A Note on the use of Benzocaine (Ethyl P-Aminobenzoate) as a Fish Anaesthetic

Abstract

Anaesthetics are used widely to facilitate the handling of fish in commercial fish farms and in experimental studies. Anaesthetized fish may be marked and measured without severe skin damage which might reduce their viability after release. Increasing the ease of handling speeds up measuring and marking, benefiting both the fish and tlie handler. In experimental studies on fish, operations such as the implantation of electrodes or gonadectomy cannot be performed without full surgical anaesthesia.The main features required of any fish anaesthetic are efficacy, wide margin of safety between anaesthetic and toxic dose levels, ease of use, safety to fish and operators and availability without prescription. The most widely used anaesthetic for fish is MS 222 (Sandoz) (Ethyl m-aminobenzoate sulphonate) which is available in crystalline form and is highly soluble in water (solubility 11 per cent). Its anaesthetic properties have been discussed by Bell (1967) and Dick (1975) and others. However, it has several disadvantageous features including high price and possible danger to the health of operatives. Wedermeyer (1970) has shown that anaesthesia with unbuffered MS 222 can result in a marked stress effect (as measured by inter-renal ascorbic acid depletion as a result of the production of adrenocorticotrophic hormone (ACTH) by the pituitary). This effect was reduced if the MS 222 solution was buffered to pH7 and was also less marked with benzocaine, another anaesthetic.Both MS 222 and benzocaine (ethyl p-aminobenzoate) are homologues of benzoic acid. Benzocaine, however, lacks the sulphonate group (H SO3CH3) which is present in MS 222. This accounts for a difference in solubility of the two compounds (benzocaine is only 0-04 per cent soluble in water) and also explains why a solution of MS 222 in distilled water, at normal working concentration (30 parts per million) is acid (pH 3.8). The addition of benzocaine to distilled water has little effect on the pH. Where MS 222 is buffered to pH7 it exists in its free base form (ethyl m-aminobenzoate) which is chemically very similar to benzocaine (ethyl p-aminobenzoate).Benzocaine does not produce the initial excitement usually seen when fish are anaesthetized with unbuffered MS 222, possibly because the pH is unchanged. The fish are rapidly sedated and quickly become less responsive to external stimuli. Recovery from benzocaine anaesthesia is as rapid as with MS 222; in one experiment a 30 cm rainbow trout which had been anaesthetized for one hour was swimming again three minutes after being returned to the water. The severity of the effects of any anaesthetic on fish depend on the concentrations used and on the exposure time and should not be so great in naturally buffered hard waters. With regard to the effects of this anaesthetic on the operator, benzocaine is widely used in human medicine, particularly in throat lozenges where doses of up to 500 mg per day are permitted.

Growth requires bone resorption at particular skeletal elements in a teleost fish with acellular bone (Oreochromis niloticus, Teleostei: Cichlidae)

“…Larvae and juveniles were classed with a width of lmm; those exceeding lOmm were classed with a width of 2mm. Animals were killed with an overdose of the narcotic 4-aminobenzoic acid ethylester (benzocaine) (Laird and Oswald 1975).…”

Enzyme histochemical characteristics of osteoblasts and mononucleated osteoclasts in a teleost fish with acellular bone ( Oreochromis niloticus , Cichlidae)

“…Reproduction of the adults and growth of larvae took place under identical conditions (temperature 30 o C; pH 7.7; HCO 3 -6.3 o dH=2.2 mmol/l; total hardness 8.2 o dH=1.4 mmol/l; magnesium 2.3 mg/l=0.1 mmol/l; calcium 52 mg/l=1.3 mmol/l), as given by Rana (1986) and Meske and Münster (1988). Larvae (n=28; 20-60 mm standard length) were killed by an overdose of 4-aminobenzoic acid ethylester (benzocaine; Laird and Oswald 1975).…”

Protein metabolism during sexual maturation in female Atlantic salmon (Salmo salar L)

“…Individual fish were weighed, anaesthetized with benzocaine (0.3 g/l) (Laird and Oswald 1975) and injected into the caudal vein with 150 mM phenylalanine containing 20 /Ci (7.4 x 106 Bq/ml) of L-[2,6,-3 H] phenylalanine/ml (specific radioactivity = 300 d.p.m./nmole phenylalanine) at a dose of 1 ml/100 g body weight. The fish were returned to aerated water and left for various times (20, 40 and 60 min for the initial time course, 45 to 50 min in the main experiment).…”