Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry.

The blood-brain barrier (BBB) is formed by the brain capillary endothelium and excludes from the brain ∼100% of large-molecule neurotherapeutics and more than 98% of all small-molecule drugs. Despite the importance of the BBB to the neurotherapeutics mission, the BBB receives insufficient attention in either academic neuroscience or industry programs. The combination of so little effort in developing solutions to the BBB problem, and the minimal BBB transport of the majority of all potential CNS drugs, leads predictably to the present situation in neurotherapeutics, which is that there are few effective treatments for the majority of CNS disorders. This situation can be reversed by an accelerated effort to develop a knowledge base in the fundamental transport properties of the BBB, and the molecular and cellular biology of the brain capillary endothelium. This provides the platform for CNS drug delivery programs, which should be developed in parallel with traditional CNS drug discovery efforts in the molecular neurosciences.

https://link.springer.com/content/pdf/10.1602%2Fneurorx.2.1.3.pdf

The Blood-Brain Barrier: Bottleneck in Brain Drug Development

The formalin test: an evaluation of the method.

The formalin test for nociception, which is predominantly used with rats and mice, involves moderate, continuous pain generated by injured tissue. In this way it differs from most traditional tests of nociception which rely upon brief stimuli of threshold intensity. In this article we describe the main features of the formalin test, including the characteristics of the stimulus and how changes in nociceptive behaviour may be measured and interpreted. The response to formalin shows an early and a late phase. The early phase seems to be caused predominantly by C-fibre activation due to the peripheral stimulus, while the late phase appears to be dependent on the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord. These functional changes seem to be initiated by the C-fibre barrage during the early phase. In mice, the behavioural response in the late phase depends on the ambient temperature. We argue that the peripheral tissue temperature as well as other factors influencing the peripheral inflammation may affect the response, possibly confounding the results obtained with the test. Furthermore, we discuss the methods of recording the response and the value of observing more than one aspect of behaviour. Scoring of several behavioural variables provides a means of assessing motor or sensorimotor function as possible causes for changes in behaviour. In conclusion, the formalin test is a valuable addition to the battery of methods available to study nociception.

The formalin test in mice is a valid and reliable model of nociception and is sensitive for various classes of analgesic drugs. The noxious stimulus is an injection of dilute formalin (1% in saline) under the skin of the dorsal surface of the right hindpaw. The response is the amount of time the animals spend licking the injected paw. Two distinct periods of high licking activity can be identified, an early phase lasting the first 5 min and a late phase lasting from 20 to 30 min after the injection of formalin. In order to elucidate the involvement of inflammatory processes in the two phases, we tested different classes of drugs in the two phases independently. Morphine, codeine, nefopam, and orphenadrine, as examples of centrally acting analgesics, were antinociceptive in both phases. In contrast, the non-steroid anti-inflammatory drugs indomethacin and naproxen and the steroids dexamethasone and hydrocortisone inhibited only the late phase, while acetylsalicylic acid (ASA) and paracetamol were antinociceptive in both phases. The results demonstrate that the two phases in the formalin test may have different nociceptive mechanisms. It is suggested that the early phase is due to a direct effect on nociceptors and that prostaglandins do not play an important role during this phase. The late phase seems to be an inflammatory response with inflammatory pain that can be inhibited by anti-inflammatory drugs. ASA and paracetamol seem to have actions independent of their inhibition of prostaglandin synthesis and they also have effects on non-inflammatory pain.

The formalin test in mice: dissociation between inflammatory and non-inflammatory pain

Calcitonin gene-related peptide promotes mechanical nociception by potentiating release of substance P from the spinal dorsal horn in rats.

Evidence that substance P and somatostatin transmit separate information related to pain in the spinal dorsal horn.

It is generally accepted that morphine exerts its analgesic effect by binding to specific opiate receptors in the brain and spinal cord1. Since Hughes et al.2 isolated and identified two endogenous pentapeptides, Met- and Leu-enkephalin, from the brain and found that they acted as agonists at opiate receptors, α-, β- and γ-endorphins, larger peptides than enkephalins and having morphine-like activity, have been identified in either the brain or pituitary of various species1. Several studies have demonstrated that enkephalins possess analgesic properties and that they are distributed in the pain-mediated pathways in the central nervous system. These findings suggest that enkephalins are important neurotransmitters or neuromodulators regulating pain transmission. We now report the isolation of a novel substance which has a Met-enkephalin releasing action. Our findings suggest the possibility of a regulating mechanism for the release of endogenous opioid peptides, especially Met-enkephalin

A novel analgesic dipeptide from bovine brain is a possible Met-enkephalin releaser.

Noradrenergic inhibition of the release of substance P from the primary afferents in the rabbit spinal dorsal horn.

Separate involvement of the spinal noradrenergic and serotonergic systems in morphine analgesia: the differences in mechanical and thermal algesic tests.

Hiroshi Takagi

Papers

Calcitonin gene-related peptide promotes mechanical nociception by potentiating release of substance P from the spinal dorsal horn in rats.

Evidence that substance P and somatostatin transmit separate information related to pain in the spinal dorsal horn.

A novel analgesic dipeptide from bovine brain is a possible Met-enkephalin releaser.

Noradrenergic inhibition of the release of substance P from the primary afferents in the rabbit spinal dorsal horn.

Separate involvement of the spinal noradrenergic and serotonergic systems in morphine analgesia: the differences in mechanical and thermal algesic tests.