Encoding of noxious heat messages in neurons of the ventrobasal thalamic complex of the rat

doi:10.1016/0006-8993(80)91125-7

Brain Research

Volume 197, Issue 2, 22 September 1980, Pages 401-413

https://doi.org/10.1016/0006-8993(80)91125-7 Get rights and content

Abstract

Ventrobasal thalamic neurons responsive to noxious mechanical stimuli were tested with noxious heat stimuli graded in temperature, surface-area and duration. Experiments were performed by plunging the tail of intact, lightly anesthetized rats into a temperature-controlled water bath.

Seventeen of 24 neurons encoded stimulus temperature by frequency of discharge although the responses of 6 of these 17 reached a plateau at the highest temperatures. Alternatively, the cell population might code stimulus temperature by a recruiting mechanism since response thresholds were distributed between 40 and 50°C.

Some units also increased their discharge in parallel with an increase in stimulus area and/or duration. Analyses of discharge patterns were performed. A decrease of discharge frequency during stimulation was not observed before several tens of seconds had elapsed. Thus, the average response of the cell population to 15, 30 and 60 sec stimuli showed no clear ‘adaptation’.

In our conditions, i.e. with most of the stimulations limited to a duration of 15 sec each, sensitization to heat was observed after 55 and 60°C, but not after 50°C.

These data indicate that noxious heat stimulus parameters are coded at the thalamic level in the rat by both an increase in discharge and a progressive recruitment of units.

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Neurones were identified on the basis of their stereotaxic location (AP +5.0 mm from lambda, Lateral 2.9 mm from midline, Depth 4.6–5.2 mm from surface) and their responses to somatosensory (nociceptive and non-nociceptive) stimuli, as described previously (Guilbaud et al., 1980; Peschanski et al., 1980, 1983; Salt and Binns, 2000). Nociceptive responses were evoked by immersion of part of either the contralateral hindpaw or the tail in water of 52C for 20 s. Responses to such stimuli were typically increases in action potential firing during the course of the stimulus and outlasting the stimulus by up to 2 min, as described previously (Peschanski et al., 1980). Similar response profiles were observed irrespective of the recording electrode type or the type of craniotomy preparation.
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The percentage of heat-responsive neurons under anesthesia is 20% in the raccoon VP55 and 32% in cat PO.56 Approximately 70–75% of mechanical nociceptive neurons are heat-responsive in rats57 and monkeys.58 Most lateral thalamic neurons can code temperatures from 43°C to 53°C,55–57 but a smaller part of these VP neurons have shown plateau responses from 50°C to 53°C.57,58
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Inhibition of pain behavior by GABA<inf>B</inf> receptors in the thalamic ventrobasal complex: Effect on normal rats subjected to the formalin test of nociception
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Citation Excerpt :
The VB receives projections from the spinothalamic tract, which neurons respond to noxious input exclusively (nociceptive specific neurons) or not (nociceptive non-specific or wide dynamic range receptive neurons; Guilbaud et al., 1994). Various studies demonstrated that numerous neurons in the VB are activated by noxious somatic stimuli and suggested that these nuclei participate in the sensory-discriminative aspect of pain (Kenshalo et al., 1980; Peschanki et al., 1980; Chung et al., 1986; Guilbaud and Kayser, 1987; Guilbaud et al., 1994). Additionally, VB neurons from animals with a chronic inflammatory or neuropathic pain condition, display significantly enhanced responses to mechanical and thermal stimulation (Guilbaud and Kayser, 1987; Guilbaud, 1991; Guilbaud et al., 1990, 1992, 1994).
The ventrobasal complex of the thalamus (VB) participates in the transmission and modulation of noxious information. Recent data suggested that GABA_B receptors in the VB might be involved in the modulation of neuronal activity in response to chronic noxious input. However, in acute inflammatory pain, the role of GABA_B receptors in the VB remains unknown. The formalin test of nociception was performed in rats stereotaxically injected in the VB contralateral to the formalin-injected paw, with saline (controls), baclofen (0.5 and 0.875 μg), a specific GABA_B receptor agonist or CGP35348 (25 μg), a GABA_B receptor antagonist. Control animals exhibited phase 1 (acute pain) and phase 2 (tonic pain) nociception-related activities as previously described. The higher dose of baclofen induced a significant decrease of all pain-related behaviors in both phases of the test and had no observable effects on the animals' motor function, while the lower dose could not reduce the total pain-related activities. Injection of CGP35348 prior to baclofen reduced the antinociceptive effect caused by baclofen during phase 2 in the paw-jerks and in total pain-related activities. CGP35348 alone had antinociceptive effects in both phases, though less pronounced than baclofen 0.875 μg in the total pain-related activities during phase 2. Data demonstrate that both the blockade and the activation of GABA_B receptors in the VB of rats induce antinociception in acute and tonic pain. An important role for GABA_B receptors on the thalamic processing of nociceptive input in the VB is suggested.

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^*: Chercheur I.N.S.E.R.M.

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Encoding of noxious heat messages in neurons of the ventrobasal thalamic complex of the rat

Abstract

Brain Research

Brain Research

Pain

Pain

Brain Research

Pain

Response of unmyelinated (C) polymodal nociceptors to thermal stimuli applied to monkey's face

J. Neurophysiol.

Responses of cutaneous sensory units with unmyelinated fibers to noxious stimuli

J. Neurophysiol.

Sensitization of myelinated nociceptive afterents that innervate monkey hand

J. Neurophysiol.

The termal sensitivity of the polymodal nociceptors in the monkey

J. Physiol. (Lond.)

The sensitization of high threshold mechanoreceptors with myelinated axons by repeated heating

J. Physiol. (Lond.)

Mesure du renouvellement du lactate chez le rat par perfusion de14CO-(l)-lactate+

Pflu¨gers Arch. ges. Physiol.

Adaptation of thermal pain in the skin

J. appl. Physiol.

Neurones responding to noxious stimulation in VB complex and PO like group in the thalamus of the rat

Neurosci. Lett.

Mesure du renouvellement du lactate chez le rat par perfusion de¹⁴CO-(l)-lactate⁺