Chemokines, immune proteins that induce chemotaxis and adhesion, and their G-protein coupled receptors distribute throughout the central nervous system (CNS), regulate neuronal patterning, and mediate neuropathology. These chemo-attractant molecules may provide a neuro-immune "link" by regulating CNS systems. The purpose of this study was to investigate the interactions of specific chemokines, stromal cell-derived factor (SDF)-1a/CXCL12, and fractalkine/CX3CL1, and their receptors, CXCR4 and CX3CR1, with the serotonin (5-hydroxytryptamine; 5-HT) and opioid systems using anatomical and electrophysiological techniques in the rat brain. In the serotonin dense midbrain raphe nuclei (RN), SDF-1a, CXCR4, fractalkine and CX3CR1 co-localize over 70% with 5-HT neurons. CX3CR1 also localizes to microglia in the RN and hippocampus. Functionally, SDF-1a (10 nM) increases spontaneous inhibitory postsynaptic current (sIPSC) frequency and evoked IPSC (eIPSC) amplitude, while decreasing paired-pulse ratio (PPR) selectively in 5-HT neurons, thus stimulating presynaptic GABA release at these neurons. Alternatively, fractalkine (10 nM) increases sIPSC and eIPSC amplitude without changing PPR selectively in 5-HT neurons, thereby elevating the postsynaptic GABA receptor number or sensitivity. These results are dose-dependent and receptor-mediated. Chemokine interactions with serotonin, a neurotransmitter regulating mood, may lead to therapies for depression comorbid with immune diseases. Additional immunohistochemical analysis in the brain shows CXCR4 and CX3CR1 neuronal co-localization with the mu-opioid receptor (MOR) in the hippocampus, cingulate cortex, periaqueductal grey (PAG), nucleus accumbens, ventral tegmental area, globus pallidus, but not in the striatum or habenular nuclei, suggesting region specific receptor interactions. Electrophysiological recordings following morphine, SDF-1?? or fractalkine in vitro treatment reveal morphine (10 ?M)-mediated hyperpolarization of the membrane potential and reduction of the input resistance of PAG neurons, however, SDF-1??and fractalkine at 10 nM do not impact either parameter. In combination, SDF-1? inhibits morphine's actions in all PAG neurons tested, and fractalkine blocks morphine-mediated changes in 60% of PAG neurons examined. Thus, CXCR4 as well as CX3CR1, although less consistently, both appear to desensitize MOR at the neuronal level. Chemokine-opioid receptor interactions may mediate novel mechanisms to treat neuro-inflammatory pain and opiate abuse. The combined anatomical and electrophysiological results support chemokines as neuromodulatory proteins that may provide communication between the nervous and immune systems. / Anatomy