Thermogenic and psychogenic recruitment of human eccrine sweat glands: Variations between glabrous and non-glabrous skin surfaces

Human eccrine sweat-gland recruitment and secretion rates were investigated from the glabrous (volar) and non-glabrous hand surfaces during psychogenic (mental arithmetic) and thermogenic stimuli (mild hyperthermia). It was hypothesised that these treatments would activate glands from both skin surfaces, with the non-thermal stimulus increasing secretion rates primarily by recruiting more sweat glands. Ten healthy men participated in two seated, resting trials in temperate conditions (25–26 °C). Trials commenced under normothermic conditions during which the first psychogenic stress was applied. That was followed by passive heating (0.5 °C mean body temperature elevation) and thermal clamping, with a second cognitive challenge then applied. Sudomotor activity was evaluated from both hands, with colourimetry used to identify activated sweat glands, skin conductance to determine the onset of precursor sweating and ventilated sweat capsules to measure rates of discharged sweating. From glandular activation and sweat rate data, sweat-gland outputs were derived. These psychogenic and thermogenic stimuli activated sweat glands from both the glabrous and non-glabrous skin surfaces, with the former dominating at the glabrous skin and the latter at the non-glabrous surface. Indeed, those stimuli individually accounted for ~90% of the site-specific maximal number of activated sweat glands observed when both stimuli were simultaneously applied. During the normothermic psychological stimulation, sweating from the glabrous surface was elevated via a 185% increase in the number of activated glands within the first 60 s. The hypothetical mechanism for this response may involve the serial activation of additional eccrine sweat glands during the progressive evolution of psychogenic sweating.     

Development and properties of the secretory response in rat sweat glands: relationship to the induction of cholinergic function in sweat gland innervation

Previous studies suggest that the sympathetic innervation of the sweat glands in the rat is initially noradrenergic and during development undergoes a transition in neurotransmitter phenotype to become cholinergic. To characterize this system and its development further, we have examined the adrenergic and cholinergic components of the secretory response in adult and immature rats and have studied the onset of sweating in the plantar sweat glands of developing rats. Stimulation of the sciatic nerve in adult rats elicited a secretory response which was completely blocked by the cholinergic antagonist, atropine, and was unaffected by adrenergic antagonists, indicating that nerve-evoked secretion was cholinergic. In adult rats, the sweat glands were quite sensitive to cholinergic agonists. In addition to acetylcholine, the mature sweat gland innervation contains vasoactive intestinal peptide (VIP). In some rats, the injection of VIP alone elicited a secretory response which was blocked by atropine, suggesting that the response to VIP was mediated cholinergically. In contrast to cholinergic agonists, the glands responded relatively infrequently and with reduced volumes of sweat to the alpha- and beta-adrenergic agonists 6-fluoronorepinephrine and isoproterenol. However, when VIP, which is a potent vasodilator, was simultaneously injected with adrenergic agonists, glands in many of the injected footpads exhibited a secretory response. The response to adrenergic agonists in combination with VIP was reduced by atropine and by phentolamine plus propranolol, but was blocked completely only by a combination of the three antagonists, indicating that both adrenergic and cholinergic mechanisms were involved. In immature rats, sweating evoked by nerve stimulation first appeared at 14 days of age in 25% of the rats tested. Both the percentage of rats sweating and the number of active glands increased rapidly. At 16 days, 50% of the rats tested exhibited some active glands, and by 21 days all rats tested exhibited a secretory response. In 16-day-old rats, nerve-evoked sweating was almost completely inhibited by local injection of 1 microM atropine, but was unaffected by phentolamine and propranolol in concentrations up to 10 microM. Similarly, the glands were sensitive to 10 microM muscarine, but they exhibited no secretory response to the alpha-adrenergic agonists, clonidine and 6-fluoronorepinephrine, nor to the beta-adrenergic agonist, isoproterenol, at concentrations up to 50 microM. The simultaneous injection of VIP with adrenergic agonists did not reveal an adrenergically mediated secretory response in 16-day-old animals.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neural control and mechanisms of eccrine sweating during heat stress and exercise.

In humans, evaporative heat loss from eccrine sweat glands is critical for thermoregulation during exercise and/or exposure to hot environmental conditions, particularly when environmental temperature is greater than skin temperature. Since the time of the ancient Greeks, the significance of sweating has been recognized, whereas our understanding of the mechanisms and controllers of sweating has largely developed during the past century. This review initially focuses on the basic mechanisms of eccrine sweat secretion during heat stress and/or exercise along with a review of the primary controllers of thermoregulatory sweating (i.e., internal and skin temperatures). This is followed by a review of key nonthermal factors associated with prolonged heat stress and exercise that have been proposed to modulate the sweating response. Finally, mechanisms pertaining to the effects of heat acclimation and microgravity exposure are presented.     

Failure to produce axon reflex-sweating in the volar skin of Japanese monkey (Macaca fuscata) and crab-eating monkey (Macaca irus)

The functional properties of the sweat glands and their innervation in the volar skin of three Japanese monkeys and two crab-eating monkeys were investigated. The sweat glands responded to both cholinomimetic and adrenomimetic agents, the former being highly predominant in the sudorific effect. Spontaneous emotional sweating was strongly or completely inhibited by atropine at 10(-8)-10(-7) g/ml, but not by dihydroergotamine at 10(-5)-10(-4) g/ml. Axon reflex sweating could not be produced by nicotine at 10(-5)-10(-4) g/ml in all of primates tested. The nerve fibers surrounding the sweat glands were histochemically confirmed to contain both acetyl- and butyrylcholinesterase.     

Function of human eccrine sweat glands during dynamic exercise and passive heat stress.

The purpose of this study was to identify the pattern of change in the density of activated sweat glands (ASG) and sweat output per gland (SGO) during dynamic constant-workload exercise and passive heat stress. Eight male subjects (22.8 +/- 0.9 yr) exercised at a constant workload (117.5 +/- 4.8 W) and were also passively heated by lower-leg immersion into hot water of 42 degrees C under an ambient temperature of 25 degrees C and relative humidity of 50%. Esophageal temperature, mean skin temperature, sweating rate (SR), and heart rate were measured continuously during both trials. The number of ASG was determined every 4 min after the onset of sweating, whereas SGO was calculated by dividing SR by ASG. During both exercise and passive heating, SR increased abruptly during the first 8 min after onset of sweating, followed by a slower increase. Similarly for both protocols, the number of ASG increased rapidly during the first 8 min after the onset of sweating and then ceased to increase further (P > 0.05). Conversely, SGO increased linearly throughout both perturbations. Our results suggest that changes in forearm sweating rate rely on both ASG and SGO during the initial period of exercise and passive heating, whereas further increases in SR are dependent on increases in SGO.     

The antidystrophic effect of the action of beta-adrenoblockaders in local damage to the nervous system]

It has been established that the lesion of the sciatic nerve, accompanied by a disturbance of normal neurotrophic provision of a kidney as a result of coming to the organ of the perverted nervous stimuli (by the neuro-conductive path through sympathetic nerves and by participation of the hypothalamus--hypophysis--peripheral glands system), leads to disturbance of functioning of mineral-corticoid receptors of kidneys. It has been also established that simultaneous pharmacological blockade of neuro-conductive and humoral pathways of transmission to the kidney of pathological stimuli from the central stump of the cut sciatic nerve prevents the development of trophic organ disturbances, tested by the state of the kidney mineral-corticoid receptor apparatus, while pharmacological stimulation of sympathetic nervous system leads to the greater disturbance of aldosterone reception by the cells of kidney channels. A valid conclusion can be made that propranolol is a substance, which may weaken possible non-adequate reactions of peripheral tissues to the action of physiologically active substances during the development of the consequences of the lesion of the nervous system and thus to prevent the development of neurogenic dystrophies.     

Morphological evidence for the innervation of apocrine sweat glands in the general hairy skin of the goat

Summary A histomorphological and histochemical study was made on the nerve supply to the apocrine sweat glands in the general hairy skin of the goat. In harmony with the previous report that the sweat glands in the goat are functionally under the control of sympathetic nervous system, the present study clearly demonstrates cholinesterase-reactive nerve fibers that closely surround the secretory portion of these glands in most of the hairy skin area, though the nerve network is fairly coarse. Analysis with cholinesterase inhibitors indicated that the sudomotor nerves in the goat contain both specific and non-specific cholinesterase.     

Cholinergic- rather than adrenergic-induced sweating play a role in developing and developed rat eccrine sweat glands

Both cholinergic and adrenergic stimulation can induce sweat secretion in human eccrine sweat glands, but whether cholinergic and adrenergic stimulation play same roles in rat eccrine sweat glands is still controversial. To explore the innervations, and adrenergic- and cholinergic-induced secretory response in developing and developed rat eccrine sweat glands, rat hind footpads from embryonic day (E) 15.5–20.5, postanal day (P) 1–14, P21 and adult were fixed, embedded, sectioned and subjected to immunofluorescence staining for general fiber marker protein gene product 9.5 (PGP 9.5), adrenergic fiber marker tyrosine hydroxylase (TH) and cholinergic fiber marker vasoactive intestinal peptide (VIP), and cholinergic- and adrenergic-induced sweat secretion was detected at P1–P21 and adult rats by starch-iodine test. The results showed that eccrine sweat gland placodes of SD rats were first appeared at E19.5, and the expression of PGP 9.5 was detected surrounding the sweat gland placodes at E19.5, TH at P7, and VIP at P11. Pilocarpine-induced sweat secretion was first detected at P16 in hind footpads by starch-iodine test. There was no measurable sweating when stimulated by alpha- or beta-adrenergic agonists at all the examined time points. We conclude that rat eccrine sweat glands, just as human eccrine sweat glands, co-express adrenergic and cholinergic fibers, but different from human eccrine sweat glands, cholinergic- rather than adrenergic-induced sweating plays a role in the developing and developed rat eccrine sweat glands.     

Compensatory hyperhidrosis following thoracic sympathectomy: a biophysical rationale

A side-effect of endoscopic thoracic sympathectomy (ETS) is compensatory hyperhidrosis (CH), characterized by excessive sweating from skin areas with intact sudomotor function. The physiological mechanism of CH is unknown, but may represent an augmented local sweat rate from skin areas with uninterrupted sympathetic innervation based on evaporative heat balance requirements. For a given combination of activity and climate, the same absolute amount of evaporation (if any) is needed to balance the rate of metabolic heat production both pre- and post-ETS. However, the rate of local sweating per unit of skin surface area with intact sudomotor activity must be greater post-ETS as evaporation must be derived from a smaller skin surface area. Under conditions with high evaporative requirements, greater degradations in sweating efficiency associated with an increased dripping of sweat should also occur post-ETS, further pronouncing the sweat rate required for heat balance. In conclusion, in addition to the potential role of psychological stimuli for increased sudomotor activity, the existence of CH post-ETS can be described by the interplay between fundamental thermoregulatory physiology and altered heat balance biophysics and does not require a postoperative alteration in physiological control.     

Identification of sudomotor activity in cutaneous sympathetic nerves using sweat expulsion as the effector response

In a warm environment at ambient temperatures between 25 degrees and 38 degrees C (relative humidity 50%-60%) the relationship between sympathetic activity in cutaneous nerves (SSA) and pulses of sweat expulsion was investigated in five young male subjects. The SSA was recorded from the peroneal nerve using a micro-electrode. Sweat expulsion was identified on the sweat rate records obtained from skin areas on the dorsal side of the foot, for spontaneous sweating and drug-induced sweating, using capacitance hygrometry. Sweat expulsion was always preceded by bursts of SSA with latencies of 2.4-3.0 s. This temporal relationship between bursts of SSA and sweat expulsion was noted not only in various degrees of thermal sweating but also in the sweating evoked by arousal stimuli, or by painful electric stimulation. The amplitude of the sudomotor burst was linearly related to the maximal rate of increase of the corresponding sweat expulsion, the amplitude of the expulsion and the integrated amount of sweat produced for the duration of the expulsion. The results provide direct evidence that sweat expulsion reflects directly centrally-derived sudomotor activity.     

Effect of cerebrospinal fluid hyperosmolality on sweating in the heat-stressed patas monkey

Dehydration increases the osmolality of body fluids and decreases the rate of sweating during thermal stress. By localizing osmotic stimuli to central nervous system tissues, this study assessed the role of central stimulation on sweating in a heat-stressed nonhuman primate. Lenperone-tranquilized patas monkeys (Erythrocebus patas n = 5), exposed to 41 +/- 2 degrees C, were monitored for calf sweat rate, rectal and mean skin temperatures, oxygen consumption, and heart rate during infusions (255-413 microliters) of hypertonic artificial cerebrospinal fluid (ACSF) into the third cerebral ventricle. ACSF made hypertonic with NaCl to yield osmolalities of 800 and 1,000 mosmol/kgH2O significantly decreased sweat rate compared with control ACSF (285 mosmol/kgH2O), achieving maximal reductions during infusion of 37 and 53%, respectively. Rectal temperature significantly increased during the recovery period, reaching elevations of 0.69 and 0.72 degrees C, respectively, at 20 min postinfusion. In contrast, ACSF made hypertonic with sucrose (800 mosmol/kgH2O) failed to change sweat rate or rectal temperature during infusion in three animals. Thus, intracerebroventricular infusions of hypertonic ACSF mimicked dehydration-induced effects on thermoregulation. The reduction in heat loss during infusion appeared to depend on an elevation in cerebrospinal fluid [Na+] and not osmolality per se.     

Target determination of neurotransmitter phenotype in sympathetic neurons

While the majority of sympathetic neurons are noradrenergic, a minority population are cholinergic. At least one population of cholinergic sympathetic neurons arises during development by a target-dependent conversion from an initial noradrenergic phenotype. Evidence for retrograde specification has been obtained from transplantation studies in which sympathetic neurons that normally express a noradrenergic phenotype throughout life were induced to innervate sweat glands, a target normally innervated by cholinergic sympathetic neurons. This was accomplished by transplanting footpad skin containing sweat gland primordia from early postnatal donor rats to the hairy skin region of host rats. The sympathetic neurons innervating the novel target decreased their expression of noradrenergif traints and developed choline acetyltransferase (ChAT) activity. In addition, many sweat gland-associated fibers acquired acetylcholinesterase (AChE) staining and VIP immunoreactivity. These studies indicated that sympathetic neurons in vivo alter their neurotransmitter phenotype in response to novel envronmental signals and that sweat glands play a critical role in the cholinergic and peptidergic differentiation of the sympathetic neurons that innervate them. The sweat gland-derived cholinergic differentiation factor is distinct from leukemia inhibitory factor and ciliary neurotrophic factor, two well-characterized cytokines that alter the neurotransmitter properties of cultured sympathetic neurons in a similar fashion. Recent studies indicate that anterograde signalling is also important for the establishment of functional synapses in this system. We have found that the production of cholinergic differentiation activity by sweat glands required sympathetic innervation, and the acquisition and maintenance of secretory competence by sweat glands depends upon functional cholinergic innervation. 1994 John Wiley & Sons, Inc.     

Effect of age on heat-activated sweat gland density and flow during exercise in dry heat

Physiological responses of eight postmenopausal older women (age 52-62 yr) and eight younger women (age 20-30 yr) were compared during moderate intensity exercise in a hot dry environment (48 degrees C dry bulb, 25 degrees C wet bulb). The age groups were matched on the basis of maximal O2 consumption (VO2max), body surface area, and body fatness. After heat acclimation the women walked at 40% VO2max for up to 2 h in the hot dry environment while heart rate (HR), rectal temperature (Tre), mean skin temperature (Tsk), whole-body sweating rate (Msw), and local sweating rates (msw; forearm, chest, and scapula) were measured. Additionally, the density of heat-activated sweat glands (HASG) was determined and average sweat gland flow (SGF) was calculated for the scapular area. Although no differences between age groups were found in HR response (when analyzed as percent of maximal HR) or Tsk, the older women had a significantly higher Tre throughout the heat-exercise session. The greater heat storage of the older women may be explained by their significantly lower Msw and msw. There were no differences between the younger and older women in the density of HASG after 30 min; therefore, the lower msw reflects a diminished output per HASG rather than a decrease in the number of sweat glands recruited. The diminished thermoregulatory ability of the older women, unrelated to differences in VO2max, appears to reflect either 1) a diminished response of the sweat glands to central and/or peripheral stimuli, or 2) an age-related structural alteration in the eccrine glands or surrounding skin cells.     

Neonatal treatment with nerve growth factor antiserum eliminates cholinergic sympathetic innervation of rat sweat glands

Most mammalian sympathetic neurons are noradrenergic, and their dependence upon nerve growth factor (NGF) for survival during development is well established. A minor population of sympathetic neurons, including those that innervate sweat glands, is cholinergic. To determine whether cholinergic sympathetic neurons, like their noradrenergic counterparts, require NGF during development, neonatal rats were treated with NGF-antiserum and 3 weeks later their sweat glands were examined for the presence of innervation. Acetylcholinesterase (AChE) staining and vasoactive intestinal polypeptide-like immunoreactivity (VIP-IR) which mark the mature sweat gland innervation were absent from the sweat glands of the anti-NGF treated animals. Further, when the glands were examined with the electron microscope, no axons or nerve terminals were evident. These observations indicate that the elaboration of the sweat gland plexus is NGF-dependent and suggest that at least one population of cholinergic sympathetic neurons in the rat requires NGF for survival. Our findings are consistent with the idea that during development NGF is a required trophic factor not only for noradrenergic sympathetic but also for cholinergic sympathetic neurons.     

Uric acid and urea in human sweat

The present study investigated whether thermal sweating may relieve elevated concentrations of serum uric acid or urea. Concentrations of uric acid and urea were measured in the sweat of sixteen male volunteers, who were treated with external heat after one hour of intense physical exercise. The same analytes were also measured in their urine and serum samples. Furthermore, creatinine and some electrolytes were determined in these specimens. The results show that the concentration of uric acid in the sweat is 24.5 micromol/L, which is only 6.3% of that in serum. The concentration of urea in the sweat is 22.2 mmol/L, which is 3.6 times that in serum. The results indicate that sweat uric acid concentration is quite minimal, and the estimated total uric acid excretion per day in normal physiological range is insignificant. However, the level of sweat urea was found at a much higher concentration than the serum level. No correlation could be established between the level of uric acid in sweat and in serum. There was also no correlation between the level of urea in sweat and that in serum. These results suggest it would not be effective to relieve the elevated serum uric acid concentration by thermal sweating when the renal excretion of uric acid is partly compromised. Nevertheless, the potential of urea excretion via profuse sweating is apparent particularly when the kidneys are damaged or their function is impaired. These findings also suggest that persons who take vigorous exercise or are exposed to hot environments should be well advised to drink adequate fluids since heavy sweating excretes only minimal uric acid, accompanied by significant diminution of urinary output and diminished urinary excretions of uric acid, which may induce elevated levels of serum uric acid.     

Cholinergic neuronal differentiation factors: evidence for the presence of both CNTF-like and non-CNTF-like factors in developing rat footpad.

Catecholaminergic sympathetic neurons are able to change their transmitter phenotype during development and to acquire cholinergic properties. Cholinergic sympathetic differentiation is only observed in fibers innervating specific targets like the sweat glands in the rat footpad. A function for ciliary neurotrophic factor (CNTF) in this process has been implied as it is able to induce cholinergic properties (ChAT, VIP) in cultured chick and rat neurons. We show here that a CNTF-like, VIP-inducing activity is present in rat footpads and that its increases 6-fold during the period of cholinergic sympathetic differentiation. Immunohistochemical analysis of P21 rat footpads demonstrated CNTF-like immunoreactivity in Schwann cells but not in sweat glands, the target tissue of cholinergic sympathetic neurons. The expression of this factor in footpads seems to be dependent on the presence of intact nerve axons, as nerve transection results in a loss of CNTF-like cholinergic activity and immunoreactivity. Immunoprecipitation experiments with rat footpad extracts provided evidence for the presence of ChAT-inducing factors other than CNTF, which may independently or together with CNTF be involved in the determination of sympathetic neuron phenotype.     

Localized β-adrenergic receptor blockade does not affect sweating during exercise

The purpose of the current study was to determine the effect of a locally administered nonselective β-adrenergic antagonist on sweat gland function during exercise. Systemically administered propranolol has been reported to increase, decrease, or not alter sweat production during exercise. To eliminate the confounding systemic effects associated with orally administered propranolol, we used iontophoresis to deliver it to the eccrine sweat glands within a localized area on one forearm prior to exercise. This allowed for determination of the direct effect of β-adrenergic receptor blockade on sweating during exercise. Subjects (n = 14) reported to the laboratory (23 ± 1°C, 35 ± 3% relative humidity) after having refrained from exercise for ≥12 h. Propranolol (1% solution) was administered to a 5-cm(2) area of the flexor surface of one forearm via iontophoresis (1.5 mA) for 5 min. A saline solution was administered to the opposing arm via iontophoresis. Each subject then exercised on a motor-driven treadmill at 75% of their age-predicted maximal heart rate for 20 min, while sweat rate was measured simultaneously in both forearms. Immediately after cessation of exercise, the number of active sweat glands was measured by application of iodine-impregnated paper to each forearm. The sweat rate for the control and propranolol-treated forearm was 0.62 ± 41 and 0.60 ± 0.44 (SD) mg·cm(-2)·min(-1), respectively (P = 0.86). The density of active sweat glands for the control and propranolol-treated forearm was 130 ± 6 and 134 ± 5 (SD) glands/cm(2), respectively, (P = 0.33). End-exercise skin temperature was 32.9 ± 0.2 and 33.1 ± 0.3°C for the control and propranolol-treated forearm, respectively (P = 0.51). Results of the current study show that when propranolol is administered locally, thus eliminating the potential confounding systemic effects of the drug, it does not directly affect sweating during the initial stages of high-intensity exercise in young, healthy subjects.     

Cutaneous vasodilatation responses synchronize with sweat expulsions

To examine whether cutaneous active vasodilatation is mediated by sudomotor nerve fibres we recorded cutaneous blood flow and sweat rates continuously with laser-Doppler flowmetry and capacitance hygrometry, respectively, from the dorsal and plantar aspects of the foot in 11 male subjects at varying ambient temperatures (T _a) between 22 and 40°C (relative humidity 40%). In a warmer environment (T _a 29–40°C), predominant responses of the blood flow curve from the sole of the foot were transient depressions (negative blood flow responses, NBR), whereas those from the dorsal foot were transient increases (positive blood flow responses, PBR). The PBR on the dorsal foot occurred spontaneously or in response to mental or sensory stimuli, and when PBR did not fuse with each other the rate of PBR was linearly related to tympanic temperature. When dorsal foot sweating was continuous, PBR on the dorsal foot almost entirely synchronized with sweat expulsion. When dorsal foot sweating was intermittent PBR sometimes occurred on the dorsal foot without corresponding sweat expulsions, but these PBR showed a complete correspondence with subthreshold sweat expulsion seen on a methacholine-treated area. The amplitude and the duration of PBR showed a significant linear relationship with the amplitude and the duration of the corresponding sweat expulsion. In a thermoneutral or cooler environment (T _a 22–29°C), PBR occurred on the sole of the foot when mental or sensory stimuli elicited sweating in that area. Thus, PBR occurred when and where sweating appeared. Atropine failed to abolish PBR on the dorsal foot. Blockade of the peroneal nerve eliminated both PBR and NBR on the dorsal foot. The results indicate that an active vasodilatation mechanism is present on the sole of the foot as well as on the dorsal foot, and thus suggest that active vasodilatation is closely related to sudomotor nerve activation.     

Regional differences in the sweating responses of older and younger men.

Ten older (60-71 yr) and nine younger (20-25 yr) active healthy men were exposed to passive heating [by placing the lower legs and feet in a 43 degrees C water bath for 60 min while sitting in a warm (35 degrees C, 45% relative humidity) chamber] in summer and winter. The increase in rectal temperature (Tre) was significantly (P less than 0.05) greater, and mean skin temperature and forearm blood flow were lower, for the older men in both seasons. Total sweating rate was lower in the older men, but significantly (P less than 0.05) so only in the summer. The Tre threshold for sweating was unaffected by either age or site (back vs. thigh). The local sweating rate (msw) on the thigh was significantly lower (P less than 0.05) for the older men throughout the exposure, whereas there were no significant age-related differences for the average or peak values of back msw, although lesser sweating on the back occurred during the first 30 min of exposure. The decreased msw on the thigh was due to a lower sweat output per heat-activated sweat gland rather than from recruitment of fewer glands. It was concluded that regional differences exist in the age-related decrement in sweat gland function. Furthermore, these findings suggest that aging leads to a decreased ability to maintain body temperature with passive heating of the extremities, which may be attributed in part to decreased regional sweat gland function.     

Evidence for neurotransmitter plasticity in vivo. II. Immunocytochemical studies of rat sweat gland innervation during development

Previous studies of the cholinergic sympathetic innervation of rat sweat glands provide evidence for a change in neurotransmitter phenotype from noradrenergic to cholinergic during development. To define further the developmental history of cholinergic sympathetic neurons, we have used immunocytochemical techniques to examine developing and mature sweat gland innervation for the presence of the catecholamine synthetic enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) and for two neuropeptides present in the mature cholinergic innervation, vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP). In 7-day old animals, intensely TH- and DBH-immunoreactive axons were closely associated with the forming glands. The intensity of both the TH and DBH immunofluorescence decreased as the glands and their innervation developed. Neither TH-IR nor DBH-IR disappeared entirely; faint immunoreactivity for both enzymes was reproducibly detected in mature animals. In contrast to noradrenergic properties, the expression of peptide immunoreactivities appeared relatively late. No VIP-IR or CGRP-IR was detectable in the sweat gland innervation at 4 or 7 days. In some glands VIP-IR first appeared in axons at 10 days, and was evident in all glands by 14 days. CGRP-IR was detectable only after 14 days. In addition to VIP-IR and CGRP-IR, we examined the sweat gland innervation for several neuropeptides which have been described in noradrenergic sympathetic neurons including neuropeptide Y, somatostatin, substance P, and leu- and met-enkephalin; these peptides were not evident in either developing or mature sweat gland axons. Our observations provide further evidence for the early expression and subsequent modulation of noradrenergic properties in a population of cholinergic sympathetic neurons in vivo. In addition, the asynchronous appearance during development of the two neuropeptide immunoreactivities raises the possibility that the expression of peptide phenotypes may be controlled independently.