Differential effects of atropine, procaine and dopamine in the rat ventral tegmentum on lateral hypothalamic...

Microinjections of the muscarinic antagonist, atropine, of dopamine, or of the local anesthetic, procaine, in the ventral tegmentum elevated frequency thresholds for lateral hypothalamic self-stimulation. The largest and most robust effects were observed following atropine (30 or 60 micrograms) microinjections. The most sensitive sites for the atropine effect were near dopamine cells. In order to determine if the effects of atropine can be reversed by pretreatment with a cholinergic agonist, carbachol (1-3 micrograms) was microinjected 15 min prior to atropine. Carbachol pretreatment attenuated the frequency threshold elevation of atropine by 47-95%. Since atropine is a local anesthetic, the effects of procaine on self-stimulation thresholds were tested as well. Procaine (100 or 250 micrograms) in ventral tegmentum elevated frequency thresholds by much less than atropine. Therefore, while atropine attenuates reward primarily through blockade of muscarinic receptors, the local anesthetic effect of atropine may enhance the threshold elevation. Dopamine (1-10 micrograms) also elevated frequency thresholds, but when dopamine injections were repeated daily, the threshold elevations were attenuated. This attenuation contrasted with the robust effects of atropine, and may reflect the development of autoreceptor subsensitivity. Hence, both dopaminergic and muscarinic receptors in ventral tegmentum are involved in lateral hypothalamic brain stimulation reward.