The Journal of Neuroscience, November 1, 1997, 17(21):8645–8655 Medial Geniculate Lesions Block Amygdalar and Cingulothalamic Learning-Related Neuronal Activity Amy Poremba1 and Michael Gabriel2 1Department of Psychology and Institute for Neuroscience, Univsity of Texas, Austin, Texas 78712, and 2Department of Psychology and Beckman Institute, University of Illinois, Urbana, Illinois 61801 This study assessed the role of the thalamic medial geniculate anterior-ventral and medial-dorsal thalamic nuclei, and the ba- (MG) nucleus in discriminative avoidance learning, wherein rab- solateral nucleus of the amygdala before training. Learning was bits acquire a locomotory response to a tone [conditioned severely impaired and TIA was abolished in all areas in rabbits stimulus (CS)1] to avoid a foot shock, and they learn to ignore with lesions. Thus learning and TIA require the integrity of the a different tone (CS2) not predictive of foot shock. Limbic MG nucleus. Only damage in the medial MG division was (anterior and medial dorsal) thalamic, cingulate cortical, or significantly correlated with the learning deficit. The lesions amygdalar lesions severely impair acquisition, and neurons in abolished the sensory response of amygdalar neurons, and these areas develop training-induced activity (TIA): more firing they attenuated (but did not eliminate) the sensory response of to the CS1 than to the CS2. MG neurons exhibit TIA during cingulothalamic neurons, suggesting the existence of extra learning and project to the amygdala. The MG neurons may geniculate sources of auditory transmission to the cingulotha- supply afferents essential for amygdalar and cingulothalamic lamic areas. TIA and for avoidance learning. To test this hypothesis, bilateral electrolytic or excitotoxic ibotenic acid MG nuclear lesions were Key words: limbic thalamus; cingulate cortex; amygdala; induced, and multiunit recording electrodes were chronically learning; instrumental conditioning; anterior ventral nucleus; implanted into the anterior and posterior cingulate cortex, the medial dorsal nucleus There is currently a great interest in the neural circuitry under- Amygdalar and MG neurons are involved in aversively moti- lying aversively motivated learning (for review, see Davis, 1992; vated instrumental conditioning processes, as well as in classical Gabriel, 1993; Lennartz and Weinberger, 1994; LeDoux, 1995; aversive conditioning. TIA develops rapidly in these areas during McGaugh et al., 1995; Maren and Fanselow, 1996). A central role discriminative avoidance learning in rabbits (Gabriel et al., 1975, of amygdalar neurons is indicated by findings that amygdala 1976, 1991b; Maren et al., 1991) and amygdalar lesions severely lesions impair the acquisition of conditioned immobility (LeDoux impair behavioral acquisition (Poremba and Gabriel, 1997). et al., 1988; Fanselow and Kim, 1994; LeDoux, 1995), autonomic Results of neuronal recording and lesion studies demon- responding (Blanchard and Blanchard, 1972; Spevack et al., 1975; strate a critical involvement of cingulate cortex and related Kapp et al., 1979; Gentile et al., 1986; Iwata et al., 1986; Helm- limbic areas [the anterior and medial dorsal (MD) nuclei] of stetter, 1992) and fear-potentiated startle behavior (Davis, 1986, the thalamus in discriminative avoidance learning (for review, 1992; Hitchcock and Davis, 1987; Sananes and Davis, 1992). Also, see Gabriel., 1993; see also Kubota et al., 1996). Intriguingly, amygdalar neurons exhibit associative, training-induced activity cingulothalamic TIA and behavioral learning are blocked in (TIA) during Pavlovian conditioning (Umemoto and Olds, rabbits with bilateral amygdalar lesions (Poremba and Gabriel, 1975; Applegate et al., 1982; Pascoe and Kapp, 1985; Nishijo et 1997), suggesting that amygdalar efferents are essential for the al., 1988; Muramoto et al., 1993; McEchron et al., 1995; Quirk cingulothalamic TIA. et al., 1995). Amygdalar TIA develops rapidly, at the outset of training, An involvement of the medial geniculate (MG) nucleus in whereas TIA in particular cingulothalamic areas develops grad- aversively motivated learning is indicated by the observation of ually, suggesting that the amygdalar efferents are needed to ini- TIA in the medial division of the MG nucleus (MGm) (Olds et tiate more gradual learning-relevant cingulothalamic coding. It al., 1972; Gabriel et al., 1975; Gabriel et al., 1976; Ryugo and has been proposed (Poremba and Gabriel, 1997) that the rapid Weinberger, 1978; Birt and Olds, 1981; Weinberger, 1982; Ede- amygdalar TIA may represent the acquisition of conditioned fear, line, 1990; Edeline and Weinberger, 1992; McEchron et al., 1995), whereas the more gradual cingulothalamic TIA development may and by impaired conditioning in animals with MG lesions (Iwata reflect changes underlying acquisition of the instrumental et al., 1986; Jarrell et al., 1986; LeDoux et al., 1986a,b; McCabe behavior. et al., 1993). Direct projections of MG neurons to the amygdala and the occurrence of MG nuclear TIA raise the possibility that MG Received June 10, 1997; revised Aug. 20, 1997; accepted Aug. 22, 1997. nuclear sensory and/or associative coding is essential for amyg- This work was supported by National Institutes of Health Grant NS26736 and by dalar and cingulothalamic TIA. If true, lesions of the MG nucleus National Science Foundation Grant BIR9504842 to M.G. will block amygdalar and cingulothalamic TIA, as well as discrim- Correspondence should be addressed to Dr. Michael Gabriel, University of Illinois, Beckman Institute, 405 N Mathews, Urbana, IL 61801. inative avoidance learning. The present study tested this Copyright © 1997 Society for Neuroscience 0270-6474/97/178645-11$05.00/0 hypothesis. 8646 J. Neurosci., November 1, 1997, 17(21):8645–8655 Poremba and Gabriel • MG Lesions, Learning, and Multisite Unit Activity Figure 1. Recording sites for anterior cingulate cortex (Area 24b), the AV thalamic nucleus, the BL amygdalar nucleus, and the MD thalamic nucleus. The sites are indicated by the white asterisks, in three coronal sections at the indicated levels in millimeters anterior (negative value) and posterior ( positive values) to begma. Preliminary results have been reported in abstract form on bare pins were positioned over each burr hole and affixed to the skull (Poremba and Gabriel, 1993). using dental acrylic. The pins were removed after the dental acrylic was set. The recording electrodes were slowly advanced to the targets by press MATERIALS AND METHODS fitting them through the holes in the Teflon guides. Wires were presol- dered to the electrodes and to each of six contact pins in a nine-pin Subjects. The subjects were 29 male New Zealand White rabbits weighing Amphenol connector, which was also affixed to the skull with dental 1.5–2.0 kg on delivery to the laboratory and maintained on ad libitum acrylic and stainless steel machine screws. An additional stainless steel water and one cup of rabbit chow daily. It has been found that mild machine screw threaded into the frontal sinus and connected to one of restriction of food intake maintains good health and prevents obesity. Surgical implantation of recording electrodes. After a minimum of 1 the Amphenol contacts served as the recording reference electrode. week for adaptation to living cages, each rabbit underwent surgery for Neuronal activity was monitored acoustically and with an oscilloscope chronic intracranial implantation of micro electrodes for recording of during electrode advancement as an aid to electrode placement. The multiunit neuronal activity. Surgical anesthesia was induced by subcuta- electrodes were not attached to the manipulator, greatly reducing the risk neous injection (1 ml/kg of body weight) of a solution containing 60 that slight movements of the rabbit (e.g., attributable to respiration) mg/ml of ketamine HCl and 8 mg/ml of xylazine, followed by hourly would damage cells. The recording sites are shown in coronal sections of injections of 1 ml of the solution. the rabbit brain in Figure 1. The stereotaxic coordinates (Girgis and Each rabbit was placed in a Kopf stereotaxic rabbit head clamp. Six Shih-Chang, 1981) were as follows: anterior-ventral (AV) nucleus: an- intracranial recording electrodes were lowered through burr holes (di- teroposterior (AP), 2.0 mm; lateral, (L), 62.3 mm, and ventral (V), 7.5 ameter, 0.5 mm) drilled in the skull over the target sites. The electrodes mm; medial-dorsal (MD) nucleus, AP, 4.6 mm; L, 61.5 mm; and V, 8.0 were made with stainless steel insect pins (00; bare shaft diameter, mm; anterior cingulate cortex (Brodmann’s area 24b): AP, 24.0 mm; L, 0.28–0.30 mm) insulated with Epoxylite. The recording surfaces were 60.8 mm; and V, 3.0 mm; and basolateral (BL) amygdalar nucleus: AP, made by removing insulation from the tip of the pin. The recording 1.5 mm; L, 65.0 mm; and V, 15.2 mm. surface lengths ranged from 10 to 50 mm, from tip to insulation, and Lesions. Bilateral electrolytic lesions of the MG nucleus were induced electrical impedances ranged from 500,000 V–2 MV. Miniature cylindri- during surgery, using electrodes made from stainless steel insect pins cal Teflon electrode guides (length, 2.5 mm; diameter, 1.5 mm) impaled coated with Epoxylite insulation. The insulation was removed from the Poremba and Gabriel • MG Lesions, Learning, and Multisite Unit Activity J. Neurosci., November 1, 1997, 17(21):8645–8655 8647 tips to uncover 0.80–0.90 mm of the metal. The lesion electrodes were chamber produced a masking noise (70 dB re: 20 N/m2) through- stereotaxically positioned in the target sites, and a 1.5 mA cathodal DC out training. The CS1 and CS2 were pure tones (0.5 sec duration, 1 or current was passed at each site for 30 sec. The target sites (six per 8 kHz) played through a loudspeaker attached to the chamber ceiling hemisphere) were AP, 6.5 mm; L, 65.6 mm; V, 12.5, 13.5, and 14.5 mm; directly above the wheel. The tone stimuli (85 dB re: 20 N/m 2)hadarise and AP, 7.5 mm; L, 65.5 mm; and V, 12.5, 13.5, and 14.5.