Forebrain stimulation from receptors in tail

Authors: Nordgreen, J et al
Journal: J Comp Physio) A 93:1235-1242
Year: 2007
Where: Beijing, Norway

An evoked potential (or "evoked response") is an electrical potential recorded from the nervous system of a human or other animal following presentation of a stimulus. They have been widely used in clinical diagnostic medicine since the 1970s.

Norwegian scientists investigated the telencephalon in fish. This is an area of the brain that is important in learning, memory, emotion, and may process pain.

Salmon were anaesthetised. A recording electrode was then placed in their brains. The tail was given electrical shocks. The electrical recordings made were of the same form that are found when pain receptors are stimulated in mammals. They have been found before in other species of fish. The experiment added to the body of literature that indicates that pain is processed by the telencephalon.

TELENCEPHALON ESSENTIAL FOR PROCESSING
EXTERNAL STIMULI ON EMOTIONAL LEVEL

"We investigated whether a galvanic stimulation of the tail base in Atlantic salmon would elicit a somatosensory evoked potential in the telencephalon. The telencephalon is central in learning and memory, and activity here may be a prerequisite for processing of external stimuli on a cognitive or emotional level."

TAIL STIMULATED AND RESPONSES MEASURED IN TELENCEPHALON

"Anaesthetized salmon (n = 11) were subjected to craniotomy and a recording electrode was inserted into the telencephalon. The fish were given stimulations of four intensities, i.e., 2,5,10 and 20 mA. A somatosensory evoked potential was elicited in the contralateral dorsal telencephalon for all intensities."

"The base of the tail was chosen as the stimulation site because fish are very sensitive to touch in this area."

"For ethical reasons, it was decided not to perform the experiments on conscious fish. All fish were euthanazed with an overdose of chlorbutanol immediately after the last series of stimulations, without being allowed to regain consciousness. In addition, local anaesthesia was used to prevent nociceptive input from the craniotomy."

"The strength of the signal measured as maximum peak amplitude and mean amplitude increased with increasing stimulus intensity whereas the peak latency did not. This kind of stimulus-response relationship is typically found for mammalian nociceptors."

"This result agrees with findings in other fish species. Furthermore, there was a significant difference between the maximum peak amplitude and mean amplitude of the somatosensory evoked potential elicited by putative non-noxious (2 mA) and putative noxious (20 mA) stimulation intensities (P<0.01). The stronger stimulation intensities also tend to introduce longer-latencies components in the somatosensory evoked potential. The results added to the body of literature indicates that the exteroceptive¹ senses are represented by processing within the telencephalon of the fish."

PATHWAY FROM PAIN RECEPTORS TO HIGHER BRAIN STRUCTURES

"One of the eight criteria for pain perception in animals is that there should be a pathway from nociceptors&sup4; in the periphery to higher brain structures. By recording the somatosensory evoked potential following noxious stimulation in the periphery, it is possible to investigate whether the signal travels to higher structures in the CNS and in which brain areas a somatosensory evoked potential is elicited. Dunlop and Laming (2005) measured single-unit activity in all main brain subdivisions, including the telencephalon, in goldfish and rainbow trout following putatively non-noxious and noxious stimulation."

TELENCEPHALON INPUT FROM THALAMUS
- A BRAIN PAIN RELAY STRUCTURE IN MAMMALS

"The telencephalon of teleosts² can be divided into a dorsal (pallial) and ventral (subpallial) part. The dorsal telencephalon receives input from several brain structures, including the thalamus. The thalamus is an important relay structure for nociceptive input in mammals."

"The dorsal telencephalon in teleosts is thought to be homologous to the pallial part of cerebrum in tetrapods³. In mammals, somatosensory information is relayed through the thalamus to the cortex. This is most likely also the case in fish, as the thalamus in teleosts receives axons from somatosensory structures (Ito et al. 1986) and projects to the dorsal part of the telencephalon."

"Dunlop and Laming (2005) found that the telencephalon is activated in response to stimulations to the periphery."

"It is known that the telencephalon receives multisensory input projecting to several areas."

TELENCEPHALON EQUIVALENT TO
HIPPOCAMPUS AND AMYGDALA IN MAMMALS

"Psychobiological studies have shown the pallial telencephalon to be important in emotionally motivated associative learning, in spatial learning and in memory. More specifically, lateral zone of the dorsal (pallial) telencephalon shows homologies to the mammalian hippocampus and medial zone of the dorsal (pallial) telencephalon is thought to be analogous to the amygdala of mammals. In humans, the hippocampus and amygdala are involved in pain perception."

Our results add to an increasing body of studies indicating that fish have many of the prerequisites for complex processing of external events."
Dunlop R, Laming P (2005) Mechanoreceptive and nociceptive responses in the central nervous system of goldfish (Carassius miraius) and trout (Oncorhynchus mykisi). J Pain 6:561-568
Ito II. Murakami T, Fukuoka T, Kishida R (1986) Thalamic fiber connections in a teleost (Sebastiscus marmoratus): Visual somatosensory, octavolateral, and cerebellar relay region to the telencephalon. J Comp Neurol 250:215-227

¹Exteroceptive = sensitivity to stimuli originating outside of the body
²Teleosts = fishes with bony skeletons
³Tetrapods = all vertebrates, except fish
4A nociceptor is a sensory receptor that reacts to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process is called nociception.


Fish Pain