A method for detecting functional activity related expression in gross brain regions, specific brain nuclei and individual neuronal cell bodies and their projections.

We have developed a system to visualize functionally activated neurons and their projections in the brain. This system utilizes a transgenic mouse, fos-tau-lacZ (FTL), which expresses the marker gene, lacZ, in neurons and their processes after activation by many different stimuli. This system allows the imaging of activation from the level of the entire brain surface, through to individual neurons and their projections. The use of this system involves detection of neuronal activation by histochemical or immunohistochemical detection of beta-galactosidase (betagal), the product of the lacZ gene. Furthermore, the underlying brain state of the FTL mice determines the basal levels of expression of betagal. Here we describe in detail our protocols for detection of FTL expression in these mice and discuss the main variables which need to be considered in the use of these mice for the detection and mapping of functionally activated neurons, circuits and regions in the brain.

Introduction

A central aim in neurobiology is to determine the neural components and circuitry which are responsible for a given brain function. Traditionally, the connections a neuron makes have been identified using techniques such as electrophysiological recordings, histochemical techniques, anterograde or retrograde tracers, and functional imaging. While these approaches have particular and powerful benefits, they also all have drawbacks that limit their usefulness in the mapping of functionally activated circuits. For example, electrophysiology and functional imaging do not visualize connectivity, and tracers are not targeted to functionally activated neurons.

One approach to determine which parts of the brain are involved in a particular function is to look for markers of neuronal activation. One group of markers, the immediate early genes, code for inducible transcription factors and are rapidly transcribed following neuronal stimulation. Of these genes, c-fos is the most studied and is induced by a wide range of different stimuli, eg depolarization, calcium influx, many different neurotransmitters (1-4). However, since c-fos is expressed exclusively in the cell nucleus, its localization does not provide information of the connectivity or the morphology of activated neurons within the nervous system. We have generated transgenic mice in which an axon targeted β-galactosidase (βgal) reporter system (5) is under the regulation of the promoter of the c-fos gene (6). In these fos-tau-lacZ (FTL) transgenic mice, neurons that express c-fos will target βgal expression to axons and dendrites via the tau sequences in the fusion protein. This permits direct visualization of neuronal cell bodies and their projections using a simple enzymatic assay (6) or immunohistochemistry. Thus, the FTL transgenic mice have certain advantages in imaging functionally activated areas and circuits in the central nervous system, in that it allows the identification of cell bodies and their projections and therefore may aid in imaging functionally activated circuitry.

Here we describe a number of different methods we have used to visualize activated neurons and their projections, activated brain nuclei, and the brain surface using the FTL mice. Depending on the resolution and brain area to be assessed, different types of immunological or histochemical detection of βgal can be used. We present our current protocols for each of these methods.

Materials and Methods

Animals

Mice were males or female transgenic heterozygote FTL mice aged between 2 and 3 months. Animals were housed in a room with a 12-hour light-dark cycle. All experimental procedures adhered and were approved by the Institutional Animal Care and Use Committee of the University of Melbourne. Food and water were supplied ad libitum. For analysis of βgal expression under basal conditions, mice were taken from their home cages and immediately injected intraperitoneally with a lethal overdose (100 μl) of Lethabarb (Virbac, Peakhurst, Australia). After the animals were deeply anaesthetised, they were transcardially perfused with 12 ml of chilled 5% sucrose in H20 followed by the 25 ml of chilled 4% paraformaldehyde in 0.1M phosphate buffer (pH 7.4; PFA). After 15 min, brains were removed and post-fixed in fresh 4% PFA for 15 min at 4°C, transferred to 20% sucrose in phosphate buffered saline (PBS) and equilibrated for 48 hr at 4°C. Brains were then frozen in Tissue-Tek (Sakura, Tokyo, Japan) and cryostat sections cut (50 or 100 μM) and collected in wells of 24 well tissue culture plates (1-2 sections/well) in PBS.

βgal enzymatic assay

For detection of βgal enzymatic activity, PBS was aspirated and sections incubated in Stain buffer (5 mM MgCl2, 5mM potassium ferrocyanide, 5 mM potassium ferricyanide, and 0.4 mg/ml 5-Bromo-4-chloro-3-indolyl-B-D-galatocpyranoside (X-gal) (Astral, Gymea, Australia)) for 24 hr at room temperature (RT). After staining, sections were rinsed in PBS and stored in 4% PFA until mounting by placing into 0.5% gelatin in H20 before transferring to slides coated with 0.5% gelatin, 10% potassium chromium alum. Sections were allowed to dry, dehydrated through graded alcohols to histolene, rehydrated through graded alcohols, rinsed in water and counterstained in 0.05% Nuclear Fast Red in 5% aluminium sulphate for 15 min. Sections were rinsed in H2O, dehydrated again and mounted in Safety Mount (Fronine, Riverstone, Australia). Sections were analyzed microscopically and areas which were βgal positive were identified by comparison with an atlas of the mouse brain (7).

Immunohistochemistry

For βgal immunohistochemistry, cryostat sections, either free-floating or mounted, were incubated overnight at 4°C in rabbit anti-βgal antisera (MP Biomedicals, Solon, USA) diluted 1:20,000 in 10% CAS-Block (Zymed, San Francisco, USA), 0.1% Triton X-100 in PBS. After washing in PBS, sections were incubated for 1 hr in Alexa Fluor 488 goat anti-rabbit IgG (Molecular Probes, Eugene, USA) diluted 1:500 in PBS. Sections were washed in PBS; if free-floating, they were mounted onto gelatinized slides, and coverslipped in Fluorescent Mounting Medium (DakoCytomation, Glostrup, Denmark). If already mounted, the sections were directly coverslipped in the same medium.

Alternatively, after washing in PBS, sections were incubated in biotinylated goat anti-rabbit IgG diluted 1:400 in 10% CAS-Block, 0.1% Triton X-100 in PBS for 1 hr. Immunoreactivity was visualized with StreptABComplex/horseradish peroxidase (DakoCytomation, Glostrup, Denmark), and Metal-Enhanced DAB (Pierce, Rockford, USA). Mounted sections, (or if free-floating, the sections were first mounted onto gelatinized slides) were air dried, dehydrated and coverslipped in Safety Mount.

Photographic imaging

Fluorescence photographs were obtained using an ImagePoint cooled CCD camera (Photometrics LTD, Tucson, AZ) and V for Windows imaging software (Digital Optics Ltd., Auckland, NZ). Images were processed with Adobe PhotoShop and Corel Draw software (Corel Corp., Dublin, Ireland). For confocal microscopy, sections were imaged with a Zeiss Axioplan 2 microscope (Zeiss, Oberkochen, Germany) fitted with a Bio-Rad 1024 confocal illuminating system (Bio-Rad, Sydney, Australia) using a x100 objective and the appropriate fluorescence filter.

Results and Discussion

We have used a number of different detection methods for the visualisation of βgal in neurons of the mice. These methods are the histochemical detection of βgal activity, and immunocytochemical detection. For the immunochemistry, we have used both fluorescence and peroxidase detection methods. Each of these methods has particular advantages. As follows, we describe the images we obtain with these methods and their particular advantages with regard to different levels of resolution and sensitivity to detect c-fos related activation in the brain.

Histochemical detection of βgal

The most convenient method of detection of βis the histochemical detection of βgal enzymatic activity. In regions containing active βgal, the cleavage of the Xgal substrate results in the deposition of a fine blue crystalline precipitate. In sections of brain of the FTL mouse, this method of detection is suitable for analysis of whole brain sections at low and medium power (Fig. 1A and B). In many parts of the brain, βgal histochemistry can resolve the structures of individual neurons (Fig. 1B). However, in areas of high density of positive neurons, such as cortex, the pattern of staining using βgal histochemistry does not resolve individual neurons and axons. In these regions, there is a lot of staining within axons, and the blue precipitate formed in the histochemical reaction does not have enough precision to accurately resolve the very fine processes of many separate axons. In these regions, the staining will appear as blue cell bodies within a uniform blue pattern. A uniform blue pattern of staining is also seen in major tracts in the brain which are βgal positive, such as anterior commissure and corpus callosum.

Fig. 1

Examples of different modes of detection of activated FTL expression in brains of FTL mice. (A)

and (B) show detection of FTL expression in neurons using βgal histochemistry. (A) shows a low power view of a coronal section of the brain (approximate Bregma - 1 mm) from an FTL mouse which had received a mild aversive stimulus (all sections were from mice which had received a footshock of 0.5 mA for 2 seconds). (B) shows higher power view of region boxed in (A), and which shows the basolateral complex of the amygdala. (C) and (D) shows detection of FTL expression in neurons using βgal peroxidase immunohistochemistry. (C) shows similar view as (B), and contains basolateral complex of amygdala. (D) shows higher power view of region boxed in (C). (E) shows FTL expression in dentate gyrus neurons using βgal fluorescence immunohistochemistry. Scale bars: A, 2 mm; B and C, 400 μm; D, 50 μm; E. 100 μm.

The blue staining pattern is easy to detect macroscopically in the FTL mouse and it is simple to determine which brain structures have βgal activity, because the FTL product is found throughout the cell body and neuronal processes. This is one of the principle advantages of the FTL mouse compared with immunohistochemistry for FOS, where only cell nuclei are labelled. Because the βgal expression fills the entire cell in the FTL mouse, the pattern of βgal expression often encompasses the brain structure which is activated (Fig. 1), whereas in sections stained for FOS, the FOS positive dots do not form such a clear pattern correlating with a particular brain structure. In FTL mice, brain tracts also stain for βgal if they are projecting from activated regions of the brain (6), whereas FOS immunohistochemistry is not detected in tracts as it is only expressed in the nucleus of neuronal cell bodies Another advantage of βgal histochemistry is that there is no background staining. This means that positive staining is easy to detect and quantitate. Our preferred method of quantification employs the determination of a brightness area product over the brain region of interest (8).

βgal immunohistochemistry

An approach which offers good resolution of individual neurons and processes involves immunohistochemistry with antibodies to βgal and detection either with peroxidase-labeled or fluorophore-labeled secondary antibodies. Immunohistochemistry with peroxidase-labeled secondary antibodies results in similar resolution of staining compared to βgal histochemistry. There are some advantages and disadvantages of this technique compared with histochemistry. The primary advantage is that there is an increase in resolution of cell bodies and processes in regions of the brain containing a high density of FTL positive neurons, in particular in cortex (9) (Fig. 1C, D shows amygdala region). A disadvantage is that there is low level background staining, so that the areas of negative to low expression are not as clean and clear compared with βgal histochemistry.

The alternative immunohistochemical detection employs fluorophore-labeled secondary antibodies (Fig. 1E). We have used this detection in the past to visualize neurons in diverse regions of the brain (6, 8-9). Very high resolution images can be obtained using confocal microscopy of thin optical sections through the neuron. For construction of neurons in three dimensions, a series of images through the depth of section can be taken and the images pooled to reconstruct a composite image of the neuron (6).

This method is reliable and has the advantage of good signal and low background. It is also very useful for double labeling studies to further identify the labeled functional circuits (9). Some problems can arise if the titer of either primary or secondary antibody is not fully optimized, which will result in either a low signal or high background. In addition, this method is normally only suitable for analysis of small regions of the brain, because the fluorescent signal is difficult to detect at low power. In some situations, where the FTL expression is particularly strong, this technique can be used for the analysis of entire brain sections (8).

Whole brain histochemistry

The overall patterns of expression in the cortex of FTL animals can be simply examined using whole-brain histochemical staining. We have used whole brain staining to look at patterns of FTL expression in response to visual stimuli, and in particular to examine light responsive regions of the cortex (9). In animals which have been housed in the dark for several days, and thus have had no light input, the primary visual cortex can be clearly distinguished as a roughly circular patch of white on the brain surface (Fig. 2A). In animals housed under normal light conditions, this area is filled with blue staining (Fig. 2B).

Fig. 2

Detection of FTL expression on the surface of the brain using whole brain βgal histochemistry.

Shown are side views of posterior cortex of brain from an FTL mouse which had been (A) housed in the dark for 3 days and (B) housed in an environment with a 12 hr dark/12 hr light cycle. The primary visual cortex is circled. Scale bar = 250 μm.

Behavioral state of mice

In our studies, we have found βgal expression in the FTL mouse brain to be a sensitive indicator of brain state. For example, if FTL mice are taken immediately from their home cage and their brains analyzed for βgal expression, some variation in expression of βgal is found between individual mice, in particular in cortical regions and hypothalamus. We believe this is due to different degrees of brain activation, and have found that if mice are given periods of exposure to an enriched environment (10), both the level of βgal staining decreases and overall variation in staining between individual mice decreases (11). This is most likely due to decrease in stress levels of the mice following environmental enrichment. We note this because it is important to point out that the behavioural state of the mice will influence their βgal expression patterns, and this needs to be taken into consideration for any experiments where these mice are used to determine which parts of the brain are activated following any given stimulus. In particular, any stimuli which may stress the mice will also affect expression in stress-responsive systems of the brain.