Depression Of Synaptic Transmission Research Articles

Effects of Halo thane on Acetylcholine Release and Sympathetic Ganglionic Transmission

Depression of synaptic transmission is one of the actions common to many general anesthetics. They appear to act primarily on the chemical transmission process itself without affecting the conduction of impulses in nerve axons. The objective of the present study was to directly examine the effects of halothane on the release of acetylcholine and overall ganglionic transmission. The effects of halothane on ganglionic transmission were studied in the stellate ganglion of the cat in vitro. The preganglionic nerves of the stellate ganglion were stimulated electrically to generate threshold and suprathreshold evoked potentials in the postganglionic nerves. The picomole levels of released acetylcholine in the superfusate were determined with a radioenzymatic method using 32P-ATP following a 5-min preganglionic stimulation before and after the addition of 0.28 and 0.59 mM halothane (vaporizer settings of 1% and 2%, respectively). Halothane at both concentrations: 1) depressed sympathetic ganglionic transmission induced by either threshold or suprathreshold stimulation of the preganglionic nerves; and 2) caused a dose-dependent decrease in acetylcholine release during threshold stimulation with no change in neurotransmitter release during suprathreshold stimulation. In summary, the interruption of synaptic transmission by halothane involves at least two mechanisms: 1) decrease in acetylcholine release during low level of synaptic transmission; and 2) postsynaptic decrease in sensitivity to acetylcholine during low and high levels of synaptic transmission.

Electrophysiological action of ethanol at the cellular level

To examine how ethanol interferes with brain function on the global levels of brain activity reflected by event-related potentials, we summarize here our recent efforts to characterize the acute cellular effects of ethanol. Four regions of the rodent brain (cerebellum, hippocampus, locus coeruleus and inferior olive) have so far been examined. The effects of acute parenteral ethanol on specific identifiable neurons within these 4 regions are highly consistent, dose-related, and spontaneously reversible. Furthermore, different patterns of response are seen in each responsive region, ranging from general increased firing in inferior olive to generally depressed synaptic transmission in hippocampus, and with more subtle effects within the cerebellum and within the locus ceruleus. This survey of consistent but differing patterns of responsiveness to ethanol at specific time points after acute exposure, suggests that the global effects of ethanol must be composed of several distinct effects both within and across many cellular systems.

The emerging pharmacology of ethanol

Historically, alcohol (ethanol) has been viewed as a non-specific CNS depressant, presumed to act equally on all bioelectric membranes. In contrast to this view, cellular electrophysiological studies, supported by anatomic and neurochemical evidence, support the emergence of a more specific pharmacological profile. Four regions of the rodent brain (cerebellum, hippocampus, locus coeruleus and inferior olive) have so far been examined. The effects of acute parenteral ethanol on specific identifiable neurons within these four regions are highly consistent, dose-related, and spontaneously reversible. Nevertheless, different patterns of effects are seen in each responsive region, ranging from general increased firing in inferior olive to generally depressed synaptic transmission in hippocampus, and with more subtle effects within the cerebellum and within the locus ceruleus. This survey of consistent but differing patterns of responsiveness to ethanol at specific time points after acute exposure, suggests that the global effects of ethanol on movement, on arousal and on emotions and memory must be composed of several distinct effects both within and across many cellular systems.

Mechanisms of halothane action on synaptic transmission in motoneurons of the newborn rat spinal cord in vitro

Action of halothane on synaptic transmission was studied on the isolated newborn rat spinal cord. Clinical doses of halothane (≦3%) suppressed mono- and polysynaptic reflexes, dorsal root reflexes, excitatory and inhibitory postsynaptic potentials as well as the spontaneous synaptic potentials caused by impulse bombardment. However, the spontaneous miniature inhibitory postsynaptic potentials observed after blocking impulse activities by tetrodotoxin were not at all suppressed by halothane. During halothane administration, the membrane potential of motoneurons was hyperpolarized by several millivolts, associated with an increase in input conductance. However, the threshold potential level for spike generation was virtually unaffected. Depression of synaptic transmission in spinal motoneurons by halothane is suggested to be due to two factors: (1) a reduction in the amount of transmitter release secondary to interference with Ca 2+ entry into nerve terminals, either by partial blockade of impulse invasion or voltage-dependent Ca 2+ channels; and (2) an increase in the depolarizing current necessary for excitation of motoneurons owing to hyperpolarization and decreased input resistance.

Ca-channel blockers and the electrophysiology of synaptic transmission of the guinea-pig olfactory cortex

Slices of guinea-pig olfactory cortex have been used to compare the potency of various Ca-blockers on the electrophysiology of synaptic transmission. Listed in the order of potency, the divalent cations Cd 2+, Ni 2+, Mn 2+, Co 2+, La 3+ and Mg 2+ depressed synaptic transmission. The organic Ca-blockers, nifedipine or nimodipine or verapamil and diltiazem were ineffective up to 0.01 nmol/l. Verapamil, D600 or diltiazem (0.1–0.3 mmol/l) depressed both synaptic transmission and the sodium-mediated presynaptic action potential. These results reaffirm the idea that “organic Ca-antagonist’ do not block all Ca-channels in brain and the high Cd 2+ sensitivity suggests the Ca-channels in post- and presynaptic membranes have dissimilar pharmacological profiles.

The effects of an adenylate cyclase inhibitor on the electrophysiological correlates of neuromuscular transmission in the frog.

The presynaptic and postsynaptic effects of MDL 12,330A, an adenylate cyclase inhibitor in several biological tissues, were studied at motor endplates in frog cutaneous pectoris nerve-muscle preparations. This agent increased both spontaneous quantal acetylcholine (ACh) release and neurally-evoked ACh release approximately twofold during the first 20-40 min of application. The increased ACh release was accompanied by a profound irreversible depression in the amplitudes of the miniature endplate potentials (m.e.p.ps) and endplate potentials (e.p.ps). The response to iontophoretically-applied ACh was reduced in parallel with the amplitude of the spontaneous m.e.p.ps, indicating that the depression of synaptic transmission was postsynaptic in origin. Endplates were voltage-clamped to study the postsynaptic depression in more detail. It was observed that the peak endplate current (e.p.c.) was depressed without concomitant changes in: the kinetics of e.p.c. decay, the relationship between peak e.p.c. and membrane potential, the ACh equilibrium potential or the voltage sensitivity of the e.p.c. decay. This suggests that MDL 12,330A reduces the postsynaptic sensitivity to ACh by a voltage-dependent block of the cholinoceptor. The presynaptic enhancement and the postsynaptic depression of junctional transmission produced by MDL 12,330A are discussed in conjunction with current theories of the role of adenylate cyclase and cyclic nucleotides at nicotinic cholinergic synapses.

Acute effects of lead at central synapses in vitro

The acute effects of lead in the rat CNS in vitro were studied on synaptic transmission in the isolated hemisected spinal cord from newborn rats and on the transport of exogenous GABA, acetylcholine and cis-3-aminocyclohexane car☐ylic acid (ACHC) from slices of cerebral cortex from adult rats. Lead had quite variable effects on monosynaptic reflexes and synaptic potentials. When it occurred, the depression of synaptic transmission by lead (typically at 18.5 μmol/liter of added lead acetate) was reversible provided exposure times were less than 15 min; furthermore, depression could be antagonised by increasing the external calcium concentration. Lead had no effect on the postsynaptic responses of motoneurons to the putative transmitters l-glutamate, GABA and glycine or to eledoisin-related peptide. The effects of lead on uptake and release of exogenous GABA and ACHC were dependent on the perfusion buffer employed: minimal effects were seen in solutions buffered with either phosphate or carbonate. When Tris HCl was used as buffer, lead inhibited the uptake of GABA and potentiated the spontaneous release of GABA with anEC 50 = 50 μmol/liter as added lead acetate. In Tris HCl buffer, lead acetate (100 μmol/liter) produced a two-fold enhancement in the spontaneous release of acetylcholine under conditions where choline and acetylcholine re-uptake was blocked by hemicholinium. The availability of free lead cations in solution is highly dependent on the concentrations of other ions (particularly phosphates) and the pH. Under the appropriate conditions, lead can inhibit CNS synaptic function acutely in a manner consistent with lead competing with calcium ions in transmitter release processes as has been established for acetylcholine release at peripheral synapses.

Noncholinesterase actions of an irreversible acetylcholinesterase inhibitor on synaptic transmission and membrane properties in autonomic ganglia.

Superfusion of the organophosphorous acetylcholinesterase inhibitor soman (pinacolyl methylphosphonofluoridate; 0.01-25 microM) produced a dose-dependent reduction of extracellularly and intracellularly recorded synaptic responses in the isolated rat superior cervical ganglia at frequencies of orthodromic stimulation that do not normally produce synaptic depression. The magnitude of depression was dependent upon the frequency of stimulation (0.02-1 Hz), was maintained after the removal of soman from the superfusion solution, and recovered by over 65% during periods of inactivity. The depression of synaptic transmission produced by soman was not dependent upon the inhibition of acetylcholinesterase (AChE) activity by this agent. Transmission was increasingly depressed by doses of soman greater than those needed to inactivate all measurable ganglionic AChE activity. Dose-dependent depression of synaptic transmission in soman also occurred after pretreatment with the irreversible AChE inhibitor diisopropylphosphofluoridate (DFP; 100 microM), which inhibited greater than 98% of the AChE activity in the ganglia. Soman produced a decline in the input resistance, resting potential, spike amplitude, and spike threshold and a reduction in the hyperpolarizing afterpotential. Soman-induced depression of synaptic transmission was not due primarily to a blockade of postsynaptic nicotinic receptors. At concentrations of soman which produced significant depression in transmission, ganglionic depolarization produced by bath-applied carbamylcholine (carbachol) was either slightly depressed or facilitated. In the presence of soman, repetitive focal application of acetylcholine or carbachol did not reveal use-dependent desensitization. Muscarinic antagonists, atropine and pirenzepine, protected against the use-dependent depression of synaptic transmission induced by soman. These results suggest that a principal site of action for soman is at the presynaptic terminal and that this site is sensitive to muscarinic receptor blockade.

Postsynaptic acetylcholine receptor efficacy is similarly increased by detergents and acetylcholinesterase inhibitors at an Aplysia synapse

At Aplysia H- and D-type cholinergic neuro-neuronal synapses, application of high concentrations of detergents (Triton X-100 and sodium deoxycholate) depressed synaptic transmission and the postsynaptic response to ionophoretic application of acetylcholine (ACh) or carbachol. However, when very low concentrations of detergents (of the order of 10 −9 M for sodium deoxycholate) were used, the nerve-evoked response as well as the ACh and carbachol ionophoretic responses were facilitated (by at least 200%), but only in H-type cells. This facilitation was similar to that previously observed in the same receptor type when acetylcholinesterase (AChE) was inhibited by various organophosphate or carbamate acetylcholinesterase inhibitor (AChEIs) 3. Indeed, the effects of AChEI and detergents were not cumulative. We propose that on H-type synapses detergents may perturb a hypothetical molecular interaction between AChE and the acetylcholine receptor (AChR) by which AChE modulates the ability of the AChR to be activated by ACh or carbachol.

Asymmetric relationships between homosynaptic long-term potentiation and heterosynaptic long-term depression.

All synaptically-based neuropsychological theories of learning postulate that there are changes resulting from neural activity which are long-lasting and confined to specific sets of synapses. In the past decade a form of synaptic strengthening known as long-term potentiation (LTP) has been found which results from high-frequency neural activity and is of sufficient duration to model as a learning mechanism. Some early tests of the synaptic specificity of LTP in area CA1 of the hippocampus indicated that although LTP was specific to the tetanized pathway, in a converging untetanized pathway it was associated with depression of synaptic transmission lasting for at least 30 min. However, others have found that this heterosynaptic depression more usually decays within 5-15 min post-tetanus despite the maintenance of LTP in the tetanized pathway. Similarly, in the dentate gyrus (DG), LTP of either the lateral (LPP) or medial (MPP) components of the perforant path afferents has been associated with only short-lasting reciprocal heterosynaptic depression. Here, using more detailed measurement of stimulus intensity curves, we report that tetanization of either MPP or LPP reliably depresses synaptic transmission in the other pathway for at least 3 h. This heterosynaptic depression, considerably smaller than the usual magnitude of LTP, was obtained regardless of whether LTP had been produced in the tetanized homosynaptic pathway. Heterosynaptic long-term depression was not observed if the test pathway had been previously tetanized.

Nicotinamide adenine dinucleotide depresses synaptic transmission in the hippocampus and has specific binding sites on the synaptic membranes

The electrical activity of transverse slices of hippocampus was used as a bioassay in which extracts of fresh brain tissue were screened for biological activity. A factor that depressed synaptic transmission was identified as nicotinamide adenine dinucleotide (NAD). This depressant action of NAD could be observed at concentrations in the range 1-10 microM and the degree of depression was monotonically related to the concentration of NAD in the bathing medium. NAD did not affect the antidromic invasion of the granule cells nor did it alter the relationship between the electrically evoked excitatory postsynaptic field potential (e.p.s.p.) and the population discharge of the granule cells (population spike). These results suggest that NAD did not affect the electrical excitability of the neuronal membranes. NAD had little effect on the sensitivity of granule cells to iontophoretically applied L-glutamate, the putative excitatory transmitter for the perforant path-granule cell pathway. Pure synaptosomal membranes, free of mitochondria, had two binding sites for NAD: a high affinity site with a Kd of 1 microM and a low affinity site with a Kd of 17 microM. These sites were similar in affinity to those of mitochondria, although the density of the high affinity sites was 5 X greater in the synaptosomal membranes. Adenosine had a relatively weak affinity for the NAD binding sites. It was concluded that NAD probably depressed synaptic transmission in the dentate gyrus by binding to sites on the presynaptic nerve terminal and reducing the amount of transmitter released by a nerve impulse. The physiological significance of this view is discussed.

The effects of halothane on sympathetic ganglionic transmission.

The effects of halothane on ganglionic transmission were studied in the stellate ganglion of the guinea pig using intracellular recordings in vitro. Depression of synaptic transmission is one of the actions common to many general anesthetics. The aim of this study was to investigate which of the processes involved in synaptic transmission are affected by halothane in concentrations comparable to those used during surgical anesthesia. The neurons of the stellate ganglion were depolarized using preganglionic nerve stimulation, postganglionic nerve stimulation, and intracellular stimulation before ad after introduction of halothane (vaporizer settings of 0.75% and 1.5% produced bath concentrations of 8 and 18 mg/dl, respectively). Halothane at both concentrations depressed sympathetic ganglionic transmission which was induced by stimulation of preganglionic nerves. Axonal transmission and the excitability of the postganglionic neurons to direct intracellular stimulation was far less sensitive to halothane than synaptic transmission. The depression of ganglionic transmission seen in the present study was most likely due to a decrease in transmitter release although alterations in postsynaptic receptor properties could have been involved as well. The decrease in sympathetic activity resulting from depression of ganglionic transmission probably contributes to the arterial hypotension seen during halothane anesthesia, along with direct myocardial depression, inhibition of catecholamine release from the adrenal medulla, direct action on vascular smooth muscle, and central sympathetic depression.

Selective depression of excitatory amino acid induced depolarizations by magnesium ions in isolated spinal cord preparations.

1. The depressant actions of Mg2+ and a range of other divalent ions on synaptic excitation and on responses produced by excitatory amino acids and other putative transmitters have been investigated in hemisected isolated spinal cords of frogs and neonatal rats. Some comparative studies were also made using the rat isolated superior cervical ganglion. 2. At concentrations above 10 microM, Mg2+ selectively antagonized N-methyl-D-aspartate (NMDA)-induced motoneurone depolarization as recorded from ventral roots of tetrodotoxin-blocked spinal cords. Depolarization evoked by quisqualate (unaffected by 20 mM-Mg2+) was resistant to the depressant action of these ions, while depolarizations evoked by other excitant amino acids were depressed to intermediate degrees. 3. Mn2+, Co2+ and Ni2+ had qualitatively similar actions to Mg2+; Mn2+ was somewhat less potent and Co2+ and Ni2+ more potent than Mg2+. The alkaline earth metal ions, Ca2+, Sr2+ and Ba2+, had very weak Mg2+-like actions. Ca2+ and Mg2+ acted additively in depressing amino acid-induced responses. 4. Mg2+ also depressed motoneurone responses evoked by noradrenaline, substance P and carbachol in the neonatal rat isolated spinal cord. However, none of these effects were as marked as the depression of NMDA-induced responses by Mg2+ in this preparation. Mg2+ did not depress motoneurone depolarization produced by 5-HT in the rat spinal cord or the depolarizing action of GABA on primary afferent terminals of the isolated frog spinal cord. 5. At concentrations producing marked depression of NMDA-induced responses, Mg2+ also depressed synaptic transmission in spinal cords in the absence of an effect on ganglionic transmission. At the same concentrations, Mn2+, Co2+ and Ni2+ depressed synaptic transmission in both preparations. 6. From the similarity in action between Mg2+ and the D-alpha-aminoadipate group of NMDA antagonists, it is suggested that the central depressant action of low concentrations of Mg2+ involves predominantly a postsynaptically mediated interference with the action of an excitatory amino acid transmitter.

Facilitation of synaptic activity in the frog spinal cord produced by 4-aminopyridine

Reflex activity in the isolated frog spinal cord is increased after adding 4-aminopyridine (4-AP) to the bath. Depression of synaptic transmission by increased magnesium or EGTA is effectively antagonized by 4-AP. It is suggested that 4-AP facilitates synaptic transmission in the central nervous system by interacting with the calcium mechanism involved in transmitter release from presynaptic nerve terminals.

The effects of nerve growth factor and its antiserum on synapses in the superior cervical ganglion of the guinea‐pig.

1. The effects of nerve growth factor (NGF) and its antiserum on synapses in the superior cervical ganglion of the guinea-pig have been examined by intracellular recording and electron microscopy. 2. Exogenous NGF, supplied locally from a silicone rubber pellet implanted near ganglia for 4-7 days, had little effect on either the function or the number of ganglionic synapses. 3. However, the depression of synaptic transmission and loss of synaptic contacts on ganglion cells which follow post-ganglionic axotomy were diminished by about 50% in the presence of exogenous NGF. 4. Other post-axotomy changes such as the development of subthreshold regenerative responses in neuronal processes, the appearance of ultrastructurally abnormal neuronal profiles suggesting rapid membrane turnover, and the cytoplasmic and nuclear changes characteristic of "chromatolysis", were also largely prevented by exogenous NGF. 5. Systemic treatment of neonatal and young adult guinea-pigs with antiserum to NGF for 4-5 days caused depression of intracellularly recorded synaptic responses within 5-8 days of the end of antiserum administration. Synapse counts in electron microscopical sections from these ganglia showed only about half as many contacts as in control ganglia from animals receiving normal rabbit serum. 6. These findings suggest that the loss of synapses from sympathetic neurones which follows axotomy results from a reduction in the amount of NGF supplied to ganglion cells. A corollary is that, among other biological roles, NGF is required by peripheral sympathetic neurones to maintain the synapses they receive.

Depression of synaptic transmission by diphenylhydantoin.

Diphenylhydantoin (phenytoin, DPH) depresses synaptic transmission at the frog neuromuscular synapse by presynaptic and postsynaptic mechanisms. In normal Ringer's solution the amplitude of the neurally evoked end-plate potentials and their quantal content are reduced. Somewhat paradoxically, miniature end-plate potential (mepp) frequency is increased by the drug. These effects could result if DPH blocked both calcium transport at the axonal membrane and intracellular calcium sequestration. Mepp amplitude is reduced, and DPH also induces nerve conduction block at high rates of stimulation. The relevance of these effects to the anticonvulsive activity of DPH is discussed.

Facilitation and depression of synaptic transmission in amphibian sympathetic ganglia

There have been few reports concerning facilitation and depression in sympathetic ganglia 9,17,40. In the present investigation, pairs of excitatory postsynaptic potentials (EPSPs) were recorded intracellulary from bullfrog paravertebral sympathetic ganglia for an analysis of the site and mechanism responsible for the phenomena of facilitation and depression of ganglionic transmission. The ratio of the amplitude of the second of a pair of EPSPs divided by the first was compared to the time interval between each pulse. These ratios demonstrated two phases: an earlier phase of facilitation (20–500 msec pulse intervals) and a later phase of depression (500 msec–10 sec). Additional parameters — rate of rise of synaptic potentials (dV/dt), synaptic currents (EPSCs), and synaptic conductances (G tr) — were determined and all confirmed the results obtained with EPSPs. Furthermore, the degree of facilitation or depression could be modulated by altering the extracellular concentration of calcium. On the other hand, comparison of the amplitude of pairs of presynaptic terminal spikes did not show any variability over similar stimulus intervals, nor were the amplitudes of miniature EPSPs significantly different before or after an evoked EPSP. Therefore, the processes of facilitation and depression of ganglionic transmission occur as a result of normal nerve terminal activity. The processes are occurring simultaneously, such that one or the other may predominate depending upon the interval between pulses, as well as the relative concentration of extracellular calcium. The results from these experiments have been interpreted by a hypothetical equation which demonstrates that facilitation of ganglionic transmission is due to activation of a ‘residual’ calcium and depression is due to depletion of readily available stores of acetylcholine.

Synaptic depression related to presynaptic axon conduction block.

1. The depression of synaptic transmission, which occurs during prolonged repetitive activation, was examined in the opener muscle of the crayfish walking leg. 2. Excitatory post-synaptic potentials (e.p.s.p.s) initially facilitated but then declined to low amplitudes after about 4000 stimulus pulses had been delivered; this depression is presynaptic in origin; 3. Axon conduction blocks occured at points of bifurcation along the entire length of the presynaptic nerve. This resulted in failure of the nerve impulse to invade some branches of the terminal arborization. 4. Nerve terminal invasion failure caused either intermittent or complete inactiviation of some synaptic release sites; this was associated with depression of the post-synaptic response. 5. The statistics of transmitter release during prolonged repetitive stimulation were examined by focal extracellular recording methods. Transmitter release could be described by binomial statistics, and depression involved a drop in m, n and p. 6. The rate of spontaneous quantal release did not decrease, however, arguing against transmitter depletion. 7. It is concluded that repetitive stimulation eventually leads to depolarization of the axon membrane. This causes impulse propagation failure which reduces the number of synaptic release sites that are activated and mimics a drop in the effective stimulation rate; both effects cause synaptic depression.

Effects of Anesthesia on Intermediary Metabolism

Major inhalational anesthetics cause inhibition in the electron transport chain in the region of Complex I resulting in decreased oxygen utilization, inhibition of metabolism of NAD-linked substrates, but not of succinate, inhibition of mitochondrial calcium uptake, and depression of synaptic transmission because of postulated changes in ACh sensitivity or GABA inhibition. Many cellular metabolic effects in CNS and other tissues are secondary to the above. Many metabolic changes noted with anesthetics occur subsequent to activation of the sympathetic nervous system either directly by the anesthetic or by surgical stimulation in the presence of light anesthesia. Many important studies remain to be done.

Effects of ketamine (CI 581) on cell responses to cutaneous stimulations in laminae IV and V in the cat's dorsal horn

The cells of laminae IV and V in the dorsal horn of the spinal cord are probably involved in the transmission of nociceptive messages to the upper centers. Pharmacological studies have shown that the analgesic proporties of some drugs could have their origin in the depression of synaptic transmission at this level. Therefore, the effect of ketamine on the activity of the laminae IV and V units was studied in spinal cats. Ketamine, 2–10 mg/kg, i.., did not modify significantly the spontaneous firing of the lamina IV cells, or their response to light cutaneous stimulations. Similarly, responses of the lamina V units to light stimulations (touch, hair movements, pressure) were not affected, while responses to nociceptive stimulations (pinching, strong electrical stimulation) were decreased. However, this effect is not sufficient to explain the whole analgesic action of ketamine and other sites of action must be found, sppecially at the subcortical and cortical levels.