizing pulse and slowly accommodating if they discharged for more than 250 ms. For Dogiel type II neurons, three of thirteen were slowly accommodating at +20 % stretch and two of four at 40 % stretch. For uniaxonal neurons the corresponding data were twelve of twenty‐six and fifteen of nineteen neurons. The slowly accommodating state was associated with increased cell input resistance in uniaxonal neurons. 5 The spontaneous action potential discharge in Dogiel type II and uniaxonal neurons ceased when the muscle was relaxed pharmacologically by nicardipine (3 μM) or isoprenaline (1 μM), although the applied stretch was maintained. At the same time, evoked spike discharge became rapidly accommodating 6 We conclude that many Dogiel type II neurons, and possibly some orally projecting uniaxonal neurons, are intrinsic, stretch‐sensitive, primary afferent neurons that respond to muscle tension with sustained action potential discharge.

Roles of peptides in transmission in the enteric nervous system.

Studies of the enteric nervous system have proved to be important in the development of new concepts of the chemical nature of transmission from neurons. In particular, they have revealed the multiplicity of influences that peptides can have on transmission, such as their action as primary transmitters, and the fact that they often act as co-transmitters in enteric neurons. However, in other cases no roles can be attributed to neuropeptides in enteric neurons, and their involvement in short-term changes in excitability seems minor.

Identification of sensory nerve cells in a peripheral organ (the intestine) of a mammal

It is commonly believed that the cell bodies of mammalian sensory neurons are contained within spinal and cranial sensory ganglia associated with the central nervous system or within the central nervous system itself. However, strong circumstantial evidence implies that some sensory neurons are contained entirely within the gastrointestinal tract. We have investigated this possibility by using intracellular methods to record the responses of myenteric neurons in the guinea-pig small intestine to physiological stimuli applied to the neighbouring mucosa. The results show that the myenteric plexus contains a population of chemosensitive sensory neurons and that these neurons correspond to neurons with AH electrophysiological properties and Dogiel type II morphology. This is the first direct evidence that some sensory neurons are contained entirely within the peripheral nervous system.

Enteric motor and interneuronal circuits controlling motility.

The enteric nervous system regulates intestinal motility. It contains intrinsic sensory neurones, several types of interneurones and excitatory and inhibitory motor neurones. This review summarizes our knowledge of motor neurones and interneurones in simple motility reflex pathways (ascending and descending excitation, descending inhibition) and it focuses on guinea‐pig ileum. Excitatory circular muscle motor neurones contain choline acetyltransferase (ChAT) and tachykinins and project orally 0.5–10 mm. They transmit via muscarinic acetylcholine receptors and tachykinins acting at NK1 and NK2 receptors. Inhibitory circular muscle motor neurones contain nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase activating peptide (PACAP), project anally up to 25 mm and transmit via ATP, nitric oxide and/or VIP. Ascending interneurones project up to 10 mm orally and contain ChAT and tachykinins. They transmit to each other via ACh at nicotinic receptors (nAChR), but to excitatory motor neurones via both nAChR and NK3 receptors. There are at least three types of descending interneurones and one transmits to inhibitory motor neurones via ATP acting at P2X receptors. NOS‐containing descending interneurones receive input via P2Y receptors from other interneurones. Transmission to and from the other descending interneurones (ChAT/5‐HT, ChAT/somatostatin) is yet to be characterized.

Electrophysiology of guinea-pig myenteric neurons correlated with immunoreactivity for calcium binding proteins

Experiments were undertaken to define the electrophysiological characteristics and shapes of neurons in the myenteric plexus of the guinea-pig ileum that are immunoreactive for calcium binding proteins. Recordings were made from the neurons with intracellular microelectrodes containing a mixture of the fluorescent dye Lucifer yellow and KCl solution. The neurons studied were filled with Lucifer yellow so that they could be re-identified after processing the tissue to reveal immunoreactivity for either the calcium binding protein (CaBP), spot 35 protein, or vitamin D-dependent CaBP. Neurons were characterized as being AH-neurons, in which each action potential is followed by a prolonged after-hyperpolarization (greater than 4 s), or S-neurons, in which the prolonged after-hyperpolarizations were not observed and focal stimulation of internodal strands evoked fast excitatory synaptic potentials. S-neurons were never immunoreactive for the CaBPs (108 cells), but most AH-neurons (62 of 74) were immunoreactive. Immunoreactive and non-immunoreactive AH-neurons were indistinguishable on the basis of their electrophysiological properties or their shapes (all the AH-neurons were Dogiel type II in shape, i.e. smooth soma and many long processes). The S-neurons had a variety of shapes, but none could be classified as Dogiel type II. It is concluded that most AH-neurons are immunoreactive for calcium binding proteins, and that these proteins are restricted to AH-neurons.

The terminals of myenteric intrinsic primary afferent neurons of the guinea-pig ileum are excited by 5-hydroxytryptamine acting at 5-hydroxytryptamine-3 receptors

The aim of this study was to identify the receptor type(s) by which 5-hydroxytryptamine applied to the intestinal mucosa excites the terminals of myenteric AH neurons. The AH neurons have been identified as the intrinsic primary afferent (sensory) neurons in guinea-pig small intestine and 5-hydroxytryptamine has been identified as a possible intermediate in the sensory transduction process. Intracellular recordings were taken from AH neurons located within 1mm of intact mucosa to which 5-hydroxytryptamine was applied. Trains of action potentials and/or slow depolarizing responses were recorded in AH neurons in response to mucosal application of 5-hydroxytryptamine (10 or 20microM) or the 5-hydroxytryptamine-3 receptor agonist, 2-methyl-5-hydroxytryptamine (1 or 3mM), and to electrical stimulation of the mucosa. The 5-hydroxytryptamine-2 receptor agonist, alpha-methyl-5-hydroxytryptamine (100microM), and the 5-hydroxytryptamine-1,2,4 receptor agonist, 5-methoxytryptamine (10microM), did not elicit such responses. The 5-hydroxytryptamine-3 receptor-selective antagonist, granisetron (typically 1microM), and the 5-hydroxytryptamine-3,4 receptor antagonist, tropisetron (typically 1microM), each reduced or abolished the responses to 5-hydroxytryptamine, while the selective 5-hydroxytryptamine-4 receptor antagonist, SB 204070 (1microM), did not. It is concluded that application of 5-hydroxytryptamine to the mucosa activates a 5-hydroxytryptamine-3 receptor that triggers action potential generation in the mucosal nerve terminals of myenteric AH neurons.