The role of an identified neuron in conditioning

A bee's brain must be able to form associations between a sucrose reward and many different other stimuli, including a variety of odours. We could, therefore, expect that a neuron that plays a key role in conditioning would have branches distributed in many different regions of the brain. One such neuron is VUMmxl (Fig. 9.4b), an unpaired neuron which has branches that are arranged symmetrically on both the left and right sides of the sub-oesophageal ganglion and brain, and Martin Hammer (1993) demonstrated experimentally that this neuron can induce conditioning. Its branches overlap the pathways that link olfactory stimuli to the extension reflex at several places. Like many other unpaired, median neurons in insects, such as DUM neurons in the thoracic ganglia (see Box 7.2, p. 187), VUMmxl appears to contain and release octopamine.

Sucrose excites VUMmxl, and this neuron continues to spike for at least half a minute after the sucrose has been removed. VUMmxl is also excited, but only for a brief time, by various odours directed at the antennae. However, Hammer found that when sucrose was delivered just after a particular odour, the next time the odour was delivered alone VUMmxl responded with a brief, intense burst of spikes followed by excitation that continued for 20 s, a similar response to that given by VUMmxl to sucrose. Like proboscis extension, this response enhancement depended strictly on the two stimuli being delivered within a short time, with the start of the odour preceding the start of sucrose delivery. The neuronal mechanism that requires a particular stimulus order is unknown.

Exciting VUMmxl by injecting current into it through a microelectrode can substitute for stimulation with sucrose in conditioning (Fig. 9.5). In forward pairing, the odour stimulus started 2 s before the electrical excitation of VUMmxl, and during backward pairing, VUMmxl excitation preceded the odour by 5 s. Activation of the proboscis-extension muscle was monitored in an electromyogram recording. After forward pairing, a test pulse of odour l0 min later produced a large electro myogram response (Fig. 9.5b). In contrast, backward pairing of VUMmxl excitation with odour did not lead to any enhancement of the response to the test odour. This experiment clearly shows that VUMmxl can condition the proboscis-extension response to a particular odour. However, it does not show that VUMmxl is solely responsible for conditioning, and it probably normally works in parallel with other similar ventral, unpaired median neurons in the suboesophageal ganglion.

VUMmxl can deliver information about a sucrose reward to the olfactory lobes and the mushroom body, the sites that the experiments with local

Figure 9.5 Electrical excitation can substitute for stimulation with sucrose in conditioning the proboscis-extension response to odours. (a) An outline of the protocol used in training experiments. First, any response by proboscis-extension muscles to a puff of odour was measured in a pre-test, Next, a puff of the odour was paired with electrical excitation of VUMmxl by a pulse of depolarising current. In forward pairing, the odour preceded the start of the current pulse, and in backward pairing the current pulse started before the odour. Finally, responses to a test pulse of odour were recorded. (b) Responses to a test puff of odour after training with either electrical excitation of VUMmxl or a sucrose reward. The response was measured as the number of spikes in a proboscis extensor motor neuron during 10 s after a test or pre-test puff of odour, and the histograms plot the median response from groups of 11-15 bees. The error bars indicate the maximum responses from each group. (a and b redrawn after Hammer, 1993.)

Figure 9.5 Electrical excitation can substitute for stimulation with sucrose in conditioning the proboscis-extension response to odours. (a) An outline of the protocol used in training experiments. First, any response by proboscis-extension muscles to a puff of odour was measured in a pre-test, Next, a puff of the odour was paired with electrical excitation of VUMmxl by a pulse of depolarising current. In forward pairing, the odour preceded the start of the current pulse, and in backward pairing the current pulse started before the odour. Finally, responses to a test pulse of odour were recorded. (b) Responses to a test puff of odour after training with either electrical excitation of VUMmxl or a sucrose reward. The response was measured as the number of spikes in a proboscis extensor motor neuron during 10 s after a test or pre-test puff of odour, and the histograms plot the median response from groups of 11-15 bees. The error bars indicate the maximum responses from each group. (a and b redrawn after Hammer, 1993.)

cooling and octopamine injection showed are important in conditioning. A key event in conditioning is that excitation ofVUMmx1 is closely associated in time with stimulation of particular odour-activated pathways. A great deal of information about the molecular mechanisms for coincidence detection in neurons has been gained in studies on the gill-withdrawal reflex of the mollusc Aplysia (Box 9.1). The mechanism by which VUMmx1 establishes conditioning is likely to involve intracellular messenger molecules such as cyclic AMP and cyclic GMP and, in Drosophila, mutations that affect production of these molecules also cause impairment of memory (Belvin & Yin, 1997). Conditioning of proboscis extension in bees is quite complex because a large range of possible stimuli can become associated with the response, and it is probable that the complexity of the mushroom bodies is in some way related with the need to be able to deal with a large number of possible stimulus configurations.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment