Differential intracellular labelling of identified neurones with two fluorescent dyes

NOP receptor mediates anti-analgesia induced by agonist-antagonist opioids

Clinical studies have shown that agonist-antagonist opioid analgesics that produce their analgesic effect via action on the kappa-opioid receptor, produce a delayed-onset anti-analgesia in men but not women, an effect blocked by co-administration of a low dose of naloxone. We now report the same time-dependent anti-analgesia and its underlying mechanism in an animal model. Using the Randall-Selitto paw-withdrawal assay in male rats, we found that nalbuphine, pentazocine, and butorphanol each produced analgesia during the first hour followed by anti-analgesia starting at ∼90min after administration in males but not females, closely mimicking its clinical effects. As observed in humans, co-administration of nalbuphine with naloxone in a dose ratio of 12.5:1 blocked anti-analgesia but not analgesia. Administration of the highly selective kappa-opioid receptor agonist U69593 produced analgesia without subsequent anti-analgesia, and confirmed by the failure of the selective kappa antagonist nor-binaltorphimine to block nalbuphine-induced anti-analgesia, indicating that anti-analgesia is not mediated by kappa-opioid receptors. We therefore tested the role of other receptors in nalbuphine anti-analgesia. Nociceptin/orphanin FQ (NOP) and sigma-1 and sigma-2 receptors were chosen on the basis of their known anti-analgesic effects and receptor binding studies. The selective NOP receptor antagonists, JTC801, and J-113397, but not the sigma receptor antagonist, BD 1047, antagonized nalbuphine anti-analgesia. Furthermore, the NOP receptor agonist NNC 63-0532 produced anti-analgesia with the same delay in onset observed with the three agonist-antagonists, but without producing preceding analgesia and this anti-analgesia was also blocked by naloxone. These results strongly support the suggestion that clinically used agonist-antagonists act at the NOP receptor to produce anti-analgesia.

Electrophysiological properties of neurones with CGRP-like immunoreactivity in rat dorsal root ganglia

Intracellular voltage recordings and fluorescent dye injections were made in vitro in 107 neurons in lumbar dorsal root ganglia (DRGs) of 6- to 8-week-old rats. Calcitonin gene-related, peptide-like immunoreactivity (CGRP-LI) was examined in these neurones, which were divided into C-, A delta-, and A alpha/beta-fibre neurones on the basis of their conduction velocities (CVs). A-fibre neurones with CGRP-LI had significantly longer mean action potential (AP) and afterhyperpolarisation (AHP) durations than those without CGRP-LI. A delta neurones with CGRP-LI had significantly longer AHP durations, slower CVs and slower maximal fibre following frequencies than those without CGRP-LI. They also had longer AP durations (not significant). The largest A delta neurones were CGRP-LI negative, whereas the smaller cells were either positive or negative. A alpha/beta neurones with CGRP-LI also had longer mean APs (not significant) and AHPs (significant) than those without CGRP-LI, and the cell size distributions were similar for positive and negative neurones. Most A-fibre neurones with CGRP-LI had inflections on the falling phase of the somatic AP. Of the A-fibre neurones with such inflections (Ai neurones), those with CGRP-LI had longer AP durations (not significant) and longer AHP durations (significant) than Ai neurones without CGRP-LI, pointing to a functionally distinct subgroup of Ai neurones. There were no significant differences in electrophysiological properties or cell size measurements between C-fibre neurones with and without detectable CGRP-LI. The patterns of electrophysiological properties of A delta neuronal somata with CGRP-LI and of most, but not all, A alpha/beta neuronal somata with CGRP-LI are similar to those reported for cutaneous nociceptors with A fibres in rat (Ritter and Mendell [1992] J. Neurophysiol. 68:2033-2041). Because rat DRG neurones that express CGRP normally also express trkA (Averill et al. [1995] Eur. J. Neurosci. 7:1484-1494), the properties described here of neurones with CGRP-LI are probably the same as those of DRG neurones with trkA.

The electrophysiological properties of rat primary afferent neurones with carbonic anhydrase activity

1. Intracellular recordings of action potentials (APs) and after-hyperpolarizations (AHPs) were made from the L3, L4 and L5 dorsal root ganglia (DRGs) of 6- to 8-week-old anaesthetized female Wistar rats in vitro at 36.5 +/- 1 degree C. Neurones were classified by their conduction velocities (CVs) as A alpha/beta (> 12 m s-1), A delta (1.3-12 m s-1) or C fibre neurones (< 1.3 m s-1). 2. Following the recording, fluorescent dye was injected intracellularly. Sections of injected neurones were tested for carbonic anhydrase (CA) activity histochemically. Reaction product intensity and cell size were measured. Control experiments showed that intracellular dye, time in vitro, axotomy and electrical stimulation did not affect proportions of CA-positive neurones or their size distributions. 3. Approximately 28-30% of DRG neurones were CA positive. Their sizes were approximately normally distributed and covered the entire size range of DRG neurones with no correlation between size and CA intensity. A greater proportion of A alpha/beta cells (62%) than of A delta (32%) or C cells (38%) were CA positive, but CA intensity was not correlated with CV. 4. In A neurones mean AP duration was significantly shorter in CA-positive cells; for CA-positive and CA-negative cells, respectively, these values were 1.6 and 2.8 ms for A delta cells; 1.1 and 1.7 ms for A alpha/beta cells; and were 1.2 and 2.3 ms for all A cells. CA intensity was negatively correlated with AP duration at base in all these groups. 5. Again in A neurones, the mean AHP durations were significantly shorter in the CA-positive cells; the mean AHP durations to 80% recovery for positive and negative cells were 8.8 and 36 ms, respectively, for A alpha/beta cells and were 8.6 and 26 ms, respectively, for all A cells. CA intensity was negatively correlated with AHP duration in A alpha/beta cells and all A cells together. 6. A fibre cells with the longer AP and AHP durations were all CA negative, while cells with the shorter durations included both CA-positive and CA-negative cells. 7. CA-positive and CA-negative A fibre neurones therefore have different electrophysiological characteristics. It is suggested that CA-negative A fibre neurones may have slower somatic firing rates and different sensory functions from the CA-positive neurones.

Primary sensory neurones: neurofilament, neuropeptides, and conduction velocity

This paper reviews and provides new data on the relationship of the peptide content in rat dorsal root ganglion (DRG) neurons to a) the neurofilament content of the soma and b) the conduction velocities of the fibres. The latter involved intracellular recordings made in vitro followed by dye injection and immunocytochemistry. Because neurofilament-poor DRG neurones have C-fibres, and A-fibre neurones are neurofilament rich, the soma neurofilament content of peptide containing neurones allowed predictions to be made about their conduction velocity ranges. Substance P-like immunoreactive (SP-LI) neurones were mostly small, neurofilament poor, but a few (15%) were neurofilament rich. From conduction velocity measurements, about half the C-fibre neurones studied and 10% of A delta-neurones but no A alpha/beta-neurones showed SP-LI. CGRP-LI neurones were also mainly neurofilament poor neurones, but 32% were neurofilament rich, including small, medium, and large neurones. Fibres of CGRP-LI neurones conducted in the C, A delta or A alpha/beta ranges. Neurones with somatostatin-LI (SOM-LI) were all neurofilament poor; preliminary data is consistent with SOM-LI neurones having C-fibres.

Comparison of three intracellular markers for combined electrophysiological, morphological and immunohistochemical analyses

Hypothalamic paraventricular and supraoptic neurons were recorded intracellularly in coronal slices and injected with Lucifer yellow, ethidium bromide or biocytin. Electrical properties, morphological staining and neurophysin immunohistochemistry were compared among the 3 markers. Lucifer yellow electrodes had a high resistance and frequently blocked during experiments. Neurons recorded with Lucifer yellow electrodes had low input resistances and low-amplitude, broad spikes. Lucifer yellow labeling in whole mount was highly fluorescent, revealing distal dendrites and axons. Of cells injected with Lucifer yellow, 64% were recovered but were faint after immunohistochemical processing. Recordings with ethidium bromide electrodes were similar to controls, although electrode blockage sometimes occurred. Only somata and proximal dendrites of ethidium bromide-filled neurons were visible in whole-mount. Forty percent of cells injected with ethidium bromide were recovered after immunohistochemical processing; these were invariably faint. Recordings with biocytin-filled electrodes were similar to control recordings. Biocytin-filled, HRP-labeled cells showed distal dendrites and often dendritic spines and axons in 50-75-microns sections. Seventy percent of biocytin-injected cells labeled with fluorescent markers were recovered and remained strongly labeled after immunohistochemical processing. Biocytin had the best electrical and staining properties for combined electrophysiological and anatomical studies.

Soma neurofilament immunoreactivity is related to cell size and fibre conduction velocity in rat primary sensory neurons

1. Intracellular recordings were made in dorsal root ganglia in vitro at 37 degrees C. The L4, L5 and L6 ganglia from 46- to 51-day-old female Wistar rats were used. In each neuron conduction velocity (CV) was measured and fluorescent dye was injected. Later the intensity of the immunoreactivity to RT97 (a monoclonal antibody to the phosphorylated 200 kDa neurofilament subunit) as well as the cell size (cross-sectional area at the nuclear level) were measured in the dye-injected neurons. RT97 was used to distinguish between the L (light, neurofilament-rich) and the SD (small dark, neurofilament-poor) neuronal somata. 2. Neurons were classified as C neurons (CV less than 1.3 m/s), C/A delta neurons (1.3-2 m/s), A delta neurons (2-12 m/s) or A alpha/beta neurons (greater than 12 m/s). 3. All A-fibre somata were RT97 positive (L) and all C-fibre somata were RT97 negative (SD), although in the C/A delta group both positive and negative neurons were seen. Thus, RT97-negative somata had C (unmyelinated) or C/A delta fibres, while RT97-positive somata had A (myelinated) or C/A delta fibres. 4. The size distributions of A neurons and C neurons were consistent with their classification as L- and SD-cell neurons respectively. The size distribution of A delta cells was skewed with a peak of small cells and a tail of medium-sized cells. 5. There was a loose positive correlation between cell size and fibre CV. 6. RT97 intensity was positively correlated with CV if all neurons were considered together, but no correlation was seen within the C, A delta or A alpha/beta CV groups. 7. RT97 intensity was positively correlated with cell size when all neurons were considered together. Although no correlation was seen within the C or the A delta CV groups, a clear positive correlation was seen for A alpha/beta neurons. 8. The relationship of RT97 intensity to cell size was not demonstrably altered by axotomy, time in vitro or the presence of intracellular dye in control experiments. 9. RT97-negative and -positive neurons could be seen in neonatal rat ganglia. Their size distributions resembled, respectively, the SD- and L-neuron populations at this age. RT97 immunoreactivity may therefore be a useful predictor of the cell type and myelinated state which a sensory cell is destined to reach in the adult rat.

Cell type and conduction velocity of rat primary sensory neurons with calcitonin gene-related peptide-like immunoreactivity

An immunocytochemical double labelling study of L4 dorsal root ganglia from rats aged seven to 10 weeks was made with an antibody to calcitonin gene-related peptide and with RT97, an anti-neurofilament antibody which specifically labels the light neuron population. Peptide immunoreactivity was found in an average of 46.5% of all neurons. Sixty-two per cent of the small dark (RT97-negative) and 30% of the light (RT97-positive) neuron populations contained the peptide. About one-third (32%) of the cells with peptide immunoreactivity were light cells and about two-thirds (68%) were small dark cells. Intracellular electrophysiological recordings were made in vitro from neurons in lumbar (L4, L5 and L6) dorsal root ganglia from six- to eight-week-old rats, followed by dye-injection and immunocytochemistry. This showed that conduction velocities of neurons with calcitonin gene-related peptide-like immunoreactivity ranged from 0.5 to 28.6 m/s. Seventy-three neurons were successfully processed. Of these, calcitonin gene-related peptide-like immunoreactivity was found in 46% of C-fibre neurons, 33% of A delta-fibre neurons and in 17% of the A alpha/beta-fibre neurons. The peptide-like immunoreactivity was found in approximately 25% of all A-fibre neurons sampled.

Conduction velocity changes along the processes of rat primary sensory neurons

Conduction velocities of rat L4, L5 and L6 dorsal root ganglion neurons were measured in vitro, from several points on the peripheral nerve and dorsal root. Conduction velocities calculated from a single stimulation point (12-26 mm from the ganglion) proved accurate for fibres conducting up to 17 m/s in the peripheral nerve and up to 14 m/s in the dorsal root, but tended to underestimate the value for faster fibres. C-fibre neurons of the L4 and L5 ganglia had a unimodal distribution of conduction velocities below 1.3 m/s. These were discontinuous with A-fibre conduction velocities, which also had a unimodal distribution and had no clear A delta peak. In contrast, conduction velocities of L6 ganglion neurons showed no discontinuity between C- and A-fibres, but had a clear A delta peak. In A-fibre neurons, dorsal root conduction velocities were on average about 14% slower than, and linearly related to, those in the peripheral nerve. However, in individual neurons the dorsal root conduction velocity could be slower or faster than that in the peripheral nerve. In C-fibre neurons dorsal root conduction velocities were almost always slower (average 28%) but not correlated with those of the peripheral nerve. Slowing of conduction velocity along the sciatic nerve was seen in most fibres conducting at less than 2 m/s, but not in faster fibres. The slowing was substantial (up to 60%), sometimes from the A delta to the C-fibre range, and sudden, occurring at a distance of between 15 and 29 mm from the ganglion.

Cell type and conduction velocity of rat primary sensory neurons with substance P-like immunoreactivity

A double labelling immunohistochemical study of rat L4 dorsal root ganglia was made with an anti-substance P antibody and with an antibody, RT97, which specifically labels the light cell population. Substance P-like immunoreactivity was found in 30% of the small dark neurons and 10% of the light neurons. Of the neurons with substance P-like immunoreactivity, 30% were light neurons and 70% were small dark neurons. A study of electrophysiologically characterized neurons in lumbar dorsal root ganglia with dye injection revealed substance P-like immunoreactivity in neurons with conduction velocities ranging from 0.5 to 9.5 m/s. It was seen in 50% of C-fibre neurons, in 20% of A delta-fibre neurons, and in no A alpha/beta neurons. Overall, substance-P-like immunoreactivity was found in 10% of A-fibre neurons sampled.