Effects of baclofen and phaclofen on receptive field properties of rat whisker barrel neurons

Extracellular single-unit recordings were made in somatosensory cortical barrels of fentanyl-sedated rats. Whiskers were deflected singly or in paired combinations. lontophoretically-applied (−)-baclofen disproportionately reduced weak responses, and phaclofen disproportionately increased them, resulting in more tightly focused or more broadly focused receptive fields, respectively. Both drugs had only minor effects on surround inhibition. In light of previous findings, we conclude that GABA_A and GABA_B mechanisms both act to enhance spatial contrast, but that the former plays a much greater role in enhancing temporal resolution.     

脊髓手术中体感诱发电位监测的临床研究

报告３０例脊髓手术中体感诱发电位（ＳＥＰ）连续监测的临床研究结果。ＳＥＰ记录包括脊髓ＳＥＰ（ＳＳＥＰ）和皮层ＳＥＰ（ＳＣＥＰ）。麻醉药物对ＳＥＰ有一定程度影响，ＳＣＥＰ相对较大。认为，至少术中ＳＥＰ潜伏期延长不超过８％和波幅降低不超过５０％，不会引起术后并发症，超过该范围由于病例较少，尚不能肯定与预后的关系，需进一步探讨。     

Differential modulation of the short- and long-latency somatosensory evoked potentials in a forewarned reaction time task.

OBJECTIVE: We investigated modulation of the short- and long-latency somatosensory evoked potentials (SEPs) in a forewarned reaction time task. METHODS: A pair of warning (auditory) and imperative stimuli (somatosensory) was presented with a 2 s interstimulus interval. In movement condition, subjects responded by grip movement with the ipsilateral hand to the somatosensory stimulation when the imperative stimulus was presented. In counting condition, they silently counted the number of imperative stimuli. The SEPs in response to the imperative stimuli were recorded. RESULTS: Frontal N30 and central N60 amplitudes were significantly smaller in the movement than in the counting or rest conditions. None of the short-latency components differed between the counting and rest conditions. In contrast to the short-latency components, P80 was significantly larger in the counting than in the rest condition, and showed a further increase from the counting to the movement condition. The N140 amplitude was significantly larger in the movement than the rest condition, but was not changed between the counting and the rest conditions. CONCLUSIONS: The attenuation of the frontal N30 and central N60, and the enhancement of the P80 and possibly the N140 resulted from the centrifugal mechanism. The present findings may show the different effects of voluntary movement on the early and subsequent cortical processing of the relevant somatosensory information requiring a behavioral response. SIGNIFICANCE: The present study demonstrated the differential modulation of short- and long-latency components of SEPs in a forewarned reaction time task.     

Neuroplastic effects of neonatal capsaicin on barrel cortex of adult rat. An electrophysiological and autoradiographic study

Capsaicin (50 mg/kg s.c./25 μl) was administered to rats on the 2nd and 5th days after birth. The animals were raised, and from the age of 3 months the properties of the evoked activity were tested in the contralateral barrelfield. This neonatal capsaicin treatment was found to induce profound changes in the responsiveness of the barrel cortex in the adult rats: (1) the receptive field of the neurons in the Cl barrel was expanded; units within a particular barrel were driven by a significantly larger number of vibrissae than in the controls. (2) The rate of discharge evoked by the related vibrissa deflection was enhanced, while (3) the angular sensitivity of the neurons was decreased. (4) The most prominent change in cortical activity was observed by autoradiography: capsaicin-treated rats exhibited an enhanced labelling of different types of neurons throughout the hemisphere (surpassing the cortical representation of stimulated vibrissae). The present observations indicate that neonatal capsaicin affects the functional activity of the rat somatosensory cortex. It is suggested that unmyelinated sensory afferents play a role in the development of the rat somatosensory system.     

Parvalbumin expression reveals a vibrissa-related pattern in rabbit SI cortex

The expression of parvalbumin-like immunoreactivity (PV-LIR) was examined in the mystacial representation within the primary somatosensory cortex (SI) of postnatal day 21 and adult rabbits. PV-LIR was expressed in a prominent vibrissa-like array of patches in layer IV despite the fact that barrels were indistinct in the cytoarchitecture. Each patch consisted of dense terminal-like PV-LIR and a preferential concentration of intensely labeled stellate neurons. Layer V contained scattered small and large intensely labeled basket cells. Layer Vb had a distinct layer of lightly labeled large pyramidal cells that received labeled basket cell terminations. Upper layer VI also contained patches of terminal-like PV-LIR that were in register with the overlying vibrissae pattern. These patches also contained a preferential distribution of labeled non-pyramidal cells as well as modified pyramidal cells. These results suggest that PV-LIR in rabbits delineates cortical modules composed of thalamorcotical afferents and inhibitory local circuits in the absence of a distinct barrel cytoarchitecure. In contrast, prior studies of rat SI cortex have revealed a distinct barrel cytoarchitecture but a uniform distribution of PV-LIR. The differences in PV-LIR between rodents and lagomorphs within the vibrissae representation in SI may be related to species differences in thalamic and local cortical circuits devoted to the whisker sense.     

Changes in somatosensory evoked responses by repetition of the median nerve stimulation

OBJECTIVE: We investigate the synaptic factor for the recovery function of evoked responses using a repetitive stimulation technique. METHODS: Somatosensory evoked cortical magnetic field (SEF) was recorded following stimulation of the median nerve using single to 6-train stimulation in 8 healthy subjects. The SEF responses after each stimulus in the train stimulation were extracted by subtraction of the waveforms. RESULTS: An attenuation of the SEF components was recognized after the second of the stimuli, but there was no significant attenuation with the third or later stimulations. The root mean square (RMS) of the 1M (peak latency at 20 ms after stimulation) and 4M (70 ms) components were smaller than that of the single stimulation during the train stimulation, while the 2M (30 ms) and 3M (45 ms) components were not attenuated, but the 3M was facilitated at the fourth to sixth stimulation. CONCLUSION: The synaptic factor was not responsible for the attenuation of the SEF components during repetitive stimulation in healthy subjects. The SEF change disclosed a functional difference among the SEF components during the train stimulation, especially among the later components.     

A slim needle-shaped multiwire microelectrode for intracerebral recording

The construction of a needle-shaped multiwire microelectrode is described. It can be made with simple mechanical tools. The presented electrode assembly consists of 12 insulated nichrome wires (core diameter 25 μm) which are embedded in epoxylite resin. The straight-cut wire tips are aligned lengthwise and have a relative spacing of 150 μm. Outer dimensions vary from 100 × 180 μm at the level of the 1st electrode channel, to 100 × 100 μm at the level of the 12th channel at the tip. The configuration of this electrode was determined by its application: the laminar analysis of evoked potentials in the cortex of the rat. However, the number of channels, the diameter of the (nichrome) wire which determines the surface area of these channels, and the channel spacing can be easily adjusted during construction to meet other experimental requirements, such as the recording of single-unit activity. The electrode which is composed of biocompatible materials is suited for the study of field potentials and multiple-unit activity, in both acute and chronic experiments, and can be used repeatedly. To demonstrate the performance of the electrode assembly, a depth profile of field potentials is presented, accompanied by the corresponding current source density distribution. The potentials were recorded in the somatosensory cortex of the rat following stimulation of the median nerve under ketamine anesthesia.     

Spinothalamic and propriospinal neurones in the upper cervical cord of the rat: terminations of primary afferent fibres on soma and primary dendrites

Summary Experiments were performed on rats to determine whether primary afferents from the upper cervical region terminate directly on Spinothalamic and propriospinal neurones. The central terminations of primary afferents from the upper cervical region were identified by diffusely filling their axons with horseradish peroxidase. Spinothalamic neurones or propriospinal neurones were identified in the same experimental animals by using retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. Approximately 3–11 % of Spinothalamic cells in laminae 4–6 of spinal segments C2–4 received apparent synaptic contacts from primary afferents on the soma or primary dendrites. Approximately 18–36% of propriospinal neurones with axons descending to lower thoracic or lumbar levels received apparent synaptic contacts on the soma or primary dendrites. These data provide anatomical evidence that Spinothalamic and long propriospinal neurones in the upper cervical cord are excited directly by primary afferents. The data also help to clarify the neural circuitry underlying somatic sensation and reflex movements evoked by neck receptors.     

Patterns of maturation of somatotopical distribution of corticospinal neurons in postnatal rats

Summary To better comprehend somatotopic development of the corticospinal projection system, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into cervical or lumbar enlargement of the spinal cord of postnatal rats. The cervical projecting neurons appeared first in the middle of the lateral surface of the posterior frontal and anterior parietal cortex on the second postnatal day (postnatal day one, P1). By P3, labeled neurons were distributed in the rostral two-thirds of the cortex, with concentrations both on the same cortex as in P1 and on the dorsomedial part of the frontal cortex. Size of the labeled area was gradually reduced between P3 and P12 to attain an adult organization: three discrete clusters of labeled neurons were isolated in the dorsomedial part of the frontal cortex, the middle of the lateral surface of the posterior frontal and anterior parietal cortex, and in the temporal cortex. The lumbar projecting neurons first appeared in the dorsomedial part of the parietal cortex, on P4. Size of the lumbar projecting area increased by P6 and decreased by P12 to attain the pattern seen in adult animals.Our findings suggest that 1) potential pioneer fibers reaching the cervical enlargement originate from the middle of the lateral surface of the posterior frontal and anterior parietal cortex, and those reaching the lumbar enlargement, from the dorsomedial part of the parietal cortex, and 2) transiently projecting areas consist of less densely distributed neurons compared to the areas destined to become the corticospinal projecting area in adult.     

Dipolar sources of the early scalp somatosensory evoked potentials to upper limb stimulation Effect of increasing stimulus rates

Brain electrical source analysis (BESA) of the scalp electroencephalographic activity is well adapted to distinguish neighbouring cerebral generators precisely. Therefore, we performed dipolar source modelling in scalp medium nerve somatosensory evoked potentials (SEPs) recorded at 1.5-Hz stimulation rate, where all the early components should be identifiable. We built a four-dipole model, which was issued from the grand average, and applied it also to recordings from single individuals. Our model included a dipole at the base of the skull and three other perirolandic dipoles. The first of the latter dipoles was tangentially oriented and was active at the same latencies as the N20/P20 potential and, with opposite polarity, the P24/N24 response. The second perirolandic dipole showed an initial peak of activity slightly earlier than that of the N20/P20 dipolar source and, later, it was active at the same latency as the central P22 potential. Lastly, the third perirolandic dipole exaplaining the fronto-central N30 potential scalp distribution was constantly more posterior than the first one. In order to evaluate the effect of an increasing repetition frequency on the activity of SEP dipolar sources, we applied the model built from 1.5-Hz SEPs to traces recorded at 3-Hz and 10-Hz repetition rates. We found that the 10-Hz stimulus frequency reduced selectively the later of the two activity phases of the first perirolandic dipole. The decrement in strength of this dipolar source can be explained if we assume that: (a) the later activity of the first perirolandic dipole can represent the inhibitory phase of a “primary response”; (b) two different clusters of cells generate the opposite activities of the tangential perirolandic dipole. An additional finding in our model was that two different perirolandic dipoles contribute to the centro-parietal N20 potential generation. Received: 5 August 1997 / Accepted: 26 November 1997