Targeting Serotonin1A Receptors for Treating Chronic Pain and Depression

The association of chronic pain with depression is becoming increasingly recognized. Treating both the conditions together is essential for an effective treatment outcome. In this regard, it is important to identify a shared mechanism involved in the association of chronic pain with depression. Central serotonin (5-hydroxytryptamine; 5-HT) neurotransmission has long been known to participate in the processing of signals related to pain. It also plays a key role in the pathogenesis and treatment of depression. Although functional responses to serotonin are mediated via the activation of multiple receptor types and subtypes, the 5-HT1A subtype is involved in the processing of nociception as well as the pathogenesis and treatment of depression. This receptor is located presynaptically, as an autoreceptor, on the perikaryon and dendritic spines of serotonin-containing neurons. It is also expressed as a heteroreceptor on neurons receiving input from serotonergic neurons. This arti-cle targets the 5-HT1A receptors to show that indiscriminate activation of pre and postsynaptic 5-HT1A receptors is likely to produce no therapeutic benefits; biased activation of the 5-HT heteroreceptors may be a useful strategy for treating chronic pain and depression individually as well as in a comorbid condition.

Presynaptic Release-Regulating mGlu1 Receptors in Central Nervous System

Group I metabotropic glutamate (mGlu) receptors consists of mGlu1 and mGlu5 receptor subtypes. These receptors are widely distributed in the central nervous system (CNS), where they preferentially mediate facilitatory signaling in neurones and glial cells, mainly by favoring phospholipase (PLC) translocation. Based on the literature so far available, group I Metabotropic glutamate receptors (mGluRs) are preferentially expressed at the postsynaptic side of chemical synapsis, where they participate in the progression of the chemical stimulus. Studies, however, have shown the presence of these receptors also at the presynaptic level, where they exert several functions, including the modulation of transmitter exocytosis. Presynaptic Group I mGluRs can be both autoreceptors regulating release of glutamate and heteroreceptors regulating the release of various transmitters, including GABA, dopamine, noradrenaline, and acetylcholine. While the existence of presynaptic release-regulating mGlu5 receptors is largely recognized, the possibility that mGlu1 receptors also are present at this level has been a matter of discussion for a long time. A large body of evidence published in the last decade, however, supports this notion. This review aims at revisiting the data from in vitro studies concerning the existence and the role of release-regulating mGlu1 receptors presynaptically located in nerve terminals isolated from selected regions of the CNS. The functional interaction linking mGlu5 and mGlu1 receptor subtypes at nerve terminals and their relative contributions as modulators of central transmission will also be discussed. We apologize in advance for omission in our coverage of the existing literature.

Differential expression of metabotropic glutamate and GABA receptors at neocortical glutamatergic and GABAergic axon terminals

Metabotropic glutamate (Glu) receptors (mGluRs) and GABAB receptors are highly expressed at presynaptic sites. To verify the possibility that the two classes of metabotropic receptors contribute to axon terminals heterogeneity, we studied the localization of mGluR1α, mGluR5, mGluR2/3, mGluR7, and GABAB1 in VGLUT1-, VGLUT2-, and VGAT- positive terminals in the cerebral cortex of adult rats. VGLUT1-positive puncta expressed mGluR1α (∼5%), mGluR5 (∼6%), mGluR2/3 (∼22%), mGluR7 (∼17%), and GABAB1 (∼40%); VGLUT2-positive terminals expressed mGluR1α (∼10%), mGluR5 (∼11%), mGluR2/3 (∼20%), mGluR7 (∼28%), and GABAB1 (∼25%); whereas VGAT-positive puncta expressed mGluR1α (∼27%), mGluR5 (∼24%), mGluR2/3 (∼38%), mGluR7 (∼31%), and GABAB1 (∼19%). Control experiments ruled out the possibility that postsynaptic mGluRs and GABAB1 might have significantly biased our results. We also performed functional assays in synaptosomal preparations, and showed that all agonists modify Glu and GABA levels, which return to baseline upon exposure to antagonists. Overall, these findings indicate that mGluR1α, mGluR5, mGluR2/3, mGluR7, and GABAB1 expression differ significantly between glutamatergic and GABAergic axon terminals, and that the robust expression of heteroreceptors may contribute to the homeostatic regulation of the balance between excitation and inhibition.

Development of brainstem 5-HT1A receptor-binding sites in serotonin-deficient mice

The sudden infant death syndrome is associated with a reduction in brainstem serotonin 5-hydroxytryptamine (5-HT) and 5-HT(1A) receptor binding, yet it is unknown if and how these findings are linked. In this study, we used quantitative tissue autoradiography to determine if post-natal development of brainstem 5-HT(1A) receptors is altered in two mouse models where the development of 5-HT neurons is defective, the Lmx1b(f/f/p) , and the Pet-1⁻/⁻ mouse. 5-HT(1A) receptor agonist-binding sites were examined in both 5-HT-source nuclei (autoreceptors) and in sites that receive 5-HT innervation (heteroreceptors). In control mice between post-natal day (P) 3 and 10, 5-HT(1A) receptor binding increased in several brainstem sites; by P25, there were region-specific increases and decreases, refining the overall binding pattern. In the Lmx1b(f/f/p) and Pet-1⁻/⁻ mice, 5-HT(1A)-autoreceptor binding was significantly lower than in control mice at P3, and remained low at P10 and P25. In contrast, 5-HT(1A) heteroreceptor levels were comparable between control and 5-HT-deficient mice. These data define the post-natal development of 5-HT(1A)-receptor binding in the mouse brainstem. Furthermore, the data suggest that 5-HT(1A)-heteroreceptor deficits detected in sudden infant death syndrome are not a direct consequence of a 5-HT neuron dysfunction nor reduced brain 5-HT levels. To elucidate the developmental relationship between serotonin (5-HT) levels and 5-HT(1A) receptors in the brainstem, we examined 5-HT(1A) binding in two 5-HT-deficient mouse models. In nuclei containing 5-HT neurons, 5-HT(1A) binding was decreased (autoreceptors), while binding was maintained in projection sites (heteroreceptors). Thus, brainstem 5-HT(1A)-heteroreceptor-binding sites do not appear developmentally sensitive to reduced brain 5-HT levels.

NMDA receptor activation modulates evoked release of substance P from rat spinal cord

The possible modulation exerted by glutamate on substance P (SP) release from the rat spinal cord has been investigated. The N-methyl-D-aspartate (NMDA) receptor agonist, NMDA (1 microM), increased SP basal outflow by 46.5+/-10.9% (n = 3, P<0.01) without changing the evoked release of the peptide. Conversely, NMDA antagonists but not 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) inhibited both electrically-evoked and capsaicin-induced release of SP. In particular, D-2-amino-5-phosphonopentanoate (D-AP5; 50 microM) inhibited electrically-evoked and capsaicin-induced release of SP by 93+/-2.4% and 93.2+/-3.8% (n = 12, P<0.01), respectively. Functional pharmacological evidence is provided for glutamate exerting a positive feedback on SP release evoked by C fibre stimulation via NMDA receptor activation.