Cs. Woolley et al., ESTRADIOL INCREASES THE FREQUENCY OF MULTIPLE SYNAPSE BOUTONS IN THE HIPPOCAMPAL CA1 REGION OF THE ADULT FEMALE RAT, Journal of comparative neurology, 373(1), 1996, pp. 108-117
The effect of estradiol to increase the density of dendritic spines an
d axospinous synapses on hippocampal CA1 pyramidal cells in the adult
female rat has been well-documented. However, presynaptic involvement
in this process of synapse elimination and formation in the adult is u
nknown. To address this issue, we have reconstructed 410 complete pres
ynaptic boutons through coded serial electron micrographs of CA1 strat
um radiatum to determine the: (1) frequency of multiple (MSB) vs. sing
le (SSB) synapse boutons; (2) number of synaptic contacts per MSB; (3)
bouton volume and surface area; and (4) types of spines in synaptic c
ontact with MSBs and SSBs in ovariectomized, estradiol-treated animals
(OVX + E) versus ovariectomized oil-treated controls (OVX + O). Quant
itative analysis of this tissue revealed that, in OVX + E animals, 45.
0% of presynaptic boutons form multiple synaptic contacts with dendrit
ic spines compared to 27.3% in controls (P <0.01); the average number
of synapses per MSB was 2.7 in OVX + E animals compared to 2.3 in cont
rols (P <0.05). This represents a 25.5% increase in the number of syna
pses formed by a given number of presynaptic boutons in estradiol-trea
ted animals (P <0.01) which largely accounts for the previously observ
ed estradiol-induced increase in axospinous synapse density. There was
no treatment effect on bouton size; however, because MSBs are larger
than SSBs, the increased frequency of MSBs in estradiol-treated tissue
results in a trend toward an estradiol-induced increase in average bo
uton size. Additionally, MSBs were found to be more irregular in shape
, i.e., significantly less spherical, than SSBs. Our results indicate
that estradiol-induced dendritic spines form synapses primarily with p
reexisting boutons in stratum radiatum and that these boutons enlarge
and change shape as they accommodate new synapses. Such findings sugge
st a relatively active role for dendrites in the process of adult syna
pse formation. (C) 1996 Wiley-Liss, Inc.