Alpha 2-adrenergic receptor inhibition of cAMP accumulation is transformed to facilitation by tumor necrosis factor-alpha

The hippocampus and TNF: Common links between chronic pain and depression

Major depression and chronic pain are significant health problems that seriously impact the quality of life of affected individuals. These diseases that individually are difficult to treat often co-exist, thereby compounding the patient's disability and impairment as well as the challenge of successful treatment. The development of efficacious treatments for these comorbid disorders requires a more comprehensive understanding of their linked associations through common neuromodulators, such as tumor necrosis factor-α (TNFα), and various neurotransmitters, as well as common neuroanatomical pathways and structures, including the hippocampal brain region. This review discusses the interaction between depression and chronic pain, emphasizing the fundamental role of the hippocampus in the development and maintenance of both disorders. The focus of this review addresses the hypothesis that hippocampal expressed TNFα serves as a therapeutic target for management of chronic pain and major depressive disorder (MDD).

Regulation of CCL2/MCP-1 production in astrocytes by desipramine and atomoxetine: involvement of α2 adrenergic receptors

Having previously observed that noradrenaline activation of β adrenergic receptors induces the synthesis of the chemokine monocyte chemoattractant protein (CCL2/MCP-1) in astrocytes, it is our interest to analyze the mechanisms involved in this process, particularly the possible effect of noradrenaline-modulating drugs. The treatment of primary rat astrocyte cultures with the noradrenaline transporter inhibitors desipramine or atomoxetine induced the expression and synthesis of CCL2/MCP-1 in these cells. This effect of both drugs in vitro suggests that CCL2/MCP-1 expression could also be modulated by some mechanism independent of the elevation of brain noradrenaline levels. This was confirmed by measuring a reduction in CCL2/MCP-1 production by the treatment with the α2 adrenergic receptor agonist clonidine. Accordingly, the blockade of α2 adrenergic receptors with yohimbine potentiated the production of MCP-1 stimulated by the activation of β receptors. While the activation of β adrenergic receptors and the subsequent elevation of cAMP levels seem to be the main pathway for noradrenaline to induce CCL2/MCP-1 in astrocytes, our data indicate that the α2 adrenergic receptors also regulate CCL2/MCP-1 expression working as inhibitory mediators.

Antidepressant-like action of intracerebral 6-fluoronorepinephrine, a selective full α-adrenoceptor agonist

The present study examined the ability of 6-fluoronorepinephrine (6FNE), a full selective α-adrenoceptor agonist, to produce antidepressant-like effects in mice. The drug, administered in the 4th ventricle, produced marked anti-immobility effects at mid-dose range in the acute forced swim, tail suspension and repeated open-space forced swim tests with minimal effect on open-field motor activity and also reversed anhedonia following lipopolysaccharide administration. Its antidepressant effects were equal to or greater than that of an established systemic antidepressant, desmethylimipramine, given subacutely. Experiments with α-adrenoceptor antagonists indicated that the drug acts primarily via the α2-receptor in contrast to endogenous catecholamines which appear to control depressive behaviour primarily via the α1-receptor. Antidepressant activity declined at higher doses signifying a possible pro-depressant effect of one of the α-adrenoceptor subtypes. Compared to the selective α2-agonist, dexmedetomidine, 6FNE showed equivalent antidepressant action in the tail suspension test but appeared to have a greater efficacy or speed of action in the repeated open-space forced swim test which produces a more sustained depression. Studies of regional brain Fos expression induced during the antidepressant tests showed that 6FNE tended to inhibit neural activity in two stress-responsive regions (locus coeruleus and paraventricular hypothalamus) but to enhance activity in two areas involved in motivated behaviour (nucleus accumbens shell and lateral septal nucleus) producing a neural pattern consistent with antidepressant action. It is concluded that 6FNE elicits a rapid and effective antidepressant and anti-stress response that may compare favourably with available antidepressants.

Uncovering molecular elements of brain-body communication during development and treatment of neuropathic pain

Integral to neuropathic pain is a reciprocal interaction between tumor necrosis factor-alpha (TNF) production and the alpha(2)-adrenergic receptor response, offering an attractive therapeutic target. The effects of varying levels of brain TNF on alpha(2)-adrenergic regulation of cyclic AMP (cAMP) production in the hippocampus and sciatic nerve were investigated during the development and amitriptyline treatment of chronic pain. Increased levels of TNF during the development of chronic pain transform alpha(2)-adrenergic inhibition of cAMP production in the brain to potentiation. While alpha(2)-adrenergic receptors regulate TNF production, they also affect descending noradrenergic pathways. Increases in levels of TNF in the brain deeply impact peripheral inflammation through regulating alpha(2)-adrenergic receptors, offering insight into brain-body interactions during neuropathic pain. Amitriptyline as an analgesic inhibits pain-induced increases in brain-associated TNF and transforms peripheral alpha(2)-adrenergic receptors. The dynamic equilibrium between TNF levels and alpha(2)-adrenergic functioning is uniquely altered during development and treatment of neuropathic pain. Proper manipulations of this interaction offer efficacious treatment of neuropathic pain.

Brain-derived tumor necrosis factor-alpha and its involvement in noradrenergic neuron functioning involved in the mechanism of action of an antidepressant

The present study documents a role for brain-derived tumor necrosis factor-alpha (TNF) in the mechanism of action of the antidepressant drug desmethylimipramine (desipramine). To establish this role, field stimulation and superfusion of rat hippocampal slices was employed to investigate the regulation of norepinephrine (NE) release by TNF. Chronic desipramine administration transforms TNF-mediated inhibition of NE release to facilitation, dependent upon alpha2-adrenergic receptor activation. Chronic i.c.v. microinfusion of polyclonal TNF antibody (pTNF-Ab) similarly transforms TNF inhibition of NE release to facilitation. To determine whether this transformation is due to desipramine-induced inhibition of TNF bioactivity in the brain, rats were i.c.v. microinfused with recombinant rat TNF (rrTNF) for 14 days, either alone or with simultaneous i.p. desipramine administration. TNF regulation of NE release in hippocampal slices isolated from these rats was compared with slices isolated from rats chronically administered desipramine alone. Although simultaneous microinfusion of rrTNF with chronic desipramine administration prevents the transformation induced by desipramine, microinfusion of rrTNF enhances TNF inhibition of NE release. These cellular events correspond to changes in immobility, analyzed by the forced swim test (FST). Intracerebroventricular microinfusion of rrTNF increases the duration of immobility of rats in the FST, compared with rats microinfused with aCSF. Desipramine administered chronically decreases immobility duration, which is mimicked by i.c.v. microinfusion of pTNF-Ab and prevented by simultaneous i.c.v. microinfusion of rrTNF. Thus, i.c.v. microinfusion of rrTNF with concomitant desipramine administration opposes decreases in neuron-associated TNF levels, required to transform presynaptic sensitivity to TNF, which is necessary for the drug to be efficacious.