Takashi A. Suzuki

Effect of dopamine and haloperidol on the c-wave and light peak of light-induced retinal responses in chick eye.

The relation between dopaminergic cells (and centrifugal fibers), the electroretinogram (ERG) c-wave, and the light peak were electrophysiologically investigated by observing the effects of a retrobulbar conduction block and intravitreal injection of either dopamine or haloperidol on these retinal responses. The retrobulbar conduction block (1% lidocaine) caused a decrease in the amplitude of the c-wave and the light peak in newly hatched chicks. Injections (2–20 μl) containing dopamine (0.1–10 mM) or haloperidol (1.3–13 mM) were given intravitreously while the responses were recorded. Although intravitreous injection of saline for control resulted in no observable change in the responses, dopamine selectively augmented the c-wave of ERGs and the light peak, but not the a-, b-, and d-waves. Haloperidol decreased first the light peak and later the c-wave. The augmentation of the retinal responses by dopamine and their reduction by haloperidol was statistically significant. The estimated threshold concentration of dopamine in the vitreous cavity was 1–3.5 μM.Since in many species the interplexiform cells have been found to contain dopamine, we hypothesize that the modulatory effects on the c-wave and the light peak in this preparation may be due to a centrifugal feed-back loop which includes the interplexiform cells to the horizontal and bipolar cells and the horizontal cells to the cones.

Physiological properties of tooth pulp-driven neurons in the first somatosensory cortex (SI) of the cat.

&NA; Tooth pulp‐driven (TPD) neurons are found in the oral area of the first somatosensory cortex (SI) of the cat. They have been classified according to their discharge patterns in response to electrical stimulation of the tooth pulp: the fast (F) type, slow (S) type, and a fast (Fa) type accompanied by afterdischarges. The characteristics of each type of TPD neuron were investigated in cats anesthetized with nitrous oxide and halothane. In surface distribution, there was no biased localization for any of the types. However, F‐type neurons receiving input from the mandibular tooth tended to be found more medially than F‐types receiving maxillary input. These TPD neurons did not change their firing pattern even when the stimulus was intensified. Mean threshold of the F‐type to tooth pulp stimulation was 7.8 ± 1.6V and tended to be lower than that of the S‐type (16.3 ± 3.0V). Graded increases in tooth pulp stimulation produced progressive increases in discharge frequency of both types of neurons. An analysis of the power function in relation to stimulus vs. response demonstrated that the exponent of the S‐type neuron was about 2.0, being significantly larger compared to the 0.8 value for the F‐type. The mean number of pulps afferent to an F‐type was 1.6, compared to 4.8 for an S‐type or 4.3 for an Fa‐type. The results suggest that F‐type TPD neurons may play a more important part in localizing pulpal pain and in recognizing the intensity than the other types.

Characteristics of the tooth pulp-driven neurons in a functional column of the cat's somatosensory cortex (SI)

SummaryThe columnar arrangement of tooth pulp-driven (TPD) neurons in the first somatosensory cortex (SI) was studied by single unit analysis in the cat anesthetized with nitrous oxide and halo thane. Tactile sensitive neurons in the oral area of SI were arranged functionally in a columnar organization. About 32% of the recorded neurons also responded to tooth pulp stimulation. Thus, the TPD neurons located in area 3b seem to be multireceptive in sensory modality. About 80% of the TPD neurons were F-type which respond with a short latency to tooth pulp stimulation and receive input from a small number of teeth. When a microelectrode penetrated through a single vertical column, most of the F-type TPD neurons encountered received common input from one or two tooth pulps. However, when an electrode was inserted across more than one column, there was no common pulpal input to the TPD neurons encountered in a track. Thus, the TPD neurons in a single column had identical peripheral and pulpal receptive fields, and the same latency of responses to tooth pulp stimulation. These characteristics would support the hypothesis that TPD (presumed nociceptive) neurons are arranged in a vertical column and it may be inferred that the columnar organization of TPD neurons in SI plays an important role in the sensory-discriminative aspects of pulpal pain.

Morphine selectively suppresses the slow response of tooth pulp-driven neurons in first somatosensory cortex (SI) of the cat

Tooth pulp-driven (TPD) neurons are found in the oral area of the first somatosensory cortex (SI) and they have been classified into fast (F-) type, slow (S-) type and fast type accompanied by afterdischarges (Fa). Effects of morphine on single unit responses of the F- and S-type neurons to pulp stimulation were examined by recording the discharges from a single neuron using a microelectrode. Intraperitoneal administration of morphine at 1.5 mg/kg markedly suppressed the response of S-type TPD neurons that fire with a long latency. This effect was completely reversed by naloxone at 0.2 mg/kg. On the other hand, the discharges of F-type neurons that fire with a short latency were not affected by application of morphine at 3 mg/kg. These findings suggest that S-type neurons in SI are concerned with pain perception.

Electrical stimulation of tooth pulp increases the expression of c-fos in the cat supraoptic nucleus but not in the paraventricular nucleus

Immunoreactivity to Fos protein was detected in the supraoptic (SON) and para-ventricular (PVN) nuclei of the cat using immunohistochemical methods. In the intact animal group, only a few Fos-positive neurons were observed in the PVN, but the SON did not contain any positive neurons. Intraperitoneal injection of pentobarbital sodium (Nembutal: 35 mg/kg) induced c-fos expression in the SON, but not in the PVN. Electrical stimulation of tooth pulp with an intensity that was 3 times the threshold of the jaw-opening reflex (200-600 microA) increased the number of Fos-positive neurons in the SON by up to 388% as compared with those of the Nembutal group, whereas the stimulation did not alter the number in the PVN. The increase was observed throughout the extent of the SON. In addition, morphine treatment (2 mg/kg, i. p.), 5 minutes before tooth pulp stimulation, considerably inhibited the increase in the SON. There were no significant differences among the 3 groups (intact, Nembutal, morphine) in the number of positive neurons in the PVN. These findings suggest that these hypothalamic nuclei have different functional roles and that the SON is involved in nociception and/or the consequent emotional and visceral reactions.

Characteristics of tooth pulp-driven neurons in the posterior group of the cat thalamus

This investigation was designed to determine the responses of neurons in the posterior group of nuclei (PO) to tooth pulp stimulation. Eighteen tooth pulp-driven (TPD) neurons were recorded in 9 cats anesthetized with nitrous oxide and halothane, 14 of them in the medial part (POM) and the remainder in the lateral part (POL) of the posterior nuclei. These TPD neurons also responded to non-noxious tactile stimuli of the orofacial region of the body. Most TPD neurons responded with a short latency of less than 20 ms to tooth pulp stimulation (mean 13.5 +/- 5.9). The number of teeth having afferents to these neurons was 4-8 (mean 6.7 +/- 1.3).

Osmotic pressure of the interstitial fluid obtained from neonatal chick eyes and its variation under light and dark adaptation

In experiments by Yoneyama et al. (1985) a hypertonic perfusing solution was used to record the c-wave from the isolated chick retina. We investigated whether the chick retina was surrounded by such hypertonic liquids. The following results were found: (1) the osmolarity of the subchoroidal humor was markedly higher than that of fluids from other areas of the eye; (2) the relative difference in osmolarity between both sides of the retina was 110–170 mOsmol in the light-adapted condition, whereas it was about 270 mOsmol in the dark-adapted condition; and (3) in the dark-adapted state the volume of the subchoroidal humor decreased about 40% but increased in osmolarity about 30–40%, whereas osmolarity of the subviteous humor decreased about 15–20 mOsmol. Results suggest the presence of an inward-directed pump mechanism for watersoluble components in the retina in addition to the proposed outward-directed pump mechanism of the pigment epithelium. The inward-directed pump may be more active in the dark-adapted state.

The projection pathway from the tooth pulp to the ipsilateral first somatosensory cortex (SI) in the cat.

About 40 per cent of tooth pulp-driven (TPD) neurones received afferent fibres from the ipsilateral tooth pulp. This ipsilateral afferent pathway was investigated in cats anaesthetized with nitrous oxide and halothane. The subcortical temperature of the brain was lowered to about 28 degrees C by perfusion of cold water within a thermode. Cooling the homotopic area contralateral to the recording site caused little change in the firing rate of short-latency TPD neurones upon ipsilateral pulp stimulation (n = 13). Microinjection (1-2 microliters) of 1 per cent lidocaine into the ipsilateral nucleus ventralis posteromedialis (VPM) caused significant diminution in the firing rate of short-latency TPD neurones (n = 11) to ipsilateral stimulation but not of long-latency TPD neurones (n = 8). About 35 per cent (13/35) of the TPD neurones distributed in the medial shell region of the VPM proper responded with short latency to ipsilateral pulp stimulation. These findings suggest that the ipsilateral input to short-latency TPD neurones in the oral area is carried via projection fibres from the ipsilateral VPM but not via commissural ones, and that the ipsilateral input to long-latency neurones is probably relayed in a site other than the ipsilateral VPM.

Gamma-aminobutyric acid- and picrotoxin-induced changes in c-wave and light peak of retinal potentials in the chick

Retinal potentials were recorded from the eyes of anesthetized and immobilized chicks by a standard direct current method. The amplitude of the electroretinogram (ERG) c-wave was measured 2 and 5 sec after the onset of the light stimulus, as indexes of the fast-rise c-wave (cF-wave) and the slow-rise c-wave (cs-wave), respectively. An intravitreal injection of gamma-aminobutyric acid (GABA) at an estimated intravitreal concentration of 10−9-10−7M resulted in an increase of the amplitude of the cs-wave, a less remarkable change in the a- and cF-waves, and a slight decrease in the b-wave. The light peak of the retinal standing potential increased in amplitude following GABA administration (10−7-10−4M). Following an intravitreal injection of picrotoxin (10−5-10−3M), the polarity of the cs-wave changed from positive to negative and a significant decrease and deformation in the light peak was observed. The amplitude of the a-wave, however, increased in the range of the higher dose, while that of the b- and cf-waves decreased markedly but no polarity reversal of the cF-wave was found. The results may suggest that the GABA-ergic synapse plays a significant role in production of the cs-wave and the light peak, along with that of the pigment epithelium.