**1. Introduction**

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Estradiol enhances some aspects of learning and memory in both humans and animal models. These enhancements are present throughout the adult lifespan (Luine, 2008) and extend into old age (Frick, 2008). While many neurochemicals and neurotrophins have been shown to be regulated by estrogen, the mechanism(s) responsible for estrogen's positive effects on cognition remain elusive. It has, however, been demonstrated that gonadal hormones, both estrogens and progestins, influence neural morphology in areas important for cognitive function such as the hippocampus and medial prefrontal cortex (PFC). Spines, which are located on the dendrites of pyramidal neurons in both of these areas, have been shown to contribute to cognitive function (Morgado-Bernal, 2011). Therefore, we will review estradiol's effects on dendritic spine density in the hippocampus and PFC in relation to cognitive function. Moreover, we also consider whether changes in spine density are important for estrogen's role in the maintenance of memory. The studies are primarily from our own laboratories, but, when available, data from other labs are compared. In our studies, spine density has been investigated by Golgi impregnation, and memory has been evaluated using the spatial memory tasks of radial arm maze and object placement, and non-spatial memory has been assessed by object recognition. In most of the studies to be discussed, both morphology and cognitive function were assessed in the subjects. This current research provides substantial data suggesting a relationship between hormones, spines and cognitive function, but we point out the need for further research to establish causal relationships between these variables and to identify how spines promote memory consolidation and are integrated into memory networks.

#### **2. Dendritic spines and memory**

Neuron to neuron communication occurs mainly when axons synapse on dendrites. Dendritic spines are small protrusions of the dendrite which receive the majority of synaptic input. Although dendritic spines are present on many neurons, they are extremely numerous on pyramidal cells of both the hippocampus and the PFC (See Figure 1, schematic

An Integrative Review of Estradiol Effects on Dendritic Spines and Memory over the Lifespan 185

Left: CA1 region of the adult rat hippocampus. CC = corpus callosum. 10x magnification. Right: Photomicrograph of golgi-impregnated dendrites taken under oil (100x). Arrows denote spines.

There is increasing evidence that the processes underlying learning and memory involve neural plasticity, which includes neurogenesis and dendritic remodeling. Ultimately memory seems to require dendritic remodeling which leads to an increase in LTP and synaptic strength (See Figure 3, schematic of a spine). This idea that memory requires alterations in dendritic spines is supported by the demonstration that the acquisition of new memories is associated with changes in dendritic spine density in the CA1 hippocampal region in adult male rats (Leuner et al, 2003; Jedlicka et al, 2008; Beltran-Campos, 2011). In addition, there is increasing evidence that existing spines undergo structural alterations that result in LTP (Jedlicka et al, 2008; Morgado-Bernal, 2011). Spine assembly involves a complex sequence of events and many proteins which have been demonstrated to be altered following memory tasks (Hotulainen and Hoogenraad, 2010; Morgado-Bernal, 2011). For example the polymerization of actin, which is highly concentrated in dendritic spines,

appears to be required for the induction of LTP (reviewed by Fortin et. al, 2011).

The hippocampal region not only contains a high density of spines, but these spines are plastic, i.e. their numbers fluctuate depending upon the state of the host. A dramatic 30%

Frankfurt, Unpublished.

**4. Estrogens and spines** 

Fig. 2. Examples of Golgi impregnation.

**3. Use of golgi impregnation techniques to study spines** 

of pyramidal neuron). Most excitatory synapses occur on dendritic spines where there is a concentration of neurotransmitter receptors.

Shown are the parts of the apical and basal trees that are used for Golgi analysis in our studies. Blue arrows denote dendritic spines. Drawn by Landry McMeans.

Fig. 1. Schematic of a pyramidal cell.

Several subtypes of dendritic spines are recognized. The classification varies depending on the author but generally dendritic spines consist of a protrusion and either have a bulbous termination (mushroom spines) or not. One may also distinguish between thin spines with a smaller head and stubby spines that lack any terminal enlargement (reviewed by Bourne and Harris 2008). Golgi impregnation is used to study spine density because it labels the cell body and the adjacent dendritic structures completely (See Figure 2, photomicrograph of a pyramidal neuron). Using this technique it has been clearly demonstrated that dendritic spine density and the type of dendritic spines observed actually change with many conditions such as hormone state (Gould et al., 1990; Kinsley, 2008; Li et al., 2004; Woolley et al., 1990), stress (Radley et al. 2008) and drug administration (Robinson et al., 2001; Robinson and Kolb, 2004; Frankfurt, et al., 2011).

Left: CA1 region of the adult rat hippocampus. CC = corpus callosum. 10x magnification. Right: Photomicrograph of golgi-impregnated dendrites taken under oil (100x). Arrows denote spines. Frankfurt, Unpublished.

Fig. 2. Examples of Golgi impregnation.

184 Sex Steroids

of pyramidal neuron). Most excitatory synapses occur on dendritic spines where there is a

Shown are the parts of the apical and basal trees that are used for Golgi analysis in our studies. Blue

Several subtypes of dendritic spines are recognized. The classification varies depending on the author but generally dendritic spines consist of a protrusion and either have a bulbous termination (mushroom spines) or not. One may also distinguish between thin spines with a smaller head and stubby spines that lack any terminal enlargement (reviewed by Bourne and Harris 2008). Golgi impregnation is used to study spine density because it labels the cell body and the adjacent dendritic structures completely (See Figure 2, photomicrograph of a pyramidal neuron). Using this technique it has been clearly demonstrated that dendritic spine density and the type of dendritic spines observed actually change with many conditions such as hormone state (Gould et al., 1990; Kinsley, 2008; Li et al., 2004; Woolley et al., 1990), stress (Radley et al. 2008) and drug administration (Robinson et al., 2001; Robinson

arrows denote dendritic spines. Drawn by Landry McMeans.

Fig. 1. Schematic of a pyramidal cell.

and Kolb, 2004; Frankfurt, et al., 2011).

concentration of neurotransmitter receptors.
