James M. Tepper

Autoreceptor-mediated changes in dopaminergic terminal excitability: Effects of striatal drug infusions

The neurophysiological correlates of autoinhibition at the terminals of nigrostriatal dopaminergic neurons were studied by measuring the changes in antidromic excitability of nigrostriatal neurons following local infusions of various catecholamine agonists and antagonists into the neostriatum. Infusions of apomorphine or amphetamine reduced terminal excitability whereas the dopamine antagonists, haloperidol, fluphenazine or sulpiride, led to increases in terminal excitability. Alterations in antidromic excitability were constrained to the terminal regions and were not observed when infusions and excitability testing were performed in the medial forebrain bundle. The alpha-2 agonist, clonidine, did not alter dopaminergic terminal excitability. Our results indicate that pharmacological manipulations which have been shown to reduce the amount of stimulation-induced transmitter release from dopaminergic terminals are associated with a dopamine autoreceptor-mediated hyperpolarization and/or alteration in ionic conductance of the terminal membranes. These results are discussed with respect to mechanisms of autoinhibition in the central nervous system.

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

Experiments were carried out to explore the view that activation of presynaptic receptors on the terminals of noradrenergic neurons is accompanied by alterations in their excitability to direct electrical stimulation. Antidromic action potentials evoked from frontal cortex of urethane anesthetized rats were recorded extracellularly from nucleus locus coeruleus. The threshold current necessary to evoke antidromic action potentials varied as a result of infusion of adrenergic agonists and antagonists into frontal cortex within 50 micrometer of the stimulating electrode. Local infusion of the alpha-adrenergic agonist clonidine produced a marked decrease in terminal excitability, while the alpha-antagonist phentolamine produced an increase in terminal excitability and was shown to reverse the effect of the agonist. Infusion of the beta-adrenergic agonist isoproterenol was without effect, although the beta-antagonist propranolol resulted in a decrease in terminal excitability. Infusions of potassium increased excitability of locus coeruleus terminals. Terminal excitability was seen to vary inversely with the rate of spontaneous or high frequency stimulation-induced firing of locus coeruleus neurons. From these observations, it may be inferred that activation or blockade of alpha-adrenergic presynaptic receptors results in changes in polarization and/or conductance of the noradrenergic synaptic endings. These results are discussed with respect to phenomena associated with the possible presynaptic regulation of neurotransmitter release.

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.

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.

Mesocortical dopaminergic neurons. 1. Electrophysiological properties and evidence for soma-dendritic autoreceptors

Mesencephalic dopaminergic neurons were electrophysiologically identified by a variety of criteria, including antidromic activation from prefrontal or cingulate cortex, neostriatum, or nucleus accumbens in urethane-anesthetized rats. The mean firing rate of 98 mesocortical dopaminergic neurons was 2.9 +/- 0.3 spikes/sec and did not differ from the mean firing rate found for nigrostriatal or nucleus accumbens dopaminergic neurons. Spontaneously active mesocortical dopaminergic neurons were inhibited by intravenous administration of either apomorphine (6 micrograms/kg) or amphetamine (0.25 mg/kg). Whereas most antidromic responses of nigrostriatal and mesoaccumbens neurons consisted of the initial segment spike only, cortically-elicited antidromic responses typically consisted of a full initial segment-soma-dendritic spike. These findings are discussed with regard to the presence of soma-dendritic autoreceptors on mesocortical dopaminergic neurons.

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.

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.

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.

Frontal cortex stimulation evoked neostriatal potentials in rats: Intracellular and extracellular analysis

Evoked potentials, action potentials and intracellular events were recorded in the neostriatum of urethane anesthetized rats to electrical stimulation of frontal cortex white matter, motor cortex and pre-limbic cortex. Five major waves of the evoked potential were identified. Wave N1 (3.9 msec latency) was small, preceded cellular events and probably represents activation of corticostriate terminals. Wave P1 (10.8 msec latency to peak following white matter stimulation) coincided with an EPSP and neuronal firing. Both wave N2 (38.0 msec latency to peak) and P2 (approximately 110 msec duration) overlapped the intracellularly recorded hyperpolarization and inhibition of cell firing. Based upon this correspondence and upon the behavior of waves N2 and P2 with changing current and during conditioning-test paired pulse stimulation, it was concluded that the waves represent different processes contributing to the cellular hyperpolarization. A late wave, N3 (175 msec onset latency) corresponded to a late rebound firing and cellular depolarization. This late wave was eliminated from the neostriatum, but not from the overlying sensorimotor cortex, by kainic acid lesions that destroyed medial thalamus but left thalamic lateral nuclei and reticular nucleus intact.

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.