Outcomes of Chronically Anticoagulated Patients Undergoing Split-Thickness Skin Grafting for Diabetic Foot Ulcers

OBJECTIVE

Split-thickness skin grafting (STSG) is commonly used for wound closure in diabetic foot ulcers (DFUs). In many cases, patients with diabetes present on long-term anticoagulation therapy. The complications associated with anticoagulants can be discouraging to surgeons considering STSG. The goal of this study was to evaluate STSG outcomes in the setting of chronic anticoagulation across a large, multicenter database.

METHODS

The authors queried the TriNetX Network, which provides access to electronic medical records for more than 75 million patients, to search for patients with a history of DFUs treated with STSG. They divided those found into two groups: long-term anticoagulant use prior to grafting and no long-term anticoagulant use. After matching, the researchers evaluated outcomes following STSG after 1 month and 5 years.

RESULTS

The authors identified 722 patients on chronic anticoagulation with DFUs who were treated with STSG; 446 of these patients were matched to 446 patients with no prior anticoagulation. One month following STSG, the anticoagulated group showed no significant increase in death, graft failure, or regrafting. At 5 years, there was no significant increase in mortality, graft failure, regrafting, or lower extremity amputation rates.

CONCLUSIONS

Chronic anticoagulation therapy does not lead to increased short- or long-term postoperative complications such as graft failure, regrafting, or increased amputation rates following STSG for wound closure. Negative outcomes following STSG for DFUs in chronically anticoagulated individuals are minimal, and grafting should be performed without hesitation.

The impact of COVID-19 on clinical outcomes of burn patients

Background

Multiple studies have shown the SARS-CoV-2 virus (COVID-19) to be associated with deleterious outcomes in a wide range of patients. The impact of COVID-19 has not been well investigated among burned patients. We suspect that patients will have worsened respiratory and thrombotic complications, ultimately leading to increased mortality. The objective of this study is to determine the impact a concurrent infection of COVID-19 has on clinical outcomes after a burn injury.

Methods

This is a retrospective, propensity matched, cohort study. We examined a de-identified database of electronic medical records of over 75 million patients across 75 health care associations in the United States for patients treated for thermal burns from 1 January 2020, to 31 July 2021, and those who also were diagnosed with COVID-19 infection within one day before or after injury based on International Classification of Disease, tenth revision (ICD-10) codes. Study participants included adults who were treated for a burn injury during the study period.

Results

We included 736 patients with burn injury and concomitant COVID-19 infection matched to 736 patients with burn injury and no concurrent COVID-19 infection (total 1472 patients, mean age 36.3 ± 24.3). We found no significant increase in mortality observed for patients with concurrent COVID-19 (OR 1.203, 95% CI 0.517-2.803; p = 0.6675). We did observe significant increase in infections (OR 3.537, 95% CI 2.798-4.471; p = 0.0001), thrombotic complications (OR 2.342, 95% CI 1.351-4.058; p = 0.0018), as was the incidence of hypertrophic scarring (OR 3.368, 95% CI 2.326-4.877; p = 0.0001).

Conclusions

We observed that concurrent COVID-19 infection was associated with an increase in infections, thrombosis and hypertrophic scarring but no increase in mortality in our cohort of burn patients.

Implications of COVID-19 Infection on Arteriovenous Fistula Thrombosis

Objective: This study aims to identify and analyze implications of COVID-19 positivity on AVF occlusion, subsequent treatment patterns, and ESRD patient outcomes. Our aim is to provide a quantitative context for vascular access surgeons in order to optimize surgical decision making and minimize patient morbidity. Methods: The de-identified national TriNetX database was queried to extracted all adult patients who had a known AVF between January 1, 2020 and December 31, 2021. From this cohort individuals who also were diagnosed with COVID-19 prior to creation of their AVF were identified. Cohorts were propensity score matched according to age at AVF surgery, gender, ethnicity, diabetes mellitus, nicotine dependence, tobacco use, use of anticoagulant medications, and use of platelet aggregation inhibitors, hypertensive diseases, hyperlipidemia, and prothrombotic states. Results: After propensity score matching there were 5170 patients; 2585 patients in each group. The total patient population had 3023 (58.5%) males and 2147 (41.5%) females. The overall rate of thrombosis of AV fistulas was 300 (11.6%) in the cohort with COVID-19 and 256 (9.9%) in the control group (OR 1.199, CI 1.005-1.43, P =.0453). Open revisions of AVF with thrombectomy were significantly higher in the COVID-19 cohort compared to the non-COVID-19 group (1.5% vs .5% P = .0002, OR 3.199, CI 1.668-6.136). Regarding the time from AVF creation to intervention, the median days for open thrombectomy in COVID-19 patients was 72 vs 105 days in controls. For endovascular thrombectomy, the median was 175 vs 168 days for the COVID-19 and control cohorts respectively. Conclusion: As for this study, there were significant differences in rates of thrombosis and open revisions of recent created AVF, however endovascular interventions remained remarkably low. As noted in this study, the persistent prothrombotic state of patients with a history of COVID-19 may persist beyond the acute infectious period of the disease.

The Burn Wound

Skin serves as a protective barrier against infection, prevents excessive fluid and electrolyte losses, performs crucial thermoregulation, and provides tactile feedback of surroundings. The skin also plays an essential role in human perception of body image, personal appearance, and self-confidence. With these many diverse functions, understanding normal anatomic composition of skin is pivotal to evaluating the extent of its disruption from burn injury. This article discusses the pathophysiology, initial evaluation, subsequent progression, and healing of burn wounds. By delineating the various microcellular and macrocellular alterations of burn injury, this review also augments providers' capacity to deliver patient-centered, evidence-based burn care.

Risk factors for opioid use disorder after severe burns in adults

INTRODUCTION

Risk factors for opioid dependence amongst burn patients have not been well-explored compared to other surgical fields.

METHODS

The TrinetX database was queried for patients diagnosed with opioid use disorder (OUD) after thermal or chemical burn. Propensity score matching was performed. Opioid and non-opioid analgesia use, ICU care, surgery, and comparative risks among common opiates were examined using descriptive and univariate regression models, including odds ratios. Subgroup analysis evaluated the impact of multimodal analgesia.

RESULTS

Odds of receiving IV opioids for acute analgesia (p = <0.0001, OR = 1.80, CI = 1.45-2.25), undergoing surgery (p = <0.0001, OR = 1.58, CI = 1.26-1.98), and ICU care (p = <0.0001, OR = 3.60, CI = 2.00-3.83) after burn injury were higher in patients who developed OUD. Patients receiving multimodal therapy within 24 hours of admission had lower odds of developing OUD (OR = 0.74, CI = 2.76-4.68, p = 0.0001) and chronic pain (OR = 0.89, CI = 0.78-1.00, p = 0.05) regardless of TBSA.

CONCLUSION

Patients who developed opioid use disorder following burn injury had higher odds of receiving opioid exclusive pain management, more frequent surgery, ICU care.

Surgical Management of Lower Extremity Wounds in the Solid Organ Transplant Patient Population: Surgeon Beware

Objective: To evaluate our institutional outcomes of surgical management of lower extremity (LE) wounds in the solid organ transplant recipient population. Approach: An 8-year retrospective review was conducted for all solid organ transplantation (SOT) recipients with LE wounds necessitating surgical management at our tertiary limb salvage center. Outcomes of interest included wound healing, surgical treatment, progression to amputation, and amputation level. Factors contributing to amputation progression were analyzed. The article adheres to the Strengthening the Reporting of Observational Studies in Epidemiology statement. Results: Sixty-four SOT recipients underwent surgical management for their LE wounds between 2010 and 2018. Median number of surgeries per patient was 5 (interquartile range = 2-8); 47 of 64 patients (73.4%) underwent amputation, and 17 of 64 patients (26.6%) underwent nonamputation surgical management. In the amputation group, the majority of primary amputations were minor (42/47, 89.4%); 24 of 42 (57.1%) patients progressed to a higher amputation level, 16 of 42 (38.1%) healed after their index procedure, and 2 of 42 (4.8%) were lost to follow-up (LTFU) after their primary minor amputation. Five of 47 (10.6%) patients undergoing amputations required primary below-knee amputations. In the nonamputation group, 15 of 17 (88.2%) healed, 1 of 17 (5.9%) expired, and 1 of 17 (5.9%) was LTFU. Innovation: To identify the outcomes of patients undergoing surgical management for LE wounds after SOT and elucidate clinical factors that impact the rate of limb salvage. Conclusions: This is the first comprehensive analysis of LE wounds in the transplant population. Our analysis indicates high rates of failed minor amputation, and frequent progression to major amputation in SOT patients. Preexisting comorbidities and immunosuppressive regimens complicate limb salvage; therefore, further research is warranted to optimize surgical LE wound management in this population.

Plantar Foot Ulcer Recurrence in Neuropathic Patients Undergoing Percutaneous Tendo-Achilles Lengthening

Equinus contracture carries 3- and 4-fold associations with diabetes and plantar foot ulceration, respectively. Percutaneous tendo-Achilles lengthening is a useful method to alleviate peak plantar pressure resulting from equinus. We aimed to evaluate the effectiveness of percutaneous tendo-Achilles lengthening and estimate the relative longevity of the approach in reducing ulcer recurrence. The medical records of patients with equinus contracture who underwent percutaneous tendo-Achilles lengthening from 2010 to 2017 were reviewed. Included patients presented with plantar ulcers and a gastroc-soleus equinus of any angle <10° of ankle dorsiflexion with the affected knee extended and flexed. Patients who received concomitant tendon lengthening procedures (including anterior tibial tendon or flexor digitorum longus) were excluded. Outcome measures included time to wound healing, time to ulcer recurrence, and development of transfer lesion. Ninety-one patients underwent percutaneous tendo-Achilles lengthening with subsequent pedal ulceration without concomitant procedures. A total of 69 (75.8%) patients had a plantar forefoot ulcer, 7 (7.7%) had midfoot ulcers, 5 (5.5%) had hindfoot ulcers, and 3 (3.3%) had ulcers in multiple locations. Seven patients received prophylactic tendo-Achilles lengthening. At a mean follow-up of 31.6 months (±26), 66 (78.6%) wounds healed at a median 12.9 weeks. A total of 29 patients (43.9%) experienced ulcer recurrence at a mean of 12 months. Twelve patients (13%) experienced a transfer lesion at a mean of 16.6 months. Tendo-Achilles lengthening can be an effective adjunctive approach to achieve wound healing and reduce long-term ulcer recurrence in patients with equinus contracture and neuropathic plantar foot ulcers. A relengthening procedure may be needed within approximately 12 months from index surgery.

Glove and instrument changing to prevent bacterial contamination in infected wound debridement and closure procedures: A prospective observational study

Many surgeons use a single table of instruments for both excisional debridement and coverage/closure of infected wounds. This study investigates the effectiveness of a two-table set-up of sterile instruments, in addition to glove exchange, to reduce instrument cross-contamination during these procedures. This is a prospective, single-site, institutional review board-approved observational study of surgical debridements of infected wounds over a 17-month period. Two separate sterile surgical tables were used for each case: Table A for initial wound debridement (debridement set-up) and Table B for wound coverage/closure (clean set-up). Swabs of each table and its respective instruments were taken after debridement but prior to coverage/closure. The primary outcome of interest was bacterial growth at 48 hours. There were 72 surgical cases included in this study. Culture results of Table A demonstrated bacterial growth in 23 of 72 (32%) cases at 48 hours compared with 5of 72 (7%) from Table B (P = .001). These data suggest that there is significant bacterial contamination of surgical instruments used for debridement of infected wounds. Use of a two-table set-up reduced instrument cross-contamination by 78%, suggesting avoidable re-contamination of the wound.

Utility of Porcine-Derived Xenograft as an Adjunct to Split-Thickness Skin Grafting in Lower-Extremity Wounds

OBJECTIVE

Porcine-derived xenograft biological dressings (PXBDs) are occasionally used to prepare chronic wound beds for definitive closure before split-thickness skin grafts (STSGs). We sought to determine whether PXBD influences rate of STSG take in lower-extremity wounds.

METHODS

Lower-extremity wounds treated with STSGs were retrospectively reviewed. Patients were included in one of two groups: wound bed preparation with PXBD before STSG or no preparation. Patients were excluded if they received wound bed preparation via another method. Patient demographics, comorbidities, wound history, wound bed preparation, and 30- and 60-day outcomes were collected.

RESULTS

There was no difference in healing outcomes between the PXBD (n = 27) and no preparation (n = 39) groups. At 30- and 60-day follow-up, percentage of STSG take was not significantly different between groups (77.9% versus 79.0%, P30 = .818; 82.2% versus 80.9%, P60 = .422). Mean wound sizes at these follow-up periods were not different (4.4 cm2 versus 5.1 cm2, P30 = .902; 1.2 cm2 versus 1.1 cm2, P60 = .689). The PXBD group had a higher mean ± SD hemoglobin A1c level (8.3 ± 3.5 versus 6.9 ± 1.6; P = .074) and age (64.9 ± 12.8 years versus 56.3 ± 11.9 years; P = .007) versus the no preparation group.

CONCLUSIONS

Application of PXBDs for wound bed preparation had no effect on wound healing compared with no wound bed preparation. The two groups varied only by mean age and hemoglobin A1c level. The PXBD may be beneficial, but these results call for randomized controlled trials to determine the true impact of PXBDs on wound healing. In addition, PXBDs may have utility outside of clinically oriented outcomes, and future work should address patient-reported outcomes and pain scores with this adjunct.

Long Term Outcomes of Split-Thickness Skin Grafting to the Plantar Foot

The most common consequence of neuropathy is a diabetic foot ulcer, which usually occurs on the plantar surface of the foot. Split-thickness skin grafting (STSG) has been shown in numerous studies to be an effective treatment for rapid coverage of diabetic ulcers. The purpose of this study is to retrospectively examine the outcomes of STSG to the plantar foot and determine the durability of this treatment compared to non-plantar surface STSG. This is a retrospective, single-center, institutional review board approved, case-control study of all patients who received STSG to their lower extremity for chronic ulcers from November 2013 to February 2017. Patients with ulcers on the plantar surface were considered cases, and non-plantar surface ulcers were considered controls. There were 182 patients who received STSG to the lower extremity, 52 to the plantar surface foot and 130 to non-plantar surface locations. Healing at 30 days was not significantly different between plantar and nonplantar ulcers (19% versus 28%, p = .199) but did become significant at 60, 90, and 365 days (21% versus 45%, p = .003; 33% versus 49%, p = .043; 38% versus 64%, p = .002, respectively). However, time to full healing was not significantly different between plantar and nonplantar groups (18.2 ± 19.5 versus 17.4 ± 21.6 weeks, mean ± standard deviation, p = .84). Recurrence was low for both groups (17% versus 10%, respectively), and there was no significant difference between groups (p = .17). Patients with plantar surface ulcers can achieve a durable coverage/closure of their wounds with STSG. When combined with appropriate patient selection and postoperative offloading, acceptable recurrence rates can be achieved.

Use of hyperbaric oxygen therapy for tissue ischemia after breast reconstruction

INTRODUCTION

Mastectomy skin flap necrosis represents a significant complication of breast reconstructive procedures and is reported to occur in 30%-52% of patients undergoing breast reconstruction. Early identification of ischemia and early initiation of hyperbaric oxygen (HBO2) therapy can mitigate the effects of ischemia and rescue otherwise non-viable breast flap tissue.

METHODS

We retrospectively examined the outcomes of HBO2 therapy in eight breasts with compromised mastectomy skin flaps between September 2015 and January 2017. Indocyanine green angiography (ICGA) was used to assess perfusion intraoperatively and post-HBO2 administration.

RESULTS

Seven patients were referred for HBO2 within 24 hours of mastectomy. One patient failed to improve despite starting hyperbaric treatment within 24 hours. All other patients manifested successful healing of their mastectomy skin flaps with acceptable cosmesis after 10 HBO2 treatments. The mean relative perfusion of the at-risk area was 13.8% (±3.7%) pre-HBO2 and 101.6% (±37.3%) post-HBO2. The average area at-risk pre-HBO2 was 17.1 cm2 and reduced to zero post-HBO2. Relative perfusion values after HBO2 were found to be 6.8 (±3.4) times greater than those measured prior to HBO2.

CONCLUSIONS

A short course of HBO2 may be sufficient to successfully rescue at risk post-mastectomy breast flaps. ICGA is a useful adjunct for evaluating post-mastectomy breast flap perfusion before and after HBO2 therapy.

Peripheral Vascular Disease Diagnostic Related Outcomes in Diabetic Charcot Reconstruction

Postreconstructive outcomes were compared in diabetic patients with Charcot neuroarthropathy (CN) who had peripheral arterial disease (PAD) diagnosed with angiography versus patients who were diagnosed clinically. A retrospective review was performed of patients with diabetic CN requiring reconstruction secondary to ulceration and/or acute infection. Of the 284 patients in the CN osseous reconstruction cohort, after accounting for exclusion criteria, 59 (20.8%) patients with PAD were included in the analyses. Forty (67.8%) of these 59 patients were diagnosed with PAD clinically and 19 (32.2%) were diagnosed with the use of angiography. Bivariate analysis was used to compare outcomes between those diagnosed with PAD via angiography versus those diagnosed clinically for the following postreconstruction outcomes: wound healing, delayed healing, surgical site infection, pin tract infection, osteomyelitis, dehiscence, transfer ulcer, new site of Charcot collapse, contralateral Charcot event, nonunion, major lower extremity amputation, and return to ambulation. Bivariate analysis found return to ambulation postreconstruction (p = .0054) to be the only statistically significant factor. There was a trend toward significance for major lower extremity amputation, with higher rates of amputation in the clinically diagnosed PAD arm. Return to ambulation indicates improved functional outcomes. The main goal of limb salvage should be focused on improving the patient's functional performance. With significantly faster rates of return to ambulation and a trend toward decreased rates of major amputation, angiography was found to be a better assessor of PAD than clinical evaluations.

Outcomes of Split-thickness Skin Grafting for Foot and Ankle Wounds in Patients With Peripheral Arterial Disease

INTRODUCTION

Tissue ischemia resulting from arterial insufficiency is a major factor affecting lower extremity wound healing in patients with peripheral arterial disease (PAD). Accelerated wound closure with split-thickness skin grafting (STSG) provides a durable barrier to infection and can prevent limb loss. Published STSG outcomes data are minimal in the post endovascular intervention population.

OBJECTIVE

In this study, the authors examine factors predictive of STSG healing in patients with PAD following vascular intervention, including the effect of non-inline flow via arterial-arterial and non-arterial collateralization.

MATERIALS AND METHODS

Patients with PAD and wounds of the foot and ankle who underwent STSG between January 2014 and December 2016 were retrospectively reviewed. All patients received angiographic evaluation and endovascular or open revascularization where necessary. Effects of extremity revascularizations, STSG percent take, and amputation rate were evaluated.

RESULTS

Thirty-five patients with 47 wounds underwent STSG. There were 21 men and 14 women with a mean age of 64 ± 13 years. Revascularization was required in 23 patients (25 extremities) before STSG, with balloon angioplasty for tibial artery lesions as the most common revascularization. Patent pedal arch was present in 8 patients; 35 patients had an absent or incomplete pedal arch. Patients with a fully patent pedal arch healed at a significantly higher rate than those with an absent or incomplete pedal arch at 1 month (62.5% vs. 17.1%, P ⟨ .05). At 90-day follow-up, 9 of 35 (25.7%) patients with 9 of 47 (19.1%) wounds were lost to follow-up, leaving 18 of 38 (47.37%) wounds healed and 20 (52.63%) still open. Ultimately, 36 of 47 (76.60%) wounds healed and 6 major amputations in 6 patients were required at a mean 502 ± 342 days follow-up.

CONCLUSIONS

These results suggest the importance of arterial-arterial connections such as the pedal arch to the healing potential of foot and ankle wounds after STSG in this high-risk patient population.

Incidence of heparin-induced thrombocytopenia in lower-extremity free flap reconstruction correlates with the overall surgical population

BACKGROUND

Lower-extremity free flap reconstruction is a growing trend in the management of lower extremity wounds. Heparin-induced thrombocytopenia (HIT) is a significant risk to free flap reconstruction. The purpose of this study was to investigate the incidence of HIT in patients receiving lower-extremity free flap surgery.

METHODS

We conducted a retrospective, single center, IRB approved cohort study in which we reviewed all patients who received lower-extremity free flap surgeries between 2011 and 2016. The 4T and HIT Expert Probability (HEP) scores were calculated to assess the likelihood of HIT.

RESULTS

One hundred patient charts revealed three patients with HIT. One patient was excluded due to a prior diagnosis of HIT. HIT incidence in patients receiving lower-extremity free flaps was between 1% and 3%, which is consistent with the national average. 4T scores indicated that two of three HIT-positive patients had a high probability of HIT (approximately 64%), and one of three HIT-positive patients had an intermediate probability (approximately 14%). HEP scoring indicated that all the three (100%) patients had HIT.

CONCLUSIONS

These data suggest that the incidence of HIT in patients receiving lower-extremity free flaps correlates with the incidence of HIT nationally. The use of available scoring methods and other algorithms, combined with patient history helps to assess the immediate perioperative risks of HIT in the absence of rapid immunologic confirmatory tests. This knowledge can allow for successful free flap salvage or for performance of free flaps in patients with a history of HIT.

Chronic Nicotine Exposure Attenuates Methamphetamine-Induced Dopaminergic Deficits

Repeated methamphetamine (METH) administrations cause persistent dopaminergic deficits resembling aspects of Parkinson's disease. Many METH abusers smoke cigarettes and thus self-administer nicotine; yet few studies have investigated the effects of nicotine on METH-induced dopaminergic deficits. This interaction is of interest because preclinical studies demonstrate that nicotine can be neuroprotective, perhaps owing to effects involving α4β2 and α6β2 nicotinic acetylcholine receptors (nAChRs). This study revealed that oral nicotine exposure beginning in adolescence [postnatal day (PND) 40] through adulthood [PND 96] attenuated METH-induced striatal dopaminergic deficits when METH was administered at PND 89. This protection did not appear to be due to nicotine-induced alterations in METH pharmacokinetics. Short-term (i.e., 21-day) high-dose nicotine exposure also protected when administered from PND 40 to PND 61 (with METH at PND 54), but this protective effect did not persist. Short-term (i.e., 21-day) high-dose nicotine exposure did not protect when administered postadolescence (i.e., beginning at PND 61, with METH at PND 75). However, protection was engendered if the duration of nicotine exposure was extended to 39 days (with METH at PND 93). Autoradiographic analysis revealed that nicotine increased striatal α4β2 expression, as assessed using [(125)I]epibatidine. Both METH and nicotine decreased striatal α6β2 expression, as assessed using [(125)I]α-conotoxin MII. These findings indicate that nicotine protects against METH-induced striatal dopaminergic deficits, perhaps by affecting α4β2 and/or α6β2 expression, and that both age of onset and duration of nicotine exposure affect this protection.

Methamphetamine treatment during development attenuates the dopaminergic deficits caused by subsequent high-dose methamphetamine administration

Administration of high doses of methamphetamine (METH) causes persistent dopaminergic deficits in both nonhuman preclinical models and METH-dependent persons. Noteworthy, adolescent [i.e., postnatal day (PND) 40] rats are less susceptible to this damage than young adult (PND90) rats. In addition, biweekly treatment with METH, beginning at PND40 and continuing throughout development, prevents the persistent dopaminergic deficits caused by a "challenge" high-dose METH regimen when administered at PND90. Mechanisms underlying this "resistance" were thus investigated. Results revealed that biweekly METH treatment throughout development attenuated both the acute and persistent deficits in VMAT2 function, as well as the acute hyperthermia, caused by a challenge METH treatment. Pharmacokinetic alterations did not appear to contribute to the protection afforded by the biweekly treatment. Maintenance of METH-induced hyperthermia abolished the protection against both the acute and persistent VMAT2-associated deficits suggesting that alterations in thermoregulation were caused by exposure of rats to METH during development. These findings suggest METH during development prevents METH-induced hyperthermia and the consequent METH-related neurotoxicity.

Methamphetamine-induced dopamine transporter complex formation and dopaminergic deficits: the role of D2 receptor activation

Methamphetamine (METH) abuse is a serious public health issue. Of particular concern are findings that repeated high-dose administrations of METH cause persistent dopaminergic deficits in rodents, nonhuman primates, and humans. Previous studies have also revealed that METH treatment causes alterations in the dopamine transporter (DAT), including the formation of higher molecular mass DAT-associated complexes. The current study extends these findings by examining mechanisms underlying DAT complex formation. The association among DAT complex formation and other METH-induced phenomena, including alterations in vesicular monoamine transporter 2 (VMAT2) immunoreactivity, astrocytic activation [as assessed by increased glial fibrillary acidic protein (GFAP) immunoreactivity], and persistent dopaminergic deficits was also explored. Results revealed that METH-induced DAT complex formation and reductions in VMAT2 immunoreactivity precede increases in GFAP immunoreactivity. Furthermore, and as reported previously for DAT complexes, pretreatment with the D2 receptor antagonist eticlopride [S-(-)-3-chloro-5-ethyl-N-[(1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxybenzamide hydrochloride] attenuated the decrease in VMAT2 immunoreactivity as assessed 24 h after METH treatment. DAT complexes distinct from those present 24 h after METH treatment, decreases in VMAT2 immunoreactivity, and increased GFAP immunoreactivity were present 48 to 72 h after METH treatment. Pretreatment with eticlopride attenuated each of these phenomena. Finally, DAT complexes were present 7 days after METH treatment, a time point at which VMAT2 and DAT monomer immunoreactivity were also reduced. Eticlopride pretreatment attenuated each of these phenomena. These findings provide novel insight into not only receptor-mediated mechanisms underlying the effects of METH but also the interaction among factors that probably are associated with the persistent dopaminergic deficits caused by the stimulant.

Defensive responses of larval Manduca sexta and their sensitization by noxious stimuli in the laboratory and field

Sensitization of defensive responses following noxious stimulation occurs in diverse species, but no demonstration of nociceptive sensitization in insects has been reported. A set of defensive behavior patterns in larval Manduca sexta is described and shown to undergo sensitization following noxious mechanical stimulation. The striking response is a rapid bending that accurately propels the head towards sharply poking or pinching stimuli applied to most abdominal segments. The strike is accompanied by opening of the mandibles and, sometimes, regurgitation. The strike may function to dislodge small attackers and startle larger predators. When the same stimuli are applied to anterior segments, the head is pulled away in a withdrawal response. Noxious stimuli to anterior or posterior segments can evoke a transient withdrawal (cocking) that precedes a strike towards the source of stimulation and may function to maximize the velocity of the strike. More intense noxious stimuli evoke faster, larger strikes and may also elicit thrashing, which consists of large, cyclic, side-to-side movements that are not directed at any target. These are sometimes also associated with low-amplitude quivering cycles. Striking and thrashing sequences elicited by obvious wounding are sometimes followed by grooming-like behavior. Very young larvae also show locomotor responses to noxious stimuli. Observations in the field of attacks on M. sexta larvae by Cardinalis cardinalis, an avian predator, suggest that thrashing decreases the success of a bird in biting a larva. In the laboratory, noxious stimulation was found to produce two forms of sensitization. Repeated pinching of prolegs produces incremental sensitization, with later pinches evoking more strikes than the first pinch. Brisk pinching or poking of prolegs also produces conventional sensitization, in which weak test stimuli delivered to another site evoke more strikes following noxious stimulation. The degree and duration of sensitization increase with more intense noxious stimulation. The most intense stimulus sequences were found to enhance strike frequency for approximately 60 min. Nociceptive sensitization generalizes to sites distant from sites of noxious stimulation, suggesting that it involves a general, but transient, arousal of defensive responses. http://www.biologists.com/JEB/movies/jeb3271.html

Limited contributions of serotonin to long-term hyperexcitability of Aplysia sensory neurons

Serotonin (5-HT) has provided a useful tool to study plasticity of nociceptive sensory neurons in Aplysia. Because noxious stimulation causes release of 5-HT and long-term hyperexcitability (LTH) of sensory neuron somata and because 5-HT treatment can induce long-term synaptic facilitation of sensory neuron synapses, a plausible hypothesis is that 5-HT also induces LTH of the sensory neuron soma. Prolonged or repeated exposure of excised ganglia to 5-HT produced immediate hyperexcitability of sensory neurons that showed little desensitization, but the hyperexcitability decayed within minutes of washing out the 5-HT. Prolonged or repeated treatment of either excised ganglia or dissociated sensory neurons with various concentrations of 5-HT failed to induce significant LTH even when long-term synaptic facilitation was induced in the same preparations. Use of a high-divalent cation solution to reduce interneuron activity during 5-HT treatment failed to enable the induction of LTH in excised ganglia. Pairing 5-HT application with nerve shock failed to enhance LTH produced by nerve shock or to reveal covert LTH produced by 5-HT. The induction of LTH by nerve stimulation was enhanced rather than inhibited by treatment with methiothepin, a 5-HT antagonist reported to block various 5-HT receptors and 5-HT-induced adenylyl cyclase activation. This suggests that endogenous 5-HT may have inhibitory effects on the induction of LTH by noxious stimulation. Methiothepin blocked immediate hyperexcitability produced by exogenous 5-HT and also inhibited the expression of LTH induced by nerve stimulation when applied during testing 1 day afterward. At higher concentrations, methiothepin reduced basal excitability of sensory neurons by mechanisms that may be independent of its antagonism of 5-HT receptors. Several observations suggest that early release of 5-HT and consequent cAMP synthesis in sensory neurons is not important for the induction of LTH by noxious stimulation, whereas later release of 5-HT from persistently activated modulatory neurons, with consequent elevation of cAMP synthesis, may contribute to the maintenance of LTH.

Activation of protein kinase A contributes to the expression but not the induction of long-term hyperexcitability caused by axotomy of Aplysia sensory neurons

Nociceptive sensory neurons (SNs) in Aplysia provide useful models to study both memory and adaptive responses to nerve injury. Induction of long-term memory in many species, including Aplysia, is thought to depend on activation of cAMP-dependent protein kinase (PKA). Because Aplysia SNs display similar alterations in models of memory and after nerve injury, a plausible hypothesis is that axotomy triggers memory-like modifications by activating PKA in damaged axons. The present study disproves this hypothesis. SN axotomy was produced by (1) dissociation of somata from the ganglion [which is shown to induce long-term hyperexcitability (LTH)], (2) transection of neurites of dissociated SNs growing in vitro, or (3) peripheral nerve crush. Application of the competitive PKA inhibitor Rp-8-CPT-cAMPS at the time of axotomy failed to alter the induction of LTH by each form of axotomy, although the inhibitor antagonized hyperexcitability produced by 5-HT application. Strong activation of PKA in the nerve by coapplication of a membrane-permeant analog of cAMP and a phosphodiesterase inhibitor was not sufficient to induce LTH of either the SN somata or axons. Furthermore, nerve crush failed to activate axonal PKA or stimulate its retrograde transport. Therefore, PKA activation plays little if any role in the induction of LTH by axotomy. However, the expression of LTH was reduced by intracellular injection of the highly specific PKA inhibitor PKI several days after nerve crush. This suggests that long-lasting activation of PKA in or near the soma contributes to the maintenance of long-term modifications produced by nerve injury.

Cyclic GMP pathway is critical for inducing long-term sensitization of nociceptive sensory neurons

Noxious stimulation can trigger persistent sensitization of somatosensory systems that involves memory-like mechanisms. Here we report that noxious stimulation of the mollusc Aplysia produces transcription-dependent, long-term hyperexcitability (LTH) of nociceptive sensory neurons that requires a nitric oxide (NO)-cyclic GMP-protein kinase G (PKG) pathway. Injection of cGMP induced LTH, whereas antagonists of the NO-cGMP-PKG pathway prevented pinch-induced LTH. Co-injection of calcium/cAMP-responsive-element (CRE) blocked both pinch-induced LTH and cAMP-induced LTH, but antagonists of protein kinase A (PKA) failed to block pinch-induced LTH. Thus the NO-cGMP-PKG pathway and at least one other pathway, but not the cAMP-PKA pathway, are critical for inducing LTH after brief, noxious stimulation.

Effect of pepper and bismuth subsalicylate on gastric pain and surface hydrophobicity in the rat

BACKGROUND

The mechanism by which dietary pepper causes dyspepsia and epigastric pain is poorly understood, as is the ability of bismuth subsalicylate (BSS) to relieve these symptoms.

AIM

To investigate the ability of black pepper, red pepper and BSS to affect gastric surface hydrophobicity and induce/relieve visceral pain in rat model systems.

METHODS

Fasted rats were administered intragastrically Vivonex containing varying concentrations of either black or red pepper (0-200 mg/mL) and gastric contact angles were read after 1-24 h. Some rats were post-treated with BSS (2.0-17.5 mg/mL) and contact angles were read after 2-18 h. To study pain sensitivity in rats treated with pepper/BSS, we compared tail-flick latencies after the application of radiant heat.

RESULTS

Both black and red pepper rapidly (< 1 h) induced a decrease in gastric surface hydrophobicity in a dose-dependent fashion. This spice-induced increase in surface wettability was long-lasting, could be enhanced in the presence of ethanol and reversed by post-treating the rats with BSS. Both black and red pepper induced an increase in pain sensitivity, consistent with the presence of gastric pain, which could also be reversed by post-treating the rats with BSS.

CONCLUSION

Both black and red pepper may induce epigastric pain by removing the stomach's hydrophobic lining and activating intramucosal pain receptors. BSS may provide relief from postprandial dyspepsia by restoring the stomach's non-wettable properties.

Functions of the LE sensory neurons in Aplysia

Mechanosensory neurons which innervate the siphon and have their cell bodies in the LE cluster of the abdominal ganglion of Aplysia have revealed many cellular and molecular processes that may play general roles in learning and memory. It was initially suggested that these cells are largely responsible for triggering the gill-withdrawal reflex evoked by weak siphon stimulation, and that most of this effect is mediated by their monosynaptic connections to gill motor neurons. This implied a simple link between plasticity at these synapses and modifications of the reflex during learning. We review more recent studies from several laboratories showing that the LE cells are not activated by very weak tactile stimuli that elicit the gill-withdrawal reflex, and that an unidentified population of siphon sensory neurons has lower mechanosensory thresholds and produces shorter latency responses. Furthermore, the direct connections between LE cells and gill motor neurons make a minor contribution when the reflex is elicited in pinned siphon preparations by light stimuli that weakly activate the LE cells. Because weak mechanical stimulation of the unrestrained siphon causes little or no LE cell activation, it is unlikely that, under natural conditions, sensitization or conditioning of reflex responses elicited by light siphon touch depends upon plasticity of LE cell synapses onto either motor or interneurons. The LE cells appear to function as nociceptors because they are tuned to noxious stimuli and, like mammalian nociceptors, show peripheral sensitization following nociceptive activation. This sensitization and the profound activity-dependent potentiation of LE synapses indicate that LE cell contributions to defensive reflexes should be largest during and after intense activation of the LE cells by noxious stimulation (with the LE cell plasticity contributing to long-lasting memory of peripheral injury). The LE sensory neurons offer special opportunities for direct tests of this and other hypotheses about specific mnemonic functions of fundamental mechanisms of neural plasticity.

Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization

Numerous studies of learning and memory in Aplysia have focused on primary mechanosensory neurons innervating the siphon and having their somata in the left E (LE) cluster of the abdominal ganglion. Although systematic analyses have been made of the responses of these LE cells to mechanical stimulation of the tightly pinned siphon, little is known about corresponding responses when the siphon is unrestrained. The present study demonstrates that LE mechanosensory thresholds in the freely moving siphon are much higher than in the pinned siphon. Light tactile stimuli adequate to activate central neurons and reflexive siphon movements often fail to activate the LE cells when the siphon is unrestrained. Because the LE cells display increasing discharge to increasing pressures, with maximal activation by crushing or tearing stimuli that cause tissue injury, they satisfy accepted definitions of nociceptor. Indeed, they show similarities to vertebrate Adelta nociceptors, including a property apparently unique (among primary afferents) to nociceptors-sensitization by noxious stimulation of their receptive field. Either pinching or pinning the siphon decreases LE cell mechanosensory threshold and enhances soma excitability. Such stimuli reduce effective tissue compliance and cause neuromodulation that enhances sensory responsiveness. These results, and recent descriptions of predatory attacks on Aplysia, suggest that LE sensory neurons are tuned to grasping and crushing stimuli that threaten or produce bodily harm. LE cell sensitization has effects, resembling hyperalgesia and allodynia, that compensate for loss of sensory function during injury and help protect against subsequent threats.

Intrinsic injury signals enhance growth, survival, and excitability of Aplysia neurons

Neurons undergo extensive changes in growth and electrophysiological properties in response to axon injury. Efforts to understand the molecular mechanisms that initiate these changes have focused almost exclusively on the role of extrinsic signals, primarily neurotrophic factors released from target and glial cells. The objective of the present investigation was to determine whether the response to axonal injury also involves intrinsic axoplasmic signals. Aplysia neurons were removed from their ganglia and placed in vitro on a substratum permissive for growth, but in the absence of glia and soluble growth factors. Under these conditions, neurites emerged and grew for approximately 4 d. Once growth had ceased, the neurites were transected. In all, 46 of 50 cells regenerated, either by resorbing the remaining neurites and elaborating a new neuritic arbor or by merely replacing the neurites that had been severed. Cut cells also exhibited enhanced excitability and, paradoxically, prolonged survival, when compared with uninjured neurons. These findings indicate that axons contain intrinsic molecular signals that are directly activated by injury to trigger changes underlying regeneration and compensatory plasticity.

Priming events and retrograde injury signals. A new perspective on the cellular and molecular biology of nerve regeneration

Successful axon regeneration requires that signals from the site of injury reach the nucleus to elicit changes in transcription. In spite of their obvious importance, relatively few of these signals have been identified. Recent work on regeneration in the marine mollusk Aplysia californica has provided several insights into the molecular events that occur in neurons after axon injury. Based on these findings, we propose a model in which axon regeneration is viewed as the culmination of a series of temporally distinct but overlapping phases. Within each phase, specific signals enter the nucleus to prime the cell for the arrival of subsequent signals. The first phase begins with the arrival of injury-induced action potentials, which act via calcium and cAMP to turn on genes used in the early stages of repair. In the next phase, MAP-kinases and other intrinsic constituents activated at the injury site are retrogradely transported through the axon to the nucleus, informing the nucleus of the severity of the axonal injury, reinforcing the earlier events, and triggering additional changes. The third phase is characterized by the arrival of signals that originate from extrinsic growth factors and cytokines released by cells at the site of injury. In the last phase, signals from target-derived growth factors arrive in the cell soma to stop growth. Because many of these events appear to be universal, this framework may be useful in studies of nerve repair in both invertebrates and vertebrates.

Zwitterionic phospholipids enhance aspirin's therapeutic activity, as demonstrated in rodent model systems

We have recently reported that the Gl toxicity of aspirin is markedly reduced when the drug is preassociated with the zwitterionic phospholipid dipalmitoylphosphatidycholine (DPPC) before intragastric administration. The present study was designed to determine whether the biological availability and therapeutic activity of aspirin were affected by chemically associating the drug with DPPC. To evaluate this, we compared the kinetics of entry of labeled aspirin into the blood, after intragastric administration of the drug in the free and lipid-associated states; we also tested the ability of the above formulations to inhibit fever, inflammation and pain in appropriate rodent model systems. We found that although the Gl absorption of free aspirin and that of the aspirin/DPPC complex were similar, in all three rodent models the complex had significantly greater antipyretic, anti-inflammatory and analgesic efficacy than aspirin alone. Dose-response analyses employing the fever model demonstrated that potency of aspirin to reduce fever was increased 5 to 10-fold when the aspirin was intragastrically administered in the lipid-associated state. We conclude that the therapeutic activity of aspirin to inhibit fever, inflammation and pain is remarkably enhanced when the drug is intragastrically administered in chemical association with the zwitterionic phospholipid DPPC. A number of molecular mechanisms have been proposed to explain the observed phospholipid-dependent increase in aspirin's therapeutic activity.

Recovery of function, peripheral sensitization and sensory neurone activation by novel pathways following axonal injury in Aplysia californica

Recovery of behavioural and sensory function was examined following unilateral pedal nerve crush in Aplysia californica. Nerve crush that transected all axons connecting the tail to the central nervous system (CNS) eliminated the ipsilateral tail-evoked siphon reflex, whose sensory input travels in the crushed tail nerve (p9). The first reliable signs of recovery of this reflex were observed within 1 week, and most animals displayed tail-evoked siphon responses within 2 weeks. Wide-dynamic-range mechanosensory neurons with somata in the ventrocaudal (VC) cluster of the ipsilateral pleural ganglion exhibited a few receptive fields (RFs) on the tail 3 weeks after unilateral pedal nerve crush, indicating that the RFs had either regenerated or been reconnected to the central somata. These RFs were smaller and sensitized compared with corresponding RFs on the contralateral, uncrushed side. Centrally conducted axon responses of VC sensory neurones to electrical stimulation distal to the nerve crush site did not reappear until at least 10 days after the crush. Because the crush site was much closer to the CNS than to the tail, the failure of axon responses to be restored earlier than the behavioural responses indicates that early stages of reflex recovery are not due to regeneration of VC sensory neurone axons into the tail. Following nerve crush, VC sensory neurones often could be activated by stimulating central connectives or peripheral nerves that do not normally contain the sensory neurone's axons. These results suggest that recovery of behavioral function after nerve injury involves complex mechanisms, including regenerative growth of axotomized VC sensory neurones, sensitization of regenerating RFs and sprouting of VC sensory neurone fibres within the CNS. Furthermore, the rapidity of behavioural recovery indicates that its initial phases are mediated by additional mechanisms, perhaps centripetal regeneration of unidentified sensory neurones having peripheral somata, or transient reconnection of proximal and distal stumps of axotomized VC cells.

Peripheral regeneration and central sprouting of sensory neurone axons in Aplysia californica following nerve injury

Morphological methods were used to examine injury-induced growth of peripheral and central axons of nociceptive mechanosensory neurones in the ventrocaudal (VC) clusters of the pleural ganglia of Aplysia californica. Pedal nerve crush transected all axons in the nerve while leaving the overlying sheath largely intact. Immunohistochemical staining was performed with an antibody to a sensory-neurone-specific peptide, sensorin-A. Following bilateral crush of pedal nerve p9, which innervates the tail, sensorin-A immunofluorescence was lost distal to the crush site within 2 days. Fine immunopositive fibres began to invade the crush region within 5 days. These fibres arborized in the crush region and gradually extended down the crushed nerve. Immunopositive fibres were found near the tail within 3 weeks. Similar results were obtained after injecting individual sensory neurone somata in the tail/p9 region of the VC cluster with biocytin. Biocytin injections and horseradish peroxidase injections 3 weeks after ipsilateral pedal nerve crush revealed new fibres projecting rostrally from the tail/p9 region of the VC cluster and entering the pleural-cerebral and pleural-abdominal connectives. Such projections were never observed in control, uncrushed preparations. These results demonstrate that nerve injury triggers extensive growth of both peripheral and central processes of the VC sensory neurones.

Motor neuronal control of tail-directed and head-directed siphon responses in Aplysia californica

1. Cutaneous stimulation of opposite ends of the body causes qualitatively different siphon responses: tail stimulation causes flaring and backward bending (the siphon T response), whereas head stimulation causes constriction and slight anterior bending (the siphon H response). This paper characterizes the motor neuronal control of siphon T and siphon H responses. 2. The siphon response to tail nerve (p9) shock in a semi-intact preparation was indistinguishable from the siphon T response in intact or parapodectomized animals. Similarly, the siphon response to head nerve (c2) shock in this preparation was indistinguishable from the siphon H response in intact or parapodectomized animals. 3. Central siphon motor neurons (SMNs) were found to cause a wider variety of movements than previously reported. The movements produced by the LFSB cells strongly resemble the flaring response of the siphon to tail or tail nerve stimulation. The movements produced by RDS and LDS1 resemble components of the constricting response of the siphon to head or head nerve stimulation. 4. Among central SMNs, the LFSB cells show the strongest activation by posterior stimulation, whereas RDS and LDS1 show the strongest activation by anterior stimulation. The LFSA cells, which produce much weaker siphon constriction, are only activated slightly by posterior stimulation and are inhibited by anterior stimulation. Peripheral SMNs are inhibited by stimulation of head and tail nerves, and thus their activity does not directly contribute to siphon T and H responses. 5. Artificially activating central SMNs with the pattern of activity previously exhibited after tail or head nerve stimulation indicated the sufficiency of the LFSB cells for the siphon T response, and of RDS and LDS1 for the siphon H response. 6. Dramatic behavioral deficits produced by hyperpolarizing the LFSB cells during tail nerve stimulation, or by hyperpolarizing RDS and LDS1 during head nerve stimulation, indicated the necessity of these cells for the expression of directed siphon responses to tail or head stimulation, respectively. 7. Because of their apparent necessity and sufficiency for directional siphon responses to anterior and posterior stimulation, these few cells provide well-defined vantage points for studying neural mechanisms underlying the motor control and transformation of siphon responses. The four LFSB cells offer a special advantage for cellular analysis because they form a homogeneous functional unit in which any sampled LFSB cell can be used as a precise monitor of the total motor output underlying the siphon T response.(ABSTRACT TRUNCATED AT 400 WORDS)

Axoplasm enriched in a protein mobilized by nerve injury induces memory-like alterations in Aplysia neurons

Axon regeneration after injury and long-term alterations associated with learning both require protein synthesis in the neuronal cell body, but the signals that initiate these changes are largely unknown. Direct evidence that axonal injury activates molecular signals in the axon was obtained by injecting axoplasm from crushed or uncrushed nerves into somata of sensory neurons with uncrushed axons. Those injected with crush axoplasm behaved as if their axons had been crushed, exhibiting increases in both repetitive firing and spike duration, and a decrease in spike afterhyperpolarization 1 d after injection. Because similar changes occur in the same cells after learning, these data suggest that some of the long-lasting adaptive changes that occur after injury and learning may be induced by common axoplasmic signals. Since the signals in axoplasm must be conveyed to the cell soma, we have begun to test the hypothesis that at least some of these signals are proteins containing a nuclear localization signal (NLS). Axoplasmic proteins at the crush site and those that accumulated at a ligation proximal to the crush were probed with an antibody to an amino acid sequence (sp) containing a NLS that provides access to the retrograde transport/nuclear import pathway. One protein, sp97, displayed properties expected of an axonal injury signal: it responded to injury by undergoing an anterograde-to-retrograde change in movement and, when the ligation was omitted, it was transported to the cell bodies of the injured neurons.

Long-term alterations induced by injury and by 5-HT in Aplysia sensory neurons: convergent pathways and common signals?

Bodily injury in Aplysia, as in mammals, produces long-lasting memory traces at various neural loci. One consequence of injury, damage to peripheral axons, produces long-term hyperexcitability, synaptic facilitation, and growth in Aplysia sensory neurons. Similar effects are induced in these cells by repeated exposure to 5-HT that is released during aversive learning. An interesting question is to what extent cellular pathways that mediate the effects of axonal injury and 5-HT overlap. One current focus is on identifying cytoplasmic signals that initiate persistent sensory alterations that contribute to both long-term sensitization and memory of injury.

Role of peri-axonal inflammation in the development of thermal hyperalgesia and guarding behavior in a rat model of neuropathic pain

Loose ligation of a rat's sciatic nerve produces hyperalgesia to thermal stimuli and elicits guarding behavior directed at the afflicted paw. The present experiments test whether localized inflammation induced by the suture material used to ligate the nerve is critical to the development of hyperalgesia. Daily injections of dexamethasone reduced the inflammatory response induced by the sutures and blocked the development of guarding behavior and thermal hyperalgesia. In a second experiment inflammation associated with cotton sutures was enhanced by soaking the sutures in Freund's adjuvant prior to ligation. This caused an augmentation of thermal hyperalgesia and guarding behavior. These results suggest that inflammation around the nerve is critical for the development of guarding behavior and thermal hyperalgesia in this model of neuropathic pain.

Retrograde transport of plasticity signals in Aplysia sensory neurons following axonal injury

Following injury to their peripheral branches, mechanosensory neurons in Aplysia display long-term plasticity that is expressed as soma hyperexcitability, synaptic facilitation, and neurite outgrowth. To investigate the nature of signals that convey information about distant axonal injury, we have investigated the development of injury-induced soma hyperexcitability in two in vitro preparations. In isolated ganglia, proximal nerve crush caused hyperexcitability to appear sooner than did distal crush, and the difference in development of hyperexcitability indicated that the injury signal moved at a rate (36 mm/d) similar to previously reported rates of retrograde axonal transport in this animal. This hyperexcitability was not due to interruption of continuous retrograde transport of trophic substances (a negative signal) because inhibitors of axonal transport applied to uncrushed nerve segments did not induce hyperexcitability. Indeed, inhibitors of axonal transport blocked crush-induced hyperexcitability, indicating that positive injury signals are involved. Crush-induced hyperexcitability was unaffected by bathing the nerve in tetrodotoxin or the ganglion in Cd2+, suggesting that the retrograde signals depend upon neither spike activity in the nerve nor synaptic transmission in the ganglion. Close excision of sensory neuron somata (which largely eliminated delays attributable to axonal transport) produced soma hyperexcitability that was expressed after 10 hr and lasted at least 17 d. These data indicate that axonal injury mobilizes signal molecules that are conveyed by retrograde axonal transport into the soma and possibly the nucleus, where they induce long-term plasticity similar to that expressed by these cells during learning and memory.

Response-specific inhibition during general facilitation of defensive responses in Aplysia

Siphon responses of Aplysia have been used to examine the neural basis of nociceptive behavioral inhibition. The authors tested the response specificity and functional significance of this inhibition. Video analysis showed that strong tail-nerve shock decreased the duration of siphon constriction evoked by weak siphon shock. Tail-nerve shock also caused the appearance of a novel flaring response to the test stimulus, which resembled the siphon response to tail-nerve shock. Novel flaring responses were expressed to both mechanical and electrical siphon stimuli. Tailshock facilitated another defensive response, inking, during the period of inhibited siphon constriction. Tailshock also facilitated tail contractions evoked by weak contralateral tail stimulation during this period. These results indicate that inhibition is not generalized across defensive responses and is specific to siphon responses that interfere with directed ink ejection toward an injured site.

Response-specific inhibition during general facilitation of defensive responses in Aplysia

Siphon responses of Aplysia have been used to examine the neural basis of nociceptive behavioral inhibition. The authors tested the response specificity and functional significance of this inhibition. Video analysis showed that strong tail-nerve shock decreased the duration of siphon constriction evoked by weak siphon shock. Tail-nerve shock also caused the appearance of a novel flaring response to the test stimulus, which resembled the siphon response to tail-nerve shock. Novel flaring responses were expressed to both mechanical and electrical siphon stimuli. Tailshock facilitated another defensive response, inking, during the period of inhibited siphon constriction. Tailshock also facilitated tail contractions evoked by weak contralateral tail stimulation during this period. These results indicate that inhibition is not generalized across defensive responses and is specific to siphon responses that interfere with directed ink ejection toward an injured site.

Induction of a cellular defense reaction is accompanied by an increase in sensory neuron excitability in Aplysia

The complexity of vertebrate immune and nervous systems makes detailed cellular analysis of neuroimmune interactions a challenging prospect. The immune systems of invertebrates, although much less complex than their vertebrate counterparts, share basic cellular defense responses to wounded self or nonself. We have developed a simple model system to study neuroimmune interactions using an invertebrate preparation well suited to detailed cellular analysis. Loose ligation of peripheral nerves in Aplysia induced a cellular defense reaction evidenced by the accumulation of large numbers of amebocytes at the ligation site. From 5 to 30 d after ligation, the excitability of the soma of sensory neurons having axons in ligated nerves was increased compared to contralateral sensory neurons with axons in nonligated nerves. Spike threshold and afterhyperpolarization were reduced, and spike amplitude and duration were increased. Spike accommodation was also decreased such that sensory neurons on the ligated side fired more spikes to a 1 sec intracellular depolarizing pulse than control sensory neurons. These effects are unlikely to be accounted for by ligation-induced injury of sensory axons since both morphological and electrophysiological evidence indicated that the axons in ligated nerves were healthy and able to conduct action potentials. Amebocytes activated by the presence of nonself may release factors that lead to a central sensitization of sensory neurons with axons in close proximity to the amebocytes.

Comparative analysis of hyperexcitability and synaptic facilitation induced by nerve injury in two populations of mechanosensory neurones of Aplysia californica

Long-term effects of nerve injury on electrophysiological properties were compared in two populations of mechanosensory neurones in Aplysia californica: the J and K clusters in the cerebral ganglia and the VC clusters in the pleural ganglia. Following crush of cerebral nerves containing their axons, the cerebral J/K sensory neurones showed long-term changes that were quite similar to alterations previously described in the VC sensory neurones after either axonal injury or aversive learning. These changes include synaptic facilitation, an increase in soma excitability and spike duration, and a decrease in spike threshold and afterhyperpolarization. In addition, simultaneous crush of both the cerebral and pedal nerves in the same animals produced alterations in the cerebral J/K sensory neurones and pleural VC sensory neurones that were virtually identical. The incidence of hyperexcitability was the same in cerebral J/K and pleural VC sensory neurones when all their axons were crushed, even though the former population includes many neurones that probably have appetitive functions while the latter population appears to be made up exclusively of neurones with defensive functions. Long-term plasticity in both sensory populations failed to occur when nerves lacking axons of the tested neurones were crushed, even when the crush site was very close to the somata of the sensory neurones. This axonal specificity argues against a role for delayed activation of facilitatory interneurones in triggering the plasticity. Several observations are consistent with a triggering role for either (1) intracellular signals released directly by axonal injury or (2) extracellular signals released locally by other axons or injured support cells, or by immunocytes attracted to the injured site.

Activity-dependent depression of mechanosensory discharge in Aplysia

1. Inhibition of action potential discharge in Aplysia mechanosensory neurons after noxious stimulation has not been described previously. The present studies investigated depressive effects of prolonged noxious stimulation and repetitive intracellular activation on the number and latency of action potentials evoked by test stimuli applied to the tail or the nerve innervating the tail. Action potential discharge was monitored in the somata of mechanonociceptors in the pleural ganglia. 2. Repeated brief pinches delivered at 5-s intervals to a sensory neuron's receptive field on the tail initially caused intense activation (10-25 spikes recorded in the soma) followed by a progressive decrease or "wind-down" of spike number during subsequent pinches. 3. Repeated application to the tail of noxious shock that caused intense activation of sensory neurons (10-22 spikes during the initial shock) produced progressive wind-down of discharge similar to that produced by repeated tail pinch. However, sensory neurons that showed lower activation (1-9 spikes) to the same shock displayed wind-up of discharge during the 10 shocks. These results suggested that prolonged, intense activation depresses subsequent action potential discharge. 4. Changes in the time required for spikes evoked in the tail to reach the central soma were used as an indicator of changes in the excitability and/or conduction velocity of peripheral branches. Repeated pinch within a sensory neuron's receptive field caused an increase in the latency of discharge elicited by test shocks within the receptive field that lasted > or = 10 min. Repetitive intracellular stimulation of the sensory neuron soma caused a similar increase in latency. 5. Repetitive soma activation decreased the number of spikes evoked 10 s later by a test shock in the sensory neuron's receptive field, indicating that spike activity depresses the initiation and/or conduction of spikes in peripheral branches. Surprisingly, repeated pinch to the receptive field caused no significant change in the number of spikes evoked by the same test shock. This difference suggests that tail pinch produces concomitant facilitatory effects that oppose the depressive effects of intense spike activity. 6. Depressive effects of repeated pinch and repetitive soma activation were expressed in the axon between the receptive field and the CNS. Spikes evoked by brief test shocks delivered to the nerve containing the axon of the recorded sensory neuron showed a transient increase in latency (perhaps due to a decrease in conduction velocity) after either procedure. Repeated pinch, but not repetitive soma activation, also caused an increase in spike threshold in the nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid amplification and facilitation of mechanosensory discharge in Aplysia by noxious stimulation

1. Little is known about modulation of action potential discharge in Aplysia mechanosensory neurons during defensive responses. The present studies examined rapid effects of noxious stimulation (occurring within 0.5-10 s) on the number of action potentials evoked by test stimuli delivered to the tail. Responses were monitored in the somata of mechanonociceptors in the pleural ganglion. A major hypothesis to be tested was that an important function of previously described alterations of membrane conductances in the sensory neuron soma is to generate an after-discharge that amplifies sensory signals during severe noxious stimulation of the cell's receptive field. 2. Discharge of spikes evoked by a moderate tap to one part of a sensory neuron's receptive field on the tail was enhanced by strong shock delivered 10 s earlier to another part of the field. Part of this enhancement appears to be due to a decrease in conduction block in central regions of the sensory neuron. 3. Repeated delivery of innocuous, moderately intense tail shock at 5-s intervals caused a progressive increase ("wind-up") of discharge, whereas repeated delivery of weak tail shock had no significant effect on discharge. In some cases the increase in action potential number involved a buildup of afterdischarge. 4. A single strong tail pinch sometimes induced an afterdischarge lasting < or = 2 s. Afterdischarge could also be induced in the isolated nervous system by intense electrical stimulation of the nerve containing the sensory neuron's main axon. 5. Several observations suggest that afterdischarge requires cooperative effects of a relatively large number of coactivated fibers in the test pathway. In contrast to pinching stimuli (which stimulated a larger part of the tail), intense, punctate stimulation with von Frey hairs failed to produce afterdischarge. Weaker tail or nerve stimulation failed to produce afterdischarge, even when short-latency, high-frequency discharge was evoked in the sensory neuron. 6. Cooperative effects on afterdischarge may differ from those involved in activity-dependent enhancement of presynaptic facilitation because simultaneous pairing of high-frequency activation of a single test sensory neuron with strong stimulation of a peripheral nerve lacking an axon of the tested sensory neuron was not sufficient to produce afterdischarge. The cooperative effects on afterdischarge may function to encode information about both the severity and spatial extensiveness of an injury. 7. Artificial hyperpolarization of the soma often reversibly reduced or abolished afterdischarge evoked by stimulating the nerve or tail. Thus the afterdischarge is often generated in or near the sensory neuron soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Similar neuronal alterations induced by axonal injury and learning in Aplysia

Learning in the marine mollusk Aplysia has been associated with enhanced sensory function, expressed in mechanosensory neurons as (i) decreases in action potential threshold, accommodation, and afterhyperpolarization, and (ii) increases in action potential duration, afterdischarge, and synaptic transmission. These alterations also occur, with a delay, after sensory axons are injured under conditions in which synaptic transmission is severely reduced. The latency and specificity of injury-induced alterations indicate that induction signals are generated at the site of injury and conveyed centrally by axonal transport. Similarities in neuronal modifications support the hypothesis that some memory mechanisms evolved from mechanisms of injury-induced sensory compensation and repair.