Effects of neonatal C-fiber depletion on discrimination of principal and adjacent whisker stimulation within rat individual cortical barrels

Role of Capsaicin in a Murine Model of Labor and Delivery

Background:

The objectives of this study were to develop a murine model of labor and delivery and to use this model to examine whether capsaicin diminishes labor pain and expedites delivery.

Methods:

To develop a murine model of labor pain, the authors identified and compared the incidence of four proposed pain behaviors in 46 mice: (1) No analgesia in labor and the postpartum period, and (2) increasing doses of an analgesic, morphine. The model was then used to examine the impact of topical cervical capsaicin on: (1) labor pain behaviors and (2) labor progress by examining its impact on the time from treatment to delivery of the first pup and on the duration of delivery per pup. The treatment was randomly allocated and the behavioral observation was blinded.

Results:

In the absence of analgesia, there was a statistically significant decrease in all four proposed pain behaviors in the postpartum period compared with labor (cumulative 55.0 ± 16.1/h vs. 16.1 ± 8.7/h; P < 0.0001). Additionally, morphine reduced their incidence during labor in a dose-dependent manner (cumulative 55.0 ± 16.1.7/h control, 46.4 ± 15.8 morphine 0.1 mg/kg/h, 34.6 ± 5.6/h, morphine 0.5 mg/kg/h; P = 0.1988, 0.0014). In addition, the incidence of identified pain behaviors was reduced by pericervical capsaicin (cumulative 55.0 ± 16.1.7/h control, 38.9 ± 15.4 capsaicin, P = 0.02).

Conclusions:

In this pilot study, the authors developed a novel mouse model of labor and delivery. Pericervical capsaicin applied days before delivery reduces labor pain behaviors.

Does reorganization in the cuneate nucleus following neonatal forelimb amputation influence development of anomalous circuits within the somatosensory cortex?

Neonatal forelimb amputation in rats produces sprouting of sciatic nerve afferent fibers into the cuneate nucleus (CN) and results in 40% of individual CN neurons expressing both forelimb-stump and hindlimb receptive fields. The forelimb-stump region of primary somatosensory cortex (S-I) of these rats contains neurons in layer IV that express both stump and hindlimb receptive fields. However, the source of the aberrant input is the S-I hindlimb region conveyed to the S-I forelimb-stump region via intracortical projections. Although the reorganization in S-I reflects changes in cortical circuitry, it is possible that these in turn are dependent on the CN reorganization. The present study was designed to directly test whether the sprouting of sciatic afferents into the CN is required for expression of the hindlimb inputs in the S-I forelimb-stump field. To inhibit sprouting, neurotrophin-3 (NT-3) was applied to the cut nerves following amputation. At P60 or older, NT-3-treated rats showed minimal sciatic nerve fibers in the CN. Multiunit electrophysiological recordings in the CN of NT-3-treated, amputated rats revealed 6.3% of sites were both stump/hindlimb responsive, compared with 30.5% in saline-treated amputated animals. Evaluation of the S-I following GABA receptor blockade, revealed that the percentage of hindlimb responsive sites in the stump representation of the NT-3-treated rats (34.2%) was not significantly different from that in saline-treated rats (31.5%). These results indicate that brain stem reorganization in the form of sprouting of sciatic afferents into the CN is not necessary for development of anomalous hindlimb receptive fields within the S-I forelimb/stump region.

Effects of neonatal C-fiber depletion on neocortical long-term potentiation and depression

Capsaicin (Cap)-induced depletion of C-fiber afferents results in plasticity of somatosensory system which is manifested as a functional alteration at different levels of the somatosensory pathway. In the present study we examined the effect of Cap-induced neonatal depletion of C-fibers on the induction of long-term potentiation (LTP) and long-term depression (LTD) in the neocortex of freely moving rats. A stimulating electrode was implanted into corpus callosum and a recording electrode was implanted in the somatosensory cortex of control (Con: normal, without electrical stimulation), trained (normal, with electrical stimulation) and Cap-treated (C-fiber depleted, with electrical stimulation) adult rats. Two weeks after the surgery, evoked field potential responses were recorded before, during (12 days) and after (1 month) the induction period of LTP and LTD. The LTP and LTD response characteristics during the time course of recording were compared between different groups. In the train group, LTP and LTD appeared after 3 days of stimulation. LTP magnitude peaked after about 6 days while LTD magnitude peaked in about 12 days. C-fiber depletion postponed the development of LTP and LTD making the highest differential levels of LTP about 6 days after the initiation of LTP induction. The impact of C-fiber depletion on slowing the time course of LTD induction was more prolonged and lasted until day 12 of the initiation of LTD induction. These results suggest that intact C-fibers are necessary for normal plasticity and long-term synaptic modification of the somatosensory system.

Effects of GABAA receptor inhibition on response properties of barrel cortical neurons in C-Fiber-depleted rats

C-fiber depletion results in expansion of low threshold somatosensory mechanoreceptive fields. In this study, we investigated the role of intact C-fibers in GABAA-mediated inhibition in barrel cortical neurons. We used electronically controlled mechanical stimulation of whiskers to quantitatively examine the responses of barrel cells to whisker displacements. After systemic injection of picrotoxin neuronal responses were recorded at 5 min intervals for 20 min and then at 10 min intervals for 100 min. Picrotoxin injection caused a 3-fold increase in response magnitude of adjacent whisker stimulation and 1.4-fold increase in response magnitude of principal whisker stimulation with a maximum enhancement 50 min after the injection. There was no significant change in spontaneous activity following picrotoxin injection. The response enhancement and receptive field expansion observed in normal rats were completely absent in the C-fiber-depleted rats. These results suggest that the GABAA-mediated inhibition that modulates the receptive field functional organization of the barrel cortex depends on intact C-fibers.