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

REM sleep inhibition by desipramine: evidence for an alpha-1 adrenergic mechanism

The acute administration of drugs that block norepinephrine (NE) reuptake suppresses rapid eye movement (REM) sleep in cats and other mammals. The mechanism is presumed to involve NE acting on cells in a pontine REM sleep-generator region. Postsynaptic noradrenergic receptor mechanisms have not been identified. In the present experiments, we tested the ability of the alpha-1 antagonist prazosin and the beta antagonist propranolol to reverse the REM sleep suppression produced by the NE reuptake blocker desipramine (DMI) in the cat. DMI reduced the number of REM sleep episodes, the REM percentage (REM sleep time/total sleep time), and the average REM sleep episode duration. The co-administration of prazosin, but not propranolol, increased the REM percentage and the average REM sleep episode duration toward the placebo level. The co-administration of the peripherally-acting, anti-hypertensive agent hydralazine did not reverse the DMI-induced REM sleep suppression. While the identity of the brain region(s) involved in mediating the alpha-1 noradrenergic suppression of REM sleep by DMI remains unclear, there is reason to consider forebrain structures including the amygdala as well as the pontine areas that generally have been implicated in REM sleep control.

Comparison of the effects of desipramine on noradrenaline- and methoxamine-evoked venoconstriction in man

1. The dorsal hand vein compliance technique was used to investigate the dual effect of tricyclic antidepressants at the noradrenergic synapse (i.e. noradrenaline uptake blockade leading to potentiation and alpha 1-adrenoceptor blockade leading to antagonism of the effect of noradrenaline). The effects of a single oral dose (100 mg) of desipramine (DMI) on venoconstrictor responses to locally infused noradrenaline and methoxamine, a selective alpha 1-adrenoceptor agonist with little affinity for the uptake mechanism, were examined. 2. Eight healthy male volunteers participated in four weekly experimental sessions. Each session was associated with one of the following treatment conditions: noradrenaline/DMI, noradrenaline/placebo, methoxamine/DMI, methoxamine/placebo. Subjects were allocated randomly to treatments and sessions according to a double-blind balanced design. Noradrenaline acid tartrate (0.33-33 ng min-1) and methoxamine hydrochloride (0.0135-135 micrograms min-1) were infused into the superficial dorsal hand vein; each dose was infused for 5-7 min with 5 min intervening washout periods. Systolic and diastolic blood pressure and pulse rate were recorded before the infusion and immediately after the infusion of the highest dose. Salivation, an index of anticholinergic activity of the antidepressant, was measured by the dental roll technique. 3. Both noradrenaline and methoxamine produced dose-dependent venoconstriction: the geometric mean ED50 for noradrenaline was 4.41 ng min-1 and for methoxamine was 2558 ng min-1; the potency ratio (noradrenaline/methoxamine) was 2884. DMI shifted the dose-response curve for noradrenaline to the left (ANOVA: P < 0.025), resulting in a dose-ratio of 0.26. DMI did not affect the dose-response curve for methoxamine significantly; the dose ratio was 1.24. 4. None of the local infusions and/or systemic treatments had any significant effects on supine systolic and diastolic blood pressure and pulse rate. 5. DMI caused a substantial (47.6%) reduction in salivary output that significantly differed from the slight statistically insignificant increase (5.8%) of salivary output recorded after placebo. 6. These results show that a single oral dose (100 mg) of DMI causes significant potentiation of the response to noradrenaline without significantly affecting the response to methoxamine. The potentiation is likely to be due to uptake blockade since the response to methoxamine was not affected. Furthermore, the lack of significant antagonism of the response to methoxamine indicates that a single oral dose (100 mg) of DMI does not cause sufficient alpha 1-adrenoceptor blockade to be detected as a pharmacodynamic change in our test system.

Use-dependent effects of acute and chronic treatment with imipramine and buspirone on excitatory synaptic transmission in the rat hippocampus in vivo

The effects of acute and long-term treatment with imipramine and buspirone on the responses of rat hippocampal neurones to low and high frequency electrical stimulation were compared. Whereas acute treatment with imipramine (10 mg/kg, i.p.) had no effect on synaptic responses to low frequency stimulation, chronic treatment for 14 days significantly reduced the amplitude of the field excitatory postsynaptic potential. Both acute and chronic imipramine treatment markedly reduced the amplitude of the nerve volley and excitatory postsynaptic potential evoked at high frequency stimulation rates in a use-dependent manner. Buspirone (0.5-3 mg/kg, i.p.) produced a significant reduction of the excitatory postsynaptic potential at high frequencies. This was enhanced after repeated administration of a dose of 0.5 mg/kg for 14 days. We previously reported a similar effect of buspirone at low frequency stimulation. Both compounds therefore share the ability to exert strong depressant effects on transmission in the hippocampus especially after chronic treatment.

Electrophysiological evidence for excitatory 5-HT2 and depressant 5-HT1A receptors on neurones of the rat midbrain tectum

It has been claimed that the aversive behaviour induced by electrical stimulation of the midbrain tectum (MT) has validity as an animal model of panic attack. A great deal of evidence obtained from behavioural studies suggests that 5-HT2 mechanisms phasically inhibit the substrates of aversion in the MT. In order to test this hypothesis we employed the technique of microiontophoresis of drugs onto neurons of the MT to assess the identity of the receptors mediating the effects of 5-hydroxytryptamine (5-HT). The results obtained show that the majority of 5-HT responsive cells in MT are cells excited by 5-HT (72%). These cells were silent or showed very low spontaneous firing activity, whereas cells depressed by 5-HT showed high spontaneous firing activity at baseline. The 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), buspirone and gepirone caused consistent reduction in the firing rate of cells depressed by 5-HT while they did not change the firing activity of cells excited by 5-HT. The excitatory effects induced by 5-HT on MT neurones were clearly attenuated by concomitant application of ketanserin, a highly specific 5-HT2 antagonist. Excitatory responses to DL-homocysteic acid were not affected by ketanserin. Previous administration of zimelidine, a selective 5-HT uptake inhibitor, caused a significant enhancement of the excitatory effects of 5-HT while similar application of gepirone did not affect the size of the excitatory responses to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Disruption of neocortical lamina V neuronal bursts by serotonin in urethane anaesthetized rats

Time-shared high speed cyclic voltametry using carbon fibre multibarrelled microelectrodes was used to monitor the concentration of 5-Hydroxytryptamine administered by iontophoresis to locations in lamina V of Sm l neocortex and to record spontaneous neuronal spike activity. In the absence of 5-Hydroxytryptamine at any one recording location the firing of two or more individual units was seen to be synchronized so that the pattern of multi-unit activity consisted of synchronized clusters of spike activity interspersed with period of neuronal silence. The repetition rate of such clusters of neuronal activity was seen to be between 0.5 and 4Hz. Maximum concentrations of 2.7 x 10(-7) M 5-Hydroxytryptamine produced by iontophoresis disrupted synchronized neuronal cluster activity. 5-Hydroxytryptamine at a concentration of 6.2 x 10(-8) M resulted in a greater than 50% inhibition of activity for 47 single units but a change in firing pattern from cluster restricted high frequency activity to a continuous mode of firing for a separate population of 11 units. Intraperitoneal administration of P-Chloroamphetamine produced similar changes of neuronal firing and hence loss of synchrony.

Evidence for excitatory 5-HT2-receptors on rat brainstem neurones

1. The technique of microiontophoresis was used to investigate the identity of the receptor mediating the excitatory effects of 5-hydroxytryptamine (5-HT) upon neurones in the midline of the medullary brainstem of the rat in vivo. 2. The 5-HT1-like receptor agonists 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) failed to excite the majority of neurones excited by 5-HT. The mobilities of 5-CT and 8-OH-DPAT when tested in vitro were found not to differ significantly from that of 5-HT, suggesting that the lack of effect of these agonists was not due to a lower rate of release from the microelectrodes. 3. The excitatory responses to 5-HT were attenuated by the 5-HT 2-receptor antagonists ketanserin and methysergide when applied microiontophoretically or administered intravenously (0.3 and 1 mg kg-1 respectively). Excitatory responses to glutamate and noradrenaline were not reduced. 4. The 5-HT3-receptor antagonist MDL 72222 failed to attenuate selectively the excitatory response to 5-HT when applied either by microiontophoresis or administered intravenously (1 mg kg-1). 5. Microiontophoretic application of the alpha 1-adrenoceptor antagonist prazosin did not attenuate excitatory responses to either 5-HT or noradrenaline. Intravenously administered prazosin (0.8 mg kg-1) also failed to attenuate excitatory responses to 5-HT, but did block excitatory responses to noradrenaline. 6. These results suggest that 5-HT2-receptors, but not 5-HT1-like receptors, 5-HT3-receptors or alpha 1-adrenoceptors, are involved in the excitatory response of midline medullary neurones to 5-HT.

Serotoninergic innervation of the cat cerebral cortex

Serotoninergic axons in the cat cerebral cortex were demonstrated immunohistochemically with a monoclonal antibody to serotonin (5-HT). Three types of 5-HT axons are distinguished at the light microscopic level by differences in their morphology. Small varicose axons are fine (less than 0.5 micron) and bear fusiform varicosities that are generally less than 1 micron in diameter. These axons extend throughout the width of the cortex and branch frequently, giving rise to widely spreading collaterals. Nonvaricose axons are smooth, show a relatively large and constant caliber (about 1 micron), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 micron or more in diameter) borne on fine-caliber fibers. These axons often form basket-like arbors around the somata of single neurons. In the simplest basket-like arbors, several large, round varicosities from a small number of axons contact the soma. In complex baskets intertwining collaterals contact the soma and apparently climb along and outline the cell's major dendrites. The patterns revealed by the climbing axons suggest that a variety of nonpyramidal cell types selectively receive dense 5-HT innervation. Serial reconstructions of the 5-HT axons within the cortex show that the large varicose axons arise as infrequent collaterals from the nonvaricose axons. A single nonvaricose parent axon gives rise to several large varicose axon collaterals that may contribute to different basket-like arbors. Conversely, a single basket-like arbor may be formed by large varicose axon collaterals from more than one nonvaricose parent axon. The small varicose axons do not appear to be related within the cortex to either the nonvaricose or large varicose axon types. The results support the hypothesis that the 5-HT projection to the cortex is organized into two subsystems, one of which may exert widespread influence in the cortex via highly divergent branches, while the other, with a more restricted distribution, acts on specific classes of cortical neurons.

beta-Phenylethylamine enhances single cortical neurone responses to noradrenaline in the rat

The firing rates of single neurones in the rat cerebral cortex were recorded using multibarrel glass microelectrodes, and the response to drugs applied by microiontophoresis was investigated. A greater number of cells responded to noradrenaline (NA) (30-66 nA) than to beta-phenylethylamine (PE) (30-100 nA). When responses were obtained to both, 90% of the neurones gave the same response to NA and PE. Applications of PE with small currents (0-12 nA) caused an increase in the response to NA without affecting the baseline firing rate or the response to acetylcholine, glutamate, GABA or 5-hydroxytryptamine. An increase was seen in both excitatory and inhibitory responses to NA. The enhancement lasted up to 39 minutes after the end of the PE application. Applications of NA with small currents (0-3 nA) failed to alter responses to NA. Possible mechanisms of the effect of PE on response to NA are discussed. These results provide further evidence for the hypothesis that trace amines can modulate catecholamine neurotransmission.

Changes in auditory evoked responses and in the inhibitory action of 5-hydroxytryptophan following chronic treatment with imipramine in the rat

The neurophysiological effects of acute and chronic treatment with the tricyclic antidepressant drug imipramine were investigated. Brainstem (BAER) and middle latency (MLR) auditory evoked responses were monitored in the alert and immobile rat. Daily injection of imipramine (10 mg/kg, IP) for 2 weeks produced a 13% increase in the latency and a 35% reduction in the amplitude of the N17 component of the MLR. Acute imipramine treatment had no effect. There was no observable change in the BAER after either acute or chronic drug administration. Serotonergic function was assessed by studying the inhibitory effects of 5-hydroxytryptophan on the BAER and MLR. Chronic application of imipramine caused an apparent reduction of the effects of 5-hydroxytryptophan (75 mg/kg, IP) on the N17 component of the MLR. This may, however, be due to the shift in baseline latency and amplitude. Inhibition of the BAER by 5-hydroxytryptophan was unaltered by long term exposure to imipramine. Acute treatment with imipramine was without effect on the modulation of the BAER and the MLR by 5-hydroxytryptophan. These results provide evidence that chronic, as opposed to acute, administration of imipramine had an inhibitory effect on auditory processing at the level of the MLR and that this may be associated with a change in the net output of the serotonergic system.

Responsiveness of suprachiasmatic and ventral lateral geniculate neurons to serotonin and imipramine: a microiontophoretic study in normal and imipramine-treated rats

The suprachiasmatic nuclei (SCN) are a major pacemaker of circadian rhythms in mammals. The SCN receive a direct retinal projection and a second optic input via the ventral lateral geniculate nucleus (vLGN). Both visual pathways mediate the entrainment of circadian rhythms, whereas both the SCN and the vLGN receive serotonergic afferents from the raphe nuclei. We investigated the effects of microiontophoretically applied serotonin (5HT) on SCN and vLGN cells in normal rats and rats chronically treated with the 5HT reuptake blocker imipramine (IMI). In the SCN of both groups over 40% of all recorded cells (N = 80) responded to 5HT with a dose-dependent suppression of their spontaneous or glutamate-evoked discharge, while twenty percent were tonically light-responsive. Except for one cell with an inconsistent 5HT response, none of the visual SCN neurons were 5HT-sensitive. In the vLGN of normal and IMI-treated rats about 60% of the cells recorded (N = 42) were inhibited by 5HT. In IMI-treated rats a few cases of excitation by 5HT were encountered in the vLGN. Visual as well as non-visual vLGN cells were responsive to 5HT. Microiontophoretic application of IMI resulted in suppression of electrical activity in both brain regions and enhanced the response induced by 5HT. Chronic IMI-treatment produced a significant increase in the sensitivity of cells in the SCN and vLGN to iontophoresed 5HT, without affecting the relative magnitude of the inhibition. The recovery from 5HT-induced inhibition was slow in these animals. Interestingly, the spontaneous discharge rate of both 5HT-sensitive and 5HT-insensitive SCN and vLGN cells was significantly lower in the imipramine-treated group.(ABSTRACT TRUNCATED AT 250 WORDS)

Rolipram, a stereospecific inhibitor of calmodulin-independent phosphodiesterase, causes beta-adrenoceptor subsensitivity in rat cerebral cortex

Prolonged pretreatment of rats with the atypical antidepressant rolipram attenuates noradrenaline (NA) sensitivity of the cerebral cortical cAMP generating system. The development of this down-regulation is time (7 d treatment required) and dose dependent (EC50 = 0.35 mg/kg). Density of beta-adrenoceptor as measured by (-)-3H-dihydroalprenolol [(-)-3H-DHA] binding is also reduced by rolipram pretreatment. The effect of rolipram is absolutely stereospecific for the (-)-enantiomer (ED50 = 0.18 mg/kg). In addition, only with this isomer, a reduction in daily weight gain was found compared to sham treated controls. Presynaptic denervation using intracerebroventricular (i.c.v.) injections of 6-hydroxydopamine (6-OHDA) prior to or during rolipram treatment did not completely block the effect of a rolipram treatment on down-regulation of cerebral cortical beta-adrenoceptors. The data favor a pre- and postsynaptic action of rolipram different from all other antidepressants studied so far in this experimental setting. Rolipram is known as inhibitor of brain phosphodiesterase. Using partially purified calmodulin-independent phosphodiesterase from brain it is shown that exclusively the (-)-enantiomer of rolipram inhibits phosphodiesterase with an IC50 of 1.25 mumol/l whereas the (+)-isomer possesses little potency. Since a marked stereospecificity for the (-)-isomer of rolipram was displayed in all pharmacological parameters tested so far with (+)- and (-)-rolipram, it is suggested that stereospecific and isozyme specific inhibition of cAMP-phosphodiesterase is, at least in part, related to the mechanism of action of the potential antidepressant drug rolipram and possibly of other antidepressants as well.

Electrophysiological and receptor studies in rat brain: effects of clorgyline

Acute high doses of clorgyline produce a rapid inhibition of monoamine oxidase type A (MAO A) in the rat brain, together with an increase in norepinephrine and a decrease in the firing rate of locus coeruleus (LC) neurones: this decrease is reversed by piperoxane, an alpha 2 antagonist. In control animals, piperoxane increases LC neuronal firing showing that these noradrenergic neurones are under alpha 2-adrenoceptor-mediated tonic inhibition. Chronic administration of clorgyline, like acute doses of this MAO A inhibitor, significantly decreases cell firing in the LC and the effect is partially reversed by piperoxane. Chronic clorgyline treatment also produces significant decreases in [3H]clonidine and [3H]dihydroalprenolol binding in cerebral cortex, receptor changes which are slightly greater in animals showing greater inhibition of LC neuronal firing: such receptor changes do not occur following a single exposure to clorgyline. Electrophysiological studies in hippocampal pyramidal cells show that the chronic clorgyline treatment does not significantly induced subsensitivity to NE in these adrenoreceptive cells.

The effect of prolonged treatment with tricyclic antidepressants on the actions of 5-hydroxytryptamine in the hippocampal slice of the rat

The effect of long-term treatment with the tricyclic antidepressants imipramine (IMI) and desmethylimipramine (DMI) on neuronal responsiveness to 5-hydroxytryptamine (5-HT) was examined in the hippocampal slice preparation from the rat. Population spikes, evoked by electrical stimulation of the stratum radiatum, were recorded in the pyramidal cell layer of the CA1 region of the isolated hippocampus. When 5-HT (10(-7) to 2 X 10(-5) M) was applied there was an initial increase followed by a decrease in the amplitude of the population spike. On washout of 5-HT the amplitude increased transiently above control levels. Daily injection of 10 mg/kg of imipramine or desmethylimipramine, intraperitoneally, into rats for 4-5 weeks was found to produce a significant decrease in the inhibitory effect of 10(-5) M 5-HT, whereas there was no apparent change in the excitatory effects. The acute application of 10(-5) M imipramine or desmethylimipramine antagonized the inhibitory effect of 10(-5) M 5-HT without affecting the excitatory effects. Acute application of the 5-HT receptor antagonists cyproheptadine (10(-5) M) and ketanserin (7.5 X 10(-6) M) completely prevented the appearance of the inhibitory effect of 10(-5) M 5-HT without affecting the excitatory effects. It was concluded that the decreased inhibitory effect of 5-HT which was produced by chronic treatment with imipramine or desmethylimipramine was probably due to a reduction in the number of 5-HT receptors or a reduction in the post-receptor effector mechanisms for 5-HT.

Antidepressants and serotonergic neurotransmission: an integrative review

The effects of acute and chronic antidepressant treatment on various aspects of 5-HT neurotransmission are reviewed, in order to assess the net effect of antidepressants on transmission across 5-HT synapses. Events considered include presynaptic effects of antidepressants (on autoreceptor function, uptake and turnover) and effects on postsynaptic receptor function (assessed by electrophysiological, neuroendocrine, behavioural, and receptor binding methods). Acute antidepressant treatment has variable effects: transmission may be enhanced, unchanged or reduced, depending mainly upon the relative contributions of 5-HT uptake blockade and 5-HT receptor antagonism. However, on chronic administration, most antidepressants appear to enhance 5-HT transmission. This effect is clearest in the case of ECS, which has little effect on 5-HT turnover, but reduces uptake and increases postsynaptic receptor function. MAOIs may be an exception: there is little evidence that MAOIs enhance 5-HT transmission following chronic treatment. Most other antidepressant drugs, including some which are powerful receptor antagonists on acute administration, reduce 5-HT receptor function briefly, but enhance receptor function if several hours elapse between the final injection and testing. Zimelidine has little effect on postsynaptic receptor function, but enhances 5-HT transmission by its powerful blockade of 5-HT uptake. Chronic treatment with antidepressant drugs has usually been found to reduce binding to 5-HT2 receptors; it is difficult to reconcile these observations with the functional studies. In general, with the possible exception of MAOIs, chronic administration of antidepressants may enhance 5-HT transmission by both pre- and post-synaptic effects, and the relative contributions vary. This conclusion supports the classical "indoleamine hypothesis of depression" rather than the more recent "hypersensitive serotonin receptor" theory.

Potentiation of the action of adenosine on cerebral cortical neurones by the tricyclic antidepressants

The effects of four tricyclic antidepressants, nortriptyline, iprindole, chlorimipramine and desipramine on adenosine-evoked depressions of the firings of rat cerebral cortical neurones has been studied. When applied iontophoretically, all four substances enhanced the depressant actions of iontophoretically applied adenosine but did not affect the depressant actions of the uptake-resistant analogue, adenosine 5'-N-ethylcarboxamide (NECA). Nortriptyline and iprindole administered intravenously (1 mg kg-1) enhanced the depressant actions of iontophoretically applied adenosine. When applied by larger iontophoretic currents, all four antidepressants inhibited the firing of cerebral cortical neurones. Chlorimipramine- and desimipramine-elicited depressions were antagonized by intravenously administered caffeine, an adenosine antagonist. Earlier studies showed the tricyclic antidepressants inhibit the uptake of adenosine by rat brain cerebral cortical synaptosomes. The present results demonstrate that four antidepressants are able to potentiate the action of adenosine and that this occurs when these compounds are given in behaviourally meaningful doses. The specificity of the potentiation is demonstrated by the failure of these compounds to potentiate the depressant actions of an uptake-resistant analogue of adenosine, NECA. Antagonism of the inhibitory effects of the antidepressants on neuronal firings by caffeine, indicates that these compounds can enhance the extracellular levels of endogenously released adenosine sufficiently to depress cell firing.

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.

Behavioural changes during withdrawal from desmethylimipramine (DMI). I. Interactions with amphetamine

Amphetamine anorexia in rats was potentiated by acute pretreatment with the tricyclic antidepressant desmethylimipramine (DMI), but was not significantly different from controls following chronic DMI pretreatment. During withdrawal from DMI, amphetamine anorexia was attenuated after 2 weeks or 2 months pretreatment, but not after 1 week of treatment. The locomotor stimulant and stereotypy inducing effects of amphetamine were slightly enhanced during withdrawal from chronic DMI. The results are discussed in relation to known neurochemical actions of DMI.

The peripheral anticholinergic activity of tricyclic antidepressants: comparison of amitriptyline and desipramine in human volunteers

The effects of three single oral doses (25 mg, 50 mg and 100 mg) of amitriptyline and desipramine, and of placebo, were compared on a range of cholinergic functions (resting pupil diameter, pilocarpine-evoked miosis, baseline-sweating, carbachol-evoked sweating, salivation, heart rate) in eight healthy volunteers. Three measures (pilocarpine-evoked miosis, carbachol-evoked sweating and salivation) reflected the antimuscarinic property of the antidepressants; in two tests (pilocarpine-evoked miosis and salivation) amitriptyline appeared to be more potent than desipramine. Resting pupil diameter was not affected by amitriptyline, whereas desipramine caused mydriasis, indicating that pupil size is not a reliable measure of anticholinergic activity in the case of drugs which also affect adrenergic mechanisms. Baseline-sweating and heart rate were not affected by the antidepressants.

Enhancement of 5-hydroxytryptamine-induced behavioral effects following chronic administration of antidepressant drugs

It has been shown recently that chronic administration of tricyclic antidepressant drugs results in enhanced responsiveness of neurones to iontophoretically applied 5-hydroxytryptamine (5-HT) in rat forebrain regions. The present investigation tested whether this efect is accompanied by an enhancement of behavioral effects of the amine. Behavioral signs of sleep in young chicks following systemic administration of 5-HT were used as an index of the acion of the amine at central receptor sites. Imipramine, desipramine, amitryptiline, pizotifen, and mianserin given 30 min before 5-HT all reduced the duration of 5-HT-induced behavioral depression. However, 6-8 day pretreatment with the same drugs resulted in an increased duration of the 5-HT-induced depression. The results suggest that the antidepressant drugs can block 5-HT receptors in the central nervous system (CNS) and that chronic blockade resulting from repeated administration of the drugs results in an increase in number or sensitivity of 5-HT receptors.

Chronic treatment with lithium or desipramine alters discharge frequency and norepinephrine responsiveness of cerebellar Purkinje cells

Cerebellar Purkinje cells were studied by electrophysiological techniques in rats treated chronically with either desipramine (DMI) or lithium chloride given intragastrically. A striking decrement occurred in discharge frequencies of simple spikes and climbing fiber bursts in both groups of animals, similar to the depression produced by iontophoresis of these agents. Chronic treatment with DMI markedly decreased responsiveness to iontophoretically applied norepinephrine (NE), whereas long-term LiCl therapy slightly enhanced response to NE; responses to gamma-aminobutyric acid were unchanged by these treatments. The inhibitory responses to locus ceruleus stimulation were unaffected by chronic LiCl treatment. The effects of these chronic treatments on responsiveness to NE are opposite to the effects these same drugs produce when administered by acute iontophoresis to single cells: DMI then potentiates and LiCl antagonizes noradrenergic responses. These results provide electrophysiological evidence for reciprocal adaptive changes in NE sensitivity, supporting results of biochemical studies.

Potentiation of responses to monoamines by antidepressants after destruction of monoamine afferents

1 Stereotaxic lesioning and microiontophoretic techniques were used to study the effects of lesions of the medial forebrain bundle (MFB) on the potentiation by antidepressant drugs of responses to monoamines of cortical neurones.2 Active uptake of noradrenaline (NA) and 5 hydroxytryptamine (5-HT) by synaptosomes from the motor and somatosensory cortex was reduced to approximately 20%, 10 to 14 days following lesion of the MFB in rats.3 Unilateral lesions of the MFB caused changes in responsiveness of neurones to NA and 5-HT, applied by iontophoresis, in the cortex ipsilateral to the lesion. Excitatory responses to both amines were observed less frequently and depression was the predominant response. Excitatory responses on the lesioned side were significantly smaller than on the unlesioned side, but the size of depressant responses was unaltered.4 Viloxazine strongly potentiated responses of cortical neurones to NA and 5-HT on both sides of the brain of MFB-lesioned rats. There were no significant differences in the potentiation of responses to monoamines on the lesioned or unlesioned sides of the brain.5 Desipramine potentiated responses to NA of neurones in the cortex ipsilateral to MFB lesions.6 Chlorimipramine potentiated responses to 5-HT of neurones in the cortex ipsilateral to MFB lesions.7 It is concluded that antidepressants can potentiate responses to monoamines despite a profound reduction in presynaptic terminals. The potentiation is unlikely to be the result of blockade of monoamine uptake into presynaptic terminals, and is probably a postsynaptic effect of the antidepressant drugs.

Tricyclic antidepressants: long-term treatment increases responsivity of rat forebrain neurons to serotonin

Long-term treatment of rats with clinically effective tricyclic antidepressant drugs induced a selective increase in the inhibitory response of forebrain neurons to serotonin applied by microiontophoresis. Long-term administration of some related drugs which lack antidepressant efficacy failed to induce such a change. The enhanced response to serotonin induced by the clinically active tricyclic drugs took 1 to 2 weeks to develop, a time course which correlates with the delayed onset of therapeutic effects in humans.

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.

End-plate voltage noise during prolonged application of acetylcholine in cat tenuissimus muscle [proceedings]

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Seizure activity in photosensitive baboons following antidepressant drugs and the role of serotoninergic mechanisms

Laboratroy and clinical evidence indicates that tricyclic antidepressants lower seizure threshold and in high doses may induce generalised seizures. In baboons with photosensitive epilepsy (Papio papio) the effects of 2 tricyclic antidepressants (imipramine and chlorimipramine) and of maprotiline and Nomi fensine have been studied (i.v. dose range 1-20 mg/kg. Imipramine, chlorimipramine and maprotiline (10 mg/kg i.v.) lowered seizure threshold to a comparable extent, whereas Nomifensine (10 mg/kg i.v-) did not enhance myoclinic responses to photic stimulation. Generalised seizures were seen 15-30 min after imipramine or chlorimipramine (20 mg/kg), and these two drugs showed no difference in their epileptogenicity. Administration of 5-hydroxytryptophan (25 mg/kg i.v.) 90 min before chlorimipramine or imipramine (10 mg/kg) completely blocked the usual augmentation of photically-induced epileptic responses. It is concluded that enhancement of serotoninergic activity following blockade of 5-HT re-uptake within the brain is unlikely to be responsible for enhanced myoclonic responses and epileptogenic seizures seen after tricyclic antidepressants. Nomifensine is significantly less epileptogenic than imipramine or chlorimipramine.

Dffects of imipramine on auditory sensitivity in the rat in relation to initial sensitivity

Rats were divided into 2 groups according to their absolute auditory thresholds, as measured by a psychophysical method of limits using an operant conditioning technique. The procedure required a water-deprived rat to bar press repeatedly until a 3 s pure tone (3 kHz) came on. VR-15 schedule was used to determined the frequency of occurrence of the tone. Only within 4 s after the tone onset could the rat get water reinforcement by sticking its nose into the dipper area. The descending method was used with tone intensities ranging from 120 to 40 dB, in 2 dB steps. Imipramine was then administered 3 times at 24-h intervals (40, 80 and 160 mg/kg, p.o., respectively), and each rat was tested at 4, 8, and 12 h after each administration. Rats with high initial thresholds showed a decrease in thresholds with the lower dose of imipramine, and those with low initial thresholds an increase with the higher dose. An analysis of variance of these data showed significant effects of the type of subject (in terms of the level of initial thresholds), dose and interval conditions, and their interaction. It was demonstrated that the lower the initial sensitivity the greater the increase in sensitivity by imipramine.

Potentiation by desipramine of neuronal responses to mescaline

The effect of desipramine on responses of single cortical neurones to mescaline was studied by the microelectrophoretic technique. Both potentiation and antagonism of responses to mescaline by desipramine were observed. The antagonism may be related to the alpha-adrenolytic action of desipramine. The potentiation is unlikely to reflect the uptake blocking action of desipramine, since desipramine does not block the uptake of mescaline in the cerebral cortex. It is suggested that the potentiation may be due to a post-synaptic action of desipramine.

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.