The Impact of Removal of Ovarian Hormones on Cholinergic Muscarinic Receptors: Examining Prepulse Inhibition and Receptor Binding

Abstract

Ovarian hormones, such as estrogens and progesterone, are known to exert beneficial effects on cognition and some psychiatric disorders. The basis of these effects is not fully understood, but may involve altered cholinergic neurotransmission. This study aimed to investigate how a lack of ovarian hormones would impact muscarinic receptor-induced deficits in prepulse inhibition (PPI) and muscarinic receptor density in several brain regions. Adult female rats were either ovariectomized, to remove the source of ovarian hormones, or left intact (sham-operated). PPI is a measure of sensorimotor gating that is typically impaired in schizophrenia patients, and similar deficits can be induced in rats by administering scopolamine, a muscarinic receptor antagonist. Our results revealed no significant effects of ovariectomy on PPI after saline or scopolamine treatment. Autoradiography was performed to measure cholinergic muscarinic receptor binding density using [³H]-pirenzepine, [³H]-AF-DX, and [³H]-4-DAMP, to label M₁, M₂/M₄, and M₃ receptors, respectively. We examined the amygdala, caudate putamen, dorsal hippocampus, motor cortex, retrosplenial cortex, and ventromedial hypothalamus. There were no significant group differences in any region for any muscarinic receptor type. These results suggest that removing peripheral ovarian hormones does not influence the cholinergic muscarinic receptor system in the context of PPI or receptor binding density.

Keywords: CHRM1; PPI; female; ovariectomy; rat; schizophrenia.

Figure 1

Example autoradiographic image of [³H]-AF-DX binding showing the traced outlines of the nine regions of interest: (1) retrosplenial cortex (granular), (2) retrosplenial cortex (dysgranular), (3) motor cortex (M1, M2), (4) dorsal hippocampus, (5) caudate putamen, (6) central amygdala, (7) basolateral amygdala, (8) medial amygdala, and (9) ventromedial hypothalamus.

Figure 2

Representative autoradiographic images showing total binding (left panels) and nonspecific binding (right panels). Three tritiated radioligands were used with the protocols described in Section 2.4.

Figure 3

Prepulse inhibition (PPI; graphs A and B) and startle responses (graph C) in intact or ovariectomized (OVX) rats (n = 10 per group). Rats were tested after treatment with saline (white bars) or 0.3 mg/kg scopolamine (grey bars). Individual data are depicted by open circles (saline) or black squares (scopolamine). %PPI refers to the difference in startle between the pulse-alone trials and the prepulse-pulse trials, divided by the response to the pulse-alone trial × 100%. Prepulse intensity (PP) refers to the trials that were 2, 4, 8, 12, or 16 dB above the 70 dB background noise followed 100 ms later by the 115 dB startling pulse. Startle amplitude refers to the average of the four blocks of eight 115 dB startling pulses. Bars represent mean ± S.E.M.