Neonatal amygdala lesions: co-occurring impact on social/fear-related behavior and cocaine sensitization in adult rats

Abstract

Neurodevelopmental abnormalities of temporal-limbic structures may underlie both adult psychiatric syndromes and increased addiction vulnerability, leading to high frequencies of "dual diagnosis" disorders. Although the amygdala is implicated in various mental disorders and drug addiction, no studies have explored the impact of early developmental damage to the amygdala on phenotypes relating to mental illness and addictions as co-occurring processes. We tested rats with neonatal amygdala lesions (NAML) vs. SHAM-operated controls in a battery of tests--novel field activity, elevated plus maze (EPM), and social interaction (SI) at baseline and after odor and restraint stress--followed by measures of cocaine sensitization (15 mg/kg vs. saline x 5 days + challenge session 2 weeks later) and remeasurement of SI. NAMLs showed increased novelty-related locomotion, less fear responding in the EPM, and resistance to predator-odor- but not to restraint-induced suppression of SI. NAMLs also had elevated cocaine sensitization profiles, and cocaine history differentially affected subsequent SI in NAMLs compared with SHAMs. NAMLs may provide models for understanding a shared neurobiological basis for and complex interactions among psychiatric symptoms, drug exposure history, and addiction vulnerability.

Figure 1

Experimental time line with respect to rat age. NAML = neonatal amygdala lesion; SI = social interaction.

Figure 2

Histological verification mapping (upper panel, left) showed minimal (dark gray) and maximal (light gray) extents of lesions in brains of subjects included in the study: 25 rats with neonatal amygdala lesions (NAML) vs. 24 SHAM-operated controls. Coordinates shown are relative to bregma based on work by Swanson, 2004. Damage was usually centered at the boundary of the central (CEA) and basolateral (BLA) nuclei but often extended into the basomedial (BMA) and medial (MEA) nuclei. Exemplary whole coronal sections from a SHAM brain and a NAML brain with bilateral CEA/BLA damage (upper panel, right) is denoted by arrows. Exemplary sections from a SHAM and 3 different NAML brains featured at 25× magnification (lower panel) show variation in lesion damage with differential extents of cellular change and frank vacuolization in the amygdala. All sections were taken from approximately the same level relative to bregma (e.g. −2.5).

Figure 3

Social interaction times of experimental rats over 5-min encounters with novel healthy partners at baseline and immediately after exposure to predator odor and restraint stress. Repeated measures analysis of variance showed significant interactions between pre-test condition and lesion (p < .05) that was carried by significant differences between 25 rats with neonatal amygdala lesions (NAML) versus 24 sham-operated controls (SHAM) after the predator odor pre-condition (p < .01, post hoc t testing). Error bars indicate ±1 SEM.

Figure 4

Activity patterns of rats in the entire first injection session taken as locomotor distance per 10-min bin over 2 hr (upper panel). Arrow indicates time of cocaine/saline injections. Differences among sham-operated saline-assigned control (SHAM-SAL, N = 11), neonatal-amygdala-lesioned saline-assigned (NAML-SAL; N = 15), SHAM cocaine-assigned (SHAM-COC, N = 13), and NAML cocaine-assigned (NAML-COC, N = 10) groups evident on this first day exemplified patterns on subsequent days. Over 5 days of cocaine sensitization (lower panel), changes in activity post-injection (post-injection hour activity minus pre-injection hour activity) demonstrated elevated cocaine sensitization patterns in NAML rats (repeated measures analysis of variance: Drug × Lesion interaction, p < .05), resulting in greater overall activation in NAML-COC compared with SHAM-COC (^*p < .05) and in NAML-COC compared with both saline groups (^***p < .001) by post hoc least significant difference testing. Saline groups were consistently below 0 because activity levels in the initial hour before injection were always greater than the hour post injection. Error bars indicate ±1 SEM.

Figure 5

Stereotypic behavior of rats in the hour post-injection over the 5 days of sensitization revealed cocaine-induced increases and sensitization of stereotypic behavior (repeated measures analysis of variance for main effect of drug, p < .001, and Drug × Day interaction, p < .001), but no differences occurred as a result of neonatal-amygdala-lesioned (NAML) rats. SHAM-SAL = sham-operated saline-assigned controls; NAML-SAL = neonatal-amygdala-lesioned saline-assigned rats; SHAM-COC = sham-operated cocaine-assigned rats; NAML-COC = neonatal-amygdala-lesioned cocaine-assigned rats. Error bars indicate ±1 SEM.

Figure 6

Two weeks after the last injection of the initial series (15 mg/kg cocaine vs. saline × 5 days), sham-operated saline-assigned control (SHAM-SAL, N = 11), neonatal-amygdala-lesioned saline-assigned (NAML-SAL; N = 15), SHAM cocaine-assigned (SHAM-COC, N = 13), and NAML cocaine-assigned (NAML-COC, N = 10) groups of rats received a single cocaine injection (15 mg/kg). Change in activity due to cocaine injection was increased in NAML rats (analysis of variance for main effect of lesion, p < .05, with a marginal effect of drug history, p = .06), and NAML rats with cocaine history activated more than all other rat groups (^**p < .01, least significant difference post hoc testing). Error bars indicate ±1 SEM.

Figure 7

Four days after the cocaine challenge, differential effects of repeated-cocaine- versus saline-dosing history emerged in social interaction (SI) among rats depending on lesion status (upper panel; analysis of variance [ANOVA] interaction between drug history and lesion, p < .05). Sham-operated (SHAM) rats with histories of multiple cocaine doses (SHAM-COC, N = 13) showed greater SI times than did SHAM rats with only saline histories prior to cocaine challenge (SHAM-SAL, N = 11) and NAML rats with histories of multiple cocaine doses (NAML-COC, N = 10; p < .05, least significant difference post hoc testing). In rats with multiple-cocaine-dose histories (lower panel), SI was differentially influenced by lesion status and pre-condition (ANOVA interaction, p < .05), with SI of NAML rats differing from that of SHAM rats immediately after predator odor exposure and 4 days after cocaine injections (^*p < .05, post hoc t testing). Error bars represent ±1 SEM.