Eyeblink conditioning in rats using pontine stimulation as a conditioned stimulus

Abstract

Previous studies using rabbits and ferrets found that electrical stimulation of the pontine nuclei or middle cerebellar peduncle could serve as a conditioned stimulus (CS) in eyeblink conditioning (Bao, Chen, & Thompson, 2000; Hesslow, Svensson, & Ivarsson, 1999; Steinmetz, 1990; Steinmetz, Lavond, & Thompson, 1985; 1989; Steinmetz et al., 1986; Tracy, Thompson, Krupa, & Thompson, 1998). The current study used electrical stimulation of the pontine nuclei as a CS to establish eyeblink conditioning in rats. The goals of this study were to develop a method for directly activating the CS pathway in rodents and to compare the neural circuitry underlying eyeblink conditioning in different mammalian species. Rats were given electrical stimulation through a bipolar electrode implanted in the pontine nuclei paired with a periorbital shock unconditioned stimulus (US). Paired training was followed by extinction training. A subset of rats was given a test session of paired training after receiving an infusion of muscimol into the anterior interpositus nucleus. Rats given paired presentations of the stimulation CS and US developed CRs rapidly and showed extinction. Muscimol infusion prior to the test session resulted in a reversible loss of the eyeblink CR. The results demonstrate that electrical stimulation of the pontine nuclei can be used as a CS in rodents and that the CS pathway is similar in rats, rabbits, and ferrets. In addition, the loss of CRs following muscimol inactivation shows that the conditioning produced with pontine stimulation depends on cerebellar mechanisms.

Fig. 1

Mean (± SE) eyeblink conditioned response (CR) percentage for rats given pontine stimulation as the conditioned stimulus during three acquisition training sessions (ACQ) and two CS-alone extinction sessions (EXT).

Fig. 2

Mean (± SE) eyeblink conditioned response (CR) onset latency (upper) and peak latency (lower) for rats given pontine stimulation as the conditioned stimulus during three acquisition training sessions (ACQ) and two CS-alone extinction sessions (EXT). The dashed line indicates the onset time of the unconditioned stimulus.

Fig. 3

Mean (± SE) eyeblink conditioned response (CR) percentage for rats given pontine stimulation as the conditioned stimulus during three acquisition training sessions (ACQ), a session following muscimol infusion into the anterior interpositus nucleus (MUS), a recovery session with no drug infusion (REC), and two CS-alone extinction sessions (EXT).

Fig. 4

Coronal section of the rat brainstem depicting the electrode placements (black dots) in the pontine region. PN, pontine nuclei.