Female rats display greater nicotine withdrawal-induced cellular activation of a central portion of the interpeduncular nucleus versus males: A study of Fos immunoreactivity within provisionally assigned interpeduncular subnuclei

Abstract

Background: The interpeduncular nucleus (>1840) (IPN) has been shown to modulate the behavioral effects of nicotine withdrawal in male rodents. To date, the contribution of this brain structure to sex differences in withdrawal is largely unexplored.

Methods: This study compared neuronal activation, as reported by observable Fos expression in the IPN of nicotine-dependent female and male rats experiencing withdrawal. We provisionally localized the Fos-expressing cells to certain IPN subnuclei within Swanson's standardized brain atlas (2018). Adult female and male rats were prepared with a pump that delivered nicotine (3.2 mg/kg/day; base) continuously. Controls received a sham surgery. Fourteen days later, the rats received administration of saline or the nicotinic receptor antagonist, mecamylamine (3.0 mg/kg; salt), and physical signs and anxiety-like behavior were assessed. The rats were then euthanized and brain sections containing the IPN were processed for Fos immunofluorescence to infer the possible IPN subnuclei displaying differential activation between sexes.

Results: Both female and male rats displayed withdrawal-induced Fos expression within the IPN. Compared to males, female rats displayed greater numbers of withdrawal-induced Fos-positive cells within a circumscribed portion of the IPN that may fall within the cytoarchitectural boundaries of the central subnucleus (>1840) (IPNc). The withdrawal-induced activation of the IPN was correlated with negative affective states in females, but not males.

Conclusion: These data suggest that a centrally located group of IPN cells, presumably situated partly or completely within the IPNc, play a role in modulating sex differences in negative affective states produced by withdrawal.

Keywords: Dependence; Immunofluorescence; Sex differences; Standardized brain atlas; c-Fos.

Figure. 1.

Photomicrographs illustrating Fos immunolabeling in coronal–plane sections containing the putatively assigned IPNi and IPNc subnuclei of the IPN. The scale bar denotes 200 μm and applies to all panels.

Figure. 2.

The data on the left side reflect Fos (+) cells (mean ± SEM) expressed in the total IPN (A) and putatively assigned IPNi (B) and IPNc (C) in mecamylamine controls and nicotine + mecamylamine-treated female and male rats. The data on the right side of this figure reflect the correlation between total IPN activation with physical signs (D) or anxiety-like behavior as measured by % time spent in the dark (E) or lit (F) side of the LDT apparatus. Asterisks (*) denote a main effect of treatment, the pound sign (#) denotes a main effect of sex, and the dagger (†) denotes a difference between nicotine + mecamylamine-treated female and male rats (p < .05).