Neurophysiological consequences of presynaptic receptor activation: changes in noradrenergic terminal excitability

Adrenoceptors Modulate Cholinergic Synaptic Transmission at the Neuromuscular Junction

Adrenoceptor activators and blockers are widely used clinically for the treatment of cardiovascular and pulmonary disorders. More recently, adrenergic agents have also been used to treat neurodegenerative diseases. Recent studies indicate a location of sympathetic varicosities in close proximity to neuromuscular junctions. The pressing question is whether there could be any effects of endo- or exogenous catecholamines on cholinergic neuromuscular transmission. It was shown that the pharmacological stimulation of adrenoceptors, as well as sympathectomy, can affect both acetylcholine release from motor nerve terminals and the functioning of postsynaptic acetylcholine receptors. In this review, we discuss the recent data regarding the effects of adrenergic drugs on neurotransmission at the neuromuscular junction. The elucidation of the molecular mechanisms by which the clinically relevant adrenomimetics and adrenoblockers regulate quantal acetylcholine release from the presynaptic nerve terminals and postsynaptic sensitivity may help in the design of highly effective and well-tolerated sympathomimetics for treating a number of neurodegenerative diseases accompanied by synaptic defects.

Sympathomimetics regulate quantal acetylcholine release at neuromuscular junctions through various types of adrenoreceptors

The studies of the interaction between the sympathetic and motor nervous systems are extremely relevant due to therapy for many neurodegenerative and cardiovascular disorders involving adrenergic compounds. Evidences indicate close contact between sympathetic varicosities and neuromuscular synapses. This raises questions about the effects of catecholamines on synaptic transmission. The currently available information is contradictory, and the types of adrenoreceptors responsible for modulation of neurotransmitter release have not been identified in mammalian neuromuscular synapses. Our results have shown that the α1A, α1B, α2A, α2B, α2C, and β1 adrenoreceptor subtypes are expressed in mouse diaphragm muscle containing neuromuscular synapses and sympathetic varicosities. Pharmacological stimulation of adrenoreceptors affects both spontaneous and evoked acetylcholine quantal secretion. Agonists of the α1, α2 and β1 adrenoreceptors decrease spontaneous release. Activation of the α2 and β1 adrenoreceptors reduces the number of acetylcholine quanta released in response to a nerve stimulus (quantal content), but an agonist of the β2 receptors increases quantal content. Activation of α2 and β2 adrenoreceptors alters the kinetics of acetylcholine quantal release by desynchronizing the neurosecretory process. Specific blockers of these receptors eliminate the effects of the specific agonists. The action of blockers on quantal acetylcholine secretion indicates possible action of endogenous catecholamines on neuromuscular transmission. Elucidating the molecular mechanisms by which clinically utilized adrenomimetics and adrenoblockers regulate synaptic vesicle release at the motor axon terminal will lead to the creation of improved and safer sympathomimetics for the treatment of various neurodegenerative diseases with synaptic defects.

Age-dependent interactive changes in serotonergic and noradrenergic cortical axon terminals in F344 rats

In the frontal cortex of aging rats, we found an increase in sprouting of the noradrenergic (NA) axons originated from the locus coeruleus (LC). The serotonergic (5-HT) axons originating from the dorsal raphe (DR) share the same cortical area and their age-dependent changes and interactions with NA axons were still unclear. To compare quantitatively the extent of axonal sprouting of DR and LC neurons in the frontal cortex, we extracellularly recorded from both DR and LC neurons in the same animals and antidromically stimulated 32 cortical sites (a pair of stimulating electrodes was moved at 100-mum intervals from 500 to 2000 microm in depth). In addition, to examine the effects of degeneration of 5-HT axons on NA axons, and vice versa, we used specific neurotoxins for 5-HT (PCA) or NA (DSP-4) axons. We also used noradrenaline uptake inhibitor (maprotiline) to verify the effects of NA on degeneration of 5-HT axons. Results suggested that 5-HT axons sprouted between 15 and 17 months of age and noradrenaline accelerated the age-dependent change of 5-HT axons.

Effects of BDNF infusion on the axon terminals of locus coeruleus neurons of aging rats

Using in vivo electrophysiological techniques and continuous local infusion methods, we examined the effects of brain-derived neurotrophic factor (BDNF) and its specific antibody (anti-BDNF) on the noradrenergic axon terminals of the locus coeruleus (LC) neurons in the frontal cortex of aging rats. Recently, we observed that LC neurons with multiple-threshold antidromic responses (multi-threshold LC neurons) increased critically between 15 and 17 months of age. To examine whether the BDNF is involved in this change occurred in the aging brain, we continuously infused BDNF into the frontal cortex for 14 days. Exogenous BDNF produced a marked increase in the multi-threshold LC neurons in the 13-month-old brain, accompanied with a decrease in threshold current. However, no morphological change in the noradrenergic axons was observed in the BDNF-infused cortex. In contrast, infusion of anti-BDNF led to a dose-dependent reduction of the multi-threshold LC neurons in the 19-month-old brain, accompanied with an increase in threshold current. These findings suggest that BDNF may contribute to functional changes in the presynaptic axon terminals of LC neurons in the aging brain.

The α2-adrenergic agonist guanfacine reduces excitability of human motor cortex through disfacilitation and increase of inhibition

OBJECTIVE

To test the acute effects of the alpha2-adrenoceptor agonist guanfacine (GFC) on motor excitability in intact humans.

METHODS

Eight healthy right-handed adults received a single oral dose of 2 mg of GFC. Motor cortex excitability was tested by focal transcranial magnetic stimulation of the hand area of the left motor cortex. Motor evoked potentials (MEP) were recorded from the right abductor pollicis brevis muscle. In addition, spinal and neuromuscular excitability were tested. All measures were obtained immediately before GFC intake (baseline), and 2, 6, and 24 h later.

RESULTS

GFC decreased the slope of the MEP intensity curve, increased paired-pulse short-interval intracortical inhibition, and decreased paired-pulse intracortical facilitation and I-wave facilitation. These effects were maximal at 2-6 h and returned to baseline at 24 h. Motor threshold, cortical silent period, and the measures of spinal (peripheral silent period, F waves) and neuromuscular excitability (maximum M wave) remained unaffected.

CONCLUSIONS

This is the first study on the effects of an anti-noradrenergic drug on human motor cortex excitability. GFC reduced cortical excitability by disfacilitation and increased inhibition. These findings support the idea that anti-noradrenergic drugs are detrimental for cortical plasticity and learning which are down-regulated by disfacilitation or increased inhibition.

Age-related changes in the release and uptake activity of presynaptic axon terminals of rat locus coeruleus neurons

Age-related changes in the release and uptake activity of presynaptic axon terminals of rat locus coeruleus (LC) noradrenergic neurons were studied in the frontal cortex using an extracellular single unit recording technique in vivo. Clonidine, a selective alpha(2) adrenergic agonist, and nisoxetine, a selective noradrenaline uptake inhibitor, were infused locally into the frontal cortex to examine the effects of these drugs on release and uptake activities of the axon terminals of LC neurons. Although the infusion of clonidine produced a marked suppression of release, the effect did not change with age. Infusion of nisoxetine caused an inhibition of uptake, but the effect was attenuated in aged rats. These results suggest that the release activity mediated by the presynaptic autoreceptor did not change with age, but the uptake activity mediated by the NA transporter declined with age in the axon terminals of LC neurons.

Changes in cortical noradrenergic axon terminals of locus coeruleus neurons in aged F344 rats

The noradrenergic innervations and noradrenaline contents of the frontal cortex in two age groups (9 and 25 months) of male F344 rats have been quantified by electrophysiological and biochemical methods. In the electrophysiological study, the percentage of locus coeruleus (LC) neurons activated antidromically from the frontal cortex decreased with age. In contrast, the percentage of LC neurons showing multiple antidromic latencies, which suggests axonal branching of individual LC neurons, increased markedly between 9 and 25 months in the frontal cortex. In the biochemical study, we found no significant difference in noradrenaline levels in the cortical terminal fields of LC neurons during aging. These results suggest that LC neurons give rise to axonal branches to retain noradrenaline levels in their target fields in the aged brain. Our findings show that LC neurons preserve a strong capability for remodeling their axon terminals even in the aged brain.

Changes in electrophysiological properties of axon terminals of locus coeruleus neurons with age in F344 rat

Age-dependent changes in electrophysiological properties of locus coeruleus (LC) neurons were studied in urethane anesthetized male F344 rats aged 8, 17 and 22 months. These properties, such as spontaneous firing rate, conduction time, and threshold currents were measured for individual LC neurons antidromically activated from terminals in the frontal cortex. We found no change in the electrophysiological properties with age except for an increased number of low-threshold LC neurons in 22-month-old rats compared to that of 8-month-old rats. These results suggested that the electrophysiological properties of cortical axon terminals of LC neurons changed with age.

Amphetamine-induced changes in nigrostriatal terminal excitability are modified following repeated amphetamine pretreatment

To investigate neural mechanisms associated with behavioral sensitization to amphetamine, we studied the effect of an intrastriatal infusion of amphetamine on nigrostriatal axon terminal electrical excitability in rats following withdrawal from repeated systemic treatment. Rats were injected with amphetamine 2.5 mg/kg s.c. or saline daily for 4 days. Either 24 h or 14 days after the last injection, extracellular recordings were obtained from dopaminergic neurons of the substantia nigra, in a blind design in which the experimenter did not know the pretreatment regime. In order to assess the electrical excitability of the nigrostriatal axonal field, neurons were activated antidromically by stimulating their terminal fields in the striatum. As previously reported, striatal infusion of amphetamine (1 microM/0.3 microliter) in control animals resulted in a significant reduction in excitability as indicated by an increase in striatal stimulus current necessary to evoke antidromic activity. In contrast, intrastriatal amphetamine administration to amphetamine-pretreated animals did not decrease excitability. Spontaneous firing rates and patterns of cell discharge did not differ between saline- and amphetamine-treated animals. The chronic amphetamine-induced change in the effect of an acute intrastriatal amphetamine infusion on nigrostriatal terminal excitability may be due to enduring alterations in the amphetamine-induced release of dopamine and other striatal neurotransmitters or to changes in the sensitivity of presynaptic hetero- and/or autoreceptors on the dopaminergic axons.

Backpropagation of action potentials generated at ectopic axonal loci: hypothesis that axon terminals integrate local environmental signals

This review deals with the fascinating complexity of presynaptic axon terminals that are characterized by a high degree of functional distinctiveness. In vertebrate and invertebrate neurons, all-or-none APs can take off not only from the axon hillock, but also from ectopic axonal loci including terminals. Invertebrate neurons display EAPs, for instance alternating with somatic APs, during survival functions. In vertebrate, EAPs have been recorded in the peripheral and central nervous systems in time relationship with physiological or pathological neuronal activities. In motor or sensory axon, EAP generation may be the cause of motor dysfunctioning or sensory perceptions and pain respectively. Locomotion is associated with rhythmic depolarizations of the presynaptic axonal membrane of primary afferents, which are ridden by robust EAP bursts. In central axons lying within an epileptic tissue EAP discharges, coinciding with paroxysmal ECoG waves, get longer as somatic discharges get shorter during seizure progression. Once invaded by an orthodromic burst, an ectopic axonal locus can display an EAP after discharge. Such loci can also fire during hyperpolarization or the postinhibitory excitatory period of the parent somata, but not during their tonic excitation. Neurons are thus endowed with electrophysiological intrinsic properties making possible the alternate discharges of somatic APs and EAPs. In invertebrate and vertebrate neurons, ectopic axonal loci fire while the parent somata stop firing, further suggesting that axon terminal networks are unique and individual functional entities. The functional importance of EAPs in the nervous systems is, however, not yet well understood. Ectopically generated axonal APs propagate backwards and forwards along the axon, thus acting as a retrograde and anterograde signal. In invertebrate neurons, somatically and ectopically generated APs cannot have the same effect on the postsynaptic membrane. As suggested by studies related to the dorsal root reflex, EAPs may not only be implied in the presynaptic modulation of transmitter release but also contribute significantly during their backpropagation to a powerful control (collision process) of incoming volleys. From experimental data related to epileptiform activities it is proposed that EAPs, once orthodromically conducted, might potentiate synapses, initiate, spread or maintain epileptic cellular processes. For instance, paroxysmal discharges of EAPs would exert, like a booster-driver, a powerful synchronizing synaptic drive upon a large number of excitatory and inhibitory postsynaptic neurons. We have proposed that, once backpropagated, EAPs are likewise capable of initiating (and anticipating) threshold and low-threshold somatodendritic depolarizations. Interestingly, an antidromic EAP can modulate the excitability of the parent soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Tandospirone and its metabolite, 1-(2-pyrimidinyl)-piperazine--II. Effects of acute administration of 1-PP and long-term administration of tandospirone on noradrenergic neurotransmission

1-(2-Pyrimidinyl)-piperazine (1-PP) is a common metabolite of the antidepressant/anxiolytic 5-HT1A agonists, tandospirone (SM-3997), gepirone, buspirone and ipsapirone. The present electrophysiological studies were undertaken to characterize in vivo the effect of 1-PP on noradrenergic (NE) neurotransmission in rat brain. At small doses, 1-PP (ED50 = 80 micrograms/kg, i.v.) reversed the depressant effect of the alpha 2-adrenoceptor agonist, clonidine (20 micrograms/kg, i.v.) on the firing activity of NE neurones of the locus coeruleus. After long-term treatment with tandospirone (10 mg/kg/day, s.c. x 14 days), the responsiveness of these NE neurones to intravenous administration of clonidine was decreased but their mean firing frequency remained within the control range. The effect of 1-PP on the postsynaptic alpha 2-adrenoceptor of pyramidal neurones in the hippocampus was investigated: intravenous administration of 1-PP (2-8 mg/kg, i.v.) reduced the effect of microiontophoretically-applied NE on CA3 pyramidal neurones of the dorsal hippocampus, without affecting their responsiveness to GABA and 5-HT. The effect of the electrical stimulation of NE neurones of the locus coeruleus in reducing firing activity of pyramidal neurones, which is mediated by postsynaptic alpha 1-adrenoceptors, was increased by 47% after acute administration of 1-PP (4 mg/kg, i.v.), presumably as a result of blockade of terminal alpha 2-autoreceptors. The effectiveness of these stimulations remained unchanged after long-term treatment with tandospirone. Furthermore, the decrease in the effectiveness of stimulation of the locus coeruleus, obtained by increasing the frequency from 1 to 5 Hz, a phenomenon due to an increased activation of terminal alpha 2-adrenergic autoreceptors by endogenous NE, remained unaltered after long-term treatment with tandospirone. In addition to the initial depressant effect, stimulation of the locus coeruleus induces a late activation of these neurones which is mediated by a beta-adrenoceptor. The degree of activation induced by stimulation of the locus coeruleus was similar in controls and in long-term tandospirone-treated rats. It is concluded that 1-PP acts as an antagonist at somatodendritic and terminal alpha 2-adrenergic autoreceptors, as well as at postsynaptic alpha 2-adrenoceptors, in the central nervous system of the rat. However, the levels of 1-PP attained after long-term administration of tandospirone were not sufficient to modify NE neurotransmission.

Axonal sprouting of noradrenergic locus coeruleus neurons following repeated stress and antidepressant treatment

Plastic changes in axon terminals of NA LC neurons following repeated stress and antidepressant treatments were examined using electrophysiological or morphological methods. For stress treatment, rats restrained in a small cage were immersed up to the neck in warm water for 10 min daily. Electrophysiological experiments were performed under urethane anesthesia on the day following the termination of stress treatment. To quantify the density of cortical axon terminals arising in the LC, the percentage of LC neurons activated antidromically from the cerebral cortex was assessed. The percentage of LC neurons showing antidromic response to cortical stimulation was increased in the animals stressed for two weeks but not for one week. Since threshold currents for antidromic activation were not changed by the stress treatment, the observed changes were interpreted as morphological (axonal sprouting) rather than physiological consequences in NA axon terminals of LC neurons. To test the ability of antidepressants to induce the regeneration of central NA axons, local injections of 6-OHDA were made bilaterally into the symmetrical sites of the FC. Two weeks after the 6-OHDA injections, the same cortical site of one hemisphere was infused with the antidepressant MPL, DMI, or MIA, and the corresponding site of the other hemisphere with SAL. The density of glyoxylic acid-induced catecholamine fibers was greater in the cortical hemisphere infused with the antidepressants than that infused with SAL. These findings indicate that repeated mild stress and antidepressant treatments induce sprouting of NA LC axons in the cerebral cortex. Axonal sprouting of LC neurons can explain both the delayed onset of the clinical response to antidepressants and subsensitivity of beta-adrenoceptors following repeated stress and antidepressant treatments, and may be a common mechanism for the clinical efficacy of antidepressant drugs and electroconvulsive shock. Furthermore, the findings suggest the possibility that axonal retraction or degeneration of central NA neurons may be involved, at least in part, in the pathology of clinical depression.

Transient elevation of amygdala alpha 2 adrenergic receptor binding sites during the early stages of amygdala kindling

Enhanced noradrenergic neurotransmission retards but does not prevent the development of kindling. We previously reported that locus coeruleus (LC) alpha 2 adrenergic receptor binding sites are transiently elevated during the early stages of kindling development. Since the firing activity of LC noradrenergic neurons is partially regulated via an alpha 2 receptor-mediated recurrent inhibition, the transient elevation in LC alpha 2 receptors could decrease LC activity and consequently facilitate the development of kindling. Transient elevation of alpha 2 receptor binding sites during early stages of kindling may also occur on noradrenergic axon terminals projecting to forebrain sites. Using in vitro neurotransmitter autoradiography techniques, we investigated this hypothesis by measuring specific [3H]idazoxan binding in 5 different areas of rat forebrain at 2 different stages of kindling development. After 2 class 1 kindled seizures, specific [3H]idazoxan binding was elevated significantly in the amygdala, but not in other forebrain regions. No differences in specific [3H]idazoxan binding were observed in any of the 5 brain regions in rats kindled to a single class 5 kindled motor seizure. Saturation of binding experiments indicated that the increase in amygdala [3H]idazoxan binding, following 2 class 1 kindled motor seizures, was due to an increase in the total number of alpha 2 receptor binding sites without a change in the affinity of the binding sites for [3H]idazoxan. Thus, the transient increase in alpha 2 receptors that occurs in the LC in the early stages of kindling also occurs in the forebrain region in which the kindled seizure originates.

Electrophysiological characterization of adrenoceptors in the rat dorsal hippocampus. III. Evidence for the physiological role of terminal alpha 2-adrenergic autoreceptors

The present electrophysiological studies were undertaken to assess the role of terminal alpha 2-adrenergic autoreceptors in regulating noradrenergic synaptic transmission in the rat CNS. The effectiveness of the electrical stimulation of the locus coeruleus (LC) in suppressing the firing activity of pyramidal neurons was determined in the dorsal hippocampus. Intravenous clonidine, an alpha 2-adrenergic agonist, decreased the effectiveness of the LC stimulation, without altering the effect of microiontophoretically applied norepinephrine. The subsequent i.v. administration of low doses of idazoxan, an alpha 2-adrenergic antagonist, reversed this effect of clonidine on the LC stimulation. To ascertain that the effect of clonidine administered i.v. was indeed attributable to its action on noradrenergic terminals, it was applied locally by microiontophoresis; it decreased the effectiveness of the LC stimulation. Another paradigm used to assess the function of terminal alpha 2-adrenoceptors was to increase the frequency of the LC stimulation from 1 to 5 Hz. This resulted in a 5-fold decrease of the effectiveness of the stimulation. That this was attributable to an enhanced activation of terminal alpha 2-adrenoceptors was suggested by the reversal of this effect of increasing the frequency of the LC stimulation by intravenous idazoxan. Furthermore, the degree of enhancement of the effectiveness of the LC stimulation by idazoxan was much greater at 5 than at 1 Hz. These results provide novel electrophysiological evidence for the potent regulatory role of terminal alpha 2-adrenoceptors on noradrenergic neurotransmission.

A mechanism for the involvement of colocalized neuropeptides in the actions of antipsychotic drugs

Evidence has accumulated to implicate neuropeptides localized within midbrain dopamine neurons (cholecystokinin, neurotensin, acetylcholinesterase) in synaptic transmission, mental disease, and pharmacotherapy. We suggest a means by which antipsychotic drugs alter the dynamics between dopamine and colocalized peptides: the intrinsic ability of these agents to stimulate dopamine neuronal activity while blocking dopamine receptors modulates the ratio of catecholaminergic to peptidergic transmission within the mesotelencephalic system. Imbalances of peptide and dopamine cotransmission and their modulation by neuroleptics may be relevant to the pathogenesis and pharmacotherapy of schizophrenia.

Electrophysiological evidence for terminal sprouting of locus coeruleus neurons following repeated mild stress

To see if repeated mild stress causes plastic changes in central noradrenergic terminal axons, the density of terminal axons arising in locus coeruleus (LC) neurons of rats was quantified by antidromic stimulation technique. After the termination of stress treatments (immersion in warm water for 10 min daily) for 1 or 2 weeks, electrophysiological experiments were performed under urethane anesthesia. The frequency of LC neurons activated antidromically from the cerebral cortex increased in rats stressed for 2 weeks but not for 1 week. Since the increased frequency of antidromic responses was not due to a change in terminal excitability, the change observed here is considered to be morphological (terminal sprouting) rather than a physiological consequence. The results suggest that LC neurons dynamically alter their terminal morphology in response to environmental stimuli.

Transient elevation of locus coeruleus alpha 2-adrenergic receptor binding during the early stages of amygdala kindling

Enhancement of noradrenergic neurotransmission retards, but does not prevent, the development of kindling. The firing activity of noradrenergic locus coeruleus (LC) neurons is partially regulated by axon collateral recurrent inhibition mediated via alpha 2-adrenergic receptors. We tested the hypothesis that LC autoinhibitory alpha 2-adrenergic receptors may change during the kindling process thereby altering LC excitability. Specific binding of the alpha 2-adrenergic receptor antagonist [3H]RX781094 (idazoxan) was measured in the LC of rats at 3 different stages of kindling development using in vitro neurotransmitter receptor autoradiography techniques. Specific [3H]RX781094 binding was elevated significantly in rats kindled to two Class 1 kindled motor seizures. No differences in binding were observed in animals kindled to Class 3 or Class 5 kindled motor seizures. Saturation of binding experiments indicated that the increase in binding following two Class 1 kindled motor seizures was due to an increase in the total number of alpha 2-receptors without a change in the affinity of the binding site for [3H]RX781094. The transient increase in number of LC alpha 2-adrenergic receptors is consistent with the idea that noradrenergic neurotransmission inhibits the early progress of kindling development, but then subsequently becomes ineffective in maintaining the inhibition during later stages of kindling development.

Mesocortical dopaminergic neurons. 2. Electrophysiological consequences of terminal autoreceptor activation

Measurement of drug- and stimulation-induced changes in the electrical excitability of dopaminergic terminals was employed to assess the effects of stimulation of dopamine terminal autoreceptors in the prefrontal cortex in urethane-anesthetized rats. Systemic or local administration of amphetamine decreased, whereas systemic administration of haloperidol increased the excitability of prefrontal cortical dopaminergic terminals of ventral tegmental area dopaminergic neurons. Mesoprefrontal dopaminergic terminal excitability was also responsive to spontaneous and stimulation-induced alterations in the rate of impulses reaching the terminal fields. These results are comparable to those previously reported for nigrostriatal and mesoaccumbens dopaminergic neurons, and are discussed with regard to the operational characteristics of autoinhibition in the mesocortical dopaminergic system.

Inhibition of K+ permeability diminishes alpha 2-adrenoceptor mediated effects on norepinephrine release

The effect of two different potassium channel blockers, 4-aminopyridine (4-AP) and quinine, on the alpha 2-adrenoceptor mediated modulation of norepinephrine (NE) release was investigated. Pairs of mouse vasa deferentia were loaded with 3H-norepinephrine (3H-NE), superfused continuously, and stimulated electrically. 4-AP (5.3 x 10(-4) M), and quinine (10(-5) M) enhanced the stimulation-evoked release of tritium significantly. The electrically induced release of radioactivity was reduced by alpha 2-adrenoceptor agonists (1-NE and xylazine) and enhanced by the alpha 2-adrenoceptor antagonist yohimbine. Both effects were affected markedly by 4-AP or quinine: the depressant action of 1-NA and xylazine was partially antagonized and the facilitatory effect of yohimbine was completely abolished during the blockade of the potassium channels. It is suggested that the blockade of the potassium permeability counteracts negative feedback modulation; therefore, it seems likely that the stimulation of alpha 2-adrenoceptors leads to an enhanced potassium permeability and hyperpolarization of varicose axon terminals.

Adaptive changes of beta-adrenergic receptors after neonatal locus coeruleus lesion: regulation of serotoninergic unit activity

Spontaneous activity of 5-hydroxytryptamine (5-HT) neurons in the dorsal raphe nucleus (DRN) was recorded in adult rats that had undergone a bilateral locus coeruleus (LC) lesion during the neonatal period. The susceptibility of this neuronal firing to beta-adrenergic manipulation was tested. Microiontophoretic application of the beta-blockers d,l-propranolol and acebutolol inhibited the firing of DRN cells in lesioned rats but not in control animals. This effect was specific to beta-receptors since the effects of pharmacological manipulation of other receptors--5-HT, gamma-aminobutyric acid (GABA), alpha-adrenoceptors--were identical in lesioned and control animals. The present data demonstrate that a neonatal noradrenergic lesion allowed the persistence of a beta-regulation of DRN neuronal firing, which in young rats is normally only transient.

Dopamine acts on D2 receptors to increase potassium conductance in neurones of the rat substantia nigra zona compacta

1. Intracellular recordings were made from neurones in the substantia nigra zona compacta in slices of rat mesencephalon in vitro. The majority of neurones fired action potentials spontaneously at 0.2-5.6 Hz. Dopamine, applied either by superfusion or from the tip of a pressurized pipette, prevented spontaneous action potential firing and hyperpolarized the membrane. 2. When the membrane potential was held negative to the threshold for action potential firing, the hyperpolarization evoked by dopamine was accompanied by a fall in input resistance. Under voltage clamp, dopamine produced an outward membrane current associated with an increase in membrane conductance. The effects of superfused dopamine on firing rate, membrane potential and membrane current were concentration dependent in the range 1-100 microM. 3. The reversal potential for the hyperpolarizations and the outward currents produced by dopamine was -109.7 +/- 1.7 mV (n = 12) when the potassium concentration was 2.5 mM and -74.0 +/- 5.0 mV (n = 4) when the potassium concentration was 10.5 mM. The change in reversal potentials in these and intermediate potassium concentrations was described by the Nernst equation. 4. The outward current induced by dopamine was reversibly reduced by barium (100-300 microM) and by high concentrations of tetraethylammonium (greater than or equal to 10 mM). Calcium-free solutions with cobalt (0.5-2 mM) did not reduce the current in response to dopamine during the first 5 min of their application. Currents and hyperpolarizations caused by dopamine were unaffected by tetrodotoxin (1 microM). 5. The hyperpolarization produced by dopamine was mimicked by the D2 receptor agonist quinpirole (LY 171555, 0.1-3 microM) and was blocked by the D2 receptor agonists domperidone and (-)-sulpiride. Agonists and antagonists at D1 receptors had no effect. 6. (-)-Sulpiride (30 nM-30 microM) produced a progressive shift to the right in the concentration-response curve to either dopamine or quinpirole. Schild analysis of the antagonism between (-)-sulpiride and quinpirole suggested competitive antagonism with a dissociation equilibrium constant for (-)-sulpiride of about 13 nM. 7. It is concluded that dopamine acts on D2 receptors on neurones of the rat substantia nigra pars compacta to increase the membrane potassium conductance.

Modulation of the excitability of septohippocampal terminals in the rat: relation to neuronal discharge rate

The excitability of the axonal terminals of medial septal neurons projecting to the dentate gyrus has been studied in the anesthetized rat under various experimental conditions: spontaneous or drug-induced variations in neuronal soma discharge rate, conditioning stimulation of afferent pathways (perforant path, commissural pathway, fimbria-fornix). It has been observed that terminals excitability is inversely correlated to the level of neuronal ongoing activity. These effects were observed on virtually all septal neurons projecting to the dentate gyrus. Since about one half of the septohippocampal neurons are likely to be cholinergic, it follows that such a phenomenon is not transmitter specific.

Presynaptic effects of scopolamine, oxotremorine, noradrenaline and morphine on [3H]acetylcholine release from the myenteric plexus at different stimulation frequencies and calcium concentrations

The inhibition by three modulators (oxotremorine, noradrenaline, morphine) of acetylcholine release from the myenteric plexus preincubated with [3H]choline was investigated at different stimulation frequencies and calcium concentrations. Moreover, [3H]acetylcholine release evoked by a low (0.1 Hz) or a high (10 Hz) stimulation rate was investigated at different calcium concentrations either in the absence or presence of scopolamine. A reduced calcium concentration (0.6 mmol/l) inhibited acetylcholine release more at 0.1 Hz (74% +/- 3%) than at 10 Hz (44% +/- 8%). Scopolamine enhanced the stimulated acetylcholine release at a calcium concentration of 1.8 mmol/l. At calcium concentrations higher than 1.8 mmol/l scopolamine failed to enhance transmitter release markedly. A reduction of the calcium concentration (less than 1.8 mmol/l) significantly enhanced the effect of scopolamine, when acetylcholine release was evoked at 0.1 Hz. Oxotremorine (10 mumol/l) completely suppressed acetylcholine release at 1 Hz (120 pulses). When 120 pulses were applied at 10 Hz the maximal effect was only a 64% inhibition and the concentration-response curve was significantly shifted to the right. However, after a reduction of both the train length or the calcium concentration oxotremorine produced a complete inhibition of acetylcholine release evoked at 10 Hz. In contrast to the effect of oxotremorine, the concentration-response curves for morphine and noradrenaline were similar at 1 Hz and 10 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Pre- and postsynaptic actions of noradrenaline and clonidine on myenteric neurons

alpha-2 adrenergic agonists inhibit nicotinic excitatory postsynaptic potentials and reduce calcium dependent action potentials in myenteric neurons. To test the hypothesis that adrenergic inhibitory effects on action potential configuration and on inhibition of acetylcholine release from the nerve terminal are analogous processes, the pharmacological characteristics and underlying mechanisms of these two effects were compared in neurons of the myenteric plexus in the guinea pig. Both clonidine and noradrenaline reduced the nicotinic fast excitatory postsynaptic potential in a concentration dependent manner, although the maximum effect produced by noradrenaline was greater. The specific alpha-2 antagonist RX781094 blocked the action of noradrenaline, with an apparent Kd value of 3.8 +/- 1 nM. Clonidine was similarly antagonized by low concentrations of this compound. The potassium channel blocker barium prevented inhibition of the fast excitatory postsynaptic potential by clonidine but not by noradrenaline. Action potentials recorded from after-hyperpolarization neurons with cesium chloride filled electrodes were prolonged in duration due to the blockade by cesium of outward potassium movement. Under these conditions, noradrenaline reduced action potential duration and slowed the rate of rise of the calcium dependent component in the presence of tetrodotoxin. RX781094 antagonized the latter effect with an estimated apparent Kd of 5.8 +/- nM. The rate of rise of the calcium dependent action potential was not affected by clonidine (30 nM to 1 microM). In the absence of potassium channel blockers noradrenaline caused hyperpolarizations which were blocked by RX781094. It has been previously shown that clonidine hyperpolarizes myenteric neurons via an adrenergically mediated increase in potassium conductance. It was concluded that the characteristics of presynaptic inhibition of release by noradrenaline and clonidine parallel the respective actions of these agonists on action potential configuration. While all clonidine effects could be explained on the basis of an increase in potassium conductance, noradrenaline exerted an additional inhibitory action which persisted in the presence of potassium channel blockade by barium or cesium.

Receptors on individual neurones

Membrane potential or ionic conductance of neurones of the mammalian central or peripheral nervous system maintained in vitro can be measured over periods of several hours. Drugs or transmitters which change potential or conductance can be applied repeatedly under equilibrium conditions, and pharmacological null methods used to characterize the receptors with which they interact. The method offers an advantage over ligand binding studies on nervous tissue because both agonist and antagonist affinities can be estimated on individual functioning cells. The results to date suggest the hypothesis that a given receptor subtype is always associated with the same change in ion conductance, and the corollary that distinct ion conductances affected by the same transmitter result from interactions with different receptor subtypes.

Autoreceptor-mediated changes in dopaminergic terminal excitability: effects of potassium channel blockers

The effects of the potassium channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA), on autoreceptor-mediated changes in dopaminergic terminal excitability were examined in urethane-anesthetized rats. Local infusions of 4-AP or TEA into neostriatal terminal fields of nigral dopaminergic neurons led to marked decreases in terminal excitability, as measured by the increase in stimulating current required to activate the neurons antidromically from the site of the infusion. The decreased excitability resulting from 4-AP could be reversed by subsequent i.v. injection of haloperidol, and was blocked in rats that had been depleted of endogenous dopamine by prior treatment with alpha-methyl-p-tyrosine (AMpT). Thus, the decrease in excitability elicited by the potassium channel-blockers was indirect, and apparently due to increased autoreceptor stimulation resulting from enhanced transmitter release. In addition, co-infusion of 4-AP and apomorphine in AMpT-treated animals led to decreased terminal excitability that did not differ from the effects of apomorphine alone, indicating that 4-AP did not block the effects of exogenous autoreceptor agonist administration. These results provide in situ electrophysiological evidence that autoreceptor-mediated processes occurring at dopaminergic terminals are not mediated by 4-AP- or TEA-sensitive potassium channels. Furthermore, our findings suggest that, as in other types of presynaptic terminals, blockade of voltage-sensitive potassium channels in dopamine terminals leads to enhanced release of transmitter.

Modulation of terminal excitability of mesolimbic dopaminergic neurons by D-amphetamine and haloperidol

Electrophysiological techniques were used to study the changes in the terminal excitability of mesolimbic DA and non-DA neurons following the infusion of D-amphetamine (D-AMP) and haloperidol (HAL) into the nucleus accumbens (NAc) of rats. The amount of current needed to evoke antidromic spikes by electrical stimulation of the NAc was used as an index of the excitability of axon terminals of these neurons. The excitability of DA neurons was decreased by D-AMP and increased by HAL. In addition, the effect produced by D-AMP was reversed by HAL. By contrast, these drugs either induced an opposite effect or were ineffective in inducing changes on the excitability of nerve terminals of mesolimbic non-DA neurons. Infusion of the vehicle or saline produced no effect. D-AMP and HAL were still effective in modulating the excitability of mesolimbic DA nerve terminals after the destruction of NAc neurons by ibotenic acid. The results suggest that the effects seen after D-AMP and HAL are mediated primarily by DA autoreceptors. It is likely that the increase in the current needed for evoking antidromic spikes after infusion of D-AMP into the terminal region is the consequence of DA autoreceptor-mediated hyperpolarization of terminal membranes. On the other hand, HAL could exert its actions by blocking autoreceptor-mediated hyperpolarization.

Clonidine and cortical plasticity: possible evidence for noradrenergic involvement

In order to test the hypothesis that noradrenergic transmission modulates ocular dominance plasticity in kitten visual cortex, we monocularly deprived kittens while administering the alpha-2 adrenergic agonist clonidine (CLON). To avoid bias in testing the hypothesis, we included, with a single blind technique, saline-treated control kittens in the series. First, using high-pressure liquid chromatography, we demonstrated that CLON treatments resulted in an average decline in cerebrospinal fluid levels of the norepinephrine metabolite, 3-methoxy-4-hydroxy phenylethylene glyolol (MHPG) of 44%. Then, single-unit recording in area 17 revealed the expected ocular dominance (OD) shift in monocularly deprived saline controls, but recording failed to find a significant shift in CLON-treated kittens. Our results support the notion that CLON treatment interferes with ocular dominance plasticity by inhibiting noradrenergic transmission in visual cortex. We discuss side effects of CLON, concluding that CLON's sedative effect may contribute to the lack of OD shift.

Amphetamine's effects on terminal excitability of noradrenergic locus coeruleus neurons are impulse-dependent at low but not high doses

The actions of amphetamine in the locus coeruleus and its terminal fields in the frontal cortex were studied using extracellular recording to measure terminal excitability, firing rate and the probability of antidromic action potential invasion of the somatodendritic region in urethane anesthetized rats. At low dose (0.25 mg/kg), amphetamine increased terminal excitability. In comparison, subsequent administration of the highest dose (5.0 mg/kg, i.v.) of amphetamine tested suppressed neuronal firing and blocked antidromic action potential invasion of the somatodendritic region. Despite the absence of impulse traffic, high dose amphetamine reversed the effect of low dose amphetamine in the terminal field and decreased terminal excitability. The alpha 2 antagonist, yohimbine (0.5 mg/kg, i.v.), reversed the effects of high dose amphetamine on terminal excitability and somatodendritic invasion without reinstating neuronal firing. Noradrenergic autoreceptor agonists are known to decrease terminal excitability, whereas antagonists are known to increase terminal excitability. Thus, since low dose amphetamine produces the same effect on terminal excitability that antagonists do, it appears that low dose amphetamine may reduce autoreceptor activation by reducing norepinephrine release in frontal cortex as a consequence of inhibiting locus coeruleus neuronal firing. In contrast, high dose amphetamine acts like autoreceptor agonists do and decreased terminal excitability. Hence high dose amphetamine may increase norepinephrine release, even in the absence of impulse traffic.

Antidromic activation of dorsal raphe neurons from neostriatum: physiological characterization and effects of terminal autoreceptor activation

Three types of neurons, distinguished on the basis of their spontaneous firing rates and patterns, extracellularly recorded waveforms and responses to neostriatal stimulation, were observed in the dorsal raphe nucleus in urethane-anesthetized rats. Type 1 neurons (presumed to be serotonergic) fired spontaneously from 0.1 to 3 spikes/s in a regular pattern, with initial positive-going bi- or triphasic action potentials. Type 1 cells exhibited long-latency antidromic responses to neostriatal stimulation (mean +/- S.E.M. 24.9 +/- 0.3 ms) that sometimes occurred at discrete multiple latencies, and supernormal periods persisting up to 100 ms following spontaneous spikes. Type 2 cells fired spontaneously in an irregular, somewhat bursty pattern from 0 to 2 spikes/s with initial negative-going biphasic spikes, and were antidromically activated from neostriatal stimulation at shorter latencies than Type 1 cells (21.8 +/- 0.9 ms). Type 3 cells were characterized by initial positive-going biphasic waveforms and displayed a higher discharge rate (5-30 spikes/s) than Type 1 or Type 2 cells. Type 3 cells could not be antidromically activated from neostriatal stimulation. The relatively long conduction time to neostriatum of the Type 1 presumed serotonergic neuron is discussed with respect to previous interpretations of the synaptic action of serotonin in the neostriatum. In conjunction with these antidromic activation studies, the neurophysiological consequences of serotonergic terminal autoreceptor activation were examined by measuring changes in the excitability of serotonergic terminal fields in the neostriatum following administration of the serotonin autoreceptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The excitability of serotonergic terminal fields was decreased by intravenous injection of 40 micrograms/kg 5-MeODMT, and by infusion of 10-50 microM 5-MeODMT directly into the neostriatum. These results are interpreted from the perspective of mechanisms underlying autoreceptor-mediated regulation of serotonin release.

Morphine and specific changes in the sensitivity of noradrenergic receptors within the "limbic" part of the feline caudate nucleus: a behaviour study

The present study describes the behaviour effects of intracerebral injections of the noradrenergic (NE) agonist oxymetazoline and the NE antagonist phentolamine into the "limbic" part of the caudate nucleus of cats primed 24 hr earlier and/or treated acutely with morphine (5 mg/kg, IP). Drug-induced changes in the morphine-specific behaviour served as dependent variables. Experiments were performed during two different periods of the year, each of them marked by a characteristic sensitivity of alpha-like NE receptors to NE agents, viz. the so-called NE "antagonist" period during which the NE receptors were sensitive to the NE antagonist phentolamine and the so-called NE "agonist" period during which the NE receptors were sensitive to NE and the NE agonist oxymetazoline. The present study demonstrates that morphine reversed the initial sensitivity to oxymetazoline respectively insensitivity to phentolamine in animals tested in the NE "agonist" period. In animals tested in the NE "antagonist" period morphine did not reverse the initial insensitivity to oxymetazoline resp. sensitivity to phentolamine. Furthermore, evidence is provided that the initial sensitivity to NE agents did not conspicuously determine the animal's response to the acute administration of morphine. The data are discussed in view of the concept that the firing rate of NE fibres determines the actual sensitivity of presynaptic and postsynaptic NE receptors to NE agonists and antagonists.

Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus in a slice of tissue cut from the rat pons. Clonidine (100 nM-10 microM), noradrenaline (10 microM-1 mM) and adrenaline (10 microM-1 mM) all reduced the duration of the spontaneously occurring action potential of the neurones. This effect was also observed on the action potential in the presence of tetrodotoxin, which results from calcium entering the cell. These concentrations of clonidine, noradrenaline and adrenaline always hyperpolarized the membrane. This hyperpolarization was prevented by two procedures which block potassium currents--intracellular caesium and extracellular barium. In conditions of potassium current blockade, noradrenaline (100 microM-1 mM) and adrenaline (20 microM-1 mM) shortened the calcium action potential but clonidine was ineffective even at 10 microM. Adrenaline and noradrenaline also suppressed inward calcium and barium currents measured under voltage clamp. This action of noradrenaline and adrenaline was not prevented by yohimbine (10 microM), propranolol (20 microM) or prazosin (1 microM); it was reduced by a concentration of phentolamine about 100 times higher than its Ke for alpha 2-adrenoceptors on locus coeruleus neurones. It is concluded that noradrenaline and adrenaline can directly inhibit calcium action potentials in locus coeruleus neurones when applied in high concentrations, but that this does not involve an alpha 2-adrenoceptor.

Mechanism of alpha 2-adrenergic inhibition of neuroeffector transmission in the mouse vas deferens

The process by which the activation of presynaptic alpha 2-adrenoceptors inhibits the release of noradrenaline from terminals of postganglionic sympathetic nerves was studied in the mouse isolated vas deferens. Clonidine was used as a prototypic agonist. Field stimulation-evoked excitatory junction potentials (e.j.p.s) were recorded from individual muscle cells. The e.j.p. amplitudes were taken as a measure of transmitter release. Changes in the external Ca2+ concentration from 2.5 to 1.25 or 5 mM caused corresponding changes in the size of e.j.p.s. When the normal Ca2+ concentration of the medium (2.5 mM) was substituted by equimolar quantities of Ba2+ or Sr2+, the e.j.p. amplitudes decreased considerably. Clonidine (0.3-30 nM) inhibited the nerve stimulation-evoked e.j.p. amplitudes in a concentration-dependent manner, without altering appreciably the frequency of spontaneous e.j.p.s. Procedures known to enhance Ca2+ entry into nerve terminals, like a high Ca2+ medium (Ca2+ 5 mM) or 4-aminopyridine 30 microM reduced the effect of clonidine. Repetitive nerve stimulation at 3 Hz, which is supposed to lead to an accumulation of free Ca2+ inside nerve terminals, similarly counteracted the effect of clonidine 10 nM. Whereas the alpha 2-adrenergic inhibition of the first e.j.p. in a train was unaffected, the inhibition of all successive e.j.p.s was gradually decreased. At 5 mM Ca2+ only the time-course of facilitation became faster, the decrease in alpha 2-adrenergic inhibition proceeded with the same pulse-dependent rate as at a normal external Ca2+ concentration, although from a lower initial level.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoreceptor-mediated changes in dopaminergic terminal excitability: effects of increases in impulse flow

The effect of spontaneous and stimulation-induced alterations in impulse flow on the antidromic excitability of nigrostriatal dopaminergic neurons were investigated in urethane-anesthetized rats. Terminal excitability was found to be inversely related to the rate of spontaneous activity of nigral neurons. Conditioning stimulation applied to dopaminergic axons in the medial forebrain bundle was found to decrease terminal excitability, but axonal conditioning stimulation was without effect on antidromic responses evoked from the medial forebrain bundle. Decreases in terminal excitability induced by medial forebrain bundle stimulation could be blocked by local infusions of haloperidol into the region of the terminal fields, suggesting that the effect was receptor-mediated. These results are consistent with the proposal that nigrostriatal dopaminergic neurons may modulate the impulse-dependent release of dopamine from striatal nerve terminals as a function of firing rate by autoreceptor-mediated alterations in the electrical properties of the terminal membrane.

Increased projection from the locus coeruleus to the lateral geniculate nucleus and visual cortex in young adult rats following unilateral enucleation

Rats of 30 and 60 days of age were subjected to removal of one eye, and electrophysiological experiments were carried out to see if the density of the projection from the locus coeruleus to the visual centers such as the lateral geniculate nucleus and visual cortex increased. In rats with an eye removed at 30 days, the projection of locus coeruleus neurons increased in the lateral geniculate nucleus contralateral and visual cortex ipsilateral to the removed eye, whereas rats with an eye removed at 60 days did not show any notable change in the density of the projection.

Noradrenaline-mediated synaptic inhibition in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus (l.c.) in slices of rat pons maintained in vitro. Focal electrical stimulation to the slice surface within the region of the l.c. evoked a synaptic depolarization followed by a hyperpolarization. These potentials were graded with stimulus intensity and were abolished in calcium-free and/or high-magnesium solutions. The nature of the hyperpolarizing synaptic potential (i.p.s.p.) was investigated. The i.p.s.p. amplitude decreased as the membrane was artificially made more negative and reversed at -114 mV. This reversal potential shifted to less negative potentials in solutions of elevated potassium ion content as predicted by the Nernst equation. The i.p.s.p. was potentiated in amplitude and its time course was prolonged by desmethylimipramine (DMI). Yohimbine (100 nM) and phentolamine (100 nM) reversibly abolished the i.p.s.p. and did not change the synaptic depolarization. Noradrenaline hyperpolarized all l.c. neurones tested, whether applied by perfusion (1-30 microM) or by pressure ejection from a micropipette placed in the solution near the recording site. The noradrenaline-induced hyperpolarization was accompanied by an increase in conductance and it reversed in polarity at -104 mV. The reversal potential of the noradrenaline hyperpolarization became less negative when the potassium ion content was increased. The noradrenaline-induced hyperpolarization was potentiated by DMI and was antagonized by yohimbine and phentolamine in the same concentrations which blocked the i.p.s.p. The results support the notion that l.c. neurones can release noradrenaline onto the somadendritic membrane of other l.c. neurones and thereby provide local feed-back inhibition.

Changes in noradrenergic terminal excitability induced by amphetamine and their relation to impulse traffic

The effects of amphetamine upon the terminal excitability of noradrenergic neurons of the nucleus locus coeruleus were studied in urethane anesthetized rats. Terminal excitability was measured by determining the stimulus currents necessary to evoke antidromic responses in locus coeruleus neurons from terminals in the frontal cortex. In most cases, terminal excitability was decreased following local infusion of amphetamine into the frontal cortex, while intravenous administration of the drug tended to increase terminal excitability. The decreased terminal excitability induced by local infusion of amphetamine appeared to be due to activation of alpha-adrenergic receptors located on the terminals of locus coeruleus neurons, since this effect mimics that of clonidine, a direct acting alpha-adrenergic agonist, and since the effect was abolished by pretreatment with alpha-methyl-p-tyrosine which disrupts the catecholamine liberating properties of amphetamine. Phentolamine, a direct acting alpha-adrenergic receptor antagonist was also found to block or reverse the effect of amphetamine. The changes in terminal excitability following intravenous injection of amphetamine appeared to be related to changes in the spontaneous activity of locus coeruleus neurons. A large decrease in spontaneous activity following intravenous administration of amphetamine was associated with increased terminal excitability, whereas when smaller changes in spontaneous activity occurred, terminal excitability was found to be decreased. These results are discussed with respect to the pharmacological properties of catecholaminergic neurons and the mechanisms of action of amphetamine.

Dopaminergic terminal excitability following arrival of the nerve impulse: the influence of amphetamine and haloperidol

Variations in the excitability of the axons and terminal fields of dopaminergic neurons of the substantia nigra were examined as a function of time following the nerve impulse in urethane-anesthetized, immobilized rats. Excitability was measured by determining the threshold, defined as the minimum current required to consistently activate dopaminergic neurons antidromically. Threshold currents were maximal immediately following the action potential and declined exponentially to a plateau. The interval during which threshold current declined, the phasic period, was significantly longer for stimulation of neostriatal terminal fields as contrasted to stimulation of axons along the medial forebrain bundle. A positive correlation was observed between antidromic response latency and the duration of this phasic period for both sites of stimulation, an observation consistent with the view that the site of initiation of the antidromic action potential is of smaller diameter in the neostriatum than in the medial forebrain bundle Amphetamine, which promotes dopamine release and/or re-uptake blockade, reduced dopaminergic terminal excitability in the neostriatum at all intervals examined. This effect increased at successively shorter intervals during the phasic portion of the excitability curve. Haloperidol, a dopamine antagonist, increased the excitability of dopaminergic terminal fields, an effect which was also more marked earlier in the phasic interval. Neither amphetamine nor haloperidol had a significant effect on the excitability of dopaminergic axons in the medial forebrain bundle. Variations in dopaminergic terminal excitability after impulse arrival, and the effects of amphetamine and haloperidol on this phenomenon suggest that terminal excitability is determined by events related to arrival of the nerve impulse including activation of presynaptic 'autoreceptors' by dopamine released from the synaptic ending.

Noradrenergic terminal excitability: effects of opioids

The local infusion of morphine or D-Ala2, Met5-enkephalinamide into the frontal cortical terminal fields of noradrenergic neurons of the nucleus locus coeruleus resulted in a decrease in the excitability of the axon terminal regions to direct electrical stimulation. These effects were concentration dependent and could be blocked or partially reversed by the local infusion of naloxone. Some evidence was obtained for a differential antagonizing effect of naloxone upon the effects of morphine and D-Ala2, Met5-enkephalinamide. These results are discussed with respect to an effect of opioids on the polarization and/or ionic conductance of the terminal fields of locus coeruleus neurons, and to the possible regulation of neurotransmitter release by presynaptic opiate receptors.