opioid receptors in the periaqueductal gray 
 ( pag ) contribute to a wide range of behaviors .
these include nociceptive 
 modulation , cardiovascular regulation , thermoregulation , and locomotor activity 
 [ 15 ] . although mu - opioid receptors ( mor ) are known to contribute to 
 pag mediated antinociception [ 6 , 7 ] , less is known about the contribution of delta - opioid receptors 
 ( dors ) .
although antinociception has been produced by the administration of dor 
 agonists into the pag , these effects are mild compared to the antinociception 
 produced by mor agonists [ 810 ] .
chronic treatment with morphine 
 causes the spinal density of dors to shift from the cytoplasm to the plasma 
 membrane [ 11 , 12 ] .
a similar shift in dors from the cytoplasm   to the plasma membrane appears to occur in the pag .
swim stress causes an 
 increase in dor density in the plasma membrane of pag neurons . in vitro recordings
show that dor agonists do not alter 
 gabaergic synaptic transmission in pag neurons from drug - naive animals [ 1517 ] , but inhibit gabaergic ipscs in mice treated chronically with 
 morphine .
the behavioral significance of enhanced dor 
 expression in the pag has not been characterized .
the increased expression of 
 dor in the pag of morphine tolerant rats could be a compensatory mechanism for 
 the loss of antinociception at the mu - opioid receptor .
increased expression of 
 dors in the spinal cord has been shown to enhance the antinociceptive effect of 
 intrathecal administration of the dor agonist deltorphin ii .
the objective of the present study was to 
 determine the behavioral consequences of activating dors in the pag following 
 induction of morphine tolerance . given the widespread effects mediated by the 
 pag , mobilization of dors to the plasma membrane could contribute to a wide 
 range of behaviors .
the enhanced antinociceptive effects of dor agonists at the 
 spinal level   suggest that the administration of dor agonists into the vpag of 
 morphine tolerant rats will produce antinociception .
this hypothesis will be 
 tested by examining the antinociceptive and locomotor effects of microinjecting 
 the dor agonist deltorphin into the vpag of rats made tolerant to morphine .
male sprague - dawley rats ( 240360 g ) were 
 anesthetized with pentobarbital and implanted with a guide cannula aimed at the 
 ventrolateral pag using stereotaxic techniques ( from lambda : ap   =   + 1.2 mm , ml   =   0.6 mm , and dv   =   4.6 mm ) .
the guide cannula was 9 mm long and affixed to two 
 screws in the skull with dental cement .
all injections and testing were conducted during 
 the dark phase of a 12-hour light / dark cycle in a dimly illuminated room . 

experiments were conducted in accordance with the national institutes of health 
 guide for the care and use of laboratory animals .
efforts were made to minimize 
 the number and potential suffering of the experimental subjects .
nociception 
 was assessed using the hot plate , tail withdrawal , and formalin tests .
the hot - plate 
 test ( iitc , woodland hills , calif , usa ) consisted of measuring the latency for 
 a rat to lick a hind paw when placed on a 52c plate .
tail withdrawal measured the latency to 
 move the tail when placed in 52c water .
the formalin test consisted of rating 
 pain behavior on a 03 scale following 
 injection of formalin ( 2% in 50 ul ) into the plantar surface of the hind paw .
the values on this 
 scale are 0   =   normal behavior ; 1   =   paw touches the ground without bearing 
 weight ; 2   =   paw does not touch the ground ; 3   =   paw is above the ground and 
 licked .
four 
 days after surgery , an injection cannula was inserted through the guide 
 cannula , but no drug was injected .
this process habituates rats to the 
 injection procedure and diminishes behavioral effects produced by cell damage 
 on the test day .
testing began one week following surgery , deltorphin ii ( 1 g/0.5 l ) 
 or saline was microinjected into the vpag .
an 11 mm injection cannula was 
 inserted into the guide cannula while the rat was gently restrained by hand . 

drugs 
 were injected at a rate of 0.1 ul/10 s. the injection cannula remained in place 
 an additional 20 seconds to minimize drug flow up the cannula track .
the stylet 
 was reinserted into the guide cannula and the rat was returned to its home 
 cage .
morphine injectionsthe objective of this experiment was to 
 determine the behavioral effects of microinjecting deltorphin into the vpag in 
 rats made tolerant to repeated subcutaneous injections of morphine .
morphine 
 ( 5 , 10 , or 20 mg / kg ) or saline ( 1 ml / kg ) 
 was administered twice a day ( at 9:30 and 15:00 ) for 3.5 days .
nociception was assessed with 
 the hot plate and tail flick tests 30 minutes after the injection on trials 1 
 and 7 , but not after injections on trials 26 .
this procedure 
 limits changes in nociception from repeated testing [ 21 , 22 ] .
six hours after the last subcutaneous injection , all rats were 
 injected with deltorphin ( 1 g/0.5 l ) into the vpag .
a subset of these rats was tested 
 again on the hot plate 50 minutes after deltorphin administration ( n   =   9 , 5 , 
 10 , and 8 for groups tested with saline , 5 , 10 , and 20 mg / kg of morphine , resp . ) . 

nociception was assessed using the formalin test in the other rats ( n   =   8 , 6 , and 
 9 for groups tested with saline , 10 , and 20 mg / kg of morphine , resp.).locomotor activity was assessed for 30 minutes 
 beginning immediately after the 20 minutes hot - plate test .
activity 
 was assessed by placing the rat into a chamber ( 25.1      47 cm ) with 7 photobeams 
 spaced 5.1 cm apart ( san diego instruments , san diego , calif , usa ) .
the average 
 number of photobeams disrupted each minute was measured and averaged over 10 
 minute intervals for 30 minutes .
the behavior of the rat was examined every 5 minutes during the 
 locomotor test in an attempt to determine the reason for the changes in 
 locomotion ( e.g. , grooming , sleeping , and freezing ) .
the objective of this experiment was to 
 determine the behavioral effects of microinjecting deltorphin into the vpag in 
 rats made tolerant to repeated subcutaneous injections of morphine .
morphine 
 ( 5 , 10 , or 20 mg / kg ) or saline ( 1 ml / kg ) 
 was administered twice a day ( at 9:30 and 15:00 ) for 3.5 days .
nociception was assessed with 
 the hot plate and tail flick tests 30 minutes after the injection on trials 1 
 and 7 , but not after injections on trials 26 .
this procedure 
 limits changes in nociception from repeated testing [ 21 , 22 ] .
six hours after the last subcutaneous injection , all rats were 
 injected with deltorphin ( 1 g/0.5 l ) into the vpag .
a subset of these rats was tested 
 again on the hot plate 50 minutes after deltorphin administration ( n   =   9 , 5 , 
 10 , and 8 for groups tested with saline , 5 , 10 , and 20 mg / kg of morphine , resp . ) . 

nociception was assessed using the formalin test in the other rats ( n   =   8 , 6 , and 
 9 for groups tested with saline , 10 , and 20 mg / kg of morphine , resp . ) .
locomotor activity was assessed for 30 minutes 
 beginning immediately after the 20 minutes hot - plate test .
activity 
 was assessed by placing the rat into a chamber ( 25.1      47 cm ) with 7 photobeams 
 spaced 5.1 cm apart ( san diego instruments , san diego , calif , usa ) .
the average 
 number of photobeams disrupted each minute was measured and averaged over 10 
 minute intervals for 30 minutes .
the behavior of the rat was examined every 5 minutes during the 
 locomotor test in an attempt to determine the reason for the changes in 
 locomotion ( e.g. , grooming , sleeping , and freezing ) .
continuous morphine administrationrats were surgically implanted with a guide 
 cannula aimed at the vpag as described in experiment 1 .
one week later , 
 tolerance was induced by implanting two 75 mg morphine pellets under the skin 
 of the upper back while rats were briefly anesthetized with halothane .
nociception was assessed using 
 the hot - plate test 2 hours following pellet implantation.rats were returned to the test room 3 days after 
 pellet implantation and allowed to habituate for 30 minutes .
nociception was 
 assessed at the end of this period using the hot - plate test to determine 
 whether tolerance had developed . following this baseline test ,
both morphine 
 and placebo - treated rats were injected with deltorphin ( 1 ug/0.5 ul ) into the 
 vpag .
nociception was assessed using the hot - plate test 30 and 60 minutes after the 
 deltorphin microinjection .
rats were surgically implanted with a guide 
 cannula aimed at the vpag as described in experiment 1 .
one week later , 
 tolerance was induced by implanting two 75 mg morphine pellets under the skin 
 of the upper back while rats were briefly anesthetized with halothane .
nociception was assessed using 
 the hot - plate test 2 hours following pellet implantation .
rats were returned to the test room 3 days after 
 pellet implantation and allowed to habituate for 30 minutes .
nociception was 
 assessed at the end of this period using the hot - plate test to determine 
 whether tolerance had developed . following this baseline test ,
both morphine 
 and placebo - treated rats were injected with deltorphin ( 1 ug/0.5 ul ) into the 
 vpag .
nociception was assessed using the hot - plate test 30 and 60 minutes after the 
 deltorphin microinjection . following testing ,
the 
 microinjection site was marked by injecting cresyl violet ( 0.2 l ) into the pag .
the brain was removed , placed 
 in formalin ( 10% ) , sectioned coronally ( 50 m ) , and viewed under a microscope to localize 
 the injection site . only rats with 
 injection sites in or immediately adjacent to the vpag were included in data 
 analysis .
the effects of morphine pretreatment were compared to 
 saline or placebo - treated controls using a t - test or analysis of 
 variance .
experiment 1 . repeated morphine injectionssystemic administration of high doses of morphine ( 5 , 
 10 , and 20 mg / kg ) produced maximal antinociception on trial 1 ( see figure 1 ) . a 
 significant decrease in antinociception was evident with repeated 
 administration from trial 1 to 7 ( f(1,56 ) = 53.446 , p   <  
.05 ) . the 
 magnitude of the decrease in antinociception was dose dependent ( f(3,56 ) = 5.536 , p   <  
the lowest dose ( 5 mg / kg ) produced the 
 least antinociception and the highest dose ( 20 mg / kg ) produced the greatest 
 antinociception in morphine tolerant rats.microinjection of deltorphin into the vpag of 
 morphine naive rats 
 produced a slight increase in hot - plate latency .
rats pretreated with saline 
 showed a significant increase in hot - plate latency following microinjection of 
 deltorphin from 10.8 ( see trial 7 in figure 1 ) to 14.5 seconds ( 20-minute test in figure 2 ) 
 ( one - tailed t(18 )   =   1.925 , p   <   .05).the effect of microinjecting deltorphin into the 
 vpag of morphine tolerant rats varied with the pretreatment dose ( see figure 2 ) .
a 2      2 anova revealed a significant difference in hot - plate latency 
 between the pretreatment groups ( f(3,84 )   =   3.203 , p   <   .05 ) , but no 
 difference in hot - plate latency between the 20- and 50-minute tests ( f(1,84 )   =   0.866 ; p   >  
the difference between groups was caused by a slight increase in the hot - plate 
 latency of rats pretreated with 20 mg / kg of morphine and a slight decrease in 
 latency in rats pretreated with 10 mg / kg ( see figure 2 ) .
although the 
 difference between these groups was statistically significant ( bonferroni , t   =   2.420 , p   <   .05 ) , neither the 10 mg / kg ( t   =   0.96 and 1.15 for 20 and 50 
 minutes , resp . ) nor the 20 mg / kg ( t   =   1.49 and 0.87 for 20 and 50 minutes , resp . ) 
 groups differed from the saline - pretreated group . moreover , these changes in 
 nociception were quite small compared to the antinociception produced by 
 systemic administration of morphine on trial 7 ( 26.2  2.7 and 31.3  2.7 seconds 
 following 10 and 20 mg / kg doses of morphine).a similar difference between pretreatment groups was evident 
 when nociception was assessed with the tail flick test 50 minutes after 
 deltorphin administration ( f(2,52 )   =   8.699 , p   <   .05 ) .
microinjection 
 of deltorphin into the pag of rats pretreated with 20 mg / kg of morphine caused 
 a small , but significant , increase in tail flick latency ( 4.4  0.2 seconds ) 
 compared to rats pretreated with saline ( 3.5  0.2 seconds ) or 10 mg / kg of 
 morphine ( 3.4  0.2 seconds ) ( tukey test , p   <   .05 for both 
 comparisons).nociception was assessed using the formalin test 
 in the rats not tested on the hot plate at 50 minutes .
formalin was injected 5 
 minutes following the 20 minute hot - plate test so the first and second phases   could be assessed 2529 and 4044 minutes after 
 microinjection of deltorphin .
in contrast to the hot - plate test , pain ratings 
 on the formalin test did not differ between the pretreatment groups on either 
 the first ( mean   ratings   =   1.65 , 1.60 , and 1.73 ) or the second ( mean ratings   =   0.40 , 0.63 , and 0.67 ) phases ( f(2,20 )   =   0.272 , p   =   .76).locomotor activity was assessed during the 30 
 minutes following the 20 minute hot - plate test in the subset of rats not 
 injected with formalin .
microinjection of deltorphin into the vpag caused a 
 significant decrease in activity in the morphine compared to the saline - pretreated 
 rats ( f(2,29 )   =   6.84 , p   <   .05 ) . this decrease was similar in rats 
 pretreated with 10 and 20 mg / kg of morphine ( see figure 3 ) .
the greatest 
 decrease in activity was evident at the latest time period ( 4150 minutes ) for 
 both pretreatment groups.the decrease in activity was accompanied by a 
 change in behavior in which rats tended to crouch or lie down in the activity 
 chamber .
an increase in the number of rats crouching or lying increased in rats 
 pretreated with 10 and 20 mg / kg of morphine beginning approximately 25 minutes after the deltorphin 
 microinjection ( see table 1 ) .
this shift in the number of rats displaying 
 normal exploratory behavior to crouching and lying increased further during the 
 last 15 minutes of the test ( 4054 minutes after 
 the deltorphin microinjection ) .
systemic administration of high doses of morphine ( 5 , 
 10 , and 20 mg / kg ) produced maximal antinociception on trial 1 ( see figure 1 ) . a 
 significant decrease in antinociception
was evident with repeated 
 administration from trial 1 to 7 ( f(1,56 ) = 53.446 , p   <   .05 ) .
the 
 magnitude of the decrease in antinociception was dose dependent ( f(3,56 ) = 5.536 , p   <  
the lowest dose ( 5 mg / kg ) produced the 
 least antinociception and the highest dose ( 20 mg / kg ) produced the greatest 
 antinociception in morphine tolerant rats .
microinjection of deltorphin into the vpag of 
 morphine naive rats 
 produced a slight increase in hot - plate latency .
rats pretreated with saline 
 showed a significant increase in hot - plate latency following microinjection of 
 deltorphin from 10.8 ( see trial 7 in figure 1 ) to 14.5 seconds ( 20-minute test in figure 2 ) 
 ( one - tailed t(18 )   =   1.925 , p   <   .05 ) . the effect of microinjecting deltorphin into the 
 vpag of morphine tolerant rats varied with the pretreatment dose ( see figure 2 ) .
a 2      2 anova revealed a significant difference in hot - plate latency 
 between the pretreatment groups ( f(3,84 )   =   3.203 , p   <   .05 ) , but no 
 difference in hot - plate latency between the 20- and 50-minute tests ( f(1,84 )   =   0.866 ; p   >  
the difference between groups was caused by a slight increase in the hot - plate 
 latency of rats pretreated with 20 mg / kg of morphine and a slight decrease in 
 latency in rats pretreated with 10 mg / kg ( see figure 2 ) . although the 
 difference between these groups was statistically significant ( bonferroni , t   =   2.420 , p   <   .05 ) , neither the 10 mg / kg ( t   =   0.96 and 1.15 for 20 and 50 
 minutes , resp . ) nor the 20 mg / kg ( t   =   1.49 and 0.87 for 20 and 50 minutes , resp . ) 
 groups differed from the saline - pretreated group . moreover , these changes in 
 nociception were quite small compared to the antinociception produced by 
 systemic administration of morphine on trial 7 ( 26.2  2.7 and 31.3  2.7 seconds 
 following 10 and 20 mg / kg doses of morphine ) .
a similar difference between pretreatment groups was evident 
 when nociception was assessed with the tail flick test 50 minutes after 
 deltorphin administration ( f(2,52 )   =   8.699 , p   <   .05 ) .
microinjection 
 of deltorphin into the pag of rats pretreated with 20 mg / kg of morphine caused 
 a small , but significant , increase in tail flick latency ( 4.4  0.2 seconds ) 
 compared to rats pretreated with saline ( 3.5  0.2 seconds ) or 10 mg / kg of 
 morphine ( 3.4  0.2 seconds ) ( tukey test , p   <   .05 for both 
 comparisons ) .
nociception was assessed using the formalin test 
 in the rats not tested on the hot plate at 50 minutes .
formalin was injected 5 
 minutes following the 20 minute hot - plate test so the first and second phases   could be assessed 2529 and 4044 minutes after 
 microinjection of deltorphin .
in contrast to the hot - plate test , pain ratings 
 on the formalin test did not differ between the pretreatment groups on either 
 the first ( mean   ratings   =   1.65 , 1.60 , and 1.73 ) or the second ( mean ratings   =   0.40 , 0.63 , and 0.67 ) phases ( f(2,20 )   =   0.272 , p   =   .76 ) .
locomotor activity was assessed during the 30 
 minutes following the 20 minute hot - plate test in the subset of rats not 
 injected with formalin .
microinjection of deltorphin into the vpag caused a 
 significant decrease in activity in the morphine compared to the saline - pretreated 
 rats ( f(2,29 )   =   6.84 , p   <   .05 ) .
this decrease was similar in rats 
 pretreated with 10 and 20 mg / kg of morphine ( see figure 3 ) .
the greatest 
 decrease in activity was evident at the latest time period ( 4150 minutes ) for 
 both pretreatment groups .
the decrease in activity was accompanied by a 
 change in behavior in which rats tended to crouch or lie down in the activity 
 chamber .
an increase in the number of rats crouching or lying increased in rats 
 pretreated with 10 and 20 mg / kg of morphine beginning approximately 25 minutes after the deltorphin 
 microinjection ( see table 1 ) .
this shift in the number of rats displaying 
 normal exploratory behavior to crouching and lying increased further during the 
 last 15 minutes of the test ( 4054 minutes after 
 the deltorphin microinjection ) .
continuous morphine administrationimplantation of morphine pellets produced an 
 increase in hot - plate latency compared to placebo - treated rats when assessed at 
 1 hour ( 36.1  3.9 versus 20.5  2.4 seconds ; t(8 )   =   3.865 , p   <  
tolerance to the antinociceptive effect of morphine was evident by day 4 as 
 indicated by a decrease in hot - plate latency to 18.8  1.8 seconds . 

microinjection of deltorphin into the pag caused a modest , but significant , 
 increase in hot - plate latency compared to baseline latency ( f(2,16 )   =   5.806 ,
although the baseline hot - plate latency for rats treated with 
 morphine pellets was greater than in placebo - treated rats ( see figure 4 ) , this 
 difference did not reach statistical significance using the bonferroni test ( t   =   1.482 nanoseconds ) .
however , microinjection of deltorphin into the vpag of 
 placebo - treated rats caused a significant increase in hot - plate latency at the 
 60-minute time point compared to the baseline ( bonferroni , 
 t   =   3.044 .
there was no difference in mean hot - plate latency following deltorphin 
 administration between morphine and placebo - treated rats ( f(1,8 )   =   0.791 , p   >   .05 ) , suggesting 
 that deltorphin administration did not increase antinociception beyond what was 
 already present in rats with morphine pellets .
implantation of morphine pellets produced an 
 increase in hot - plate latency compared to placebo - treated rats when assessed at 
 1 hour ( 36.1  3.9 versus 20.5  2.4 seconds ; t(8 )   =   3.865 , p   <   .05 ) . 

tolerance to the antinociceptive effect of morphine was evident by day 4 as 
 indicated by a decrease in hot - plate latency to 18.8  1.8 seconds . 

microinjection of deltorphin into the pag caused a modest , but significant , 
 increase in hot - plate latency compared to baseline latency ( f(2,16 )   =   5.806 , p   <   .05 ) .
although the baseline hot - plate latency for rats treated with 
 morphine pellets was greater than in placebo - treated rats ( see figure 4 ) , this 
 difference did not reach statistical significance using the bonferroni test ( t   =   1.482 nanoseconds ) .
however , microinjection of deltorphin into the vpag of 
 placebo - treated rats caused a significant increase in hot - plate latency at the 
 60-minute time point compared to the baseline ( bonferroni , 
 t   =   3.044 .
there was no difference in mean hot - plate latency following deltorphin 
 administration between morphine and placebo - treated rats ( f(1,8 )   =   0.791 , p   >   .05 ) , suggesting 
 that deltorphin administration did not increase antinociception beyond what was 
 already present in rats with morphine pellets .
the 
 present data demonstrate that repeated morphine administration alters the response 
 of vpag neurons to the dor agonist deltorphin .
although pag neurons contribute 
 to a wide range of behaviors , the change in response 
 to deltorphin microinjection was specific to locomotor activity .
microinjection 
 of deltorphin into the vpag produced a consistent decrease in activity in rats 
 pretreated with morphine .
this decrease in activity was caused by a drastic 
 change in behavior from exploring and grooming to crouching and lying along the 
 edge of the cage .
in contrast , microinjection of deltorphin produced a mild 
 antinociception that was not altered in a consistent manner by prior morphine 
 administration .
the 
 descending modulatory system that runs from the pag to rostral ventromedial 
 medulla ( rvm ) to spinal dorsal horn plays an important role in the 
 antinociceptive effects of both mor and dor agonists [ 9 , 2528 ] .
the antinociception 
 produced by microinjection of dor agonists into the pag is weak compared to 
 morphine administration [ 9 , 10 ] . 

however , dors are located in the pag   and these receptors 
 appear to move from the cytoplasm to the plasma membrane following stress .
the density of dors on 
 the membrane has also been 
 shown to increase in spinal neurons following chronic exposure to morphine [ 11 , 12 ] .
the present data show 
 that microinjection of deltorphin into the vpag had modest effects on 
 nociception .
rats pretreated with 10 mg / kg of morphine showed a slight 
 hyperalgesia compared to saline - pretreated rats injected with deltorphin into 
 the vpag .
rats pretreated with 
 20 mg / kg of morphine showed a slight increase in hot - plate latency following 
 deltorphin microinjection .
the lack of effect of deltorphin in modulating nociception is 
 surprising given that spinal administration of deltorphin following chronic 
 morphine administration produces antinociception .
the 
 lack of a consistent change in nociception following deltorphin administration 
 could be caused by an inability of the morphine administration procedure to 
 mobilize dors to the plasma membrane .
although possible , this explanation seems 
 unlikely given that a decrease in activity was produced by microinjection of 
 deltorphin into the pag . moreover , we used two different procedures to induce 
 tolerance ( repeated injections and continuous administration ) that closely 
 match previous studies reporting changes in dors [ 11 , 12 , 18 ] .
finally , 
 mobilization of dors to the plasma membrane is associated with morphine 
 tolerance and the rats in the present study showed clear signs of tolerance to 
 the antinociceptive effects of morphine . 
 in vitro electrophysiological 
 recordings
reveal dor - mediated inhibition of gabaergic ipscs in tissue from 
 mice pretreated with morphine , but no effect of dor 
 agonists in pag slices from animals that have not been exposed to morphine [ 1517 ] .
this inhibition of gaba input 
 is similar to the effect produced by administration of mor agonists into the 
 pag .
these data suggest 
 that microinjection of dor agonists into the pag of morphine tolerant rats 
 should produce antinociception .
however , the present data show no consistent 
 antinociceptive effect following deltorphin microinjection into the pag of 
 morphine tolerant rats . in 
 contrast
, microinjection of deltorphin into the vpag of morphine pretreated - rats 
 caused a clear and consistent decrease in locomotor activity .
thus , it appears that 
 dors compensate for the locomotor , but not the antinociceptive effects 
 associated with morphine tolerance in the vpag .
the immobility produced by 
 morphine microinjection into the vpag appears to be part of a defensive 
 freezing response [ 3033 ] .
however , the decrease 
 in activity produced by microinjection of deltorphin into the vpag reported 
 here does not appear to be caused by fear - induced freezing .
microinjection of 
 deltorphin caused rats to crouch and lay along the edge of the cage as if the 
 rats were ill or dysphoric .
this effect appears to be consistent with previous 
 research showing that deep tissue pain sufficient to induce recuperative 
 behavior activates vpag neurons .
given that stress 
 increases the density of dors on the plasma membrane , activation of these 
 receptors may contribute to recuperative behavior .
one hypothesis is that 
 the recuperative behavior mediated by the vpag is part of a coordinated response triggered by 
 severe hemorrhage that includes hypotension .
severe blood loss has been shown 
 to activate neurons in the vpag , and inactivation of 
 the vpag [ 2 , 36 ] 
 or microinjection of the dor antagonist naltrindole into the pag   blocks the hypotension 
 produced by hemorrhage .
future studies are needed to determine whether 
 activation of dors in the pag alters blood pressure .
the decrease in 
 locomotor activity caused by microinjection of deltorphin into the vpag of 
 morphine tolerant rats is consistent with previous data showing that dor 
 density on the plasma membrane increases following chronic morphine 
 administration .
pag dors could contribute to morphine tolerance [ 3840 ] , behavioral changes 
 related to stress , or hypovolemic shock [ 35 , 37 ] , but 
 do not appear to contribute to antinociception .
that is microinjection of 
 deltorphin into the pag did not produce antinociception regardless of how 
 tolerance was induced ( repeated injections or continuous administration ) , test 
 used to assess nociception ( hot plate , tail flick , and formalin tests ) , or test 
 times ( 20 and 50 minutes ) .
 chronic morphine administration shifts delta - opioid receptors ( dors ) from the cytoplasm to the plasma membrane . given that microinjection of morphine into the pag 
 produces antinociception , it is hypothesized that the movement of dors to the membrane will allow antinociception to the dor agonist deltorphin ii as a way to compensate for morphine tolerance . 
 tolerance was induced by twice daily injections of morphine ( 5 , 10 , or 20 mg / kg , subcutaneous ) for 3.5 days . 
 microinjection of deltorphin into the vpag 6 hours after the last morphine injection produced a mild antinociception that did not vary in a consistent manner across morphine pretreatment doses or nociceptive tests . 
 in contrast , deltorphin caused a decrease in activity in morphine tolerant rats that was associated with lying in the cage . 
 the decrease in activity and change in behavior indicate that chronic morphine administration alters dors in the vpag . 
 however , activation of these receptors does not appear to compensate for the decrease in antinociception caused by morphine tolerance . 
1. Introduction
2. Methods
3. Results
4. Discussion
5. Conclusion