Frequency dependent intermittency and ionic basis of impulse conduction in postganglionic sympathetic fibres of guinea-pig vas deferens

Sex and chronic stress differentially alter phosphorylated mu and delta opioid receptor levels in the rat hippocampus following oxycodone conditioned place preference

Following oxycodone conditioned place preference (CPP) in naïve female and male Sprague Dawley rats, delta- and mu-opioid receptors (DORs and MORs) redistribute in hippocampal CA3 pyramidal cells and GABAergic interneurons in a manner that would promote opioid-associative learning processes, particularly in females. MORs and DORs similarly redistribute in CA3 and hilar neurons following chronic immobilization stress (CIS) in females, but not males, essentially "priming" the opioid system for oxycodone-associative learning. Following CIS, only females acquire oxycodone CPP. The present study determined whether sex and CIS differentially affect the levels of phosphorylated MORs and DORs (pMORs and pDORs) in the hippocampus following oxycodone CPP as phosphorylation is important for opioid receptor internationalization and trafficking. In naïve oxycodone-injected (Oxy) female rats, the density of pMOR-immunoreactivity (ir) was increased in CA1 stratum oriens and CA3a,b strata lucidum and radiatum compared to saline-injected (Sal)-females. Additionally, the density of pDOR-ir increased in the pyramidal cell layer and stratum radiatum of CA2/3a in Oxy-males compared to Sal-males. In CIS females that acquire CPP, pDOR-ir levels were increased in the CA2/3a. These findings indicate only rats that acquire oxycodone CPP have activated MORs and DORs in the hippocampus but that the subregion containing activated opioid receptors differs in females and males. These results are consistent with previously observed sex differences in the hippocampal opioid system following Oxy-CPP.

The influences of reproductive status and acute stress on the levels of phosphorylated mu opioid receptor immunoreactivity in rat hippocampus

Opioids play a critical role in hippocampally dependent behavior and plasticity. In the hippocampal formation, mu opioid receptors (MOR) are prominent in parvalbumin (PARV) containing interneurons. Previously we found that gonadal hormones modulate the trafficking of MORs in PARV interneurons. Although sex differences in response to stress are well documented, the point at which opioids, sex and stress interact to influence hippocampal function remains elusive. Thus, we used quantitative immunocytochemistry in combination with light and electron microscopy for the phosphorylated MOR at the SER375 carboxy-terminal residue (pMOR) in male and female rats to assess these interactions. In both sexes, pMOR-immunoreactivity (ir) was prominent in axons and terminals and in a few neuronal somata and dendrites, some of which contained PARV in the mossy fiber pathway region of the dentate gyrus (DG) hilus and CA3 stratum lucidum. In unstressed rats, the levels of pMOR-ir in the DG or CA3 were not affected by sex or estrous cycle stage. However, immediately following 30 minutes of acute immobilization stress (AIS), males had higher levels of pMOR-ir whereas females at proestrus and estrus (high estrogen stages) had lower levels of pMOR-ir within the DG. In contrast, the number and types of neuronal profiles with pMOR-ir were not altered by AIS in either males or proestrus females. These data demonstrate that although gonadal steroids do not affect pMOR levels at resting conditions, they are differentially activated both pre- and post-synaptic MORs following stress. These interactions may contribute to the reported sex differences in hippocampally dependent behaviors in stressed animals.

Cellular and subcellular localization of androgen receptor immunoreactivity relative to C1 adrenergic neurons in the rostral ventrolateral medulla of male and female rats

In male and female rats, high androgen levels can increase blood pressure. The C1 area of the rostral ventrolateral medulla (RVLM), which is crucial for blood pressure regulation, contains estrogen receptors (ERs) in pre- and postsynaptic neuronal compartments and is modulated by estrogens (Wang et al. [2006] Brain Res 1094:163-178). In this study, the cellular and subcellular localization of androgen receptors (ARs) in the C1 area was examined in sections from male, proestrus (high estrogen) and diestrus (low estrogen) female rat brains that were immunocytochemically labeled for AR and tyrosine hydroxylase (TH). By light and electron microscopy, AR-labeled nuclei were scattered among TH-labeled somata in the RVLM; significantly more AR-labeled nuclei were seen males compared to females. Electron microscopy revealed that extranuclear AR-immunoreactivity (ir) was in similar profile types in male and female rats. AR-ir was almost exclusively in myelinated and unmyelinated axons and in glia. Rarely, AR-ir was in axon terminals that contacted TH-containing dendrites. AR-labeled axon terminals had large diameters and contained numerous dense-core vesicles, resembling peptide-containing hypothalamic or solitary tract inputs. No nuclear or extranuclear AR-ir was found in TH-labeled perikarya and dendrites although a few non-TH- labeled dendrites contained AR-ir. Qualitatively, more axonal profiles appeared to be present in males compared to females. These studies suggest that, unlike ERs, ARs in male and female rats are almost exclusively positioned on afferents and glia, suggesting that androgens modulate RVLM C1 neurons, and thus blood pressure, through presynaptic and glial signaling.

Ultrastructural localization of estrogen receptor beta immunoreactivity in the rat hippocampal formation

Several lines of evidence indicate that estrogen affects hippocampal synaptic plasticity through rapid nongenomic mechanisms, possibly by binding to plasma membrane estrogen receptors (ERs). We have previously shown that ERalpha immunoreactivity (ir) is in select interneuron nuclei and in several extranuclear locations, including dendritic spines and axon terminals, within the rat hippocampal formation (Milner et al., [2001] J Comp Neurol 429:355). The present study sought to determine the cellular and subcellular locations of ERbeta-ir. Coronal hippocampal sections from diestrus rats were immunolabeled with antibodies to ERbeta and examined by light and electron microscopy. By light microscopy, ERbeta-ir was primarily in the perikarya and proximal dendrites of pyramidal and granule cells. ERbeta-ir was also in a few nonprincipal cells and scattered nuclei in the ventral subiculum and CA3 region. Ultrastructural analysis revealed ERbeta-ir at several extranuclear sites in all hippocampal subregions. ERbeta-ir was affiliated with cytoplasmic organelles, especially endomembranes and mitochondria, and with plasma membranes primarily of principal cell perikarya and proximal dendrites. ERbeta-ir was in dendritic spines, many arising from pyramidal and granule cell dendrites. In both dendritic shafts and spines, ERbeta-ir was near the perisynaptic zone adjacent to synapses formed by unlabeled terminals. ERbeta-ir was in preterminal axons and axon terminals, associated with clusters of small, synaptic vesicles. ERbeta-labeled terminals formed both asymmetric and symmetric synapses with dendrites. ERbeta-ir also was detected in glial profiles. The cellular and subcellular localization of ERbeta-ir was generally similar to that of ERalpha, except that ERbeta was more extensively found at extranuclear sites. These results suggest that ERbeta may serve primarily as a nongenomic transducer of estrogen actions in the hippocampal formation.

Effects of Heptanol on Neurogenic Contractions of Vas Deferens: A Comparative Study of Stimulation Frequency in Guinea Pig and Rat

This study examines the role of gap junctional communication in smooth muscle in relation to the frequency of stimulation and the innervation density of the tissue in the generation of neurogenic contractions. Toward this end the effects of heptanol, a gap junctional blocker, on the neurogenic contractions of guinea pig and rat vas deferens at different frequencies of stimulation (single pulse, 5, 10, 20, 40, 60, and 80 Hz) were studied. In both the prostatic and epididymal halves of these tissues, heptanol abolished the neurogenic contractions at the lower frequencies of stimulation. At higher frequencies, contractions were resistant to heptanol action. The effect of heptanol on the neurogenic contractions was found to decrease with increasing stimulation frequency. The neurogenic contractions of rat vas deferens were more resistant to heptanol than those of guinea pig vas deferens. Our data indicate that gap junctional communication is significant in the generation of neurogenic contractions in both guinea pig and rat vas deferens in a frequency-dependent manner, and we discuss the mechanisms underlying these findings.

Dynamics of fast synaptic excitation during trains of stimulation in myenteric neurons of guinea-pig ileum

Fast excitatory postsynaptic potentials (fEPSPs) occur in bursts in the myenteric plexus during evoked motor reflexes in the guinea-pig ileum in vitro. This study used electrophysiological methods to study fEPSPs during stimulus trains to mimic bursts of synaptic activity in vitro. The amplitude of fEPSPs or fast excitatory postsynaptic currents (EPSCs) declined (rundown) during stimulus trains at frequencies of 0.5, 5, 10 and 20 Hz. At 0.5 Hz, fEPSP or fEPSC amplitude declined by 50% after the first stimulus but remained constant for the remainder of the train. At 5, 10 and 20 Hz, synaptic responses ran down completely with time constants of 0.35, 0.21 and 0.11 s, respectively. Recovery from rundown occurred with a time constant of 7 s. Mecamylamine, a nicotinic cholinergic receptor antagonist, or PPADS, a P2X receptor antagonist, reduced fEPSP amplitude, but they had no effect on rundown. Responses caused by trains of ionophoretically applied ATP or ACh (to mimic fEPSPs) did not rundown. Blockade of presynaptic inhibitory muscarinic, adenosine A1, opioid, alpha2-adrenergic and 5-HT1A receptors or pertussis toxin (PTX) treatment did not alter rundown. Antidromic action potentials followed a 10-Hz stimulus train. Iberiotoxin (100 nM), a blocker of large conductance calcium activated K+ (BK) channels, did not alter rundown. These data suggest that synaptic rundown is not due to: (a) action potential failure; (b) nicotinic or P2X receptor desensitization; (c) presynaptic inhibition mediated by pertussis-toxin sensitive G-proteins, or (d) BK channel activation. Synaptic rundown is likely due to depletion of a readily releasable pool (RRP) of neurotransmitter.

Ultrastructural evidence that androgen receptors are located at extranuclear sites in the rat hippocampal formation

Like estrogens in female rats, androgens can affect dendritic spine density in the CA1 subfield of the male rat hippocampus [J Neurosci 23:1588 (2003)]. Previous light microscopic studies have shown that androgen receptors (ARs) are present in the nuclei of CA1 pyramidal cells. However, androgens may also exert their effects through rapid non-genomic mechanisms, possibly by binding to membranes. Thus, to investigate whether ARs are at potential extranuclear sites of ARs, antibodies to ARs were localized by light and electron microscopy in the male rat hippocampal formation. By light microscopy, AR immunoreactivity (-ir) was found in CA1 pyramidal cell nuclei and in disperse, punctate processes that were most dense in the pyramidal cell layer. Additionally, diffuse AR-ir was found in the mossy fiber pathway. Ultrastructural analysis revealed AR-ir at several extranuclear sites in all hippocampal subregions. AR-ir was found in dendritic spines, many arising from pyramidal and granule cell dendrites. AR-ir was associated with clusters of small, synaptic vesicles within preterminal axons and axon terminals. Labeled preterminal axons were most prominent in stratum lucidum of the CA3 region. AR-containing terminals formed asymmetric synapses or did not form synaptic junctions in the plane of section analyzed. AR-ir also was detected in astrocytic profiles, many of which apposed terminals synapsing on unlabeled dendritic spines or formed gap junctions with other AR-labeled or unlabeled astrocytes. Collectively, these results suggest that ARs may serve as both a genomic and non-genomic transducer of androgen action in the hippocampal formation.

Ultrastructural evidence for mu-opioid modulation of cholinergic pathways in rat dentate gyrus

Within the rat hippocampal formation, cholinergic afferents and mu-opioid receptors (MORs) are involved in many crucial learning processes, including those associated with drug reward. Pharmacological data, and the overlapping distributions of cholinergic and mu-opioid systems, particularly in the dentate gyrus, suggest that MOR activation is a potential mechanism for endogenous opioid modulation of cholinergic activity. To date, anatomical evidence supporting this has not been reported. To delineate the relationship between cholinergic afferents and MOR-containing processes in the dentate gyrus, hippocampal sections were dually immunolabeled for vesicular acetylcholine transporter (VAChT) and MOR-1 and examined by electron microscopy. VAChT immunoreactivity was in unmyelinated axons and axon terminals, and was most often associated with small synaptic vesicles. MOR immunoreactivity was found in axons, axon terminals and, to a lesser extent, perikarya, which resembled GABAergic basket cells. Semi-quantitative ultrastructural analysis revealed that from 5% to 13% (depending on laminar location) of VAChT-immunoreactive (ir) presynaptic profiles contained MOR immunoreactivity. Additionally, 7% of VAChT-ir presynaptic profiles directly apposed MOR-ir axons and terminals, and there were almost no appositions to MOR-ir dendrites. These data suggest that opioids may directly and indirectly modulate acetylcholine release and/or reuptake. In the hilus and molecular layer, 4% of VAChT-ir terminals contacted dendritic shafts that were also contacted by MOR-ir terminals. This suggests that cholinergic afferents and MOR-containing afferents can converge on granule cell dendrites (which are restricted to the molecular layer) and on interneuron dendrites in the hilus. The results of this study provide ultrastructural evidence for direct and indirect modulation of cholinergic systems by mu-opioids in the hippocampal formation.

Characterization of action potential-evoked calcium transients in mouse postganglionic sympathetic axon bundles

1. Action potential-evoked Ca(2+) transients in postganglionic sympathetic axon bundles in mouse vas deferens have been characterized using confocal microscopy and Ca(2+) imaging. 2. Axonal Ca(2+) transients were tetrodotoxin sensitive. The amplitude depended on both the frequency of stimulation and the number of stimuli in a train. 3. Removal of extracellular Ca(2+) abolished the Ca(2+) transient. Cd(2+)(100 microM) inhibited the Ca(2+) transient by 78 +/- 10 %. The N-type Ca(2+) channel blocker omega-conotoxin GVIA (0.1 microM) reduced the amplitude by -35 +/-4 %, whereas nifedipine (10 microM; L-type) and omega-conotoxin MVIIC (0.1 microM; P/Q type) were ineffective. 4. Caffeine (10 mM), ryanodine (10 microM), cyclopiazonic acid (30 microM) or CCCP (10 microM) had no detectable effects. 5. Blockade of large and small conductance Ca(2+)-dependent K+ channels with iberiotoxin (0.1 microM) and apamin (1 microM), respectively, or Ca(2+)-dependent Cl(-) channels by niflumic acid (100 microM) did not alter Ca(2+) transients. 6. In contrast, the non-specific K+ channel blockers tetraethylammonium (10 mM) and 4-aminopyridine (10 mM) markedly increased the amplitude of the Ca(2+) transient. Blockade of delayed rectifiers and A-like K+ channels, by tityustoxin-K (alpha) (0.1 microM) and pandinustoxin-K (alpha) (10 nM), respectively, also increased the Ca(2+) transient amplitude. 7. Thus, Ca(2+) transients are evoked by Na(+)-dependent action potentials in axons. These transients originate mainly from Ca(2+) entry through voltage-dependent Ca(2+) channels (80 % Cd(2+) sensitive of which 40 % was attributable to N-type). Twenty per cent of the Ca(2+) transient was not due to Ca(2+) entry through voltage-gated Ca(2+) channels. Intracellular stores and mitochondria were not involved in the generation of the transient. Ca(2+) transients are modulated by A-like K+ channels and delayed rectifiers (possibly K(V)1.2) but not by Ca(2+)-activated ion channels.

Cellular relations between mu-opioid receptive, GABAergic and reticulospinal neurons in the rostral ventrolateral medulla

Physiological studies have suggested that mu-opioid receptor (MOR) activation can both excite and inhibit reticulospinal neurons in the rostral ventrolateral medulla (RVL), possibly via influences on GABAergic neurons. Thus, to determine the cellular relationships of MORs to GABAergic neurons in the RVL, two experimental approaches were used. First, single sections through the RVL were labeled for MOR using immunoperoxidase detection and for GABA using immunogold detection and examined by electron microscopy. These studies revealed that MOR-immunoreactive (IR) terminals were smaller on average than GABA-IR terminals and formed both asymmetric and symmetric synapses, whereas GABA-IR terminals formed exclusively symmetric synapses. MOR and GABA immunoreactivities rarely co-localized. Interactions between axons and terminals containing MOR or GABA immunoreactivity were primarily: (1) direct appositions with each other; or (2) convergence onto a common dendritic target that sometimes contained either MOR or GABA immunoreactivity. Since the identity of these target dendrites mostly was unknown, a second study was designed to determine if they might be reticulospinal neurons. For this study, reticulospinal neurons were identified with a retrograde tracer and both MOR and GABA were localized in the same sections of the RVL. These studies revealed that numerous GABA-IR terminals formed symmetric synapses on the perikarya and proximal dendrites of reticulospinal neurons. In contrast, few MOR-IR terminals contacted reticulospinal perikarya and large dendrites although they were often found nearby. These results provide anatomical evidence that MOR activation by endogenous or exogenous agonists may indirectly alter GABAergic neurotransmission in the RVL either through presynaptic interactions between cells or through competing influences on postsynaptic targets.

Neurotransmitter release mechanisms in sympathetic neurons: past, present, and future perspectives

In 1969, Paton and Vizi described the inhibitory actions of noradrenaline on acetylcholine release from the innervation of the guinea-pig ileum longitudinal muscle. They concluded "that acetylcholine output by the nervous networks of the longitudinal strip is under the normal control of the sympathetic by a species of presynaptic inhibition mediated by <==> receptors". This work was carried out in the Pharmacology Department at Oxford University. Clearly, a period in the 'Dreaming Spires' of Oxford sufficiently inspired Sylvester to take up a life long career in scientific research. He has published more than 300 papers on a wide range of topics but clearly has a strong interest in neurotransmitter release mechanisms and recently, non-synaptic interactions between neurons. It seems fitting therefore to write a brief review on the continuing studies on neurotransmitter release mechanisms in sympathetic neurons in a volume honoring the now distinguished Professor Vizi.

Ultrastructural evidence for presynaptic mu opioid receptor modulation of synaptic plasticity in NMDA-receptor-containing dendrites in the dentate gyrus

Physiological studies have demonstrated that long-term potentiation (LTP) induction in N-methyl-D-aspartate (NMDA) receptor containing dentate granule cells following lateral perforant path stimulation is opioid dependent, involving mu-opioid receptors (MORs) on gamma-aminobutyric acid (GABA)-ergic neurons. To determine the cellular relationships of MORs to postsynaptic NMDA receptor-containing dendrites, immunoreactivity (-I) against MOR and the NMDA receptor subunit 1 (NMDAR1) was examined in the outer molecular layer of the dentate gyrus using electron microscopy. MOR-I was predominantly in axons and axon terminals. NMDAR1-I was almost exclusively in spiny dendrites, but was also in a few terminals. Using immunogold particles to localize precisely NMDAR1, one-third of the NMDAR1-I was detected on the dendritic plasmalemma; in dendritic spines plasmalemmal immunogold particles were near synaptic densities. Many MOR-labeled axons and terminals contacted NMDAR1-labeled dendrites. MOR-labeled terminals formed symmetric (inhibitory-type) synapses on NMDAR1-labeled dendritic shafts or nonsynaptically contacted NMDAR1-labeled shafts and spines. MOR-labeled axons often abutted NMDAR1-containing dendritic spines which received asymmetric (excitatory-type) synapses from unlabeled terminals. Occasionally, MOR-labeled terminals and dendrites were apposed to NMDAR1-containing terminals. These results provide anatomical evidence that endogenous enkephalins or exogenous opioid agonists could inhibit GABAergic terminals that modulate granule cell dendrites, thus boosting depolarizing events in granule cells and facilitating the activation of NMDA receptors located on their dendrites.

Quantal evoked depolarizations underlying the excitatory junction potential of the guinea-pig isolated vas deferens

1. The effects of a putative gap junction uncoupling agent, heptanol, on the intracellularly recorded junction potentials of the guinea-pig isolated vas deferens have been investigated. 2. After the stimulation-evoked excitatory junction potentials (EJPs) had been suppressed by heptanol (2.0 mM) to undetectable levels, a different pattern of evoked activity ensued. This consisted of transient depolarizations that were similar to EJPs in being stimulus locked and in occurring at a fixed latency, but differed from EJPs in that they occurred intermittently and had considerably briefer time courses. 3. Analysis of the amplitudes and temporal parameters of the rapid residual depolarizations revealed a close similarity with spontaneous EJPs (SEJPs). There was no statistically significant difference between the rise times, time constants of decay and durations of the rapid residual depolarizations and of SEJPs. 4. Selected evoked depolarizations were virtually identical to SEJPs occurring in the same cell. Evoked depolarizations of closely similar amplitude and time course also occurred, usually within a few stimuli of each other. 5. These depolarizations appear to represent the individual quantal depolarizations that normally underlie the EJP and are therefore termed 'quantal excitatory junction potentials' (QEJPs) to distinguish them from both EJPs and SEJPs. 6. We examined the possibility that heptanol revealed QEJPs by disrupting electrical coupling between cells in the smooth muscle syncytium. Heptanol (2.0 mM) had no effect on the amplitude distribution, time courses, or the frequency of occurrence of SEJPs. Intracellular input impedance (Rin) of smooth muscle cells was left unaltered by heptanol. 7. 'Cable' potentials of the vas deferens, recorded using the partition stimulation method, also remained unchanged in the presence of heptanol. Thus, heptanol appeared not to modify syncytial electrical properties of the smooth muscle in any significant way. 8. Our observations show directly that the quantal depolarizations underlying the EJP in syncytial smooth muscle are SEJP-like events. However, no unequivocal statement can be made about the mechanism by which heptanol unmasks QEJPs from EJPs.

[Cys(O2NH2)2]enkephalin analogues and dalargin: selectivity for delta-opioid receptors

To investigate the structure-activity relationships for potent and selective action of enkephalins at the delta-opioid receptors, two newly synthesized analogues, [Cys(O2NH2)2,Leu5]enkephalin and [Cys(O2NH2)2, Met5] enkephalin and the hexapeptide [D-Ala2,Leu5]enkephalyl-Arg (dalargin) were tested and compared with [Leu5]enkephalin and [Met5]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective delta-opioid receptors) and the guinea pig ileum (mu- and kappa-opioid receptors). The mouse vas deferens assays included evaluation of the effects of opioid agonists on the first, purinergic, and the second, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-resistant responses. The opioids tested inhibited in a similar manner: (i) the purinergic and the adrenergic components of the electrically evoked contractions; and (ii) the ATP- and noradrenaline-induced postjunctional responses of the mouse vas deferens. Extremely low IC50 values (of 2-5 orders) were found for [Cys(O2NH2)2,Leu5] enkephalin, whose relative potency was between 239 and 1316 times higher than that of [Leu5]enkephalin. The order of potency for the other peptides in this tissue was: [Cys(O2NH2)2,Met5]enkephalin > [Leu5]enkephalin > dalargin > [Met5]enkephalin. The highest IC50 values in the guinea pig ileum assays, indicating the lowest affinity for mu-/kappa-opioid receptors, were obtained for the cysteine sulfonamide analogues, while dalargin showed a potency four times higher than that of [Met5]enkephalin. The order of potency in this tissue was: dalargin > [Met5]enkephalin > [Leu5]enkephalin > [Cys(O2NH2)2,Met5]enkephalin > [Cys(O2NH2)2,Leu5]enkephalin. The ratio, IC50 in guinea pig ileum: IC50 in mouse vas deferens, indicating selectivity of the respective peptide for delta-opioid receptors, was extremely high for [Cys(O2NH2)2,Leu5]enkephalin and especially for the adrenergic component of the responses. This ratio for [Cys(O2NH2)2,Met5]enkephalin was higher than the ratios for dalargin, [Leu5]enkephalin and [Met5]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O2NH2) in the enkephalin molecule greatly increases the potency and selectivity of the analogues at delta-opioid receptors, while both D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of [Leu5]enkephalin decrease them.

Geometry, kinetics and plasticity of release and clearance of ATP and noradrenaline as sympathetic cotransmitters: roles for the neurogenic contraction

The paper compares the microphysiology of sympathetic neuromuscular transmission in three model preparations: the guinea-pig and mouse vas deferens and rat tail artery. The first section describes the quantal release of ATP and noradrenaline from individual sites. The data are proposed to support a string model in which: (i) most sites (> or = 99%) ignore the nerve impulse and a few (< or = 1%) release a single quantum of ATP and noradrenaline; (ii) the probability of monoquantal release is extremely non-uniform; (iii) high probability varicosities form 'active' strings; and (iv) an impulse train causes repeated quantal release from these sites. Analogy with molecular mechanisms regulating transmitter exocytosis in other systems is proposed to imply that coincidence of at least two factors at the active zone, Ca2+ and specific cytosolic protein(s), may be required to remove a 'fusion clamp', form a 'fusion complex' and trigger exocytosis of a sympathetic transmitter quantum, and that the availability of these proteins may regulate the release probability. The second section shows that clearance of noradrenaline in rat tail artery is basically > or = 30-fold slower than of co-released ATP, and that saturation of local reuptake and binding to local buffering sites maintain the noradrenaline concentration at the receptors, in spite of a profound decline in per pulse release during high frequency trains. The third section describes differences in the strategies by which mouse vas deferens and rat tail artery use ATP and noradrenaline to trigger and maintain the neurogenic contraction.

The structural relations between nerve fibres and muscle cells in the urinary bladder of the rat

Intramuscular nerve fibres in the bladder of adult female rats were investigated by means of serial sections. The following observations were made. (1) Upon penetrating into the musculature the nerve bundles branch repeatedly, and almost all turn into single fibres; their axons become varicose, the Schwann cell sheath is attenuated, incomplete or absent, and the separation between axonal membrane and muscle cell membrane is reduced to tens of nanometres. (2) All single axons, and some of those within bundles, are varicose, but the characteristic of being varicose is expressed by degrees, and is not an all-or-none state. (3) Varicosities contain vesicles (mostly of the agranular type), microtubules (with little connection with the axolemma or the vesicles), some neurofilaments (scarce or absent in the best developed varicosities), mitochondria (whose size is on average smaller than those of the perikaryon, and a minute amount of endoplasmic reticulum. (4) Terminal varicosities, the true anatomical ending of an axon, are often devoid of Schwann cell sheath, are packed with vesicles, rarely contain microtubules or neurofilaments, and lie close to a muscle cell: the gap is often reduced to approximately 10 nm. (5) Schwann cells accompany the axons within the muscle strands. Unlike the area of the axonal profiles, the area of glial sheath changes little along the length of the nerve fibre, except towards its end. (6) The Schwann cell sheath around a varicosity is often incomplete; the area of the axolemma thus exposed is covered by the basal lamina, and is here referred to as a 'window'. While some varicosities have a window only a few tens of nanometres in width, others have more than one window, and some are devoid of Schwann cell altogether, so that their entire axolemma is in contact with the basal lamina. The Schwann cell never extends beyond the axon, whereas very often (and possibly always) the axon extends beyond the Schwann cell. (7) Intervaricose segments vary in length and diameter, the narrowest ones accompanying the more clear-cut varicosities. Some intervaricose segments are as small as 50 nm in diameter, contain a single microtubule and lack a Schwann cell sheath. Others, sheathed by a Schwann cell, contain a single neurofilament or no organelles at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Impulse conduction in sympathetic nerve terminals in the guinea-pig vas deferens and the role of the pelvic ganglia

Focal extracellular recording techniques were used to study nerve impulse propagation and the intermittent transmitter release mechanism in sympathetic nerve terminals of the guinea-pig vas deferens in vitro. In particular, the nature of impulse propagation in postganglionic nerve fibres was characterized following pre- or postganglionic stimulation. Conventional intracellular recording techniques were also used to study directly ganglionic transmission in cell bodies in the anterior pelvic ganglia. When brief electrical stimuli were applied to the hypogastric nerve trunk close to the prostatic end of the vas deferens, the nerve terminal impulses recorded extracellularly could be evoked either directly by stimulation of the parent axon (i.e. postganglionically) or indirectly by stimulation of the preganglionic nerve fibre. In 364 separate recordings, nerve terminal impulse conduction failure was not observed during trains of stimuli at 1 Hz. However, apparent "intermittent conduction" of nerve impulses was noted on 16 occasions. In these fibres, the degree of intermittent conduction decreased as the frequency of stimulation was increased. Conduction in these intermittent fibres was reversibly interrupted by removing calcium from the Krebs' solution or by the addition of the ganglion blocker, hexamethonium (30-100 microM). Thus, the cause of intermittent conduction is failure of the transmission of excitation in the sympathetic ganglia. Impulses evoked by postganglionic stimulation never failed to propagate into the nerve terminals, and changes in the shape or amplitude of the nerve terminal impulse during trains of stimuli were not detected. One effect of stimulation was a frequency-dependent increase in the latency of the nerve terminal impulse which developed during the train of stimuli. Thus, intermittence of transmitter release from individual varicosities cannot be attributed to failure of impulse propagation in sympathetic nerve terminals. Transmission in the anterior pelvic ganglia was investigated directly by making intracellular recordings from cell bodies whose terminals projected to the vas deferens. Many cell bodies received a strong synaptic input which generated an action potential in the postganglionic cell body on a one-to-one basis. However, in some cell bodies there was a low safety factor for the generation of the action potential by the excitatory postsynaptic potential. The safety factor for generating an action potential in the postganglionic cell body was raised by increasing the frequency of stimulation. These findings suggest that peripheral ganglia are not simple one-to-one relay stations, but may well play an important role in controlling the patterns of nerve impulse traffic in postganglionic sympathetic neurons.

A calcium-dependent component of the action potential in sympathetic nerve terminals in rat tail artery

A pharmacological approach was employed in order to visualize a Ca2(+)-dependent component of the extracellularly recorded nerve terminal impulse in the secretory regions of the sympathetic postganglionic nerves in the rat tail artery. Application of potassium-channel-blocking agents within the recording electrode caused the nerve terminal impulse to acquire a delayed negative deflection, which we have termed the late negative component (LNC) of the nerve terminal impulse. The time course and the latency of the LNC differed from that of the postjunctional transmitter-induced excitatory junction current, and the LNC persisted when the excitatory junction current was blocked by adenosine [alpha,beta-methylene]triphosphate, and was resistant to the alpha 1-antagonist prazosin and the alpha 2-antagonist yohimbine. Probably, therefore, the LNC was exclusively prejunctional in origin. For the following reasons it seems likely that the LNC, at least in part, was caused by influx of Ca2+ into the secretory regions of these nerves: (a) the LNC occurred only when potassium-blocking agents were present within the recording electrode; (b) the LNC amplitude increased with the Ca2+ concentration inside the recording electrode and was reduced by the removal of Ca2+; (c) the LNC was enhanced by replacing Ca2+ in the medium inside the recording electrode with Ba2+; (d) the LNC was depressed by the inorganic Ca2(+)-channel blocker cadmium or the Ca2(+)-channel-blocking peptide omega-conotoxin added within the recording electrode only, or by addition of cadmium or cobalt (but not the organic Ca2(+)-channel blocker nifedipine) inside and outside the recording electrode.(ABSTRACT TRUNCATED AT 250 WORDS)

K+ and Ca2+ channel blockers may enhance or depress sympathetic transmitter release via a Ca(2+)-dependent mechanism "upstream" of the release site

Extracellular recording of the pre- and postjunctional electrical activity in guinea-pig or mouse vas deferens or rat tail artery was employed to study the mechanisms by which the K+ channel blockers, tetraethylammonium and 4-aminopyridine and the Ca2+ channel blockers, Cd2+, Mn2+ or nifedipine influence the nerve stimulation-induced release of adenosine 5'-triphosphate as a sympathetic co-transmitter. The K+ and Ca2+ channel blocking agents examined had no effect on the spontaneous quantal release of adenosine 5'-triphosphate. However, addition of tetraethylammonium and 4-aminopyridine inside the recording electrode broadened the nerve terminal action potential and caused it to become more resistant to local application of tetrodotoxin, and dramatically increased the magnitude and tetrodotoxin resistance of adenosine 5'-triphosphate release within the patch. Surprisingly, tetraethylammonium and 4-aminopyridine were equally effective when added outside the recording electrode; now they did not increase the duration of the nerve terminal action potential inside the patch but increased its resistance to locally applied tetrodotoxin and dramatically increased the magnitude as well as the tetrodotoxin resistance of adenosine 5'-triphosphate release from sites inside the patch. Both tetraethylammonium and 4-aminopyridine contributed to these effects, with a strong potentiating interaction. Nifedipine was without effect, but application of 1-100 microM Cd2+ or 1-5 mM Mn2+ either inside or outside the recording electrode blocked adenosine 5'-triphosphate release inside the patch. The results indicate: (i) that the nerve terminal action potential is generated by activation of voltage-gated, regenerative Na+ channels but also has a small component carried by influx of Ca2+ and that it is "normally" terminated by activation of voltage- as well as Ca(2+)-dependent K+ channels; (ii) that the release probability is tonically depressed by the resting K+ efflux, and promoted by the resting Ca2+ influx, "upstream" of the release sites; and (iii) that the upstream control of the release probability may involve both changes in properties of ionic channels in the nerve terminal membrane, and effects on the cytoskeleton leading to changes in the availability of releasable quanta in varicosities within the patch.

"Upstream" regulation of the release probability in sympathetic nerve varicosities

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

The effect of substance P on nerve action potential propagation and cholinergic transmission in the myenteric plexus of the guinea-pig ileum

The effect of substance P on nerve terminals in myenteric plexus of the guinea-pig ileum was investigated. Neurogenic twitches of the myenteric plexus longitudinal muscle strip were recorded. Twitches of the strip portion where excitation involved the most distal parts of cholinergic nerve terminals were more increased by local application of substance P (0.1 and 0.4 nmol/l) than twitches of the portion where excitation involved both distal and proximal parts of nerve terminals. Substance P addition to a portion of the strip conducting nerve action potentials to invade the neighbouring strip portion also augmented twitches of the latter portion so that the interference with the propagation process was considered. The effect of substance P was poorly antagonized by the addition of a substance P antagonist, (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-substance P. Compound nerve action potentials were evoked in strands of fibres of the myenteric plexus by low-frequency train stimulation (1 Hz). The addition of substance P prevented a decrease of the amplitude of responses observed under control conditions. Using high-frequency train stimulation (30 Hz) the amplitude of responses to impulses 2-7 was augmented over that to the first impulse; substance P further increased such facilitation regularly. It seems that substance P might promote nerve action potential invasion of the distal parts of nerve terminals.

The topographical basis of cholinergic transmission in guinea-pig ileum myenteric plexus

Myenteric plexus-longitudinal muscle strips were used to study nerve action potential propagation and transmission and their differences between the proximal and the distal regions of cholinergic terminals. Neurogenic twitches of a portion of the strip were evoked by focal electrical stimulation. Twitches mediated by the distal regions of cholinergic nerve terminals were more influenced by drugs affecting Ca2+ "utilization" (Bay K 8644, kappa opiate ligand ethylketocyclazocine, changes in extracellular Ca2+ or Co2+ concentration) in contrast to twitches mediated by proximal regions of these terminals which were more influenced by drugs affecting sodium-potassium spike (tetrodotoxin, dendrotoxin, 4-aminopyridine, tetraethylammonium). Post-tetanic potentiation of twitches was prominent with that portion of the strip where the distal regions of nerve terminals were involved. Drugs interfering with Na+/K+ spikes indiscriminately influenced both the twitch height and post-tetanic potentiation whereas changes in extracellular Ca2+ concentration affected selectively only post-tetanic potentiation. Release of [3H]acetylcholine from pre-labelled strips evoked by 1 Hz continuous stimulation or by train stimulation at 30 Hz was measured selectively from portions containing either proximal and distal or only distal regions of nerve terminals. The release from portions containing the distal regions was relatively higher when evoked by 30 Hz than by 1 Hz. The distal regions of nerve terminals might be thus recruited to participate in transmission by a frequency-dependent process. Nerve impulses were recorded from strands of nerve fibres in the myenteric plexus. At 1 and 5 mm distance from the stimulation focus nerve impulses were completely suppressed by tetrodotoxin. At 5 mm, in some strands the amplitude of nerve impulses was also subject to the effect of drugs affecting Ca2+ "utilization"; facilitation of nerve impulse amplitude during 30 Hz train stimulation was always influenced by drugs affecting Ca2+ "utilization". Propagation of nerve impulses in the distal region of cholinergic nerve terminals was found to be Ca-sensitive and frequency-dependent; this might form the basis for facilitation and post-tetanic potentiation of muscarinic transmission.

A model for the fast 4-aminopyridine effects on amphibian myelinated nerve fibres. A study based on voltage-clamp experiments

The effect of 4-aminopyridine (4-AP) on the potassium currents in the node of Ranvier in myelinated nerve fibres was investigated with the voltage-clamp technique. The potential and time dependence of the currents in solutions with a high potassium concentration (114.5 mM) were studied. The block of the tail current was found to be less than that of the corresponding current during a potential step. Also, the time-course of the tail current was modified. In order to explain these findings, kinetic models of the 4-AP action were constructed and analysed numerically. A simple four-state model, in which 4-AP interacts only with open channels and where the binding is diphasically potential dependent, was found to account for the effects.

Inhibition of transmitter release from sympathetic nerve endings by omega-conotoxin

1. The effects of the calcium channel blocker, omega-conotoxin, on sympathetic neuroeffector function in the guinea-pig vas deferens have been investigated using a combination of mechanical and electrophysiological recording techniques. 2. Biphasic contractions evoked by electrical field stimulation were irreversibly abolished by omega-conotoxin (10-100 nmol/L). 3. Electrically evoked excitatory junction potentials and currents were irreversibly blocked by omega-conotoxin (10-100 nmol/L). Spontaneous excitatory junction potentials and currents were unaffected by this treatment. 4. omega-Conotoxin did not block impulse propagation in the nerve terminals. However, in three of four experiments omega-conotoxin caused a decrease in the size of the nerve terminal impulse. 5. These findings support the suggestion the omega-conotoxin acts prejunctionally to inhibit sympathetic neuroeffector by interfering with depolarization-secretion coupling.

Some pharmacological applications of an extracellular recording method to study secretion of a sympathetic co-transmitter, presumably ATP

Extracellular recording in guinea-pig or mouse vas deferens or rat tail artery was used to study the effects of some pharmacological agents on the nerve terminal spike (NTS) and the secretion of a sympathetic co-transmitter (presumably ATP), as reflected in the excitatory junction current (EJC). A negative-going EJCi (i for inside) was assumed to reflect release from sites inside, and a positive-going EJCo (o for outside) release from sites outside the recording electrode. Passage into or out of the electrode seemed to be slow. Tetrodotoxin (TTX) in the outer medium blocked the NTS and ECJo as well as EJCi; TTX in the pipette blocked stimulus-evoked but not spontaneous EJCi. The dihydropyridine Ca2+ channel blocking agent, nifedipine, was without effect, but Cd2+ in the external medium blocked EJCo and also, by an effect apparently 'upstream' of varicosities, inhibited EJCi (i.e. release within the patch) but not the NTS. When present in the outer medium the alpha 2-adrenoceptor agonists, clonidine and xylazine, blocked both EJCo and EJCi, but not the NTS. The effects of clonidine were blocked by yohimbine, which in itself increased the EJCo by about 50%. Neuropeptide Y and met-enkephalin in the outer medium blocked EJCo; the effect of met-enkephalin was blocked by naloxone. The K+ channel blocking agents, tetraethylammonium and 4-aminopyridine, inside or outside the electrode, increased dramatically the size of EJCi or EJCo, respectively.

A technique for simultaneously comparing the release of noradrenaline evoked by field stimulation and by propagated nerve impulses in rabbit ear arteries

1. A technique has been described which utilized radiotracer methods to measure the release of transmitter noradrenaline, simultaneously, from segments of rabbit ear artery subjected to field stimulation and propagated nerve impulses. 2. The release of radioactivity from arteries labelled with 3H-noradrenaline was much greater in the segment subjected to field stimulation than in the segment stimulated by propagated nerve impulses. 3. The release of radioactivity from segments invaded by nerve impulses decreased progressively with increases in frequency through the range of 10-50 Hz, using 150 pulses at 10, 15, 20, 30 and 50 Hz. However, the release remained constant in the field stimulated segments throughout the frequency range used.

Comparison of the effects of phenoxybenzamine and uptake blockade on noradrenaline efflux from rabbit ear arteries evoked by field stimulation and propagated nerve impulses

1. A comparison has been made of the effects of blockade of prejunctional alpha-adrenoreceptors and blockade of transmitter noradrenaline uptake in segments of rabbit ear arteries subjected to field stimulation or neuronally propagated impulses. 2. The relationship between evoked release and frequency of stimulation differed in artery segments subjected to field stimulation and those receiving propagated nerve impulses. However, the effectiveness of phenoxybenzamine in increasing stimulation-induced efflux of radioactivity decreased as the frequency of stimulation increased in artery segments subjected to either field stimulation or neuronally propagated impulses. 3. Blockade of neuronal and extraneuronal uptake had no effect on evoked efflux from field-stimulated artery segments but it did produce a marked and significant enhancement of release evoked by propagated nerve impulses.

Physiological and morphological changes in developing peripheral nerves of rat embryos

Physiological and morphological properties of intercostal nerves were studied in rat embryos of 13-21 days of gestation (birth is at 21-22 days). Rat intercostal nerves emerged from the spinal cord at day 13 of gestation when they were surrounded by a few Schwann cells. The nerve bundles contained growth cones, and large and small diameter unmyelinated profiles. At 13-17 days of gestation, large diameter profiles and irregularly shaped growth cones made up a significant portion of the axon population in intercostal nerves. At day 13 electrical stimulation of the nerve evoked action potentials. Thus, intercostal nerves are excitable at day 13 in utero, prior to the formation of functional nerve-muscle contacts (day 14-15). A functional nerve-muscle junction is, therefore, not a prerequisite for conduction of action potentials. From the onset of excitability Na+ was the major carrier of the action potential inward current, but there was also a small Ca2+ current that lasted until 18 days of gestation. At 17-18 days of gestation, Schwann cell proliferation and extension of Schwann cell cytoplasm into the nerve subdivided it into numerous smaller bundles. Axons located in the periphery of many bundles were wrapped by Schwann cell cytoplasm and were probably the first ones to become myelinated. During the same period extracellularly recorded action potentials consisted of multiple negative peaks, and conduction velocity increased about 2-fold. Myelination began at day 22 and was completed within 3 weeks after birth when the ratio of myelinated-to-unmyelinated axons reached its adult value.

Electrical activity at the sympathetic neuroeffector junction in the guinea-pig vas deferens

1. The relationship between the nerve terminal action potential and transmitter release from sympathetic postganglionic nerve terminals has been studied in vitro by focal extracellular recording. 2. In the absence of stimulation, 'spontaneous excitatory junction currents' (SEJCs) were recorded with amplitudes up to 500 microV, durations of 50-80 ms and frequencies of occurrence of 0.3-0.05 Hz; SEJCs of unusually long time course were also observed. The SEJCs were not recorded in tissues pre-treated with 6-hydroxydopamine to destroy sympathetic nerves, were unaffected by tetrodotoxin (TTX), the competitive alpha-adrenoceptor antagonists, prazosin and phentolamine, the irreversible alpha-adrenoceptor antagonist benextramine but were blocked by alpha,beta-methylene ATP which desensitizes P2-purinoceptors. 3. During trains of supramaximal stimuli at 0.1-4 Hz stimulus locked 'excitatory junction currents' (EJCs) were evoked intermittently from the population of varicosities located under the suction electrode with a probability of occurrence of 0.005-0.8. Although EJCs occurred intermittently, they were always preceded by an associated, non-intermittent, nerve impulse (delay less than or equal to 3 ms). 4. The EJCs reflect transmitter release from nerves because they were abolished by TTX, removal of calcium from the bathing medium, exposure to alpha-beta-methylene ATP and exhibited frequency-dependent facilitation. 5. Amplitude distributions of SEJCs and EJCs recorded in the same attachment were similar and skewed towards low-amplitude events. Individual SEJCs and EJCs could be found which were identical in amplitude and time course. 6. Locally applied TTX blocked impulse propagation and transmitter release in the terminal region; electrotonic invasion of the terminals from the point of block did not activate the transmitter release process. 7. These studies indicate that (1) intermittence of transmitter release is caused by a low probability of release in the invaded varicosity and is not caused by conduction failure in the terminal regions, (2) only a single quantum is normally secreted when the release mechanism of a varicosity is activated by the nerve impulse and (3) active invasion of the terminals is necessary for transmitter release to occur.

Contractile responses of isolated guinea-pig vas deferens to trains of electrical stimuli and influence of prostaglandin E2

The contractile responses of isolated guinea-pig vas deferens to long trains of 300 stimuli (0.3 ms, 90 V) applied at a frequency of 8 Hz or 20 Hz were diphasic contractions comprising a non-adrenergic, probably ATP-mediated component (phase I) and an adrenergic component (phase II). Stimulation with short trains of 10 stimuli (1 ms, 90 V) at a frequency of 10 Hz elicited monophasic responses. The contractions evoked by either long or short trains of stimuli were due to excitation of sympathetic nerve terminals without, however, being purely adrenergic ones. Prostaglandin E2 (PGE2) in increasing concentrations of 1 nmol/l to 100 nmol/l reduced phase I of the responses to long trains of stimuli at a frequency of 8 Hz, as well as the monophasic responses to short trains of stimuli. Both phases of the contractions evoked by long trains of stimuli at a frequency of 20 Hz were reduced by the lowest and potentiated by the highest concentrations of PGE2. The high concentration of PGE2 also potentiated phase II of the responses to long trains of stimuli at a frequency of 8 Hz and the diphasic contractions, resulting from the simultaneous application of ATP and noradrenaline. The results suggest that in guinea-pig vas deferens, PGE2 in low concentrations presynaptically inhibits the non-adrenergic components of the contractile responses to electrical stimulation of the sympathetic nerve terminals, while in high concentrations postsynaptically potentiates mainly the adrenergic components of these responses.

The topography of cholinergic transmission in the mechanism of drug action at muscarinic synapses of the guinea-pig ileum

The pharmacology of cholinergic neurogenic responses evoked by the participation of only the endings of axon terminals was compared to that of responses evoked by participation of the more proximal parts of the terminals also. Myenteric plexus-longitudinal muscle strips of the guinea-pig ileum were drawn through narrow orifices in 2 rubber membranes dividing a bath into 3 separate compartments. Oral segments were stimulated electrically by single impulses or by trains and local neurogenic contractions were evoked. The contractions of the aboral segment due to nerve impulses transmitted from the oral segment via the middle segment were also recorded. The opioid ligands ketocyclazocine and [D-Ala2,MePhe4,Met(O)5-ol]enkephalin and noradrenaline inhibited the twitches of the aboral segment evoked by oral segment stimulation more than the local twitches of the oral segment when these agents were applied directly to the respective compartments. The twitches of the aboral segment were also inhibited by the application of these drugs into the middle compartment adjusted to 10 mm width. Verapamil and the alkaline earth metal ions cobalt and lanthanum had similar effects. 4-Aminopyridine increased twitch amplitude more in the aboral segment than in the oral segment when applied directly; similar effects in the aboral segment were seen when the agents were applied to the middle compartment. The action of atropine, papaverine, d-tubocurarine and prostigmine did not discriminate between twitches in the oral and aboral segment when applied directly and all drugs except prostigmine were without effect when applied to the middle compartment.(ABSTRACT TRUNCATED AT 250 WORDS)

Axonal projections and conduction properties of olfactory peduncle neurons in the armadillo (Chaetophractus vellerosus)

Extracellular unit recording was employed to study the axonal properties and efferent projections of antidromically identified neurons in the olfactory peduncle (OP) region of a primitive eutherian macrosmatic mammal, the south american armadillo (Chaetophractus vellerosus). Of 72 cells which satisfied the criteria for antidromic invasion, 55 (76%) and 17 (24%) responded to ipsi- and contralateral olfactory bulb (IOB; COB) stimulation, respectively. The absolute refractory period (3.25 +/- 0.3 ms; mean +/- SE) and the conduction velocity (CV; 1.94 +/- 0.2 m/s; mean +/- SE) of IOB and COB driven neurons were negatively correlated (r = -0.52; p less than 0.001). In paired-shock tests (8-1950 ms interval), and early supernormal period (SPN) of increased CV and excitability was found following the relative refractory period in 82% of tested cells (N = 50); this period was followed by a late subnormal phase (SBN) of decreased CV and increased threshold in 58% of neurons (N = 50). Significant correlations were found to exist between: CV and absolute magnitude of latency variation (r = -0.55; p less than 0.001; n = 43), CV and duration of SPN and SBN periods (r = -0.60; p less than 0.002; n = 24 and r = 0.58; p less than 0.02; n = 19, respectively) and between duration of SPN and SBN phases (r = 0.79; p less than 0.001; n = 30). Maximum latency variation during the SPN and SBN periods was attained in a gradual, additive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium channel antagonists on action potential conduction and transmitter release in the guinea-pig vas deferens

The effects of the Ca2+ channel antagonists amlodipine, cobalt, diltiazem, nifedipine and verapamil and the local anaesthetic lignocaine were investigated on action potential conduction in and on evoked transmitter release from sympathetic nerves in the guinea-pig isolated vas deferens. Transmitter release was investigated by measurement of evoked (trains of pulses at 1 and 2 Hz, 0.1-0.5 ms supramaximal voltage) excitatory junction potentials (e.j.ps) using microelectrodes; tension was recorded simultaneously; tritium [3H] overflow from vasa preincubated (37 degrees C, 30 min) in Krebs solution containing either [3H]-noradrenaline (NA, 25 microCi ml-1, 2 X 10(-6) M NA) or [3H]-adenosine (50 microCi ml-1, 1 X 10(-6) M adenosine). Amlodipine (0.5-2 X 10(-4) M), verapamil (0.5-2 X 10(-4) M), diltiazem (1-8 X 10(-4) M), lignocaine (0.1-2 X 10(-3) M) and cobalt (2-6 X 10(-2) M) in descending order of potency, but not nifedipine (1-5 X 10(-3) M), increased the latency and inhibited, then abolished, the amplitude and number of action potentials in a concentration-dependent manner. Amlodipine (0.5-1 X 10(-4) M), verapamil (1-2 X 10(-4) M), diltiazem (1-5 X 10(-4) M) and cobalt (1 X 10(-3) M), in descending order of potency, but not nifedipine (5 X 10(-4) M), inhibited then abolished evoked e.j.ps in a concentration-dependent manner. Cobalt inhibited e.j.ps at a lower concentration than that (2-6 X 10(-2) M) required to block action potential conduction. In unstimulated tissues, the resting [3H] overflow following preincubation with [3H]-NA consisted largely of 4-hydroxy 3-methoxymandelic acid (VMA), 4-hydroxy 3-methoxy phenylglycol (MOPEG), 3,4 dihydroxyphenylglycol (DOPEG) and NA; stimulated tissues (300 pulses at 20 Hz, 0.5 ms supramaximal voltage) released mainly NA. Verapamil (0.1-1 X 10(-4) M), amlodipine (0.05-1 X 10(-4) M) and nifedipine (1-5 X 10(-4) M), but not cobalt (2 X 10(-3) M), increased, significantly, the resting overflow of 3H comprising mainly DOPEG. Cobalt (2 X 10(-3) M) inhibited, significantly, the stimulation-evoked overflow of 3H. Verapamil (1 X 10(-4) M) had little effect on the resting overflow of 3H following preincubation with [3H]-adenosine. Diltiazem (5 X 10(-4) M) and cobalt (2 X 10(-3) M) both inhibited evoked 3H overflow. Nifedipine (5 X 10(-4) M) was ineffective. 6 The effectiveness of Ca2+ channel antagonists at pre- and postjunctional sites differ; the results are discussed in terms of the selectivity of these drugs for each site and their differential effects on transmitter release.

Neuropeptide Y--a cotransmitter with noradrenaline and adenosine 5'-triphosphate in the sympathetic nerves of the mouse vas deferens? A biochemical, physiological and electropharmacological study

A combination of biochemical, physiological and electropharmacological methods was employed to examine the occurrence of neuropeptide Y and the pre- and postjunctional effects of this peptide on sympathetic neuromuscular transmission in the mouse vas deferens. This tissue had a high content of neuropeptide Y-like immunoreactive material, suggesting a dense innervation by neuropeptide Y-containing nerve fibres. Addition of neuropeptide Y at concentrations from 5 X 10(-9) to 5 X 10(-7) M induced both pre- and postjunctional effects in vitro. Neuropeptide Y per se induced a rise in the resting tension, and "instantly" potentiated the contractile effects of exogenous noradrenaline and of the stable adenosine 5'-triphosphate (ATP) analogue, alpha,beta-methylene ATP. Neuropeptide Y reduced the secretion of [3H]noradrenaline evoked by electrical nerve stimulation, and selectively depressed the stimulus-evoked, but not the spontaneously occurring excitatory junction potentials in smooth muscle cells. Further, neuropeptide Y reduced the amplitudes of the twitch contractions evoked by electrical field stimulation with short stimulus trains at 10 Hz, and also (although to a smaller extent) the delayed contractile response to longer trains of nerve stimuli. The pre- and postjunctional effects of neuropeptide Y were not changed by alpha- or beta-adrenoceptor blocking agents, or by tachyphylaxis to the effects of ATP, or by the calcium channel blocker nifedipine.

IN CONCLUSION

sympathetic neuromuscular transmission in the mouse vas deferens may be mediated not only by noradrenaline and ATP, but also by neuropeptide Y.(ABSTRACT TRUNCATED AT 250 WORDS)

Non monotonic morphometric changes produced at mouse neuromuscular junctions following in vivo stimulation at various frequencies

Morphometric changes have been studied at the mouse neuromuscular junction (NMJ) as a function of the frequency of stimulation. In vivo stimulation was made at either 10, 25, 50 or 100 Hz. No significant change in the area of NMJ profiles is observed after stimulation. This indicates that stimulation does not change the volume of the nerve terminal. There is also no change of the volume density of mitochondria. However, there are significant reductions of the numerical density on area (NA, number/micron2) of the clear vesicles following stimulation at 25, 50 or 100 Hz. The NA of clear vesicles is not further reduced following stimulation at 50 or 100 Hz than after the stimulation at 25 Hz. The numerical density on area of the coated vesicles was significantly increased following the stimulation at 25 Hz and 100 Hz. It is not increased more following stimulation at 50 Hz or 100 Hz than following stimulation at 25 Hz. One possible interpretation of the clear vesicle and the coated vesicle results is that less vesicles are liberated per sec by the NMJ with stimulation at 50 and 100 Hz than at 25 Hz because the action potential may fail to invade some terminal arborizations. The total surface density (i.e. surface of membrane in micron2 per micron3 of nerve terminal) of the clear vesicles, the coated vesicles, the vacuoles and the presynaptic plasma membrane is not changed significantly following stimulation at any frequency. This indicates that there is no net loss of membrane at any of the frequencies investigated.

Interactions between the duration of stimulation and noradrenaline on cholinergic transmission in the myenteric plexus-smooth muscle preparation

Output of acetylcholine (ACh), electromyogram (EMG) recordings and contractions of myenteric plexus-longitudinal muscle strip preparations from the guinea-pig ileum were studied during stimulation by single impulses or by trains (30 Hz; 2 to 128 impulses) under control conditions and in the presence of noradrenaline (NA). During supramaximal stimulation NA (2.5 microM) inhibited both contractions of the smooth muscle and the release of ACh evoked by single impulses more effectively than those evoked by train stimulation so that in a train of 4 impulses the output of ACh per impulse after the 2nd to 4th impulses was 69 to 104% higher than the output after the 1st impulse. During submaximal stimulation, contractions and ACh release evoked by single impulses were almost completely inhibited by NA. The neurogenic EMG, a direct consequence of the localized action of released transmitter (ACh), was recorded in the longitudinal muscle 4 and 10 mm aborally from the focal stimulation site. The incidence of the neurogenic response was much higher at the proximal (4 mm) than at the distal (10 mm) site and was proportional to the number of impulses in a train (100 Hz). NA inhibited propagation of the neurogenic response evoked by single impulses whereas its effect during train stimulation was less. It is concluded that in the course of train stimulation, sites of transmission more distant from the stimulation focus was recruited, and consequently the secretion of ACh in succeeding impulses was enhanced. NA could interfere with this process; it might inhibit the invasion by action potentials of cholinergic nerve terminal varicosities, thereby reducing the release of ACh.

Influence of temperature on stimulus-secretion coupling in the sympathetic nerves and on neuromuscular transmission, in guinea-pig vas deferens

In guinea-pig isolated vas deferens moderate cooling (within the range 16-41 degrees C) improved, while further cooling depressed, sympathetic neuromuscular transmission. Moderate cooling: (a) enhanced the contractile responses both to electrical nerve stimulation and (although less strongly) to exogenous noradrenaline or to the adenosine 5'-triphosphate (ATP) analogue, alpha, beta-methylene ATP; (b) enhanced the overflow of [3H]noradrenaline evoked by electrical nerve stimulation at 1-8 Hz, but not that caused by direct depolarization of varicosities with 80 mM potassium; (c) reduced the depressing effect of exogenous noradrenaline, as well as the enhancing effect of the alpha-adrenoceptor blocking agent phentolamine, on the secretory responses to electrical nerve stimulation; (d) while not altering the maximal secretion of tracer noradrenaline at infinitely high frequency or external calcium, lowered the frequency or calcium concentration required for half-maximal secretion, both in the absence and in the presence of phentolamine.

IN CONCLUSION

moderate cooling improves sympathetic neuromuscular transmission in guinea-pig vas deferens, both prejunctionally, by enhancing transmitter secretion, and post-junctionally, by increasing the responsiveness of the smooth muscle to transmitter.

Unequal branch point filtering action in different types of dorsal root ganglion neurons of frogs

The influence of the intraganglionic branch point on impulse conduction in single neurons of frog dorsal root ganglia (DRG) has been determined by measuring the least interval at which it will conduct two action potentials into the dorsal root. At 21-23 degrees C, branch points of myelinated fibers had long least conduction intervals and low safety factors for orthodromic impulse conduction compared to nodes of Ranvier in peripheral nerve. DRG neurons with broad somatic spikes with a shoulder on the falling phase and slowly conducting myelinated or non-myelinated axons had the longest least conduction intervals (lowest safety factors). DRG neurons with brief somatic spikes with little or no shoulder on the falling phase had short least conduction intervals (higher safety factor) regardless of their conduction velocity. The results indicate that certain DRG neurons have found a way to minimize branch point filtering action.