Amino acid systems in the interpeduncular nucleus are altered in a sex-dependent manner during nicotine withdrawal

Abstract

Prior work in male rodents established that the medial habenula-interpeduncular nucleus (MHb-IPN) pathway modulates nicotine withdrawal. Specifically, withdrawal severity has been closely associated with inhibitory tone in the IPN via interneurons that release γ-aminobutyric acid (GABA). Inhibitory tone in the IPN is regulated by projections from the MHb that co-release glutamate and acetylcholine. Within the IPN, inhibitory tone is also regulated via corticotropin-releasing factor type 1 (CRF1) receptors that control GABA release from local interneurons. This study extends previous work by comparing sex differences in GABA, glutamate, as well serotonin levels in the IPN during precipitated nicotine withdrawal. Sex differences in withdrawal-induced neurochemical effects were also compared following systemic administration of a CRF1 receptor antagonist. The results revealed that there were no group differences in serotonin levels in the IPN. A major finding was that females displayed a larger withdrawal-induced increases in GABA levels in the IPN than males. Also, withdrawal increased IPN glutamate levels in a similar manner in females and males. Blockade of CRF1 receptors produced a larger suppression of the withdrawal-induced increases in GABA levels in the IPN of females versus males, an effect that was likely related to the robust increase in glutamate following administration of the CRF1 receptor antagonist in females. These data suggest that amino acid systems in the IPN modulate sex differences in the behavioral effects of nicotine withdrawal. Furthermore, our data imply that medications that target stress-induced activation of the IPN may reduce withdrawal severity, particularly in females.

Keywords: GABA; IPN; RRID: RGD_737960; RRID: SCR_002865; antalarmin; glutamate; sex differences.

FIGURE 1

This figure describes the experimental groups and timeline of the dialysis studies. The top part (Initial Dialysis Study) illustrates the procedures for comparing sex differences in the IPN of control and nicotine-treated rats following administration of mecamylamine to precipitate withdrawal. The bottom part (Subsequent Dialysis Study) illustrates the procedures for comparing sex differences in mecamylamine-induced neurochemical changes in the IPN of nicotine-treated rats following administration of antalarmin.

FIGURE 2

GABA levels (nM ± SEM) in the IPN of female (control n = 6; nicotine-treated n = 6) and male (control n = 6; nicotine-treated n = 6) rats. The top panels reflect raw dialysate levels and the bottom panel reflects percentchange in nicotine-treated female and male rats relative to their respective controls. Each point reflects a 20-min sampling period during baseline and then following mecamylamine administration to precipitate withdrawal (shaded area). Asterisks (*) denote a difference from controls, daggers (†) denote a difference from baseline, and pound signs (#) denote a sex difference between female and male rats (p ≤ 0.05)

FIGURE 3

Glutamate levels (μM ± SEM) in the IPN of female (control n = 6; nicotine-treated n = 6) and male (control n = 6; nicotine-treated n = 6) rats. The top panels reflect raw dialysate levels and the bottom panel reflects percentchange in nicotine-treated female and male rats relative to their respective controls. Each point reflects a 20-min sampling period during baseline and then following mecamylamine administration to precipitate withdrawal (shaded area). Asterisks (*) denote a difference from controls (p ≤ 0.05)

FIGURE 4

Serotonin levels (nM ± SEM) in the IPN of female (control n = 6; nicotine-treated n = 6) and male (control n = 6; nicotine-treated n = 6) rats. The top panels reflect raw dialysate levels and the bottom panel reflects percentchange in nicotine-treated female and male rats relative to their respective controls. Each point reflects a 20-min sampling period during baseline and then following mecamylamine administration to precipitate withdrawal (shaded area)

FIGURE 5

GABA, glutamate, and serotonin levels in the IPN of nicotine-treated rats following antalarmin administration. The graph depicts nicotine-treated rats that did not receive antalarmin from Figures 1–3. Data include female (nicotine-treated n = 6; nicotine-treated + antalarmin n = 7) and male (nicotine-treated n = 6; nicotine-treated + antalarmin n = 6) rats. Each point reflects a 20-min sampling period during baseline and then following mecamylamine administration (shaded area). Asterisks (*) denote a significant difference from controls (p ≤ .05)