Advanced Modeling of Peripheral Neuro-Effector Communication and -Plasticity.

The peripheral nervous system (PNS) plays crucial roles in physiology and disease. Neuro-effector communication and neuroplasticity of the PNS are poorly studied, since suitable models are lacking. The emergence of human pluripotent stem cells (hPSCs) has great promise to resolve this deficit. hPSC-derived PNS neurons, integrated into organ-on-a-chip systems or organoid cultures, allow co-cultures with cells of the local microenvironment to study neuro-effector interactions and to probe mechanisms underlying neuroplasticity.

Angiotensinergic innervation of the kidney: present knowledge and its significance.

Intrarenal neurotransmission implies the co-release of neuropeptides at the neuro-effector junction with direct influence on parameters of kidney function. The presence of an angiotensin (Ang) II-containing phenotype in catecholaminergic postganglionic and sensory fibers of the kidney, based on immunocytological investigations, has only recently been reported. These angiotensinergic fibers display a distinct morphology and intrarenal distribution, suggesting anatomical and functional subspecialization linked to neuronal Ang II-expression. This review discusses the present knowledge concerning these fibers, and their significance for renal physiology and the pathogenesis of hypertension in light of established mechanisms. The data suggest a new role of Ang II as a co-transmitter stimulating renal target cells or modulating nerve traffic from or to the kidney. Neuronal Ang II is likely to be an independent source of intrarenal Ang II. Further physiological experimentation will have to explore the role of the angiotensinergic renal innervation and integrate it into existing concepts.

Neuroeffector apparatus in gastrointestinal smooth muscle organs.

Control of gastrointestinal (GI) movements by enteric motoneurons is critical for orderly processing of food, absorption of nutrients and elimination of wastes. Work over the past several years has suggested that motor neurotransmission is more complicated than simple release of transmitter from nerve terminals and binding of receptors on smooth muscle cells. In fact the 'neuro-effector' junction in the tunica muscularis might consist of synaptic-like connectivity with specialized cells, and contributions from multiple cell types in integrated post-junctional responses. Interstitial cells of Cajal (ICC) were proposed as potential mediators in motor neurotransmission based on reduced post-junctional responses observed in W mutants that have reduced populations of ICC. More recent studies on W mutants have contradicted the original findings, and suggested that ICC may not be significant players in motor neurotransmission. This review examines the evidence for and against the role of ICC in motor neurotransmission and outlines areas for additional investigation that would help further resolve this controversy.

Non-synaptic transmission at autonomic neuroeffector junctions.

Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction is described. The essential features are that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions; muscle effectors are bundles rather than single smooth muscle cells, that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission.

Transmission at autonomic neuroeffector junctions.

For organs innervated by the autonomic nervous system, it is generally held that neuroeffector transmission is achieved by varicosities releasing transmitters some distance from the membranes of target cells. Transmitters are thought to diffuse through the extracellular space and interact with post-junctional receptors that are widely distributed over the cell membranes. This article presents an alternative view, suggesting that transmission can occur at organized neuroeffector contacts, that transmitters interact with restricted pools of specialized junctional receptors, and that many receptors on target cells are not involved in neuroeffector transmission.

Modulator role of neuropeptide Y in human vascular sympathetic neuroeffector junctions.

Reverse transcription polymerase chain reaction (RT-PCR) studies identified the mRNA coding for the Y1 and Y2 receptors in human mammary artery/vein and saphenous vein biopsies. Y1 receptors are expressed in vascular smooth muscles and potentiate the contractile action of sympathetic co-transmitters, adenosine triphosphate (ATP) and noradrenaline (NA); BIBP 3226, a competitive Y1 receptor antagonist, blocked the neuropeptide Y (NPY)-induced modulation. The Y2 receptor is expressed in sympathetic nerves terminals and modulates the pool of sympathetic co-transmitters released at the neuroeffector junction. NPY plays a dual role as a modulator of sympathetic co-transmission; it facilitates vascular smooth muscle reactivity and modulates the presynaptic release of ATP and NA. Sympathetic reflexes regulate human vascular resistance, where NPY plays a modulator role of paramount importance following increased sympathetic discharges, such as stress and vascular disease.

Voltage-gated calcium channels in autonomic neuroeffector transmission.

Calcium influx through voltage-gated calcium channels (VGCCs) is required for neurotransmitter release. Recent research has characterised several pharmacologically and electrophysiologically distinct VGCC subtypes, some of which are involved in neurotransmitter release. Transmitter release from autonomic neurons can be coupled to calcium entry through N-, P/Q- and/or R-type VGCCs; the precise combination of VGCC subtypes appears to vary according to the neurotransmitter, tissue and species. L-type channels rarely appear to be important in autonomic neurotransmitter release. There does not appear to be a general rule regarding the nature of the VGCCs coupled to release of a particular transmitter in different tissues or species. Release of the same neurotransmitter from different populations of neurons often reveals a different pattern of involvement of VGCCs. Transmitters released from the same population of neurons are sometimes coupled to calcium influx through different VGCC subtypes. However, release of transmitters thought to be co-localised within vesicles is coupled to calcium influx through the same VGCCs. The role of VGCC subtypes in transmitter release can be altered by mode of nerve stimulation. Different VGCC subtypes may be coupled to transmitter release at low versus high electrical stimulation frequencies, or in response to potassium depolarization or chemical stimulation. In certain disease processes, voltage-gated calcium channels on autonomic neurons can be targeted; for example antibodies to P/Q-type VGCCs in Lambert-Eaton myasthenic syndrome downregulate VGCCs, thereby inhibiting autonomic neuroeffector transmission.

Dynamic responses of electrically coupled systems.

An identified pair of electrically coupled neurons in the buccal ganglion of the freshwater snail Helisoma trivolvis is an experimentally accessible model of electrical synaptic transmission. In this investigation, electrical synaptic transmission is characterized using sinusoidal frequency (Bode) responses computed by Laplace transforms and responses to brief stimuli. The frequency response of the injected neuron shows a 20-dB/decade attenuation and a phase shift from 0 degree at low frequencies to -90 degrees at high frequencies. The response of a coupled cell shows a 40-dB/decade attenuation and a phase shift from 0 degrees at low frequencies to -180 degrees at high frequencies. A simple mathematical model of electrical synaptic transmission is described that displays each of these crucial features of the measured frequency responses. Methods are described to estimate the frequency responses of coupled systems based on presynaptic measurements. The responses of the coupled system to brief pulses of current were computed using the principle of superposition. The electrical properties of coupled systems impose a minimum delay in reaching a peak in all postsynaptic responses. The delays in the postsynaptic responses to brief stimuli are related to the electrical and anatomical parameters of coupled networks.

Modulation of peripheral sympathetic nerve transmission.

The past 15 years have been witness to a remarkable growth in knowledge regarding the modulation of "sympathetic traffic" to neuroeffector organs, including vascular tissue. The release of norepinephrine from peripheral sympathetic neurons is now known to be under both negative and positive feedback control. Norepinephrine, when released from peripheral neurons, acts on presynaptic alpha 2-receptors to inhibit further neurotransmission. Vascular postsynaptic alpha 2-receptors, sensitive to circulating catecholamines, subserve vasoconstriction. The antihypertensive agents clonidine, guanabenz and guanfacin likely reduce blood pressure by acting centrally on alpha 2 postsynaptic neurons to limit sympathetic transmission to blood vessels. Clonidine can produce venoconstriction and thereby improve orthostatic hypotension by activating venous alpha 2-receptors. Additional presynaptic dopaminergic receptors (DA2), muscarinic receptors (acetylcholine), opioid receptors, prostaglandin receptors, adenosine receptors (A1) and histamine (H2) receptors are present on sympathetic nerve membranes and, when engaged with the appropriate ligand, can limit the exocytotic process. Gamma-aminobutyric acid and serotonin demonstrate similar roles in reducing sympathetic nerve activity. In contrast to these inhibitory presynaptic mechanisms, facilitation of norepinephrine release appears to occur by way of neuronal angiotensin II receptor activation and perhaps through stimulation of sympathetic nerve membrane beta 2-receptors. An appreciation of these inhibitory and facilitator mechanisms is useful in the treatment of a variety of clinical conditions, including hypertension, heart failure, orthostatic hypotension, septic shock and a number of common withdrawal syndromes.

Peptides and vasomotor mechanisms.

The multiple and diverse roles played by neuropeptide Y, vasoactive intestinal polypeptide, substance P, calcitonin gene-related peptide and other biologically active peptides in the cardiovascular system are considered. A model of the vascular neuroeffector junction is described, which illustrates the interactions of peptidergic and nonpeptidergic transmitters that are possible at pre- and postjunctional sites. The effects of peptides on specific endothelial receptors are also described, which highlights the ability of these agents to act as dual regulators of vascular tone at both adventitial and intimal surfaces, following local release from nerves, or from endothelial cells themselves. Changes in expression of vascular neuropeptides that occur during development and aging in some disease situations and following nerve lesion are discussed.

Brain macrophages: neurotoxic or neurotrophic effector cells?

The development of the central nervous system and various pathological contexts imply remodeling or alteration of neuronal networks associated with tissue recruitment of mononuclear phagocytes. Purification and culture of brain macrophages have provided a tool for investigating the functions of these cells. We discuss different mechanisms whereby macrophages could directly influence the survival of neurons and the growth of their processes.

Neuroeffector connections of giant multimodal neurons in the African snail Achatina fulica.

A new method of making preparations was used to analyse the neuroeffector connections of the paired giant neurons of the African snail Achatina fulica. These neurons were found to induce postsynaptic potentials in the muscles of the mantle, heart, the wall of the pulmonary cavity, and the muscular elements of the renal complex, the pericardium, the sexual apparatus, the walls of the cerebral arteries, the filaments of the columellar muscles, the wall of the abdomen, and the tentacle retractor muscles. Rhythmic neuron activity led to the development of marked facilitation and long-term potentiation of synaptic potentials. The possible significance of the multiple neuroeffector connections of giant neurons is discussed.

Autonomic neuroeffector junctions--reflex vasodilatation of the skin.

A general model of the autonomic neuroeffector junction is proposed. In this model, emphasis is placed on the muscle effector bundle with electrotonic coupling between individual cells via gap junctions (or nexuses) and en passage release of transmitter from autonomic nerve varicosities. This release results in transmission to effector cells across junctional clefts ranging from about 20 nm in the vas deferens and iris to as much as 2000 nm in some large arteries. The ultrastructural identification of different autonomic nerve types is described. Current theories on the synthesis, storage, release, and inactivation of transmitter during cholinergic, adrenergic, and purinergic transmission are summarized. Some speculations are made about the possible involvement of purinergic nerves in the innervation of vessels and mast cells in the skin, and whether this involvement results in a functional link between ATP, histamine, bradykinin, and prostaglandin in cutaneous vasodilatation. Another possibility considered as the basis for this reflex is the release of substance P from sensory (pain) nerve collaterals in the skin.

Arterial neuroeffector responses in early and mature spontaneously hypertensive rats.

Intramural sympathetic neuroeffector responses and presynaptic regulation of neurotransmission by amine uptake and alpha 2-adrenergic receptors were examined in young (5-week-old) and mature (12-week-old) spontaneously hypertensive rats (SHR) and were compared with those of age-matched Wistar-Kyoto (WKY) control rats. Electrical field stimulation (20 V, 0.2-msec pulse width, 3-second pulse train each minute, 5-100 Hz) elicited contractile responses from isolated mesenteric arteries mounted in a myograph. There was a significant difference between the sensitivity of arteries to electrical field stimulation in the two age groups, with arteries from 12-week-old rats being more sensitive than arteries from 5-week-old animals. Also, there was a significant age-strain interaction: the sensitivity of arteries from SHR to electrical field stimulation increased dramatically with age compared with that of WKY rat arteries. Cocaine significantly increased the sensitivity to electrical field stimulation after inhibition of presynaptic alpha 2-adrenergic receptors, and had a significantly greater effect in arteries from 5-week-old SHR compared with WKY controls. This would reflect an overactive neuronal amine uptake mechanism in young SHR. At 12 weeks there was no significant interstrain difference in the effect of cocaine. Yohimbine increased the sensitivity to electrical field stimulation both before and after inhibition of neuronal amine uptake, but there was no difference in its effect with age or strain. Therefore, although sensitivity to sympathetic nerve stimulation varies with age in the SHR, there is no evidence that this can be ascribed to alpha 2-adrenergic receptor function.

Action of the polyamine beta-philanthotoxin on neuromuscular transmission in insects.

The beta-fraction of the venom of the solitary wasp Philanthus triangulum inhibited glutamate-induced potentials in muscle fibres of the locust in a dose-dependent manner (50% block at 2 micrograms/ml). Single channel behaviour and miniature excitatory postsynaptic currents were not affected. The blocking effect disappeared after treatment of the muscle with the lectin concanavalin A which prevents desensitization of glutamate receptors. Block of potentials induced by iontophoretic application of glutamate was strongly dependent on the frequency of stimulation: no block occurred at stimulus intervals of 30 sec or more. The nature of this inhibition is discussed and proposed to be a delay of recovery from desensitization.

A functional study of the development of the cardiac sympathetic neuroeffector junction in the SHR.

We studied the function of the cardiac sympathetic nerve varicosity in isolated right atrial preparations of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 4, 9, 14, 20 and 50 weeks of age. Cumulative concentration-atrial period response (C-R) curves to isoprenaline showed similar maximum response and sensitivity (EC50) at all ages but there was an age-related fall in resting atrial rate. Similar results were found for methoxamine (alpha 1-adrenoceptor agonist) although the maximum response was significantly less than for isoprenaline. The time-dependent recovery (T 1/2) of the fall in atrial period in response to sympathetic nerve stimulation from electrical field pulses (1-32 at 1 Hz) was enhanced by neuronal uptake inhibition by desipramine (0.1-1 mumol/l), to a similar degree at all ages. Pre-junctional alpha 2-adrenoceptor stimulation by clonidine caused progressively more inhibition of the number of field pulses-fall in period relationship with age. SHR atria were similar to WKY rat atria at all ages except for a further impairment of the development of pre-junctional alpha 2-adrenoceptors. These studies indicate that the function of the cardiac sympathetic varicosity matures early (by 4 weeks) and overall there is very little impairment in SHR versus WKY rat atria.

On the factors which determine the time-courses of junction potentials in the guinea-pig vas deferens.

The time-courses of junction potentials and junction currents at the sympathetic neuroeffector junction of guinea-pig vas deferens were investigated using simultaneous intracellular and focal extracellular recording. Excitatory junction potentials produced in the smooth muscle cells following electrical stimulation of the sympathetic nerves had duration of 600-1000 ms and were prolonged in time-course compared with the underlying excitatory junction currents which had durations of 40-150 ms. The time-course of the excitatory junction potential could be predicted accurately from the time-course of the underlying excitatory junction current by assuming that the smooth muscle cells were isopotential during the excitatory junction potential. In contrast, the time-course of the spontaneous excitatory junction potential, produced by the action of a single quantum of transmitter, was identical to the time-course of the underlying spontaneous excitatory junction current, both events having durations of 40-150 ms. Local application of 10(-4) M adenosine 5'-triphosphate in the vicinity of an intracellular microelectrode produced depolarizations ("adenosine 5'-triphosphate potentials") of durations ranging between 400 and 2500 ms. When adenosine 5'-triphosphate was applied close to the rim of an extracellular electrode, negative-going signals ("adenosine 5'-triphosphate currents") were produced which reflected the time-course of postjunctional membrane currents evoked by adenosine 5'-triphosphate. Adenosine 5'-triphosphate currents were abolished by alpha, beta-methylene adenosine 5'-triphosphate (10(-6) M) but were unaffected by tetrodotoxin (3 x 10(-7) M or 10(-6) M) and the alpha-adrenoceptor blockers prazosin (10(-6) M) and phentolamine (10(-6) M). The time-courses of adenosine 5'-triphosphate currents were similar to the time-courses of excitatory junction currents recorded in the same preparation. Adenosine 5'-triphosphate currents were generally brief compared with simultaneously recorded adenosine 5'-triphosphate potentials which had durations or greater than or equal to 1000 ms. The time-courses of relatively brief adenosine 5'-triphosphate potentials (duration less than or equal to 800 ms) followed the time-courses of the underlying adenosine 5'-triphosphate currents. These results indicate that the time-course of decay of the excitatory junction potential in the guinea-pig vas deferens is determined mainly by the passive membrane properties of the smooth muscle cells, the duration of underlying transmitter action being brief. However, the factors determining the time-course of adenosine 5'-triphosphate potentials are less clear, and may include passive membrane properties and diffusion of locally applied adenosine 5'-triphosphate.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiology of neuroeffector transmission in the isolated, innervated trachea of the guinea-pig.

Intracellular recordings were made from cells of the guinea-pig trachealis muscle. Some cells were electrically quiescent while others exhibited spontaneous slow waves. In quiescent cells, stimulation of the cervical vagus nerve evoked transient depolarization. Occasionally there was a single depolarization, but more often there were several fluctuations in potential. In spontaneously active cells, vagal stimulation induced a transient increase in amplitude of the slow waves without affecting their frequency. Depolarizing responses could be obtained with a single pulse applied to the vagus nerve, and responses increased in amplitude with number of pulses (up to 16 pulses), and with frequency of stimulation (up to 20 Hz). Depolarization did not give rise to spike discharge. Responses to vagal stimulation were blocked by atropine. In the presence of neostigmine, vagally-mediated depolarization was augmented and abortive spikes were observed in a number of cells. In quiescent cells, repetitive stimulation of the sympathetic stellate ganglion evoked slight hyperpolarization. In spontaneously active cells, sympathetic stimulation evoked attenuation, or temporary cessation of slow wave discharge, with or without hyperpolarization. Sympathetic-induced hyperpolarization and suppression of slow waves were both blocked by propranolol, but unaffected by phentolamine. Electrical changes associated with sympathetic stimulation may be of minor importance in the initiation of relaxation.

An examination of the pre- and postsynaptic alpha-adrenoceptors involved in neuroeffector transmission in rabbit aorta and portal vein.

1 The alpha-adrenoceptor agonists, clonidine and xylazine, reduced and the alpha-antagonists, yohimbine an rauwolscine, increased the stimulation-evoked tritium overflow from rabbit aorta and portal vein pre-incubated with [3H]-noradrenaline. 2 Based on an order of agonist potency of clonidine greater than xylazine greater than phenylephrine and antagonist potency of rauwolscine = yohimbine greater than prazosin, the presynaptic receptor mediating these effects is of the alpha 2 type. 3 In the aorta, stimulation-evoked contractions were abolished by prazosin (0.1 micrometers) and potentiated by rauwolscine and yohimbine in concentrations that increased the stimulation-evoked overflow tritium. 4 In the portal vein, prazosin was less potent in reducing, and rauwolscine and yohimbine failed to potentiate, the stimulation-evoked contraction. 5 In experiments in which tissues were pre-exposed to phenoxybenzamine (30 nM) to block some of the postsynaptic alpha-receptors, rauwolscine in concentrations that increased stimulation-evoked tritium overflow, reduced the evoked contraction in the portal vein but not in the aorta. 6 It is concluded that presynaptic alpha 2-autoreceptors are present in both tissues and that the postsynaptic alpha-receptors which mediate nerve stimulation-evoked contractions are alpha 1 in the aorta but a mixture of alpha 1 and alpha 2 in the portal vein.

Relaxation of smooth muscle following contraction elicited by sympathetic nerve stimulation in vivo.

1. A method for studying in vivo the process of neuroeffector transmission in the nictitating membrane and nasal blood vessels of the cat is described. 2. Administration of desmethylimipramine or cocaine caused increases in both the amplitude and duration of the nasal and membrane responses which may be explained by inhibition of neuronal uptake of noradrenaline. 3. Phenoxybenzamine depressed the responses to nerve stimulation, but had little effect on the relationship between response amplitude and rate of recovery. 4. The relationship between response amplitude and rate of recovery is discussed and related to the sigmoid shape of a log concentration-response curve.