Cholinergic modulation of pavlovian fear conditioning: Effects of intrahippocampal scopolamine infusion

Cholinergic neurotransmission has been implicated in the acquisition of a v ariety of tasks, including Pavlovian fear conditioning. To more precisely d efine the role of cholinergic modulation in this process, the effect of sit e-specific cholinergic antagonism was assessed. Male Long-Evans rats were i mplanted with chronic, bilateral cannulae aimed at the dorsal hippocampus. Infusions of scopolamine hydrobromide (50 mug bilaterally) or phosphate-buf fered saline (PBS) were made immediately prior to a signaled Pavlovian fear conditioning procedure. On consecutive days following training, all rats w ere given independent tests assessing freezing to both the training context and the tone conditional stimulus (CS). Relative to PBS infused controls, rats that received intrahippocampal infusions of scopolamine showed a signi ficant attenuation of contextual freezing but comparable levels of freezing to the tone CS. Neither shock sensitivity nor general activity levels diff ered between rats infused with scopolamine or PBS. These findings suggest t hat fear conditioning to context, but not discrete CS, requires intact chol inergic neurotransmission in the hippocampus. Hippocampus 2001;11:371-376. (C) 2001 Wiley-Liss, Inc.