Choline acetyltransferase of the common type immunoreactivity in the rat brain following different heroin treatments: A pilot study

Previous studies suggest that behavioral consequences of heroin treatment depend on the drug history of the animals and that cholinergic neurotransmission is involved in both behavioral and motor sensitization induced by heroin and other drugs of abuse. Immunohistochemistry, using a recently developed antiserum, specific for choline acetyl-transferase of the common type (cChAT), was applied to four different groups of rats, differing in drug regimens. Two groups of rats were submitted to the same schedule of heroin sensitization and then challenged for vehicle or heroin before sacrifice, obtaining two distinct groups, namely heroin-vehicle (HV) and heroin-heroin (HH). The same challenge was applied to another group of rats, previously submitted to a treatment with vehicle, obtaining other two groups, vehicle-vehicle (VV) and vehicle-heroin (VH), respectively. The number of cChAT-positive neurons is significantly increased (p < 0.05) in the diagonal band nuclei (with a consequent increase of cChAT positive fibers in the dentate gyrus) and notably, even not significantly (p > 0.05), increased in the nucleus accumbens core of heroin-sensitized rats (HV, HH). Instead, acute heroin treatment significantly increase (p < 0.05) the number of cChAT-positive cells in the nucleus accumbens shell of both heroin-naïve (VH) and heroin-sensitized (HH) rats. In heroin-sensitized rats (HV, HH), moreover, staining intensity of cChAT-positive fibers is significantly increased in the dorsal striatum, and basolateral amygdala (p < 0.05). Unlikely, cChAT positive fibers in the central amygdala are significantly increased (p < 0.05) by acute heroin treatments (VH, HH). The increase of cholinergic fibers in the dentate gyrus of the heroin sensitized rats (HV, HH) seems accompanied by a evident reduction in calretinin immunoreactive neurons in the same area. Our results, in a small group of animals, support the view that cholinergic mechanisms are intimately associated with the development of addictive phenotype. Furthermore, they suggest that cholinergic system is differentially engaged, following different heroin treatments.

Research highlights▶ Immunohistochemical visualization of the different engagement of cholinergic system following different heroin treatments in the rats, using a new polyclonal antibody against choline acetyl-transferase of the common type (cChAT). ▶ Results: Increase of cChAT positive fibers in the basolateral nucleus of the amygdala (BLA), in nucleus accumbens core, in the striatum and in the dentate gyrus of hippocampus, after heroin sensitization; increase of cChAT positive neurons in the medial septal nucleus (MS) and in the diagonal band nuclei, after heroin sensitization. Selective increase of cChAT positive fibers in the nucleus accumbens shell and in the central nucleus of the amygdala after an acute injection of heroin to drug naïve rats. Reduction of calretinin immune-reactive cells in the dentate gyrus of heroin sensitized rats. ▶ Discussion highlights: link between cholinergic system and drug dependence; evidence that different areas of the brain are involved in the different stages of addiction.