br Introduction As a strong analgesic morphine is often used

Introduction As a strong analgesic, morphine is often used to treat acute and chronic pain. However, morphine is a substance with low lipid solubility that slowly enters the nerve tissue through the blood–brain barrier and produces neurotoxicity. Morphine-induced neurotoxicity can be produced after just a few days of use, leading to neuronal dysfunction or death, and resulting in irreversible damage. Neurotoxic mechanisms include morphine-induced neuronal apoptosis, oxidative stress, and inhibition of neuron production. A previous study has shown that morphine exposure damages learning and memory (Lu et al., 2010); therefore, improving our understanding of how morphine can lead to learning and memory disorders could help to limit adverse reactions in clinical applications. The midbrain periaqueductal gray (PAG) is a central region that integrates morphine dependence, withdrawal responses (Liu et al., 2016), tolerance, and reward behavior (Sante et al., 2000). Morphine addiction and learning and memory have a common molecular mechanism (Hu and Lu, 2006). However, little research has been conducted to investigate the relationship between the midbrain PAG and learning and memory. Toll-like receptors (TLRs) are expressed in microglia (Olson and Miller, 2004), astrocytes (Bowman et al., 2003), oligodendrocytes (Bsibsi et al., 2002), and neurons (Préhaud et al, 2005). TLR4 activation is related to many of the side effects of morphine, such as reward effects (Chen et al., 2017), withdrawal responses (Liu et al., 2016), opioid relapse (Schwarz and Bilbo, 2013), and morphine tolerance (Qu et al., 2017). Moreover, TLR4 activation is associated with neurodegenerative diseases that cause cognitive impairment (Wang et al., 2018, Tang et al., 2018, Khajevand-Khazaei et al., 2018). For example, Alzheimer’s disease model rats show increased TLR4 expression in the hippocampus and have decreased learning and memory ability in the Morris water maze. Furthermore, increased expression of TLR4/nuclear factor-κB (NF-κB) in the hippocampus of traumatic SJB3-019A injury rats leads to nerve damage as well as learning and memory impairments (Tang et al., 2018). In addition, in a study that used lipopolysaccharide (LPS)-induced systemic inflammation animal models, Khajevand-Khazaei et al. (2018) revealed impaired spatial memory ability in the Y-maze due to increased hippocampal TLR4 and NF-κB expression. When endogenous ligands bind to TLR4, the signal transduces to the Toll/interleukin-1 receptor (TIR) region, activating NF-κB and mitogen-activated protein kinases (MAPKs), which induce inflammatory responses (Yan, 2006). Administration of atorvastatin reduces TLR4 mRNA and protein expression and improves learning and memory (Wang et al., 2018). These findings suggest that increased expression of TLR4 is associated with opioid side effects and cognitive impairment, and mediates the inflammatory response. The cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/cAMP-responsive-element-binding protein (CREB) cascade is a classical G protein-coupled receptor signal transduction pathway, and is a common pathway in drug addiction, learning, and memory (Hu and Lu, 2006). In the nervous system, an increase in PKA expression promotes neuronal growth, survival, and synaptic plasticity of nerve endings (Chierzi et al., 2005), induces neurotransmitter release (Cho et al., 2015) and regulates synaptic transmission, and plays an important regulatory role in the formation of long-term memory (Bollen et al., 2014). Activation of the cAMP/PKA cascade also mediates opiate dependence. Chronic morphine exposure or withdrawal enhances the expression of hippocampal PKA, and the blockade of PKA attenuates the symptoms of physical opiate withdrawal (García-Pardo et al., 2016). Research by Fang et al. (1999) revealed that long-term exposure (2 min–36 h) to morphine (100 μmol/L) could induce biphasic changes in PKA levels. Morphine treatment for 2 min–30 min caused a rapid and significant decrease in PKA, before levels gradually recovered and apparently increased at 36 h.