Neurokinin-1 (NK-1) receptor and brain-derived neurotrophic factor (BDNF) gene expression is differentially modulated in the rat spinal dorsal horn and hippocampus during inflammatory pain

To date, pain-induced peripheral and central sensory activation has been well characterized; however, little emphasis has been placed on studying the physiological mechanisms of the stress-like component of pain and its relationship to mood or affect. The importance of the emotional aspects of chronic pain and their impact on cognition and the overall perception of the nociceptive stimuli is augmented by clinical observations that majority of chronic pain patients often suffer from various forms of depressive illnesses [1‐4]. The hippocampus, one of the main regulators of affect within the limbic system, has been previously shown to exhibit a robust stress-induced neurodegenerative plasticity related to the pathophysiology of depression [5‐8]. Furthermore, the hippocampus has also been associated with the processing of pain-related information, particularly its potential role in shaping the affective-motivational response to noxious sensory stimulation. For example, peripheral administration of formalin was shown to attenuate levels of Fos protein in the rat hippocampus [9], while microinjections of lidocaine or glutamate receptor antagonists directly into the dorsal hippocampal formation decreased formalin-related nociceptive behaviors [10, 11].

The tachykinin neuropeptide substance P (SP) and brain-derived neurotrophic factor (BDNF), each expressed by a subset of primary sensory neurons, are known modulators of nociceptive processing within the CNS [12‐14]. Upon tissue injury or noxious stimulation, SP and BDNF are released into laminae I and II of the spinal cord dorsal horn, where through activation of their respective postsynaptic receptors, neurokinin-1 (NK-1) and tyrosine kinase B (trkB), contribute to development of hyperalgesia and central sensitization associated with chronic pain [15‐18]. Both NK-1 receptors and BDNF are also highly expressed in the limbic system, primarily the amygdala, the hippocampus and the hypothalamus [14, 19, 20]. Their potential involvement in the processing of mood/affect has been suggested by clinical observations that NK-1 receptor antagonists have antidepressant properties [21, 22], while amplification of hippocampal BDNF levels is considered to be a possible common down-stream effect of various antidepressant psychopharmacotherapies [23, 24]. However, the influences of these neuromediators on modulation of neuronal plasticity following chronic pain, particularly their functional differences in the spinal dorsal horn vs. the hippocampus, are still largely undefined.

Previous studies have shown that intracellular transcriptional regulation of NK-1 receptor and BDNF genes, during either spinal nociceptive processing or stress-related stimulation of hippocampus, may be modulated by transduction pathways involving activation of extracellular signal-regulated kinases (ERK)/cAMP response element binding protein (CREB) cascades [25‐29]. Once activated by wide variety of extracellular signals through dual phosphorylation at threonine (Thr²⁰²) and tyrosine (Tyr²⁰⁴) sites [30], p-ERK proteins translocate from the cytoplasm into the nucleus and activate transcription factors such as CREB. Subsequently, CREB phosphorylated at serine¹³³ (p-CREB) further induces transcription of genes containing cAMP response element (CRE) binding sites in their promoter regions [31], such as c-fos, NK-1, BDNF, and trkB [32‐34]. The ERK/CREB-dependent cascade represents one of many intracellular pathways through which the extracellular stimuli, such as pain, may be transduced into post-translational and transcriptional responses within the neuronal tissue [25].

To address nociceptive regulation of regions of the CNS related to potentially distinct sensory vs. affective functions, we measured NK-1 receptor and BDNF gene expression in the spinal cord and the hippocampus following administration of complete Freund's adjuvant (CFA) into the rat hind paw. Furthermore, Western blot analysis was used to assess whether the changes in transcription of these two genes was correlated with CFA-evoked alterations in amounts of nuclear p-ERK and p-CREB proteins.

The mechanisms by which inflammatory nociception is processed within the spinothalamic pathways of the CNS have been well established at both cellular and molecular levels. However, the impact of painful stimulation on the higher brain centers, mainly the emotion- and cognition-processing components of the CNS, is still unclear. Besides the sensory aspects of nociception, a negative impact on affect is essential for accurate characterization of a stimulus as "painful". Therefore, improved understanding of how pain affects the mood-controlling regions of the brain would be very beneficial to treatment of chronic pain in the clinical setting.

The experiments in this study were designed to conduct a direct comparison of CFA-induced peripheral nociception on the expression of NK-1 receptor and BDNF in the spinal cord and hippocampal formation, one of the central limbic regions involved in the regulation of affect. Current results demonstrate that NK-1 receptor and BDNF mRNA levels are initially increased in the ipsilateral dorsal horn at 24 h and 4 days post CFA (Fig. 1), consistent with results of other studies [35, 39, 40]. Previous reports have suggested that SP and BDNF may co-localize in the same large dense-core vesicles (LDCVs) of the small diameter primary nociceptors, and that their release is dependent on the activation of tyrosine kinase A (trkA) receptors due to enlarged levels of nerve growth factor (NGF) in the periphery [14]. Increases in NK-1 receptor and BDNF gene expression in the dorsal horn likely represent a secondary effect resulting from facilitated activation of NMDA, NK-1 and trkB receptors located on the secondary afferents. This effect is considered to be directly related to increased SP and BDNF release from the terminals of primary nociceptors in response to peripheral inflammation [15].

Another important aspect of this study was to address the long-term effects of nociception on CNS gene plasticity. Thus, 10 and 21 day time points were included in the experimental design. Previously we reported that a single injection of CFA produces thermal and mechanical hyperalgesia for about 10–12 days post injection; however, when CFA was administered three times (weekly injections) over 21 day period, hyperalgesia was present throughout the entire time course [41]. Therefore, we also compared the effects of single vs. triple CFA treatments on gene expression over a 21 day period. Spinal NK-1 receptor gene expression was not affected at these later time points (Fig. 1A), while the BDNF mRNA levels continued to increase, with the most robust response occurring after three weekly administrations of CFA (Fig. 1B). The discrepancy in pain-induced regulation of these two genes at later time points could be due to an array of factors, ranging from differences in susceptibility during long-term sensory regulation to diverse intracellular conditions required for induction of transcription or stabilization of the mRNAs. Besides having roles in activity-dependent excitability, BDNF is also important for neuronal survival, regeneration, outgrowth and overall maintenance [42], suggesting that continuous expression of the BDNF gene may be more important in development of latent stages of chronic pain. Overall, the initial nociception-induced up-regulation of NK-1 receptor and BDNF gene expression in the spinal cord supports the requirement of these neuromodulators for the maximum amplification of ascending pain signaling during central sensitization.

Nociceptive information from the periphery and the spinal cord is transmitted to the brain mainly through spinothalamic and parabrachial ascending pathways [43]. Furthermore, complex neuronal networks connect the parabrachial area or the thalamus to the limbic regions such as the hippocampus, the amygdala and the hypothalamus, which can modulate spinal nociceptive processing through activation of descending monoaminergic pathways from the brain stem [44]. However, the limbic areas of the brain, particularly the hippocampus, are also involved in control of stress responses and are considered to be the main regulators of mood or affect. CFA evoked robust decreases in NK-1 receptor and BDNF mRNA levels in the hippocampal formation, contrary to the prominent up-regulation seen in the spinal cord. A similar type of hippocampal plasticity was previously observed after either peripheral formalin injections or immobilization stress, as both of these stimuli were shown to down-regulate hippocampal NK-1 and BDNF gene expression [36, 38, 41]. The mechanisms responsible for the similarity between nociception- and stress-evoked effects in the hippocampus are not fully understood, but previous results suggest that maintenance of the nociceptive regulation of the hippocampus is not dependent upon continuous activation of the HPA axis [41]. The current results provide additional evidence that persistent nociception may have a significant effect on modulation of the higher brain centers, further suggesting that both the NK-1 receptor and BDNF may play a prominent role in pain processing within limbic structures such as the hippocampus.

Previous studies have suggested that CNS events leading to central sensitization during peripheral inflammation, hippocampal regulation due to stress, or long-term potentiation associated with spatial learning and memory may be dependent on similar intracellular transduction mechanisms, primarily the activation of cytosolic ERK and subsequent phosphorylation of nuclear CREB [29, 45‐50]. Both of these proteins are important for activity-dependent gene expression, and are thought to play a key role in synaptic plasticity contributing to translation of acute stimuli into long-term events such as the development and maintenance of chronic pain or depression. Western blot analysis showed that 24 h or 21 days after CFA administration, p-ERK levels were generally not altered in either the dorsal horn or the hippocampus, except for slight increases in spinal p-ERK2 (Fig. 4). Previous reports suggest that robust activation of spinal ERK following CFA [25, 26, 50] may be transient, since p-ERK levels usually peak within minutes after application of painful stimulus and then start to diminish due to increased dephosphorylation by various MAP kinase phosphatases [25, 26, 50]. We addressed initial p-ERK activation by assessing protein levels at 10 min after 50 μL of CFA (data not shown); however, changes in the regulation of either spinal or hippocampal p-ERK levels were not observed at this time point either. The discrepancy with previous reports may be due to a lesser amount of CFA used in our experiments, methods and/or quantitiative end-points used in each study, or differences in specific time points when protein changes were measured. However, eventual activation of transduction proteins such as ERKs and other intracellular kinases often initiates long-term synaptic plasticity through modulation of post-translational and transcriptional events that outlast both the initiating stimulus and kinase activation [25]. This may, in part, explain increased levels of nuclear p-CREB in the ipsilateral dorsal horn at both 24 h and 21 days (3×) after CFA treatments (Fig. 5). Elevated activation of CREB in the spinal cord further supports the role of this transcription factor in persistent nociception. Similar to NK-1 receptor and BDNF gene expression, CFA diminished hippocampal p-CREB levels, a phenomenon reminiscent of that previously reported during stress and depression [24, 51], possibly by triggering intracellular events common to both types of stimuli. Besides ERKs, the activation of CREB may be coupled to other transduction factors, primarily the calcium/calmodulin-dependent protein kinase (CaM kinases)- or protein kinase A (PKA)-dependent cascades [27, 52], to trigger the transcription of target genes such as NK-1 receptor and BDNF. Due to their similar patterns of nociception-evoked regulation, these data imply that expression of NK-1 receptor and BDNF genes could be at least partially controlled by CREB-dependent intracellular signaling in both the sensory and affective systems of the CNS [32]. However, the transcription of NK-1 receptor gene at the later phase of inflammation is probably dependent on other regulatory pathways, since no changes were observed in the dorsal horn 21 days after CFA.

Our original hypothesis was that NK-1 receptor and BDNF gene expression may be co-regulated by peripheral nociception, but the results of this study demonstrated that is not the case, particularly at the longest time points tested in our chronic pain model. However, diverse patterns of gene expression suggest that NK-1 receptors and BDNF may have different roles in various regions of the CNS during chronic pain, since the spinal cord is an integral component of the sensory system, while the hippocampus is a part of the limbic system and a key contributor in regulation of affect. Novel information about the modulation of sensitivity, function, and plasticity of the genes encoding relevant neurotransmitters and their receptors, which contribute to the overall perception of pain, may give us a better idea of how to control the negative effects of pain on the mood and may provide a major insight into the development of improved therapeutic regimens for treating depression-like aspects of chronic pain.