Estrogens
also appear to modulate the activity of the direct and indirect pathways via

their
effects on dopamine receptors. In cultured striatal neurons, treatment with
17?-estradiol

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

had
several effects: it blocked cellular response to D2 binding, but approximately
doubled the

effect of
D1 binding on enzymatic activity in neurons (Maus et al., 1989) and decreased
current

through
calcium channels in striatal MSN’s (Mermelstein, Becker, & Surmeier, 1996).

Additionally,
when estradiol benzoate was injected into both OVX and CAST rats, there was a

rapid
(30-minutes following injection) decrease in dopamine binding at D2 receptors
in striatum,

but only
in OVX rats (Bazzett & Becker, 1994). Additionally, both acute (Levesque
& DiPaolo,

1988) and
subchronic (Tonnaer et al., 1989) administration of estradiol in OVX rats
decreases

the ratio
of high to low affinity striatal D2 receptors. In contrast, OVX leads to
decreases in D1

receptor
density compared to intact females (Levesque, Gagnon, & DiPaolo, 1989).

Together,

these data
suggest that the presence of estradiol in striatal motor pathways (in female
rats only)

may
enhance the functioning of the direct pathway, either by enhancing D1 activity,
or

disinhibiting
the direct pathway by blocking D2 binding or limiting GABA-ergic inputs from
the

indirect
pathway.