Zsuzsanna Wiesenfeld-Hallin

Allodynia limits the usefulness of intraspinal neural stem cell grafts; directed differentiation improves outcome

Several studies have reported functional improvement after transplantation of neural stem cells into injured spinal cord. We now provide evidence that grafting of adult neural stem cells into a rat thoracic spinal cord weight-drop injury improves motor recovery but also causes aberrant axonal sprouting associated with allodynia-like hypersensitivity of forepaws. Transduction of neural stem cells with neurogenin-2 before transplantation suppressed astrocytic differentiation of engrafted cells and prevented graft-induced sprouting and allodynia. Transduction with neurogenin-2 also improved the positive effects of engrafted stem cells, including increased amounts of myelin in the injured area, recovery of hindlimb locomotor function and hindlimb sensory responses, as determined by functional magnetic resonance imaging. These findings show that stem cell transplantation into injured spinal cord can cause severe side effects and call for caution in the consideration of clinical trials.

Neuropeptide expression in rat dorsal root ganglion cells and spinal cord after peripheral nerve injury with special reference to galanin.

The temporal course of changes in peptide expression in the dorsal root ganglia L4 and L5 and in the dorsal horn of the spinal cord has been studied in rats subjected to a sciatic nerve transection at a mid-thigh level following different survival times. Galanin-, substance P-, vasoactive intestinal polypeptide-, peptide histidine-isoleucine- and calcitonin gene-related peptide-like immunoreactivities have been studied both by immunohistochemistry and radioimmunoassay. Galanin messenger ribonucleic acid has also been studied by in situ hybridization in the dorsal root ganglia of normal and lesioned animals. In addition, a group of animals with a sciatic nerve crush was studied to compare possible differences in peptide expression after both types of lesions. The results show that the transection induces an increase in the number of cell bodies expressing galanin-like immunoreactivity in the ganglia, and that the galanin levels rise about 120-fold after three and 14 days of survival. This increase reflected increased synthesis of the peptide, since there was a rise in the galanin messenger ribonucleic acid already at 24 h post-lesion, which was maintained for at least 60 days. In the spinal cord there was an increase of staining in the midportion of the outer layers of the dorsal horn that corresponded to fibers thought to arise from cells of the dorsal root ganglia affected by the transection. Also a depletion of substance P-like and an increase in vasoactive intestinal polypeptide- and peptide histidine-isoleucine-like immunoreactivities in the dorsal root ganglia were confirmed. These changes were shown to be rapidly detectable and were paralleled by similar changes in the dorsal horn of the spinal cord. For calcitonin gene-related peptide the immunohistochemistry was inconclusive, and the radioimmunoassay showed no detectable changes. After nerve crush a transient increase in the number of galanin immunoreactive neurons was observed, as well as a decrease in the number of neurons showing substance P-like immunoreactivity. These changes were most noticeable between six and 14 days of survival. After this, peptide expression seemed to return slowly to normal, that is by day 45 post-crush only a few cells showed galanin-like, and many sensory neurons expressed substance P-like immunoreactivity. The results demonstrate that when primary sensory neurons are peripherally lesioned they respond in a complex manner, altering their normal production of peptides by increasing or decreasing their synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperalgesia, anxiety, and decreased hypoxic neuroprotection in mice lacking the adenosine A1 receptor

Caffeine is believed to act by blocking adenosine A1 and A2A receptors (A1R, A2AR), indicating that some A1 receptors are tonically activated. We generated mice with a targeted disruption of the second coding exon of the A1R (A1R−/−). These animals bred and gained weight normally and had a normal heart rate, blood pressure, and body temperature. In most behavioral tests they were similar to A1R+/+ mice, but A1R−/− mice showed signs of increased anxiety. Electrophysiological recordings from hippocampal slices revealed that both adenosine-mediated inhibition and theophylline-mediated augmentation of excitatory glutamatergic neurotransmission were abolished in A1R−/− mice. In A1R+/− mice the potency of adenosine was halved, as was the number of A1R. In A1R−/− mice, the analgesic effect of intrathecal adenosine was lost, and thermal hyperalgesia was observed, but the analgesic effect of morphine was intact. The decrease in neuronal activity upon hypoxia was reduced both in hippocampal slices and in brainstem, and functional recovery after hypoxia was attenuated. Thus A1Rs do not play an essential role during development, and although they significantly influence synaptic activity, they play a nonessential role in normal physiology. However, under pathophysiological conditions, including noxious stimulation and oxygen deficiency, they are important.

Sex differences in pain perception

BACKGROUND A number of studies have demonstrated a higher prevalence of chronic pain states and greater pain sensitivity among women compared with men. Pain sensitivity is thought to be mediated by sociocultural, psychological, and biological factors. OBJECTIVE This article reviews laboratory studies that provide evidence of sex differences in pain sensitivity and the response to analgesic drugs in animals and humans. The biological basis of such differences is emphasized. METHODS The literature from this relatively new field was surveyed, and studies that clearly illustrate the differences in pain mechanisms between the sexes are presented. Using the search terms sex, gender, and pain, a review was conducted of English-language literature published on MEDLINE between January 1980 and August 2004. RESULTS Although differences in pain sensitivity between women and men are partly attributable to social conditioning and to psychosocial factors, many laboratory studies of humans have described sex differences in sensitivity to noxious stimuli, suggesting that biological mechanisms underlie such differences. In addition, sex hormones influence pain sensitivity; pain threshold and pain tolerance in women vary with the stage of the menstrual cycle. Imaging studies of the brain have shown differences between men and women in the spatial pattern and intensity of response to acute pain. Among rodents, females are more sensitive than males to noxious stimuli and have lower levels of stress-induced analgesia. Male rodents generally have stronger analgesic response to mu-opioid receptor agonists than females. Research on transgenic mice suggests that normal males have a higher level of activity in the endogenous analgesic system compared with normal females, and a human study has found that mu-receptors in the healthy female brain are activated differently from those in the healthy male brain. The response to kappa-opioids, which is mediated by the melanocortin-1 receptor gene in both mice and humans, is also different for each sex. CONCLUSION Continued research at the genetic and receptor levels may support the need to develop gender-specific drug therapies.

Substance P and calcitonin gene-related peptide synergistically modulate the gain of the nociceptive flexor withdrawal reflex in the rat

A comparison has been made between the effects of intrathecally administered L-glutamate, L-aspartate, substance P and calcitonin gene-related peptide (CGRP) on the excitability of rat flexor alpha-motoneurons activated monosynaptically by Ia afferents and polysynaptically by high mechano-threshold cutaneous afferents. At doses that do not modify the monosynaptic reflex, substance P, CGRP and some factor released from sural C-fibres increase the excitability of the nociceptive flexion reflex for prolonged periods in a multiplicative fashion. The excitatory amino acids have no such action. We suggest that one role for C-afferent neuropeptides is a long-acting gain modulation of nociceptive inputs into the spinal cord.

The systemic administration of local anaesthetics produces a selective depression of C-afferent fibre evoked activity in the spinal cord.

&NA; An electrophysiological analysis of the antinociceptive effects of systemic lidocaine and its longer acting primary amine congener, tocainide, has been performed in the decerebrate‐spinal unanaesthetised rat. Neither of these local anaesthetic drugs, when administered systemically in doses of up to 10 mg/kg (lidocaine) or 100 mg/kg (tocainide), produced any evidence of a block in the conduction of action potentials in A&bgr;, A&dgr; or C primary afferents. The local anaesthetics also failed to reduce mustard oil induced neurogenic extravasation, a test of cutaneous C‐fibre terminal function. Lidocaine produced a transient (1–2 min) depression in monosynaptic reflexes at doses of ≥ 1 mg/kg while tocainide had no effect on this reflex at any dose up to a 100 mg/kg. Both drugs, however, significantly suppressed the C‐fibre evoked polysynaptic reflex generated by stimulating the sural nerve. The tocainide effect was longer lasting with less action on the short latency A&bgr;‐evoked reflex than lidocaine. The reflex activity in hamstring flexor &agr;‐motoneurones evoked by pinching the toes of the ipsilateral hind paw was reduced by both drugs but not abolished. Thermal and noxious chemical evoked reflexes were, however, completely suppressed by the local anaesthetic drugs, again with a longer action from tocainide. These results demonstrate that the systemic administration of drugs which increase the inactivation of sodium channels can produce a selective central block of certain types of afferent evoked activity in the spinal cord. There are resemblances between the selective C‐fibre suppressing actions of systemically administered local anaesthetics and the pharmacological actions of narcotic opiates which may represent a similar mechanism for the analgesic action of these quite different classes of drugs.

Reduced antinociception and plasma extravasation in mice lacking a neuropeptide Y receptor

Neuropeptide Y (NPY) is believed to exert antinociceptive actions by inhibiting the release of substance P and other ‘pain neurotransmitters’ in the spinal cord dorsal horn. However, the physiological significance and potential therapeutic value of NPY remain obscure. It is also unclear which receptor subtype(s) are involved. To identify a possible physiological role for the NPY Y1 receptor in pain transmission, we generated NPY Y1 receptor null mutant (Y1-/-) mice by homologous recombination techniques. Here we show that Y1-/- mice develop hyperalgesia to acute thermal, cutaneous and visceral chemical pain, and exhibit mechanical hypersensitivity. Neuropathic pain is increased, and the mice show a complete absence of the pharmacological analgesic effects of NPY. In the periphery, Y1 receptor activation is sufficient and required for substance P release and the subsequent development of neurogenic inflammation and plasma leakage. We conclude that the Y1 receptor is required for central physiological and pharmacological NPY-induced analgesia and that its activation is both sufficient and required for the release of substance P and initiation of neurogenic inflammation.

Further studies on galanin-, substance P-, and CGRP-like immunoreactivities in primary sensory neurons and spinal cord: Effects of dorsal rhizotomies and sciatic nerve lesions

The peptides galanin (GAL), substance P (SP), and calcitonin gene-related peptide (CGRP) were analyzed with immunohistochemistry and radioimmunoassay in the spinal cord, dorsal root ganglia, dorsal roots, and sciatic nerve of normal rats and rats subjected to several experimental procedures, including ligation, crush, and/or sectioning of nerves. The results show that peripheral nerve transection induces a dramatic increase in GAL content both in dorsal roots and sciatic nerve, demonstrating that this lesion causes an increased out-transport of the newly synthesized peptide both into the central and peripheral branches of the primary sensory neurons. In contrast evidence was obtained for decreased out-transport of SP and CGRP. The functional significance of these findings remains to be analyzed.

Substance P and somatostatin modulate spinal cord excitability via physiologically different sensory pathways

The effect of intrathecally injected substance P and somatostatin on spinal flexion reflex excitability was examined in decerebrate, spinalized, unanaesthetized rats. Substance P increased the excitability of the spinal cord to mechanical and thermal stimuli suprathreshold for C-afferents. Somatostatin had a similar effect with thermal, but not with mechanical stimuli. It is suggested that both peptides are released in association with C-afferent activation. Substance P may be released by polymodal nociceptors whereas somatostatin may be released by thermosensitive C-afferents.

Combined opioid-NMDA antagonist therapies. What advantages do they offer for the control of pain syndromes?

SummaryThe potential beneficial effect of coadministration of opiates with antagonists of the N-methyl-D-aspartate (NMDA) receptor for glutamate are discussed. There is a growing body of experimental data indicating that selective NMDA antagonists acting at various receptor sites and clinically available drugs with some affinity for the NMDA receptor potentiate the analgesic effect of opiates and may block or reduce the development of tolerance following long term opiate administration. In the majority of clinical studies addressing this issue, ketamine, which has NMDA antagonist properties, has been found to reduce the need for opiates for the treatment of severe pain conditions. Thus, the combination of clinically available NMDA antagonists with opiates may be of benefit in treating a number of painful conditions.