Phenytoin normalizes exaggerated fear behavior in p-chlorophenylalanine (PCPA)-treated rats

Abstract

Temporal lobe epilepsy may be associated with emotional difficulties such as depression and anxiety. Because the amygdala is involved in both epilepsy and emotion, common neural mechanisms in this temporal lobe structure may underlie the emotional disturbances observed in people with epilepsy. The neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) is implicated in many psychopathologies, and 5-HT also modulates amygdala excitability. Therefore, the present study uses the fear-potentiated startle (FPS) paradigm to investigate the effect of neuronal excitability on fear behavior in rats treated with p-chlorophenylalanine (PCPA) to chronically inhibit 5-HT synthesis. PCPA treatment selectively enhanced FPS in individually housed rats. The exaggerated FPS response was reduced to control level by the anticonvulsant phenytoin at 10mg/kg, and phenytoin at 30mg/kg further decreased FPS behavior. These data suggest that a subseizure state of neuronal excitability mediated by low 5-HT in brain fear circuits may be associated with pathological fear behavior.

Figure 1

DL-p-chlorophenylalanine (PCPA) increases fear-potentiated startle (FPS) selectively in rats housed individually. Rats were assigned to one of four treatment groups in a 2 x 2 design with housing and drug treatment as between-groups factors: group/saline (n = 10), group/PCPA (n = 10), individual/saline (n = 12), or individual/PCPA (n = 13). Animals were either group or individually housed for 13 days, and received either PCPA (300 mg/kg i.p.; black bars) or isovolumetric 0.9% saline (SAL; white bars) on days 1 and 10. FPS was tested on day 13 and is expressed as percent (mean ± SE) potentiated startle amplitude (Methods). In the group housed condition, there is no significant difference in FPS between saline (33 ± 7%) and PCPA (35 ± 7) treated animals (p > 0.05). However, in the individually housed condition PCPA treated rats show 60 ± 8 % FPS compared to 38 ± 7 % in saline treated controls (p < 0.05). Thus, PCPA increased FPS selectively in the individually-housed rats. * indicates significant difference from individually-housed, SAL-treated control (P < 0.05, Bonferroni multiple comparisons post-hoc test).

Figure 2

Phenytoin (PHN) reduces FPS selectively in PCPA-treated rats. Individually housed rats received i.p. injections of either saline (SAL) or PCPA (300 mg/kg) on days 1 and 10. FPS was measured 30 min. after injection of PHN (0, 10, or 30 mg/kg i.p.) within subjects on days 13, 15, and 17. On days 14 and 16, animals were retrained with CS-US pairing to minimize extinction. In individually-housed, PCPA-treated rats FPS was 47 ± 7% in control (n = 22), and reduced to 26 ± 10% (n = 8) and 6 ± 7% (n = 8) by 10 and 30 mg/kg PHN, respectively. In individually housed, SAL-treated rats FPS was 26 ± 6% in control (n = 20), and 28 ± 13% (n = 8) and 19 ± 9% (n = 8) after 10 and 30 mg/kg PHN, respectively. ANOVA shows that phenytoin significantly reduced FPS in PCPA-treated animals (p < 0.05), but had no effect in saline treated controls (p > 0.05). * indicates significant difference (p < 0.05, Bonferroni multiple comparison post-hoc test).

Figure 3

Phenytoin does not alter baseline startle responses in individually housed rats. Rats were individually housed for 13 days prior to testing, and received either PCPA (300 mg/kg i.p.) injections on days 1 and 10 or isovolumetric injections of 0.9 % saline. Baseline startle was measured 30 min after injection of PHN (0, 10, or 30 mg/kg i.p.) on days 13, 15, and 17 within subjects, and is expressed as percent (mean ± SE) of control (group housed, saline, 0 mg/kg PHN) startle amplitude during leaders and trailers (Methods). The baseline startle amplitude of saline treated animals was 970 ± 130, 900 ± 70, and 990 ± 80 at 0, 10, and 30 mg/kg PHN, respectively. In PCPA treated rats, baseline startle amplitude was 870 ± 110, 800 ± 70, and 890 ± 100 at 0, 10, and 30 mg/kg PHN, respectively. Two-way ANOVA shows no significant main effect of PHN (p > 0.05) or PCPA (p > 0.05) and no significant interaction (p > 0.05). However, the matching within subjects was significant (p < 0.05), indicating that phenytoin did not alter baseline startle reactivity in these animals.