Analgesia for orofacial nociception produced by morphine microinjection into the spinal trigeminal complex 1

14s

Pain. 15 (1983) 145-155 Elsevier Biomedical Press

Analgesia for Orofacial Nociception Produced by Morphine Microinjection into the Spinal Trigeminaf Complex ’ 3. Peter Rosenfeld, Catherine Pickrel and James G. Broton Departments of Neurobiolog,, Physiology and Psychology, Northwestern Unrvers~ty, Cresup Neuroscience Laboratory, Evanston, Ill. 60201 (U.S.A.) (Received

10 February

1982, accepted

9 June 1982)

Summa~ Morphine sulfate (0.75 pg) was microinjected into (rat) rostra1 and caudal trigeminal areas singly and simultaneously, using cannula-electrode combinations. Also, 0.5 pg or 1.0 pg of morphine was injected into nucleus reticularis paragigantocellularis (PGC). Both single trigeminal nuclear microinjections significantly elevated the latency to a defensive face-rub reaction to noxious facial heat, bilaterally. There was no summation effect with the conjoint injection of 1.5 pg total to rostra1 and caudal nuclear areas. The 0.5 pg injection in PGC had a significantly Iarger effect than did the 0.75 pg trigeminal injections. The caudal but not the rostra1 trigeminal injections did elevate the threshold for aversive reaction to caudal trigeminal nuclear stimulation of the injected tissue. This finding contrasts with the previously reported fact that as much as 1.O pg of morphine in PGC has no effect on this aversive reaction threshold to brain stimulation.

Introduction Analgesic doses of systemic morphine (6-10 “g/kg) consistently elevate the threshold for aversive reaction to electrical stimulation of the primary afferent synaptic region in rostra1 and caudal regions of the trigeminal nuclear complex [ 12,13,15,16]. Presumably, the systemic injection is effective because it accesses one or more established substrates for opiate analgesia such as periaqueductal gray (PAG [lo]), nucleus raphe magnus (RM [4]), and nucleus reticularis paragigantocel’ Supported

by NIH Grants

030~3959/83/~-~/~03.~

DE05204

and GM23696.

b 1983 Eisevier Biomedical

Press

146

lularis (PGC [17]). Nevertheless we have found that microinjections in these areas which do produce facial analgesia fail to elevate the threshold for aversive reaction to stimulation of rostra1 and caudal trigeminal regions. These thresholds in the same rats were altered by systemic injection [ 12.131. One as yet untested possible substrate for the systemic effect is the trigeminal nuclear complex itself. Although there is good evidence that the primary afferent terminal area in the spinai cord is an effective opiate analgesia substrate [7.20], evidence of a comparable role for the trigeminal homolog of the cord is as yet lacking. Thus the two related major purposes of this study were to ( 1) xz~e~~ the effect of trigeminal nuclear opiate microinjection on orofacial nociception. and (2) determine whether or not the trigeminal nuclear complex could be a substrate for the systemic opiate effect on aversive trigeminal nuclear stimulation. Also investigated are the questions of which s&nuclear region(s) (rostra1 and/or caudal) of the long trigeminal complex might be an analgesia substrate. and whether or not conjoint injections of different trigeminal areas might produce summatin~ effects, as we saw previously with simultaneous injections of midbrain and bulbar reticular substrates [ 131. Methods Ten albino rats (450-600 g) were surgically implanted with cannula-electrode combjnations aimed at the subnuclei oralis and caudalis of the trigenlina1 nuclear complex. However, since a majority of the rostra1 trigeminal electrode locations could not be identified positively, rostra1 brain stimulation effects will not be discussed further. Twisted 78 pm nichrome wire pairs for electrical bipolar brain stimulation (as described elsewhere [ 12,131) were fastened with epoxylite to 25gauge stainless steel outer guide cannulae. The electrode pairs of 1 mm intertip distance extended 6 mm below the end of the guide cannulae -which ended 6 mm above targets. The outer cannulae were implanted with 31-gauge obturator wires ending flush with the end of the guide cannulae. During microinjection, a 31-gauge injection cannula would replace the obturator and extend 6 more mm below the end of the guide. The aim of this arrangement was to provide the ability to stimulate the same region of a trigeminal nuclear locus previously opiate-injected. For comparison, simple cannulae were implanted in PGC with 1 mm distances between inner and outer cannula tips (as in ref. 12). All injections were in 0.5 ~1 of solution injected over 2 min. Doses will be given in the results. Our orofacial peripheral analgesia index was the face-rub latency: installed on the rats’ faces between eye and upper lip, bilaterally, were 10 ft, 4 mm X 1 mm diameter. l/8 W resistors. These devices were in contact with the skin through facial fur. When a 150-250 mA, DC current is applied to such a resistor, it becomes aversively hot, producing a non-stereotyped, face-rubbing escape reaction within 10 sec. Previous work has established the reliability and validity of the face-rub response latency as a non-reflexive orofacial nociception index [ 11- 141. It is noted that the heat is discontinued the instant the rat responds. The test is given once a day per resistor.

147

To find the threshold for aversive responses to brain stimulation in trigeminal nucleus caudalis (TNC), we gave 200 Hz gradually rising, sinusoidal bursts of about 0.5 set duration every 5 set, increasing the peak-to-peak current of each successive burst by about 5 PA, until an aversive reaction was seen. This reaction was defined to involve a non-stereotyped movement of all 4 limbs in an apparent escape attempt as agreed by two independent observers 95% of the time {as in refs. 12,13,15,16). One or more of the following behaviors accompanied this reaction 85% of the time: defecation or urination, squealing, face rubbing, and teeth gnashing. The current level just producing the aversive reaction was recorded as the aversive reaction threshold. We have previously reported that this judged threshold will sustain goal-directed, passive instrumental avoidance; this validates the judged aversive property of the threshold current. (These stimulation parameters and procedures are explained in ref. 16.) It is noted that the aversive series is given only in the ascending direction and only once per day. The first time the rat shows aversive reactions and distress signs, the series is terminated and the current settings recorded as the threshold. The aim here is to minimize the rat’s discomfort. The rats’ weights were recorded daily during all phases of the study. From 3 to 6 days following surgery and thereafter until sacrifice, only one rat showed a weight loss. Two weeks after recovery from surgery the rats with rostra1 and caudal trigeminal cannula-electrodes received in randomized order the following microinjections, one every other day: (1) isotonic saline first control; both trigeminal sites were saline-injected; (2) single rostra1 trigeminal nuclear morphine (sulfate) injection, 0.75 pg; (3) single caudal nuclear microinjection 0.75 fig; (4) systemic (s.c.) morphine microinjection, 6 mg/kg; (5) isotonic saline second control (like (1)); (6) sham injection; both trigeminal cannulae were utilized but no substance was injected; (7) conjoint trigeminal microinjection; procedures (2) and (3) were done simultaneously so that a total of 1.5 pg of morphine was given. Behavioral testing followed each of the above microinjection treatments by 10 min and followed the systemic injection by 60 min. Behavioral data were either latencies in seconds or aversive stimulation threshold currents in PA. For all rats the average of the 2 baseline control days and of the sham day was taken as the denominator of a fraction whose numerator would be a value obtained on an experimental drug or saline control day. This fraction x 100 gives the drug effect expressed as percent of control (%C); 100% = no change, > 100% = analgesia, ( 100% = hyperalgesia. If under any drug, no reaction to a heat stimulus occurred after 300% of control latency expired or if 25 set lapsed, whichever came first, the heat was stopped and the value 300% or the percent change at 25 set was recorded. This procedure was used to avoid injurious heat levels and yields a conservative bias in calculated means. For brain stimulation, 300% of baseline current was also considered a cut-off level. In determining doses for microinjections we attempted to find lower doses which would produce analgesia but not total block, otherwise it would be impossible to interpret a total block produced by conjoint microinjection. On double opiate microinjection days, only face-rub data were obtained. Data analysis utilized 2-tailed t tests. Effects of particular injections were analyzed in repeated measures (wit~n-subject) t test comparisons with saline controls. Com-

14x

parison of different experimental injections from different groups involved independent-group (between-subject) r tests. Additional non-parametric tests, as described below, were done when deemed appropriate. Histological verification of all placements concluded the studies. Immediately after rats lost consciousness due to the sacrificing injection. 0.5 ~1 of saturated Fast Green dye was injected for 2 min through both cannulae. Brains were removed within 10 min.

Results

Since each of the 10 rats had a heater on each side of his face, there were 20 possible measures for each injection. One electrode pair per location for each rat restricted the sample size for brain stimulation to a maximum of 10. In the case of the rostra1 trigeminal electrodes. in 5 rats the tips could not be positively localized. (The damage from attached cannulae apparently obscured tip loci.) Two other electrodes were not well localized in the nucleus, but were medial to it. Therefore no effects on rostra1 stimulation are presented. The Fast Green marking procedure allowed identification of on-target injection-cannula tips. Numbers of usable cases in various categories are shown in the denominators of the fractions next to each bar in Figs. 1 and 2. Data from off-target cannulae were discarded. Caudal trigeminal nuclear cannulae (CAUD in figures) resulted in dye dispersions extending from 1 mm above obex to 2 mm below, and medio-laterally at most, from the lateral edge of the primary trigeminal tract to about 0.5 mm medial to the medial margin of the trigeminal nucleus (i.e.. in the nuclei reticularis dorsalis and ventralis). Dorso-ventrally, 4 dye dispersion patterns extended from the brain stem dorsal undersurface to just above the ventral inner surface. In 6 cases, more local dispersions were seen, mainly involving tract and nucleus and not extending more than 0.2 mm ventral to the tract margin. Rostra1 cannulae (ROST in figures) mediated dispersions extending from the lower third of the main sensory nucleus to the upper third of subnucleus interpolaris and always including oralis. Dorso-ventral extents typically involved the surface of the brain stem, extending downward variable amounts but below the trigeminal tract’s ventral margin by at least 0.3 mm. Medio-laterally. the dispersions extended from the restiform body to the middle of nucleus reticularis parvocellularis. Two cases with dye impinging on nucleus reticularis gigantocellularis (another possible opiate substrate [3]) were eliminated. One caudal nuclear stimulating electrode pair was medial to the nucleus and the data not used. Of the remaining 9 placements which yielded data that were used, 7 were within the trigeminal nucleus or on its lateral margin on the tract. and 2 were in the medial nuclear border region, impinging on the medially adjacent reticular formation. Other sources of attrition involved cannula-plugging at variable points in the procedures. as well as loss of facial resistors in one rat.

149

Microinjection effects on the face-rub (FR) Fig. 1 shows effects of various injections on face-rub latencies expressed as percent of control (SC). The fractions near each bar give the number of total blocks (300 + %C) over the number of cases for that bar. Variation indicators are standard deviations. FR scores for heaters ipsilateral and contralateral to microinjection sites are combined in view of our first and unexpected finding that no significant differences between ipsi- and contralateral injections obtained. The ipsilateral mean %C for caudal injection was 247 + 75% versus 194 _t 74% for the contralateral mean. Both effects compared to saline control means were significant (P -c0.05). The comparable respective rostra1 %Cs were 159 + 71% (ipsilateral) and 206 + 69% (contralateral). It is noted that in 20 separate FR tests with sham injection and in 36 separate FR tests with saline injection, only one failure to respond was seen. Fig. 1 shows that microinjections of either rostra1 (ROST) or caudal (CAUD) sites produced significant facial analgesia in the average amounts of 186%C + 71 (P < 0.01) and 222%C + 77 (P < O.OOl), respectively. The simultaneous injections of both sites (BOTH in Fig. 1) also produced a significant 197% f 92 (P < 0.01) effect, but not significantly larger than that of either single microinjection. The caudal and double injections produced roughly similar proportions (8/16 and 5/16, respectively) of total blocks, both of which appear greater than the 2/15 proportion seen with rostra1 injections. For comparison with these data, 3 rats were additionally implanted with PGC cannulae and bilateral face-rub devices (yielding 6 measurements) and microinjected with 0.5 pg of morphine sulphate and 2 saline control solutions on alternate days. Fig. 1 shows that this lower dose (compared to the 0.75

330

FACE-RUB

LQT

310 % p

290 270 250

;

230

T”

210 190

!

170

L

150 130 110 .75UC.75UG

1.5UG.SUG

BMG/KG’

SITE/DOSE

Fig. 1. Effects of various morphine microinjections in the doses and at the sites shown on the latency to defensive face-rub response to noxious facial heat; UG is microgram. ROST and CAUD are single rostra1 and caudal trigeminal injection effects; BOTH is the conjoint injection effect; PGC is the nucleus reticularis paragigantocellularis effect; SUBCUT is the subcutaneous injection effect. Ordinate is percent of control latency (100% = no effect). Variation indicators are standard deviations. Asterisks (*) over bars indicate degrees of statistically significant difference (in parametric tests) from controls; between bar asterisks indicate difference between experimental groups: * P < 0.08, ** P i 0.05, *** P c 0.01, **** P < 0.001. Non-parametric test results in text. Fractions near each bar give proportion of total blocks (no responses) over total tests.

TABLE

1

FACE-RUB LATENCIES IN SECONDS (_t SD.) FOR THE VARIOUS INJECTION TYPES GIVEN SEPARATELY FOR IPSILATERAL AND CONTRALATERAL FACE SIDES -_--. _.~.__ ____ Drug treatment

Face aide ..-____l__--

. ._

tpsilateral

C‘ontralateral ._-

Sham injection Saline injection Rostra1 morphine Caudal morphine Double morphine -..

injection injection injection

4.9 * 2.2 3.6i2.8 5. I i 3.9 I1.7_+8.1 9.1 +x.0

-5.4 $3.8 4.8 * 3.0 x.4+5.1 9.0% 5.0 8.4i5.1 ~__....

,ug single trigeminal injection) in PGC produced a larger (P -C0.001) %C mean (265% i 65), and a larger proportion of total blocks (4/6) than did any of the other microinjections. The PGC microinjection effects resembled those of the 6 mg/kg systemic injection of morphine (SUBCUT. in Fig. 1). An independent groups comparison of the %C scores for CAUD (0.75 pg) versus PGC (0.5 pg) was significant at P < 0.0s. Table 1 shows the averaged FR latencies in seconds for the various experimental and control treatments. These values largely reflect the trends seen in the transformed data discussed above and shown in Fig. 1. Some of the means in Table I (as in Fig. 1) are artificially low due to the use of values taken at cut-off points. While this truncation probably distorts the parametric f tests we used in a conservative direction, it was nevertheless deemed appropriate to also apply in certain cases non-parametric sign-tests for correlated samples [5. p. 3241 and for independent samples (‘median test’ [S, p. 3231). These do not test hypotheses about means so that neither test sets are redundant. The first result of these tests was that again no significant ipsilateral-contralateral differences were obtained. To further pursue this surprising equivalence, a test of the significance of the difference between correlated proportions [5, p. 1621 of total blocks for ipsiiateral versus contralateral caudal injections was done. Despite the numerically large value of FR latency for caudal contralateral versus ipsilateral injection (Table I), this difference was not significant in the present sample. Comparison of rostral, caudal and double injection effects (each versus saline effects), using the non-transformed data from Table I in sign-tests for correlated samples [5, p. 3241 yielded significant values of z = 2.41 (P < 0.02). 3.25 P < 0.01). and 1.99 (P < 0.05), respectively. These tests were done with ipsilateral and contralateral data combined within injection groups. Microinjection effects on aversive stimulation threshold Fig. 2 shows the effects of various injections on the thresholds to aversive stimulation in caudal trigeminal nuclear sites. It is seen that the caudal microinjection (CAUD) produced an average elevation which though variable (178%C & 98) fell just short of statistical significance; (f = 2.05. df= 7, 0.05 < P < 0.10). There were 2 of 9 total blocks and one other case where the response was exactly at 300%.

The %C score was at about the same level as that for 6 mg/kg systemic (SUBCUT.) injection, although the proportion of total blocks for the latter, 7 of 9, was greater than that for the caudal microinjection. The effect of 6 mg/kg systemic morphine CAUDQL

280 %

260

TRIG.

STIM

< _-

***----

UG

.75UG .75UG SITE/DOSE

i

200

HZ

240

:

220

6

200

r

180

;

161

L

140 120 100 1

6NG/KG

Fig. 2. Effects of various morphine microinjections on the threshold current level for aversive reaction to caudal trigeminal nuclear brain stimulation at 200 Hz, sinusoidal. The other abbreviations are as in the Fig. 1 caption.

was significant (P < 0.05) though at 172X + 82, not as large as those previously reported (> 2OO%C) with larger dosages [12,13,15,16]. Fig. 2 also shows that the rostra1 microinjection failed to significantly affect the threshold for aversive caudal trigeminal stimulation. Also shown (replotted from a previous study [12]) for comparison in Fig. 2 is the lack of effect of 1.0 pg of PGC-microinjected morphine on aversive reaction threshold. An independent-groups, 2-tailed t test of the difference between the mean %C for the 1.0 pg PGC injection (101% + 6) and the mean %C for the 0.75 pg caudal trigeminal injection was significant (t = 3.77, df= 12, P -c 0.01). Table II shows results with non-transformed results in microamperes of current.

TABLE

II

AVERSIVE REACTION NAL NUCL .EAR (TNC)

THRESHOLDS IN MICROAMPERES BRAIN STIMULATION IN VARIOUS

Drug treatment

200 Hz caudal

Sham injection Saline injection Rostra1 morphine Caudal morphine

16.4* 15.7* 18.4& 24.8*

injection injection

18 11 14 16

stimulation

OF CURRENT CONDITIONS

FOR

TRIGEMI-

Discussion

The orofacial analgesic effect produced by trigeminal nuclear (morphine) microinjection implicates this brain stem primary afferent terminal region as an opiate substrate in addition to previously established. homologous cord areas (7.201. Two possible artifacts that could account for the effects should be ruled out: (1) The trigeminal nuclear complex is close to the surface of the brain stem. Drugs may have passed from the injection sites to the surface accessing other analgesia substrates. The bilateral effects of trigeminal injecton could be based on this type of occurrence. (2) Medial diffusion from the trigeminal nucleus to the medial bulbar substrates (PGC. RM) could have carried the FR analgesia. With regard to the first point, the center of the trigeminal nucleus is no closer to the lateral surface of the brain stem than is PGC from the ventral surface. Indeed. in a previous study [12]. intended PGC cannulae placements histologically localized to the lateral edge of the pyramid and within the inferior olive (i.e., 0.5 mm from surface) failed to mediate elevation of the threshold for TNC stimulation. If diffusion from TNC to PGC is likely via an extracranial route, then diffusion in the reverse direction is no less likely. However, we consistently fail to see PGC injections, even very ventral ones. affect the threshold for aversive reaction to TNC stimulation, whereas TNC injection is effective. The same fact applies to the second point noted above: if material can diffuse medially from TNC to PGC and RM so that FR analgesia is carried by medial activation. then PGC application should diffuse laterally to access TNC and should produce analgesia for TNC stimulation, which in fact does not happen. Similarly, if the TNC-mediated biiateral FR effect is carried by an improbably long-distance diffusion across brain to the contralateral TNC. then the medial structures in the path of this diffusion would be activated en route and one would then expect to see the much more potent FR analgesia one typically obtains with low dose applications to these medial sites. In contrast. TNC was found here to produce a smaller effect, relative to PGC. on FR. Finally, it should be recalled that using higher volumes, col~centratioIls and injection rates than those used here. Lomax [8] systematically studied diffusion of opiates from injection sites in brain stem and concluded that morphine diffusion is probably confined to a 1 mm sphere centered at the injector tip. The PGC nucleus appears to be a more sensitive substrate than the trigeminal nucleus since a smaller microinjection in PGC produces a larger effect. This may be because some diffusion from PGC to RM (another nearby opiate substrate) occurred leading to a combined activation of independent descending inhibitory paths from both RM and PGC [Z,lS], This effect would be in addition to descending paths from RM possibly affected by activation in PGC of PGC-RM connections [6]. Dye diffusion studies here suggest that such diffusion did not occur. The discussion about diffusion in the preceding paragraph is relevant here as is the Lomax [g] study cited there. Additionally. we have previously reported that medially off-target. intended PGC electrodes (i.e., closer to and/or including RM) did not have larger effects than laterally off-target placements [ 12.13f. A different possible reason for PGC’s apparently greater sensitivity is that at least two routes for descending connections may

I53

leave PGC for primary afferent terminal regions: direct connections [2] and connections through RM ]6] just noted. Whether the functional trigeminal opiate receptors are on higher trigeminal projection neurons or on mediating intratrigeminal interneurons (or on both), the descending paths from PGC would appear to access more inhibitory synaptic surface area in the trigeminal complex. It is also possible that opiate receptor-carrying PGC neurons may have unknown inhibitory access to other ascending pain paths not accessed by opiate sensitive trigeminal neurons. Rostra1 trigeminal injections were also effective for facial analgesia but less so than were caudal injections. In this case, it is more difficult to rule out diffusion from ineffective rostra1 sites (the least rostral) to nearby effective caudal sites (the least caudal). Such a hypothesis is also consistent with the facts that (1) caudal but not rostra1 microinjections produced elevation of aversive reaction thresholds to near-significantly different levels; (2) no summation effects of rostra1 and caudal injections were observed. No summation would be expected if the rostra1 injections were simply diffusing to and redundantIy activating sensitive caudal sites. It is possible, however, that neural connections from rostra1 to caudal neurons could redundantly activate caudal neurons directly accessible to caudal injections. Another possible diffusion route would be extracranial: in the case of several rostra1 dye injections, we did observe involvement of the brain stem dorsal surface. A direct effect of caudal but not rostra1 microinjections is consistent with the fact that dense concentrations of opiate receptors are located in caudal but not rostra1 areas [ 11. The caudal injections produced suggestive elevations in brain stimulation thresholds. Although this effect was of just borderline statistical significance compared to saline control, the trigeminal injection effect was significantly greater than that of PGC injection which was found here and before [ 121 to be at control levels. It is acknowledged, however, that the statistical test used here compared results of two experiments done 6 months apart in our lab and it is remotely possible that sampling biases carried the effects. On the other hand. there were 4 cases here where caudal injection produced > 200% elevations (including two total blocks). We have never observed a total block of aversive reaction to trigeminal nuclear stimulation with saline injection, and the largest (probably chance) elevation of reaction threshold with 1 pg morphine PGC injection we have ever seen was < 120%. The caudal injection effects on stimulation thresholds here were more variable than those usually observed. This may be due to the possibility that the injection cannulae (of the cannula-electrode assembly) damaged stimulation substrates. Also, some of the stimulation sites were relatively medial in the trigeminal nuclei. Our typical experience with trigeminal stimulation is that as the placements of stimulation electrodes are more medial, systemic morphine produces less elevation of reaction threshold, falling to zero at the medial border region of the nucleus [ 13,161. The average elevation of about 172% here for systemic injection was somewhat lower than the near 200 + %C values we have reported previously [ 12,131, although the present systemic injection effect did exceed significance and included a large proportion of total blocks. The present results and previous work [ 12,131 show that while PAG and PGC are excellent opiate substrates for facial analgesia, opiate injections in these sites are

154

without effect on aversive trigeminal stimulation. Of the central sites now established as substrates for peripheral opiate analgesia, the present results leave the trigeminal area as the best candidate substrate for mediating morphine’s systemic effect on aversive trigeminal stimulation. It remains to account for the greater sensitivity of PGC with respect to peripheral analgesia coexisting with its total insensitivity with respect to trigeminal brain stimulation. It is suggested that PGC may give rise to descending inhibitory pathways with predominantly presynaptic inhibitory action on trigeminal primary terminals. Sessle et al. [ 191 have reported that conditioning stimulation in PAG, RM and lateral areas corresponding to PGC produces a partially naloxone-reversible inhibition in trigeminal neurons, with a time course implicating presynaptic mechanisms. PGC connects to RM [6] in any case. Lovick and Wolstencroft [9] have, however. noted that both pre- and postsynaptic mechanisms may relate RM to the trigeminal complex. Nuclear (postsynaptic) stimulation thresholds would be unaffected by presynaptic action. On the other hand, the opiate substrates in the trigeminal nucleus (on interneurons going to projection neurons or directly on projection neurons themselves) would have to mediate predominantly postsynaptic effects in order for the direct application of opiates in the trigeminal nucleus to raise the threshold for effects of stimulation of the nucleus. It would follow that systemic injections, which should access all functional opiate substrates, would have larger effects than those of any single microin.jection. This was the case here and previously [ Il. 121. Finally. the equivalent ipsi- and contralateral effects of trigeminal nuclear microinjection on the face-rub latency need to be explained. There are documented connections between left and right trigeminal nuclear complexes (e.g., see ref. 14 and references cited) but somatotopic organization in opiate analgesia systems has only recently been studied [ 1 I- 131. and there has been no work, to our knowledge, dealing with left-right organization. Many studies of opiate microinjection effects involve injection of central structures (e.g., ventromedial periaqueductal gray. RM, etc.) and the response system often studied is the tail flick so that bilateral effects are not determinable. We reported [14] that lesions of primary descending trigeminal tract produce contralateral as well as ipsilateral facial analgesia, whereas lesions of the main sensory nucleus and/or subnucleus oralis produce only ipsilateral analgesia. These data strongly implicate the existence of crossed mechanisms of orofacial pain perception although their relationship to opiate substrates remains to be studied.

Acknowledgement This work was supported

by NIH Grants

DE05204

and GM23696

to J.P.R.

References I Atweh. S.F. and Kuhar. M.J.. Autoradiographic localuation cord and lower medulla, Brain Res.. 124 (1977) 53-67.

of opiate receptors

m rat brain.

1. Spinal

155

2 Basbaum, A.I. and Fields, H.L., The origin of descending pathways in the dorsolateral funicuhis of the spinal cord of the cat and rat: further studies on the anatomy of pain modulation, J. camp. Neurol., 187 (1979) 513-532. 3 Chan, S.H.H., Participation of the nucleus reticularis gigantocellularis in the morphine suppression of jaw-opening reflex in cats, Brain Res., 160 (1979) 377-380. 4 Dickenson. A.D., Oliveras, J.-L. and Besson, J.-M., Role of the nucleus raphe magnus in opiate analgesia as studied by the microinjection technique in the rat, Brain Res., 170 (1979) 95-l 11. 5 Fergusson, G.A., Statistical Anaiysis in Psychology and Education. 3rd edition, McGraw-Hill, New York, 1971. 6 Gallager, D.W. and Pert, A., Afferents to brainstem nuclei (brainstem raphe, n. reticularis pontis caudalis, and n. gigantocellularis) in the rat as demonstrated by microiontophoretically applied horseradish peroxidase, Brain Res., 144 (1978) 257-275. 7 Homma, E., Collins, J.G. and Kitahata, L.M., Effects of intrathecal morphine on activity of dorsal horn neurons activated by noxious heat, Pain, Suppl. 1 (1981) S249. 8 Lomax, P., The distribution of morphine following intracerebral microinjection, Experientia (Basel), 22 (1966) 249-250. 9 Lovick, T.A. and Wolstencroft, J.H., lnhibito~ effects of nucleus raphe magnus on neuronai responses in the spinal trigeminal nucleus to nociceptive compared with non-nociceptive inputs, Pain, 7 (1979) 135-149. 10 Mayer, D.J. and Price, D.D., Central nervous system mechanisms of analgesia, Pain, 2 (1976) 379-404. 11 Rosenfeld, J.P. and Keresztes-Nagy, P., Differential effects of intracerebrally microinjected enkephalin analogs on centrally versus peripherally induced pain, and evidence for a facial versus lower body analgesic effect, Pain, 9 (1980) 171-181. 12 Rosenfeld, J.P. and Stocco, S.. Differential effects of systemic versus intracranial injection of opiates on central. orofacial and lower body nociception: somatotypy in bulbar analgesia systems, Pain. 9 (1980) 307-318. 13 Rosenfeld, J.P. and Stocco, S., Effects of midbrain, b&bar, and combined morphine microinjections and systemic injections on orofacial nociception and rostra1 trigeminal stimulation: independent midbrain and bulbar opiate analgesia systems? Brain Res., 215 (1981) 342-348. 14 Rosenfeld, J.P., Clavier, R.M. and Broton, J.G., Bilateral and unilateral antinociceptive effects of rostra1 trigeminal nuclear complex lesions in rats, Brain Res., 157 (1978) 147- 152. 15 Rosenfeld, J.P. and Holzman, B.S., Differential effect of morphine on stimulation of primary versus higher order trigeminal terminals, Brain Res., 124 (1977) 367-372. 16 Rosenfeld, J.P. and Vickery, J.L.. Differential effect of morphine on trigeminal nucleus versus reticular aversive stimulation: independence of negative effects from stimulation parameters, Pain. 2 (1976) 405-416. 17 Satoh, M., Akaike, A. and Takagi, H., Excitation by morphine and enkephalin of single neurons of nucleus reticularis paragigantocellularis in the rat: a probable mechanism of analgesic action of opioids, Brain Res., 169 (1979) 406-410. 18 Satoh, M., Akaike, A., Nakazawa, T. and Takagi, H., Evidence for involvement of separate mechanisms in the production of analgesia by electrical stimulation of the nucleus reticularis paragigantocellularis and nucieus raphe magnus in the rat, Brain Res., 194 (1980) 525-529, 19 Sessle, B.J., Hu, J.W., Dubner, R. and Lucier, G., Functional properties of neurons in cat trigeminal subnucleus caudalis (rn~uil~ dorsal horn). II. Modulation of responses to noxious and non-noxious stimuli by periaqueductal gray, nucleus raphe magnus, cerebral cortex and afferent influences, and effect of naloxone, J. Neurophysiol., 45 (1981) 193-207. 20 Yaksh, T.L. and Rudy, T.A., Analgesia mediated by a direct spinal action of narcotics. Science. 192 (1976) 1357-1358. 21 Yaksh, T.L., Yeung, J.C. and Rudy, T.A., Systematic examination in the rat of brain sites sensitive to the direct application of morphine: observation of differential effects within the periaqueductal gray, Brain Res., 114 (1976) 83-103.