- Abstract: Dopamine D2-like agonists
induce penile erection (PE) and yawning in a
variety of species, effects that have recently
been suggested to be specifically mediated by
the D4 and D3 receptors, respectively. The
current studies were aimed at characterizing a
series of D2, D3, and D4 agonists with respect
to their capacity to induce PE and yawning in
the rat, as well as the pro-erectile effects of
apomorphine in wild-type and D4 receptor (R)
knock-out (KO) mice.
-
- All D3 agonists induced dose-dependent
increases in PE and yawning over a similar range
of doses, whereas significant increases in PE or
yawning were not observed with any of the D4
agonists. Likewise, D2, D3, and D4 antagonists
were assessed for their capacity to alter
apomorphine- and pramipexole-induced PE and
yawning. The D3 antagonist, PG01037, inhibited
the induction of PE and yawning, whereas the D2
antagonist, L-741,626, reversed the inhibition
of PE and yawning observed at higher doses. The
D4 antagonist, L-745,870, did not alter
apomorphine- or pramipexole-induced PE or
yawning. A role for the D3 receptor was further
supported as apomorphine was equipotent at
inducing PE in wild-type and D4R KO mice,
effects that were inhibited by the D3
antagonist, PG01037, in both wild-type and D4R
KO mice.
-
- Together, these studies provide strong
support that D2-like agonist-induced PE and
yawning are differentially mediated by the D3
(induction) and D2 receptors (inhibition). These
studies fail to support a role for the D4
receptor in the regulation of PE or yawning by
D2-like agonists.
-
-
- Introduction: The involvement of
dopamine in the regulation of penile erection
(PE) has been a longstudied phenomenon (Hyyppa
et al., 1970). Systemic administration of the
non-selective dopamine agonist, apomorphine, is
known to induce PE and yawning in a variety of
species including rats (Benassi-Benelli et al.,
1979), mice (Rampin et al., 2003), monkeys
(Gisolfi et al., 1980), and man (Lal et al.,
1987), suggesting that the receptor regulation
of these effects may be similar across species.
Several D3-preferring agonists, including
7-OH-DPAT, pramipexole, and quinpirole have been
shown to induce PE over low doses with
inhibition of PE occurring at higher doses
(Melis et al., 1987; Ferrari et al., 1993;
Ferrari and Giuliani, 1995), as has previously
been demonstrated for yawning (e.g., Collins et
al., 2005; Collins et al., 2007).
-
- D2-like agonist-induced PE and yawning are
thought to be centrally mediated as they are
inhibited by relatively nonselective, centrally
active, D2-like antagonists such as haloperidol,
sulpiride, and clozapine, but not the peripheral
D2-like antagonist domperidone (Benassi-Benelli
et al., 1979; Gower et al., 1984; Doherty and
Wisler, 1994; Hsieh et al., 2004). Moreover, a
significant body of literature supports a common
role for the paraventricular nucleus (PVN) in
the induction of PE and yawning by both
physiologic and pharmacologic means (e.g.,
Argiolas and Melis, 1998; Melis and Argiolas,
1999; Melis and Argiolas, 2003; Argiolas and
Melis, 2005), however, the specific receptor(s)
mediating the pro-erectile effects of D2-like
agonists are yet to be elucidated. Recently, a
specific role for the D4 receptor in the
induction of PE by D2-like agonists has been
suggested. Dose-dependent increases in the
percent incidence of PE were reported following
systemic administration of D4-selective agonists
(Hsieh et al., 2004).
-
- Similar dose-dependent inductions of PE
following systemic (Brioni et al., 2004;
Enguehard-Gueiffier et al., 2006; Melis et al.,
2006) or intra-PVN (Melis et al., 2005; Melis et
al., 2006) administration of a variety of
D4-selective agonists (e.g., ABT-724, CP226269,
PD-168,077 and PIP3EA), with the D4-selective
antagonist, L745,870, reported to block
PD-168,077- and PIP3EA-induced PE (Melis et al.,
2005; Enguehard- Gueiffier et al., 2006; Melis
et al., 2006). Although these findings support a
role for the D4 receptor in the mediation of PE,
it should be noted that D4-selective agonists
have generally been reported to induce fewer
erections compared to less selective D2-like
agonists such as apomorphine, and L-745,870 has
been shown to be ineffective at altering the
induction of PE by apomorphine (Melis et al.,
2006), suggesting that other receptor(s) are
also involved in the mediation of D2-like
agonist-induced PE.
-
- Interestingly, a variety of D3-preferring
agonists (e.g., (+)-3-PPP, 7-OH-DPAT,
pramipexole, quinelorane, and quinpirole) have
also been reported to increase PE (Melis et al.,
1987; Ferrari et al., 1993; Doherty and Wisler,
1994; Ferrari and Giuliani, 1995) suggesting
that D3 receptors may be involved in the
induction of PE by D2-like agonists. The current
studies were aimed at characterizing the roles
of the D2, D3, and D4 receptors in the
regulation of D2-like agonist-induced PE. Thus,
in vitro binding affinities for a series of
D2-like agonists and antagonists with varying
degrees of selectivity for the D2, D3, and D4
receptors were first determined to compare
receptor selectivity. Agonists were then
assessed for their capacity to induce PE and
yawning, and antagonists were assessed for their
capacity to alter the induction of PE and
yawning by apomorphine or pramipexole in rats.
Similarly, the pro-erectile effects of
apomorphine were evaluated in D4R WT and KO mice
alone, and in combination with the D3
antagonist, PG01037. Convergent evidence from
the characterization of the pro-erectile effects
of D2-like agonists, as well as the
agonist-antagonist interactions in rats and D4R
WT and KO mice, supports the notion that the
induction of PE and yawning by D2-like agonists
used herein are similarly mediated by the D3
receptor, whereas the inhibition of PE and
yawning observed at higher doses results from a
concomitant activation of the D2 receptor.
-
- Discussion: These studies were aimed
at characterizing the receptors involved in the
regulation of the pro-erectile effects of
D2-like agonists in rats and mice. Convergent
evidence from the pharmacologic evaluation of
the effects of a series of D2-like agonists with
varying degrees of selectivity for the D2, D3,
and D4 receptors alone, and in combination with
D2-, D3-, and D4-selective antagonists suggest
that the induction of PE is mediated by an
activation of the D3 receptor, whereas the
inhibition of PE observed at higher doses
results from the concomitant activation of the
D2 receptor, as has previously been described
for D2-like agonist induced yawning (Collins et
al., 2005; Collins et al., 2007).
-
- These studies failed to support a role for
the D4 receptor in the mediation of D2-like
agonist-induced PE as D4-selective agonists
failed to induce PE, and D4-selective
antagonists failed to inhibit PE in rats,
whereas apomorphine was equally effective at
inducing PE in WT and D4R KO mice. In agreement
with previous reports, apomorphine, pramipexole,
and quinpirole induced PE and yawning with
inverted U-shaped dose-response curves, and 75
to 87.5% of rats displaying at least one PE at
the peak dose, however, these studies are the
first to report a similar pro-erectile effect
for the D3-preferring agonist, PD-128,907. The
results of these studies suggest that the
capacity of these agonists to induce PE is
related to their activity at the D3, but not D4
receptor as increases in yawning and PE were
observed over a similar range of low doses even
though large differences exist between their in
vitro selectivities for the D3 compared to D4
receptor (e.g., apomorphine D4/D3 Å 0.05 and
PD-128,907 D4/D3 Å 1280; Table 1). In agreement
with this notion, but contrary to previous
findings (Brioni et al., 2004; Melis et al.,
2005; Enguehard-Gueiffier et al., 2006), all of
the highly selective D4 agonists failed to
induce PE. It should be noted however, that the
maximal PE responses for apomorphine,
quinpirole, and pramipexole were lower than some
previous reports (e.g., Melis et al., 2006),
suggesting procedural differences may have
affected the PE response.
-
- Nevertheless, the percent incidence of PE
for apomorphine and the D3-preferring agonists
were similar to previous reports (e.g., Hsieh et
al., 2004), suggesting that any procedural
differences only affected the maximal number of
PEs observed, not the absolute capacity of the
agonists to induce PE. The effects of D2-, D3-,
and D4-selective antagonists on apomorphine- and
pramipexole-induced PE and yawning further
support specific roles for the D3 and D2
receptors in the mediation of D2-like
agonist-induced PE. When given at behaviorally
active doses (Collins et al., 2005;
Enguehard-Gueiffier et al., 2006; Collins et
al., 2007), the D3-selective antagonist,
PG01037, and D2-selective antagonist, L-741,626,
differentially affected apomorphine- and
pramipexole-induced PE and yawning, whereas the
D4-selective antagonist, L-745,870, did not
alter the induction or inhibition of PE or
yawning. Similar to the effects of the D3 and D2
antagonists on yawning, PG01037 produced a
selective rightward and/or downward shift of the
ascending limb, whereas L- 741,626 produced a
selective rightward shift of the descending limb
of the PE doseresponse curves for apomorphine
and pramipexole with respect to both the
absolute number, and percent incidence of
PE.
-
- Together with previous reports describing
specific roles for the D3 and D2 receptors in
the regulation of D2-like agonist-induced
yawning (Collins et al., 2005; Collins et al.,
2007), the differential and selective effects of
the D3 and D2 antagonists on PE, combined with
the fact that both apomorphine and a variety of
D3-preferring agonists were equipotent at
inducing PE and yawning suggest that the
induction of PE and yawning by D2-like agonists
is mediated by the D3 receptor, whereas the
inhibition of PE and yawning observed at higher
doses results from a concomitant activation of
the D2 receptor. It should be noted, however,
that unlike pramipexole apomorphine also has
activity at D1-like receptors which may also
influence the PE response, although the precise
role of D1 receptors in the modulation of PE is
currently unclear (Melis et al., 1987;
Zarrindast and Jamshidzadeh, 1992; D'Aquila et
al., 2003; Hsieh et al., 2004), and may involve
peripheral rather than central D1 receptors
(El-Din et al., 2007). A role for the D3
receptor in the induction of PE and yawning is
further supported by the dose-response analysis
of a series of D2-like antagonists on
pramipexole-induced PE and yawning.
-
- Dose-dependent inhibition of
pramipexole-induced PE was observed following
pretreatment with D3-selecitve (PG01037 and
SB-277011A), non-selective D2/D3 (raclopride),
non-selective D2-like (haloperidol), and
D2-selective (L-741,626) antagonists, an effect
that was correlated with their capacity to
inhibit yawning, but not observed with the
D4-selective antagonists (L-745,870 and Ro
61-6270). Furthermore, all of the D2-like
antagonists inhibited PE and yawning with
similar potencies regardless of the fact that
large differences exist with respect to their in
vitro selectivity for D3 compared to D4
receptors (e.g., PG01037 D4/D3 Å 1.3 x 1004,
raclopride D4/D3 Å 64, and haloperidol D4/D3 Å
0.1; Table 1), whereas antagonists highly
selective for D4 compared to D3 receptors (e.g.,
L-745,870 D4/D3 Å 1.7 x 10-04 and Ro 61-6270
D4/D3 Å 9.1 x 10-05; Table 1) failed to alter
pramipexole-induced PE or yawning.
-
- Although Ro 61-6270 has not been extensively
characterized (Clifford and Waddington, 2000),
L-745,870 has been shown to possess favorable
pharmacokinetics (0.3 mg/kg; p.o. is thought to
be sufficient to occupy ~90% of D4 receptors;
Patel et al., 1997), and has been shown to
inhibit PD-168,077- and PIP3EA-induced PE at a
dose of 1.0 mg/kg (Enguehard- Gueiffier et al.,
2006; Melis et al., 2006), suggesting that the
doses used in the current studies were
sufficient to block D4 receptors. Together with
previous reports that L- 745,870 was unable to
alter apomorphine-induced PE (Melis et al.,
2006), the current studies suggest that the
pro-erectile effects of D2-like agonists (e.g.,
apomorphine and pramipexole) are mediated by
activation of the D3, but not D4 receptor.
Despite the distinct and differential effects of
PG01037 and L-741,626 observed in the current
studies, the fact that relatively large doses of
the D3-selective antagonists (PG01037 and
SB-277011A) were required to inhibit
pramipexole-induced yawning and PE, whereas
similar effects were observed with relatively
low doses of non-selective (raclopride and
haloperidol) and selective (L-741,626) D2
antagonists, effects that may suggest that the
inhibition of PE is mediated by antagonist
activity at receptor(s) other than the D3
receptor.
-
- These are not, however, the first studies to
suggest a disconnect between the in vitro and in
vivo potencies of the D3-antagonists, PG01037
and SB- 277011A. In fact, a number of previous
studies have reported similar in vivo potencies
when these antagonists have been evaluated in a
variety of operant procedures (3.2- 24.0 mg/kg;
Andreoli et al., 2003; Xi et al., 2004; Gilbert
et al., 2005; Cervo et al., 2007). Moreover,
previous studies aimed at characterizing the in
vivo selectivity of D2-like agonists and
antagonists, suggest that PG01037 and SB-277011A
are devoid of significant D2, cholinergic, and
serotonergic antagonist activities at doses up
to 56.0 mg/kg, whereas L-741,626 displays a much
more limited in vivo D2-selectivity with
significant D3 antagonist activity observed at
doses as low as 3.2 mg/kg (Collins et al., 2005;
Collins et al., 2007). Perhaps the strongest
evidence in support of a specific role for the
D3 receptor in the induction of PE by D2-like
agonists was provided by the evaluation of
apomorphineinduced PE in the WT and D4R KO
mice.
-
- Not only was apomorphine equally effective
at inducing PE in the WT and D4R KO mice, but
the pro-erectile effect of apomorphine was also
dose-dependently inhibited by the D3-selective
antagonist, PG01037, in both the WT and D4R KO
genotypes. Although species differences
precluded comparisons of the effects of agonists
and antagonists on yawning and PE to be made in
mice as D2-like agonists do not induce yawning
in mice (Li et al., submitted), when taken
together with the pharmacologic data collected
in rats, these data provide strong support for a
role for the D3, but not D4 receptor in the
induction of PE by D2-like agonists in rodents.
To summarize, a series of D2-like agonists and
antagonists with varying degrees of selectivity
for the D2, D3, or D4 receptors were assessed
for their capacity to modulate PE and yawning in
rats. Similar to the effects of apomorphine, all
D3-preferring agonists induced dose-dependent
increases in PE and yawning over a similar range
of low doses, with the inhibition of PE and
yawning occurring at higher doses; D4-selective
agonists failed to induce PE or yawning.
-
- The D3-selective antagonist, PG01037, and
D2- selective antagonist, L-741,626, had similar
effects on PE and yawning, with PG01037
selectively shifting the ascending limbs, and
L-741,626 selectively shifting the descending
limbs of the dose-response curves for
apomorphine- and pramipexoleinduced PE and
yawning. Additionally, dose-dependent inhibition
of pramipexoleinduced PE was observed with a
series of D2-like antagonists with a wide range
of selectivities for the D3 and D2 receptors, an
effect that corresponded to their capacity to
inhibit pramipexole-induced yawning, but was not
observed with D4-selective antagonists.
Furthermore, the pharmacologic evaluation of the
pro-erectile effects of D2- like agonists was
validated in D4R KO mice. Not only was
apomorphine was equally effective at inducing PE
in both WT, and D4R KO mice, but the induction
of PE by apomorphine was dose-dependently
inhibited by the D3-selective antagonist,
PG01037 in both genotypes. In conclusion,
although inferences with regard to the receptors
mediating the pro-erectile effects of
D4-selective agonists could not be made, these
studies provide convergent evidence in support
of a role for the D3 receptor in the induction
of PE by D2-like agonists, with the inhibition
of PE observed at higher doses resulting from
the concomitant activation of the D2
receptor.
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- Distribution
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the human forebrain:
- comparison
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neurons
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Based on studies in the
rat, Sokoloff et al. have made the valuable
suggestion that the D3 receptor is a
particularly important target for antipsychotics
in the mesolimbic DA system. These study in the
human demonstrates that the distribution of D3
receptors and D3 mRNA-bearing neurons is
consistent with relative segregation of the D3
subtype to the limbic striatum as well as it
primary and secondary targets and many sources
of its afferents.
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