Short-term monocular deprivation weakens the synaptic connections of the visual conduction pathway in the deprived eye, resulting in a significant decline in vision . Form deprivation during the sensitive period of visual development leads to amblyopia. Our experiment was carried out during the sensitive period of visual development in kittens to ensure that this effect of form deprivation was observed. During the sensitive period of visual development, amblyopia and a decrease in VIP expression in the LGBd were caused by the unequal input of binocular visual information, and this decrease in VIP expression promoted the development of amblyopia. After 3 weeks of nasal administration of VIP, the PVEP results showed improvements, and VIP expression in the LGBd was increased. These results suggest that nasally administered VIP can reach the cerebral cortex and have an effect. VIP intervention can improve the metabolism of lateral geniculate neurons and enhance visual function during the sensitive period of visual development in amblyopic kittens.
After monocular deprivation, the expression of VIP in the LGBd of the deprivation group was significantly lower than that of the control group, indicating that VIP expression depended on normal illumination. Immunohistochemistry showed that the ability of LGBd neurons to express the VIP protein was decreased by form deprivation, and in situ hybridization demonstrated that form deprivation also affected the production of endogenous VIP mRNA in the LGBd. VIP, as a neurotransmitter in the central system [16,17], is widely distributed in neurons  and binds to receptors VPAC1, VPAC2, and PAC1 . After binding to the receptor, VIP plays a physiological role through a series of signal transduction pathways, including the cAMP-dependent protein kinase pathway, alcohol phospholipid pathway, ornithine decarboxylase polyamine pathway and Ras pathway. Down-regulated expression of VIP in the LGBd will lead to a reduction in the binding of VIP to its associated receptors, thereby probably inhibiting the expression of the corresponding function. At the same time, the decrease in VIP reduces the diurnal discharge frequency of neurons, thereby affecting the long-term electrical activity in the suprachiasmatic nucleus of the central system [20,21].The down-regulation in VIP also inhibits electrical transmission between neurons via the VPAC2-mediated cAMP pathway , thereby hindering the information transmission in the visual nervous system. These effects, in turn, accelerate the emergence of amblyopia.
After 3 weeks of VIP intervention, the expression of VIP in lateral geniculate neurons increased, and increased VIP expression promoted the recovery of visual function through its physiological effects. VIP can counteract the decrease in neurons caused by electrical conduction block , inhibit the apoptosis of neurons by reducing the translocation of cytochrome C from the mitochondria to the cytoplasm , and promote the proliferation of neurons. VIP can inhibit the production of interleukin-1 β, tumour necrosis factor α, β-amyloplast and other inflammatory and neurotoxic factors produced by microglia in the inflammatory environment and plays a protective role in neurons [25,26]. VIP provides indirect nutritional support to neurons by acting on astrocytes  and provides nutrition for neurons undergoing division . VIP has also been found to induce glycogen decomposition in the cerebral cortex of mice, potentially through increasing glucose utilization by promoting the formation of AMP . VIP is also involved in regulating the secretion of intracellular pancreatic polypeptide, adiponectin, insulin and other metabolic hormones, thereby affecting cell metabolism . VIP can also activate other excitatory intermediate neurons to produce excitatory postsynaptic potentials, which further increase the excitability of the central nervous system. VIP affects the biological metabolism of neurons in the visual nervous system by regulating the proliferation and differentiation of neurons, the synthesis of a variety of cytokines, the secretion of related hormones and the nutritional support of neurons to improve visual function.
Arden et al.  designed the checkerboard square reversal stimulation visual evoked potential test for clinical amblyopia examination. The latency of the P100 wave in amblyopia eyes is longer and the amplitude is lower than in normal eyes. Visual evoked potentials have been widely used as a diagnostic and therapeutic evaluation of amblyopia [32,33]. In our experiment, the latency of the P100 wave in amblyopic eyes of amblyopic kittens was longer and the amplitude was lower than those in the contralateral eyes and the ipsilateral eyes of the control group, consistent with the results of previous studies. Hubel and Wiesel  found that the visual plasticity of kittens was highly sensitive before 8 weeks after birth, gradually decreased after 8 weeks, and disappeared at the 3rd month. After 3 months, the effect of monocular deprivation on the size of neurons in the LGBd was basically negligible . Therefore, we performed monocular deprivation at the 3rd week after birth to ensure its effect on visual development. Because dark environments affect the plasticity of the visual cortex of kittens, increasing the amplitude of P100 waves  and improving vision , dark environments may also affect the plasticity of the LGBd. All experimental animals were maintained in a 24-hour light environment, and observation revealed that maintaining light at night did not affect sleep. Correction of ametropia was performed during PVEP to avoid interference with P100 latency and amplitude . Gozes et al.  administered VIP to the nasal mucosa of rats through inhalation to treat Alzheimer's disease and found that the concentration of VIP in the brains of rats was similar to that found with direct intraventricular injection. In contrast, intravenous administration of VIP results in significantly lower concentrations in the brain and blood . Therefore, VIP (containing 10% Sefsol and 40% isopropanol) was given through nasal mucosa, in which Sefsol and isopropanol were used as penetration enhancers. The latency and amplitude of the P100 wave in the amblyopia Sefsol intervention group were similar to those in the amblyopia non-intervention group, with differences that were not statistically significant; therefore, the effects of Sefsol and isopropanol as penetration enhancers on the experimental results were excluded. At the same time, innovative covering methods were used to reduce the risk of skin infection due to traditional eyelid suture and avoid the adverse consequences of repeated PVEP detection on eyelid skin, such as corneal irritation and suture rupture.
However, due to the short intervention time of VIP in this experiment, the full pharmacological effect of VIP may not have been observed; the P100 latency and amplitude in the VIP intervention group remained significantly different from those in the normal control group. Moreover, there was no significant difference in the average optical density of VIP immunohistochemically positive cells between the amblyopia Sefsol intervention group and the amblyopia non-intervention group. However, significant differences were still seen in the number of VIP-positive cells. Based on the experimental results, VIP undeniably has a therapeutic effect on the visual nervous system. Some studies have found that LGBd function in form-deprived amblyopic kittens can be partially restored after the sensitive period of visual development , which is worthy of our next study.