Full text of DOI:10.1016/S0924-977X(00)00069-9

European Neuropsychopharmacology 10 (2000) 177–188
www.elsevier.com/locate/euroneuro
Effects of the co-administration of mirtazapine and paroxetine on
serotonergic neurotransmission in the rat brain
*
`
Agnes Besson, Nasser Haddjeri, Pierre Blier , Claude de Montigny
´
´
Neurobiological Psychiatry Unit, McGill University, 1033 Pine Avenue, West Montreal, Quebec, Canada H3A 1A1
Received 23 November 1999; received in revised form 18 January 2000; accepted 25 January 2000
Abstract
The a2-adrenoreceptor antagonist mirtazapine, which is also a 5-HT2, 5-HT3 and H1 receptors antagonist and the selective serotonin
(5-HT) reuptake inhibitor paroxetine are effective antidepressant drugs which enhance 5-HT neurotransmission via different mechanisms.
The present studies were undertaken to determine whether the mirtazapine–paroxetine combination could induce an earlier and/or a
greater effect on the 5-HT system than either drug alone. Using in vivo electrophysiological paradigms, the firing activity of dorsal raphe
5-HT neurons was decreased by 70% in rats treated with paroxetine (10 mg/kg/day, s.c.) for 2 days and was back to normal after 21
days. In contrast, a 2-day treatment with mirtazapine (5 mg/kg/day, s.c.) did not alter the firing of 5-HT neurons whereas it was increased
by 60% after 21 days of treatment. A low dose of mirtazapine (5 mg/kg/day, s.c.32 days) failed to offset the decremental effect of
paroxetine on the 5-HT neuron firing activity, but a higher dose (10 mg/kg/day, s.c.32 days) did attenuate the decremental effect of
paroxetine. In the dorsal hippocampus, neither mirtazapine (5 mg/kg/day, s.c.) nor a paroxetine (10 mg/kg/day, s.c.) treatment altered
the responsiveness of 5-HT1A receptors to microiontophoretically-applied 5-HT. Both in controls and in rats treated for 2 days with
paroxetine alone, the administration of the 5-HT1A antagonist WAY 100635 (25–100 mg/kg, i.v.) did not change the firing activity of
dorsal hippocampus CA3 pyramidal neurons. However, WAY 100635 increased significantly the firing activity of these neurons in rats
treated with mirtazapine alone but to a greater extent with both mirtazapine and paroxetine for 2 days. After 21 days of treatment, WAY
100635 increased to a greater degree the firing rate of CA3 pyramidal neurons in rats which received the combination over rats given
either drug alone. It is concluded that the mirtazapine–paroxetine combination shortened the delay in enhancing the tonic activation of
postsynaptic 5-HT1A receptors and produced a greater activation of the postsynaptic 5-HT1A receptors than either drug given alone. The
present results suggested that mirtazapine may have a faster onset of action than a SSRI, and that the co-administration of mirtazapine and
paroxetine may accelerate the antidepressant response and as well as being more effective than either drug alone.
Science B.V. All rights reserved.
2000 Elsevier
Keywords: Mirtazapine; Paroxetine; Serotonin; Dorsal raphe; Dorsal hippocampus
1. Introduction
sive shock (ECS) administration lead to an enhanced 5-HT
neurotransmission through a sensitization of the postsynap-
tic 5-HT1A receptors (de Montigny and Aghajanian, 1978;
de Montigny, 1984; Welner et al., 1989; Nowak and
Dulinski, 1991; Stockmeier et al., 1992). Long-term treat-
ment with either monoamine oxidase inhibitors (MAOIs)
or selective 5-HT reuptake inhibitors (SSRIs) results in a
desensitization of the somatodendritic 5-HT1A autoreceptor
of 5-HT neurons in the dorsal raphe nucleus, thereby
allowing their firing rate to recover in the presence of the
drugs (Blier et al., 1986; Chaput et al., 1986). In addition,
long-term SSRI treatment also desensitizes terminal
5-HT1B (in the rat) and 5-HT1D (in the guinea-pig)
Although the physiopathology of major depression is not
fully defined, there is a growing body of evidence suggest-
ing the implication of the serotonin (5-HT) system in the
therapeutic effect of antidepressant treatments (Heninger
and Charney, 1987; Price et al., 1990; Van Praag et al.,
1990; Cummings, 1993; Blier and de Montigny, 1994;
Maes and Meltzer, 1995). It has been shown that long-term
tricyclic antidepressant (TCA) and repeated electroconvul-
*Corresponding author. Tel.: 11-514-398-7304; fax: 11-514-398-
4486.
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178
A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
autoreceptors, whereas long-term MAOI treatment desen-
sitizes terminal a2-adrenergic heteroreceptors located on
5-HT terminals (Blier and Bouchard, 1994; Mongeau et
al., 1994), the role of both receptors being to inhibit 5-HT
water–ethanol solution (80:20, v/v and 50:50, v/v, for the
2 day- and 21 day treatments, respectively). For each
series of experiments, controls were implanted with a
minipump filled with the same vehicle as the corre-
sponding treated groups. Rats receiving the mirtazapine–
paroxetine combination were implanted with two
minipumps. All the experiments were carried out with the
minipumps on board. None of these treatments altered the
normal behavior of the animals in their cages or upon
handling, and the weight gain was similar in the treated
groups than in controls. The regimens above have been
chosen on the basis of previous experiments indicating a
marked blockade of 5-HT reuptake with 10 mg/kg/day of
¨
release (Gothert, 1980). Long-term treatment with the
antidepressant mirtazapine, an a2-adrenoceptor antagonist,
increases 5-HT neurotransmission, as a result of a sus-
tained increase in the firing activity of 5-HT neurons in
presence of decreased function of a2-adrenergic
heteroreceptors located on 5-HT terminals (Haddjeri et al.,
1997). Finally, long-term treatment with 5-HT1A receptor
agonists, such as gepirone, desensitizes the 5-HT1A au-
toreceptor on 5-HT neurons, but not the postsynaptic
5-HT1A receptors located on CA3 pyramidal neurons
(Blier and de Montigny, 1987; Schechter et al., 1990;
Godbout et al., 1991; Fanelli and McMonagle-Strucko,
1992; Dong et al., 1998; Haddjeri et al., 1999).
˜
paroxetine (Pineyro et al., 1994), and a blockade of a2-
adrenoceptors by 5 mg/kg/day of mirtazapine after a
long-term administration (Haddjeri et al., 1997).
Recently, novel direct evidence of an enhanced 5-HT
neurotransmission by antidepressant treatments have been
provided (Haddjeri et al., 1998). This study showed that
long-term treatment with either the TCA imipramine, the
SSRI paroxetine, the selective and reversible MAO-A
befloxatone, the a2-adrenergic antagonist mirtazapine, the
5-HT1A receptor agonist gepirone or repeated ECS therapy
enhanced the tonic activation of postsynaptic 5-HT1A
receptors in the dorsal hippocampus, as put into evidence
by the enhanced disinhibition produced by the acute
administration of the selective 5-HT1A receptor antagonist
WAY 100635 (Haddjeri et al., 1998).
The present study was undertaken to determine whether
the association of mirtazapine with the SSRI paroxetine
could act in synergy to enhance 5-HT neurotransmission.
This was deemed of interest because these drugs have been
shown to modulate the 5-HT system via different mecha-
nisms. Thus, using in vivo electrophysiological paradigms
in the rat dorsal raphe and dorsal hippocampus, the effects
of short-term and long-term treatments with mirtazapine
and paroxetine, given alone and in combination, on the
firing activity 5-HT neurons and on the degree of activa-
tion of postsynaptic 5-HT1A receptors have been assessed.
2.2. Electrophysiological procedures
Rats were anesthetized with chloral hydrate (400 mg/kg,
i.p.) and were mounted in a stereotaxic apparatus. Addi-
tional doses (100 mg/kg, i.p.) were given to maintain the
anesthesia along the experiment.
2.2.1. Extracellular unitary recordings from dorsal
raphe 5-HT neurons
A hole was drilled 1 mm anterior to lambda on the
midline. 5-HT neurons were recorded with single-barreled
glass micropipettes filled with a 2 M NaCl solution. Their
impedance range was between 2 and 4 MV. The dorsal
raphe nucleus was encountered at a depth of 5–6 mm
below the surface of the brain. The 5-HT neurons were
identified according to the criteria of Aghajanian (1978),
by their slow (0.5–2.5 Hz) and regular firing rate and
long-duration (0.8–1.2 ms) positive action potentials. Once
5-HT neurons were identified, their spontaneous firing
activity was recorded for 50–100 s to determine their mean
baseline firing rate.
2.2.2. Extracellular recording and microiontophoresis
from dorsal hippocampus CA3 pyramidal neurons
A hole was drilled 4.2 mm lateral and 4.2 mm anterior
to lambda. Extracellular recordings of dorsal hippocampus
CA3 pyramidal neurons and microiontophoretic applica-
tions of 5-HT were performed with five-barreled glass
micropipettes pulled in the conventional manner in order to
achieve a tip diameter of 10–15 mm. The central recording
barrel was filled with a 2 M NaCl solution saturated with
fast green dye. One of the side-barrels was filled with
quisqualic acid (1.5 mM in 400 mM NaCl, pH 8) since a
leak or a small ejection current (12 to 24 nA) of
quisqualate was needed to activate the CA3 hippocampus
pyramidal neurons within their physiological range in
anesthetized animals. These neurons were identified ac-
cording to the criteria of Kandel and Spencer (1961): large
amplitude and long duration complex spike discharges.
Two side-barrels were filled with 5-HT (10 mM in 200
2. Materials and methods
2.1. Animals and treatments
Male Sprague–Dawley rats (Charles-River, Quebec,
Canada) weighing 300620 g on the day of the experiment
were used. The animals were maintained on a 12:12 h
light–dark cycle with free access to food and water. Rats
were treated for 2 or 21 days with either mirtazapine (5 or
10 mg/kg/day), paroxetine (10 mg/kg/day), or the
combination (mirtazapine–paroxetine), using osmotic
minipumps (Alza, Palo Alto, CA, USA) implanted subcuta-
neously on the dorsal region of the rat under halothane
anesthesia. Mirtazapine and paroxetine were dissolved in a

A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
179
mM NaCl, pH 4). Two ejection currents of 5-HT were
used (5 and 10 nA), each ejection period lasting for 50 s.
The fourth barrel was filled with 2 M NaCl and served as
an automatic current balance.
region, WAY 100635 would restore 5-HT neuronal firing
activity. However, because WAY 100635 was given sys-
temically, it would be simultaneously blocking the effects
of 5-HT on postsynaptic neurons, thereby cancelling out
the effect of WAY 100635 on the somatodendritic auto-
receptors. Indeed, if the action of WAY 100635 at the
somatodendritic 5-HT1A autoreceptors was influencing the
activity of the hippocampal neurons, it would serve to
further inhibit their firing due to an increased release of
5-HT into the target area. Thus, it can be assumed that any
increment in the firing activity of hippocampus pyramidal
neurons would reflect an increased level in the tonic
activation of the postsynaptic 5-HT1A receptors and the
degree to which WAY 100635 disinhibits this firing would
be a direct measure of the tonic level of activation of these
receptors by extracellular 5-HT.
In order to assess the blockade of the 5-HT transporter
produced by paroxetine, the RT50 values after microion-
tophoretic applications of 5-HT (5 and 10 nA) were
determined. The RT50 value corresponds to the time (in
seconds) required to achieve a 50% recovery of the firing
activity of the neuron once the microiontophoretic applica-
tion of 5-HT has been stopped (de Montigny et al., 1980).
This parameter is a reliable index of the in vivo activity of
monoamine reuptake process in the rat hippocampus and
previous studies have shown an increase of the RT50
values for 5-HT after acute intravenous paroxetine in-
jections or after the lesion of the 5-HT pathways by the
˜
neurotoxin 5,7-dihydroxytryptamine (Pineyro et al., 1994).
We mainly focused our interest on overall 5-HT trans-
mission by assessing the tonic activation of the postsynap-
tic 5-HT1A receptors of the dorsal hippocampus CA3
pyramidal neurons. To this end, we compared the firing
rate of dorsal hippocampus CA3 pyramidal neurons before
and after the systemic injection of the selective 5-HT1A
receptor antagonist, WAY 100635, via a lateral tail vein
cannula in control and treated groups. It is well established
that the suppression of the firing activity of CA3 pyramidal
neurons by microiontophoretic applied 5-HT is mediated
through the activation of 5-HT1A receptors (de Montigny
et al., 1984). If a tonic activation of postsynaptic 5-HT1A
receptors exists, the firing activity of hippocampus CA3
pyramidal neurons will be lowered even though the
responsiveness of postsynaptic 5-HT1A receptors to 5-HT
is unchanged. Then, the blockade of these 5-HT1A re-
ceptors by WAY 100635 will disinhibit the CA3 hippocam-
pus pyramidal neurons resulting in an increase of their
firing activity (Haddjeri et al., 1998). In this series of
experiments the current of quisqualate was chosen in order
to obtain a firing rate around 3 Hz in order to more readily
allow the detection of enhancements in firing following
administration of WAY 100635. The baseline firing was
recorded for at least 2 min before the intravenous injection
of WAY 100635. An intravenous injection of saline
preceded the first injection of WAY 100635 to eliminate
any effect due to the injection by itself. The saline
injection corresponds to the 0 mg/kg dose. WAY 100635
was administered in incremental doses of 25 mg/kg at time
intervals of 1–2 min. To avoid residual drug effects, only
one cell was studied in each rat. Since WAY 100635,
administered intravenously, did not modify the firing rate
of 5-HT neurons in the dorsal raphe nucleus of anes-
thetized rats (Forster et al., 1995; Lejeune and Millan,
1998; Haddjeri et al. unpublished observations), it can be
assumed that any changes in the firing activity of hip-
pocampus pyramidal neurons would reflect an increased
level in the tonic activation of the postsynaptic 5-HT1A
receptors. Therefore, in treated animals, where there would
be increased extracellular levels of 5-HT in the raphe
2.3. Drugs
Mirtazapine (ORG 3770) was provided by Organon
(Oss, The Netherlands), paroxetine by SmithKline Beech-
am (Harlow, UK) and WAY 100635 by Wyeth–Ayerst
(Princeton, NJ, USA). 5-HT creatinine sulfate and quis-
qualic acid were purchased from Sigma (St. Louis, MO,
USA).
2.4. Statistical analyses
Results were expressed as mean6S.E.M. ANOVA fol-
lowed by the Student Newman–Keuls test with a 0.05
statistic level of significance for the pairwise comparisons
were used for all the statistical analyses except for the
analysis of the results obtained with the short-term treat-
ments in the dorsal raphe. In this latter series of experi-
ments two doses of mirtazapine (5 and 10 mg/kg/day)
were used. However, we were a priori interested in
comparing the effects of the mirtazapine–paroxetine
combination with either drug given alone and not in
comparing the effect of one dose of mirtazapine to the
other. For this reason, it was more appropriate to use
Student’s t-tests followed by the Bonferroni correction for
this set of data.
One- and two-way repeated measure ANOVA were used
to determine the effects of the different treatments on the
firing rate of dorsal hippocampus CA3 pyramidal neurons
when repeated doses of WAY 100635 were given.
3. Results
3.1. Effects of mirtazapine and paroxetine on the
spontaneous firing activity of 5-HT neurons of the dorsal
raphe nucleus
Following a 2-day treatment with paroxetine (10 mg/
kg/day, s.c.), there was a 75% reduction of the firing
activity of dorsal raphe 5-HT neurons (Fig. 1). In rats

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A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
Fig. 1. Integrated firing rate histograms of 5-HT neurons in the dorsal raphe nucleus showing their spontaneous firing activity in control rats and in rats
treated with MIR, PRX or both for 2 days (A). The number above each neuron indicates the depth from the ventral border of the sylvius aqueduct at which
the neuron was recorded. The time base applies to all traces. (B) Effects of short-term treatments with MIR, PRX and their combination on the spontaneous
firing activity of dorsal raphe 5-HT neurons (means6S.E.M.). Numbers in the bottom of the columns indicate the number of neurons tested. *P,0.05
between the treated group and the corresponding control group. *P,0.05 between the MIR-PRX treated group and the group treated with PRX alone. The
Student’s t test with the Bonferroni correction was used for the statistical analyses.
treated with mirtazapine (5 or 10 mg/kg/day, s.c.) for 2
days, the firing activity of 5-HT neurons appeared to be
increased in a dose-dependent manner (12% and 19% with
5 and 10 mg/kg/day of mirtazapine, respectively; Fig. 1),
although this did not reach statistical significance. When
the low dose of mirtazapine (5 mg/kg/day, s.c.) was
co-administered with paroxetine for 2 days, the firing rate
of 5-HT neurons was similar to that observed with of
paroxetine alone (Fig. 1). However, the co-administration
of the high dose of mirtazapine (10 mg/kg/day, s.c.)
significantly attenuated the reduction of the serotonergic
firing activity produced by paroxetine (Fig. 1).
21 days, there was a significant increase (60%) of the
firing rate of dorsal raphe 5-HT neurons. In the mir-
tazapine–paroxetine group, the firing activity of 5-HT
neurons was similar to that of controls (Fig. 2).
3.2. Recovery time of dorsal hippocampus pyramidal
neurons after microiontophoretic applications of 5-HT
Both the 2- and 21-day treatments with paroxetine (10
mg/kg/day, s.c.) significantly prolonged the RT50 follow-
ing the microiontophoretic application of 5-HT with both
ejections currents used (5 and 10 nA), whereas mirtazapine
(5 mg/kg/day, s.c.) was without effect on this parameter.
The paroxetine-induced prolongation of the RT50 was not
modified by the co-administration of mirtazapine (Fig. 3).
In rats treated with paroxetine (10 mg/kg/day, s.c.) for
21 days, the firing activity of dorsal raphe 5-HT neurons
was not significantly different from the control group (Fig.
2). In rats treated with mirtazapine (5 mg/kg/day, s.c.) for

A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
181
Fig. 2. Integrated firing rate histograms of 5-HT neurons in the dorsal raphe nucleus showing their spontaneous firing activity in control rats and in rats
treated with MIR, PRX or both for 21 days (A). The number above each neuron indicates the depth from the ventral border of the sylvius aqueduct at
which the neuron was recorded. The time base applies to all traces. (B) Effects of long-term treatments with MIR, PRX and their combination on the
spontaneous firing activity of dorsal raphe 5-HT neurons (means6S.E.M.). Numbers in the bottom of the columns indicate the number of neurons tested.
*P,0.05 between the treated group and the corresponding control group. The one-way ANOVA followed by Student’s t test for the 232 comparisons was
used for the statistical analyses.
3.3. Tonic activation of the postsynaptic 5-HT1A
receptors on the dorsal CA3 hippocampus pyramidal
neurons by mirtazapine and paroxetine
100635 (38%, 68%, 62% from baseline with 50, 75, 100
mg/kg of WAY 100635, respectively; Fig. 6A). In fact,
these increases did reach statistical significance (P50.008,
using the Kruskal–Wallis one-way ANOVA on ranks).
Moreover, following the 2-day co-administration of mir-
tazapine and paroxetine for 2 days, WAY 100635 induced a
clear dose-dependent increase of the firing activity of
hippocampus CA3 pyramidal neurons (Fig. 6A).
As for the 21-day treatments with either mirtazapine or
paroxetine alone, WAY 100635 induced a dose-dependent
increase of the mean firing activity of CA3 pyramidal
neurons (Fig. 5). The degrees of increased in firing activity
of these neurons induced by WAY 100635 in the mir-
tazapine- and in the paroxetine-treated groups were similar
(Fig. 6B). When mirtazapine and paroxetine were co-
administered, the increase of the firing activity of CA3
pyramidal neurons in response to WAY 100635 was much
As illustrated in Fig. 4, the injection of saline did not
alter the firing activity of the dorsal hippocampus CA3
pyramidal neurons in any of the groups. In control rats, the
intravenous injection of the 5-HT1A antagonist WAY
100635 (Haddjeri et al., 1998) did not modify the firing
activity of CA3 pyramidal neurons within a dose range of
25–100 mg/kg (Rueter et al., 1998; Figs. 4A and 6A).
Similarly, in rats treated with paroxetine for 2 days, the
intravenous injection of WAY 100635 failed to modify the
firing activity of CA3 pyramidal neurons (Figs. 4C and
6A). In rats treated with mirtazapine for 2 days, an increase
in the mean firing rate of CA3 pyramidal neurons was
observed in response to intravenous injection of WAY

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A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
Fig. 3. Recovery time of dorsal hippocampus CA3 pyramidal neurons, expressed as RT50 values (means6S.E.M.), from microiontophoretic applied 5-HT
in control rats and in rats treated with MIR, PRX or both for 2 days (A) or 21 days (B). The numbers in the bottom of the columns indicate the number of
neurons tested. *P,0.05 between the treated group and the control group, using the one-way ANOVA followed by Student’s t test for the 232
comparisons.
greater than that observed with paroxetine or mirtazapine
alone (Figs. 5 and 6B).
Romero et al., 1996). The decreased firing activity of 5-HT
neurons induced by short-term treatments with various
SSRIs has been suggested to result from a greater activa-
tion of somatodentritic 5-HT1A autoreceptors due to the
increased levels of extracellular 5-HT in the dorsal raphe
produced by SSRIs (Bel and Artigas, 1992; Hjorth et al.,
1995; Romero et al., 1996). The recovery of firing activity
upon pursuing the SSRI treatment results from a desensiti-
zation of these somatodentritic 5-HT1A autoreceptors (see
Blier and de Montigny, 1994 for review).
Previous studies have established that the firing activity
of 5-HT neurons in the dorsal raphe nucleus is dependent
on the tonic activation of a1-adrenoreceptors located on
the soma and dendrites of these neurons (Svensson et al.,
1975; Baraban and Aghajanian, 1980a,b). The transient
increase of the firing activity of dorsal raphe 5-HT neurons
induced by the acute administration of mirtazapine (250
mg/kg, i.v.) has been shown to be due to the enhancement
of noradrenaline (NA) release, leading to a greater activa-
tion of their a1-adrenoreceptors (Haddjeri et al., 1996). In
the present study, the 2-day treatment with mirtazapine did
4. Discussion
Two main points of interest emerge from the present
electrophysiological studies concerning the concomitant
administration of the a2-adrenoreceptor antagonist mir-
tazapine and the SSRI paroxetine. First, the delay for
obtaining an increased tonic activation of postsynaptic
5-HT1A receptors is shortened by the combination of
mirtazapine and paroxetine. Second, the combination
induced a greater enhancement of the tonic activation of
postsynaptic 5-HT1A receptors than either drug given
alone.
The marked decrease of the firing activity of dorsal
raphe 5-HT neurons and followed by recovery, after the
short-term and the long-term administration of paroxetine
respectively (Figs. 1 and 2), is in agreement with previous
studies (see Blier and de Montigny, 1994 for review;

.
1
2
177
188
Fig. 4. Integrated firing rate histograms of dorsal hippocampus CA3 pyramidal neurons showing their responsiveness to microiontophoretic applications of 5-HT and to acute injections of cumulative
doses of WAY 100635 (25–100 mg/kg, i.v.) in control rats and in rats treated with MIR, PRX and their combination for 2 days. The neurons were activated by microiontophoretic applications of
quisqualate (11 to 23 nA). White horizontal bars indicate the duration of 5-HT applications. Arrows indicate the moment of the injection of WAY 100635 (25 mg/kg, i.v.).The time base applies to all
traces.

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A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
not increase significantly the firing activity of 5-HT
neurons. This contrasts with the 80% increase of the firing
activity of dorsal raphe 5-HT neurons produced by an
acute injection of mirtazapine (250 mg/kg, i.v.; Haddjeri et
al., 1996). This apparent discrepancy might be due to
lower cerebral concentrations of mirtazapine attained after
a 2-day subcutaneous administration than after an intraven-
ous bolus. However, after 21 days of administration,
mirtazapine (5 mg/kg/day, s.c.) significantly increased
(60%) the spontaneous firing activity of dorsal raphe 5-HT
neurons, possibly due to a desensitization of a2-adrenergic
heteroreceptors located on 5-HT terminals (Haddjeri et al.,
1997), as previously reported.
When the two drugs were co-administered for 2 days,
the low dose of mirtazapine (5 mg/kg/day, s.c.) failed to
reverse the paroxetine-induced reduction of the firing
activity of dorsal raphe 5-HT neurons. However, the high
dose of mirtazapine (10 mg/kg/day, s.c.) did attenuate the
suppressant effect of paroxetine (Fig. 1). When the two
antidepressant drugs were co-administered during 21 days,
the mirtazapine-induced increase of the firing rate of 5-HT
neurons was no longer present. Altogether these results
suggest that a physiological opposition between the 5-
HT1A autoreceptor and the postsynaptic a1-adrenoceptor
may exist, that is the opposing actions of paroxetine and
mirtazapine on 5-HT neuronal firing activity would tend to
cancel each other.
A pharmacokinetic interaction between mirtazapine and
paroxetine is unlikely to account for the above results since
mirtazapine does not alter the activity of the cytochrome
P450 isoenzymes (Delbressine and Vos, 1997), and the
inhibitory effect of paroxetine on the activity of CYP2D6
(Bauman, 1996; Lane, 1996) would rather lead to in-
creased plasma levels of mirtazapine. A competitive
interaction at the receptor level can also be ruled out since
both mirtazapine and paroxetine have negligible affinity
for 5-HT1A receptors (Tulloch and Johnson, 1992; De
Boer, 1996; Goodwin, 1996), and paroxetine has no
affinity for adrenoreceptors (Goodwin, 1996; Frazer,
1997).
An interaction at the level of the locus coeruleus can,
however, be involved in the effects of the combination of
mirtazapine and paroxetine (Fig. 2). NA neurons of the
locus coeruleus receive a dense 5-HT input (Steinbusch,
1984) and send projections to the dorsal raphe nucleus
(Loizou, 1969; Anderson et al., 1977). Previous studies
from our laboratory have provided evidence for a de-
creased firing activity of locus coeruleus NA neurons
following a long-term treatment with paroxetine (10 mg/
kg/day, s.c.; Szabo et al., 1999). Preliminary results
suggest that when mirtazapine (5 mg/kg/day, s.c.) and
paroxetine (10 mg/kg/day, s.c.) are co-administered, the
increased firing activity of locus coeruleus NA neurons
induced by mirtazapine alone is prevented by concomitant
paroxetine administration. Thus, the decremental effect of
paroxetine on the firing activity of locus coeruleus NA
neurons might dampen the incremental one of mirtazapine
which acts as antagonist at somatodentritic a2-adrenergic
autoreceptors on these neurons. This would prevent the
enhancement of NA release in the dorsal raphe nucleus and
the consequent activation of the a1-adrenoceptors located
on 5-HT which would prevent the mirtazapine–paroxetine
combination from enhancing 5-HT neuronal firing activity
(Fig. 2).
The results obtained in the dorsal hippocampus indicated
that the inhibition of 5-HT transporter by paroxetine was
not modified either by short- or long-term mirtazapine
administration. The assertion that the inhibitory effect of
5-HT on the firing activity of CA3 pyramidal neurons is
mediated by the 5-HT1A receptor subtype is supported by
the observation that the selective 5-HT1A receptor antago-
nist WAY 100635 produced a marked reduction of the
effect of microiontophoretically-applied (Figs. 4 and 5).
Should there be a greater tonic activation of these post-
synaptic 5-HT1A receptors, one would expect their bloc-
kade by WAY 100635 to result in a greater enhancement of
the baseline firing activity of CA3 pyramidal neurons.
WAY 100635 (25–100 mg/kg, i.v.) did not produce such
disinhibition in control rats (Figs. 4–6). As previously
reported (Haddjeri et al., 1998), this result suggests a lack
of a tonic activation of postsynaptic 5-HT1A receptors in
the anesthetized untreated rats. Interestingly, it was recent-
ly reported that WAY 100635 disinhibit CA1 pyramidal
neurons firing activity, but not in 5-HT depleted, freely
moving rats (Suzuki et al., 1999). In the present study, in
rats receiving paroxetine alone for 2 days, WAY 100635
did not induce any significant disinhibition of CA3 pyrami-
dal neurons. However, it did induce a significant disinhibi-
tion of the firing activity of these neurons in rats treated
with mirtazapine or paroxetine for 21 days. This is in
keeping with recent studies from our laboratory showing
that WAY 100635 produces a disinhibition of CA3 pyrami-
dal neurons following long-term administration of various
antidepressant treatments (Haddjeri et al., 1998; Rueter et
al., 1998). The enhanced tonic activation of postsynaptic
5-HT1A receptors observed after the long-term treatment
with paroxetine can be accounted for by higher levels of
extracellular 5-HT in the hippocampus, as a result of the
recovery of the firing rate of 5-HT neurons associated with
the blockade of 5-HT reuptake. This assumption is con-
sistent with previous studies showing increased extracellu-
lar concentrations of 5-HT in terminal brain areas (frontal
cortex, hypothalamus) after long-term SSRI treatments
(Bel and Artigas, 1993; Rutter et al., 1994). The enhanced
tonic activation of postsynaptic 5-HT1A receptors observed
after the long-term treatment with mirtazapine can also be
explained by increased levels of 5-HT in the hippocampus
but, in contrast to that produced by paroxetine, it would
result from the increase of 5-HT firing activity and the
desensitization of a2-adrenergic heteroreceptors located on
terminal 5-HT neurons (Haddjeri et al., 1997).
A striking finding was the marked enhanced tonic

.
1
2
177
188
Fig. 5. Integrated firing rate histograms of dorsal hippocampus CA3 pyramidal neurons showing their responsiveness to microiontophoretic applications of 5-HT and to acute injections of cumulative
doses of WAY 100635 (25–100 mg/kg, i.v.) in control rats and in rats treated with MIR, PRX and their combination during 21 days. The neurons were activated by microiontophoretic applications of
quisqualate (11 to 23 nA). White horizontal bars indicate the duration of 5-HT applications. Arrows indicate the moment of the injection of WAY 100635 (25 mg/kg, i.v.). The time base applies to all
traces.

186
A. Besson et al. / European Neuropsychopharmacology 10 (2000) 177–188
Fig. 6. Dose–response curves of the effects of intravenous injections of WAY-100635 in control rats and in rats treated with MIR, PRX and their
combination for 2 days (A) or 21 days (B). *P,0.05 between the treated groups and the control group. **P,0.05 between the group treated with the
MIR–PRX combination and the group treated with PRX or treated with MIR alone. Number of neurons tested: 9–12 per group.
activation of postsynaptic 5-HT1A receptors observed after
the short-term treatment with the mirtazapine–paroxetine
combination while the firing activity of dorsal raphe 5-HT
neurons was still significantly reduced (Fig. 1). In addition,
the 2-day mirtazapine treatment, which did not modify
5-HT neuronal firing, also increased the disinhibitory
action of WAY 100635, albeit to a lesser extent than in the
combined treatment group. Taken together, these results
suggest an enhanced 5-HT release in the dorsal hippocam-
pus which is independent of the 5-HT neuronal firing rate
in the dorsal raphe nucleus (Haddjeri et al., 1998; Rueter et
al., 1998). Another interesting finding was the greater tonic
activation of postsynaptic 5-HT1A receptors observed
following the long-term treatment with mirtazapine–parox-
etine compared to each drug given alone (Fig. 6). A higher
degree of 5-HT neuronal firing activity cannot account for
this observation as the mean firing rate of dorsal raphe
5-HT neurons in mirtazapine–paroxetine treated rats was
not increased (Figs. 1 and 2). Altogether, these results
suggest that the co-administration of mirtazapine with
paroxetine enhances the tonic activation of postsynaptic
5-HT1A receptors due to the differential effects of the two
drugs on 5-HT neurotransmission in the CA3 hippocam-
pus: the blockade of terminal 5-HT transporters and
desensitization of 5-HT1B autoreceptors by paroxetine
In conclusion, the present study provides evidence for
an earlier increase in 5-HT neurotransmission with mir-
tazapine than with a SSRI. At least one SSRI-controlled
trial supports this possibility of a more rapid onset of
action of mirtazapine (Thompson, 1999). The combination
of mirtazapine and paroxetine induced a more rapid and
greater enhancement of 5-HT neurotransmission than
either drug given alone. In the light of previous evidence
of the key role of postsynaptic 5-HT1A receptors site in the
therapeutic effect of antidepressant drugs, the co-adminis-
tration of mirtazapine and paroxetine to patients presenting
major depression might accelerate the antidepressant re-
sponse and/or might prove more effective than either drug
alone. A double-blind controlled trial is presently under-
way at our center to assess these possibilities.
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