Bob,
There's always fluorescence--these authors used 100 um Vibratome 
sections and found that the primary antibody may make a difference too.

Journal of Neuroscience Methods 170 (2008) 165–178
Associative image analysis: A method for automated quantification
of 3D multi-parameter images of brain tissue
Christopher S. Bjornsson, Gang Lin, Yousef Al-Kofahi, Arunachalam 
Narayanaswamy, Karen L. Smith, William Shain, Badrinath Roysamb

Simple avidinylated HRP may be smaller than ABC complexes and penetrate 
better.  People have also used DMSO 1-10% in incubation solutions, and 
ethanol extraction steps on cut thick sections, reported to improve 
penetration (both in histonet archives).

Good histo,
Mike King
UF Pharmacology & Therapeutics

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Message: 3
Date: Tue, 27 May 2008 10:04:52 -0800
From: "Bob Nienhuis" 
Subject: [Histonet] Thick section penetration for IHC
To: Histonet 

It has been suggested that the ABC detection system may not be the 
optimal choice for IHC in thick brain sections (~100 microns). 
Apparently, the large avidin-biotin complexes may not penetrate tissue 
too well.

What would you suggest instead, LSAB, polymer or...?

Bob Nienhuis
Neurobiology Research
UCLA / VA Medical Center
Los Angeles