Chronic exposure to lead acetate affects the development of protein kinase C activity and the distribution of the PKCgamma isozyme in the rat hippocampus

This study has examined the effect of chronic inorganic lead exposure on phospholipid-dependent protein kinase C (PKC) activity, and the distribution of its alpha (alpha), beta II (betaII), gamma (gamma), and zeta (zeta) isozymes in subcellular fractions of the developing rat hippocampus. Dams were exposed to either 0 or 1000 ppm lead acetate in their drinking water for one week and mated. Offspring were exposed to lead in utero, via lactation, and directly in the drinking water after weaning. The offspring were sacrificed at postnatal days 1 (P1), 8 (P8), 15 (P15), and 29 (P29). PKC activity was determined in the post-synaptosomal supernatant (PSS) and synaptosomal (P-2) membrane fractions by an in vitro assay using histone as the phosphate acceptor. The selected PKC isozymes were detected by immunoblotting techniques. In control animals, PKC activity (pmole/min/mg total protein) in both subcellular fractions substantially increased between P1 and P8. In chronically exposed rats exhibiting clinically relevant blood lead concentrations, this marked increase in PKC activity on P8 was significantly attenuated in both subcellular fractions. On this postnatal day, the amount of immunodetectable PKC gamma was significantly higher in the synaptosomal membrane fraction of lead-exposed rats. Other isozymes were unaffected. These results imply that in lead-exposed animals the PKC gamma isozyme was inactive even though it was associated with the membrane. These results also suggest that prolonged exposure to the heavy metal attenuated PKC activity at an important developmental time to potentially adversely affect normal hippocampal function.

Symposium overview: chemical modulation of neuroreceptors and channels via intracellular components

Whereas the roles of G proteins and protein kinases in various neuroreceptors and ion channels have been studied extensively, their roles in the actions of drugs and toxicants on these receptors and channels remain to be elucidated. Almost all drugs and toxicants exert multiple actions on multiple target sites, and there is no reason to assume that a chemical modulates a receptor/channel via a single mechanism. In fact, experimental evidence is slowly but steadily being accumulated to indicate that certain drugs and toxicants modulate neuroreceptor/channel functions through interactions with intracellular components such as G proteins and protein kinases. Multiple actions of a toxicant on various receptors/channels may be explained on the basis of its interaction with the G protein/kinase system that is a common denominator of the target sites. This is a virgin field that promises a quantum leap in the coming years. Each presentation and discussion will focus on expected future developments and potential significance in the field of neurotoxicology.

Diminished blocking effect of acute lead exposure on high-threshold voltage-gated calcium currents in PC12 cells chronically exposed to the heavy metal

Rat pheochromocytoma (PC12) cells were grown in 0, 10, 25, or 50 microM lead-containing growth media for up to twelve weeks. High-threshold whole-cell calcium currents from these PC12 cells were recorded in lead-free recording media (control), then in 1 microM lead-containing recording media (acute challenge), and finally again in lead-free recording media (wash). The acute lead challenge decreased calcium currents in all treatment groups (including 0 microM lead). However, this blocking effect of acute lead application diminished with prolonged chronic exposure to 25 and 50 microM lead. Although the acute lead challenge mainly caused a decrease in calcium currents, in some chronically exposed PC12 cells increased calcium currents were recorded during the application of 1 microM lead acetate. In other chronically exposed PC12 cells, the acute lead challenge caused the peak of the current-voltage curve to shift from +10 mV to 0 mV. The number of cells exhibiting either an increase in calcium current or a shift in the current-voltage relationship following acute lead challenge increased with prolonged chronic exposure to the heavy metal. The time-dependent increase in calcium influx may be responsible for at least one manifestation of lead neurotoxicity.

Prenatal and postnatal chronic exposure to low levels of inorganic lead attenuates long-term potentiation in the adult rat hippocampus in vivo

Dams were exposed to 0, 100, 500, or 1000 ppm lead acetate in their drinking water for 1 week and mated. Offspring were exposed via lactation and in their drinking water after weaning. At 13 weeks, a male and a female pup from each litter were anesthetized with urethane and prepared for recording of CA3-evoked field potentials in the hippocampal CA1 region. In control and 100 ppm lead-treated animals, high-frequency stimulation (50 Hz) induced long-term potentiation (LTP) throughout a 4 h recording period. In contrast, 1000 ppm lead-treated pups showed little LTP, while those exposed to 500 ppm exhibited LTP initially but then failed to maintain it. Chronic developmental lead exposure prevented the full expression of LTP in the CA1 region of the rat hippocampus.

Chronic exposure to inorganic lead increases high-threshold voltage-gated calcium currents in rat pheochromocytoma (PC12) cells

Rat pheochromocytoma (PC12) cells were exposed to lead acetate (0, 10, 25 and 50 microM) in their growth media for up to 12 weeks. High-threshold voltage-gated calcium currents were recorded each week from nerve growth factor-differentiated PC12 cells using the whole-cell patch-clamp technique. Chronic exposure for 1 month did not modify peak or sustained calcium current amplitudes in lead-treated cells when compared to sister control cultures. Two month exposure to 25 and 50 microM significantly increased peak and sustained calcium current amplitudes, while 10 microM had little effect. During the third month of exposure, peak and sustained calcium current amplitudes remained increased in the cells exposed to 25 and 50 microM lead acetate. By the end of the second month of exposure to 25 and 50 microM lead acetate, the voltage at which maximal current amplitude was attained shifted from + 10 mV to 0 mV. The observed effects of toxicologically relevant lead concentrations on high-threshold calcium currents in chronically exposed mammalian cells provide further support for the notion that at least one cellular target of the heavy metal's neurotoxic action may be the voltage-gated calcium channel.

Acute exposure to inorganic lead modifies high-threshold voltage-gated calcium currents in rat PC12 cells

Acute exposure to 1, 10 and 50 microM lead acetate solutions irreversibly decreased calcium currents in 21 of 30 nerve growth factor-differentiated PC12 cells. In five cells, however, calcium currents irreversibly increased following lead exposure. Lead was equally effective at blocking the peak and sustained components of the calcium current. These data suggest complex interactions between neurotoxicologically relevant lead concentrations and high-threshold calcium currents in mammalian cells. They provide further support for the notion that at least one target of lead's toxic action is the voltage-gated calcium channel.

The effect of continuous morphine analgesia on chronic thermal hyperalgesia due to sciatic constriction injury in rats

We employed hindfoot withdrawal latencies to radiant heat to assess the analgesic effect of prolonged morphine infusion on thermal hyperalgesia induced by chronic constriction injury (CCI) of the rat sciatic nerve. All CCI rats developed thermal hyperalgesia while sham-operated animals did not. Continuous systemic infusion of morphine dose-dependently reversed the thermal hyperalgesia in the CCI rats. In contrast, thermal hyperalgesia persisted in saline-treated CCI rats. Tolerance to morphine's analgesic effect did not develop over a period of seven days of morphine infusion, which is considered long-term for animal models. These data suggest that morphine acts rapidly and effectively to reduce behavioral signs of hyperalgesia in rats with sciatic CCI, without the concomitant development of tolerance. Scheduled administration of morphine might be an appropriate treatment regimen for relief of neuropathic pain, and the infrequent use of opioids in equivalent human clinical pain syndromes due to fear of opioid unresponsiveness and tolerance might need to be re-evaluated.

Responses of neurons in the ventrolateral orbital cortex to noxious cutaneous stimulation in a rat model of peripheral mononeuropathy

The responses of ventrolateral orbital cortex neurons to noxious cold pressor were compared in rats with loose ligatures tied around their sciatic nerve with those in rats in which the sciatic nerve was exposed but not ligated. In ligated rats more cells responded to cold pressor and their average afterdischarges were longer. There were no differences in the background firing rate or the magnitude of response to the cold pressor between the two groups.

Responses of rat nucleus submedius neurons to enkephalins applied with micropressure

The purpose of this study was to determine what effects leucine-enkephalin and D-Ala2-D-Leu5-enkephalin have on both the background and naturally evoked activity of thalamic nucleus submedius neurons responsive to mechanical cutaneous stimulation. Thirty-five neurons in the nucleus submedius were fully characterized during single-unit extracellular recordings as nociceptive, low-threshold mechanoreceptive (LTM) or unresponsive. Micropressure was used to apply the opioids. Eighteen neurons were inhibited; 13 of these were nociceptive and one was LTM. Six units were activated; two of these were nociceptive and three were LTM. The remaining 11 units were unaffected. Opioid responses were tested for antagonism by naloxone in 12 neurons; eight of these responses were antagonized by naloxone. Statistical analyses indicated that the effects of enkephalins on nociceptive neurons were selective for neuronal modality. The opioids also altered the response of some nociceptive neurons to receptive field stimulation. The presence of nociceptive neurons in the nucleus submedius that are selectively inhibited by opioids provides additional support for the involvement of submedius neurons in nociception. The results of this study suggest that this involvement is more than merely transmission of nociceptive input, since the opioids may be selectively modulating the type of information that is transmitted to the cortex.

Spontaneous activity of cat spinal ganglion neurons in vivo

Recordings were made from L4-S1 spinal ganglion neurons of anesthetized cats while their associated dorsal root and the sciatic nerve were left intact, locally anesthetized, or locally anesthetized and sectioned. In all three experimental conditions spontaneous discharges were recorded. These discharges occurred in the absence of any electrical stimulation of the dorsal root or sciatic nerve, and were not due to peripheral exploration of receptive fields or sustained firing in joint or muscle afferents. The spontaneous discharges were relatively rhythmic, and their firing frequency ranged from 5 to 100 impulses per s. Interactions between spontaneous and electrically evoked discharges were observed that depended on the impulse's frequency of firing. High frequency discharges always abolished low frequency impulses regardless of whether these latter were spontaneous or evoked. Extra spikes and postspike events that followed impulses evoked by stimulation of the dorsal root or sciatic nerve were also recorded from some spinal ganglion neurons. These results suggest that spontaneous discharges may originate within the spinal ganglion itself, and that they can occur under normal circumstances.

Characteristics of action potentials recorded from cat spinal ganglion neurons in vivo

Action potentials were recorded intracellularly from L4-S1 dorsal root ganglion (DRG) neurons of anesthetized cats. Based on the shape of their waveforms the action potentials were classified as either typical (n = 49) or atypical (n = 8). The atypical potentials were characterized by a slowly rising, well-defined early depolarization of small amplitude and long duration. This kind of prepotential randomly triggered spike potentials with varying latencies. The average rise time, duration, and area of atypical potentials were significantly different than those of typical ones. In some DRG neurons, the afterhyperpolarization was preceded by a delayed repolarization, or was followed by postspikes and long-lasting afteroscillations. In others, small depolarizations could be recorded during subthreshold stimulation. These depolarizations arose slightly later than the spike potential, and were all or none, with relatively stable onset latency and size. These results are explained by postulating the presence of chemical synapses and/or electrotonic coupling in the DRG.

Diverse actions of 5-hydroxytryptamine on frog spinal dorsal horn neurons in vitro

The effects of 5-hydroxytryptamine on the membrane potential and input resistance of 86 dorsal horn neurons were studied using intracellular recordings in isolated, hemisected spinal cords of adult frogs (Rana pipiens). Bath application of serotonin (5-100 microM) caused membrane depolarizations in 58 (67%) neurons, hyperpolarizations in 12 (14%) cells, biphasic responses in nine (11%) neurons, and no detectable change in seven (8%) cells. In some neurons depolarized by serotonin, the amine's responses could be mimicked by the selective 5-HT2 agonist (+/-)-1(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride and the 5-HT1C/2 agonist alpha-methyl-5-hydroxytryptamine, and blocked by the 5-HT1C/2 antagonists ketanserin and mianserin. In other neurons depolarized by serotonin, the 5-HT3 agonist 2-methyl-5-hydroxytryptamine mimicked, and the 5-HT3 antagonist, 3-tropanyl-3,5-dichlorobenzoate, blocked the serotonin-induced responses. Depolarizing responses due to activation of 5-HT1C/2 receptors were generally accompanied by increases in the membrane input resistance, whereas depolarizations mediated by 5-HT3 receptors were associated with a decreased membrane input resistance. Superfusion with tetrodotoxin or low-Ca2+/high-Mg(2+)-containing media abolished about half of the depolarizing responses. Hyperpolarizations caused by serotonin were associated with a decrease in membrane input resistance, and might have been due to activation of a potassium conductance. These responses persisted in bathing solutions containing tetrodotoxin or low-Ca2+/high-Mg2+. The 5-HT1A agonist 8-hydroxy-2-(di-N-propylamine)tetralin hydrobromide mimicked, whereas the 5-HT1A antagonist spiroxatrine blocked, these hyperpolarizing responses. Other antagonists selective for 5-HT1C/2 or 5-HT3 receptors were without effect. Serotonin-produced biphasic responses consisted of either an initial depolarization followed by a hyperpolarization or the reverse. The selective 5-HT2 agonist (+/-)-1(2,5-dimethyoxy-4-iodophenyl)-2-aminopropane hydrochloride could only mimic the depolarizations, whereas the 5-HT1A agonist 8-hydroxy-2-(di-N-propylamine)tetralin hydrobromide produced only the hyperpolarizations. Spiroxatrine, a 5-HT1A antagonist, blocked only the hyperpolarizations without affecting the depolarizations, and methysergide, a non-specific 5-HT receptor antagonist, depressed both the depolarizations and hyperpolarizations. Serotonin also appeared to affect spinal dorsal horn neurons indirectly because it produced excitatory postsynaptic potentials, inhibitory postsynaptic potentials, and a mixture of both.(ABSTRACT TRUNCATED AT 400 WORDS)

Retrograde tracing of projections between the nucleus submedius, the ventrolateral orbital cortex, and the midbrain in the rat

The fluorescent tracers fluoro-gold and 1,1'-dioctadecyl-3,3,3,3-tetramethyl indocarbocyanine perchlorate were used as retrograde markers to examine reciprocal connections between the rat nucleus submedius and the ventrolateral orbital cortex. In addition, midbrain projections to each of these regions were examined. In the prefrontal cortex, we found that input from the nucleus submedius terminates rostrally within the lateral and ventral areas of the ventrolateral orbital cortex. Conversely, the cortical input to the nucleus submedius originates from the medial and dorsal parts of the ventrolateral orbital cortex. Our data also demonstrated that neurons from the ventrolateral periaqueductal gray and the raphe nuclei project to the midline nuclei of the thalamus, including a small projection to the nucleus submedius. We further determined that regions within the ventrolateral periaqueductal gray and raphe nuclei project to the ventrolateral orbital cortex, and that these regions overlap with those that project to the nucleus submedius. These findings suggest that the nucleus submedius might be part of a neural circuit involved in the activation of endogenous analgesia.

5-Hydroxytryptophan (5-HTP) augments spontaneous and evoked phrenic motoneuron discharge in spinalized rats

Experiments on anesthetized, spinalized rats were conducted to determine the effects of systemic 5-hydroxytryptophan (5-HTP) administration on: (1) spontaneous phrenic nerve activity and (2) evoked phrenic responses to short latency, non-serotonergic synaptic inputs elicited by electrical stimulation of lateral funiculus. 5-HTP augmented spontaneous phrenic activity and allowed expression of a second, longer latency evoked response. Both effects were antagonized by methysergide. Our results suggest that spinal serotonin increases the efficacy of synaptic inputs to phrenic motoneurons.

A new qualitative method for detecting IgD in unconcentrated cerebrospinal fluid

We report a method which is capable of demonstrating the isoelectric focusing (IEF) pattern of immunoglobulin D in unconcentrated cerebrospinal fluid (CSF) samples containing as little as 0.1-0.5 ng of total IgD. The method used was an immuno-sandwich technique, with alkaline phosphatase enzyme amplification. Oligoclonal and polyclonal IgD patterns were seen in CSF samples. No cross-reactivity with other immunoglobulins (IgG, IgA and IgM) was detected.

Responses of neurons in the rat ventrolateral orbital cortex to phasic and tonic nociceptive stimulation

Responses of ventrolateral orbital cortex (VLO) neurons to innocuous touch and pressure, and noxious pinch and cold were examined by extracellular recordings. Eight neurons increased their firing rate in a graded fashion to all stimuli applied, 4 units decreased their discharges, and 6 neurons remained unaffected. All responsive units displayed the greatest change in firing rate during noxious cold, and all exhibited whole-body receptive fields. These data further implicate the VLO in nonciception.

Electrophysiological properties of frog spinal dorsal horn neurons and their responses to serotonin: an intracellular study in the isolated hemisected spinal cord

Frog dorsal horn neurons and their responses to serotonin (5-HT) were studied in intracellular recordings from isolated hemisected spinal cords. Electrophysiological properties were comparable to those of mammals. Bath application of 5-HT (10-50 microM) increased the excitability and caused membrane depolarizations in 7/14 cells, reduced or abolished activity and caused hyperpolarizations in 4 neurons, exerted a biphasic effect in two cells, and produced no detectable change in one neuron. The multiple effects of 5-HT may be mediated through different 5-HT receptor subtypes.

Bulbospinal serotoninergic pathways in the frog Rana pipiens

We combined retrograde fluorescent tracing with rhodamine immunofluorescence to identify the origin of serotoninergic neurons with descending projections to the spinal cord of frogs. After injections of Fluoro-gold into the spinal cord, retrogradely labeled immunoreactive serotoninergic neurons were detected in the caudal part of the brainstem from the level of the obex through the level of the VIII nerve. These doubly labeled cells were distributed along the midline throughout the rostrocaudal extent of the dorsal portion of the raphe nuclear region. Doubly labeled neurons were more numerous in the rostral than in the caudal part of the raphe area. The fluorescent tracer 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) was then placed in and around the middle and rostral raphe nuclear region. Anterogradely labeled fibers could be traced bilaterally in the lateral portion of the dorsal funiculus and the lateral and ventral funiculi. These fibers were seen terminating in the dorsal and ventral horns, as well as in the intermediate grey matter. After placement of DiI in the caudal raphe area, labeled fibers were found only in the intermediate grey and ventral horn. These findings suggest that the organization of bulbospinal serotoninergic pathways in the frog is similar to that of mammals, and that an isolated amphibian spinal cord preparation could be a useful model for pharmacological and physiological studies of the action of serotonin (5HT) in the spinal cord.

Responses of type A cat spinal ganglion neurons to repetitive stimulation of their central and peripheral processes

Intracellular recordings were made from L7-S1 type A spinal ganglion neurons of anesthetized cats while electrical stimulation was delivered repetitively to their associated dorsal root and the sciatic nerve. The general response pattern of these neurons changed during stimulation at progressively higher rates. The changes were observable as jitter in onset latency of the evoked spikes, inability of evoked responses to follow electrical stimuli in a 1:1 manner (spike failure), reduction in action potential amplitude, and decomposition of the full spike into its non-myelinated and myelinated components. The frequency following ability of these spike components was in the order of full spike less than non-myelinated less than myelinated. In jitter in onset latency and inability to follow high frequency stimulation was determined only for the full spike, as is typical for antidromicity criteria, a wide frequency following spectrum was obtained for our sample of spinal ganglion neurons. Less than a third of the cells were able to follow stimulation rates in excess of 200 Hz, and about a fifth of the neurons failed to follow any rates greater than 20 Hz. Most of the neurons activated from both the dorsal root and sciatic nerve responded with the same pattern of stimulus-evoked responses. However, some of these cells exhibited strikingly different patterns to dorsal root and sciatic stimulation, including the presence of prepotentials following stimulation of one, but not the other, process. These prepotentials occurred in the depolarizing direction, at threshold stimulation were all-or-none in nature, generated spikes that varied in onset latency, and failed to occur at even low-to-moderate rates of stimulation. The results indicate that the frequency following spectrum of cat type A ganglion neurons is wide, and that it is their somata that are most vulnerable to high frequency stimulation. It is possible that some of the observed prepotentials are functional manifestations of synaptic contacts in spinal ganglia.

Responses of neurons in the rat nucleus submedius to noxious and innocuous mechanical cutaneous stimulation

Extracellular recordings were used to characterize responses to cutaneous mechanical stimulation of 78 neurons in the rat nucleus submedius (SM). Thirty-nine of these units were activated by some type of cutaneous mechanical stimulation. Eighteen cells were activated exclusively by noxious stimuli. In 13 of these cells, responses were of swift onset and relatively rapid termination following stimulus application. In contrast, in three neurons responses were delayed both in onset and termination, and in two the response was immediate, but the markedly increased evoked activity outlasted stimulus application by 13 min. Receptive fields (RFs) of these nociceptive neurons were generally large, although none were bilateral. Four SM neurons were activated by innocuous stimuli, but their maximal response was obtained only after noxious stimulation. Responses of all of these neurons were of immediate onset and recovery, and their RFs were large (two were bilateral). Twelve SM neurons were activated maximally by innocuous stimuli. Responses of seven of these cells were immediate in onset and termination, while that of three were delayed in both onset and termination. Two of the 12 innocuous-only neurons quickly became unresponsive to repeated stimulus applications, and could be reactivated only after a rest period during which no stimuli were applied. RFs of these units were also generally large, and in three cases were bilateral. Five SM neurons responded by decreasing, or completely ceasing, their firing subsequent to noxious-only (n = 2), or innocuous-only (n = 3) stimulation. Four of these units had large RFs (two were bilateral). The remaining 39 SM neurons could not be activated by any type of mechanical cutaneous stimulation we tried. Electrical stimulation of the ventrolateral orbital cortex (VLO) was employed to examine frontal cortical projections of 21 SM neurons. Ten of these units were activated, although all of them synaptically rather than antidromically, and two were inhibited. There was no clear-cut relationship between neuronal location, physiological type, RF site, or VLO stimulation effects among the 39 SM neurons. These results provide further support for the involvement of SM neurons in nociceptive information signaling, and suggest additionally that the role of the nucleus is not limited to nociception but encompasses a wider range of cutaneous sensations.

Identification of distinct C3b and C4b recognition sites in the human C3b/C4b receptor (CR1, CD35) by deletion mutagenesis

Complementary DNA clones encoding the NH2-terminal region of human CR1 have been isolated and sequenced. The deduced complete amino acid sequence of the F allotype of human CR1 contains 2,039 residues, including a 41-residue signal peptide, an extracellular domain of 1,930 residues, a 25-amino acid transmembrane domain, and a 43-amino acid cytoplasmic region. The extracellular domain is composed exclusively of 30 short consensus repeats (SCRs), characteristic of the family of C3/C4-binding proteins. The 28 NH2-terminal SCRs are organized as four long homologous repeats (LHRs) of seven SCRs each. The newly sequenced LHR, LHR-A, is 61% identical to LHR-B in the NH2-terminal two SCRs and greater than 99% identical in the COOH-terminal five SCRs. Eight cDNA clones were spliced to form a single construct, piABCD, that contained the entire CR1 coding sequence downstream of a cytomegalovirus promoter. COS cells transfected with piABCD transiently expressed recombinant CR1 that comigrated with the F allotype of erythrocyte CR1 on SDS-PAGE and that mediated rosette formation with sheep erythrocytes bearing C4b and C3b. Recombinant CR1 also had factor I-cofactor activity for cleavage of C3(ma). Analyses of six deletion mutants expressed in COS cells indicated that the NH2-terminal two SCRs of LHR-A contained a site determining C4 specificity and the NH2-terminal two SCRs of LHR-B and -C each had a site determining C3 specificity. The presence of these three distinct sites in CR1 may enable the receptor to interact multivalently with C4b/C3b and C3b/C3b complexes generated during activation of the classical and alternative pathways.

Iontophoretic application of calcitonin gene-related peptide produces a slow and prolonged excitation of neurons in the cat lumbar dorsal horn

Calcitonin gene-related peptide (CGRP) was applied by iontophoresis onto physiologically characterized neurons. CGRP (20-100 nA) activated both wide-dynamic-range (5/8) and low-threshold mechanoreceptive units (3/12), but had no effect on nociceptive-specific neurons (0/4). The excitation was of slow onset (30 s to 3 min) and prolonged duration (up to 10 min). In none of the tested units did CGRP cause inhibition. The slow and prolonged action suggests a neuromodulatory role for CGRP in spinal cord sensory processes.

Enkephalin-like immunoreactivity in the nucleus submedius of the cat and rat thalamus

In the present study, we employed the peroxidase-antiperoxidase immunocytochemical technique to study the presence and distribution of enkephalin in the nucleus submedius of both cats and rats at the light- and electron-microscopic levels. The enkephalin-like immunoreactive (ENK-LI) fibers were present in a concentrated, albeit limited, manner in the nucleus submedius of both species. These fibers were located close to the dorsal and caudal edge of the nucleus, and were confined to a small area that never exceeded 350 microns in the rostrocaudal or 250 microns in the dorsoventral direction. Mediolaterally, however, the fibers extended some 700 microns. No ENK-LI cell bodies were seen in the nucleus submedius, even in colchicine-treated animals. At the electron-microscopic level, the ENK-LI terminals were seen to synapse on dendrites. These data indicate a previously unsuspected role of enkephalin in synaptic transmission processes within the nucleus submedius, and provide additional support for the role of this nucleus in the processing of nociceptive information at medial thalamic levels.

Immunoreactive enkephalin is contained within some trigeminal and spinal neurons projecting to the rat medial thalamus

In order to determine whether enkephalin is contained within rat trigeminothalamic and spinothalamic neurons, we have combined the techniques of retrograde labeling and immunocytochemistry. Injection sites were limited to medial thalamic nuclei. Retrogradely labeled neurons were most often seen contralaterally in trigeminal and spinal laminae I, V, VI and VII. After immunocytochemical processing, about 10% of the retrogradely labeled cells exhibited enkephalin immunoreactivity. These double-labeled neurons were located in laminae VI and VII.