Neonatal exposure to permethrin pesticide causes lifelong fear and spatial learning deficits and alters hippocampal morphology of synapses

Abstract

Background: During the neurodevelopmental period, the brain is potentially more susceptible to environmental exposure to pollutants. The aim was to determine if neonatal exposure to permethrin (PERM) pesticide, at a low dosage that does not produce signs of obvious abnormalities, could represent a risk for the onset of diseases later in the life.

Methods: Neonatal rats (from postnatal day 6 to 21) were treated daily by gavage with a dose of PERM (34 mg/kg) close to the no-observed-adverse-effect level (NOAEL), and hippocampal morphology and function of synapses were investigated in adulthood. Fear conditioning, passive avoidance and Morris water maze tests were used to assess cognitive skills in rats, whereas electron microscopy analysis was used to investigate hippocampal morphological changes that occurred in adults.

Results: In both contextual and tone fear conditioning tests, PERM-treated rats showed a decreased freezing. In the passive avoidance test, the consolidation of the inhibitory avoidance was time-limited: the memory was not impaired for the first 24 h, whereas the information was not retained 72 h following training. The same trend was observed in the spatial reference memories acquired by Morris water maze. In PERM-treated rats, electron microscopy analysis revealed a decrease of synapses and surface densities in the stratum moleculare of CA1, in the inner molecular layer of the dentate gyrus and in the mossy fibers of the hippocampal areas together with a decrease of perforated synapses in the stratum moleculare of CA1 and in the inner molecular layer of the dentate gyrus.

Conclusions: Early-life permethrin exposure imparts long-lasting consequences on the hippocampus such as impairment of long-term memory storage and synaptic morphology.

Figure 1

Fear conditioning tests in PERM-treated and control groups. A) Locomotor activity, expressed as distance moved, during 3-minute habituation period preceding the conditioning session. B) Freezing on conditioning session expressed as 1-minute averages for the period before (baseline) and after each of ten tone-shock conditioning trials. C) Freezing on context (CX) and cued (CS) fear conditioning sessions during an 8-minute extinction test. All data are means ± SEM. Group sizes: control (N = 13), PERM (N = 15). *P < 0.05 versus control group; **P < 0.01 versus control group; ***P < 0.001 versus control group.

Figure 2

Performance of PERM-treated and control group on the Morris water maze task in the probe tests at 24 or 72 hours after the last training session. The time spent in the quadrant target without the platform was measured in 90 seconds. All data are means ± SEM. Group sizes: control (N = 9), PERM (N = 9). **P < 0.01 versus control group.

Figure 3

Performance of permethrin (PERM)-treated and control group on passive avoidance task. Latency time to enter into the dark sector of the chamber was measured before, 24 hours and 72 hours after shock conditioning. All data are means ± SEM. Group sizes: control (N = 9), PERM (N = 9). *P < 0.05 versus control group.

Figure 4

Electron microscopic picture. A) synaptic junctions preferentially stained by ethanol-phosphotungstic acid (E-PTA) procedure in the rat hippocampal SMCA1. Pre- and postsynaptic membrane appositions are clearly evidenced as sharp parallel black lines: dotted (arrow) and full line, respectively, against an unstained background. Bar: 0.2 μm; B) perforated synapse in rat hippocampal SMCA1 stained by means of E-PTA technique. The evident discontinuity (arrowhead) in the giant contact zone is currently supposed to represent a perforation. Bar: 0.3 μm.