Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Effects of trazodone treatment on alpha-2 adrenoceptor function in depressed patients

Tricyclic antidepressant drugs reduce the sensitivity of alpha 2 adrenoceptors during long-term treatment. In the present study, the alpha 2 adrenergic agonist clonidine was administered to 11 depressed patients before and during treatment with the triazolopyridine antidepressant trazodone (TRZ). Clonidine's ability to decrease blood pressure (BP) and plasma levels of the norepinephrine metabolite 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), and to increase sedation and plasma growth hormone (GH), were measured. TRZ had little effect on these indices of pre- and postsynaptic alpha 2 receptor function, suggesting that the antidepressant properties of TRZ are not related to changes in alpha 2 adrenoceptor sensitivity.

Noradrenergic and opioid mediation of tricyclic-induced reversal of escape deficits caused by inescapable shock pretreatment in rats

The present study was undertaken to investigate the possible involvement of a noradrenergic and opioid mediation of the reversal by tricyclic antidepressants of escape deficits produced by inescapable shock pretreatment. Rats were first exposed to 60 inescapable shocks (15 s duration, 0.8 mA, every min +/- 15 s) and 48 h later subjected, to daily shuttle-box sessions (30 trials/day, ITI: 30 s) during 3 consecutive days. Twice-daily IP injection of desipramine or clomipramine (total daily dose: 32 mg/kg) prevented escape deficits. Penbutolol (0.125; 0.25; 0.5 mg/kg), prazosin (1; 2 mg/kg) and naloxone (0.5; 1 mg/kg) given once a day dose-dependently attenuated the beneficial effect of tricyclic antidepressants in reducing the number of escape failures in rats exposed to shock pretreatment. In agreement with data obtained in the forced-swimming model, these findings support the notion that activation of noradrenergic and opioid receptors is an important factor in the mediation of the effects of tricyclic antidepressants in animal models of depression.

Central serotonergic responses and behavioural adaptation to repeated immobilisation: the effect of the corticosterone synthesis inhibitor metyrapone

Rats were immobilised for 2 h/day. Twenty-four hours after the 1, 3 or 7 immobilisation periods they were injected with the 5HT agonist 5-methoxy-N,N-dimethyltryptamine (5MeODMT; 5 mg/kg i.p.) and behavioural responses (i.e. hind limb abduction, forepaw treading, head weaving, tremor, Straub tail) compared with those of a control group. As we have previously observed after 7 (but not after 1 or 3 immobilisations) forepaw treading and tremor were enhanced and the other responses unaffected. Pretreatment with metyrapone (a corticosterone synthesis inhibitor 150 mg/kg i.p., 3 h before each immobilisation) did not affect the above responses to 1 immobilisation, increased tremor after 3 immobilisations and also increased forepaw treading, hind limb abduction and Straub tail after 7 immobilisations but decreased head weaving under the latter conditions. Metyrapone without immobilisation had no effect on responses to 5MeODMT. Twenty four hours after 1 or 3 (but not 7) immobilisation periods, rats placed for the first time in an open field showed less locomotion and rearing and more defaecation than control animals. Rats also given metyrapone exhibited normal open field behaviour after only 3 immobilisations. The drug also accelerated the return to normal on repeated immobilisation of the impairment of food intake and growth rate which occurred after a single immobilisation. The results as a whole suggest that metyrapone promotes behavioural adaptation to repeated immobilisation and that this is associated with enhanced postsynaptic responses to 5HT. These findings suggest that immobilisation stress-induced changes might be relevant as an animal model for depression which incorporates reported biochemical abnormalities in the illness and is of relevance to proposals concerning its precipitation by stress.

Frequency-dependent block of field potentials in the rat hippocampal slice caused by tricyclic antidepressants

The effect of the tricyclic antidepressants imipramine and desipramine were studied on field potentials in the rat hippocampal slice. The electrically evoked stratum radiatum nerve volley, excitatory postsynaptic potential (e.p.s.p.) and pyramidal cell layer population spike (PS) were recorded in the CA1 region. At concentrations of 10(-6)M to 10(-5)M, impramine did not affect the amplitude of the nerve volley, e.p.s.p. or PS at low frequencies of stimulation (0.01 Hz). At higher frequencies of stimulation (1-100 Hz), imipramine caused a frequency-dependent block of the nerve volley, e.p.s.p. and PS. The time course of onset of the frequency-dependent block in the presence of imipramine was very slow. Maximum inhibition was reached after 3-4 h treatment with imipramine. Desipramine (10(-6)-10(-5)M) also caused a frequency-dependent block of the hippocampal field potentials. Only slight frequency-dependent block was observed in slices from rats injected in vivo with desipramine (10 mg kg-1) for 14 days.

Subsensitivity of enteric cholinergic neurones to alpha 2-adrenoceptor agonists after chronic sympathetic denervation

The concentration-effect relationships of noradrenaline, dopamine and clonidine in inhibiting resting and stimulated acetylcholine output have been studied in intact and in sympathetically denervated preparations of guinea pig isolated distal colon. The order of potencies for the inhibition of resting acetylcholine release in intact preparations was clonidine greater than dopamine greater than noradrenaline while the order of intrinsic activities was noradrenaline greater than dopamine greater than clonidine. Sympathetic denervation was able to modify the potency of either clonidine, dopamine and noradrenaline. Noradrenaline was 6 times more potent in inhibiting resting acetylcholine release in denervated than in intact preparations, while clonidine and dopamine underwent a 18-fold and a 11-fold decrease in potency after denervation. The potency of clonidine relative to noradrenaline was 110 in intact preparations and only 1.2 in denervated organs. The intrinsic activities of noradrenaline, dopamine and clonidine were almost unchanged in denervated organs. A dose-dependent facilitatory effect of yohimbine on both the resting acetylcholine output and the peristaltic reflex could be observed in intact but not in sympathetically denervated preparations at concentrations ranging from 2.5 X 10(-8) M to 2.5 X 10(-7) M. Yohimbine was able to counteract the inhibitory effect of dopamine and to remove the inhibitory effect of periarterial nerve stimulation on both acetylcholine release and the peristaltic reflex. Our results are consistent with the existence of a tonic physiological modulation of enteric cholinergic neurones by postganglionic sympathetic fibres. The order of potencies of adrenoceptor agonists and the antagonism by yohimbine is consistent with such a modulation being entirely carried out through alpha 2-heteroceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenoreceptor interactions of the enantiomers and metabolites of mianserin: are they responsible for the antidepressant effect?

Mianserin is a tetracyclic antidepressant whose postulated mechanism of action involves release of noradrenaline mediated via cortical alpha 2-adrenergic autoreceptor blockade. This property resides stereoselectively in the S(+)-enantiomer of mianserin, which is also more potent in behavioural tests indicative of antidepressant activity, and in the reversal of clonidine-induced effects. Cortical receptor binding studies have indicated that although a similar stereoselectivity prevails for the inhibition of both alpha 2-binding (clonidine) and alpha 1-binding (prazosin), mianserin and its enantiomers are more potent antagonists at alpha 2- than at alpha 1-binding sites. However, no stereoselectivity is apparent for the antagonism of cortical alpha 2-heteroreceptors controlling serotonin release. Following chronic administration, (+/-)- and S(+)-mianserin, but not the R(-)-enantiomer, produce functional supersensitivity at alpha 2-autoreceptors which is unaccompanied by changes in clonidine binding. Neither mianserin nor its enantiomers alter the sensitivity of alpha 2-heteroreceptors following chronic administration. Like mianserin and its S(+)-enantiomer, but unlike R(-)-mianserin and the 8-hydroxy metabolite, the desmethyl metabolite inhibits noradrenaline uptake in vitro. 8-hydroxymianserin and, to a lesser extent, desmethylmianserin release noradrenaline from cortical slices via alpha 2-autoreceptor antagonism, but only the 8-hydroxy metabolite blocks alpha 2-autoreceptors and alpha 2-heteroreceptors in synaptosomal preparations. It is likely that S(+)-mianserin, desmethylmianserin, and 8-hydroxymianserin contribute substantially to the overall facilitating effect of mianserin on noradrenergic transmission in vivo. As yet it is unclear whether this effect is exclusively responsible for the antidepressant activity of mianserin or whether the stereoselectivity also shown by the mianserin enantiomers towards serotonin receptors plays a complementary role.

Chlorimipramine, electroconvulsive shock and combination thereof: differential effects of chronic treatment on apomorphine-induced behaviours and on striatal and mesocortical dopamine turnover

We studied the influence of different pretreatment regimens (Chlorimipramine-Cmi, electroconvulsive shock-ECS, and Cmi + ECS all regimens being applied for either 2 or 15 days) on the open field behaviour, on the striatal and on the prefrontal dopamine-PFC DA turnover in rats injected with either apomorphine-AP 25 micrograms/kg (stimulating presynaptic DA receptors), AP 200 micrograms/kg (stimulating post-synaptic DA receptors), or vehicle (control). In the controls, AP 25 micrograms/kg reduced the locomotor activity and the striatal, but not the PFC DA turnover. AP 200 micrograms/kg increased the locomotor activity and reduced the striatal but not the PFC DA turnover. Short-term pretreatment: ECS and Cmi + ECS prevented the decrease of striatal DA turnover after AP 25 micrograms/kg. No other influence of any pretreatment on behaviour or DA-turnover became significant. Long-term pretreatment: Chronic Cmi: marginally increased the open field behaviour and marginally decreased the PFC DA turnover; significantly increased the effect of AP (200 micrograms/kg) on striatal DA turnover and the effect of AP (25 and 200 micrograms/kg) on PFC DA turnover. Repeated ECS: decreased locomotion and rearing and increased PFC DA turnover; increased the effect of AP (200 micrograms/kg) on locomotion and on striatal DA turnover; decreased the effect of AP (25 and 200 micrograms/kg) on PFC DA turnover. Chronic Cmi + ECS: decreased locomotion and rearing and marginally decreased PFC DA turnover; increased the effect of AP on hyperlocomotion and on striatal DA turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased synaptic markers in hippocampus of depressed patients

The neuronal adhesion molecule, D2-protein, and the marker for mature synapses, D3-protein, were studied by crossed immunoelectrophoresis in postmortem samples of human brain hippocampus and frontal cortex. The samples were obtained from 6 patients with endogenous depression, 6 patients with Parkinson's disease, and 9 controls without known neurological or psychiatric disorders. Both D2-protein and D3-protein were significantly increased in hippocampus of depressed patients compared to controls (32% and 30%, respectively) and compared to Parkinson's disease patients (31% and 24%, respectively). However, no significant change was observed in frontal cortex. Combined with the previously observed increase in the level of D2-like protein in blood plasma from depressed patients the findings may indicate that, at least in some regions of the brain, synaptic turnover is increased during depression.

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.

The acute effects of antidepressant drugs on the performance of conditioned avoidance behavior in rats

The effects of acute administration of 10 different antidepressant drugs were examined on the performance of a two-way conditioned avoidance response in rats. The antidepressant drugs impaired avoidance behavior by decreasing avoidance responding and increasing the number of escape failures. The order of effectiveness for increasing overall response latency at a common dose of 10 mg/kg was: desipramine, maprotiline, protriptyline, (+) oxaprotiline, nortriptyline, imipramine, amitriptyline, (-) oxaprotiline, fluoxetine, and chlorimipramine. Avoidance behavior was impaired most by those antidepressant drugs that are also potent inhibitors of norepinephrine uptake.

Performance and extinction of lever press behavior following chronic administration of desipramine to rats

The effect of repeated administration of desipramine (DMI) on the acquisition, performance, and extinction of a lever press response for food reward was studied. Chronic administration of DMI caused a reduction in pressing under a CRF schedule both in naive and well-trained rats. Responding during extinction sessions did not differ between saline-treated rats and rats given DMI chronically. In addition, chronic administration of DMI reduced the body weight and food intake of rats on either a free-feeding or a restricted-feeding schedule. Consequently, lever pressing was also studied in a group of rats whose body weight was regulated to match the body weight of rats were administered DMI chronically. In comparison to this control group, rats administered DMI chronically responded significantly less during both reinforcement and extinction sessions. These results fail to replicate earlier reports that chronic DMI administration produces increased resistance to extinction. The results also show that assessment of food-motivated performance in rats treated chronically with DMI is difficult because of long-term changes in body weight and food intake.

A stimulatory effect of intraaccumbens injections of noradrenaline on the behavior of rats in the forced swim test

Intraaccumbens injections of catecholamines noradrenaline and dopamine, though not of serotonin, stimulated locomotion by rats in an open field, 10-15 min later. Similar effects were observed 5 min after microinjection of apomorphine whereas clonidine only attenuated locomotor activity. On the other hand, intraaccumbens administration of phenylephrine, isoproterenol and quipazine, in doses similar to an effective dose of noradrenaline, did not alter rat open field behavior. The escape-directed activity of rats in the forced swim test (FST) was stimulated 5 min after local administration of noradrenaline, phenylephrine, isoproterenol or apomorphine only. No effects in the FST were observed 15 min after noradrenaline injection or after intracaudate noradrenaline administration. The stimulatory effects of intraaccumbens noradrenaline injection in the FST were antagonized by the local pretreatment of rats with phentolamine, though not with propranolol. Accordingly, it is possible to conclude that both catecholamines, but not serotonin, play complex and probably distinct roles within the nucleus accumbens in the stimulation of activity by rats in the FST and the open field test.

Agonist interactions with beta-adrenergic receptors following chronic administration of desipramine or the atypical antidepressants, iprindole and mianserin

Desipramine (DMI), decreased the maximum number of beta-adrenergic receptors by approximately 10, 20, 30, and 20% in groups of rats treated i.p. with 5 mg/kg for 14 days or 10 mg/kg for 7, 14, or 21 days, respectively. In studies of agonist competition for beta-adrenergic receptors labelled with [125I]-CYP, chronic DMI administration caused a selective decrease in those receptors normally found in the high affinity conformation in proportion to the dose of DMI administered. No change was observed in either the number of receptors in the agonist low affinity conformation or in the affinity of any drug for the high or low affinity conformations of the receptors. Therefore, chronic DMI caused a selective decrease in the beta-adrenergic receptors linked to adenylate cyclase but did not appear to change other properties of the receptors that would be manifested as a change in their ability to interact with adrenergic agonists. Neither iprindole (15 mg/kg i.p., 14 days) nor mianserin (10 mg/kg i.p., 14 days) decreased the number of receptors, the proportions of agonist high or low affinity receptors, or the affinity of competitor drugs for these receptors, suggesting a different mechanism for the reported loss of adenylate cyclase activity following these drugs than the down-regulation of receptors observed with chronic DMI treatment.

Tricyclic antidepressant drugs affect histamine receptors in human leukocytes

Tricyclic antidepressant drugs bind to histamine receptors in rodent and other mammalian tissues. However, a readily accessible, normal human tissue for studies of these drugs has not been available. We showed that freshly isolated human leukocytes can be used for this purpose. Following isolation by dextran sedimentation, leukocytes were incubated for 30 sec. in histamine concentrations from 1.0 nM to 1.0 mM. A dose-related increase in intracellular cyclic AMP was observed. When the cells were preincubated for 10 min. in the tricyclic, nortriptyline, and then challenged with histamine, the concentration of histamine needed for a comparable cyclic AMP increase was elevated 100-fold over non-preincubated cells. These results indicate that the tricyclic drug interacts with histamine receptors on leukocytes, and that these cells may be used for biochemical and clinical studies of these drugs.

Influence of long-term zimelidine treatment on LSD-induced behavioural effects

The effect of lysergic acid diethylamide (LSD) on copulatory and exploratory behaviour in hormone-treated, ovariectomized rats was investigated 4 weeks after chronic treatment with either saline or zimelidine (ZIM). A decrease in sensitivity to the lordosis-inhibiting effect of LSD was observed in the group treated with ZIM. The two groups also differed in their reaction to LSD in the exploratory test. The ZIM-treated animals seemed to be less sensitive to LSD. We therefore suggest that long-term treatment with ZIM reduces the sensitivity of the postsynaptic serotonin receptor.

Treatment with clorgyline and pargyline differentially decreases clonidine-induced hypotension and bradycardia

A study was made of the effects of acute and chronic treatment with monoamine-oxidase (MAO) inhibitors on the peripheral and central cardiovascular response induced by clonidine in anaesthetized normotensive rats. Clonidine (30 nmoles X kg-1 i.v.) produced a biphasic change in mean blood pressure; an initial transient increase was followed by a prolonged hypotensive effect, coinciding with the maximal bradycardia. Twenty-four hours after acute (single) or chronic (daily for 7 days) administration of MAO inhibitors (pargyline 10 mg X kg-1 SC or clorgyline 0.3 mg X kg-1 SC) there was no effect either on the basal cardiovascular parameters or on the initial pressor response induced by clonidine. Chronic but not acute treatment with clorgyline, an inhibitor of type A MAO, greatly decreased the hypotension and bradycardia induced by clonidine for as long as 5 days after its discontinuation. On the other hand, after chronic administration of pargyline (10 mg X kg-1), a preferential type B MAO inhibitor, the hypotension and bradycardia caused by clonidine were differently affected. There was a reduction in the bradycardia up to the third day following the discontinuation of pargyline, whereas the hypotensive response induced by clonidine was only attenuated for 24 h and unaffected with a lower dose of pargyline (0.3 mg X kg-1). It is concluded that chronic administration of the type A MAO inhibitor, clorgyline, attenuates the central responses to clonidine through the reduction in sensitivity of brain alpha-adrenoceptors. Pargyline, that preferentially inhibits type B MAO, reduces only the bradycardia induced by clonidine. This result may indicate a different modulation of the receptors involved in this response to clonidine.

An increase in sensitivity of rat cingulate cortical neurones to substance P occurs following withdrawal of chronic administration of antidepressant drugs

The sensitivity of neurones in the cingulate cortex of the rat to iontophoretically applied substance P (SP) was tested one hour after a single dose of various antidepressant drugs and also 1 day following the termination of a chronic dosing schedule (14 once daily injections) of the same agents. One hour after a single injection of desipramine (DMI), chlorimipramine (CMI), trimipramine (TMI) or zimelidine ( ZIM ) (all at 10 mg kg-1 i.p.) there was no change in the mean size of excitatory responses to SP compared to those before the injection. There was a tendency towards a decrease in the TMI group. One day following the last of 14 consecutive daily injections (10 mg kg-1 i.p.) of the above agents there was a significant increase in the size of excitatory responses to SP compared to those in rats receiving daily saline injections. However, 14 days of treatment with DMI did not alter the responses to L-glutamate. Similar chronic dosing schedules with either diazepam (5 mg kg-1) or fluphenazine (5 mg kg-1) did not affect the responses to SP. Thus chronic but not acute administration of antidepressant drugs results in an increase in the sensitivity of neurones, in the cingulate cortex of the rat, to SP.

A study of the effects of clonidine on the EEG in rats treated with single and multiple doses of antidepressants

The influence of repeated and single administrations of desipramine, amitryptiline, and mianserin on the EEG effects of clonidine has been investigated in rats implanted with chronic cortical electrodes. Clonidine induced a dose-dependent EEG synchronization in control animals. Signs of behavioral depression occurred after administration of moderate (0.1 mg/kg) and higher (0.2 mg/kg) doses of clonidine. Single doses of desipramine and amitryptiline attenuated the clonidine effect, while mianserine potentiated clonidine-induced synchronization. Antidepressants given once daily for 14 days completely (desipramine and amitryptiline) or partially (mianserin) reduced the effect of clonidine. Antidepressants alone produced only a slight effect on cortical EEG pattern.

Some effects of chronic antidepressant treatments on rat brain monoaminergic systems

A range of established and putative antidepressant therapies were studied for the effect of their long-term administration on two facets of presynaptic monoaminergic functioning in rat brain, namely NE, DA, and 5-HT turnover and alpha 2-adrenoceptor sensitivity. Unless stated otherwise drugs (10 mg/kg) were injected i.p. twice daily for 14 days. ECT (100 mA for 1 s) was applied once daily for 10 days. Changes in turnover were indirectly assessed by measuring levels of metabolites. Brain levels of MHPG-SO4 were unchanged by chronic amitriptyline, imipramine, nisoxetine (20 mg/kg), nortriptyline, salbutamol (5 mg/kg), and ECT. Amitriptyline elicited a slight, but significant, increase in brain DOPAC content. Brain levels of 5-HIAA were increased by amitriptyline, imipramine, salbutamol, and ECT. An overall view of the results indicates that no common pattern of change was elicited by the range of antidepressant therapies studied. Central alpha 2-adrenoceptor sensitivity was assessed by investigating the effect of various therapies on the ability of clonidine (25 mg/kg i.p.) to decrease rat brain MHPG-SO4 content. The clonidine-induced fall was attenuated by desipramine, imipramine, and ECT. Amitriptyline, iprindole, mianserin, nisoxetine, nortriptyline, Org 6582 (10 mg/kg once daily), pargyline (25 mg/kg once daily), salbutamol, and trazodone were ineffective. The following chronic antidepressant therapies were investigated for their effect on rat frontal cortex 3H-clonidine binding: amitriptyline, desipramine, imipramine, iprindole, mianserin, nisoxetine, nortriptyline, pargyline, salbutamol, and ECT. CHronic, but not acute, pargyline decreased 3H-clonidine binding and this was due to a diminished number of binding sites. The induction of subsensitive presynaptic alpha 2-adrenoceptors in rat brain is not a property common to all forms of antidepressant therapies. Hence it cannot be the fundamental mode of action of antidepressants. No correlation exists between the changes in rat cortical 3H-clonidine binding and the observed changes in the sensitivity of central presynaptic alpha 2-adrenoceptors.