The role of physical and biological factors in determining the time course of neuronal responses

Basolateral amygdala noradrenergic influences on memory storage are mediated by an interaction between beta- and alpha1-adrenoceptors

Extensive evidence indicates that norepinephrine modulates memory storage through an activation of beta-adrenoceptors in the basolateral nucleus of the amygdala (BLA). Recent findings suggest that the effects of beta-adrenergic activation on memory storage are influenced by alpha1-adrenoceptor stimulation. Pharmacological findings indicate that activation of postsynaptic alpha1-adrenoceptors potentiates beta-adrenoceptor-mediated activation of cAMP formation. The present study examined whether inactivation of alpha1-adrenoceptors in the BLA would alter the dose-response effects on memory storage of intra-BLA infusions of a beta-adrenoceptor agonist, as well as that of a synthetic cAMP analog. Male Sprague Dawley rats received bilateral microinfusions into the BLA of either the beta-adrenoceptor agonist clenbuterol (3-3000 pmol in 0.2 microliter) or 8-bromoadenosine 3':5'-cyclic monophosphate (8-bromo-cAMP) (0.2-7 nmol in 0.2 microliter) alone or together with the alpha1-adrenoceptor antagonist prazosin (0.2 nmol) immediately after training in an inhibitory avoidance task. Retention was tested 48 hr later. Clenbuterol induced a dose-dependent enhancement of retention, and prazosin attenuated the dose-response effects of clenbuterol. Posttraining intra-BLA infusions of 8-bromo-cAMP also induced a dose-dependent enhancement of retention latencies. However, concurrent infusion of prazosin did not alter the dose-response effects of 8-bromo-cAMP. These findings are consistent with the view that alpha1-adrenoceptors affect memory storage by modulating beta-adrenoceptor activation in the BLA. Moreover, these findings are consistent with those of pharmacological studies indicating that beta-adrenoceptors modulate memory storage by a direct coupling to adenylate cyclase, whereas alpha1-receptors act indirectly by influencing the beta-adrenoceptor-mediated influence on cAMP formation.

Involvement of M1-muscarinic receptors in the excitation of neocortical neurones by acetylcholine

The technique of microelectrophoresis was used to investigate the cholinoceptor pharmacology of spontaneously active single neurones in the parietal cortex of the rat. Acetylcholine, carbachol and the selective M1-muscarinic receptor agonist, McN-A-343, were each potent excitants (rank order of apparent potency: carbachol greater than acetylcholine greater than McN-A-343). When measured in vitro, the apparent mobilities of carbachol and acetylcholine were similar although significantly less than that of McN-A-343, suggesting that the lower potencies of acetylcholine and McN-A-343 probably reflect a genuine biological phenomenon. In addition to excitation, carbachol also evoked biphasic (excitation/depression) and depressant responses. In contrast to the other cholinoceptor agonists, nicotine produced weak and inconsistent excitations. Excitatory responses to acetylcholine and carbachol were significantly attenuated by the selective M1-muscarinic receptor antagonist, pirenzepine, at a time when the excitatory response to McN-A-343 was also significantly reduced. Responses to phenylephrine were not diminished. On several cells an excitatory response to carbachol was converted to a depression by pirenzepine. These results suggest that the excitatory responses of cortical neurones to cholinoceptor agonists are mediated predominantly by M1-muscarinic receptors. The identity of the receptor mediating the depressant response to carbachol remains uncertain, although nicotinic cholinoceptors do not appear to be involved.

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.

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.

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.

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.

Dose-dependent biphasic alterations in the spontaneous activity of neurons in the rat neostriatum produced by d-amphetamine and methylphenidate

A dose-response analysis was performed on D-amphetamine- and methylphenidate-induced changes in neuronal activity in the neostriatum of immobilized, phenidate-induced changes in neuronal activity in the neostriatum of immobilized, locally anesthetized rats. Whereas a marked depression of firing rate characterized the response to intraperitoneal injections of 2.5 mg/kg D-amphetamine or 10 mg/kg methylphenidate, increasing the dose (5.0--7.5 mg/kg D-amphetamine and 20--25 mg/kg methylphenidate) shifted this response pattern to a prolonged increase in activity. Both stimulant-induced increases and decreases in neostriatal activity are reversed by 2.0 mg/kg haloperidol. In contrast to the response of neostriatal neurons, the firing rate in the substantia nigra pars compacta was inhibited by both stimulants, even at doses that increased the activity of neurons in the neostriatum. The possible mechanisms underlying these drug-induced changes in firing rate are discussed along with the behavioral implications of a stimulant-induced dose-dependent shift in neostriatal unit activity.

The pharmacology of adrenergic neuronal responses in the cerebral cortex: evidence for excitatory alpha- and inhibitory beta-receptors

1. The technique of microelectrophoresis was used to compare the actions of a range of adrenoceptor agonists on single cortical neurones in the rat anaesthetized with halothane. 2. Phenylephrine and methoxamine were exclusively excitatory, whereas salbutamol was entirely depressant. Noradrenaline and isoprenaline could evoke both excitatory and depressant responses. Lower doses of isoprenaline usually evoked depressions, whereas higher doses, on the same cell, evoked excitatory responses. 3. The alpha-adrenoceptor blocking agents, phentolamine and phenoxybenzamine, reversibly antagonized excitatory responses to adrenoceptor agonists, without affecting depressant responses to adrenoceptor agonists or excitatory responses to acetylcholine. 4. The beta-adrenoceptor blocking agents, propranolol and sotalol, reversibly antagonized both depressant and excitatory responses to adrenoceptor agonists, without affecting responses to acetylcholine. When the effect of sotalol on excitatory and depressant responses to adrenoceptor agonists was compared on the same cell, the depressant responses could be selectively antagonized, without affecting the excitatory responses. 5. It is concluded that (a) responses of cortical neurones to adrenoceptor agonists are mediated by both alpha- and beta-receptors; (b) these alpha- and beta-receptors give rise to opposite effects: the alpha-receptors being excitatory and the beta-receptors being inhibitory; and (c) responses of many neurones reflect the presence of both types of receptor.

The action of microelectrophoretically applied L-3,4-dihydroxyphenylalanine (DOPA) on single cortical neurones

The technique of microelectrophoresis was used in order to compare the actions of L-3,4-dihydroxyphenylalanine (DOPA) and noradrenaline on single neurones in the cerebral cortices of cats and rats. DOPA could both excite and depress cortical neurones. Cells excited by DOPA were also excited by noradrenaline and cells depressed by DOPA were also depressed by noradrenaline. In the case of both excitatory and depressant responses, DOPA appeared to be less potent than noradrenaline. Responses to DOPA and noradrenaline could be antagonized by phentolamine and propranolol. Responses to acetylcholine were not affected. Responses to acetylcholine, but not responses to DOPA, were antagonized by atropine. The results indicate that locally applied DOPA may mimic the actions of noradrenaline on cortical neurones. Possible mechanisms for these effects of DOPA are discussed.

Effects of iprindole on responses of single cortical and caudate neurones to monoamines and acetylcholine

1 The technique of microelectrophoresis was used to study the effects of iprindole on single neurones in the cerebral cortex and caudate nucleus of the rat. 2 Iprindole, when applied for a brief period, did not affect the firing rate of the vast majority of neurones tested. 3 Both potentiation and antagonism of neuronal responses to noradrenaline, dopamine, and 5-hydroxytryptamine could be observed after a brief application of iprindole. Potentiation and antagonism often occurred after the same application of iprindole, antagonism always preceding potentiation. 4 Responses to acetylcholine were affected by iprindole similarly: both potentiation and antagonism of the responses could be observed. 5 Responses to glutamate were not affected by iprindole. 6 It is concluded that the potentiation of responses to monoamines by iprindole cannot be explained on the basis of uptake blockade; this potentiation may be due to the blockade of masked receptors on the post-synaptic cell. 7. It is suggested that the common pharmacological action of the tricyclic antidepressants may be the ability to block both monoamine and acetylcholine receptors in the brain.

Effects of desipramine on neuronal responses to dopamine, noradrenaline, 5-hydroxytryptamine and acetylcholine in the caudate nucleus of the rat

1 The sensitivity of single neurones to microelectrophoretically applied dopamine, noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholing (ACh) was investigated in the caudate nucleus of the rat, anaesthetized with halothane. Both excitatory and depressant responses could be observed to each of the agonists. There was a high correlation between the direction of responses to dopamine and noradrenaline, whereas there was no significant correlation between the direction of responses to dopamine and ACh. 2 The effect of desipramine was studied on both excitatory and depressant responses to dopamine, NA and 5-HT, and on excitatory responses to ACh. Both potentiation and antagonism of neuronal responses to monoamines and ACh could be observed after a brief application of desipramine. 3 Excitatory responses to glutamate were not affected by desipramine. 4 The observation that responses to dopamine and NA can be potentiated by desipramine in the caudate nucleus suggests that uptake blockade is not a prerequisite for potentiation. 5 It is suggested that the potentiation of neuronal responses to dopamine by desipramine may be responsible for the therapeutic efficacy of desipramine in Parkinson's disease.

The effect of tricyclic antidepressants on cholinergic responses of single cortical neurones

1 The technique of microelectrophoresis was used in order to study the effects of tricyclic antidepressants on responses of single cortical neurones to acetylcholine. 2 Both potentiation and antagonism of excitatory responses to acetylcholine could be observed after a brief application of imipramine or desipramine. A higher dose of the antidepressant was required to evoke antagonism than to evoke potentiation. 3 Responses to carbachol were affected by desipramine similarly, suggesting the inhibition of cholinesterase is not responsible for the potentiation of cholinergic responses. 4 A brief application of atropine also had a dual effect on responses to acetylcholine. 5 It is suggested that the potentiation of excitatory cholinergic responses by atropine and the antidepressants may be due to the blockade of masked inhibitory receptors.

Effects of imipramine and desipramine on responses of single cortical neurones to noradrenaline and 5-hydroxytryptamine

1 The technique of microelectrophoresis was used in order to study the effects of imipramine and desipramine on single neurones in the somatosensory cortex of the cat, anaesthetized with halothane.2 Imipramine and desipramine, when applied for a brief period, did not affect the firing rate of the vast majority of the neurones tested.3 Both potentiation and antagonism of excitatory responses to noradrenaline could be observed after a brief application of either of the antidepressants. Four drug-interaction patterns could be distinguished: potentiation of immediate onset; potentiation reaching its maximum after a delay; antagonism followed by potentiation; antagonism followed by recovery.4 When different doses of the same antidepressant were applied, it was found that the drug-interaction patterns were related to the dose of antidepressant applied, a lower dose causing potentiation, and a higher dose antagonism of the response.5 Both potentiation and antagonism of depressant responses to noradrenaline could be observed.6 Both excitatory and depressant responses to 5-hydroxytryptamine were modified by imipramine and desipramine: a smaller dose of the antidepressant potentiated, and a higher dose antagonized the responses.7 Excitatory responses to glutamate were not affected by imipramine and desipramine.