Opiate and non-opiate analgesia induced by inescapable tail-shock: Effects of dorsolateral funiculus lesions and decerebration

Brain Research. 291 (1984) 325-336 Elsevier

325

Opiate and Non-Opiate Analgesia Induced by Inescapable Tail-Shock: Effects of Dorsolateral Funiculus Lesions and Decerebration L. R. WATKINS1, R. DRUGAN2. R. L. HYSON2, T. B. MOYE 2, S. M. RYAN2, D. J. MAYER I and S. F, MAIER 2

1Dept. of Physiology and Biophysics, Medical College of Virginia, Richmond, VA 23298 and 2Dept. of Psychology, University of Colorado, Boulder, CO 80309 ( U. S. A. ) (Accepted May 24th, 1983)

Key words. opiate analgesia - - non-opiate analgesia - - dorsolateral funiculus - - decerebration - inescapable shock - - learned helplessness

Previous studies have demonstrated that inescapable tail-shock can produce either non-opiate or opiate short-term analgesia, dependent on the number of shocks delivered. Additionally, extended exposure to inescapable tail shock can produce long-term, opiate analgesic effects. Several lines of investigation suggest that the psychological dimension of perceived controllability may powerfully influence these phenomena in that each form of opiate analgesia can only be produced following exposure to inescapable, rather than equal amounts and distribution of escapable, shock. This has suggested that these opiate analgesias result from the organism's learning that it has no control over shock. Although it has been assumed that the opiate and non-opiate analgesias induced by tail shock may be subserved by neural circuitry similar to that mediating morphine analgesia and other forms of environmentally induced analgesia, no direct evidence exists to support this assumption. The present study sought to provide an initial attempt at defining the neural circuitry involved in these phenomena by examining the effect of bilateral dorsolateral funiculus (DLF) lesions and decerebration. These experiments revealed that pathways within the spinal cord DLF are critical for the production of short-term non-opiate analgesia, shortterm opiate analgesia, and long-term opiate analgesia since bilateral DLF lesions abolished all three pain inhibitory effects. Additionally, it was found that decerebration did not attenuate either the short-term non-opiate or short-term opiate analgesia induced by inescapable tail shock. Combining the observations that these non-opiate and opiate short-term effects are not reduced by decerebration yet are abolished by DLF lesions clearly delimits the source of descending pain inhibition as being within the caudal brainstem. INTRODUCTION

high doses of systemic morphine3,7,44, morphine microinjected into the periaqueductal gray44, carbachol

It is now generally recognized that neural systems originating within the brainstem can powerfully modulate sensitivity or reactivity to pain. Electrical and/or chemical stimulation of a n u m b e r of hrainstem structures can inhibit the response of spinal cord dorsal horn neurons to noxious stimuli and, at a behavioral level, produce potent analgesia (for reviews, see refs. 5,18 and 64). Since spinal nociceptive reflexes and evoked activity of dorsal horn neurons are blocked by these manipulations, this implies that the inhibition must be mediated by descending spinal cord pathways which originate in supraspinal structures. Of the spinal pathways which could potentially be involved, the dorsolateral funiculus (DLF) has been repeatedly implicated in descending pain modulation in that D L F lesions have been demonstrated to block the pain inhibitory effects of all but

(X)06-8993/84/$03.00 © 1984 Elsevier Science Publishers B.V.

microinjected into the parabrachial region22,2s, and electrical stimulation of the periaqueductal gray:, periventricular grays substantia nigraS, nucleus raphe magnuslT,40, and nucleus reticularis paragigantocellularis40. Although many brainstem areas do modulate pain via descending D L F pathways, this does not imply that all of these effects are mediated by a common neural circuit. Recent work has made it clear that multiple opiate and non-opiate pain inhibitory systems exist within the central nervous system (for reviews, see refs. 18,31 and 64). Opiate and non-opiate centrifugal pain control systems can be selectively activated not only by electrical and chemical stimulation of discrete nuclei but also by exposure of the organism to a variety of environmental stimuli (for reviews, see refs. 9,31 and 64). In the case of environ-

326 mentally induced analgesias (EIA), the situation is made more complex by the fact that. dependent upon the specific stimulus chosen, production of analgesia appears to be either dependent on, or independent of, hormonal factors derived from the pituitary and/or adrenal glands 9-31.33,58-64. The occurrence of multiple forms of EIA9,31.64 requires analysis of the nature of the events that selectively determine which type of EIA is activated. To date, the majority of research has been aimed at identifying factors which determine whether opiate or non-opiate E I A will be produced. One important factor which has been identified is the degree to which the organism can exert control (e.g. alter the onset, offset, duration, or temporal pattern) over the noxious environmental stimuli. Maier et al. 3~ exposed one group of rats to a series of 80 tail-shocks each of which terminated when the subject performed a wheel turn escape response. A second group of rats was simply given a series of identical but inescapable tail-shocks. For these subjects, wheel turning had no effect on the shock. Thus the two groups experienced identical amounts and distributions of shock, but one had control over shock via the availability of an escape response while the other did not. Both escapable and inescapable shock produced short-term E I A measured immediately after the 80 shocks, but the analgesia following inescapable shock was far more sensitive to the opiate antagonist naltrexone. Moreover. exposure to the inescapable tail-shocks sensitized the organism such that exposure to brief, less intense foot-shock 24 h later in a different apparatus reinstated the analgesia 27. while the analgesia following equal amounts of escapable shock was not reinstated by this second tail-shock exposure. Emphasizing the importance of the controllability of the stressor, experiencing escapable shock before or after inescapable shock prevented the occurrence of this 'long-term' reinstated E I A 42. This long-term E I A , which occurs after exposure to inescapable tail-shocks, was completely blocked by opiate antagonists 34 and completely cross-tolerant with morphine 13. The importance of the controllability of the stressor suggests a relationship between E I A and another behavioral phenomenon in which controllability is critical; that is. 'learned helplessness'. Exposure to inescapable shocks identical to those used in the E I A

studies described above interferes with the organism's later learning to escape shock in a different situation where escape is possible, while experience with equal amounts of escapable shock does not have this effectS. This is called the learned helplessness effect 38. and has been argued to result from the subject's learning that the shock cannot be controlled37.38. This parallel with learned helplessness and a variety of other experiments t4,39 suggest that the organism's learning that it has no control over shock might be a critical factor in activating opiate systems. If this is so. then parameters such as the number of shocks delivered ought to be critical. Learning that shock is not escapable is a complex Iorm of learning and ought to require many exposures. Thus a series of shocks should produce, if anything, an early nonopiate analgesia before the subject has learned that the shock cannot be controlled, followed by a late opiate analgesia after such learning has occurred. Consistent with this argument, naltrexone did not diminish the short-term analgesic response measured after 5 or 20 inescapable tail-shocks 15A9,26. but completely blocked the analgesia observed after 60 or 80 shocks of exactly the same type 19,z~. Again. learning about the uncontrollability of shock was critical for the opiate form of E I A since exposure to 60 or 80 escapable tail-shocks does not produce opiate analgesia 15. Prior experience with escapable shock might be expected to interfere with the subject's learning that shock is uncontrollable during the series of 80 inescapable shocks, since learning that behavior and shock termination are related interferes with later learning that they are unrelated 54. Correspondingly, Moye el al. 43 found that exposure to escapable shock 4 h before the series of 80 inescapable shocks blocked the occurrence of the opiate analgesia which normally appears after 60 or 80 shocks, but had no effect on the occurrence of the non-opiate analgesia after 5 and 20 shocks. Prior exposure to inescapable shock did not have this effect. Similarly, Lewis et al. 29 found 3 rain of inescapable connnuous foot-shock to produce an EIA unaffected by naloxone, while 20 min of inescapable intermittent foot-shock produced a naloxone-sensitive EIA. Maier el al. 39 have presented evidence indicating that 20 min of intermittent inescapable foot-shock is sufficient to lead to the organism's learning that the shock cannot be con-

327 trolled, while 3 rain of inescapable continuous footshock is not. In sum, studies of inescapable tail-shock have revealed the existence of an opiate long-term analgesia and both non-opiate and opiate short-term analgesias, and have suggested a role for the psychological dimension of controllability in the regulation of these phenomena. However, the implication of these results for an understanding of the activation of descending pain inhibition systems and EIA in general is not clear. It has been assumed that the differing analgesias produced by the inescapable tail-shock are mediated by descending pathways such as those described earlier, but there is no direct evidence for this assumption. Nothing is known about the neural circuitry mediating these effects save that spinallymediated nociceptive reflexes are inhibited. The tailflick to radiant heat has typically been used to measure pain sensitivity/reactivity in the above studies. Thus the same body part (the tail) is both shocked and tested. This raises the possibility that the analgesia is a local sensory effect. Alternatively, the inhibition of the spinally-mediated tail-flick nociceptive reflex 6'~ could easily be produced by intraspinal mechanisms rather than by pathways originating within the brain and descending to the cord. Such a possibility is made plausible by the finding that intraspinal as well as descending mechanisms have been implicated in other E1A preparations 5v,$9. Moreover, exposure to inescapable shock of the sort used in the aforementioned EIA studies has been argued to produce catecholaminergic and cholinergic changes sufficient to produce a general motor activation deficit 2,~6. Thus the increased tail-flick latencies induced by 80 inescapable tail-shocks may be a reflection of a general deficit in the initiation of motor activity rather than the activation of a pain inhibitory pathway as with other forms of EIA. The purpose of the present study was two-fold: (1) to identify whether these opiate and non-opiate analgesias induced by inescapable tail-shock are mediated via descending pathways by examining the effect of specific spinal cord lesions, and (2) to determine the origin of the pain inhibitory system(s) by examining the effect of neural transections.

EXPERIMENT 1. EFFECT OF DLF LESIONS ON SHORTTERM AND LONG-TERM OPIATE ANALGESIA Methods Since the dorsolateral funiculus (DLF) of the spinal cord has been repeatedly implicated in pain modulation (see refs. 5,57 and 64 for review), the effect of bilateral DLF lesions was examined. Forty-seven adult male rats (Holtzman) were used to determine the effect of bilateral DLF lesions on the short-term and long-term analgesic effects of inescapable tailshock. Rats were anesthetized with Metofane (Pitman-Moore) and a laminectomy was performed at the second thoracic (T2) vertebral level. Following reflection of the dura, the DLF was identified by locating the dorsal root entry zone. Eleven animals then received bilateral DLF lesions using the minimum pressure necessary to sever the axons with microscissors. The other 36 rats served as sham-operated controls; in these animals, the dura was reflected but no lesions were made. The exposed spinal cords were then covered with Gel-Foam powder (Upjohn) and the wounds closed. All animals were treated with gentamicin (Elkins-Sinn) as required. Behavioral testing began approximately one week after surgery. Twenty-four of the sham-operated rats were used to verify opiate mediation of the shortterm analgesia. Twenty minutes prior to testing, half of these animals received 14 mg/kg naltrexone (s.c.); the remaining animals received equivolume saline (s.c.). This dose has previously been shown to completely block the analgesia produced by the present shock conditionsl'~. 26. All 47 rats were then tested for baseline pain responsivity using the tail-flick test 12. The radiant heat source consisted of a 150 W projector spotlight (General Electric) focused through a condenser lens upon the dorsal aspect of the tail. A lateral movement of the tail of at least 5 mm activated a photocell receiver, automatically terminating the trial. Bulb voltage was adjusted to attain baseline responses of approximately 7 s in naive animals. Following baseline determinations, each rat was placed in a plexiglass restraining tube (23.4 cm length, 7.0 cm width) and the tail taped to a plexiglass rod which extended from the rear of the restrainer. Electrodes augmented with electrolyte paste were then taped to the tail. Unscrambled constant current

328 shock (5 s, 1.0 mA) was delivered through the tail electrodes on a variable interval 60 s schedule (range of 5--200 s). Pain sensitivity was assessed using the tail-flick test after the delivery of 20, 40 and 80 shocks. Tail-flick trials were terminated at 15.0 s if no response occurred in order to avoid tissue damage. The DLF-lesioned rats and the drug-naive shamoperated rats were tested for the existence of longterm analgesia 24 h after the initial shock exposure. In accordance with the protocol of Jackson et al. 27. brief re-exposure to shock preceded testing of pain responsivity. Scrambled constant current shock (0.6 mA) was delivered through the grid floor of two-way shuttle boxes (34.5 x 20.5 x 19.5 cm). The boxes were subdivided into two equal compartments by a metal panel. Passage between the compartments was possible only through a 5.5 x 5.5 cm (H × W) archway cut in the dividing panel. Re-exposure to shock

consisted of 5 single-crossing shuttle box escape trials presented on a variable interval 60 s schedule (range of 5-200 s). Upon completion of these trials, analgesia was assessed using the tail-flick test (see above). The results of two successive tail-flick trials (approx. 1 min inter-trial interval) were averaged to attain a mean response latency. The tail-flick latencies recorded after shock (Days 1 and 2), were expressed as a percent of maximal possible effect (%MPE) using the following equation20: %MPE = [ ( T L - - B L ) / ( 1 5 . 0

BL)] ×100

where TL = the test latency recorded after shock and BL = mean baseline latency. Analysis of variance was utilized to compare the time-course of analgesia observed for each group on Day l. The mean %MPE for each post-shock test (Days 1 and 2~ was compared

A H01

H03

Ho8

Hll

1-105

H14

HO2

HUO

MI~

I1 !o

rtu~

r= ~ •

B J06

J15

J12

J05

J04

J 21

J16

J32

J 25

J 22

Jll

Fig. 1. Illustration of the bilateral dorsolateral funiculuslesions (DLF) of animals used in Experiments 1 (A, top two rows) and 2 (B, bottom two rows). Drawingswere made at the maximal extent of the lesions, whichwere clearly identifiable by myelin disruption and gfiosis,

329

Results

between groups using the Student's t-test; Satterthwaite's approximation of the Student's t was used when significant differences in sample variance occurred 6s. Paired t-tests comparing post-shock and baseline latencies were also calculated for each group. Upon completion of behavioral testing, rats were overdosed with sodium pentobarbital and perfused transcardially with 10% formalin. The exposed thoracic cords of control animals were visually examined for any evidence of spinal damage. The cords of lesioned animals were removed from the vertebral column and post-fixed in 10% formalin. Following paraffin embedding, these cords were cut into 15 y m sections and stained using the Klfiver-Barrera method. Drawings were then made of the maximal extent of the lesions, which were clearly identifiable by gliosis and myelin disruption. All lesion analyses were performed blind with respect to the behavioral results.

I00

A

B

The bilateral D L F lesions are illustrated in Fig. 1A. In agreement with previous reports23,57,,s% bilateral D L F lesions did not reliably alter baseline pain responsivity from that observed for controls (P > 0.25); baseline pain responsivity was also unaltered by naltrexone administration (P > 0.6). However, both D L F lesions (P < 0.000l) and naltrexone (P < 0.0005) markedly reduced tail-shock induced analgesia (Day 1) compared to saline controls (Fig. 2B). Whereas saline-injected rats exhibited reliable analgesia after both 20 (P < 0,001) and 40 (P < 0.001) shocks, D L F lesions totally blocked the induction of analgesia (P > 0.25 compared to BL for each test). Naltrexone also greatly reduced tailshock induced analgesia at all times tested; a small, though reliable, elevation in tail-flick latency was observed only after 20 shocks (P < 0.005). In fact, tail-

r-1 S H A M •

SHAM-

O DLF

C

SALINE NALTRE XONE LESION

75 w n

~E z w (.2 rr

50

T

25

w o_

0

- L.

_

I

I

I

I

I

3

5

20

I

40

NUMBER OF TAIL SHOCKS (DAYI)

80

SHAM DL F

REINSTATE (DAY2)

Fig. 2. Effect of DLF lesions on short-term (Day 1) non-opiate analgesia (A), short-term opiate analgesia (B), and long-term (Day 2, reinstatement) analgesia (C). A: non-opiate analgesia was observed after presentation of 3 tail shocks, as demonstrated by lack of antagonism of analgesia in naltrexone-treated (filled squares) compared to saline-treated (open squares) sham-operated controls, Analgesia was abolished by bilateral DLF lesions; post-shock tail-flick latencies of DLF-lesioned animals (open circles) were never reliably elevated above baseline values. B: opiate analgesia was observed following 20 and 40 tail shocks, as demonstrated by the marked reduction of analgesia in naltrexone-treated (filled squares) compared to saline-treated (open squares) sham-operated controls. Bilateral DLF lesions abolished this short-term opiate analgesic state; DLF-lesioned animals (open circles) never exhibited reliable analgesia at any time tested. C: long-term (reinstatement) analgesia was induced only in sham-operated animals (open bar). No long-term analgesia was elicited from DLF-lesioned animals (hatched bar).

330 flick latenoes of the naltrexone-treated group never reliably differed from those of DLF-lesioned animals (P > 0.2 for each test). The observation that naltrexone reliably reduced analgesia compared to saline controls as early as 20 shocks (P < 0.05) is in marked contrast to previous studies15,19,26 which found that non-opiate analgesia occurs at this time. It appears that the normal timecourse of analgesia has been disrupted by the experimental procedures followed in the present study. As reviewed in the Introduction. tail-shock has previously been observed to reliably produce a bimodal effect wherein two distinct and successive peaks of analgesia occur: non-opiate analgesia and opiate analgesia occur after 20 and 80 shocks, respectivelylS,19,26. However. in the present experiment, no sueh~bimodal pattern was observed. It appears that the occurrence of the opiate peak was temporally shifted, in that the maximum opiate analgesia occurred at 40 shocks (P < 0.0005. comparing naltrexone and saline groups). Also, differing from previous studies, analgesia was no longer observed at 80 shocks (P > 0.1 sham-saline; P > 0.1. sham-naltrexone). Examination of the long-term analgesia induced by prior exposure to inescapable tail-shock (Fig. 2C) revealed that bilateral DLF lesions also abolished this effect (P < 0.005). Whereas sham-operated controls exhibited reliable analgesia (P < 0.05), no analgesia was observed in DLF-lesioned animals (P > 0.25). EXPERIMENT 2. EFFECT OF DLF LESIONS ON SHORTTERM NON-OPIATE ANALGESIA Methods

The results of Experiment 1 suggest that the timecourse of analgesia induced by inescapable tail-shock was altered by the experimental procedures used. Comparison of the protocol followed in Experiment 1 with those used in previous studies performed in this laboratorylS,19,26 revealed that the only apparent difference between these procedures was the use in the present study of animals that had undergone surgical stress. Since: (1) both non-opiate and opiate analgesia elicited by inescapable tail-shock are dependent upon endocrine factors 36. and (2) endocrine

responsivity to environmental stressors has been observed to be facilitated by prior exposure to stress52, 53, it is possible that the surgical interventions used in Experiment 1 were sufficient to temporally alter the occurrence of the non-opiate and opiate peaks. Thus. it becomes plausible that inescapable tail-shock still produces a bimodal effect, but that the occurrence of non-opiate and opiate analgesia is temporally shifted as to appear after fewer shock exposures. We therefore examined the effects of nattrexone and DLF lesions on analgesia induced by 1.3 and 5 tail shocks. Surgical manipulations, drug administration. testing procedures, and statistical analyses were identical to those outlined in Experiment I. Results

Baseline pare responsivity was again unaltered by either DLF lesions (P > 0.4) or naltrexone administration (P > 0.2), compared to saline controls. Brief exposure to inescapable tail-shock was sufficient to induce analgesia in both the saline and naltrexone sham groups (Fig. 2A). Tail-flick latencies of the saline group were reliably elevated above baseline measures after 1 (P < 0.05) and 3 (P < 0,05} shocks: latencies of the nattrexone group were reliably elevated after both 3 (P < 0.05) and 5 ~P < 0.05) shocks. Since naltrexone failed to reduce the analgesia compared to saline controls (P > 0.3), this effect appears to be mediated by non-opiate pathways. Analgesia exhibited by both the saline (P < 0.0005) and naltrexone (P < 0.0001) groups was markedly greater than that observed in DLF-lesioned animals. In fact, bilateral DLF lesions (Fig. IB) abolished the nonopiate analgesic response. Post-shock tail-flick latencies of DLF-lesioned animals were never reliably elevated above baseline (P > 0,4 for each time tested). EXPERIMENT 3. EFFECT OF DECEREBRAT1ON ON SHORT-TERM OPIATE ANALGESIA Methods

The prior two experiments demonstrate that the short-term non-opiate and opiate analgesic effects of inescapable tail-shock are mediated by centrifugal pathways within the D L F of the spinal cord. These

331 results are consistent with previous investigations which demonstrated that analgesias induced by such diverse environmental stimuli as brief front paw or hind paw shock 57 and vaginal stimulation 59 are mediated by descending D L F pathways. Interestingly, the neural circuitry underlying analgesia elicited by these paw-shock62 and vaginal stimulation 59 paradigms exists within the caudal brainstem and spinal cord since decerebration failed to markedly reduce these effects. Thus the question arises as to whether the neural circuitry mediating the short-term analgesic effects of inescapable tail-shock is also extant to the caudal brainstem and spinal cord, or whether the integrity of more rostral structures is required. Forty-six adult male rats (Holtzman) were used to determine the effect of decerebration on the shortterm analgesic effects of inescapable tail-shock. All surgery was performed under Metofane, a short-acting, inhalant anesthetic. Twenty animals served as sham-operated controls; for each of these animals, a bone flap was removed from the skull without lesioning the brain. Midcollicular decerebration was attempted in 26 animals using suction under visual guidance. Upon completion of brain transection, Oxycel (Parke-Davis) was placed in the wound to aid coagulation and the wound was then closed. Behavioral testing was delayed until approximately 10-12 h after surgery. At this time, half of the sham and decerebrate animals received 14 mg/kg naltrexone (s.c.) 20 min prior to behavioral testing', the remainder received equivolume saline (s.c.). All rats were then tested for baseline pain responsivity using the tail-flick test (see Experiment 1). Shock delivery and behavioral testing was as described in Experiment 1, with the exception that tail-flick latencies were recorded after 5, 20 and 40 shocks. Statistical analyses were as described previously. Upon completion of behavioral testing, animals were asphyxiated using carbon dioxide gas and the heads of the rats placed in 10% formalin for approximately 1 week. At this time, the brains of decerebrated rats were carefully dissected free of the cranium and examined for incomplete transection. The brains of successfully decerebrated rats were then sectioned on a freezing microtome and stained with neutral red to determine the level of decerebration. Any animal whose brain was transected rostral to the

level of the superior colliculus was excluded from statistical analyses. All surgical verifications were performed blind as to the behavioral results.

Results In agreement with previous reportsS'~, 62 decerebration (n = 11) did not reliably alter baseline pain responsivity compared to saline controls (n = 11, P > 0.5); naltrexone (n = 10) also had no effect on baseline tail-flick latencies (n = 10 saline controls: P > 0.5). As seen in Fig. 3B, opiate analgesia was observed in response to this tail shock procedure in both sham-operated and decerebrate groups. The opiate peak was greatest at 5 shocks in sham animals (P < 0.01, saline vs naltrexone) and at 20 shocks in decerebrate animals (P < 0.05, saline vs naltrexone). I00 n • 0 •

SHAM-SALINE SHAM- NALTREXONE DECEREBRATE- SALINE DECER E B RATE- NALTRE XONE

75

50,

w ~A

~.

25

A

8 1

I

I

I

r

I

'I

3

5

20

40

N U M B E R OF T A I L SHOCKS

Fig. 3. Effect of deeerebration on short-term non-opiate (A)

and opiate (B) analgesias. A: non-opiate analgesia was induced by presentation of 3 tail shocks; naltrexone (filled symbols) failed to antagonize analgesia either in sham-operated (squares) or decerebrated (circles) animals, compared to saline controls (open symbols). Decerebration (open circles) did not reliably reduce analgesia compared to sham-operated controls (open squares), demonstrating that the supraspinal neural circuitry required for induction of this non-opiate analgesia exists within the caudal brainstem. B: opiate analgesia, as defined by naltrexone reversibility, was induced in both sham-operated (squares) and decerebrated (circles) animals by presentation of 5 or 20 tail shocks. Decerebration (open circles) again failed to reliably reduce analgesia, compared to sham controls (open squares).

332 Decerebration failed to reliably reduce analgesia. compared to sham-saline controls (P > 0.84). However, naltrexone did not have as great an antagonistic effect in decerebrate animals as it did in sham animals; the decerebrate-naltrexone group showed greater analgesia than did the sham-naltrexone group (P < 0.01). EXPERIMENT 4. EFFECT OF DECEREBRATION ON SHORT-TERM NON-OPIATE ANALGESIA

Methods' The results of the previous experiment again indicate a temporal shift in the occurrence of the opmte peak, such that an early non-opiate peak was not observed. Thus, the experiment was repeated with post-shock behavioral testing occurring after 1 and 3 shocks. All other procedures were identical to those described in Experiment 3.

Results Decerebration (P > 0.4) and naltrexone administration (P > 0.5) again failed to alter baseline pain responsivity. All groups exhibited a reliable elevation in tail-flick latency after 3 shock exposures (P < 0.05 in each case). Non-opiate pathways appear to mediate this response: analgesia exhibited bv sham-saline (n = 8) and sham-naltrexone (n - 9) groups did not reliably differ (P > 0.25), nor did the analgesia shown by decerebrate-saline (n = 7) and decerebrate-naltrexone (n -- 8) groups (P > 0.951. Decerebration did not reliably reduce this non-opiate analgesia, compared to sham controls (P > 0.5. decerebrate-saline vs sham-saline: P > 0.1. decerebrate-naltrexone vs sham-naltrexone). DISCUSSION The results of the present study demonstrate that bilateral DLF lesions abolish all three forms of environmentally induced analgesia examined: long-term opiate analgesia, short-term opiate analgesia, and short-term non-opiate analgesia. Thus it can be concluded that pathways within the DLF are critical for the production of these pain inhibitory effects, Although it is not possible to unequivocally state

that descending, rather than ascending DLF pathways mediate these analgesias, a review of the literature reveals that descending pathways within the DLF are by far the most likely to be involved. The possibility that DLF lesions abolish analgesia by interruption of ascending pain transmission pathways appears to be unlikely since: (1) DLF lesions do not result in an elevation of pain thresholds as measured by the tail-flick test (Expts. 1 and 2. refs, 23,57 and 59); (2) the behavioral responses of DLF-lesioned and sham animals to tail-shock (vocalization, struggling) were indistinguishable; and (3) electrophysiological investigations indicate that. in the rat. ascending spinal cord pathways involved in pain transmission lie primarily within the ventrolateral funiculus 67. In contrast, strong evidence exists which indicates that descending pathways within the DLF can potently inhibit pain transmission: (1) as noted previously, bilateral DLF lesions abolish behavioral analgesia induced by electrical stimulation of the periaqueductat gray v, morphine mlcromiection into the periaqueductal gray 44. low doses of systemic morphine 3,7,44. and carbachol microinjection into the dorsal parabrachial regionZ2~28: (2) DLF lesions block inhibition of dorsal horn nociceptors produced by electrical stimulation of the nucleus raphe magnuslT, 40. nucleus reticularis paragigantocellutaris ~. substantia nigraS, periventricular gray s, periaqueductal gray 7 and lateral hypothalamus (E.E. Carstens, pers. commun.): and (3) bilateral DLF lesions attenuate a variety of environmentally induced analgesias (EIAs) including transcutaneous footshock of all 4 paws 32, vaginal probing 59. classical conditioning5% front paw shock57, and hind paw shockSL Of these forms of EIA. it has been possible to clearly show that descending, rather than ascending, D L F pathways underlie both classicattv conditioned analgesia ~ and front paw foot-shock-induced analgesiaS2. Importantly. in both of these studies, the DLF lesions could be placed caudal to the body region exposed to the analgesia-producing stimulus. Despite the fact that no ascending pathways were disrupted. analgesia produced by front paw shock and classical conditioning were both abolished by DLF lesions, thereby demonstrating that descending pain inhibitory pathways within the D L F can be activated in response to environmental stimuli Thus the weight of evidence reviewed above

333 strongly suggests that bilateral DLF lesions abolish the long-term analgesic effects of inescapable tailshock by the interruption of descending pain inhibitory pathways. Since the present study incorporated a behavioral paradigm typically used in learned helplessness experiments, these data imply that the analgesic state induced by learned helplessness is mediated by a centrifugal pathway within the DLF of the spinal cord. The observations that this long-term analgesic response (1) is mediated by a DLF pathway, (2) can be antagonized by either systemic 34 or intratheca141 naltrexone, and (3) demonstrates crosstolerance to morphine 13, provide several parallels between this phenomenon and analgesias induced by other environmental manipulations, including classical conditioning5% front paw shock 55,57.63, prolonged shock of all 4 paws ~'9,30,32, and vaginal probing 51,59 (Steinman, Roberts and Komisaruk, unpublished observations). The demonstration that the analgesia produced by conditions which induce learned helplessness is mediated by mechanisms similar to those isolated for other forms of E1A is of some importance. It suggests that variables important in producing learned helplessness such as the controllability of the stressor may be generally important in the production of EIA. Additionally, it is known that the long-term analgesic response to inescapable tail-shock is dependent upon the pituitaryadrenal cortical axis. This opiate analgesia can be abolished by either hypophysectomy, adrenalectomy, or dexamethasone administration 33. Importantly, the effect of adrenalectomy on long-term analgesia can be reversed by corticosterone administration. The fact that DLF lesions abolish long-term analgesia implies that the adrenal corticosteroids are not solely responsible for inhibition of pain transmission at the spinal level. While neural pathways within the DLF are clearly involved, it remains unclear whether the corticosteroids are exerting their effects at supraspinal and/or spinal levels. It is interesting to note that activation of this descending pain inhibitory pathway appears to be profoundly influenced by prior surgical stress. Previous studies performed in this laboratory using surgically naive animals demonstrated that non-opiate and opiate tail-shock induced analgesias occur after 20 and 8(I shock exposures, respectively~5,~9.26. However, in the present series of experiments, sham-oper-

ated animals exhibited non-opiate and opiate analgesia after 3 and 5-20 shocks, respectively. While any hypotheses forwarded to account for this effect are clearly speculative, it seems possible that the tissue damage associated with the surgical procedures serves as an inescapable aversive stimulus; that is, the subjects are in pain following surgery, and the pain cannot be eliminated by their performance of instrumental acts. Since chronic inescapable stressors enhance the responsivity of the pituitary-adrenal cortical and sympathetic-adrenal medullary axes to further stress 11,52,53, hyper-reactivity of these endocrine glands could plausibly facilitate activation of pain inhibitory processes in response to inescapable tail-shock. The observation that bilateral adrenalectomy antagonizes tail-shock induced analgesia 36, despite the fact that hypophyseal response to shock is enhanced following adrenalectomy 24, suggests that the source of this facilitation derives from adrenal rather than hypophyseal factors. Pathways within the DLF also mediate the shortterm analgesic responses to inescapable tail-shock. The results of the present study demonstrate that the non-opiate analgesia observed after brief shock as well as the opiate analgesia observed after prolonged shock are abolished by bilateral DLF lesions. The descending pain inhibitory pathways mediating these effects arise within the caudal brainstem, since both the non-opiate and opiate analgesias could still be elicited in decerebrate animals. This result strongly implies that the cerebral cortex, thalamus, hypothalamus and the majority of the midbrain are not necessary for, though in intact animals may influence, the production of analgesia observed in response to inescapable shock. Combining the observations that non-opiate and opiate short-term analgesias are not blocked by decerebration yet are abolished by DLF lesions clearly delimits the source of descending pain inhibition as being within the most caudal portion of the midbrain, pons, and/or medulla. Recent anatomical studies have identified the supraspinal structures which send projections through the DLF of the spinal cord. Within the caudal brainstem, the nucleus reticularis gigantocellularis pars alpha, nucleus raphe alatus (NRA), and the paralemniscal area each densely project to the spinal cord through the DLF~,.s~,m. Of these areas, the NRA has been repeatedly implicated

334 in descending pain modulation. This ventral medullary area projects to all spinal levels4, 60 and terminates primarily within laminae I, II, and V of the spinal cord dorsal horn 4.25. Lesions within N R A have been shown to attenuate analgesia induced by such diverse manipulations as systemic morphine 47,70, intracerebral (periaqueductal gray) morphine microinjections 70, naloxone-sensitive electrical brain stimulation to, acupuncture 16, front paw shock 65, hind paw shock 65, and classical conditioning 65. Additionally, electrical stimulation of N R A can produce either opiate or non-opiate-mediated inhibition of pain transmission at the level of the spinal cord 45,58-6°,71,72. These observations, plus the fact that neurons within N R A are activated, either directly or indirectly~ by noxious peripheral stimulP 7.2~, make this ventral medullary area the most likely source of the critical D L F projection mediating tail shock induced analgesia. The decerebration results also comment on the argument that opiate analgesia after prolonged shock results from the organism's learning that it has no control over shock. It will be recalled that this hypothesis derived from the findings that only prolonged inescapable but not escapable shock lead to both long- and short-term opiate analgesia 19,35, and that prior exposure to escapable shock blocked the occurrence of this opiate analgesia. However, here opiate analgesia after prolonged shock occurred in decerebrate animals. This indicates that either (t)

REFERENCES 1 Abols, I. A. and Basbaum, A. I., Afferent connections of the rostral medulla of the cat: a neural substrate for midbrain-medullary interactions in the modulation of pain, J. comp. Neurol., 201 (1981) 285--297. 2 Anisman, H., Time-dependent variations in aversively motivated behaviors: non-associative effects of eholinergic and catecholaminergic activity, Psychol. Rev., 82 (1975) 359-385. 3 Barton, C., Basbaum, A. I. and Fields, H. L., Dissociation of supraspinal and spinal actions of morphine: a quantitative evaluation, Brain Research, 188 (1980) 487-498. 4 Basbaum, A. I., Clanton, C. H. and Fields, H. L,, Three bulbospinal pathways from the rostral medulla of the cat; an autoradiographic study of pain modulating systems, J: cornp. Neurol., 178 (t978) 209-224. 5 Basbaum, A. I. and Fields, H. L., Endogenous pain control systems: review and hypothesis, Ann. Neurol., 4 (1978) 451--462. 6 Basbaum, A. I. and Fields, H. L., The origin of descending

decerebrate rats are capable of higher order learning such as uncontrollability, (2) the opiate analgesia observed in decerebrate animals has a different basis than that in normal subjects, or (3) the learning of uncontrollability hypothesis is incorrect. Neither (1) or (2) above seem likely. However. the dimension of controllability is clearly important. If it is not the learning about uncontrollabilit~ that produces the phenomenon described above, how are they to be explained? One possibility would be that extensive exposure to a stressor, independent of learning, activates opiate analgesia systems. However. learning that the stressor is controllable 43 might initiate a process that blocks the activation of. or counteracts the output from. these systems. Thus the 'baseline' condition may be one of no control, which is then modulated by the learning of controllability, rather than the converse. The controllability of stressors may still be important in modulating analgesia systems, but the essential psychological factor may be learning about control rather than the lack of control. ACKNOWLEDGEMENTS We would like to thank Ms. lngrid Kinscheck and Ms. Holly Schumaker for their technical assistance. Supported in part by PHS Grant D A 00576 to D.J.M. and NSF Grant BNS 78-00508 to S.F.M.. who is the recipient of Research Scientist Development Award MH 00314.

pathways in the dorsolateral funicutus ofthe spinal cord of the cat and rat: further studies on the anatomy of pain modulation, J. comp. Neurol., 187 (1979) 513-532. 7 Basbaum. A.. Marley, N. J. E.. O'Keefe, J. and Clanton, C H.. Reversal of morphine and stimulation-produced analgesia by subtotal spinal cord lesions. Pain. 3 (1977) 43-56. 8 Blinn, G., Hienz, G. and Jurna. I.. Effects of substantia nigra stimulation on suralis-evoked spinal reflex activity: comparison with the effects of morphine andstimulation in the periaqueductal gray matter, Neuropharmacology, 19 (t980) 75-85. 9 Bodnar. R. J., Kelly, D. D., Brutus, M. and Glusman, M. Stress-induced analgesia: neural and hormonal determinants. Neurosci. Biobehav. Rev,. 4 (1979),87-100. 10 Cannon. J. T.. Prieto, G. J. and Liebeskind. J. C.. Disruption of stimulation-produced analgesia by iesionsof the nucleus raphe magnus, Soc. Neurosci. Abstr.. 6 (1980) 320. 11 Dallman, M. F. and Jones, M. T., Corticosteroid feedback control of ACTH secretion: effeet of stress-induced corticosterone secretion on subsequent stress responses. Endocri-

335

nology, 92 (1973) 1357-1375. 12 D'Amour, F. E. and Smith, D. L., A method for determining loss of pain sensation, J. Pharmacol. exp. Ther., 72 (141) 74-79. 13 Drugan, R. C., Grau, J. W., Maier, S. F., Madden, J. and Barchas, J. D., Cross tolerance between morphine and the long-term analgesic reaction to inescapable shock, Pharmacol. Biochem. Behav., 14 (1981) 67%682. 14 Drugan, R. G. and Maier, S. F., Analgesia and opioid involvement in the shock-elicited activity and escape deficits produced by inescapable shock, Learn. Motivation, in press. 15 Drugan. R. C., Moye, T. B. and Maier, S. F., Opioid and non-opioid forms of stress-induced analgesia: some environmental determinants and characteristics, Behav. Neural Biol., 35 (1982) 251-264. 16 Eh, S., Participation of descending inhibition on acupuncture analgesia. In National Symposia of Acupuncture and Moxibustion and Acupuncture Anaesthesia, Beijing, 1979., p. 27. 17 Fields, H. L., Basbaum, A. I., Clanton, C. H. and Anderson, S. D., Nucleus raphe magnus inhibition of spinal cord dorsal horn neurons, Brain Research, 126 (1977) 441-453. 18 Gebhart, G. F., Opiate and opioid peptide effects on brain stem neurons: relevance to nociception and antinociceptive mechanisms, Pain, 12 (1982) 93-140. 19 Grau, J. W., Hyson, R. L., Maier, S. F., Madden, J. and Barchas, J. D., Long-term stress-induced analgesia and activation of an opiate system, Science, 203 (1981) 1409-1412. 20 Grumbach, L., The prediction of analgesic activity in man by animal testing. In R. S. Knighton and P. R. Dunkc (Eds.), Pain, Little Brown, Boston, 1966, pp. 163-182. 21 Guilbaud, G., Peschanski, M., Gautron, M. and Binder, D., Responses of neurons of the nucleus raphe magnus to noxious stimuli, Neurosci. Lett., 17 (1980) 149-154. 22 Hayes, R. L., Katayama, Y. and Watkins, L. R., Effects of dorsolateral funiculus lesions on cholinergic analgesia in the cat, in preparation. 23 Hayes, R. L., Price, D. D., Bennett, G. J., Wilcox. G. L. and Mayer, D. J., Differential effects of spinal cord lesions on narcotic and non-narcotic suppression of nociceptive reflexes: further evidence for the physiologic multiplicity of pain modulation, Brain Research, 155 (1978) 91-101. 24 Hodges, J. R. and Jones, M. T., Changes in pituitary corticotrophic function in the adrenalectomized rat, J. Physiol. (Lond.). 173 (1964) 190-200. 25 Holstage, G. and Kuypers, H. G. J. M., The anatomy of brain stem pathways to the spinal cord in cat. A labelled amino acid tracing study. In H. G. J. M. Kuypers and G. F. Martin (Eds.), Descending Pathways to the Spinal Cord, Progress in Brain Research, Vol. 57, Elsevier Biomedical Press, Amsterdam, 1982, pp. 145-175. 26 Hyson, R. L.. Ashcraft, L. J., Drugan, R. C., Grau. J. W. and Maier, S. F., Extent and control of shock affects naltrexone sensitivity of stress-induced analgesia and reactivity to morphine, Pharmacol Biochern. Behav., 17 (1982) 1019-1025. 27 Jackson, R. L., Coon, D. J. and Maier, S. F., Long-term analgesic effects of inescapable shock and learned helplessness, Science, 206 (1979) 91-94. 28 Katayama, Y., Watkins, L. R., Becket, D. P. and Hayes, R. L., Cholinoceptive cells in the pontine parabrachial region mediate non-opiate analgesia in the cat, submitted.

29 Lewis, J. W., Cannon, J. T. and Liebeskind, J. C., Opioid and non-opioid mechanisms of stress analgesia, Science, 208 (1980) 623-625. 30 Lewis, J. W., Sherman, J. E. and Liebeskind, J. C., Opioid and non-opioid stress analgesia: assessment of tolerance and cross-tolerance with morphine, J. Neurosci., 1 (1981) 358-363. 31 Lewis, J. W., Stapleton, J. M., Catiglioni, A. J. and Liebeskind, J. C., Stimulation-produced analgesia and intrinsic mechanisms of pain suppression. In G. Finck and L. J. Whalley (Eds.), Neuropeptides, Churchill-Livingstone, Edinburgh, in press. 32 Lewis, J. W., Terman, G. W., Watkins, L. R., Mayer, D. J. and Liebeskind, J. C., Opioid and non-opioid mechanisms of footshock analgesia: role of the spinal dorsolateral funiculus, Brain Research, 267 (1983) 13%144. 33 MacLennan, A. J., Drugan, R. C., Hyson, R. L., Maier, S. F., Madden, J. and Barchas, J. D., Hypophysectomy and dexamethasone block the analgesic but not shuttlebox escape consequences of inescapable shock, J. comp. physiol. Psychol., 96 (1982) 904-912. 34 Maier, S. F., Davies, S., Grau, J. W., Jackson, R. L., Morrison, D., Moye, T., Madden. J. and Barchas, J. D., Opiate antagonists and the long-term analgesic reaction induced by inescapable shock, J. comp. physiol. Psychol., 94 (1980) 1172-1184.

35 Maier, S. F., Drugan, R. C. and Grau, J. W., Controllability, coping behavior and stress-induced analgesia, Pain, 12 (1982) 47-56. 36 Maier, S. F., Drugan, R. C., Hyson, L. L., Ryan, S., Moye, T. B., Watkins, L. R. and Mayer, D. J., The role of the pituitary-adrenal axis in inescapable shock produced analgesia, in preparation. 37 Maier, S. F. and Jackson, R. L., Learned helplessness: all of us were right (and wrong): inescapable shock has multiple effects. In G. H. Bower (Ed.), Psychology of Learning and Motivation, Vol. 13, Academic Press, New York, 1979, pp. 155-219. 38 Maier, S. F. and Seligman, M. E. P., Learned helplessness: theory and evidence, J. exp. Psychol.: General, 105 (1976) 3-46. 39 Maier, S. F., Sherman, J. E., Lewis, J. W., Terman, G. W. and Liebeskind, J. C., The opioid/nonopioid nature of stress-induced analgesia and learned helplessness, J. exp. Psychol.: Anim. Behav. Proc., 9 (1983) 80-90. 40 McCreery, D. B., Bloedel, J. R. and Hames, E. G., Effects of stimulating in raphe nuclei and in reticular formation on response of spinothalamic neurons to mechanical stimuli, J. Neurophysiol., 42 (1979) 166-182. 41 Milner, S., Drugan, R. C., Hyson, R. L., Moye, T. B., Watkins, L. R., Mayer, D. J. and Maier, S. F., Intrathecal naltrexone reverses the long term analgesic effect of inescapable shock, in preparation. 42 Moye, T. B., Grau, J. W., Coon, D. J. and Maier, S. F., Therapy and immunization of long-term analgesia in the rat, Learn. Motivation, 12 (1981) 133-149. 43 Moye, T. B., Hyson, R. L., Grau, J. W. and Maier, S. F., Immunization of opioid analgesia: effects of prior escapable shock on subsequent shock-induced and morphine-induced antinociception, Learn. Motivation, in press. 44 Muffin, R., Bennett, G. J. and Mayer, D. J., The effects of dorsolateral spinal cord (DLF) lesions on analgesia from morphine microinjected into the periaqueductal grey matter(PAG) of the rat, Soc. Neurosci. A bstr., 2 (1976) 946.

336 45 Oliveras. J. L., Hosobuchi. Y.. Redjemi, F., Guilbaud. G. and Besson. J. M., Opiate antagonist, naloxone, strongly reduces analgesia induced by stimulation of a raphe nucleus (centralis inferior), Brain Research, 120 (1977) 221-229 46 Overmier. J. B. and Seligman. M. E. P., Effects of inescapable shock upon subsequent escape and avoidance learning, J. comp. physiol. Psychol., 63 (1967) 23-33. 47 Proudfit. H. K., Time-course of alterations in morphine-induced analgesia and nociceptive threshold following medullary raphe lesion, Neuroscience. 6 (1981) 945-95l. 48 Rivot. J. P.. Chaouch. A. and Besson. J. M,. Effects of naloxone on the inhibitory actions induced by stimulation of the raphe magnus on responses of spinal cord dorsal horn cells. Neurosci. Lett. Suppl. 1 (1978) $324, 49 Rivot. J. P.. Chaouch, A. and Besson. J. M., The influence of naloxone on the C fiber response of dorsal horn neurons and their inhibitory control by raphe magnus stimulation. Brain Research. 176 (1979) 355-364. 50 Satoh, M.. Akaike. A.. Nakazaawa. T. and Takagi, H.. Evidence for involvement of separate mechanisms in the production of analgesia by electrical stimulation of the nucleus reticularis paraglgantocellularis and nucleus raphe magnus in the rat. Brain Research. 194 (1980) 525-529. 51 Steinman, J. L., Roberts. L. A. and Komisaruk. B. R.. Evidence that endogenous opiates contribute to the mediation of vaginal stimulation-produced antinociception in rats. Soc. Neurosci. Abstr.. 8 (1982) 265. 52 Tache. Y.. DuRuisseau. P.. Ducharme, J. R. and Collu, R., Adenohypophyseal hormone response to chronic stress in dexamethasone-treated rats. In J. Girard (Ed.), Hormone Research, Vol. 11. Karger. Basel, 1979, pp. 101-108. 53 Taehe. Y.. DuRuisseau. P.. Ducharme, J. R. and Collu. R.. Pattern of adenohypophyseal hormone changes m male rats following chronic stress. Neuroendocrinology, 26 (1978) 208-219. 54 Volpicelli, J. R.. U l m R R., Altenor, A, and Seligman, M. E. P.. Learned mastery in the rat. Learn. Motivation. in press. 55 Watkins, L. R., Cobelli. D. A., Faris, P.. Aceto. M. D. and Mayer. D. J., Opiate vs. non-opiate footshock induced analgesia (FSIA): the body region shocked is a critical factor. Brain Research. 242 (1982) 299-308. 56 Watkins, L. R.. Cobelli, D. A. and Mayer. D. J., Classical conditioning of front paw and hind paw footshock induced analgesia (FSIA): naloxone reversibility and descending pathways. Brain Research, 243 (1982) 119-132. 57 Watkins. L. R., Cobelli, D. A. and Mayer, D. J., Opiate vs. non-opiate footshock induced analgesia (FSIA): descending and intraspinal components. Brain Research. 245 (1982) 97-106. 58 Watkins. L. R.. Cobelli. D. A., Newsome. H. H. and Mayer. D. J.. Footshock induced analgesia is dependent neither on p~tmtary nor sympathetic activation. Brain Research. 245 (1982) 81-96.

59 Watkins. L. R., Faris, P. L.. Mayer, D. J and Komisaruk, B. R.. Anatomical evidence that supraspinal efferents to the spinal cord mediate vaginal stimulation-induced antinociception in rats. Soc. Neurosci. Abstr.. 8 (1982) 771 60 Watkins, L R., Griffin, G., Leichnetz. G R and Mayer, D. J., The somatotop~c organization of the nucleus raphe magnus and surrounding brain stem structures as revealed by HRP slow-release gels. Brain Research. 181 (1980~ t-15. 61 Watkins, L. R., Griffin, G.. Leichnetz. G. R. and Mayer. D. J., Identification and somatotopic organization of nuclei projecting via the dorsolateral funiculus in rats: a retrograde tracing study using HRP slow-release gels. Brain Research. 223 (1981) 237-255. 62 Watkins. L. R., Kinscheck. I. B. and Mayer, D. J., Failure of decerebration or periaqueductal gray lesions to abolish opiate or non-opiate footshock analgesia. Brain Research, m press. 63 Watkins. L. R. and Mayer. D. J.. Involvement ot spinal opioid systems in footshock induced analgesia: antagonism by naloxone is possible only before induction of analgesia. Brain Research, 242 (t982) 309-316. 64 Watkins. k R. and Mayer. D. J., Organization of endogenous opiate and non-opiate pain control systems, Science, 216 (1982) 1185-1192. 65 Watkins, L R., Young, E. G.. Kinscheck, 1. B. and Mayer. D. J., The neural basis of footshock analgesia: the role of specific ventral medullary nuclei. Brain Research, in press. 66 Weiss, J. M. Glazer. H. I. and Pohorecky, L. A , Coping behavior and neurochemical changes: an alternative explanation for the original 'learned helplessness' experiments. In G. Serban and A. Kling (Eds.), Animal Models in Human Psychobiology, Plenum Press. New York. 1976. pp. 42-99. 67 Willis, W. D. and Coggeshatl. R. E,, Sensory Mechanisms of the Spinal Cord, Plenum Press, New York. 1978. 68 Winer, B. J.. Statistical Principles in Experimental Design, McGraw-Hill. New York, 1962. p. 37. 69 Winter. C. A. and Flataker, L.. The effect of cortisone, deoxycorticosterone, and adrenocorticotrophic hormone upon the response of animals to analgesic drugs, J. Pharmacol. exp. Ther., 103 (t950) 93-105. 70 Young, E. G., Watkins. L. R. and Mayer, D, J., Comparison of the effects of ventral medullary lesions on systemic and microinjection morphine analgesia, Brain Research. m press. 71 Zorman. G.. Belcher. G., Adams, J. E. and Fields, H. L., Lumbar intrathecal naloxone blocks analgesia produced by microstimulation of the ventromedial medulla in the rat. Brain Research, in press. 72 Zorman. G.. Hentall, I. D,, Adams. J. E. and Fields. H. L.~ Naloxone-reversible analgesia produced by microstimutation in the rat medulla. Brain Research. 219 f1981) 137-148.