Dorsal raphe stimulation, 5-HT and morphine microiontophoresis effects on noxious and nonnoxious identified neurons in the medial thalamus of the rat

Regional Differences Within the Anterior Cingulate Cortex in the Generation Versus Suppression of Pain Affect in Rats

The anterior cingulate cortex (ACC) modulates emotional responses to pain. Whereas, the caudal ACC (cACC) promotes expression of pain affect, the rostral ACC (rACC) contributes to its suppression. Both subdivisions receive glutamatergic innervation, and the present study evaluated the contribution of N-methyl-d-aspartic acid (NMDA) receptors within these subdivisions to rats' expression of pain affect. Vocalizations that follow a brief noxious tail shock (vocalization afterdischarges, VAD) are a validated rodent model of pain affect. The threshold current for eliciting VAD was increased in a dose-dependent manner by injecting NMDA into the rACC, but performance (latency, amplitude, and duration) at threshold was not altered. Alternately, the threshold current for eliciting VAD was not altered following injection of NMDA into the cACC, but its amplitude and duration at threshold were increased in a dose-dependent manner. These effects were limited to Cg1 of the rACC and cACC, and blocked by pretreatment of the ACC with the NMDA receptor antagonist d-2-amino-5-phosphonovalerate. These findings demonstrate that NMDA receptor agonism within the cACC and rACC either increases or decreases emotional responses to noxious stimulation, respectively. PERSPECTIVE: NMDA receptor activation of the rostral and caudal ACC respectively inhibited or enhanced rats' emotional response to pain. These findings mirror those obtained from human neuroimaging studies; thereby, supporting the use of this model system in evaluating the contribution of ACC to pain affect.

Distributions of different types of nociceptive neurons in thalamic mediodorsal nuclei of anesthetized rats

Mediodorsal thalamic nucleus (MD) is a critical relay of nociception. This study recorded responses of MD neurons to noxious mechanical and thermal stimuli in isoflurane anesthetized rats. We found the threshold of noxious mechanical stimulation was 141 gw and that of noxious heat stimulation was 46 °C. A significantly higher percentage of noxious inhibitory neurons were found in the medial and central part of the MD, whereas a higher percentage of noxious excitatory neurons were found in the lateral part of the MD and adjacent intralaminar nuclei. The differential distribution of excitatory and inhibitory neurons implies functional differentiation between the medial and lateral part of the MD in nociception processing. Furthermore, by an analysis of the stimulus-response function (SRF), we found 80% of these excitatory neurons had a step-function or hat-shape-like SRF. This suggests that most of the MD neurons may serve as a system to distinguish innocuous versus noxious stimuli.

Functional connectivity of the dorsal and median raphe nuclei at rest

Serotonin (5-HT) is a neurotransmitter critically involved in a broad range of brain functions and implicated in the pathophysiology of neuropsychiatric illnesses including major depression, anxiety and sleep disorders. Despite being widely distributed throughout the brain, there is limited knowledge on the contribution of 5-HT to intrinsic brain activity. The dorsal raphe (DR) and median raphe (MR) nuclei are the source of most serotonergic neurons projecting throughout the brain and thus provide a compelling target for a seed-based probe of resting-state activity related to 5-HT. Here we implemented a novel multimodal neuroimaging approach for investigating resting-state functional connectivity (FC) between DR and MR and cortical, subcortical and cerebellar target areas. Using [(11)C]DASB positron emission tomography (PET) images of the brain serotonin transporter (5-HTT) combined with structural MRI from 49 healthy volunteers, we delineated DR and MR and performed a seed-based resting-state FC analysis. The DR and MR seeds produced largely similar FC maps: significant positive FC with brain regions involved in cognitive and emotion processing including anterior cingulate, amygdala, insula, hippocampus, thalamus, basal ganglia and cerebellum. Significant negative FC was observed within pre- and postcentral gyri for the DR but not for the MR seed. We observed a significant association between DR and MR FC and regional 5-HTT binding. Our results provide evidence for a resting-state network related to DR and MR and comprising regions receiving serotonergic innervation and centrally involved in 5-HT related behaviors including emotion, cognition and reward processing. These findings provide a novel advance in estimating resting-state FC related to 5-HT signaling, which can benefit our understanding of its role in behavior and neuropsychiatric illnesses.

Role of gonadal hormones on mu-opioid-stimulated [³⁵S]GTPγS binding and morphine-mediated antinociception in male and female Sprague-Dawley rats

RATIONALE

Male rats are more sensitive to morphine-mediated antinociception than female rats. A role for gonadal hormones in this sex difference has not been clearly defined.

OBJECTIVES

To test the hypothesis that in vivo manipulation of gonadal hormones alters morphine-mediated G protein activation and leads to changes in morphine-mediated antinociception.

METHODS

Adult male and female rats were gonadectomized and treated with either estradiol or testosterone in the females or testosterone in the male for up to 10 days. The ability of morphine and the peptidic mu-opioid agonist [D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin (DAMGO) to stimulate [(35)S]GTPγS binding was measured in brain slices. In separate groups of identically treated rats, the antinociceptive response to morphine was determined using the warm-water tail-withdrawal assay.

RESULTS

In the thalamus, morphine- and DAMGO-stimulated [(35)S]GTPγS binding was reduced by estradiol treatment of gonadectomized females compared to gonadectomized females treated with vehicle or testosterone. In the nucleus accumbens, the morphine-stimulated [(35)S]GTPγS binding was increased by estradiol treatment of gonadectomized females. In males, castration caused an increase in agonist-stimulated binding in the thalamus and a reduction in the amygdala compared with intact males. No significant changes were seen in mu-opioid agonist-stimulated [(35)S]GTPγS binding in other brain regions. There was no difference in antinociception following the systemic administration of morphine across the different hormonal manipulation conditions and the greater sensitivity of males was maintained irrespective of the treatment conditions.

CONCLUSIONS

The modulation of mu-opioid receptor activation of G proteins by manipulation of sex hormones is region-specific and not reflected in antinociceptive responsiveness to morphine.

Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The medial thalamic parafascicular nucleus (PF) and the rostral anterior cingulate cortex (rACC) are implicated in the processing and suppression of the affective dimension of pain. The present study evaluated the functional interaction between PF and rACC in mediating the suppression of pain affect in rats following administration of morphine or carbachol (acetylcholine agonist) into PF. Vocalizations that occur following a brief noxious tailshock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by injection of morphine or carbachol into PF. Vocalizations that occur during tailshock were suppressed to a lesser degree, whereas, spinal motor reflexes (tail flick and hindlimb movements) were only slightly suppressed by injection of carbachol into PF and unaffected by injection of morphine into PF. Blocking glutamate receptors in rACC (NMDA and non-NMDA) by injecting D-2-amino-5-phosphonovalerate (AP-5) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. Carbachol-induced increases in vocalization thresholds were not affected by injection of either glutamate receptor antagonist into rACC. The results demonstrate that glutamate receptors in the rACC contribute to the suppression of pain affect produced by injection of morphine into PF, but not to the suppression of pain affect generated by intra-PF injection of carbachol.

Contribution of the periaqueductal gray to the suppression of pain affect produced by administration of morphine into the intralaminar thalamus of rat

The parafascicular nucleus (nPf) of the intralaminar thalamus is implicated in the processing of pain affect in both animals and humans. Administration of morphine into nPf results in preferential suppression of the affective reaction to noxious tail shock in rats. The involvement of the ventrolateral periaqueductal gray in mediating the antinociceptive action of morphine injected into nPf was evaluated. Vocalizations that occur after tail shock offset (vocalization afterdischarges) are a validated rodent model of pain affect and were preferentially suppressed by injection of morphine into nPf. Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of morphine into nPf. Inactivation of the vPAG via the microinjection of muscimol (GABA(A) agonist) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. The results demonstrate that a functional link between the nPf and vPAG in generating the antinociceptive action of morphine injected into nPf.

PERSPECTIVE

Microinjection of morphine into nucleus parafascicular preferentially suppressed rats' affective reaction to noxious stimulation. This affective analgesia was reversed by inactivation of the ventrolateral periaqueductal gray. Understanding the neurobiology underlying the suppression of pain affect will provide insights into new treatments for pain and its associated affective disorders.

Intracerebroventricular tryptophan increases heating and heat storage rate in exercising rats

The role of increased hypothalamic tryptophan (TRP) availability on thermoregulation and rates of core temperature increase and heat storage (HS) during exercise was studied in normal untrained rats running until fatigue. The rats were each anesthetized with 2.5% tribromoethanol (1.0 ml kg(-1) ip) and fitted with a chronic guiding cannula attached to the right lateral cerebral ventricle 1 week prior to the experiments. Immediately before exercise, they were randomly injected through these cannulae with 2.0 microl of 0.15 M NaCl (SAL; n=6) or 20.3 microM L-TRP solution (n=7). Exercise consisted of running on a treadmill at 18 m min(-1) and 5% inclination until fatigue. Body temperature was recorded before and during exercise with a thermistor probe implanted into the peritoneal area. Rates of core temperature increase (HR, degrees C min(-1)) and heat storage (HSR, cal min(-1)) were calculated. TRP-treated rats showed a rapid increase in body temperature which was faster than that observed in the saline-treated group during the exercise period. The TRP group also showed a higher rate of core temperature increase and HS. TRP-treated rats that presented higher HR and HSR also fatigued much earlier than saline-treated animals (16.8+/-1.1 min TRP vs. 40+/-3 min SAL). This suggests that the reduced running performance observed in TRP-treated rats is related to increased HR and HSR induced by intracerebroventricular injection of TRP in these animals.

Effect of strain and sex on mu opioid receptor-mediated G-protein activation in rat brain

Strain and sex differences in mu opioid-mediated antinociception have been reported in rodents. The present studies evaluated mu opioid receptor-mediated G-protein activation in Lewis and Fischer 344 (F344) male and female rats using agonist-stimulated [35S]GTPgammaS binding. Compared to Lewis rats, F344 rats exhibited a 35% higher level of net DAMGO-stimulated [35S]GTPgammaS binding in striatum. Basal [35S]GTPgammaS binding was approximately 30% lower in thalamus of Lewis than F344 rats. Female Lewis rats also exhibited slightly ( approximately 15%) lower basal [35S]GTPgammaS binding in cingulate cortex relative to F344 rats of either sex. The relative efficacies of the mu partial agonists, morphine and buprenorphine, were also examined. Buprenorphine exhibited approximately 40% lower relative efficacy in the periaqueductal gray in Lewis compared to F344 rats, but no other relative efficacy differences were found between strains or sexes. Moreover, regional differences in the relative efficacy of buprenorphine were also detected in Lewis but not F344 rats. In contrast to these results, the only difference found between sexes was the 13% lower basal [35S]GTPgammaS binding in the cingulate cortex of female compared to male Lewis rats. These results suggest that differences in mu opioid receptor-mediated G-protein activation may contribute to strain differences in opioid antinociception, whereas sex differences may result predominantly from other mechanisms.

Evidence that tryptophan reduces mechanical efficiency and running performance in rats

It has been reported that exercise increases brain tryptophan (TRP), which is related to exhaustive fatigue. To study this further, the effect of increased TRP availability on the central nervous system (CNS) with regard to mechanical efficiency, oxygen consumption (VO(2)) and run-time to exhaustion was studied in normal untrained rats. Each rat was anesthetized with thiopental (30 mg/kg ip b. wt.) and fitted with a chronic guiding cannula attached to the right lateral cerebral ventricle 1 week prior to the experiments. Immediately before exercise, the rats were randomly injected through these cannulae with 2.0 microl of 0.15 M NaCl (n=6) or 20.3 microM L-TRP solution (n=6). Exercise consisted of running on a treadmill at 18 m min(-1) and 5% inclination until exhaustion. TRP-treated rats presented a decrease in their mechanical efficiency (21.25+/-0.84%, TRP group vs. 24.31+/-0.98%, saline-treated group; P< or =.05), and increased VO(2) at exhaustion (40.3+/-1.6 ml kg(-1) min(-1), TRP group vs. 36.0+/-0.8 ml kg(-1) min(-1), saline group; P< or =.05), indicating that the metabolic cost of exercise was higher in the former group. In addition, a highly significant reduction was also observed in run-time to exhaustion of TRP animals compared to those of the saline-treated group (15.2+/-1.52 min, TRP group vs. 50.6+/-5.4 min, saline group; P< or =.0001). It can be deduced from the data that intracerebroventricular TRP injection in rats increases O(2) consumption and reduces mechanical efficiency during exercise, diminishing running performance.

Responses of thalamic neurons to input from the male genitalia

There have been relatively few electrophysiological studies, in any species, describing the supraspinal processing of inputs from the male genital tract. The thalamus was the focus of the present study. In 11 urethan-anesthetized male rats, subregions of the thalamus were surveyed for neuronal responses to the search stimulus, bilateral electrical stimulation of the dorsal nerve of the penis (DNP). A total of 133 DNP-responsive neurons were found and further tested for degree of somatovisceral convergence from other peripheral structures. Histological reconstruction of the recording sites revealed that the penile-responsive neurons were distributed among various thalamic subregions. These thalamic subregions included the medial-dorsal nuclei and ventral and lateral thalamic subregions (majority of neurons responsive to both tactile and pinch stimulation of the penis) as well as intralaminar, posterior and reticular subregions (majority responsive to pinch only). Taken together, the data demonstrate the existence of thalamic neurons with inputs from the male genitalia with widespread somatovisceral convergence. These neurons likely contribute to the neural circuitries underlying various aspects of penile sensation associated with reproductive and nociceptive events.

Modulation of noradrenergic and serotonergic transmission by noxious stimuli and intrathecal morphine differs in the dorsal raphe nucleus of anesthetized rat: in vivo voltammetric studies

We examined the effects of cutaneous noxious heat as well as the intrathecal administration of morphine on the oxidation current of peaks 1 and 2 in the dorsal raphe nucleus (DRN) of anesthetized rats. Differential normal pulse voltammetry with carbon fiber electrodes identified distinct oxidation currents at +120 mV (peak 1: catechol signals) and +280 mV (peak 2: 5-hydroxyindole signals). The catechol signal was significantly increased by 22.9 +/- 4.2% after applying cutaneous noxious heat at 52 degrees C. The 5-hydroxyindole signal was decreased by 39.8 +/- 4.3 and by 25.2 +/- 4.7% after stimulation with cutaneous noxious heat at 52 and 45 degrees C, respectively. A low dose of morphine (2.5 microg) potentiated the increase in the catechol signal and the decrease in the 5-hydroxyindole signal induced by noxious heat, and a high dose (10.0 microg) attenuated both. The effects of morphine at low (2.5 microg) and high doses (10.0 microg) were antagonized by naloxone (0.5 mg/kg, i.p.). These results indicate that noxious heat stimulation increased the catechol signal and decreased the 5-hydroxyindole signal in the DRN. The intrathecal administration of morphine affects the noxious stimulation-induced activity of noradrenergic and serotonergic neurotransmission in the DRN.

Antinociceptive effects of morphine injected into the nucleus parafascicularis thalami of the rat

The antinociceptive action of morphine microinjected into the nucleus parafascicularis thalami (nPf) on pain behaviors organized at different levels of the neuraxis was examined in the rat. Behaviors organized at spinal (spinal motor reflexes, SMRs), medullary (vocalizations during shock, VDSs), and forebrain (vocalization afterdischarges, VADs) levels were elicited by noxious tailshock. Morphine administered into nPf generated dose-dependent increases in thresholds of VDS and VAD, but failed to elevate SMR thresholds. Increases in vocalization thresholds were reversed in a dose-dependent manner by the microinjection of the mu-opiate receptor antagonist, methylnaloxonium, into nPf. Results are discussed in terms of the relative influence of nPf-administered morphine on nociceptive processing at spinal versus supraspinal levels of the neuraxis.

Ultrastructural localization of mu-1 opioid receptor in the dorsal raphe nucleus of the rat

A simple pre-embedding avidin-biotin-peroxidase complex technique was used to study the ultrastructural localization of mu-1 opioid receptor in the rat dorsal raphe nucleus. Using low concentrations of the first antiserum for incubation with a short reaction time to 3,3'-diaminobenzidine, the immunostaining was faint at the light microscopic level. However, at the electron microscopic level strong immunoreaction was observed. Mu-1 opioid receptors were found to be localized on the postsynaptic membrane of dendrites, extra-synaptic plasma membrane, and the surface of the small, clear vesicles in axon terminals. Of the total 407 immunopositive profiles observed, 76.4% (311/407) were dendrites and 18.9% (77/407) were axon terminals. The immunostained myelinated axons and perikarya were relatively rare, with frequencies of 1.0% (4/407) and 3.7% (15/407), respectively. About 50.8% of the immunopositive dendrites (158/311) were immunostained having their MOR-LI results beneath the postsynaptic membrane, although about 19.6% of them (31/158) also exhibited MOR-LI on other components, including the extrasynaptic plasma membrane. Other immunopositive dendrites showed staining in some other contents, including extrasynaptic plasma membrane (82/311, 26.4%) or not on the plasma membranes (71/311, 22.8%). Less than half of the immunopositive axon terminals (35/77, 45.5%) were found to make synapses with nonimmunoreactive dendrites (31/77, 40.3%) or immunopositive dendrites (4/77, 5.2%); none were found to make synapses with immunoreactive perikarya. The present study shows that mu-1 opioid receptor in the dorsal raphe nucleus plays a role at both synapse or not.

The effect of morphine on responses of mediodorsal thalamic nuclei and nucleus submedius neurons to colorectal distension in the rat

In halothane-anesthetized rats, we characterized the responses of single neurons in the nuclei of medial thalamus (MT), specifically the mediodorsal thalamic nucleus (MD) and the nucleus submedius (Sm), to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of intravenous morphine (Mor) on these responses using standard extracellular microelectrode recording techniques. 62 MD and 46 Sm neurons were isolated on the basis of spontaneous activity. 47 of the MD neurons (76%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. 38 of the Sm neurons (83%) responded to CRD, of which 89% had excitatory and 11% had inhibitory responses. Responses of MD and Sm neurons excited by CRD were related significantly to distension pressure (20-100 mmHg), with maximum excitation occurring at 60 and 100 mmHg, respectively. MD neurons inhibited by CRD also had graded responses to graded CRD, with maximum inhibition occurring at 80 mmHg. The responses to noxious (pinch, heat) and nonnoxious (tap, brush) cutaneous stimuli were studied in 59 of the MD and 44 of the Sm neurons isolated. 22 of the MD neurons (37%) studied had cutaneous receptive fields, of which 59% were large and bilateral, 41% were small and usually contralateral receptive fields. 55% of these neurons were nociceptive-specific, 45% responded to both noxious and nonnoxious cutaneous stimulation. 29 of the Sm neurons (66%) studied had cutaneous receptive fields, of which 72% were large and usually bilateral, 14% were small and bilateral, 14% were small and contralateral receptive fields. 90% of these neurons were nociceptive-specific, 10% responded to both noxious and nonnoxious stimulation. No MD or Sm neurons responded exclusively to nonnoxious cutaneous stimulation. Mor (0.125, 0.25, 0.5 and 1 mg/kg I.V.) attenuated MD and Sm neuronal excitatory responses to CRD in a dose-dependent fashion, abolishing evoked activity with a dose of 0.5 mg/kg (p < 0.05) and 1 mg/kg (p < 0.05), respectively. Naloxone (0.4 mg/kg I.V.) reversed the effects of Mor. Mor and naloxone had no effects on spontaneous activity. These data support the involvement of MD and Sm neurons in visceral nociception, and are consistent with a role of Sm in affective-motivational, and MD in both sensory-discriminative and affective-motivational aspects of nociception.

Ultrastructural localization of delta-1 opioid receptor in the dorsal raphe nucleus of the rat

The ultrastructural localization of delta-1 opioid receptor in the rat dorsal raphe nucleus was studied by the preembedding avidin-biotin-peroxidase complex technique. With application of a low concentration of the first antiserum in incubation and control of short-time reaction to 3,3'-diaminobenzidine, the immunoreaction seemed to be faint at the light microscopic level. At the electron microscopic level, however, delta-1 opioid receptor immunoreaction products were found to be localized specifically on the postsynaptic membrane of dendrites, dense-cored vesicles, and the surface of the small, clear vesicles in axon terminals with strong immunoreactivity. Of the total 659 immunopositive profiles observed, up to 62.4% (411/659) were dendrites, whereas 33.8% (223/659) were axon terminals. The immunostained myelinated axons and perikarya were relatively rare, with the frequencies 0.8% (5/659) and 3.0% (20/659), respectively. Most of the immunopositive dendrites (338/411, 82.2%) were immunostained only at the postsynaptic membranes. Other immunoreactive dendrites showed their immunoreaction products also in some other contents besides the postsynaptic membranes (44/411, 10.7%) or only in those contents but not the postsynaptic membranes (25/411, 6.1%). Only four dendrites showed their immunoreactive results only at the membrane not related to synapse (4/267, 1.0%). No dendrite was found immunostained in all the contents. About half of the immunopositive axon terminals (125/223, 56.1%) were found to make synapse with nonimmunoreactive dendrites (76/223, 34.1%) or immunoreactive dendrites (49/223, 22.0%), while only one was found to make contact with immunoreactive perikarya. The present study showed that delta-1 opioid receptor in the dorsal raphe nucleus is mostly localized on postsynaptic membrane; the main function of the delta-1 receptor in the dorsal raphe nucleus is to receive signals from the opioid-containing axon terminals through synapses.

Serotonin modulates retinotectal and corticotectal convergence in the superior colliculus

A dense serotonin (5-HT)-containing projection to the superficial layers of the superior colliculus (SC) has been demonstrated in diverse mammalian species, but how 5-HT may affect visual signals within these laminae is largely unknown. This study undertook to investigate the distribution of 2 types of 5-HT receptors in the SC and to ascertain their physiological effects on transmission of visual signals to the SC from the retinotectal and corticotectual pathways. Autoradiography of tissue sections exposed to [3H]-8-OH-DPAT (8-hydroxy-dipropylaminotetraline) or to [125I]cyanopindolol plus isoproterenol showed that 5-HT1A and 5-HT1B receptors, respectively, were present in the superficial SC layers. In unilaterally enucleated animals, binding of ligand to 5-HT1B receptors was greatly reduced on the deafferented (contralateral) side, which is consistent with the possibility that these receptors are located on preterminal axons. Binding to 5-HT1A receptors was unaltered by enucleation. In recordings of superficial layer neurons from SC slices, application of 5-HT during blockade of 5-HT1A receptors with spiperone reduced the amplitude of EPSPs evoked by stimulation of the optic tract. The 5-HT concentration for a 50% reduction in EPSP amplitude was 6 microM. Under these conditions, there were no significant alterations in either membrane potential or input resistance concurrent with 5-HT mediated reduction in EPSPs. During extracellular in vivo recordings, 5-HT, applied by iontophoresis or micropressure or by endogenous release produced by electrical stimulation of the dorsal raphé nucleus, strongly suppressed visual activity in SC neurons. The effectiveness of 5-HT application was significantly stronger on responses evoked by electrical stimulation of the optic chiasm (an average response decrement of 92.2%) than on these evoked in the same neurons by stimulation of visual cortex (an average response reduction of 32.3%). These results support the following conclusions. The 5-HT1B receptors are located preferentially on optic axon terminals and exert presynaptic inhibition of retinotectal inputs. Secondly, 5-HT1A receptors probably have a postsynaptic localization and may affect activity of SC neurons irrespective of the source of input. The combined effect of 5-HT at both subtypes would bias SC visual activity toward information received from the corticotectal pathway.

The dorsal raphe: an important nucleus in pain modulation

The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. It has abundant 5-HT neurons and many other neurotransmitter and/or neuromodulator containing neurons. Its vast fiber connections to other parts of the central nervous system provide a morphological basis for its pain modulating function. Its descending projections, via the nucleus raphe magnus or directly, modulate the responses caused by noxious stimulation of the spinal dorsal horn neurons. In ascending projections, it directly modulates the responses of pain sensitive neurons in the thalamus. It can also be involved in analgesia effects induced by the arcuate nucleus of the hypothalamus. Neurophysiologic and neuropharmacologic results suggest that 5-HT neurons and ENKergic neurons in the DRN are pain inhibitory, and GABA neurons are the opposite. The studies of the intrinsic synapses between ENKergic neurons, GABAergic neurons, and 5-HT neurons within the DRN throw light on their relations in pain modulation functions, and further explain their functions in pain mediation.

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.

Enkephalinergic innervation of GABAergic neurons in the dorsal raphe nucleus of the rat

The preembedding double immunoreaction method was used to study interrelations of enkephalinergic and GABAergic neuronal elements in the dorsal raphe nucleus of the Wistar albino rat. The enkephalin-like neuronal elements were immunoreacted by the peroxidase-antiperoxidase method and silver-gold intensified, which showed strongly and was specific. The GABA-like immunoreactive neurons were immunoreacted by the peroxidase-antiperoxidase method only. GABA-like neural somata were postsynaptic to both the enkephalin-like immunoreactive and the non-immunoreactive axon terminals. The enkephalin-like immunoreactive axon terminals were also found to synapse GABA-like immunoreactive dendrites. The GABA-like immunoreactive neuronal elements were also found to receive synapses from other non-immunoreactive as well as GABA-like immunoreactive axon terminals. Almost all of the synapses appeared to be asymmetrical. Possible functional activity of interactions among the enkephalinergic, GABAergic, and serotonergic neuronal elements in the dorsal raphe nucleus are discussed.

GABAergic innervation of serotonergic neurons in the dorsal raphe nucleus of the rat studied by electron microscopy double immunostaining

A double immunocytochemical method combining the preembedding PAP technique and the postembedding immunogold technique was used to examine interactions between GABAergic and serotonergic neurons in the same tissue sections of the dorsal raphe nucleus of the rat. A large number of immunogold stained GABAergic axon terminals were found to be presynaptic to strongly PAP immunostained serotonergic perikarya and dendrites. The types of synapses were mostly symmetrical although a few asymmetrical ones were also found. No axo-axonic synapse between the GABAergic axon terminals and the serotonergic neuronal profiles was found. These results suggest that GABAergic neurons could modulate serotonergic neurons in the dorsal raphe nucleus through synaptic relations.

Effect of superior colliculus lesions on sensory unit responses in the intralaminar thalamus of the rat

The effects of kainic acid lesions of the intermediate and deep layers of the superior colliculus on the sensory input to the intralaminar thalamus of the rat were determined. Ipsiversive circling and contralateral sensory neglect were consistently seen after lesion placement. Two to 7 days later, the intralaminar thalamus was systematically explored for extracellular mechanoreceptive unit responses to high threshold and low threshold stimuli. On the side ipsilateral to the lesion the number of responsive units was reduced by 51%. The loss was particularly marked for nociceptive units (80%), and low threshold and complex units with orofacial receptive fields (73%). This effect may involve a partial deafferentation of the intralaminar thalamus as well as altered excitatory thresholds of thalamic neurons. It is suggested that the functionally distinct direct tectothalamic projection as well as the indirect tecto-reticulo-thalamic pathway are implicated.

Sensory responses of intralaminar thalamic neurons activated by the superior colliculus

The intralaminar thalamus of anesthetized rats was explored for neurons activated by stimulation of the superior colliculus and responsive to sensory inputs. Neurons activated by stimulation of the intermediate and deep collicular layers were distributed throughout the intralaminar thalamus. Approximately one half of them responded to tectal as well as sensory inputs. The majority were nociceptive or had a more complex response pattern including responses to auditory stimulation. A small population of low threshold units had contralateral orofacial receptive fields and responded to light taps; these units were preferentially localized anteriorly in the central lateral and paracentral nuclei. Neurons responsive to tectal and sensory stimulation were randomly intermingled with other neurons which had no detectable sensory input. The results indicate that ascending projection neurons of the intermediate and deep layers of the superior colliculus provide an input to functionally diverse subpopulations of intralaminar thalamic neurons. In view of its projections to motor cortex and basal ganglia, the intralaminar thalamus appears directly implicated in basal ganglia and superior colliculus related mechanisms of attention, arousal and postural orienting.

Serotonin and substance P afferents to parafascicular and central medial nuclei

Potent analgesia is elicited by electrical stimulation of the periaqueductal gray (PAG), dorsal raphe nucleus (DRN) and intralaminar thalamus. Horseradish peroxidase conjugated wheat germ agglutinin (HRP-WGA) was stereotaxically pressure injected into the parafascicular (PF) or central medial (CM) nucleus to identify brainstem afferents to the intralaminar thalamus. WGA-immunoreactive (-ir) neurons were identified in the DRN, PAG and lateral dorsal tegmentum (LDTg) after PF and CM injections. Many retrogradely labeled cells in the DRN and ventral PAG were also serotonin-ir, and a portion of WGA-ir cells in the LDTg were substance P-ir. These results substantiate previous studies implicating the intralaminar thalamus and periaqueductal region, as well as serotonin and substance P, in antinociception.

Subcortical projections to the centromedian and parafascicular thalamic nuclei in the cat

The primary objective of this study is to identify the totality of input to the centromedian and parafascicular (CM-Pf) thalamic nuclear complex. The subcortical projections upon the CM-Pf complex were studied in the cat with three different retrograde tracers. The tracers used were unconjugated horseradish peroxidase (HRP), horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), and rhodamine-labeled fluorescent latex microspheres (RFM). Numerous subcortical structures or substructures contained labeled neurons with all three tracing techniques. These labeled structures included the central nucleus of the amygdala; the entopeduncular nucleus; the globus pallidus; the reticular and ventral lateral geniculate nuclei of the thalamus; parts of the hypothalamus including the dorsal, lateral, and posterior hypothalamic areas and the ventromedial and parvicellular nuclei; the zona incerta and fields of Forel; parts of the substantia nigra including the pars reticularis and pars lateralis, and the retrorubral area; the pretectum; the intermediate and deep layers of the superior colliculus; the periaqueductal gray; the dorsal nucleus of the raphe; portions of the reticular formation, including the mesencephalic, pontis oralis, pontis caudalis, gigantocellularis, ventralis, and lateralis reticular nuclei; the nucleus cuneiformis; the marginal nucleus of the brachium conjunctivum; the locus coeruleus; portions of the trigeminal complex, including the principal sensory and spinal nuclei; portions of the vestibular complex, including the lateral division of the superior nucleus and the medial nucleus; deep cerebellar nuclei, including the medial and lateral cerebellar nuclei; and lamina VII of the cervical spinal cord. Moreover, the WGA-HRP and rhodamine methods (known to be more sensitive than the HRP method) revealed several afferent sources not shown by HRP: the anterior hypothalamic area, ventral tegmental area, lateral division of the superior vestibular nucleus, nucleus interpositus, and the nucleus praepositus hypoglossi. Also, the rhodamine method revealed labeled neurons in laminae V and VI of the cervical spinal cord.

Enkephalinergic synaptic axon terminals on serotoninergic neurons in the dorsal raphe nucleus of the rat: an electron microscopic study by the double immunostaining method

Met-enkephalinergic synaptic axon terminals on serotoninergic neurons in the dorsal raphe nucleus of the rat were observed with double immunostaining at the electron microscopic level. Met-enkephalin-like immunoreactive axon terminals were found to make asymmetrical or symmetrical synaptic contacts with serotonin-like immunoreactive nerve cell bodies and dendrites. The findings suggest that opioid-containing neurons modulate serotoninergic neurons through synapses in the dorsal raphe nucleus.

Restricted cortical termination fields of the midline and intralaminar thalamic nuclei in the rat

The projections from the midline and intralaminar thalamic nuclei to the cerebral cortex were studied in the rat by means of anterograde tracing with Phaseolus vulgaris-leucoagglutinin. The midline and intralaminar nuclear complex taken as a whole projects to widespread, predominantly frontal, cortical areas. Each of the constituent thalamic nuclei has a restricted cortical projection field that overlaps only slightly with the projection fields of adjacent midline and intralaminar nuclei. The projections of the intralaminar nuclei cover a larger cortical area than those of the midline nuclei. The laminar distributions of fibres from individual midline and intralaminar thalamic nuclei are different and include both deep and superficial cortical layers. The parataenial, paraventricular and intermediodorsal midline nuclei each project to circumscribed parts of the prefrontal cortex and the hippocampal and parahippocampal regions. In the prefrontal cortex, the projections are restricted to the medial orbital, infralimbic, ventral prelimbic and agranular insular fields, and the rostral part of the ventral anterior cingular cortex. In contrast to the other midline nuclei, the rhomboid nucleus projects to widespread cortical areas. The rostral intralaminar nuclei innervate dorsal parts of the prefrontal cortex, i.e. the dorsal parts of the prelimbic, anterior cingular and dorsal agranular insular cortical fields, the lateral and ventrolateral orbital areas, and the caudal part of the ventral anterior cingular cortex. Additional projections are aimed at the agranular fields of the motor cortex and the caudal part of the parietal cortex. The lateral part of the parafascicular nucleus sends fibres predominantly to the lateral agranular field of the motor cortex and the rostral part of the parietal cortex. The medial part of the parafascicular nucleus projects rather sparsely to the dorsal part of the prelimbic cortex, the anterior cingular cortex and the medial agranular field of the motor cortex. Individual midline and intralaminar thalamic nuclei are thus in a position to directly influence circumscribed areas of the cerebral cortex. In combination with previously reported data on the organization of the midline and intralaminar thalamostriatal projections and the prefrontal corticostriatal projections the present results suggest a high degree of differentiation in the convergence of thalamic and cortical afferent fibres in the striatum. Each of the recently described parallel basal ganglia-thalamocortical circuits can thus be expanded to include projections at both the cortical and striatal levels from a specific part of the midline and intralaminar nuclear complex. The distinctive laminar distributions of the fibres originating from the different nuclei emphasize the specificity of the midline and intralaminar thalamocortical projections.

The antinociceptive action of supraspinal opioids results from an increase in descending inhibitory control: correlation of nociceptive behavior and c-fos expression

In an earlier report, we demonstrated that subcutaneous injection of formalin in the rat hindpaw evokes a characteristic pattern of expression of the fos protein product of the c-fos protooncogene in spinal cord neurons, and that systemic morphine reversed the fos-like immunoreactivity in a dose-dependent, naloxone-reversible manner. The present study compared the effects of intracerebroventricular administration of the mu-selective opioid ligand [D-Ala2, NMe-Phe4, Gly-ol5] enkephalin, on the pain behavior and spinal cord fos-like immunoreactivity produced by subcutaneous formalin. Formalin injection produced a biphasic pain behavioral response which lasted about 1 h. There was a significant correlation between the formalin pain score and overall fos-like immunoreactivity in the lumbar enlargement. The greatest numbers of labeled cells and most intense fos-like immunoreactivity were found in laminae I, IIo and V of the L4-5 segments, ipsilateral to the formalin-injected paw. Considerable staining was also found in the ipsilateral ventral horn laminae VII and VIII. [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin produced a dose-related, naloxone-reversible inhibition of both the formalin-evoked pain behavior and fos expression in the cord. The behavioral response to formalin, however, could be completely blocked without eliminating the expression of fos in spinal neurons. Moreover, subpopulations of neurons were differentially regulated. Thus, 100% inhibition of pain behavior was produced at a dose of [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin which reduced fos-like immunoreactivity in the superficial laminae by only 64% and in the neck and ventral cord by 85%. Furthermore, the dose of [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin which produced approximately 50% inhibition of fos-like immunoreactivity in the neck and ventral regions of the spinal cord was without effect in the superficial dorsal horn. Since the potencies for inhibition of pain behavior and fos-like immunoreactivity in the neck and ventral horn were comparable, these data suggest that the activity of neurons in these regions is directly related to the pain behavior produced by nociceptive inputs. Finally, we found that bilateral, midthoracic lesions of the dorsal part of the lateral funiculus blocked both the antinociception and fos suppression produced by intracerebroventricular [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin. These results are consistent with the hypothesis that the analgesic action of supraspinally administered opiates results from an increase in descending inhibitory controls that regulate the firing of subpopulations of spinal cord nociresponsive neurons.

Organization of the thalamostriatal projections in the rat, with special emphasis on the ventral striatum

The organization of the thalamic projections to the ventral striatum in the rat was studied by placing injections of the retrograde tracer cholera toxin subunit B in the ventral striatum and small deposits of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in individual midline and intralaminar thalamic nuclei. In order to provide a complete map of the midline and intralaminar thalamostriatal projections, PHA-L injections were also made in those parts of the intralaminar nuclei that project to the dorsal striatum. The relationship of thalamic afferent fibres with the compartmental organization of the ventral striatum was assessed by combining PHA-L tracing and enkephalin immunohistochemistry. The various midline and intralaminar thalamic nuclei project to longitudinally oriented striatal sectors. The paraventricular thalamic nucleus sends most of its fibres to medial parts of the nucleus accumbens and the olfactory tubercle, whereas smaller contingents of fibres terminate in the lateral part of the nucleus accumbens and the most ventral, medial, and caudal parts of the caudate-putamen complex. The projections of the parataenial nucleus are directed towards central and ventral parts of the nucleus accumbens and intermediate mediolateral parts of the olfactory tubercle. The intermediodorsal nucleus projects to lateral parts of the nucleus accumbens and the olfactory tubercle and to ventral parts of the caudate-putamen. The projection of the rhomboid nucleus is restricted to the rostrolateral extreme of the striatum. A diffuse projection to the ventral striatum arises from neurons ventral and caudal to the nucleus reuniens rather than from cells inside the nucleus. Fibres from the central medial nucleus terminate centrally and dorsolaterally in the rostral part of the nucleus accumbens and medially in the caudate-putamen. Successively more lateral positions in the caudate-putamen are occupied by fibres from the paracentral and central lateral nuclei, respectively. The lateral part of the parafascicular nucleus projects to the most lateral part of the caudate-putamen, whereas projections from the medial part of this nucleus terminate in the medial part of the caudate-putamen and in the dorsolateral part of the nucleus accumbens. Furthermore, a rostral to caudal gradient in a midline or intralaminar nucleus corresponds to a dorsal to ventral and rostral to caudal gradient in the striatum. In the ventral striatum, thalamic afferent fibres in the "shell" region of the nucleus accumbens avoid areas of high cell density and weak enkephalin immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of nociceptively identified neurons in thalamic parafascicularis by chemical stimulation of dorsal raphe with glutamate, morphine, serotonin and focal dorsal raphe electrical stimulation

The properties of local application of glutamate, morphine and serotonin in the dorsal raphe (DR) area and the effects of DR electrical stimulation on the spontaneous activity and on the nociceptive responses of 135 parafascicularis (PF) neurons were studied. It was observed that local glutamate application within the DR exerts an effect upon the "nociceptive-on" PF neuronal activity similar to that induced by focal electrical stimulation of the DR in intact animals and in animals after dorsal spinal cord section. In addition, local application of morphine and serotonin in the DR area elicits different effects on the spontaneous activity versus the nociceptive responses of PF neurons. These observations suggest that opioids and serotonin at least in part participate in modulation of pathways from DR to PF. This observation is consistent with the hypothesis that the DR ascending path modulates nociceptive input to the PF (at least in part) via activations of both opioid and serotonergic receptors.