Electrical stimulation of the rat ventral midbrain elicits antinociception via the dorsolateral funiculus

The pain-suppressive effects of focal electrical stimulation of sites throughout the ventral midbrain were examined in awake rats. Chronic bipolar electrodes were implanted in medial and lateral regions of the midbrain. Current thresholds for suppression of the tail-flick reflex in response to noxious heat were determined for both a biphasic and a monophasic stimulation parameter at each site. Stimulation of areas throughout the ventral midbrain produced tail-flick suppression (TFS), but no one area was consistently effective in all animals. Monophasic and biphasic stimulation were qualitatively equal in the duration of TFS and the distribution of effective sites. The production of TFS was not correlated with other behavioral reactions to brain stimulation. TFS appeared to be mediated by non-opiate pathways since naloxone administration (10 mg/kg) had no discernible effect on the production of TFS. The current threshold for producing TFS was extremely variable over both short (one half hour) and long (one week) intervals. The incidence of TFS from previously effective sites was significantly less following bilateral dorsolateral funiculus (DLF) lesions, indicating that the antinociceptive effects of ventral midbrain stimulation are mediated by this spinal pathway.

Partial purification and characterization of a cellular protein that binds to the SV40 core origin of DNA replication

A monkey cell factor that interacts specifically with double- and single-stranded DNA sequences in the early domain of the simian virus 40 (SV40) core origin of replication was identified using gel-retention assays. The protein was enriched over 1200-fold using ion-exchange and affinity chromatography on single-strand DNA cellulose. Binding of protein to mutant origin DNA restriction fragments was correlated with replication activity of the mutant DNAs. Exonuclease footprint experiments on single-stranded DNA revealed prominent pause sites in the early domain of the core origin. The results suggest that this cellular protein may be involved in SV40 DNA replication.

B vitamins suppress spinal dorsal horn nociceptive neurons in the cat

To explore the role of vitamin B in neural mechanisms of analgesia, we investigated the effect of a compound of vitamins B1, B6 and B12 (Neurobion, E. Merck) on the nociceptive responses of single neurons in the spinal cord dorsal horn in anesthetized cats. Intrathecal superfusion of Neurobion, using a small pool placed on the spinal surface, produced a significant dose-dependent depression in the responses evoked by noxious skin heating (50 or 52 degrees C, 10 s) of hindfoot skin, but not of spontaneous activity in dorsal horn neurons. These results indicate that the therapeutic effect of vitamin B compounds in the clinical management of pain may involve a suppression of nociceptive transmission at the spinal level.

Microinjections of glutamate or morphine at coincident midbrain sites have different effects on nociceptive dorsal horn neurons in the rat

Responses of single lumbar spinal neurons to noxious skin heating (50 degrees C, 10 s) were electrophysiologically recorded in barbiturate-anesthetized rats. Responses of all neurons were suppressed by electrical stimulation in the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF). Microinjection of glutamate (GLU, 0.1-0.3 microliter, 0.5 M) into the PAG rapidly (within 15 s) suppressed (to 13-55% of control) the responses of 6/16 neurons with recovery within 8 min. The remainder were affected less at even higher doses (0.5-1 microliter). Responses of 4/10 neurons were suppressed following GLU microinjected into the LRF. We also tested effects of microinjection of morphine (MOR, 5 micrograms/0.5 microliter) into GLU-sensitive and insensitive PAG sites. Responses of 4 neurons were unaffected, 4 were enhanced (to 130-155%), and 2 suppressed (to 43 and 57%) following MOR in PAG, with enhancement or suppression beginning within 12-20 min and lasting 40 to over 70 min. The differing effects of GLU and MOR may reflect different mechanisms for the descending modulation of spinal nociceptive transmission.

Antinociception vs motor effects of intrathecal vasopressin as measured by four pain tests

The effects of intrathecal (i.t.) vasopressin (VP) on nociception were quantitatively tested in rats using 4 pain tests: tail flick, tail shock vocalization, hot plate, and formalin. In addition, motor effects of VP were examined qualitatively. I.t. VP produced a prolonged antinociception lasting at least 40 min on the tail flick (2.5 and 25 ng) and formalin (25 ng) tests, and a brief antinociception lasting less than 20 min on the tail shock (25 ng) and hot plate (25 ng) tests. Those rats not responding to the pain tests showed no signs of perceiving the pain stimulus, such as orientation to the stimulus or vocalization. In addition, i.t. VP produced scratching bouts (2.5 and 25 ng) and suppressed hindbody motor function (25 ng). The motor inhibitory effects of VP, although severe in some rats, were brief, lasting less than 15 min. In conclusion, i.t. VP produces antinociception in addition to its motor effects, and these properties appear to be due to separate mechanisms.

Inhibition of the responses of cat dorsal horn neurons to noxious skin heating by stimulation in medial or lateral medullary reticular formation

Responses of single lumbar dorsal horn units to noxious radiant heating (50 degrees C, 10 s) of glabrous footpad skin were recorded in cats anesthetized with sodium pentobarbital and 70% nitrous oxide. The heat-evoked responses of 37/40 units were reduced during electrical stimulation (100 ms trains, 100 Hz, 3/s, 25-600 microA) in the medullary nucleus raphe magnus (NRM) and/or in laterally adjacent regions of the medullary reticular formation (MRF). Inhibition was elicited by stimulation in widespread areas of the medulla, but with greatest efficacy at ventrolateral sites. The magnitude of inhibition increased with graded increases in medullary stimulation intensity. Mean current intensities at threshold for inhibition or to produce 50% inhibition were higher for NRM than for MRF sites. Units' responses to graded noxious heat stimuli increased linearly from threshold (42-43 degrees C) to 52 degrees C. During NRM (5 units) or ipsilateral MRF stimulation (7 units), responses were inhibited such that the mean temperature-response functions were shifted toward higher temperatures with increased thresholds (1.5 degrees and 1 degree C, respectively) and reduced slopes (to 60% of control). Contralateral MRF stimulation had a similar effect in 4 units. Inhibitory effects of NRM and MRF stimulation were reduced (by greater than 25%) or abolished in 4/6 and 5/12 units, respectively, following systemic administration of the serotonin antagonist methysergide. Inhibitory effects from NRM, ipsi- and contralateral MRF were reduced or abolished in 2/9, 4/8 and 6/9 cases, respectively, following systemic administration of the noradrenergic antagonist phentolamine. These results confirm and extend previous studies of medullospinal inhibition and the role of monoamines, and are discussed in terms of analgesic mechanisms.

The dorsal raphe nucleus: a re-evaluation of its proposed role in opiate analgesia systems

Previous studies have concluded that (a) electrical stimulation in the periaqueductal gray/dorsal raphe nucleus (PAG/DRN) region specifically produces either non-opiate or opiate forms of antinociception dependent upon the dorsoventral level of stimulation and (b) the 'opiate' form of stimulation-produced analgesia (SPA) arising from the ventral PAG/DRN region shows cross-tolerance with opiate forms of footshock analgesia, implying common neural substrates. This latter conclusion in turn implies that SPA elicited from the ventral PAG/DRN region would be expected to be antagonized by scopolamine, since this muscarinic cholinergic antagonist blocks opiate footshock analgesia. The present study demonstrates instead that neither 10 mg/kg naloxone nor 10 mg/kg scopolamine had any effect on SPA elicited from sites histologically verified to lie within the presumptive 'opiate' ventral PAG/DRN region. These data bring into question both the site specificity of opiate SPA and the common mediation of ventral PAG/DRN SPA and opiate forms of footshock analgesia.

Parametric and pharmacological studies of midbrain suppression of the hind limb flexion withdrawal reflex in the rat

These experiments quantitatively analyzed effects of electrical midbrain stimulation on a nociceptive hind limb flexion reflex in rats anesthetized with sodium pentobarbital. We recorded the force of isometric hind limb flexion withdrawal, and related flexor electromyographic (EMG) activity, elicited by noxious heat (42-54 degrees C, 10 sec) applied to the ventral hind paw. Several hind limb flexors including biceps femoris were active during the reflex. Quantified reflex responses to identical noxious heat stimuli delivered every 2 min were constant in magnitude and were reduced or abolished during stimulation (100 msec trains at 100 Hz, 3/sec, 15-325 microA) in the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF). LRF was significantly more effective than PAG stimulation in suppressing reflex responses. The magnitude of the reflex responses increased with graded increases in the temperature of the noxious heat stimulus. The slope of the temperature-response relationship was significantly reduced during PAG stimulation, whereas it was shifted toward higher temperatures with significantly increased threshold during LRF stimulation. To investigate possible transmitters involved, we tested if PAG- or LRF-evoked reflex suppression was affected following systemic administration of the opiate antagonist naloxone, the serotonin antagonist methysergide, the noradrenergic antagonist phentolamine, or the cholinergic antagonist scopolamine. Naloxone had little effect, while methysergide and phentolamine reduced PAG- and LRF-evoked reflex suppression in about one-half of the cases. Scopolamine largely reduced PAG- and LRF-evoked reflex suppression (in 8/9 and 4/6 rats, respectively). These results indicate that the flexion reflex is under parametrically but not pharmacologically distinct inhibitory midbrain controls.

Inhibition of rat spinothalamic tract neuronal responses to noxious skin heating by stimulation in midbrain periaqueductal gray or lateral reticular formation

The descending inhibitory effects of electrical midbrain stimulation on identified lumbar spinothalamic tract (STT) neurons were investigated in rats anesthetized with sodium pentobarbital. STT units were identified by their antidromic response to stimulation in the contralateral ventrobasal thalamus or medial lemniscus (mean conduction velocity: 18-20 m/sec). Thirty-two of 43 dorsal horn multireceptive STT units gave reproducible responses to noxious heat stimuli (42-56 degrees C, 10 sec) applied to the ventral hind paw. All STT units' responses were suppressed during stimulation (100 msec trains at 100 Hz, 3/sec, 15-300 microA) in PAG or LRF bilaterally. On- and offset of inhibition was rapid (less than 1 sec). STT unit responses to noxious heat were more effectively suppressed by LRF than PAG stimulation, based on statistically significant differences in mean thresholds for inhibition, slopes of current-inhibition plots, and mean current intensities evoking 50% inhibition. Mapping experiments revealed lowest-threshold (less than 25 microA) sites for inhibition to be in ventral PAG, subjacent tegmentum, and LRF bilaterally. STT unit responses increased with graded increases in stimulus temperature from threshold (mean: 46 degrees C) to 56 degrees C. Slopes of temperature-response functions were significantly reduced with little change in threshold during PAG stimulation, whereas these functions were shifted in a parallel manner toward higher temperatures during LRF stimulation. The serotonin antagonist methysergide reduced or blocked PAG- or LRF-evoked inhibition in 3/5 and 5/12 STT units, respectively. These results indicate that STT units are subject to different inhibitory controls from PAG and LRF.

Medial hypothalamic stimulation suppresses nociceptive spinal dorsal horn neurons but not the tail-flick reflex in the rat

This study investigated the potential analgesic effects of medial hypothalamic stimulation (HS) on a measure of nocifensive behavior (tail-flick test (TF] in awake rats, and potential inhibitory effects of identical HS on spinal dorsal horn neuronal responses to noxious skin heating in the same animals anesthetized with sodium pentobarbital. Sixty-five male Sprague-Dawley rats implanted with a bipolar stimulation electrode in histologically verified medial hypothalamic sites were tested behaviorally for TF suppression during HS (100 ms trains at 100 Hz, 3/s, 100-1100 microA) in 2-4 consecutive weekly test sessions. Thirty-three of these rats were then used in electrophysiological experiments to record responses of 36 dorsal horn units to noxious skin heating (48-54 degrees C, 10 s/2 min) of the hindfoot pad in the absence of and during HS. Behaviorally, 31/65 rats had no TF suppression at the highest HS intensity tested (1100 microA), 24/65 rats exhibited aversive behavior or motor activity which disallowed reliable TF testing, and only 10/65 rats showed TF suppression in at least one test session. In electrophysiological experiments, the heat-evoked responses of 25/36 dorsal horn units were inhibited to at least 50% of control during HS. The responses of 11 units remained at 65-100% of the control responses during HS of up to 1100 microA. In rats demonstrating TF suppression, 4/7 units were inhibited. In rats with no TF suppression, 10/15 units were inhibited, and in rats showing aversive behavior, 11/14 units were inhibited by HS. These data indicate that although HS suppresses spinal nociceptive neurons, it does not cause reliable TF suppression in unanesthetized rats and bring into question the often-held assumption that stimulation-evoked descending inhibition of spinal nociceptive neurons implies behavioral analgesia.

Effects of midbrain stimulation and iontophoretic application of serotonin, noradrenaline, morphine and GABA on electrical thresholds of afferent C- and A-fibre terminals in cat spinal cord

We have used the single-fibre excitability testing method to investigate whether electrical stimulation in midbrain periaqueductal gray or lateral reticular formation, as well as intraspinal iontophoretic application of the suspected inhibitory neurotransmitters serotonin (5-hydroxytryptamine), noradrenaline, the opiate morphine, or gamma-aminobutyric acid (GABA), exert presynaptic actions at the central terminals of cutaneous afferent unmyelinated or myelinated fibres. Thresholds to antidromically excited 42 single unmyelinated and 18 myelinated fibres in the sural nerve by intraspinal microstimulation were determined before and during periaqueductal gray or lateral reticular formation stimulation (3 100 ms trains/s at 100 Hz; 100-900 microA) or intraspinal iontophoretic application (40-300 nA) of 5-hydroxytryptamine, noradrenaline, morphine or GABA from a multibarrel micropipette. Periaqueductal gray or lateral reticular formation stimulation had mixed effects on unmyelinated and myelinated fibre thresholds, with most threshold measurements within +/- 10% of control. There was a tendency for thresholds to increase more during periaqueductal gray than lateral reticular formation stimulation. Thresholds of unmyelinated fibres were predominantly raised during iontophoretic application of 5-hydroxytryptamine (20/29 fibres), noradrenaline (10/13) and morphine (15/21), while GABA had mixed effects; thresholds of nearly all myelinated fibres were raised by each drug. Both periaqueductal gray or lateral reticular formation stimulation and application of 5-hydroxytryptamine, noradrenaline or morphine tended to raise thresholds in the majority of the 53 unmyelinated and myelinated fibres tested. Methodological problems in interpreting the physiological significance of these results for presynaptic modulation are discussed.

Inhibition of the responses of neurons in the rat spinal cord to noxious skin heating by stimulation in midbrain periaqueductal gray or lateral reticular formation

Single units responsive to noxious heating of glabrous hindfoot skin were recorded in the lumbar spinal cord of rats anesthetized with sodium pentobarbital. Unit responses to heat stimuli (e.g. 50 degrees C, 10 s) delivered at 2-min intervals were stable and were markedly suppressed during stimulation (100-ms pulse trains at 100 Hz, 3/s, 25-400 microA) in the midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF). Inhibition did not appear to outlast the midbrain stimulation period. By systematically varying the position of an array of 3 or 5 stimulating electrodes, we observed that stimulation in PAG and subjacent tegmental areas, as well as in widespread regions of the LRF bilaterally, suppressed unit responses to noxious skin heating. The degree of suppression of unit responses increased with graded increases in PAG or LRF stimulation intensity. LRF appeared to be more efficacious than PAG stimulation, based on lower currents at threshold, as well as on significantly greater slopes in current-inhibition plots for LRF compared to PAG stimulation. Unit responses increased linearly with graded increases in stimulus temperature from 42 to 54 degrees C. Slopes of temperature-response lines were reduced during PAG stimulation with no change in response threshold, while temperature-response lines were shifted in a parallel manner toward higher temperatures during LRF stimulation with an increase in response threshold. The results suggest that differential inhibitory systems are activated by PAG or LRF stimulation and are discussed in relation to previous findings in the cat and as a possible mechanism of stimulation-produced analgesia.

Hypothalamic inhibition of rat dorsal horn neuronal responses to noxious skin heating

The responses of single lumbar dorsal horn neurons to noxious radiant heat stimuli (42-54 degrees C, 10 sec, 1/2 min) applied to glabrous hind paw skin were recorded in rats anesthetized with sodium pentobarbital. Unit responses to 50 or 52 degrees C stimuli were constant over time and were consistently and powerfully inhibited during bipolar stimulation (three 100 msec trains/sec at 100 Hz, 200 microA) in the medial hypothalamus. Inhibition was also evoked by stimulation in medial and ventrobasal thalamic nuclei, lateral hypothalamus and adjacent cerebral peduncle, and amygdala. Inhibition increased with graded increases in intensity of hypothalamic stimulation, with a mean inhibitory threshold of 71 +/- 43 (S.D.) microA for 13 units. The responses of dorsal horn units to graded increases in the temperature of noxious heat stimuli were inhibited during hypothalamic stimulation, such that slopes of the linear temperature-response functions were reduced with no change in response threshold (mean: approximately 44 degrees C). Inhibition was blocked or reduced in 4/7 units following systemic administration of the 5-hydroxytryptamine (5-HT) antagonist methysergide. The results confirm and extend previous work in the cat and are discussed in relation to analgesic mechanisms.

[New derivatives of 10,11-dihydro-5H-dibenz[b,f]azepine with antiarrhythmic action]

After an introduction about the importance of the 10, 11-Dihydro-5H-dibenz[b,f]azepine system for the drug research and according to experiences about the change of pharmacodynamic effects in the field of the phenothiazine bases by dialkylaminoacyl substitution in the following report some new 3-carbalkoxyamino-5-omega-aminoacyl-10,11-dihydro-5H-dibenz[b,f]++ +azepines and their intermediates for synthesis are described. In result of pharmacological investigations which showed antiarrhythmic activities the substance 3-carbethoxyamino-5-dimethylaminoacetyl-10,11-Dihydro-5H-dib enz[b, f]azepine X HCl (GS 015, AWD 19-166, Bonnecor) was selected for clinical tests. For studying the biotransformation of this substance the expected main metabolites were prepared by chemical synthesis.

Neuronal responsiveness to gonadotropin-releasing hormone and its correlation with sexual receptivity in the rat

In the present study, an attempt was made to correlate the neuronal responsiveness of individual preoptic-septal (POA/S) units to iontophoretically applied GnRH with the onset of sexual receptivity. In both behavioral and electrophysiological studies, ovariectomized, estrogen-primed rats were used. In behaviorally tested rats, lordosis quotients (LQ) were determined at varying times following progesterone (P) injection. For electrophysiological studies, P was given 1 hr after the start of recording. GnRH was iontophoretically applied for 30 sec at 16 nA on spontaneously discharging cells. A unit was deemed excited or inhibited if a repeatable 30% change in discharge rate was observed. From 2-10 hours as the LQ increased from 17 to 90 the total number of GnRH sensitive cells did also. The majority of responsive cells were excited by the peptide. As receptivity displayed a sharp increase from 2 to 6 hours the mean responsiveness of cells excited by GnRH was significantly elevated over inhibitory responses. These findings confirm the E/P biasing effect on POA/S unit responses to GnRH. Moreover, they suggest that a dynamic relationship exists between GnRH responses at the cellular level and sexual behavior throughout the course of steroid-induced receptivity.

Naloxone does not consistently affect inhibition of spinal nociceptive transmission produced by medial diencephalic stimulation in the cat

In cats anesthetized with sodium pentobarbital and artificially ventilated with 70% N2O, the response of single lumbar dorsal horn units to noxious radiant heat stimuli (50 degrees C, 10 sec) applied to glabrous footpad skin were recorded with microelectrodes. Unit heat-evoked responses were markedly suppressed by electrical stimulation (100 msec pulse trains at 100 Hz, 3/sec, 50-300 microA) at sites in the medial basal diencephalic periventricular gray, lateral hypothalamic area, preoptic area, and posterior basal telencephalon. Inhibition of dorsal horn unit responses produced by brain stimulation was not consistently affected following systemic administration of the opiate antagonist naloxone (0.5-1 mg/kg i.v.), indicating that the endogenous opioid peptides are not primarily involved in the mediation of this descending inhibition under the experimental conditions.

Serotonin involvement in descending inhibition of spinal nociceptive transmission produced by stimulation of medial diencephalon and basal forebrain

The responses of single lumbar dorsal horn neurons to noxious radiant heat stimuli (50 degrees C, 10 sec) applied to glabrous footpad skin were recorded in cats anesthetized with sodium pentobarbital and 70% N2O. Responses were markedly reduced during electrical stimulation (100-msec trains at 100 Hz, 3/sec, up to 400 microA) at sites in the medial diencephalic periventricular gray (PVG), preoptic area, and basal forebrain. A role for serotonin (5-hydroxytryptamine, 5-HT) was investigated by determining whether descending inhibition from these areas could be affected by (1) acute systemic administration of the 5-HT antagonist methysergide, or (2) depletion of central 5-HT levels by pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA; 500 mg/kg, i.p.). Inhibition produced by stimulation at these sites was reduced or abolished in 22 cases following administration of methysergide (0.2 to 1 mg/kg) to non-pretreated cats. In the PCPA-pretreated cats, stimulation in preoptic or basal forebrain areas inhibited the responses of 26 units to noxious skin heating to varying degrees; PVG stimulation inhibited the responses of 14 of 26 units, while the remainder were unaffected. The mean current threshold for inhibition produced by PVG or preoptic/basal forebrain stimulation was significantly higher, while mean inhibition at 200 microA was significantly lower, in units from PCPA-pretreated cats compared to those from non-pretreated cats. The results indicate that 5-HT may be involved in the mediation of spinal inhibition produced by medial diencephalic and basal forebrain stimulation.

Inhibition of spinal dorsal horn neuronal responses to noxious skin heating by lateral hypothalamic stimulation in the cat

The responses of single lumbar dorsal horn units to noxious radiant heating (50 degrees C, 10 s) of glabrous foot pad skin were recorded in cats anesthetized with sodium pentobarbital and 70% N2O. The heat-evoked responses of each of 38 units were markedly reduced during electrical stimulation (100-ms trains at 100 Hz, 3/s, 25-300 microA) in the lateral hypothalamic area (LH). LH sites at which stimulation inhibited dorsal horn unit heat-evoked responses were mapped by systematically varying the position of the stimulating electrode. Inhibition was generated at posterior through anterior hypothalamic levels in a region extending laterally from the periventricular gray (PVG) to the cerebral peduncles on both sides and ventrally to the base of the brain. The magnitude of inhibition increased with graded increases in LH stimulation intensity. Respective mean current intensities at threshold for generating inhibition were 27.6 +/- 17.4 (SD) microA for contralateral and 30.1 +/- 23.7 microA for ipsilateral LH stimulation. Dorsal horn unit responses to a series of graded noxious heat stimuli generally increased linearly from threshold (38-45 degrees C) to 52 degrees C. The slopes of such linear temperature-response functions were reduced, with no significant change in threshold, when the temperature series was repeated during concomitant ipsilateral LH stimulation. Contralateral LH stimulation produced similar slope reductions but additionally produced a significant mean threshold increase of 1.7 degrees C. The inhibitory effect of LH stimulation was significantly reduced in nine units from a mean of 28 +/- 18% of control to a mean of 59 +/- 18% following systemic administration of the serotonin (5-hydroxytryptamine, 5-HT) antagonist, methysergide (0.3-1 mg/kg). Possible functional relationships of LH with brain stem inhibitory systems and its role in analgesic mechanisms are discussed.

Inhibition of spinal dorsal horn neuronal responses to noxious skin heating by medial preoptic and septal stimulation in the cat

1. Responses of single lumbar dorsal horn units to controlled noxious radiant heating of glabrous hindfoot skin were recorded in cats anesthetized with sodium pentobarbital and 70% N2O. The heat-evoked responses of all units studied were markedly suppressed during concomitant electrical stimulation (mean, 30 Hz; 25-300 microA) of medial preoptic and ventromedial septal areas. 2. Brain sites at which stimulation inhibited spinal neuronal heat-evoked responses were mapped by systematically varying the depth of the stimulating electrode in tracks at anteroposterior levels +14 through +18. At each stimulation site, the magnitude of the spinal neuronal response to heat (50 degrees C, 10 s, 1 per 3 min) during brain stimulation was expressed as a percentage of the control response (no brain stimulation), which was stable in size over repeated trials. Sites at which stimulation markedly reduced the heat-evoked response were located in the medial preoptic area and in the ventromedial septum (diagonal band of Broca) up to anterior level +17. 3. The magnitude of inhibition increased with graded increases in brain-stimulation intensity. For 15 units, the mean current threshold to generate inhibition was 25 microA. 4. Responses of dorsal horn neurons to a series of graded noxious heat stimuli increased linearly from threshold (40-45 degrees C) to 52 degrees C. The slopes of such linear temperature-response curves were significantly reduced, without a change in the response threshold, when the temperature series was repeated during concomitant preoptic or septal stimulation. 5. The possible relationship of the medial preoptic and septal areas to inhibitory systems in the brain stem, and their possible role in analgesic mechanisms, are discussed.

Effect of iontophoretically applied 5-hydroxytryptamine on the excitability of single primary afferent C- and A-fibers in the cat spinal cord

Excitability testing of single sural afferent C- and A-fibers was employed to study possible presynaptic effects of 5-hydroxytryptamine (5-HT) applied iontophoretically at the intraspinal point of lowest threshold for their antidromic activation in anesthetized or decerebrate spinalized cats. Threshold for single fibers recorded in the sural nerve was measured prior to and during iontophoretic application of 5-HT through a micropipette positioned in close proximity to the intraspinal stimulating electrode. 5-HT produced dose-related increases in threshold for antidromic activation in 21 or 30 C-fibers. Six of 9 A delta, and 4 of 7 A beta-fibers were similarly affected.