Responses of single cortical neurones to noradrenaline and dopamine

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Neuromodulatory actions of dopamine in the neostriatum are dependent upon the excitatory amino acid receptor subtypes activated

In the mammalian neostriatum, dopamine modulates neuronal responses mediated by activation of excitatory amino acid receptors. The direction of this modulation varies with the specific subtype of excitatory amino acid receptor activated. Responses evoked by iontophoretic application of glutamate (Glu) and the non-N-methyl-D-aspartate (NMDA) agonists quisqualate and alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid were significantly attenuated when dopamine was applied. In contrast, responses evoked by NMDA were markedly potentiated. The enhancement of NMDA-evoked excitations was mimicked by bath application of SKF 38393, a D1 receptor agonist. The D1 receptor antagonist SCH 23390 blocked the dopamine enhancement of NMDA-induced excitations. Quinpirole, a D2 receptor agonist, attenuated responses evoked by both NMDA and non-NMDA receptor agonists. These results indicate that the complex modulatory actions of dopamine in the neostriatum are a function of the excitatory amino acid receptor as well as the specific dopamine receptor subtype activated. These findings are of clinical relevance since the actions of dopamine and excitatory amino acids have been implicated in neurological and affective disorders.

Differential modulation by dopamine of responses evoked by excitatory amino acids in human cortex

The responses of human neocortical neurons to iontophoretic application of excitatory amino acids and their modulation by dopamine (DA) were studied in vitro. Brain slices were obtained from children undergoing surgery for intractable epilepsy. Application of N-methyl-D-aspartate (NMDA) to the slices induced slow depolarizations accompanied by decreased input conductances and sustained action potentials in cortical neurons. Glutamate produced rapid depolarizations and firing with few changes in input conductances. Quisqualate also induced depolarization and firing, but input conductances increased during the rising phase of the membrane depolarization. Iontophoretic application of DA alone produced no change in membrane potential or input conductance. However, when DA was applied in conjunction with the excitatory amino acids, it produced contrasting effects. With either bath application of DA or when iontophoresis of DA preceded application of NMDA, the amplitude of the membrane depolarizations and the number of action potentials were increased, whereas the latency of these responses decreased. In contrast, DA decreased the amplitude of the depolarizations and the number of action potentials evoked by glutamate or quisqualate. The fact that DA affects responses to NMDA and glutamate or quisqualate in opposite directions is of considerable importance to the understanding of cellular mechanisms of neuromodulation and the role of DA in cognitive processing and in epilepsy.

Comparative analysis of the effects of iontophoretically applied dopamine in different regions of the rat brain, with special reference to the cingulate cortex

A systematic comparison of the effects of iontophoresed dopamine (DA) was carried out in the neostriatum (NS), nucleus accumbens (Acb) and anterior cingulate (ACg), prefrontal (PF) and parietal (Par) cortex of urethane-anesthetized rats, before and after treatment with the specific DA uptake blockers GBR 12909 and Bupropion. Similar experiments were also conducted after DA denervation with 6-hydroxydopamine and after DA depletion with alpha-methyl-p-tyrosine. The average rate of spontaneous neuronal firing was comparable in all regions, except in the NS after DA depletion. A majority of the units were inhibited by DA in every region and condition tested. As assessed with the IT50 index, the responsiveness to DA was not markedly different between regions, indicating that the postsynaptic sensitivity to this amine is independent of the density of DA receptors and of DA innervation. In contrast, the average duration of DA inhibitions (RT90) was considerably longer (5-fold) in the intact ACg than in the PF, Par, NS, or Acb. Moreover, treatment with both DA uptake blockers reduced the duration of DA inhibitions in ACg (4- to 9-fold); while lengthening it in PF, NS and Acb; and having no apparent effect in Par. DA depletion and DA denervation also reduced the duration of the DA inhibitions in ACg without effect in Par. Taken together, these results provide further evidence for the existence of a presynaptic, positive-feedback mechanism in ACg, triggered by DA, and favouring the further release of this transmitter upon its reuptake in DA nerve terminals.

Organization and actions of the noradrenergic input to the hamster's superior colliculus

Immunocytochemistry using antisera directed against dopamine-beta-hydroxylase (DBH) was used to determine the organization of the noradrenergic (NE) input to the hamster's superior colliculus (SC). Immunocytochemistry for DBH was combined with retrograde transport of fluorogold (FG) to determine the sources of NE input to SC. Microiontophoretic techniques were used together with extracellular single unit recording and receptive field mapping techniques to determine the manner in which NE influenced the responses of individual SC neurons. The hamster's SC contained numerous DBH-positive fibers but no immunopositive cells. These fibers formed a plexus that was most dense in the lower stratum griseum superficiale (SGS). The density of DBH-positive fibers was very low in the stratum opticum (SO) and increased in density in the stratum griseum intermediale (SGI) and the other deep layers. When FG injections into the SC were combined with immunocytochemical detection of DBH, double-labeled cells were observed in the contralateral locus ceruleus. DBH-positive neurons were observed in several other portions of the mesencephalon and pons, but none of these were labelled with FG. The effects of NE iontophoresis were assessed for a total of 135 SC neurons. In 74% (N = 100), NE reduced spontaneous and/or stimulus evoked activity. In 3% (N = 4 cells), NE increased activity, and in 23% (N = 31 cells) it had no effect. These percentages were essentially the same for superficial layer visual cells and somatosensory neurons in the deep laminae. The effect of NE iontophoresis upon signal to noise ratios was assessed for 46 visual and 56 somatosensory neurons. For 54% (N = 25) of the visual cells and 16% (N = 9) of the somatosensory cells, NE iontophoresis decreased signal to noise ratios. For 13% (N = 6) of the visual cells and 21% (N = 12) of the somatosensory cells, NE iontophoresis increased signal to noise ratios. The effects of NE on the responsivity of SC neurons were antagonized by propranolol (86% of the 21 cells tested), sotalol (67% of the six cells tested), and atenolol (effective in the single cell tested). All these agents are beta-adrenergic antagonists. The single alpha-adrenergic antagonist that we evaluated, corynanthine, potentiated the effects of NE on the responsivity of the two SC neurons that we tested.

Pharmacological characterization of the receptor mediating the adrenergic inhibition of responses to substance P in the cingulate cortex

The excitatory responses of neurones in the anterior cingulate cortex of the rat to iontophoretically applied substance P (SP) are reduced by noradrenaline (NA) applied iontophoretically or released from noradrenergic pathways. In order to determine the receptor involved in this inhibitory effect we have studied the effects of a number of receptor-specific adrenergic agonists and antagonists on responses of cingulate neurones to SP in rats anaesthetized with chloral hydrate. Low iontophoretic currents (0-15 nA) of NA, adrenaline and the beta-agonist, clenbuterol, all strongly reduced responses to SP. Isoprenaline was also effective but less consistently so, although problems were experienced with its iontophoretic release from micropipettes. The alpha 1-agonists, phenylephrine and methoxamine were also able to reduce responses to SP. However, this reduction required higher iontophoretic currents (15-60 nA) and was associated with depressant effects on baseline firing rate. The alpha 2-agonist clonidine was only weakly active at high currents and this too was associated with depression of baseline firing. Similar weak effects were noted with dopamine. The inhibitory effects of NA on SP responses were convincingly blocked or reversed by the beta-antagonist, practolol, but not by the alpha 1-antagonist, prazosin. The reduction of SP responses by phenylephrine was also blocked by practolol but unaffected by prazosin. Finally, reduction of SP excitations by activation of the coeruleocortical pathway was also blocked by practolol applied iontophoretically to the cortical cells. These results are consistent with the hypothesis that the effect of NA on SP responsiveness in the cingulate cortex is mediated by beta-adrenoreceptors.

Excitatory neuronal responses to dopamine in the cerebral cortex: involvement of D2 but not D1 dopamine receptors

The technique of microelectrophoresis was used to evaluate the relative contribution of D1 and D2 dopamine receptors towards the mediation of the excitatory response of single neurones to dopamine in the somatosensory cortex of the rat. The selective D1 dopamine receptor agonist, SKF 38393, failed to excite any of the cells to which it was applied. In contrast, the selective D2 dopamine receptor agonist, LY 171555, excited the majority of cells tested. The apparent potency of LY 171555 was significantly lower than that of dopamine. When the mobilities of SKF 38393 and LY 171555 were assessed by an in vitro method, they were found to be at least as great as those of dopamine and phenylephrine, suggesting that the lack of effect of SKF 38393 and the lower apparent potency of LY 171555 compared to dopamine reflect genuine biological phenomena. The alpha 1-adrenoceptor antagonist, prazosin, discriminated between excitatory responses to the alpha 1-adrenoceptor agonist, phenylephrine, and LY 171555: responses to phenylephrine were more susceptible to antagonism than were those to LY 171555. The dopamine receptor antagonist, haloperidol, produced the reverse discrimination: responses to LY 171555 were more affected than were those to phenylephrine. Neither antagonist reduced the response to the control agonist, acetylcholine. When applied continuously with low ejecting currents, LY 171555 antagonized the excitatory response to dopamine while the response to phenylephrine was relatively preserved. The response to acetylcholine was unaffected. When similarly applied, SKF 38393 had no selective action on the response to dopamine. 6 These results suggest that D2 dopamine receptors are involved in mediating the excitatory neuronal response to dopamine in the cerebral cortex, whereas DI dopamine receptors are unlikely to be involved. LY 171555 appears to act as a partial agonist at D2 dopamine receptors in this test system.

Effects of ionophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex

Changes in spontaneous activity of rat S1 cortical neurones with identified receptive fields were investigated in reply to ionophoresed noradrenaline (NA). Extracellular levels of NA were maintained constant by continuous electrochemical analysis at the carbon fibre recording tip of the multibarrel micro-electrode. In the absence of NA there were clear differences in spike amplitude, firing rate and pattern of firing of deep (800-1400 micron) and superficial (0-800 micron) cells. Superficial cells responded to low (5 X 10(-8) to 5 X 10(-7) M) NA concentrations with simple inhibition. Recovery occurred within a minute or so of extracellular NA concentrations falling below detectable (10(-8) M) levels. Increases in local concentration merely stopped cells firing. In contrast, cells located in the deep zone could often be excited by very low NA concentrations (less than 10(-8) M), with inhibition occurring at levels 10-100 times greater. Most cells, however, were inhibited, with threshold doses for a 50% change in firing rate much higher than for superficial cells. Some cells in the deep zone showed sustained increases in firing rate following an ionophoretic trial. This could occur for periods of up to 1 h after ceasing a trial. Such effects could be produced by levels as low as 10(-7) M-NA. Interspike interval analysis for deep cells suggested that their spontaneous activity resembled that established for slow-wave sleep. During and after excitation by NA the pattern of firing of small groups of these cells changed to that established for the waking state. The effect could persist for up to 1 h following a short (2-5 min) ionophoretic trial.

Dissimilar responses of cortical neurons to chronic trazodone or desipramine treatment

The effects of acute and chronic trazodone or desipramine treatment on the spontaneous firing rate of cortical neurons in chloral hydrate anesthetized rats were determined. Either trazodone (30 or 60 mg/kg, po) or desipramine (5 or 10 mg/kg, po) was administered daily for periods of up to 11 or 16 days respectively. A significant reduction in the firing rate of cortical neurons was observed after acute and chronic 10 mg/kg desipramine treatment. Chronic treatment with 5 mg/kg desipramine had no effect on activity. Neither acute nor chronic treatment with trazodone produced any significant alteration in cortical unit activity. The results suggest that the spontaneous activity of randomly encountered cortical neurons is not involved in the mechanism of the therapeutic action produced by chronic treatment with the two clinically effective antidepressants. The response of the cortical neurons to antidepressant treatment might help differentiate the relative effects of the drugs on monoaminergic receptors. Also, the depressant effect of desipramine on the spontaneous rate of cortical neurons does not appear to be a specific effect of chronic treatment since it occurred after a single dose.

The effect of microelectrophoretically applied clonidine on single cerebral cortical neurones in the rat. Evidence for interaction with alpha 1-adrenoceptors

The technique of microelectrophoresis was used in order to examine the effects of clonidine on single neurones in the somatosensory cortex of the rat, and to compare its actions with those of noradrenaline and phenylephrine. Clonidine evoked only excitatory responses on cortical neurones. The clonidine-sensitive neurones were also excited by noradrenaline and phenylephrine. Clonidine had a consistently lower apparent potency than either noradrenaline or phenylephrine. Responses to clonidine had a slower time-course than responses to the other two adrenoceptor agonists, both the latencies to onset and the recovery times being longer for responses to clonidine than for responses to noradrenaline and phenylephrine. When the mobilities of clonidine and phenylephrine were compared using an in vitro method, no significant difference was found between the mobilities of the two ionic species, suggesting that they have similar transport numbers. Thus the difference between the potencies and time-courses of responses to clonidine and phenylephrine are presumably of biological origin. Responses to clonidine were antagonised by microelectrophoretically applied prazosin; responses to phenylephrine were equally antagonised, while responses to acetylcholine were not affected. Clonidine could reversibly antagonise excitatory responses to both noradrenaline and phenylephrine, without affecting responses to acetylcholine. The results suggest that clonidine may act as a partial agonist at excitatory alpha 1-adrenoceptors on cortical neurones.

Action of dextroamphetamine on dopamine sensitive cells in the snail brain

Presynaptic and postsynaptic actions of dextroamphetamine (DEX) were studied on dopamine (DA) sensitive neurons of the subesophageal ganglion of the garden snail Helix aspersa utilizing standard microelectrode techniques. Dextroamphetamine (5.5 X 10(-7)-10(-4)M) produced effects on DA-sensitive neurons similar to that caused by DA (5.5 X 10(-7)-10(-4)M). On cells excited by DA, surfused DEX (5.5 X 10(-7)M) caused an excitation that could be blocked by chlorpromazine (0.5-1 X 10(-6)M) or haloperidol (0.5-1 X 10(-6)M). Elevating the extracellular Mg2+ from 4 to 20 mM reduced the depolarization caused by DEX from 11 to 2.5 mV without affecting the response to DA. The response remaining is attributed to a direct response to DEX on DA receptors. Surfused DEX caused an inhibition of cells inhibited by DA. Both DA and DEX effects were selectively blocked by dihydroergotamine (0.5-1 X 10(-6)M). Elevating the [Mg2+] decreased the hyperpolarization caused by DEX from 11 to 3 mV without affecting the DA response. The effect of elevated magnesium in decreasing responses to surfused DEX suggests that the primary action of DEX is at the nerve terminal to cause DA release. Iontophoretic application of DEX caused minimal excitation or inhibition of DA neurons. This is attributed to the fact that DA receptors at the site of drug application are not associated with synaptic innervation. The response obtained with iontophoretically applied DEX suggest a weak direct action on DA receptors.

Comparison of the effects of methoxamine with those of noradrenaline and phenylephrine on single cerebral cortical neurones

1 The technique of microelectrophoresis was used to compare the actions of methoxamine, noradrenaline and phenylephrine on single neurones in the somatosensory cerebral cortex of the rat.2 Methoxamine evoked only excitatory responses on cortical neurones. The methoxamine-sensitive cells were also excited by phenylephrine; cells excited by methoxamine could either be excited or depressed by noradrenaline.3 Methoxamine appeared to be less potent than either noradrenaline or phenylephrine in evoking excitatory responses.4 Responses to methoxamine had a slower time course than responses to either noradrenaline or phenylephrine, both the latencies to onset and the recovery times being longer for responses to methoxamine than for responses to noradrenaline or phenylephrine.5 When the absolute mobilities of methoxamine, noradrenaline and phenylephrine were compared using an in vitro method, no significant differences were found between the mobilities of the three ionic species, suggesting that the three drugs have similar transport numbers. Thus the differences in potency between methoxamine and the other two drugs, and the difference between the time courses of responses to methoxamine and the other two drugs, are presumably of biological origin.6 The alpha-adrenoceptor antagonist, phenoxybenzamine, antagonized equally excitatory responses to methoxamine and noradrenaline, and responses to methoxamine and phenylephrine, without affecting responses to acetylcholine.7 When responses to methoxamine and noradrenaline and responses to methoxamine and acetylcholine were summated on the same cells, the net responses were smaller than those expected on the basis of additive effects; the deviation from additivity was greater in the case of the summation of responses to methoxamine and noradrenaline than in the case of summation of responses to methoxamine and acetylcholine. This observation is consistent with the hypothesis that the interaction between methoxamine and noradrenaline follows the model of competitive dualism, whereas the interaction between methoxamine and acetylcholine follows the model of functional synergism.8 The results suggest that methoxamine may act as a partial agonist at excitatory alpha-adrenoceptors on cerebral cortical neurones.

A procedure for comparing the mobilities of unlabeled drugs used in microelectrophoresis experiments

A novel method for comparing the absolute mobilities of unlabeled compounds released from micropipettes in microelectrophoresis experiments is described. The method is based on the principle that the introduction of a "foreign" ion into an electrolyte reduces the transport number of a "reference" ion present in the electrolyte. Using [14C]-noradrenaline as the "reference" ion, the mobilities of two "foreign" ions, methoxamine and phenylephrine, were compared. No significant difference was found between the mobilities of the two drugs. It was concluded that the two drugs probably have similar transport numbers when released from solutions of equal molarity in microelectrophoresis experiments in vivo, and thus the previously reported difference between the apparent potencies of the two drugs is presumably of biological origin. The method described here may be of use in comparing the mobilities of other compounds, the radiolabeled forms of which are either unavailable or prohibitively expensive.

Some electrophysiological and pharmacological properties of the cortical, noradrenergic projection of the locus coeruleus in the rat

The effect of repetitive stimulation of the locus coeruleus (LC) on the discharge rate of spontaneously active neurons of the visual, rostral and cingulate cortex was investigated in untreated and catecholamine-depleted rats under chloral hydrate anesthesia. In untreated animals, the inhibitory transsynaptic effects predominated over the excitatory ones. In catecholamine-depleted rats, the percentage of inhibited cells was significantly reducted in all areas. The vast majority of spontaneously active neurons in all cortical regions was depressed by microiontophoretically applied noradrenaline (NA). A few cells were resistant to NA; no excitatory effects were noticed on any cell. The transsynaptically mediated depression of the discharge rate of cells in all three cortical areas was reversibly antagonized by the iontophoretically administered beta-receptor blocking drug practolol. On the contrary the a-receptor blocking drugs piperoxane and WB4101 were ineffective in this respect. Thus, we tentatively conclude from these data that the NA-elicited depression of cells in the cortex is mediated by a receptor of the beta-type. Repetitive stimulation of the reticular formation elicited a desynchronizing effect on the EEG of chloral hydrate anaesthetized rats. LC stimulation, in contrast, hardly produced any modification of the EEG as judged by visual examination of the recordings.

Activation of lateral geniculate neurons by norepinephrine: mediation by an alpha-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1-15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic alpha-adrenergic receptors: epinephrine greater than NE greater than phenylephrine greater than or equal to alpha-methylnorepinephrine greater than dopamine greater than isoproterenol. The alpha-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (less than 10 nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The beta-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic alpha-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brain alpha1-adrenoceptors labeled with [3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic or alpha1-adrenergic receptor.

The action of microelectrophoretically applied (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI) on single cortical neurones

1. The technique of microelectrophoresis was used in order to compare the actions of the imidazoline derivative, (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI), with those of dopamine and phenylephrine on single neurones in the cerebral cortex of the rat anaesthetized with halothane. 2. DPI and phenylephrine were almost exclusively excitatory, whereas dopamine could evoke both excitatory and depressant responses. 3. In the case of excitatory responses, DPI appeared to be more potent than dopamine, and was approximately equipotent with phenylephrine. 4. The dopamine antagonist, haloperidol, could discriminate between excitatory responses to DPI and dopamine: responses to dopamine were abolished, whereas responses to DPI, and to a control agonist, acetylcholine, were unaffected. 5. The alpha-adrenoceptor antagonist, phenoxybenzamine, antagonized equally excitatory responses to DPI and phenylephrine. Responses to acetylcholine were not affected. 6. It is concluded that DPI does not stimulate dopamine receptors on cortical neurones; the excitatory responses of these cells to DPI may be mediated by alpha-adrenoceptors.

Comparison of the responses of single cortical neurones to tyramine and noradrenaline: effects of desipramine

1 The technique of microelectrophoresis was used in order to compare the actions of tyramine and noradrenaline on single neurones in the cerebral cortex of the rat.2 Tyramine could both excite and depress cortical neurones. Each tyramine-sensitive cell was also sensitive to noradrenaline. There was a high correlation between the directions of responses to tyramine and noradrenaline, most cells excited by tyramine being excited by noradrenaline, and most cells depressed by tyramine being depressed by noradrenaline.3 In the case of both excitatory and depressant responses, tyramine appeared to be less potent than noradrenaline.4 Tyramine evoked ;slower' responses than noradrenaline, both the latencies to onset and the recovery times being longer for responses to tyramine than for responses to noradrenaline.5 When the rates of release of tyramine and noradrenaline from micropipettes were measured in vitro, no significant difference could be observed between the transport numbers of the two drugs. Thus the difference in potency between the two drugs, and the difference in the time courses of responses to the two drugs, are presumably of biological origin.6 Desipramine could discriminate between neuronal responses to tyramine and noradrenaline: responses to tyramine were antagonized, while responses to noradrenaline were either potentiated or unaffected. Responses to DL-homocysteic acid were not affected by desipramine.7 The results are consistent with the hypothesis that tyramine is an indirectly acting sympathomimetic amine in the brain, and desipramine acts by blocking the uptake of both tyramine and noradrenaline into presynaptic noradrenergic nerve terminals.