5-HT2C and GABAB receptors influence handling-induced convulsion severity in chromosome 4 congenic and DBA/2J background strain mice

Abstract

Progress towards elucidating the underlying genetic variation for susceptibility to complex central nervous system (CNS) hyperexcitability states has just begun. Genetic mapping analyses suggest that a gene(s) on mid-chromosome 4 has pleiotropic effects on multiple CNS hyperexcitability states in mice, including alcohol and barbiturate withdrawal and convulsions elicited by chemical and audiogenic stimuli. We recently identified Mpdz within this chromosomal region as a gene that influences alcohol and barbiturate withdrawal convulsions. Mpdz encodes the multi-PDZ domain protein (MPDZ). Currently, there is limited information available about the mechanism by which MPDZ influences drug withdrawal and/or other CNS hyperexcitability states, but may involve its interaction with 5-HT2C and/or GABAB receptors. One of the most useful tools we have developed thus far is a congenic strain that possesses a segment of chromosome 4 from the C57BL/6J (donor) mouse strain superimposed on a genetic background that is >99% from the DBA/2J strain. The introduced segment spans the Mpdz gene. Here, we demonstrate that handling-induced convulsions are less severe in congenic vs. background strain mice in response to either a 5-HT2C receptor antagonist (SB242084) or a GABAB receptor agonist (baclofen), but not a GABAA receptor channel blocker (pentylenetetrazol). These data suggest that allelic variation in Mpdz, or a linked gene, influences SB242084- and baclofen-enhanced convulsions. Our results are consistent with the hypothesis that Mpdz's effects on CNS hyperexcitability, including alcohol and barbiturate withdrawal, involve MPDZ interaction with 5-HT2C and/or GABAB receptors. However, additional genes reside within the congenic interval and may also influence CNS hyperexcitability.

Figure 1. SB242084, a selective 5-HT_2C receptor antagonist, produced less severe handling-induced convulsions (HICs) in chromosome 4 congenic (○) vs. D2 background strain (●) mice

The mice were scored for baseline (pre-drug) handling-induced convulsions immediately before i.p. administration of SB242084 (A) 1 mg/kg or (B) 10 mg/kg (the arrow marks injection of SB242084 at time 0) and hourly for 10 hr after SB242084 administration. Data represent the mean scores ± SEM scores for baseline and post-SB242084 HICs (n=17 congenic and n=18 D2 strain mice). In some cases the error bars are smaller than the symbol. Inset: SB242084-enhanced HIC severity was calculated as the area under curve (AUC) corrected for individual differences in baseline HIC severity, and was significantly lower in congenic vs. D2 background strain mice after 1 or 10 mg/kg SB242084 (*p < 0.001).

Figure 2. Baclofen, a selective GABA_B receptor agonist, produced less severe handling-induced convulsions (HICs) in chromosome 4 congenic (○) vs. D2 background strain (●) mice

The mice were scored for baseline (pre-drug) handling-induced convulsions immediately before i.p. injection of baclofen (A) 5 mg/kg or (B) 10 mg/kg (the arrow marks injection of baclofen at time 0), and scored every 30 minutes for 6 hours after baclofen administration. Data represent the strain mean ± SEM for baseline and post-baclofen HICs (n=26 congenic and n=31 D2 mice). In some cases the error bars are smaller than the symbol. Inset: Baclofen-induced HIC severity was calculated as the area under curve (AUC) corrected for individual differences in baseline HIC severity, and was significantly lower in congenic vs. D2 background strain mice after 10 mg/kg baclofen (*p < 0.05), and showed a trend in the same direction after 5 mg/kg baclofen (p < 0.1).

Figure 3. Pentylenetetrazole (PTZ), which blocks GABA_A receptor channels, produced comparably severe handling-induced convulsions (HICs) in chromosome 4 congenic (○) and D2 background (●) strain mice

Mice were scored for baseline (pre-drug) handling-induced convulsions immediately before i.p. injection of PTZ (A) 5 mg/kg, 15 mg/kg (not shown) (B) or 30 mg/kg and then 1, 3, 5, 8, 12, 15, 20 and 60 min after PTZ administration. Arrow marks injection of PTZ at time 0. Data represent the strain mean ± SEM for baseline and post-PTZ HICs (n=54 congenic and n=48 D2 mice). In some cases the error bars are smaller than the symbol. PTZ-enhanced HIC severity (AUC) was calculated as the area under curve corrected for individual differences in baseline HIC severity, and was not significantly different between congenic and D2 background mice after administration of PTZ (5, 15 or 30 mg/kg).