Our present results showed that, in SoCx, WAG/Rij rats had a higher density of dendritic spine when compared to healthy Wistar rats. Furthermore, our results showed for the first time that sensory experiences in early developmental period can cause permanent changes in dendritic spine density in WAG/Rij rats. A decrease occurred in the density of dendritic spines in the deep layer of SoCx in adult WAG/Rij rats when subjected to TS and MS during neonatal period. Additionally, sensory experiences induced changes in spine morphology within the SoCx. In the deep pyramidal cells of maternally separated animals there were less thin spines than tactile-stimulated animals.
The SoCx in rats that are genetically predisposed to absence epilepsy is regarded to trigger epileptic discharges. In general, it was concluded that the spike-wave discharges firstly appear at SoCx and then rapidly spreads to the remaining parts of the cortex and cortico-thalamic network. Seizure activity initially takes place in the deep-pyramidal cells, afterwards in the superficial layer pyramidal neurons and spreads towards ipsilateral cortical areas [19].
The SoCx in WAG/Rij rats is characterized by synaptic hyperexcitability [8]. In absence seizures, electrophysiological intracellular recordings demonstrated that the pyramidal cells in the deep layers of SoCx show fast activation, hyperexcitability and hypersynchronizing characteristics [19]. Karpova et al. (2005) demonstrated significant structural changes in dendritic and axonal arborization in pyramidal neurons SoCx of WAG/Rij rats. In WAG/Rij rats, it was found that there were longer dendrites and had less branching in pyramidal neurons of SoCx compared with non-epileptic control rats [20]. According to these data that were collected in the upper layers, it is suggested that epileptic rats might have atypically characterized neurons in the SWD generation site with greater arborizations and more synaptic connections between neurons. These features might facilitate the initiation and spreading of SWD [1].
In our experiment, different from previous study (Karpova et al., 2005), the neuronal organization of deep layer of SoCx was investigated by Golgi-Cox staining in WAG/Rij rats with genetic epilepsy and in non-epileptic control rats with the same age. Similarly, we found quantitative differences in the density of dendritic spines between WAG/Rij rats and non-epileptic control rats in the deep layers of SoCx. It is likely that dendritic abnormalities are both the cause and result of seizures in this genetic model.
Dentritic spines are thin protrusions that emerge from the surface of various neurons, specifically postsynaptic structures, which play mostly an excitatory role in synaptic communications [21, 22]. The morphology and density of dentritic spine is crucial for synaptic plasticity. Physiological and pathological conditions are reported to be associated with the spine morphology and density [23, 22]. Dendritic spine abnormalities are generally reported in the brain specimens of epileptic patients in hippocampal tissue of epilepsy patients with temporal lobe, decrease in dendritic spine density is frequently reported. However, alterations in dendritic length, shape, and branching patterns and focal increase in dendritic spines are less frequently reported in cortical and hippocampal tissues. In various animal models that involve acute seizures or chronic epilepsy, similar dendritic changes have been observed, which are primarily loss of dendritic spines and less frequently increase in dendritic spines [24].
Generally, the majority of excitatory synaptic inputs are received and integrated by dendritic spines in the central nervous system, and therefore have influence on neuronal excitability. In certain kinds of epilepsy, hyperexcitable circuits and seizures might result from dendritic spine abnormalities. Therefore, it is safe to assume that high dendritic spine number and associated excitatory synaptic input disturb the balance between excitation and inhibition in the WAG/Rij rat brain, causing seizure. In addition, types of spine may have distinct functions and alterations in the spine type ratio may cause a significant effect on neuronal excitability and function [25, 26].
In our study, all types of spines were reduced in epileptic rats exposed to TS and MS. However, MS in WAG/Rij rats decreased the most in thin spines. It is known that mushroom spines are more stable whereas thin spines are newly formed [27, 28]. Therefore, our findings indicate that tactile stimulations not only modulate structural plasticity in the SoCx by decreasing spine numbers, but also by changing the ratio of new/mature spines.
It is not surprising that tactile stimulations led to change spine density in the SoCx, where tactile sensation gets processed [17].Various data confirm the beneficial effects of enriched environment on synaptic plasticity in different animal models [29, 30].
Studies showed that TS, which is an enriching positive experience that mimics maternal licking and grooming, has the potential to affect the neuroanatomic organization of the brain
[12, 16, 31, 32, 33].
Richards et al., (2012) showed that TS early in life increased spine density, dendritic branching, and dendritic length in prefrontal cortex and amigdala of rats [12]. In another study, TS treatment, by increasing dendritic complexity, length and synaptic contact in all cortical areas and amygdala, reversed neuroanatomical alterations caused by prenatal valproic acid exposure in rats [31]. However, Kolb and Gibb (2010) demonstrated that TS may differently alter synaptic organisation in different brain areas [32]. According to their study, TS treatment early in life decreases spine density and dendritic length in the parietal cortex. The same environmental exposure can alter spine density differently in accordance with manipulation age and manipulations early in life lead to decrease in spine density while those given in adulthood cause it to increase [34].
Interestingly, in SoCx, maternally separated animals showed dendritic density similar to that of tactile-stimulated animals. This result seems to be related to short MS sessions (non-stresful) and the increased compensatory licking and grooming behaviors (tactile stimulations) by the dams to the pups after the separation period [35].
It has been suggested that possible mechanisms that underlie the neural and behavioural effects of TS include endocrine function alterations, increased production of neurotrophic factors (insulin-like growth factor, brain-derived neurotrophic factor, and fibroblast growth factor-2) and altered gene methylation [12].
Further studies are required to clarify mechanisms underlying the seen effects of TS on the morphology and density of dendritic spines in the deep layer of SoCx in epileptic rat’s brain.