Administration of clonidine can reduce the doses of opioid analgesics required for pain control. Clonidine is also used to counteract symptoms of opioid withdrawal. How does this work?
Clonidine is an alpha-2-adrenoceptor agonist. Clonidine activates presynaptic alpha-2-adrenoceptors serving as autoreceptors on both central and peripheral nervous system noradrenergic nerve terminals. Activation of these autoreceptors reduces release of nordrenaline. Clonidine also activates alpha-2-adrenoceptors on the neurones of the locus coeruleus, the major source of noradrenergic innervation in the brain, to inhibit locus coeruleus neurone firing and further reduce central nervous system noradrenergic neurotransmission. By these mechanisms, clonidine is an indirect sympatholytic agent and has been used as an antihypertensive drug.
Clonidine is also a direct adrenoceptor agonist at presynaptic alpha-2-adrenoceptors serving as heteroreceptors on the primary afferent neurone nerve terminals bringing nociceptive signals into the spinal cord and at postsynaptic alpha-2-adrenoceptors on secondary spinal cord neurones relaying pain information up to the brain. The descending systems gating pain transmission through the spinal cord include noradrenergic neurones releasing noradrenaline to activate the presynaptic alpha-2-adrenoceptor heteroreceptors on the primary afferent neurone nerve terminals preventing them from releasing their neurotransmitters and transmitting their nociceptive signals. Meanwhile, the noradrenergic descending projections also active postsynaptic alpha-2-adrenoceptors on secondary spinal cord neurones, inhibiting these neurones, and preventing them from relaying the nociceptive signals up to the brain. Therefore, clonidine, which activates these alpha-2-adrenoceptors, has analgesic properties.
The descending pain gating systems also activate local engodenous opioid peptide releasing interneurones within the spinal cord. These interneurones inhibit the secondary spinal cord neurones relaying the nociceptive information up to the brain and so further block transmission of nociceptive signals through the spine. There is therefore a good additive effect between clonidine and the opioid analgesics, which produce spinal analgesia by mimicking the action of the endogenous opioid peptides. Administering clonidine can reduce the doses of opioid analgesics required to control pain.
Another use for clonidine is in controlling symptoms of opioid withdrawal. Part of the reason why clonidine helps is that by its non-opioid analgesic mechanisms it controls the pain associated with opioid withdrawal. Opioid receptors also normally inhibit the neurones of the locus coeruleus and opioid withdrawal is also associated with over activation of the locus coeruleus and the brain noradrenergic system. This results in symptoms such as anxiety, agitation, irritability, and mood swings. Clonidine activates alpha-2-adrenoceptors inhibiting the cells of the locus coeruleus and presynaptic alpha-2-adrenoceptor autoreceptors reducing noradrenaline release.
